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NICHOLAS DIBELLA: So last
time, at the end of last class,

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00:00:23,950 --> 00:00:29,380
I began to talk to you about
quantum tunneling, which

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00:00:29,380 --> 00:00:34,030
allows for a lots of
crazy events to occur.

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It allows me to walk
through chalkboards.

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00:00:37,280 --> 00:00:40,000
It allows me to fly.

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00:00:40,000 --> 00:00:45,610
It allows for various
types of magic to be true.

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And it's just so wonderful
that quantum mechanics

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allows for these really
strange things to occur.

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00:00:51,700 --> 00:00:55,360
It kind of gives us hope--
it gives possibility--

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00:00:55,360 --> 00:01:02,830
that pretty much whatever
you can imagine can occur.

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00:01:02,830 --> 00:01:07,390
And today, I'd like to expand
on this theme of possibility,

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00:01:07,390 --> 00:01:10,690
and talk to you all
about parallel universes.

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00:01:15,370 --> 00:01:21,640
So you're all here today
in this room at MIT,

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00:01:21,640 --> 00:01:24,800
watching me talk
about some stuff.

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00:01:24,800 --> 00:01:28,580
But would you believe
that in a far,

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00:01:28,580 --> 00:01:35,680
far away galaxy there is an
exact copy of this classroom

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00:01:35,680 --> 00:01:40,360
with ab exact copy of each
one of you-- each person--

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00:01:40,360 --> 00:01:43,840
with each person having every
single characteristic of you,

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00:01:43,840 --> 00:01:49,870
having the exact DNA, the exact
memories, the exact preference

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00:01:49,870 --> 00:01:52,900
of ice cream, the
exact everything,

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00:01:52,900 --> 00:01:57,710
and an exact copy of me
talking to you about stuff?

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00:01:57,710 --> 00:02:02,260
Well, according to some
parallel universe theories,

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00:02:02,260 --> 00:02:04,590
this actually is the case.

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00:02:04,590 --> 00:02:09,190
Whereas, in this universe,
I simply mention that fact

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00:02:09,190 --> 00:02:11,890
and continue to talk
about some other things,

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00:02:11,890 --> 00:02:17,170
in another universe I
could mention that fact

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00:02:17,170 --> 00:02:18,310
and just leave the room.

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00:02:21,220 --> 00:02:23,160
[DOOR CREAKING]

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00:02:23,160 --> 00:02:26,070
[MURMURS]

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00:02:34,320 --> 00:02:35,896
Hi.

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00:02:35,896 --> 00:02:37,500
We're not in that
universe, however.

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00:02:40,170 --> 00:02:41,010
OK.

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00:02:41,010 --> 00:02:45,810
Now, what do I mean
by parallel universes?

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00:02:45,810 --> 00:02:48,450
Well, first I need to be a
little bit more precise on what

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00:02:48,450 --> 00:02:53,790
I mean by universe, because it's
used in a few different ways.

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00:02:53,790 --> 00:02:56,370
Now, usually when we
think about the universe,

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00:02:56,370 --> 00:02:59,550
we think about "the
universe"-- the everything

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00:02:59,550 --> 00:03:00,540
that's out there.

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00:03:00,540 --> 00:03:03,340
We think of that as
being the universe.

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00:03:03,340 --> 00:03:07,270
Well, in this phrase,
parallel universes,

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00:03:07,270 --> 00:03:09,360
the type of universe
that I'm referring to

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00:03:09,360 --> 00:03:11,200
is a little bit different.

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00:03:11,200 --> 00:03:14,004
And there's one type
of parallel universe

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00:03:14,004 --> 00:03:15,420
that I'd like to
talk about first,

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00:03:15,420 --> 00:03:18,850
and that has a very specific
meaning of universe.

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00:03:18,850 --> 00:03:21,870
And that's the
observable universe,

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00:03:21,870 --> 00:03:23,440
which I'll write down here.

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00:03:33,050 --> 00:03:37,480
So as you all know, the
universe is pretty big--

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00:03:37,480 --> 00:03:39,070
pretty big.

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00:03:39,070 --> 00:03:42,550
But there's only a certain
fraction of it that we can see.

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00:03:42,550 --> 00:03:46,210
And that's because
of the big bang.

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00:03:46,210 --> 00:03:50,460
And let me tell you
briefly about the big bang.

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00:03:50,460 --> 00:03:54,280
You'll get a lot more about
this in a couple of weeks,

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00:03:54,280 --> 00:03:58,490
but let me just briefly
go over the big bang.

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So a long time ago, all the
galaxies, and all the stars,

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and all the matter were a lot
closer than they were today--

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extremely close.

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00:04:12,040 --> 00:04:13,660
Infinitely close, in fact.

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And it's speculated that about
14 billion years ago, all

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of the matter and all of
the energy in the universe

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00:04:21,850 --> 00:04:24,590
was concentrated
at a single point.

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And for some reason,
these bits of matter

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just started to expand
away from each other.

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00:04:34,750 --> 00:04:39,750
And the reason for the expansion
was that space itself expanded.

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00:04:39,750 --> 00:04:42,010
And as a result,
everything sitting on space

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00:04:42,010 --> 00:04:45,700
would get farther
apart from each other.

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And that might
sound kind of weird,

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00:04:47,560 --> 00:04:51,860
so let me give you an analogy.

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Suppose you're an ant and you're
living on top of a volleyball.

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So this is you.

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And according to
you, your universe

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consists only of two dimensions.

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You can either
walk one way or you

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can walk another way,
similar to how we move around

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on the surface of the Earth.

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Relative to the ground
there's only, essentially,

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00:05:32,380 --> 00:05:33,864
two directions we can move.

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Well, that's what the universe
is like for this ant living

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on a volleyball.

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Of course, we, the
omniscient observers,

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00:05:41,140 --> 00:05:42,670
see the whole
volleyball, and so we

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00:05:42,670 --> 00:05:44,770
recognize that
the ant's actually

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living on the surface of a
three dimensional object.

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00:05:50,380 --> 00:05:52,570
But the ant doesn't know that.

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00:05:52,570 --> 00:05:58,524
Well, what would happen if we
expanded this ant's universe?

93
00:05:58,524 --> 00:06:00,565
What would happen if we
expanded this volleyball?

94
00:06:08,450 --> 00:06:10,090
We still want to
see it of course.

95
00:06:10,090 --> 00:06:14,960
Let's say this is before,
and this is after.

96
00:06:19,910 --> 00:06:22,130
Of course, the ant is
still the same size,

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00:06:22,130 --> 00:06:26,300
it's just that the volleyball
has gotten much bigger.

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00:06:26,300 --> 00:06:31,220
So the ant's still lower
here, but something

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00:06:31,220 --> 00:06:33,720
has changed for the ant.

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00:06:33,720 --> 00:06:37,230
On this universe there
could be, for instance,

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00:06:37,230 --> 00:06:43,160
lots of other ants just
specked all around,

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00:06:43,160 --> 00:06:45,440
just sitting around
various points.

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00:06:48,855 --> 00:06:51,510
We didn't completely
raise erase perfectly,

104
00:06:51,510 --> 00:06:53,999
so let me make this
a little bit better.

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00:06:53,999 --> 00:06:55,790
So there are these
points on the volleyball

106
00:06:55,790 --> 00:06:58,160
that the ant observes,
and they're all

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00:06:58,160 --> 00:07:01,880
separated at certain
distances from each other.

108
00:07:01,880 --> 00:07:04,550
And once you expand
this volleyball,

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00:07:04,550 --> 00:07:08,180
these points, as a result of
the expansion of the volleyball,

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00:07:08,180 --> 00:07:10,470
get farther apart
from each other.

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00:07:10,470 --> 00:07:11,930
So I drew five dots there.

112
00:07:11,930 --> 00:07:16,490
Now there are still five dots,
but they are farther apart

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00:07:16,490 --> 00:07:18,930
from each other.

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00:07:18,930 --> 00:07:22,250
And then that's something
that the ant would observe.

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00:07:22,250 --> 00:07:27,470
Well, suppose right
at the expansion,

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00:07:27,470 --> 00:07:34,190
one of these other ants--
suppose these dots are ants--

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00:07:34,190 --> 00:07:43,080
sent a signal-- send some
light beam to our friendly ant,

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00:07:43,080 --> 00:07:44,960
the ant that we
started off with.

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00:07:44,960 --> 00:07:50,070
So right before the expansion,
some ant sent a signal.

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00:07:50,070 --> 00:07:54,640
So the signal travels
from this ant to that ant.

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00:07:54,640 --> 00:07:57,980
Well, suppose the
ant sends right

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00:07:57,980 --> 00:08:00,710
when the expansion starts.

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00:08:00,710 --> 00:08:05,810
Well, it's going to take
some time for that signal

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00:08:05,810 --> 00:08:11,399
to reach the original
ant, because light travels

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00:08:11,399 --> 00:08:13,940
at a certain speed, and it takes
a certain time to get there.

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00:08:16,790 --> 00:08:21,440
Similarly, in our universe,
and our universe's very early

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00:08:21,440 --> 00:08:29,720
history, light from other
stars and other galaxies

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00:08:29,720 --> 00:08:35,159
has taken time to reach
us since the big bang.

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00:08:35,159 --> 00:08:41,880
And there are some
stars that have since

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00:08:41,880 --> 00:08:43,950
moved very far away from us--

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00:08:43,950 --> 00:08:45,630
extremely far away from us--

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00:08:45,630 --> 00:08:47,250
and the light from
those stars simply

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00:08:47,250 --> 00:08:49,630
hasn't had enough
time to reach us yet.

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00:08:49,630 --> 00:08:53,090
And so, there's a certain
distance away from us

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00:08:53,090 --> 00:08:56,770
that is the maximum distance
that we can observe.

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00:08:56,770 --> 00:09:00,560
And this distance
defines a whole sphere

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00:09:00,560 --> 00:09:04,050
called the Hubble
sphere, which we refer

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00:09:04,050 --> 00:09:07,390
to as the observable universe.

139
00:09:07,390 --> 00:09:12,090
So there's only a small
section of the whole universe

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00:09:12,090 --> 00:09:16,000
that we can observe, and we call
that the observable universe.

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00:09:16,000 --> 00:09:17,700
AUDIENCE: What's the radius of--

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00:09:17,700 --> 00:09:20,180
NICHOLAS DIBELLA: What's the
radius of the Hubble sphere?

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00:09:20,180 --> 00:09:21,840
The radius of the
observable universe?

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00:09:21,840 --> 00:09:26,260
I think it's 40
billion light years.

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00:09:26,260 --> 00:09:28,890
And that's and that's different
from the age of the universe,

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00:09:28,890 --> 00:09:33,600
because actually
the space between us

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00:09:33,600 --> 00:09:36,000
has actually expanded.

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00:09:36,000 --> 00:09:42,210
And there are some
effects that give rise

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00:09:42,210 --> 00:09:44,280
to that increased distance.

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00:09:44,280 --> 00:09:47,340
And in terms of meters,
I have that for you.

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00:09:47,340 --> 00:09:50,160
In terms of meters,
it's about 10

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00:09:50,160 --> 00:09:54,270
to the 27th meters,
which is pretty big.

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00:09:54,270 --> 00:09:57,060
It would take it would take
lights about 40 billion years

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00:09:57,060 --> 00:10:00,630
to reach from here until the
end of the Hubble volume--

155
00:10:00,630 --> 00:10:02,850
to the end of the
observable universe.

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00:10:02,850 --> 00:10:07,230
Now, as each year
passes by, light

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00:10:07,230 --> 00:10:09,270
has an extra year to travel.

158
00:10:09,270 --> 00:10:11,850
And so, for each
year that passes by,

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00:10:11,850 --> 00:10:16,087
the observable universe
increases by one light year.

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00:10:16,087 --> 00:10:18,420
So the observable universe
is getting bigger and bigger.

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00:10:20,950 --> 00:10:25,290
Now, strictly speaking,
we don't really

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00:10:25,290 --> 00:10:28,980
know if there's anything outside
of the observable universe,

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00:10:28,980 --> 00:10:30,960
because we just
can't observe it.

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00:10:30,960 --> 00:10:36,840
And so, you might say
that anything that's

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00:10:36,840 --> 00:10:38,970
outside of the
observable universe

166
00:10:38,970 --> 00:10:44,610
is just subject to
mystical speculations

167
00:10:44,610 --> 00:10:48,180
or metaphysical speculations,
and science can't do anything

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00:10:48,180 --> 00:10:51,790
about those places.

169
00:10:51,790 --> 00:10:58,950
Well, actually, surprisingly,
we can say some things

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00:10:58,950 --> 00:11:01,950
about what's outside of
the observable universe,

171
00:11:01,950 --> 00:11:05,874
although the things that
we say are more indirect.

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00:11:05,874 --> 00:11:11,130
Now, there's the question
that space might be finite

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00:11:11,130 --> 00:11:13,920
or it might be infinite.

174
00:11:13,920 --> 00:11:19,520
And we now have a kind of
good answer to that question.

175
00:11:19,520 --> 00:11:22,830
But first, what does it mean
for space to be finite or space

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00:11:22,830 --> 00:11:23,880
to be infinite?

177
00:11:23,880 --> 00:11:31,830
Well, if space were finite, you
might imagine that you take off

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00:11:31,830 --> 00:11:34,660
from a rocket ship on
earth, and then you travel

179
00:11:34,660 --> 00:11:37,706
and you keep traveling
farther and farther away.

180
00:11:37,706 --> 00:11:43,740
And then, you eventually
reach a sign that says,

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00:11:43,740 --> 00:11:46,860
this is the end of the universe.

182
00:11:46,860 --> 00:11:48,990
Dead end.

183
00:11:48,990 --> 00:11:50,850
You might reach a
sign that says that.

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00:11:50,850 --> 00:11:55,150
Well, that's not what I
mean by space being finite.

185
00:11:55,150 --> 00:11:56,790
What I mean by
space being finite,

186
00:11:56,790 --> 00:12:02,910
is something like this
volleyball universe of the ant.

187
00:12:02,910 --> 00:12:04,260
It's finite for--

188
00:12:07,110 --> 00:12:09,339
I mean, we, the
omniscient observer,

189
00:12:09,339 --> 00:12:11,880
look at this volleyball and we
see that it has a finite size.

190
00:12:15,000 --> 00:12:17,190
Of course, this is an analogy.

191
00:12:17,190 --> 00:12:23,670
It's an analogy to the real
world, where our universe might

192
00:12:23,670 --> 00:12:28,980
possibly be some four
dimensional hypersphere

193
00:12:28,980 --> 00:12:30,780
that we're living on top of.

194
00:12:30,780 --> 00:12:37,310
So it's a four dimensional
extension to this volleyball.

195
00:12:37,310 --> 00:12:41,340
In that sense, the
universe might be finite.

196
00:12:41,340 --> 00:12:44,790
And one consequence
of this is actually

197
00:12:44,790 --> 00:12:49,500
that if you started at some
point in the universe--

198
00:12:49,500 --> 00:12:51,520
supposed you started here.

199
00:12:51,520 --> 00:12:54,120
If you started at this point in
the universe and you traveled,

200
00:12:54,120 --> 00:12:58,110
say, this direction, and
just kept traveling straight

201
00:12:58,110 --> 00:13:01,080
forward, eventually
you would return

202
00:13:01,080 --> 00:13:03,870
to where you started off.

203
00:13:03,870 --> 00:13:08,130
And that seems kind
of weird, right?

204
00:13:08,130 --> 00:13:12,420
If I keep traveling this
way, then ordinarily I

205
00:13:12,420 --> 00:13:14,700
wouldn't think that
eventually I'd return here,

206
00:13:14,700 --> 00:13:17,550
but actually we know that
the earth is earth is round,

207
00:13:17,550 --> 00:13:19,770
so eventually we're
going to reach back

208
00:13:19,770 --> 00:13:21,180
at our original point.

209
00:13:21,180 --> 00:13:26,700
And that's similar to what would
happen in a finite universe.

210
00:13:26,700 --> 00:13:31,140
Well, it looks like this
actually isn't the case.

211
00:13:31,140 --> 00:13:35,150
There are some very
interesting measurements

212
00:13:35,150 --> 00:13:39,230
that people have done
about some radiation

213
00:13:39,230 --> 00:13:42,200
that we observe in the sky
called the cosmic microwave

214
00:13:42,200 --> 00:13:45,290
background radiation.

215
00:13:45,290 --> 00:13:47,972
People have done
measurements of the stuff.

216
00:13:47,972 --> 00:13:50,180
I'm not going to get into
exactly what that stuff is,

217
00:13:50,180 --> 00:13:53,570
but in a couple of weeks
you'll know more about it.

218
00:13:53,570 --> 00:13:55,525
But there is this
radiation that we just

219
00:13:55,525 --> 00:13:57,650
see when we look out-- when
we point our telescopes

220
00:13:57,650 --> 00:13:59,160
and look at the sky.

221
00:13:59,160 --> 00:14:00,710
There's this radiation.

222
00:14:00,710 --> 00:14:05,120
And the way that this
radiation looks actually

223
00:14:05,120 --> 00:14:09,950
gives us an indication to
the geometry of the universe.

224
00:14:09,950 --> 00:14:11,580
There are these
spots, for instance,

225
00:14:11,580 --> 00:14:13,520
in the radiation
that would indicate

226
00:14:13,520 --> 00:14:16,430
whether the universe
is spherical,

227
00:14:16,430 --> 00:14:20,930
or it might be hyperbolic,
or it might be flat.

228
00:14:20,930 --> 00:14:26,510
It might just be one big line--

229
00:14:26,510 --> 00:14:30,260
one hyperplane we
would have to say.

230
00:14:30,260 --> 00:14:33,590
And it looks like the
universe actually is infinite.

231
00:14:33,590 --> 00:14:36,860
And it looks like
the universe is flat,

232
00:14:36,860 --> 00:14:40,460
which is probably the least
interesting case that's

233
00:14:40,460 --> 00:14:41,120
conceivable.

234
00:14:41,120 --> 00:14:43,230
But that's what it looks like.

235
00:14:43,230 --> 00:14:46,610
So it looks like the whole
universe is infinite and flat.

236
00:14:49,330 --> 00:14:52,140
So let's assume that's the case.

237
00:14:52,140 --> 00:14:54,190
It's supported by observations.

238
00:14:54,190 --> 00:15:02,980
And I think it's
actually seems--

239
00:15:02,980 --> 00:15:06,700
it's kind of hard to
reason from intuition

240
00:15:06,700 --> 00:15:08,555
that the universe
should be infinite,

241
00:15:08,555 --> 00:15:10,930
so I would try to reason from
intuition that the universe

242
00:15:10,930 --> 00:15:11,830
should be infinite.

243
00:15:11,830 --> 00:15:13,900
But maybe it seems
natural to some of you

244
00:15:13,900 --> 00:15:19,840
that the universe should be
infinite rather than finite.

245
00:15:19,840 --> 00:15:21,680
In that case, you'd
probably like this.

246
00:15:21,680 --> 00:15:24,410
You'd probably like this fact.

247
00:15:24,410 --> 00:15:25,210
Question?

248
00:15:25,210 --> 00:15:27,444
AUDIENCE: What do you
mean the universe is flat?

249
00:15:27,444 --> 00:15:29,860
NICHOLAS DIBELLA: Oh, what do
I mean the universe is flat?

250
00:15:29,860 --> 00:15:36,200
Well, this is what I mean by
the universe being curved.

251
00:15:36,200 --> 00:15:38,750
For these ants, their
universe is curved.

252
00:15:38,750 --> 00:15:42,890
AUDIENCE: Oh, so it's not
like it's a flat plane,

253
00:15:42,890 --> 00:15:45,492
just it doesn't curve?

254
00:15:45,492 --> 00:15:47,200
NICHOLAS DIBELLA:
Yeah, it doesn't curve.

255
00:15:47,200 --> 00:15:50,350
If it went one direction,
you wouldn't return

256
00:15:50,350 --> 00:15:51,780
to where you started off.

257
00:15:51,780 --> 00:15:54,220
That's what I mean by
the universe being flat.

258
00:15:54,220 --> 00:15:58,168
And it looks like the
universe is flat and infinite.

259
00:15:58,168 --> 00:15:59,788
AUDIENCE: In all
geometries, would you

260
00:15:59,788 --> 00:16:02,060
be turned toward where
you started though?

261
00:16:02,060 --> 00:16:03,020
NICHOLAS DIBELLA: In all
geometries, would what?

262
00:16:03,020 --> 00:16:04,130
AUDIENCE: Like, if
you're on a hypersphere,

263
00:16:04,130 --> 00:16:06,088
then unless you come back
to where you started,

264
00:16:06,088 --> 00:16:08,112
it's not like that some
different geometry it

265
00:16:08,112 --> 00:16:11,635
would be possible for it to
be curved, but [INAUDIBLE]

266
00:16:11,635 --> 00:16:12,760
NICHOLAS DIBELLA: Oh, yeah.

267
00:16:12,760 --> 00:16:15,530
It could be possible, that
the geometry is curved

268
00:16:15,530 --> 00:16:18,010
but we don't return
where we started.

269
00:16:18,010 --> 00:16:19,810
You can imagine something like--

270
00:16:19,810 --> 00:16:21,271
I don't know.

271
00:16:25,637 --> 00:16:26,470
Something like this.

272
00:16:30,400 --> 00:16:32,050
So I go from 1D to
2D, and then you

273
00:16:32,050 --> 00:16:33,550
have to use your
minds to make a 3D.

274
00:16:33,550 --> 00:16:36,271
And then you have to use
your minds to make it 4D.

275
00:16:36,271 --> 00:16:38,020
Yeah, you can imagine
something like this.

276
00:16:38,020 --> 00:16:40,900
You travel this way, keep
going, keep going, keep going,

277
00:16:40,900 --> 00:16:43,420
and then you never return
to where you started.

278
00:16:43,420 --> 00:16:45,270
Yeah, that's possible.

279
00:16:45,270 --> 00:16:49,150
You could have fun thinking
about possible geometries

280
00:16:49,150 --> 00:16:50,890
the universe could be.

281
00:16:50,890 --> 00:16:53,080
But that's not going to
correspond to reality

282
00:16:53,080 --> 00:16:53,702
unfortunately.

283
00:16:53,702 --> 00:16:54,910
We can fun thinking about it.

284
00:16:54,910 --> 00:16:56,770
And for a long time, this is
all that people were able to do,

285
00:16:56,770 --> 00:16:57,603
just think about it.

286
00:16:57,603 --> 00:16:59,200
We didn't have
data long time ago.

287
00:16:59,200 --> 00:17:00,908
We don't have measurements
that indicated

288
00:17:00,908 --> 00:17:03,130
that the universe is infinite.

289
00:17:03,130 --> 00:17:04,780
Question?

290
00:17:04,780 --> 00:17:06,818
AUDIENCE: How is 4D
supposed to look?

291
00:17:06,818 --> 00:17:08,859
NICHOLAS DIBELLA: OK, how
is 4D supposed to look.

292
00:17:08,859 --> 00:17:11,050
I can't tell you how
4D is supposed to look.

293
00:17:11,050 --> 00:17:13,089
Nobody can tell you
what 4D looks like.

294
00:17:13,089 --> 00:17:15,670
You just have to have
to try to visualize

295
00:17:15,670 --> 00:17:19,480
an analogy of what some
properties of it would be like.

296
00:17:19,480 --> 00:17:22,930
You can't actually see
it, and that's something

297
00:17:22,930 --> 00:17:25,240
that we have to live
with unfortunately.

298
00:17:25,240 --> 00:17:29,050
I mean, sorry.

299
00:17:29,050 --> 00:17:31,658
Yeah, question?

300
00:17:31,658 --> 00:17:33,606
AUDIENCE: There are
actually diagrams

301
00:17:33,606 --> 00:17:38,640
of what they think it might
possibly look like, except not.

302
00:17:38,640 --> 00:17:40,850
They kind of explain--

303
00:17:40,850 --> 00:17:42,620
NICHOLAS DIBELLA:
Well, I know there

304
00:17:42,620 --> 00:17:47,165
are some diagrams I've heard
of that, once you look at them,

305
00:17:47,165 --> 00:17:48,790
you kind of get an
illusion that you're

306
00:17:48,790 --> 00:17:49,750
seeing the fourth dimension.

307
00:17:49,750 --> 00:17:51,375
Is that kind of what
you're describing?

308
00:17:51,375 --> 00:17:54,330
AUDIENCE: No, like, diagrams
that show hypercubes.

309
00:17:54,330 --> 00:17:55,840
NICHOLAS DIBELLA: Oh, oh, oh.

310
00:17:55,840 --> 00:17:58,940
Well, those diagrams that
you're talking about are--

311
00:17:58,940 --> 00:18:03,100
they're four dimensional
objects that are embedded

312
00:18:03,100 --> 00:18:04,790
onto a two dimensional surface.

313
00:18:04,790 --> 00:18:06,570
So you're not really seeing it.

314
00:18:06,570 --> 00:18:10,992
AUDIENCE: And how
would they think

315
00:18:10,992 --> 00:18:13,581
that it's flat and infinite?

316
00:18:13,581 --> 00:18:15,580
NICHOLAS DIBELLA: How,
do they actually do this?

317
00:18:15,580 --> 00:18:16,930
How do they actually
infer from the data

318
00:18:16,930 --> 00:18:18,130
that it's flat and infinite?

319
00:18:18,130 --> 00:18:18,976
Well, that's--

320
00:18:18,976 --> 00:18:21,642
AUDIENCE: If you can only see as
far as the observable universe,

321
00:18:21,642 --> 00:18:23,522
then that means that
no matter what you see,

322
00:18:23,522 --> 00:18:26,125
you can't tell what's beyond
that, and it could be.

323
00:18:26,125 --> 00:18:27,000
NICHOLAS DIBELLA: OK.

324
00:18:27,000 --> 00:18:29,200
Well, we can't
directly observe what's

325
00:18:29,200 --> 00:18:31,360
outside of the
observable universe,

326
00:18:31,360 --> 00:18:37,659
but we can make
indirect inferences.

327
00:18:37,659 --> 00:18:39,284
AUDIENCE: But nothing
comes from there.

328
00:18:39,284 --> 00:18:40,595
Like, you can't see radiation.

329
00:18:40,595 --> 00:18:41,470
You can't--

330
00:18:41,470 --> 00:18:43,261
NICHOLAS DIBELLA:
Nothing comes from there,

331
00:18:43,261 --> 00:18:45,310
but the universe--
the total universe--

332
00:18:45,310 --> 00:18:48,410
has a certain geometrical shape.

333
00:18:48,410 --> 00:18:53,710
And there are clever ways
of making these inferences.

334
00:18:53,710 --> 00:19:00,850
I don't have time to get into
them now, but we'll talk later.

335
00:19:00,850 --> 00:19:04,720
So for the purpose of the
rest of today's class,

336
00:19:04,720 --> 00:19:08,809
assume that the
universe is infinite.

337
00:19:08,809 --> 00:19:10,600
This is the assumption
we're going to make.

338
00:19:29,870 --> 00:19:35,810
This has so many wonderful
consequences, the assumption

339
00:19:35,810 --> 00:19:37,490
that the universe is infinite.

340
00:19:37,490 --> 00:19:42,810
I should say "the
universe" is infinite.

341
00:19:45,750 --> 00:19:48,830
there are some theories
about the early--

342
00:19:48,830 --> 00:19:51,830
well, pretty much
all the theories

343
00:19:51,830 --> 00:19:55,880
about the early
universe speculate

344
00:19:55,880 --> 00:20:02,900
that a long time ago after
this massive expansion

345
00:20:02,900 --> 00:20:05,030
following the big bang--

346
00:20:05,030 --> 00:20:07,520
well, during this expansion--

347
00:20:07,520 --> 00:20:12,880
all the material
that was present

348
00:20:12,880 --> 00:20:20,750
dispersed randomly and formed
certain arrangements of matter.

349
00:20:20,750 --> 00:20:24,890
And not only did it form
some types of arrangements

350
00:20:24,890 --> 00:20:28,550
of matter, but
actually this expansion

351
00:20:28,550 --> 00:20:33,170
created all different possible
arrangements of matter

352
00:20:33,170 --> 00:20:35,820
to spread out.

353
00:20:35,820 --> 00:20:39,140
So imagine a very
long time ago, you've

354
00:20:39,140 --> 00:20:42,772
got all every possible
arrangement of matter.

355
00:20:42,772 --> 00:20:48,650
So in this room, for instance,
there are air molecules,

356
00:20:48,650 --> 00:20:51,520
there are molecules
making up you,

357
00:20:51,520 --> 00:20:53,750
there are atoms and
subatomic particles,

358
00:20:53,750 --> 00:20:57,630
and there's a finite number
of particles in this room.

359
00:20:57,630 --> 00:21:00,860
And there is a
finite number of ways

360
00:21:00,860 --> 00:21:02,728
that you can arrange
those particles.

361
00:21:05,390 --> 00:21:07,700
I could, for instance--

362
00:21:07,700 --> 00:21:09,390
I'll make an analogy.

363
00:21:09,390 --> 00:21:15,950
Suppose I've got a
rectangular universe.

364
00:21:15,950 --> 00:21:17,420
Before I had a
volleyball universe,

365
00:21:17,420 --> 00:21:20,320
now suppose I have a
rectangular universe.

366
00:21:20,320 --> 00:21:22,070
Suppose I have a
rectangular universe that

367
00:21:22,070 --> 00:21:26,020
has four different
types of particles.

368
00:21:28,840 --> 00:21:32,720
No-- let's say there are two
different types of particles.

369
00:21:32,720 --> 00:21:34,330
So there's particle
1 and particle 2.

370
00:21:37,495 --> 00:21:38,870
These are all the
different types

371
00:21:38,870 --> 00:21:46,910
of particles in our fake
universe-- our play universe.

372
00:21:46,910 --> 00:21:48,530
We have 1 and 2.

373
00:21:48,530 --> 00:21:55,220
And suppose there's only
space in our universe for four

374
00:21:55,220 --> 00:21:57,220
different particles.

375
00:21:57,220 --> 00:21:59,900
Or yeah, say there's only
space for four of them.

376
00:21:59,900 --> 00:22:01,670
Then what can we do?

377
00:22:01,670 --> 00:22:05,300
We can put 1 here.

378
00:22:05,300 --> 00:22:08,000
We can put 2 here.

379
00:22:08,000 --> 00:22:10,190
We can put another 2 here.

380
00:22:10,190 --> 00:22:13,165
And we can put another
particle 1 there.

381
00:22:13,165 --> 00:22:14,540
I know it looks
like there's more

382
00:22:14,540 --> 00:22:16,352
there's more space
for more particles,

383
00:22:16,352 --> 00:22:17,810
but let's say that
this is the most

384
00:22:17,810 --> 00:22:23,539
that our universe our fake
universe can accommodate.

385
00:22:23,539 --> 00:22:25,830
So this is one possible
arrangement of those particles.

386
00:22:25,830 --> 00:22:27,800
Another possible arrangement
of those particles

387
00:22:27,800 --> 00:22:32,430
would be to have 1, 2, 1, 1.

388
00:22:32,430 --> 00:22:35,960
Another one would be 1, 2, 1, 2.

389
00:22:35,960 --> 00:22:38,195
In fact, how many
different arrangements

390
00:22:38,195 --> 00:22:39,320
are there in this universe?

391
00:22:39,320 --> 00:22:39,620
AUDIENCE: 16.

392
00:22:39,620 --> 00:22:40,670
NICHOLAS DIBELLA: 16.

393
00:22:40,670 --> 00:22:41,630
Yeah, it's 16.

394
00:22:41,630 --> 00:22:44,596
There's two for this one,
there's two for this one,

395
00:22:44,596 --> 00:22:46,220
two for this one,
and two for this one.

396
00:22:46,220 --> 00:22:48,200
Two, times two, times
two, times two, is 16.

397
00:22:48,200 --> 00:22:50,570
So there are 16 different
possible arrangements

398
00:22:50,570 --> 00:22:52,250
of these particles.

399
00:22:52,250 --> 00:22:54,270
In this universe.

400
00:22:54,270 --> 00:22:56,840
And so, there are really
16 different types

401
00:22:56,840 --> 00:22:59,960
of realities in this universe.

402
00:23:02,800 --> 00:23:08,300
Reality is defined
essentially by how

403
00:23:08,300 --> 00:23:10,760
various particles are arranged.

404
00:23:10,760 --> 00:23:15,350
And somehow the
way these particles

405
00:23:15,350 --> 00:23:21,030
are arranged gives rise to how
we perceive them and so forth.

406
00:23:21,030 --> 00:23:24,170
So the arrangement of particles
gives rise to the reality

407
00:23:24,170 --> 00:23:29,450
that those particles
impart to us.

408
00:23:29,450 --> 00:23:37,395
Now, our universe, of course,
is not rectangular like this.

409
00:23:37,395 --> 00:23:39,020
Well, that's actually
not so important.

410
00:23:39,020 --> 00:23:41,450
But our universe is much
bigger than this universe

411
00:23:41,450 --> 00:23:43,046
that we have here.

412
00:23:43,046 --> 00:23:44,170
I'll go here to this board.

413
00:23:50,050 --> 00:23:54,460
Our observable
universe is a sphere.

414
00:23:54,460 --> 00:23:58,900
Remember, the whole universe we
suspect is flat and infinite,

415
00:23:58,900 --> 00:24:01,450
but our observable universe
is some finite sphere.

416
00:24:01,450 --> 00:24:07,810
And our observable universe
has a certain radius.

417
00:24:07,810 --> 00:24:09,960
And this radius happens to be--

418
00:24:09,960 --> 00:24:10,960
I have it written down--

419
00:24:15,670 --> 00:24:20,150
8 times 10 to 26th meters.

420
00:24:20,150 --> 00:24:26,670
Now, suppose a particle
has a radius of about--

421
00:24:26,670 --> 00:24:30,070
OK, what we want to do
now is fit particles

422
00:24:30,070 --> 00:24:30,951
into this universe.

423
00:24:30,951 --> 00:24:32,950
And we're going to ask
how many particles can we

424
00:24:32,950 --> 00:24:35,170
fit inside of it.

425
00:24:35,170 --> 00:24:36,670
It's going to be a
lot of particles,

426
00:24:36,670 --> 00:24:39,400
because the observable
universe is really big

427
00:24:39,400 --> 00:24:40,960
and the particles
are really small.

428
00:24:40,960 --> 00:24:45,370
But we can get a rough
estimate for how many particles

429
00:24:45,370 --> 00:24:47,750
we can put inside of it.

430
00:24:47,750 --> 00:24:52,690
So a particle-- say this
is a little particle--

431
00:24:52,690 --> 00:24:55,264
looks like a TIE fighter.

432
00:24:55,264 --> 00:24:55,930
What am I doing?

433
00:24:59,945 --> 00:25:11,880
OK, say this is a particle, and
it's got a length of about 10

434
00:25:11,880 --> 00:25:14,800
to the minus 13 meters,
which is significantly

435
00:25:14,800 --> 00:25:19,077
smaller than the length of
the observable universe.

436
00:25:19,077 --> 00:25:21,160
Well, now you can ask, how
many of these particles

437
00:25:21,160 --> 00:25:23,370
can you fit inside
of this universe?

438
00:25:23,370 --> 00:25:25,480
And to do that, you
can simply calculate

439
00:25:25,480 --> 00:25:28,030
the volume of this
universe and divide

440
00:25:28,030 --> 00:25:31,630
by the volume of a
subatomic particle, which

441
00:25:31,630 --> 00:25:36,280
might be a proton, or a
neutron, or something like that.

442
00:25:36,280 --> 00:25:41,850
Simply take those two
volumes and divide them.

443
00:25:41,850 --> 00:25:43,950
Take the radius, cube it.

444
00:25:43,950 --> 00:25:46,720
Use the formula for the volume
of a sphere and then just

445
00:25:46,720 --> 00:25:49,990
divide by that formula
applied with this radius.

446
00:25:49,990 --> 00:25:57,840
And you get about 10
to the 118th particles.

447
00:25:57,840 --> 00:25:59,347
AUDIENCE: That's
more than a google.

448
00:25:59,347 --> 00:26:01,430
NICHOLAS DIBELLA: That's
more than a google, yeah.

449
00:26:01,430 --> 00:26:04,615
But it's not more
than a googleplex.

450
00:26:04,615 --> 00:26:05,756
Particles--

451
00:26:05,756 --> 00:26:07,800
AUDIENCE: What's a googleple?

452
00:26:07,800 --> 00:26:12,970
NICHOLAS DIBELLA: A googleplex
is 10 to the 10 to 100.

453
00:26:12,970 --> 00:26:16,040
So it's 10 to the google.

454
00:26:16,040 --> 00:26:18,410
So a google is 1
followed by 100 zero's,

455
00:26:18,410 --> 00:26:21,150
but imagine a number of
followed by a Google of zeros.

456
00:26:24,280 --> 00:26:25,245
Yeah.

457
00:26:25,245 --> 00:26:26,620
There's not even
there's not even

458
00:26:26,620 --> 00:26:29,350
space for that many particles
in the observable universe.

459
00:26:29,350 --> 00:26:30,790
That's how big a googleplex is.

460
00:26:34,369 --> 00:26:36,660
Did you guys hear that the
term googleplex was actually

461
00:26:36,660 --> 00:26:39,214
proposed by a nine-year-old
or something like?

462
00:26:39,214 --> 00:26:40,380
AUDIENCE: Somebody's nephew.

463
00:26:40,380 --> 00:26:42,090
NICHOLAS DIBELLA: Yeah,
some mathematician's nephew

464
00:26:42,090 --> 00:26:43,140
or something like that.

465
00:26:43,140 --> 00:26:43,620
AUDIENCE: What's a
googleplex again?

466
00:26:43,620 --> 00:26:45,240
NICHOLAS DIBELLA: What's
a googleplex again?

467
00:26:45,240 --> 00:26:46,156
It's 10 to the google.

468
00:26:46,156 --> 00:26:49,180
And a google is 10 to the 100.

469
00:26:49,180 --> 00:26:50,790
That's just cool stuff--

470
00:26:50,790 --> 00:26:54,110
cool math terms to know.

471
00:26:54,110 --> 00:26:56,340
Like zenzizenzic
and other stuff.

472
00:26:58,850 --> 00:26:59,730
Disregard that.

473
00:26:59,730 --> 00:27:01,530
Disregard that.

474
00:27:01,530 --> 00:27:07,058
10 to the 118th particles
can fit in our universe.

475
00:27:12,890 --> 00:27:14,400
That's how many
particles can fit.

476
00:27:14,400 --> 00:27:16,525
But now we can ask, how
many different arrangements

477
00:27:16,525 --> 00:27:19,720
of particles can there
be in this universe?

478
00:27:19,720 --> 00:27:21,930
And that's an
extremely large number.

479
00:27:25,170 --> 00:27:28,646
Over here in this
simple universe,

480
00:27:28,646 --> 00:27:30,270
we had space for four
particles, and we

481
00:27:30,270 --> 00:27:32,550
found that the number of
different arrangements

482
00:27:32,550 --> 00:27:37,210
was just 2 to the power of 4.

483
00:27:37,210 --> 00:27:38,346
And that was 16.

484
00:27:38,346 --> 00:27:39,884
AUDIENCE: [INAUDIBLE]

485
00:27:39,884 --> 00:27:41,925
NICHOLAS DIBELLA: Oh yeah,
you can also have no--

486
00:27:41,925 --> 00:27:43,020
so it would be three.

487
00:27:43,020 --> 00:27:44,490
Yeah, it would be three.

488
00:27:44,490 --> 00:27:46,170
Yeah, you could have
no particles there,

489
00:27:46,170 --> 00:27:47,700
so it would be three.

490
00:27:47,700 --> 00:27:51,240
But once we apply
that idea to this--

491
00:27:51,240 --> 00:27:55,050
2, 3, raised to a huge number.

492
00:27:55,050 --> 00:27:58,830
It's not going to make a
whole lot of difference.

493
00:27:58,830 --> 00:28:01,900
And oh, yeah, and here I'm just
thinking about one particle.

494
00:28:01,900 --> 00:28:04,080
So I should have
said 3 to the fourth,

495
00:28:04,080 --> 00:28:08,820
because you could have
zero particle at a slot.

496
00:28:08,820 --> 00:28:12,240
So 10 to the 118th particles
can fit in our universe.

497
00:28:12,240 --> 00:28:17,130
And that means that
there are 2 to the 10,

498
00:28:17,130 --> 00:28:19,816
to 118th different arrangements.

499
00:28:31,730 --> 00:28:35,075
And that number is actually
greater than a googleplex.

500
00:28:35,075 --> 00:28:35,700
AUDIENCE: Whoa.

501
00:28:35,700 --> 00:28:37,325
NICHOLAS DIBELLA:
And that's actually--

502
00:28:37,325 --> 00:28:39,310
I mean, I said 2 versus 10.

503
00:28:39,310 --> 00:28:41,393
That didn't really make a
whole lot of difference.

504
00:28:41,393 --> 00:28:44,335
Now, that number's
actually about 10 to the 10

505
00:28:44,335 --> 00:28:45,520
to the 118th.

506
00:28:45,520 --> 00:28:46,520
It's approximately that.

507
00:28:50,490 --> 00:28:51,510
That's how huge it is.

508
00:28:55,950 --> 00:29:03,900
Now, I can ask, if the
universe started out

509
00:29:03,900 --> 00:29:08,050
with every different possible
arrangements of particles--

510
00:29:08,050 --> 00:29:11,880
If we started out with
this vastly huge number

511
00:29:11,880 --> 00:29:18,340
of different ranges of
particles, then by now

512
00:29:18,340 --> 00:29:21,060
you'd still expect to find
every possible arrangement

513
00:29:21,060 --> 00:29:21,780
of particles.

514
00:29:21,780 --> 00:29:24,200
You'd still expect to find that.

515
00:29:24,200 --> 00:29:27,180
The laws of physics will
allow that to happen.

516
00:29:27,180 --> 00:29:28,740
So now I can ask--

517
00:29:28,740 --> 00:29:33,090
well, if you do have an
infinite universe, then

518
00:29:33,090 --> 00:29:36,210
how far do I have
to travel before I

519
00:29:36,210 --> 00:29:41,010
reach an exact copy of
our observable universe?

520
00:29:41,010 --> 00:29:44,130
How far do I have to
travel to get to that?

521
00:29:44,130 --> 00:29:48,900
Well, there are 10
to the 10 to the--

522
00:29:48,900 --> 00:29:49,800
is that right?

523
00:29:49,800 --> 00:29:51,030
Is that approximation good?

524
00:29:51,030 --> 00:29:54,150
I think it is, right?

525
00:29:54,150 --> 00:29:56,496
Maybe I made a mistake.

526
00:29:56,496 --> 00:29:57,770
[INTERPOSING VOICES]

527
00:29:57,770 --> 00:30:00,014
NICHOLAS DIBELLA: I have
things written down.

528
00:30:00,014 --> 00:30:03,353
[INTERPOSING VOICES]

529
00:30:05,477 --> 00:30:07,560
NICHOLAS DIBELLA: Well,
actually, I think it's OK.

530
00:30:07,560 --> 00:30:11,620
I think it's OK Yeah,
doesn't matter a whole lot.

531
00:30:11,620 --> 00:30:14,460
But now I can ask--

532
00:30:14,460 --> 00:30:20,130
eventually, as I travel
from our observable universe

533
00:30:20,130 --> 00:30:26,100
to another universe to another
universe to another universe,

534
00:30:26,100 --> 00:30:31,157
eventually, you'd expect
these arrangements to repeat.

535
00:30:31,157 --> 00:30:32,490
Because they have to eventually.

536
00:30:32,490 --> 00:30:38,440
If you come across 2 to
10 to 118th arrangements

537
00:30:38,440 --> 00:30:43,680
and then you get to another one,
well, you have to repeat it,

538
00:30:43,680 --> 00:30:46,290
because there's only that
many different arrangements.

539
00:30:46,290 --> 00:30:48,820
And so, if you
travel far enough,

540
00:30:48,820 --> 00:30:51,690
then applying that reason
to our own universe,

541
00:30:51,690 --> 00:30:55,320
you'd expect to find an exact
copy of our own universe.

542
00:30:55,320 --> 00:30:58,140
And you'd have to travel about--

543
00:30:58,140 --> 00:31:03,450
well, you could take the size
of our observable universe

544
00:31:03,450 --> 00:31:05,067
and just multiply
it by the number

545
00:31:05,067 --> 00:31:06,150
of different arrangements.

546
00:31:06,150 --> 00:31:24,660
And so, you'd expect to find
an exact copy of our universe

547
00:31:24,660 --> 00:31:30,970
at about 10 to 10th to
the 118th meters away.

548
00:31:30,970 --> 00:31:32,470
I mean this is a
really huge number,

549
00:31:32,470 --> 00:31:37,470
so we don't have to be a
whole lot precise about how

550
00:31:37,470 --> 00:31:38,800
we manipulate them.

551
00:31:38,800 --> 00:31:42,875
So you'd expect to not only
find a copy of yourself,

552
00:31:42,875 --> 00:31:46,410
but you also expect to
find a copy of New York.

553
00:31:46,410 --> 00:31:48,990
You expect to find a
copy of the planet Mars.

554
00:31:48,990 --> 00:31:52,500
You expect to find a copy
of the Andromeda galaxy.

555
00:31:52,500 --> 00:31:54,630
You'd expect to
find an exact copy

556
00:31:54,630 --> 00:32:00,600
of every single piece of
matter in the huge observable

557
00:32:00,600 --> 00:32:02,652
universe.

558
00:32:02,652 --> 00:32:06,570
In another you are having
this exact same conversation.

559
00:32:06,570 --> 00:32:07,920
OK, OK.

560
00:32:07,920 --> 00:32:10,680
In another-- now, here years
where quantum comes in.

561
00:32:10,680 --> 00:32:13,650
Here's where quantum
mechanics comes in.

562
00:32:13,650 --> 00:32:21,150
According to quantum
mechanics, there's a--

563
00:32:21,150 --> 00:32:24,390
what I'll do next,
my next action,

564
00:32:24,390 --> 00:32:27,990
isn't predicted
with 100% certainty.

565
00:32:27,990 --> 00:32:31,440
My next action could be
that I throw a chalk at you,

566
00:32:31,440 --> 00:32:36,090
or it could be that you use
some sort of mental powers

567
00:32:36,090 --> 00:32:38,910
to pull the chalk
from me somehow,

568
00:32:38,910 --> 00:32:42,030
through some very complicated
contrived quantum tunneling

569
00:32:42,030 --> 00:32:44,850
mechanism.

570
00:32:44,850 --> 00:32:48,710
But the fact is
that my future isn't

571
00:32:48,710 --> 00:32:51,370
known with complete certainty.

572
00:32:51,370 --> 00:32:54,596
And so, there are a
number of possibilities

573
00:32:54,596 --> 00:32:55,720
of what my future would be.

574
00:32:55,720 --> 00:32:57,030
And there's a number
of possibilities

575
00:32:57,030 --> 00:32:58,280
of what your future would be--

576
00:32:58,280 --> 00:33:00,210
and your future,
and your future,

577
00:33:00,210 --> 00:33:05,190
and the entire future of the
observable universe actually.

578
00:33:05,190 --> 00:33:07,230
And actually, there's
an infinite number

579
00:33:07,230 --> 00:33:10,470
of possible futures
of every particle

580
00:33:10,470 --> 00:33:13,620
for the observable universe.

581
00:33:13,620 --> 00:33:17,910
And there's a
non-zero probability

582
00:33:17,910 --> 00:33:21,300
associated with each one
of these possible outcomes.

583
00:33:21,300 --> 00:33:24,480
And that means, that if
you travel far enough,

584
00:33:24,480 --> 00:33:28,530
you'd eventually not only
reach an exact duplicate of her

585
00:33:28,530 --> 00:33:31,950
of our universe at this
instant, but you'd also

586
00:33:31,950 --> 00:33:34,350
find an exact duplicate
of our universe

587
00:33:34,350 --> 00:33:39,190
that has the same exact
future of 10 minutes from now,

588
00:33:39,190 --> 00:33:44,910
and the same exact history,
and same exact everything.

589
00:33:44,910 --> 00:33:49,150
And you'd expect to find
infinitely many of those.

590
00:33:49,150 --> 00:33:52,620
It's crazy, but it's true.

591
00:33:52,620 --> 00:33:56,070
And actually, this is the
least controversial type

592
00:33:56,070 --> 00:33:58,050
of parallel universe.

593
00:33:58,050 --> 00:33:59,790
This is the least
controversial type.

594
00:33:59,790 --> 00:34:10,090
This is only what's called
a level I multiverse.

595
00:34:10,090 --> 00:34:16,130
So the multiverse is
the whole collection

596
00:34:16,130 --> 00:34:19,880
of all these universes, which we
say are parallel-- we call them

597
00:34:19,880 --> 00:34:21,139
parallel universes.

598
00:34:21,139 --> 00:34:26,159
The multiverse is the
collection of all of these.

599
00:34:26,159 --> 00:34:28,006
And this is the simplest
type of multiverse.

600
00:34:28,006 --> 00:34:29,434
AUDIENCE: So are
all the different

601
00:34:29,434 --> 00:34:30,714
really possible though?

602
00:34:30,714 --> 00:34:33,215
Because, don't they have to
obey laws of physics and stuff

603
00:34:33,215 --> 00:34:33,530
like that?

604
00:34:33,530 --> 00:34:34,040
NICHOLAS DIBELLA: Yeah, yeah.

605
00:34:34,040 --> 00:34:34,699
OK.

606
00:34:34,699 --> 00:34:38,810
Every arrangement that obeys
the laws of physics is possible.

607
00:34:38,810 --> 00:34:42,050
But the laws of physics
are pretty flexible.

608
00:34:42,050 --> 00:34:42,630
[LAUGHTER]

609
00:34:42,630 --> 00:34:43,130
No.

610
00:34:43,130 --> 00:34:46,040
I mean, flexible in the
sense that if you conceive

611
00:34:46,040 --> 00:34:50,699
of something happening,
then if you're smart enough,

612
00:34:50,699 --> 00:34:54,590
then you can figure out
a way for it to happen.

613
00:34:54,590 --> 00:35:01,880
And actually, precisely proving
that it's possible for me to,

614
00:35:01,880 --> 00:35:04,510
I don't know, as a
result of lifting up

615
00:35:04,510 --> 00:35:06,440
my foot I cause
one of the students

616
00:35:06,440 --> 00:35:09,410
in the back to think
about Harry Potter,

617
00:35:09,410 --> 00:35:11,377
to prove that that's
possible is really hard.

618
00:35:11,377 --> 00:35:12,960
And it might actually
not be possible.

619
00:35:12,960 --> 00:35:15,440
But I suspect that
it's possible,

620
00:35:15,440 --> 00:35:21,830
because you have all these
different things that

621
00:35:21,830 --> 00:35:22,610
could happen.

622
00:35:22,610 --> 00:35:24,800
I could somehow affect
the air molecules here

623
00:35:24,800 --> 00:35:27,500
and cause them to do
something that will propagate,

624
00:35:27,500 --> 00:35:30,170
reaching the
students in the back,

625
00:35:30,170 --> 00:35:35,100
and then affecting his
neurons in some way.

626
00:35:35,100 --> 00:35:36,810
It would be it would
be very complicated,

627
00:35:36,810 --> 00:35:38,360
but it's really
hard to prove that.

628
00:35:38,360 --> 00:35:40,100
But I hope it's true.

629
00:35:40,100 --> 00:35:43,760
And I hope I'm not
crazy for hoping

630
00:35:43,760 --> 00:35:45,710
that it's true,
because I think it

631
00:35:45,710 --> 00:35:47,750
would be so cool if it's true.

632
00:35:47,750 --> 00:35:50,900
Some people object
to these ideas

633
00:35:50,900 --> 00:35:52,570
because they find them weird.

634
00:35:52,570 --> 00:35:55,640
But I actually hope they're
true because they're weird.

635
00:35:55,640 --> 00:35:58,850
And I think it's really cool.

636
00:35:58,850 --> 00:36:00,427
Yeah, question?

637
00:36:00,427 --> 00:36:01,010
AUDIENCE: Yes.

638
00:36:01,010 --> 00:36:04,082
So all this depends on that
that the universe is infinite?

639
00:36:04,082 --> 00:36:05,040
NICHOLAS DIBELLA: Yeah.

640
00:36:05,040 --> 00:36:07,250
This all that depends
on the assumption

641
00:36:07,250 --> 00:36:09,060
that the universe is infinite.

642
00:36:09,060 --> 00:36:09,560
OK.

643
00:36:09,560 --> 00:36:11,934
If the universe is big enough
however, but not infinite--

644
00:36:11,934 --> 00:36:14,000
if it's really large,
but still finite--

645
00:36:14,000 --> 00:36:16,710
then it's still possible to find
an exact copy of our universe.

646
00:36:16,710 --> 00:36:18,700
But it wouldn't be guaranteed.

647
00:36:18,700 --> 00:36:20,375
It wouldn't be guaranteed.

648
00:36:24,020 --> 00:36:25,805
Question?

649
00:36:25,805 --> 00:36:28,715
AUDIENCE: Do you mean the
universe or the amount

650
00:36:28,715 --> 00:36:30,922
of matter in the universe?

651
00:36:30,922 --> 00:36:32,630
NICHOLAS DIBELLA:
Well, necessarily both.

652
00:36:32,630 --> 00:36:34,046
AUDIENCE: Space
could be infinite,

653
00:36:34,046 --> 00:36:35,270
but there might not be--

654
00:36:35,270 --> 00:36:36,320
NICHOLAS DIBELLA: Oh,
that's true, that's true.

655
00:36:36,320 --> 00:36:38,153
Space could be infinite,
but there might not

656
00:36:38,153 --> 00:36:39,980
be an infinite amount of matter.

657
00:36:39,980 --> 00:36:42,440
So you can imagine,
for instance, what's

658
00:36:42,440 --> 00:36:44,780
been called an island universe.

659
00:36:44,780 --> 00:36:49,370
You have an infinite
universe, but there's

660
00:36:49,370 --> 00:36:56,150
only matter and energy at a very
small island in the universe.

661
00:36:56,150 --> 00:37:00,380
And if that's true, then
you can make predictions

662
00:37:00,380 --> 00:37:05,360
about what you'd observe
with various astronomical

663
00:37:05,360 --> 00:37:06,416
measurements.

664
00:37:06,416 --> 00:37:07,790
You can make
definite predictions

665
00:37:07,790 --> 00:37:09,279
with that type of model.

666
00:37:09,279 --> 00:37:10,820
And people have made
some predictions

667
00:37:10,820 --> 00:37:12,110
with that type of model.

668
00:37:12,110 --> 00:37:14,180
And it turns out
that those models

669
00:37:14,180 --> 00:37:16,040
are ruled out by observation.

670
00:37:16,040 --> 00:37:18,088
The island universe is
ruled out by observation.

671
00:37:18,088 --> 00:37:19,838
AUDIENCE: So there has
to be enough matter

672
00:37:19,838 --> 00:37:21,052
to fill the entire universe?

673
00:37:21,052 --> 00:37:22,010
NICHOLAS DIBELLA: Yeah.

674
00:37:22,010 --> 00:37:23,610
There has to be--

675
00:37:23,610 --> 00:37:26,720
matter has to persist
throughout the whole universe.

676
00:37:26,720 --> 00:37:29,091
And there's good
evidence that it does.

677
00:37:29,091 --> 00:37:30,507
AUDIENCE: But how
could there ever

678
00:37:30,507 --> 00:37:33,050
have been an infinite amount
of matter in one point?

679
00:37:33,050 --> 00:37:33,890
NICHOLAS DIBELLA:
How could there ever

680
00:37:33,890 --> 00:37:36,077
have been an infinite amount
of matter at one point?

681
00:37:36,077 --> 00:37:37,701
AUDIENCE: [INAUDIBLE]
had to create it.

682
00:37:37,701 --> 00:37:39,080
So it must have been all--

683
00:37:39,080 --> 00:37:41,270
NICHOLAS DIBELLA: Well,
that's a hard question.

684
00:37:41,270 --> 00:37:48,170
And actually, a process called
inflation might explain that.

685
00:37:48,170 --> 00:37:51,380
And I'm going to talk about
inflation in a minute.

686
00:37:51,380 --> 00:37:53,050
Where am I on time?

687
00:37:53,050 --> 00:37:56,480
Let me see.

688
00:37:56,480 --> 00:37:58,071
Yeah, we could
take a break here.

689
00:37:58,071 --> 00:37:59,070
We'll take a break here.

690
00:37:59,070 --> 00:38:01,340
I'll return talking
about inflation

691
00:38:01,340 --> 00:38:03,640
and the level II multi-verse.

692
00:38:11,840 --> 00:38:14,810
I mentioned that the
level one multi-verse

693
00:38:14,810 --> 00:38:18,330
is the least controversial,
and it's the least weird.

694
00:38:18,330 --> 00:38:21,902
So if you thought that was
crazy, well, listen to this.

695
00:38:21,902 --> 00:38:28,893
Now, I'm going to discuss
the level II multi-verse.

696
00:38:34,810 --> 00:38:42,550
So in the level I
multi-verse, the only kind

697
00:38:42,550 --> 00:38:45,460
of weird thing that
happened was that there

698
00:38:45,460 --> 00:38:48,190
were universes
outside of our own

699
00:38:48,190 --> 00:38:52,300
that had different
arrangements of matter.

700
00:38:52,300 --> 00:38:54,850
And there's only a finite
number of arrangers of matter

701
00:38:54,850 --> 00:38:57,740
and a finite amount of
space that are possible.

702
00:38:57,740 --> 00:39:00,040
So if you travel far
enough, then eventually you

703
00:39:00,040 --> 00:39:02,350
reach a copy of our
arrangement of matter,

704
00:39:02,350 --> 00:39:05,560
and you'd find a copy of
our history and our future.

705
00:39:05,560 --> 00:39:09,091
And that's pretty much all there
is to a level I multiverse.

706
00:39:09,091 --> 00:39:09,590
Question?

707
00:39:09,590 --> 00:39:10,965
AUDIENCE: There's
no way anything

708
00:39:10,965 --> 00:39:13,430
could get more complicated
than what we just did.

709
00:39:13,430 --> 00:39:14,380
NICHOLAS DIBELLA: There's
no way that anything

710
00:39:14,380 --> 00:39:15,040
could get more complicated?

711
00:39:15,040 --> 00:39:15,340
AUDIENCE: It's not possible

712
00:39:15,340 --> 00:39:16,280
NICHOLAS DIBELLA:
It's not possible.

713
00:39:16,280 --> 00:39:16,780
OK.

714
00:39:16,780 --> 00:39:20,200
Well, in a level II
multiverse, not only

715
00:39:20,200 --> 00:39:26,470
are there different arrangements
of matter in finite Hubble

716
00:39:26,470 --> 00:39:40,200
volumes, but we also have
different dimensionalities

717
00:39:40,200 --> 00:39:41,435
of space and time.

718
00:39:58,340 --> 00:40:05,830
We also have different
properties of particles.

719
00:40:08,650 --> 00:40:10,840
For instance, the
mass of the electron

720
00:40:10,840 --> 00:40:20,440
is different in a parallel level
II universe than it is here.

721
00:40:20,440 --> 00:40:29,800
Different properties
of particles,

722
00:40:29,800 --> 00:40:32,660
and different
physical constants.

723
00:40:48,310 --> 00:40:51,280
I'm going to talk more about
this in the second, what

724
00:40:51,280 --> 00:40:54,770
each one of these means.

725
00:40:54,770 --> 00:41:02,140
But first, let me tell
you how this could happen.

726
00:41:05,660 --> 00:41:10,404
So there's a-- hi.

727
00:41:15,400 --> 00:41:20,950
There's a popular theory
that's proven enormously

728
00:41:20,950 --> 00:41:25,030
successful to explain
the early universe,

729
00:41:25,030 --> 00:41:28,840
and this theory goes by
the name of inflation.

730
00:41:28,840 --> 00:41:34,210
And inflation refers to an
extremely, extremely rapid

731
00:41:34,210 --> 00:41:39,580
expansion of space in
the early universe.

732
00:41:39,580 --> 00:41:46,420
So you can imagine,
a very long time ago,

733
00:41:46,420 --> 00:41:50,290
you had the universe,
for instance.

734
00:41:50,290 --> 00:41:54,235
Say this is the whole universe,
and it expands very rapidly.

735
00:41:57,620 --> 00:42:02,320
It's hard to visualize that, but
just try to think in your heads

736
00:42:02,320 --> 00:42:06,310
that the distance between
points expands, and expands

737
00:42:06,310 --> 00:42:07,360
really rapidly.

738
00:42:07,360 --> 00:42:09,660
That's what I mean
by space expanding.

739
00:42:09,660 --> 00:42:12,550
So space expands everywhere.

740
00:42:12,550 --> 00:42:14,830
Space everywhere expands.

741
00:42:14,830 --> 00:42:20,860
However, there are some
regions in the universe

742
00:42:20,860 --> 00:42:23,740
where space stops expanding.

743
00:42:23,740 --> 00:42:26,470
And it stops expanding
just because of what are

744
00:42:26,470 --> 00:42:28,300
called quantum fluctuations.

745
00:42:28,300 --> 00:42:31,180
There is a quantum
probability that space here

746
00:42:31,180 --> 00:42:35,710
will stop expanding, and there
there's a possibility here

747
00:42:35,710 --> 00:42:37,530
that space will stop expanding.

748
00:42:37,530 --> 00:42:41,770
And so, there are
lots of regions

749
00:42:41,770 --> 00:42:43,900
that pop up around
the universe where

750
00:42:43,900 --> 00:42:46,290
space has stopped expanding.

751
00:42:46,290 --> 00:42:48,520
And we call these
regions bubbles.

752
00:42:55,460 --> 00:42:57,085
So the universe has
expanded, and there

753
00:42:57,085 --> 00:43:03,892
are some bubbles where
space has stopped expanding.

754
00:43:07,130 --> 00:43:10,810
And there are some other
regions where space continues

755
00:43:10,810 --> 00:43:13,030
to expand really fast.

756
00:43:13,030 --> 00:43:20,080
Well, it turns out that when
this happens a very mysterious

757
00:43:20,080 --> 00:43:24,490
process called
symmetry breaking--

758
00:43:24,490 --> 00:43:26,502
spontaneous symmetry breaking--

759
00:43:38,880 --> 00:43:40,170
occurs.

760
00:43:40,170 --> 00:43:46,300
And it occurs in each one
of these bubble universes.

761
00:43:46,300 --> 00:43:48,520
So this is a bubble.

762
00:43:48,520 --> 00:43:51,180
It occurs in each
one of these bubbles.

763
00:43:51,180 --> 00:43:54,330
And as a result, the symmetry
breaking, what happens

764
00:43:54,330 --> 00:43:59,460
is that these values
become fixed--

765
00:43:59,460 --> 00:44:02,460
the dimension of space becomes
fixed, the dimension of time

766
00:44:02,460 --> 00:44:03,240
becomes fixed.

767
00:44:03,240 --> 00:44:05,670
I mean, the number of
dimensions of space

768
00:44:05,670 --> 00:44:08,240
and the number of dimensions
of time become fixed.

769
00:44:08,240 --> 00:44:11,160
So in our universe,
for instance,

770
00:44:11,160 --> 00:44:15,660
there are probably three
dimensions of space

771
00:44:15,660 --> 00:44:17,370
and just one dimension of time.

772
00:44:17,370 --> 00:44:19,920
And that's exactly
what we observe.

773
00:44:19,920 --> 00:44:26,600
I can walk forward, backwards,
upwards, downwards, sidewards,

774
00:44:26,600 --> 00:44:28,390
rightwards, whatever.

775
00:44:28,390 --> 00:44:31,167
So there are three different
directions we can travel,

776
00:44:31,167 --> 00:44:33,000
and those are the three
dimensions of space.

777
00:44:33,000 --> 00:44:36,030
And there also appears just
to be one dimension of time

778
00:44:36,030 --> 00:44:38,946
that we travel, and we
travel into the future.

779
00:44:38,946 --> 00:44:41,490
So there's one
dimension of time.

780
00:44:41,490 --> 00:44:44,670
And that's what's symmetry
breaking did for our bubble.

781
00:44:44,670 --> 00:44:52,620
It fixed the dimensionality of
space and time to be 3 and 1.

782
00:44:52,620 --> 00:44:55,710
Symmetry breaking also fixed--

783
00:44:55,710 --> 00:44:58,830
by fixed, I mean it
determined-- determined

784
00:44:58,830 --> 00:45:01,380
the different
properties of particles.

785
00:45:01,380 --> 00:45:07,687
For example, it fixed the value
of the electron mass, which

786
00:45:07,687 --> 00:45:11,310
is, as you know, about 9.1 times
10 to the minus 31 kilograms.

787
00:45:11,310 --> 00:45:17,730
It fixed the mass of the proton,
which is about 1.67 times 10

788
00:45:17,730 --> 00:45:19,860
to the minus 27 kilograms.

789
00:45:19,860 --> 00:45:21,441
It's fixed the mass of--

790
00:45:24,600 --> 00:45:27,020
those only the two ones
that are the mass I know.

791
00:45:27,020 --> 00:45:29,800
It fixed the mass of lots of
other different particles,

792
00:45:29,800 --> 00:45:33,024
and it also fixed things
like charges-- like,

793
00:45:33,024 --> 00:45:33,690
electric charge.

794
00:45:36,240 --> 00:45:38,134
Why?

795
00:45:38,134 --> 00:45:39,550
Well, I'll get to
why in a minute.

796
00:45:39,550 --> 00:45:41,050
But it's also fixed
electric charge.

797
00:45:41,050 --> 00:45:43,800
It also fixed some
property called

798
00:45:43,800 --> 00:45:48,150
spin, which is a type of
intrinsic angular momentum.

799
00:45:48,150 --> 00:45:50,320
Don't worry about it if
you haven't heard of it.

800
00:45:50,320 --> 00:45:52,470
It also fixed-- well,
it fixed a whole bunch

801
00:45:52,470 --> 00:45:54,840
of properties of particles.

802
00:45:54,840 --> 00:45:57,450
There are certain properties
that particles have,

803
00:45:57,450 --> 00:46:00,330
and they have these properties
because a type of symmetry

804
00:46:00,330 --> 00:46:03,390
breaking occurred in
the early universe

805
00:46:03,390 --> 00:46:04,965
as a result of inflation.

806
00:46:04,965 --> 00:46:07,120
Let me write this
down-- inflation.

807
00:46:13,630 --> 00:46:17,190
And so, different properties
of particles were determined,

808
00:46:17,190 --> 00:46:20,442
and different dimensionalities
space and time

809
00:46:20,442 --> 00:46:21,150
would determined.

810
00:46:21,150 --> 00:46:23,850
And also, different physical
constants are determined.

811
00:46:23,850 --> 00:46:27,580
I mean, the speed of light
is about 2.99792458 times 10

812
00:46:27,580 --> 00:46:31,710
to the 8 meters per
second, for example.

813
00:46:31,710 --> 00:46:36,990
And that value was determined
by symmetry breaking.

814
00:46:36,990 --> 00:46:41,850
It's also conceivable
that our bubble universe

815
00:46:41,850 --> 00:46:46,650
could have been--

816
00:46:46,650 --> 00:46:48,300
what's the word
I'm looking for--

817
00:46:48,300 --> 00:46:53,100
given, or could have been
imparted with a different set

818
00:46:53,100 --> 00:46:55,540
of these numbers.

819
00:46:55,540 --> 00:47:01,500
Our universe could have had
a dimensionality of space

820
00:47:01,500 --> 00:47:02,186
being four.

821
00:47:02,186 --> 00:47:03,810
There could be four
dimensions of space

822
00:47:03,810 --> 00:47:06,660
and one dimension of time.

823
00:47:06,660 --> 00:47:09,690
And if that were the
case, then our world

824
00:47:09,690 --> 00:47:12,370
would be quite different.

825
00:47:12,370 --> 00:47:16,860
And actually, we probably
wouldn't exist in this world.

826
00:47:16,860 --> 00:47:22,410
First, it's easy to visualize
what four dimensions of space

827
00:47:22,410 --> 00:47:25,265
would mean in terms
of how we can move.

828
00:47:25,265 --> 00:47:27,390
We would have four different
independent directions

829
00:47:27,390 --> 00:47:28,512
that we can move.

830
00:47:28,512 --> 00:47:29,970
That's what it
would mean for there

831
00:47:29,970 --> 00:47:33,210
to be four dimensions of space.

832
00:47:33,210 --> 00:47:36,330
But actually, you can
make some predictions

833
00:47:36,330 --> 00:47:42,840
about how atoms would behave
if the laws of physics

834
00:47:42,840 --> 00:47:45,630
are the same but
the dimensionalities

835
00:47:45,630 --> 00:47:48,030
of space and time are different.

836
00:47:48,030 --> 00:47:52,650
And you can make these
predictions by simply fixing up

837
00:47:52,650 --> 00:47:57,150
the equations of
the laws of physics

838
00:47:57,150 --> 00:48:02,260
in an appropriate manner,
and then get at some results.

839
00:48:02,260 --> 00:48:05,400
By the way, all of
these bubble universes

840
00:48:05,400 --> 00:48:07,230
have the same laws of physics.

841
00:48:07,230 --> 00:48:08,724
They have the same
laws of physics.

842
00:48:08,724 --> 00:48:10,140
The only things
that are different

843
00:48:10,140 --> 00:48:14,380
are the dimensionalities
of space and time,

844
00:48:14,380 --> 00:48:18,420
the properties of matter,
and the physical constants,

845
00:48:18,420 --> 00:48:20,160
like the speed of light.

846
00:48:20,160 --> 00:48:22,040
Also what could differ--

847
00:48:22,040 --> 00:48:25,680
Well, within each one
of these universes

848
00:48:25,680 --> 00:48:31,140
there could be a whole
slew of type I universes.

849
00:48:31,140 --> 00:48:35,190
So type I is kind of
contained in type II.

850
00:48:35,190 --> 00:48:37,040
So you have infinite
number of type I,

851
00:48:37,040 --> 00:48:41,580
an infinite number of type
I there, and so forth.

852
00:48:41,580 --> 00:48:44,710
But they all have the
same laws of physics.

853
00:48:44,710 --> 00:48:46,980
Now, you can make
certain predictions

854
00:48:46,980 --> 00:48:50,640
about what the world would
be like in a universe

855
00:48:50,640 --> 00:48:54,400
with different dimensionalities
from that of our own.

856
00:48:54,400 --> 00:48:58,510
For example, if the
dimension of space--

857
00:48:58,510 --> 00:49:10,320
let's say there are four
space dimensions and five time

858
00:49:10,320 --> 00:49:12,300
dimensions.

859
00:49:12,300 --> 00:49:14,700
Which is conceivable.

860
00:49:14,700 --> 00:49:16,790
Its hard to imagine,
but it's conceivable.

861
00:49:16,790 --> 00:49:20,010
And you can fix the
equations of physics

862
00:49:20,010 --> 00:49:23,640
to include for this
number of dimensions.

863
00:49:23,640 --> 00:49:26,081
Let's say there are
five time dimensions.

864
00:49:28,850 --> 00:49:32,910
Well, such a world would be
very different from our own.

865
00:49:32,910 --> 00:49:38,280
Because it turns out that if our
universe were like this, then

866
00:49:38,280 --> 00:49:42,450
it would be absolutely
impossible to make a prediction

867
00:49:42,450 --> 00:49:48,750
about any event with a
non infinite uncertainty.

868
00:49:48,750 --> 00:49:50,265
So we couldn't make
any prediction.

869
00:49:53,151 --> 00:49:54,900
We're able to make
pretty good predictions

870
00:49:54,900 --> 00:49:56,370
in our own universe, right?

871
00:49:56,370 --> 00:49:58,890
I mean, I predict that
this chalk, once I drop it,

872
00:49:58,890 --> 00:50:03,800
will fall on this paper with
an uncertainty of say 1 mile.

873
00:50:03,800 --> 00:50:04,520
I don't know.

874
00:50:04,520 --> 00:50:07,410
So there's an
uncertainty of one mile

875
00:50:07,410 --> 00:50:10,990
of where this chalk will fall.

876
00:50:10,990 --> 00:50:12,180
OK, It worked.

877
00:50:12,180 --> 00:50:15,189
My uncertainty was non-infinite.

878
00:50:15,189 --> 00:50:16,980
It was pretty big, but
it was non infinite.

879
00:50:16,980 --> 00:50:19,740
And it turned out to
correspond to what we observed.

880
00:50:19,740 --> 00:50:22,620
Well, if these were
the dimensionalities

881
00:50:22,620 --> 00:50:25,770
of space and time,
you wouldn't be

882
00:50:25,770 --> 00:50:29,130
able to get any better
than infinite uncertainty.

883
00:50:29,130 --> 00:50:34,320
You can also can imagine there
being just one space dimension,

884
00:50:34,320 --> 00:50:37,230
and there being like
four time dimensions.

885
00:50:37,230 --> 00:50:42,140
So let say one
space and four time.

886
00:50:45,800 --> 00:50:49,990
Well, in this universe with
these dimensionalities,

887
00:50:49,990 --> 00:50:56,720
it turns out that all
atoms are unstable.

888
00:50:56,720 --> 00:50:58,650
They're immediately unstable.

889
00:50:58,650 --> 00:51:01,720
If I had an atom it would
just exist for a split second

890
00:51:01,720 --> 00:51:03,380
and then that would
be the end it.

891
00:51:03,380 --> 00:51:07,860
It would be impossible
for molecules to form.

892
00:51:07,860 --> 00:51:09,650
It would be impossible
for atoms just

893
00:51:09,650 --> 00:51:15,950
to sit there, much less than
stick together to other atoms.

894
00:51:15,950 --> 00:51:19,760
And it would be impossible
for macromolecules to form.

895
00:51:19,760 --> 00:51:23,600
It would be impossible
for simple cells to form--

896
00:51:23,600 --> 00:51:24,915
What's that?

897
00:51:24,915 --> 00:51:26,810
AUDIENCE: No
sentient life there.

898
00:51:26,810 --> 00:51:28,130
NICHOLAS DIBELLA: Yeah, no.

899
00:51:28,130 --> 00:51:29,420
No life there.

900
00:51:29,420 --> 00:51:31,910
This type of universe
would be completely

901
00:51:31,910 --> 00:51:36,237
devoid of intelligent
people like us.

902
00:51:36,237 --> 00:51:37,520
[LAUGHTER]

903
00:51:37,520 --> 00:51:38,217
Yeah?

904
00:51:38,217 --> 00:51:40,994
AUDIENCE: What would it be
like outside of these bubbles?

905
00:51:40,994 --> 00:51:42,785
NICHOLAS DIBELLA:
Outside of these bubbles?

906
00:51:42,785 --> 00:51:45,260
AUDIENCE: Would the
amount of dimensions

907
00:51:45,260 --> 00:51:47,240
and all properties
of physical constant

908
00:51:47,240 --> 00:51:49,524
be changing all the time?

909
00:51:49,524 --> 00:51:52,024
They would never be the same?

910
00:51:52,024 --> 00:51:53,190
NICHOLAS DIBELLA: You mean--

911
00:51:53,190 --> 00:51:55,030
AUDIENCE: If you
left the bubble,

912
00:51:55,030 --> 00:51:58,080
and you were just in
the rest of space,

913
00:51:58,080 --> 00:52:00,130
I guess continuing to expand.

914
00:52:02,794 --> 00:52:06,800
NICHOLAS DIBELLA:
I'm not exactly sure.

915
00:52:06,800 --> 00:52:10,460
I think-- Yeah,
I'm not sure what

916
00:52:10,460 --> 00:52:13,300
it's like outside the bubbles.

917
00:52:13,300 --> 00:52:16,770
I don't completely
understand this.

918
00:52:16,770 --> 00:52:18,760
I'm telling you the
stuff that I do know.

919
00:52:18,760 --> 00:52:19,400
AUDIENCE: Do you
know if it would

920
00:52:19,400 --> 00:52:21,800
be like just one moment
they're inside the bubble

921
00:52:21,800 --> 00:52:24,040
and everything is normal,
and the next minute they're

922
00:52:24,040 --> 00:52:26,450
outside the bubble and
it's completely different?

923
00:52:26,450 --> 00:52:28,030
NICHOLAS DIBELLA:
Oh, oh, oh, oh.

924
00:52:28,030 --> 00:52:28,820
OK.

925
00:52:28,820 --> 00:52:30,410
Well, I can kind of answer that.

926
00:52:30,410 --> 00:52:31,760
I can kind of answer that.

927
00:52:31,760 --> 00:52:34,560
These bubbles are
infinite in size.

928
00:52:34,560 --> 00:52:38,415
So once you're in one of these
bubbles you can't get out.

929
00:52:38,415 --> 00:52:40,040
You're not going to
be able to get out.

930
00:52:40,040 --> 00:52:42,080
And you can't interact with the
other bubbles unfortunately.

931
00:52:42,080 --> 00:52:43,705
AUDIENCE: How are
they infinite in size

932
00:52:43,705 --> 00:52:45,605
if there's more
than one of them?

933
00:52:45,605 --> 00:52:47,480
NICHOLAS DIBELLA: That's
kind hard to imagine

934
00:52:47,480 --> 00:52:50,860
how they can be infinite in size
and with more than one of them.

935
00:52:50,860 --> 00:52:53,420
But that's simply--

936
00:52:53,420 --> 00:52:56,570
I know it's hard to visualize.

937
00:52:56,570 --> 00:52:59,690
But infinity is a
really weird concept,

938
00:52:59,690 --> 00:53:03,915
and it does weird things to
you when you think about it.

939
00:53:03,915 --> 00:53:06,650
AUDIENCE: But the entire
universe in its entirety

940
00:53:06,650 --> 00:53:10,482
is infinitely larger than
infinitely large things.

941
00:53:10,482 --> 00:53:11,440
NICHOLAS DIBELLA: Yeah.

942
00:53:11,440 --> 00:53:16,520
You have to be pretty careful
when you use the word infinity.

943
00:53:16,520 --> 00:53:18,850
You might have heard about
different types of infinity,

944
00:53:18,850 --> 00:53:21,710
like countable infinity,
uncountable infinity.

945
00:53:21,710 --> 00:53:24,170
For instance, there are
more irrational numbers

946
00:53:24,170 --> 00:53:26,570
than there are rational numbers.

947
00:53:26,570 --> 00:53:30,620
And I'll let you all
wonder about that.

948
00:53:34,420 --> 00:53:40,220
The point is, the way you deal
with infinity is kind of weird,

949
00:53:40,220 --> 00:53:43,400
and it's kind of confusing, and
it does weird things to you.

950
00:53:43,400 --> 00:53:46,690
AUDIENCE: [INAUDIBLE] any
amount of any number--

951
00:53:46,690 --> 00:53:49,670
any combination of sounds
has to be a number.

952
00:53:49,670 --> 00:53:51,514
NICHOLAS DIBELLA: Any
combination of what?

953
00:53:51,514 --> 00:53:53,366
AUDIENCE: Any combination
of sounds ever

954
00:53:53,366 --> 00:53:55,440
has to be [INAUDIBLE]
of a number.

955
00:53:55,440 --> 00:53:56,880
But there are infinite numbers.

956
00:53:56,880 --> 00:53:58,944
And there are less--

957
00:53:58,944 --> 00:54:00,455
you have to have--

958
00:54:00,455 --> 00:54:02,830
NICHOLAS DIBELLA: I don't know
what you're talking about.

959
00:54:02,830 --> 00:54:03,355
What?

960
00:54:03,355 --> 00:54:05,590
Talk to me after class.

961
00:54:05,590 --> 00:54:07,100
I don't know.

962
00:54:07,100 --> 00:54:08,960
OK.

963
00:54:08,960 --> 00:54:11,270
Well, getting back to this.

964
00:54:11,270 --> 00:54:14,556
In this universe with
these dimensionalities,

965
00:54:14,556 --> 00:54:15,680
atoms would cease to exist.

966
00:54:15,680 --> 00:54:17,138
They'd just be
completely unstable.

967
00:54:19,770 --> 00:54:27,740
Now, this level of
multiverse actually provides

968
00:54:27,740 --> 00:54:30,870
a nice explanation
for the question,

969
00:54:30,870 --> 00:54:36,920
why is there one
dimension of time

970
00:54:36,920 --> 00:54:38,990
and why are there three
dimensions of space?

971
00:54:38,990 --> 00:54:41,160
Why is the mass of an
electron this number?

972
00:54:41,160 --> 00:54:43,070
Why is the speed of
light this number?

973
00:54:43,070 --> 00:54:46,310
Why is Newton's gravitational
constant this number?

974
00:54:46,310 --> 00:54:49,880
It provides a nice
explanation for that.

975
00:54:49,880 --> 00:54:56,805
Because there are
infinitely many bubbles.

976
00:54:56,805 --> 00:54:59,180
And there are infinitely many
different types of symmetry

977
00:54:59,180 --> 00:55:00,870
breakings that can occur.

978
00:55:00,870 --> 00:55:02,720
So you'd expect to
find some probability

979
00:55:02,720 --> 00:55:05,660
distribution of various
dimensionalities of space

980
00:55:05,660 --> 00:55:06,852
and time.

981
00:55:06,852 --> 00:55:08,810
You'd expect to find a
probability distribution

982
00:55:08,810 --> 00:55:11,510
of various properties
of particles

983
00:55:11,510 --> 00:55:15,830
and various values of
physical constants.

984
00:55:15,830 --> 00:55:20,070
And so, it just kind
of occurred by chance

985
00:55:20,070 --> 00:55:24,875
that our universe has
these certain values.

986
00:55:27,710 --> 00:55:33,650
If it were the case that these
other bubbles didn't exist.

987
00:55:33,650 --> 00:55:37,910
If it were the case that they
were just one universe with one

988
00:55:37,910 --> 00:55:41,450
set of numbers for
physical constants,

989
00:55:41,450 --> 00:55:43,160
one set of numbers
for properties

990
00:55:43,160 --> 00:55:45,845
of particles, one set
of dimensionalities

991
00:55:45,845 --> 00:55:46,880
for space and time.

992
00:55:46,880 --> 00:55:50,540
If there were just one universe
with those very specific sets

993
00:55:50,540 --> 00:55:58,820
of numbers, then it's very hard
to explain why those numbers

994
00:55:58,820 --> 00:56:01,560
and why not other numbers.

995
00:56:01,560 --> 00:56:05,480
Well, the level II multiverse
provides a very elegant

996
00:56:05,480 --> 00:56:07,820
explanation--

997
00:56:07,820 --> 00:56:09,510
it just occurred by probability.

998
00:56:09,510 --> 00:56:12,690
It's just a result of
symmetry breaking occurring

999
00:56:12,690 --> 00:56:14,780
in the early universe.

1000
00:56:14,780 --> 00:56:18,220
That's a wonderful explanation.

1001
00:56:18,220 --> 00:56:21,350
Another explanation that's
been traditionally proposed

1002
00:56:21,350 --> 00:56:28,470
is that something
fine-tuned those values.

1003
00:56:28,470 --> 00:56:32,060
Well, that's a kind
of controversial view

1004
00:56:32,060 --> 00:56:37,391
that I think is less elegant
than this multiverse theory.

1005
00:56:37,391 --> 00:56:37,890
Question?

1006
00:56:37,890 --> 00:56:39,858
AUDIENCE: Well, you were
saying that all this

1007
00:56:39,858 --> 00:56:41,810
is happening from our universe.

1008
00:56:41,810 --> 00:56:44,554
So from our big bang
all this happened?

1009
00:56:47,372 --> 00:56:48,330
NICHOLAS DIBELLA: Yeah.

1010
00:56:48,330 --> 00:56:50,550
From our big bang,
this all happened.

1011
00:56:50,550 --> 00:56:51,050
OK.

1012
00:56:51,050 --> 00:56:55,035
So there's one huge universe,
which is "the universe."

1013
00:56:55,035 --> 00:56:57,560
Which you could say
is "the multiverse."

1014
00:56:57,560 --> 00:57:02,660
And inside the multiverse there
are various infinite bubbles

1015
00:57:02,660 --> 00:57:05,270
that arised as a
result of this process

1016
00:57:05,270 --> 00:57:06,649
called inflation occurring.

1017
00:57:06,649 --> 00:57:08,690
And a result of this
symmetry breaking occurring,

1018
00:57:08,690 --> 00:57:10,100
fixed these various numbers.

1019
00:57:10,100 --> 00:57:13,400
AUDIENCE: But does that mean
that all this happening now,

1020
00:57:13,400 --> 00:57:15,400
is that-- if space is
infinite, could there

1021
00:57:15,400 --> 00:57:16,550
be another big bang?

1022
00:57:16,550 --> 00:57:18,800
NICHOLAS DIBELLA: Oh, could
there be another big bang?

1023
00:57:18,800 --> 00:57:21,660
Yeah, there there could be.

1024
00:57:21,660 --> 00:57:22,994
There could be another big bang.

1025
00:57:26,270 --> 00:57:29,690
This whole subject of
talking about the universe

1026
00:57:29,690 --> 00:57:32,330
as a whole, this is the
subject of cosmology.

1027
00:57:32,330 --> 00:57:35,850
And there are a lot of really
cool ideas in cosmology.

1028
00:57:38,940 --> 00:57:40,580
You can talk about
multiple big bangs.

1029
00:57:40,580 --> 00:57:42,860
You could talk about
the universe expanding

1030
00:57:42,860 --> 00:57:45,740
and then contracting, and
then being a cyclic universe.

1031
00:57:45,740 --> 00:57:48,230
There are lots of cool ideas.

1032
00:57:48,230 --> 00:57:51,020
It's possible that there
could be multiple big bangs.

1033
00:57:51,020 --> 00:57:56,960
And I think it might be
possible within this theory.

1034
00:57:56,960 --> 00:57:59,524
And that might even--

1035
00:57:59,524 --> 00:58:01,190
there are various
versions of inflation,

1036
00:58:01,190 --> 00:58:03,440
and there are probably
lots of versions where

1037
00:58:03,440 --> 00:58:05,330
multiple big bangs do occur.

1038
00:58:05,330 --> 00:58:07,700
But I'm not an expert
on it, and so I'm just

1039
00:58:07,700 --> 00:58:10,120
feeding you what I know.

1040
00:58:10,120 --> 00:58:11,350
Was there another question?

1041
00:58:11,350 --> 00:58:11,850
Yeah.

1042
00:58:11,850 --> 00:58:13,490
AUDIENCE: How many
meters is on kilometer?

1043
00:58:13,490 --> 00:58:15,615
NICHOLAS DIBELLA: How many
meters is one kilometer?

1044
00:58:15,615 --> 00:58:17,562
1,000.

1045
00:58:17,562 --> 00:58:20,584
AUDIENCE: OK, that's it.

1046
00:58:20,584 --> 00:58:22,750
NICHOLAS DIBELLA: Very deep,
philosophical question.

1047
00:58:25,470 --> 00:58:26,400
OK.

1048
00:58:26,400 --> 00:58:28,580
Does anybody have the time?

1049
00:58:28,580 --> 00:58:29,870
I'm not going to look at my--

1050
00:58:29,870 --> 00:58:30,810
AUDIENCE: 2:39

1051
00:58:30,810 --> 00:58:34,560
NICHOLAS DIBELLA: 2:39 OK.

1052
00:58:34,560 --> 00:58:36,150
Let's see.

1053
00:58:36,150 --> 00:58:38,090
So this is the
level II multiverse.

1054
00:58:38,090 --> 00:58:42,005
I think I'm going to skip level
III and going to just go to IV.

1055
00:58:42,005 --> 00:58:43,880
I don't think we have
time to do all of them.

1056
00:58:43,880 --> 00:58:48,030
But level III would actually
take a long time to explain,

1057
00:58:48,030 --> 00:58:52,680
because, well, level III has
to do with the many worlds

1058
00:58:52,680 --> 00:58:54,270
interpretation of
quantum mechanics,

1059
00:58:54,270 --> 00:58:55,853
and I would have to
explain that first

1060
00:58:55,853 --> 00:58:59,610
and then talk about
crazy consequences.

1061
00:58:59,610 --> 00:59:03,150
But actually, the
level III multiverse

1062
00:59:03,150 --> 00:59:07,290
doesn't add any
qualitatively new features.

1063
00:59:07,290 --> 00:59:12,360
Like, you we still have these
different sets of numbers.

1064
00:59:12,360 --> 00:59:14,735
You still have different
arrangements of matter.

1065
00:59:14,735 --> 00:59:16,110
So I'm just going
to push forward

1066
00:59:16,110 --> 00:59:20,450
to level IV, which is the
highest level multiverse.

1067
00:59:20,450 --> 00:59:22,154
You guys ready?

1068
00:59:22,154 --> 00:59:22,695
AUDIENCE: No.

1069
00:59:22,695 --> 00:59:24,180
AUDIENCE: Always.

1070
00:59:24,180 --> 00:59:25,170
AUDIENCE: Never.

1071
00:59:25,170 --> 00:59:26,160
AUDIENCE: It's scary.

1072
00:59:26,160 --> 00:59:26,790
NICHOLAS DIBELLA: It's scary?

1073
00:59:26,790 --> 00:59:27,414
AUDIENCE: Yeah.

1074
00:59:27,414 --> 00:59:30,066
It's [INAUDIBLE].

1075
00:59:30,066 --> 00:59:33,807
How did they even come to this?

1076
00:59:33,807 --> 00:59:35,640
NICHOLAS DIBELLA: How
did they come to this?

1077
00:59:35,640 --> 00:59:41,965
Well, inflation
was discovered by--

1078
00:59:41,965 --> 00:59:44,006
AUDIENCE: We're still
discovering things on Earth

1079
00:59:44,006 --> 00:59:45,902
that we don't know.

1080
00:59:45,902 --> 00:59:47,997
How do we figure out--

1081
00:59:47,997 --> 00:59:50,330
NICHOLAS DIBELLA: There are
a lot of really smart people

1082
00:59:50,330 --> 00:59:51,746
that have thought
of these things,

1083
00:59:51,746 --> 00:59:55,400
and inflation was actually
discovered by physicists

1084
00:59:55,400 --> 00:59:57,380
here at MIT, Alan Guth.

1085
00:59:57,380 --> 01:00:01,750
He discovered this
inflation mechanism.

1086
01:00:01,750 --> 01:00:06,830
And it actually it explains
a whole lot of phenomena

1087
01:00:06,830 --> 01:00:09,680
that are really hard to explain,
and impossible to explain

1088
01:00:09,680 --> 01:00:12,131
with other mechanisms.

1089
01:00:12,131 --> 01:00:18,200
It explains why, for
example, the universe

1090
01:00:18,200 --> 01:00:21,740
is roughly uniform.

1091
01:00:21,740 --> 01:00:24,412
Why the universe
roughly looks the same

1092
01:00:24,412 --> 01:00:25,370
no matter where you go.

1093
01:00:25,370 --> 01:00:26,453
It looks roughly the same.

1094
01:00:26,453 --> 01:00:28,490
It explains lots of
things like that.

1095
01:00:28,490 --> 01:00:28,990
Question?

1096
01:00:28,990 --> 01:00:30,220
AUDIENCE: Is he still here?

1097
01:00:30,220 --> 01:00:31,716
NICHOLAS DIBELLA: Yeah, yeah.

1098
01:00:31,716 --> 01:00:35,000
I have his book [INAUDIBLE].

1099
01:00:35,000 --> 01:00:35,630
Oh, good.

1100
01:00:35,630 --> 01:00:36,858
Cool, cool.

1101
01:00:36,858 --> 01:00:38,767
AUDIENCE: So you must ask him--

1102
01:00:38,767 --> 01:00:39,725
NICHOLAS DIBELLA: Yeah.

1103
01:00:39,725 --> 01:00:40,940
AUDIENCE: Is he a teacher.

1104
01:00:40,940 --> 01:00:43,587
NICHOLAS DIBELLA: Yeah,
actually next semester

1105
01:00:43,587 --> 01:00:45,170
I'm taking a class
that he's teaching,

1106
01:00:45,170 --> 01:00:48,030
called The Early Universe.

1107
01:00:48,030 --> 01:00:52,520
And that's really
cool that we students

1108
01:00:52,520 --> 01:00:55,190
get to take classes taught
by the absolute experts

1109
01:00:55,190 --> 01:00:56,295
on these fields.

1110
01:00:56,295 --> 01:00:58,478
So yeah, come to MIT.

1111
01:00:58,478 --> 01:00:59,980
[LAUGHTER]

1112
01:00:59,980 --> 01:01:00,480
OK.

1113
01:01:04,560 --> 01:01:07,630
So now level IV multi-verse.

1114
01:01:15,019 --> 01:01:16,060
I looking doing that too.

1115
01:01:19,230 --> 01:01:23,430
Now, level II multi-verse, we
had different dimensionalities

1116
01:01:23,430 --> 01:01:26,020
of space and time, different
properties of particles,

1117
01:01:26,020 --> 01:01:30,040
different physical constants,
but the same laws of physics.

1118
01:01:30,040 --> 01:01:32,530
Well guess what's going to
be different in the level IV

1119
01:01:32,530 --> 01:01:35,260
universe?

1120
01:01:35,260 --> 01:01:38,274
Different laws of physics, yeah.

1121
01:01:38,274 --> 01:01:39,256
AUDIENCE: That's weird.

1122
01:01:58,920 --> 01:02:04,930
NICHOLAS DIBELLA: It's been
noticed for a long time that--

1123
01:02:04,930 --> 01:02:09,020
well, the universe is
explained very well

1124
01:02:09,020 --> 01:02:10,325
by mathematical models.

1125
01:02:13,790 --> 01:02:16,310
A lot of you probably haven't
taken any mathematical classes

1126
01:02:16,310 --> 01:02:19,370
on physics, but there
are lots of equations

1127
01:02:19,370 --> 01:02:22,550
in physics that give rise
to these various predictions

1128
01:02:22,550 --> 01:02:25,500
that I'm talking about.

1129
01:02:25,500 --> 01:02:27,920
And I gave you some equations
in the second class.

1130
01:02:27,920 --> 01:02:29,090
Maybe I shouldn't have--

1131
01:02:29,090 --> 01:02:31,900
but somebody influenced me.

1132
01:02:31,900 --> 01:02:35,360
But there are various
equations in physics.

1133
01:02:35,360 --> 01:02:36,860
And there are various
equations that

1134
01:02:36,860 --> 01:02:41,510
are very specific to a
particular set of laws

1135
01:02:41,510 --> 01:02:43,100
of physics.

1136
01:02:43,100 --> 01:02:46,700
And we say that
these laws of physics

1137
01:02:46,700 --> 01:02:50,660
have a particular
mathematical structure.

1138
01:02:50,660 --> 01:02:52,760
And it's been a
view for a long time

1139
01:02:52,760 --> 01:02:56,720
that these mathematical
structures merely

1140
01:02:56,720 --> 01:02:58,460
approximate the universe.

1141
01:02:58,460 --> 01:03:05,510
Maybe the universe is impossible
to describe with math.

1142
01:03:05,510 --> 01:03:13,574
Our models predict
very well what

1143
01:03:13,574 --> 01:03:14,990
we should observe
in the universe,

1144
01:03:14,990 --> 01:03:16,260
but they're not perfect.

1145
01:03:16,260 --> 01:03:19,100
In fact, we know for a fact
that we don't have the precise--

1146
01:03:19,100 --> 01:03:25,450
we don't have the absolute
correct model for our universe.

1147
01:03:25,450 --> 01:03:27,200
Well, because, actually,
quantum mechanics

1148
01:03:27,200 --> 01:03:32,330
and general relativity
don't work with each other.

1149
01:03:32,330 --> 01:03:33,570
They contradict each other.

1150
01:03:33,570 --> 01:03:36,590
And this is a problem
in modern physics

1151
01:03:36,590 --> 01:03:38,510
that people are
working on solving.

1152
01:03:38,510 --> 01:03:41,390
String theory is one hope for--

1153
01:03:41,390 --> 01:03:41,890
what's that?

1154
01:03:41,890 --> 01:03:43,521
AUDIENCE: So only one is right?

1155
01:03:43,521 --> 01:03:44,520
NICHOLAS DIBELLA: Well--

1156
01:03:44,520 --> 01:03:45,942
AUDIENCE: Or is it mixable?

1157
01:03:45,942 --> 01:03:46,889
[INAUDIBLE]

1158
01:03:46,889 --> 01:03:48,680
NICHOLAS DIBELLA: The
question is, is there

1159
01:03:48,680 --> 01:03:53,570
only one mathematical structure
that can model the universe?

1160
01:03:53,570 --> 01:04:01,370
Well, you could probably think
of different sets of equations

1161
01:04:01,370 --> 01:04:02,890
to give the same results.

1162
01:04:02,890 --> 01:04:06,350
But if that's the
case, then it might

1163
01:04:06,350 --> 01:04:10,070
have to be that they're
all really manifestations

1164
01:04:10,070 --> 01:04:11,050
of the same thing.

1165
01:04:11,050 --> 01:04:12,200
AUDIENCE: OK.

1166
01:04:12,200 --> 01:04:14,158
NICHOLAS DIBELLA: They
might all be isomorphic,

1167
01:04:14,158 --> 01:04:15,910
is the technical term--

1168
01:04:15,910 --> 01:04:18,344
the term the mathematicians use.

1169
01:04:18,344 --> 01:04:20,804
AUDIENCE: So how would
they decide which model

1170
01:04:20,804 --> 01:04:22,280
is the right or wrong one?

1171
01:04:22,280 --> 01:04:23,480
NICHOLAS DIBELLA: How would
they decide which model

1172
01:04:23,480 --> 01:04:24,650
is right for our universe?

1173
01:04:24,650 --> 01:04:27,549
Well, first of all, you to
have a consistent model.

1174
01:04:27,549 --> 01:04:29,090
You have to have a
consistent theory.

1175
01:04:29,090 --> 01:04:30,890
It has to be consistent.

1176
01:04:30,890 --> 01:04:34,820
General relativity plus quantum
mechanics is not consistent.

1177
01:04:34,820 --> 01:04:39,050
When you try to talk about
the quantum mechanics

1178
01:04:39,050 --> 01:04:41,060
inside of black
holes for instance,

1179
01:04:41,060 --> 01:04:43,049
you get at results like
infinite probabilities.

1180
01:04:43,049 --> 01:04:44,840
And what does an infinite
probability mean?

1181
01:04:44,840 --> 01:04:45,548
It's meaningless.

1182
01:04:45,548 --> 01:04:47,220
It doesn't really mean anything.

1183
01:04:47,220 --> 01:04:50,870
And so, the theory we have
right now is inconsistent.

1184
01:04:50,870 --> 01:04:52,940
And people are searching
for a consistent theory.

1185
01:04:52,940 --> 01:04:55,730
And so far, string theory
looks like it's our best hope.

1186
01:04:55,730 --> 01:04:57,330
But that's not worked out yet.

1187
01:04:57,330 --> 01:04:59,480
But it's doing better
than other theories,

1188
01:04:59,480 --> 01:05:01,310
and so, it receives
a lot more attention.

1189
01:05:01,310 --> 01:05:04,490
But it's still the
fundamental problem.

1190
01:05:04,490 --> 01:05:06,690
AUDIENCE: How do they
determine consistency then?

1191
01:05:06,690 --> 01:05:08,815
NICHOLAS DIBELLA: How do
you determine consistency?

1192
01:05:08,815 --> 01:05:11,390
Well, you check to see,
here are two things,

1193
01:05:11,390 --> 01:05:13,676
do they agree with each other?

1194
01:05:13,676 --> 01:05:15,050
If this is true
and this is true,

1195
01:05:15,050 --> 01:05:16,258
it implies that this is true.

1196
01:05:16,258 --> 01:05:19,060
AUDIENCE: Is it consistency
with experiments

1197
01:05:19,060 --> 01:05:20,060
of what we already know?

1198
01:05:20,060 --> 01:05:20,930
NICHOLAS DIBELLA: OK.

1199
01:05:20,930 --> 01:05:22,180
Various types of consistency--

1200
01:05:22,180 --> 01:05:23,090
OK.

1201
01:05:23,090 --> 01:05:26,264
First and foremost, it has
to be consistent in itself.

1202
01:05:26,264 --> 01:05:27,680
If it's not
consistent in itself--

1203
01:05:27,680 --> 01:05:28,160
AUDIENCE: That's bad.

1204
01:05:28,160 --> 01:05:29,618
NICHOLAS DIBELLA:
Then it gets bad.

1205
01:05:29,618 --> 01:05:35,060
And secondly, we hope that it
models the actual universe.

1206
01:05:35,060 --> 01:05:38,780
I can think of a theory
right now that's consistent,

1207
01:05:38,780 --> 01:05:40,700
but it doesn't
model the universe.

1208
01:05:40,700 --> 01:05:43,820
I could propose a theory
that the whole universe

1209
01:05:43,820 --> 01:05:49,400
is determined by
how I move this pen.

1210
01:05:49,400 --> 01:05:52,970
I can think of various
rules of how it works.

1211
01:05:52,970 --> 01:05:56,099
But it doesn't agree
with experiment.

1212
01:05:56,099 --> 01:05:58,140
So we need something that
agrees with experiment.

1213
01:06:06,250 --> 01:06:09,040
But it's been asked, why--

1214
01:06:09,040 --> 01:06:12,340
well, actually, it's been
remarked by Einstein,

1215
01:06:12,340 --> 01:06:15,340
that perhaps the
most incomprehensible

1216
01:06:15,340 --> 01:06:19,270
thing about the universe is
that it is comprehensible.

1217
01:06:19,270 --> 01:06:24,430
And it's comprehensible
with the aid of mathematics.

1218
01:06:24,430 --> 01:06:28,450
And many people have wondered,
why does math work so well?

1219
01:06:28,450 --> 01:06:31,610
Why does it work so well?

1220
01:06:31,610 --> 01:06:34,060
Maybe it's the case that
the universe actually

1221
01:06:34,060 --> 01:06:35,440
is mathematical.

1222
01:06:35,440 --> 01:06:37,120
Math isn't merely
an approximation,

1223
01:06:37,120 --> 01:06:40,750
but the universe
is mathematical.

1224
01:06:40,750 --> 01:06:49,490
And that's the main idea behind
this level IV multiverse.

1225
01:06:49,490 --> 01:06:55,350
So in our universe there is--

1226
01:06:55,350 --> 01:07:00,120
well, if it's true that the
universe is mathematical,

1227
01:07:00,120 --> 01:07:02,950
then in our universe there's
a mathematical structure

1228
01:07:02,950 --> 01:07:05,960
that is the universe.

1229
01:07:05,960 --> 01:07:12,990
So the main assumption
here is that the universe

1230
01:07:12,990 --> 01:07:14,150
is mathematical.

1231
01:07:24,650 --> 01:07:27,680
Well, what kind of
mathematical models?

1232
01:07:27,680 --> 01:07:32,230
Well general relativity
is, for instance,

1233
01:07:32,230 --> 01:07:36,170
some sub branch of
differential geometry

1234
01:07:36,170 --> 01:07:43,400
and spacetime is a
pseudo-Riemannian manifold.

1235
01:07:43,400 --> 01:07:45,260
It's very complicated math.

1236
01:07:45,260 --> 01:07:47,840
They're very complicated
mathematical structures.

1237
01:07:47,840 --> 01:07:50,480
But the fact is that they
are mathematical structures.

1238
01:07:50,480 --> 01:07:52,640
And the assumption behind
the level IV multiverse

1239
01:07:52,640 --> 01:07:55,790
is that the universe
is mathematical.

1240
01:07:55,790 --> 01:08:01,520
Now, we can ask, why this
mathematical structure and not

1241
01:08:01,520 --> 01:08:03,950
other mathematical structures?

1242
01:08:03,950 --> 01:08:07,610
And a nice answer
to that question,

1243
01:08:07,610 --> 01:08:10,100
similar to the answer
to the question, why

1244
01:08:10,100 --> 01:08:13,580
these dimensionalities, why
these concepts and not others,

1245
01:08:13,580 --> 01:08:17,990
is that there are many other
universes with different laws

1246
01:08:17,990 --> 01:08:21,140
of physics, which correspond
to different mathematical

1247
01:08:21,140 --> 01:08:23,359
structures.

1248
01:08:23,359 --> 01:08:25,880
So I just proposed
one before with the

1249
01:08:25,880 --> 01:08:28,760
had to do with this pen.

1250
01:08:28,760 --> 01:08:34,760
If I could formulate that into
a mathematical theory that's

1251
01:08:34,760 --> 01:08:39,470
consistent, then according
to level for multiple IV

1252
01:08:39,470 --> 01:08:43,580
multiverse theories
there exist universes

1253
01:08:43,580 --> 01:08:47,960
that obey the laws of physics
as dictated by this pen.

1254
01:08:50,910 --> 01:08:52,200
In our universe--

1255
01:08:52,200 --> 01:08:54,660
AUDIENCE: And when
you say there exists,

1256
01:08:54,660 --> 01:08:56,859
would that mean that if
you travelled far enough

1257
01:08:56,859 --> 01:08:59,359
or something, or if you search
the whole physical universe,

1258
01:08:59,359 --> 01:09:00,279
you say, "there exists"--

1259
01:09:00,279 --> 01:09:00,899
NICHOLAS DIBELLA: "There
exists," yeah, yeah.

1260
01:09:00,899 --> 01:09:02,000
AUDIENCE: You're going
to find it somewhere?

1261
01:09:02,000 --> 01:09:03,840
NICHOLAS DIBELLA: You're
going to find it somewhere.

1262
01:09:03,840 --> 01:09:04,349
Yeah.

1263
01:09:04,349 --> 01:09:06,140
But you probably won't
be able to get to it

1264
01:09:06,140 --> 01:09:06,830
because it's so far away.

1265
01:09:06,830 --> 01:09:07,120
AUDIENCE: Yeah.

1266
01:09:07,120 --> 01:09:08,729
NICHOLAS DIBELLA: And because
it's moving away from a--

1267
01:09:08,729 --> 01:09:09,380
AUDIENCE: A different
bubble or something.

1268
01:09:09,380 --> 01:09:11,420
NICHOLAS DIBELLA: Yeah,
a different bubble.

1269
01:09:11,420 --> 01:09:12,609
You probably won't be
able to reach these.

1270
01:09:12,609 --> 01:09:13,984
AUDIENCE: Kind of
like, existence

1271
01:09:13,984 --> 01:09:16,000
is a iffy sort of a thing.

1272
01:09:16,000 --> 01:09:18,290
NICHOLAS DIBELLA: Existence
is an iffy sort of thing.

1273
01:09:18,290 --> 01:09:23,979
This is the most controversial
level of multiverse.

1274
01:09:23,979 --> 01:09:26,479
The first one that I described
you seemed crazy at the time,

1275
01:09:26,479 --> 01:09:27,770
but now we have different
laws of physics,

1276
01:09:27,770 --> 01:09:28,603
different constants.

1277
01:09:28,603 --> 01:09:32,689
And level I we're at home with.

1278
01:09:32,689 --> 01:09:33,189
question?

1279
01:09:33,189 --> 01:09:34,890
AUDIENCE: In the
multiverse, is it

1280
01:09:34,890 --> 01:09:39,140
possible for
universes to overlap?

1281
01:09:39,140 --> 01:09:42,260
NICHOLAS DIBELLA: Is it possible
for universes to overlap?

1282
01:09:42,260 --> 01:09:44,810
Well, it depends on how
you define universe.

1283
01:09:44,810 --> 01:09:47,990
Is it possible-- are
you asking for there

1284
01:09:47,990 --> 01:09:53,349
to be two different level
IV universes overlapping?

1285
01:09:53,349 --> 01:09:54,890
Because if that's
the case, then that

1286
01:09:54,890 --> 01:10:01,950
would require the overlapped
universe to obey--

1287
01:10:01,950 --> 01:10:04,760
well, it might
require it to obey

1288
01:10:04,760 --> 01:10:07,010
two different mathematical
laws at the same time,

1289
01:10:07,010 --> 01:10:09,590
or might just be an
amalgam of the two.

1290
01:10:09,590 --> 01:10:10,003
AUDIENCE: Well if
you have the two

1291
01:10:10,003 --> 01:10:12,002
different mathematical
structures though, right,

1292
01:10:12,002 --> 01:10:13,880
sometimes mathematical
structures do overlap.

1293
01:10:13,880 --> 01:10:14,838
NICHOLAS DIBELLA: Yeah.

1294
01:10:14,838 --> 01:10:17,510
Yeah-- so, it's-- yeah.

1295
01:10:17,510 --> 01:10:18,590
Yeah.

1296
01:10:18,590 --> 01:10:19,354
Question?

1297
01:10:19,354 --> 01:10:24,695
AUDIENCE: Two things--
one, how fast or far

1298
01:10:24,695 --> 01:10:26,339
can a modern rocket go?

1299
01:10:26,339 --> 01:10:27,630
NICHOLAS DIBELLA: How far what?

1300
01:10:27,630 --> 01:10:30,485
AUDIENCE: How far or fast
can a modern rocket go?

1301
01:10:30,485 --> 01:10:32,110
NICHOLAS DIBELLA: I
don't know how fast

1302
01:10:32,110 --> 01:10:33,214
a modern rocket can go.

1303
01:10:33,214 --> 01:10:34,380
AUDIENCE: How far can it go?

1304
01:10:34,380 --> 01:10:34,750
NICHOLAS DIBELLA:
How far can it--

1305
01:10:34,750 --> 01:10:37,380
well, if you just leave it,
if you wait long enough,

1306
01:10:37,380 --> 01:10:38,610
it'll go arbitrarily far--

1307
01:10:38,610 --> 01:10:39,497
a rocket.

1308
01:10:39,497 --> 01:10:41,485
AUDIENCE: OK.

1309
01:10:41,485 --> 01:10:43,970
AUDIENCE: It can go fast
enough to cause the Doppler--

1310
01:10:43,970 --> 01:10:45,791
I know that things
[INAUDIBLE] that had

1311
01:10:45,791 --> 01:10:47,460
the Doppler effect [INAUDIBLE].

1312
01:10:47,460 --> 01:10:48,835
NICHOLAS DIBELLA:
OK, yeah, yeah.

1313
01:10:48,835 --> 01:10:50,046
Yeah.

1314
01:10:50,046 --> 01:10:51,420
Yeah, rockets can
go pretty fast.

1315
01:10:51,420 --> 01:10:53,080
Let me get back to
multiverses though.

1316
01:10:53,080 --> 01:10:55,230
OK.

1317
01:10:55,230 --> 01:10:57,904
What time is it?

1318
01:10:57,904 --> 01:10:59,332
AUDIENCE: 2:52

1319
01:10:59,332 --> 01:11:00,290
NICHOLAS DIBELLA: 2:52.

1320
01:11:00,290 --> 01:11:00,540
OK.

1321
01:11:00,540 --> 01:11:01,690
I have few more minutes.

1322
01:11:01,690 --> 01:11:03,870
OK.

1323
01:11:03,870 --> 01:11:08,220
So in this level
IV multi-verse you

1324
01:11:08,220 --> 01:11:11,010
have different laws of physics.

1325
01:11:11,010 --> 01:11:12,660
Our universe has
one specific type

1326
01:11:12,660 --> 01:11:15,600
that what we hope
to one day find.

1327
01:11:15,600 --> 01:11:18,510
We might not find it, but
we hope we will find it.

1328
01:11:18,510 --> 01:11:21,450
If we do find it, then
that would be evidence

1329
01:11:21,450 --> 01:11:24,635
for this level IV multiverse.

1330
01:11:24,635 --> 01:11:26,364
It would be evidence
for the assumption

1331
01:11:26,364 --> 01:11:27,780
that the universe
is mathematical.

1332
01:11:27,780 --> 01:11:30,540
It we have evidence
because we then

1333
01:11:30,540 --> 01:11:34,680
would have a mathematical
structure for our universe.

1334
01:11:34,680 --> 01:11:39,780
And if other universes exist
with different laws of physics,

1335
01:11:39,780 --> 01:11:42,360
then than that would be
a rather nice explanation

1336
01:11:42,360 --> 01:11:46,440
for why these laws not others.

1337
01:11:46,440 --> 01:11:49,920
Well, it's because,
every conceivable set

1338
01:11:49,920 --> 01:11:52,560
of physical laws would exist.

1339
01:11:52,560 --> 01:11:55,080
Every conceivable set of
laws would exist would,

1340
01:11:55,080 --> 01:11:58,760
and there's some
probability distribution

1341
01:11:58,760 --> 01:12:03,780
that your universe is equipped
with these laws and not

1342
01:12:03,780 --> 01:12:05,054
some others.

1343
01:12:05,054 --> 01:12:06,720
AUDIENCE: How would
you ever determine--

1344
01:12:06,720 --> 01:12:07,730
NICHOLAS DIBELLA: How
did you ever determine

1345
01:12:07,730 --> 01:12:08,720
those probabilities?

1346
01:12:08,720 --> 01:12:09,599
I don't know.

1347
01:12:09,599 --> 01:12:10,140
I don't know.

1348
01:12:10,140 --> 01:12:12,630
And I don't know how
you would actually

1349
01:12:12,630 --> 01:12:18,630
figure out if the level IV
multiverse theories are true.

1350
01:12:18,630 --> 01:12:24,750
Other than that they answered
some questions rather nicely,

1351
01:12:24,750 --> 01:12:26,330
like why these laws
and not others.

1352
01:12:26,330 --> 01:12:29,240
It might actually be completely
impossible the answer.

1353
01:12:29,240 --> 01:12:33,160
It might be forever in
the realm of metaphysics.

1354
01:12:33,160 --> 01:12:37,050
But it provides a
nice sense of closure

1355
01:12:37,050 --> 01:12:40,320
to all of these universes.

1356
01:12:40,320 --> 01:12:43,410
We can explain
everything so nicely

1357
01:12:43,410 --> 01:12:45,330
with the aid of
parallel universes.

1358
01:12:45,330 --> 01:12:52,170
Things it seems the things that
seem once to be so mysterious,

1359
01:12:52,170 --> 01:12:54,640
now they just fall under
the rug or something.

1360
01:12:54,640 --> 01:12:55,365
Is that a--

1361
01:12:55,365 --> 01:12:56,344
I don't know.

1362
01:12:56,344 --> 01:12:57,510
Am I using the phrase right?

1363
01:12:57,510 --> 01:12:58,301
Fall under the rug?

1364
01:12:58,301 --> 01:12:59,380
Swept under the rug?

1365
01:12:59,380 --> 01:13:01,267
Is swept under the
rug a bad phrase?

1366
01:13:01,267 --> 01:13:02,100
Like, a mean phrase?

1367
01:13:02,100 --> 01:13:02,599
OK.

1368
01:13:02,599 --> 01:13:04,540
Well, these questions
are answered so nicely

1369
01:13:04,540 --> 01:13:07,020
is what I'm trying to say
with parallel universes.

1370
01:13:07,020 --> 01:13:11,060
And they seem crazy,
and they are crazy.

1371
01:13:11,060 --> 01:13:14,410
But there is lots of
evidence for them,

1372
01:13:14,410 --> 01:13:19,440
particularly the lower
level type multiverses.

1373
01:13:19,440 --> 01:13:21,270
And so, we should
take them seriously.

1374
01:13:21,270 --> 01:13:23,085
And even if you don't
take them seriously,

1375
01:13:23,085 --> 01:13:24,990
you should at least
appreciate them,

1376
01:13:24,990 --> 01:13:27,920
that they give us nice
things to talk about,

1377
01:13:27,920 --> 01:13:29,790
nice things to think about.

1378
01:13:29,790 --> 01:13:33,160
And they give us lots of more
opportunities to make jokes.

1379
01:13:33,160 --> 01:13:34,680
That's always one
of my favorites.

1380
01:13:34,680 --> 01:13:36,210
AUDIENCE: Do you have any joke?

1381
01:13:36,210 --> 01:13:37,793
NICHOLAS DIBELLA:
Do I have any jokes?

1382
01:13:37,793 --> 01:13:38,970
Well, sure.

1383
01:13:38,970 --> 01:13:41,900
I have lots of jokes.

1384
01:13:41,900 --> 01:13:45,060
I had one joke--

1385
01:13:45,060 --> 01:13:47,280
I was planning on
saying, one other type

1386
01:13:47,280 --> 01:13:51,670
of conceivable universe
that's level IV.

1387
01:13:51,670 --> 01:13:55,050
In our universe,
gravitation is not

1388
01:13:55,050 --> 01:13:57,380
responsible for people
falling in love,

1389
01:13:57,380 --> 01:13:59,910
but it's conceivable that in
another universe gravitation

1390
01:13:59,910 --> 01:14:01,960
is responsible for
people falling in love.

1391
01:14:01,960 --> 01:14:03,810
[LAUGHTER]