A
Physicist’s Physicist Ponders the Nature of Reality
Edward Witten reflects on
the meaning of dualities in physics and math, emergent space-time, and the
pursuit of a complete description of nature.
November 28 2017
Among the brilliant
theorists cloistered in the quiet woodside campus of the Institute for Advanced
Study in Princeton, New Jersey, Edward Witten stands out as a kind of high
priest. The sole physicist ever to win the Fields Medal, mathematics’ premier
prize, Witten is also known for discovering M-theory, the leading candidate for
a unified physical “theory of everything.” A genius’s genius,
Witten is tall and rectangular, with hazy eyes and an air of being only
one-quarter tuned in to reality until someone draws him back from more abstract
thoughts.
During a visit this fall, I spotted Witten on the
Institute’s central lawn and requested an interview; in his quick, alto voice,
he said he couldn’t promise to be able to answer my questions but would try.
Later, when I passed him on the stone paths, he often didn’t seem to see me.
Physics luminaries since Albert Einstein, who lived
out his days in the same intellectual haven, have sought to unify gravity with
the other forces of nature by finding a more fundamental quantum theory to
replace Einstein’s approximate picture of gravity as curves in the geometry of
space-time. M-theory, which Witten proposed in 1995, could conceivably offer
this deeper description, but only some aspects of the theory are known.
M-theory incorporates within a single mathematical structure all five versions
of string theory, which renders the
elements of nature as minuscule vibrating strings. These five string theories
connect to each other through “dualities,” or mathematical equivalences. Over
the past 30 years, Witten and others have learned that the string theories are
also mathematically dual to quantum field theories — descriptions of particles
moving through electromagnetic and other fields that serve as the language of
the reigning “Standard Model” of particle physics. While he’s best known as a
string theorist, Witten has discovered many new quantum field theories and
explored how all these different descriptions are connected. His physical
insights have led time and again to deep mathematical discoveries.
That’s extremely strange,
that the world is based so much on a mathematical structure that’s so
difficult.
Researchers pore over his work and hope he’ll take
an interest in theirs. But for all his scholarly influence, Witten, who is 66,
does not often broadcast his views on the implications of modern theoretical
discoveries. Even his close colleagues eagerly suggested questions they wanted
me to ask him.
When I arrived at his office at the appointed hour
on a summery Thursday last month, Witten wasn’t there. His door was ajar.
Papers covered his coffee table and desk — not stacks, but floods: text
oriented every which way, some pages close to spilling onto the floor.
(Research papers get lost in the maelstrom as he finishes with them, he later
explained, and every so often he throws the heaps away.) Two girls smiled out
from a framed photo on a shelf; children’s artwork decorated the walls, one
celebrating Grandparents’ Day. When Witten arrived minutes later, we spoke for
an hour and a half about the meaning of dualities in physics and math, the
current prospects of M-theory, what he’s reading, what he’s looking for, and
the nature of reality. The interview has been condensed and edited for
clarity.
Physicists
are talking more than ever lately about dualities, but you’ve been studying
them for decades. Why does the subject interest you?
People keep finding new facets of dualities.
Dualities are interesting because they frequently answer questions that are
otherwise out of reach. For example, you might have spent years pondering a
quantum theory and you understand what happens when the quantum effects are
small, but textbooks don’t tell you what you do if the quantum effects are big;
you’re generally in trouble if you want to know that. Frequently dualities
answer such questions. They give you another description, and the questions you
can answer in one description are different than the questions you can answer
in a different description.
What are some of these
newfound facets of dualities?
It’s open-ended because there are so many different
kinds of dualities. There are dualities between a gauge theory [a theory, such
as a quantum field theory, that respects certain symmetries] and another gauge
theory, or between a string theory for weak coupling [describing strings that
move almost independently from one another] and a string theory for strong
coupling. Then there’s AdS/CFT duality, between a gauge theory and a
gravitational description. That duality was discovered 20 years ago, and it’s amazing to what
extent it’s still fruitful. And that’s largely because around 10 years ago, new
ideas were introduced that rejuvenated it. People had new insights about
entropy in quantum field theory — the whole story about “it from qubit.”
The AdS/CFT
duality connects a theory of gravity in a space-time region called
anti-de Sitter space (which curves differently than our universe) to an
equivalent quantum field theory describing that region’s gravity-free boundary.
Everything there is to know about AdS space — often called the “bulk” since
it’s the higher-dimensional region — is encoded, like in a hologram, in quantum
interactions between particles on the lower-dimensional boundary. Thus, AdS/CFT
gives physicists a “holographic” understanding of the quantum nature of
gravity.
That’s the idea that
space-time and everything in it emerges like a hologram out of information
stored in the entangled quantum states of particles.
Yes. Then there are dualities in math, which can
sometimes be interpreted physically as consequences of dualities between two
quantum field theories. There are so many ways these things are interconnected
that any simple statement I try to make on the fly, as soon as I’ve said it I
realize it didn’t capture the whole reality. You have to imagine a web of
different relationships, where the same physics has different descriptions,
revealing different properties. In the simplest case, there are only two
important descriptions, and that might be enough. If you ask me about a more
complicated example, there might be many, many different ones.
Given this web of
relationships and the issue of how hard it is to characterize all duality, do
you feel that this reflects a lack of understanding of the structure, or is it
that we’re seeing the structure, only it’s very complicated?
I’m not certain what we should hope for.
Traditionally, quantum field theory was constructed by starting with the
classical picture [of a smooth field] and then quantizing it. Now we’ve learned
that there are a lot of things that happen that that description doesn’t do
justice to. And the same quantum theory can come from different classical
theories. Now, Nati Seiberg [a theoretical
physicist who works down the hall] would possibly tell you that he has faith
that there’s a better formulation of quantum field theory that we don’t know
about that would make everything clearer. I’m not sure how much you should
expect that to exist. That would be a dream, but it might be too much to hope
for; I really don’t know.
There’s another curious fact that you might want to
consider, which is that quantum field theory is very central to physics, and
it’s actually also clearly very important for math. But it’s extremely
difficult for mathematicians to study; the way physicists define it is very
hard for mathematicians to follow with a rigorous theory. That’s extremely
strange, that the world is based so much on a mathematical structure that’s so
difficult.
What do you
see as the relationship between math and physics?
I prefer not to give you a cosmic answer but to
comment on where we are now. Physics in quantum field theory and string theory
somehow has a lot of mathematical secrets in it, which we don’t
know how to extract in a systematic way. Physicists are able to come up with
things that surprise the mathematicians. Because it’s hard to describe
mathematically in the known formulation, the things you learn about quantum
field theory you have to learn from physics.
I find it hard to believe there’s a new formulation
that’s universal. I think it’s too much to hope for. I could point to theories
where the standard approach really seems inadequate, so at least for those
classes of quantum field theories, you could hope for a new formulation. But I really
can’t imagine what it would be.
You can’t imagine it at
all?
No, I can’t. Traditionally it was thought that
interacting quantum field theory couldn’t exist above four dimensions, and
there was the interesting fact that that’s the dimension we live in. But one of
the offshoots of the string dualities of the 1990s was that it was discovered
that quantum field theories actually exist in five and six dimensions. And it’s
amazing how much is known about their properties.
If there’s a radically
different dual description of the real world, maybe some things physicists
worry about would be clearer, but the dual description might be one in which
everyday life would be hard to describe.
I’ve heard about the
mysterious (2,0) theory, a quantum field theory describing particles in six
dimensions, which is dual to M-theory describing strings and gravity in
seven-dimensional AdS space. Does this (2,0) theory play an important role in
the web of dualities?
Yes, that’s the pinnacle. In terms of
conventional quantum field theory without gravity, there is nothing quite like
it above six dimensions. From the (2,0) theory’s existence and main
properties, you can deduce an incredible amount about what happens in
lower dimensions. An awful lot of important dualities in four and fewer
dimensions follow from this six-dimensional theory and its
properties. However, whereas what we know about quantum field theory is
normally from quantizing a classical field theory, there’s no reasonable
classical starting point of the (2,0) theory. The (2,0) theory has properties
[such as combinations of symmetries] that sound impossible when you first hear
about them. So you can ask why dualities exist, but you can also ask why is there
a 6-D theory with such and such properties? This seems to me a more fundamental
restatement.
Dualities sometimes make it
hard to maintain a sense of what’s real in the world, given that there are
radically different ways you can describe a single system. How would you
describe what’s real or fundamental?
What aspect of what’s real are you interested in?
What does it mean that we exist? Or how do we fit into our mathematical
descriptions?
The latter.
Well, one thing I’ll tell you is that in general,
when you have dualities, things that are easy to see in one description can be
hard to see in the other description. So you and I, for example, are fairly
simple to describe in the usual approach to physics as developed by Newton and
his successors. But if there’s a radically different dual description of the
real world, maybe some things physicists worry about would be clearer, but the
dual description might be one in which everyday life would be hard to describe.
What would you say about
the prospect of an even more optimistic idea that there could be one single
quantum gravity description that really does help you in every case in the real
world?
Well, unfortunately, even if it’s correct I can’t
guarantee it would help. Part of what makes it difficult to help is that the
description we have now, even though it’s not complete, does explain an awful
lot. And so it’s a little hard to say, even if you had a truly better
description or a more complete description, whether it would help in practice.
Are you speaking of
M-theory?
M-theory is the candidate for the better
description.
You proposed M-theory 22
years ago. What are its prospects today?
Personally, I thought it was extremely clear it
existed 22 years ago, but the level of confidence has got to be much higher
today because AdS/CFT has given us precise definitions, at least in AdS
space-time geometries. I think our understanding of what it is, though, is
still very hazy. AdS/CFT and whatever’s come from it is the main new
perspective compared to 22 years ago, but I think it’s perfectly possible that
AdS/CFT is only one side of a multifaceted story. There might be other equally important facets.
What’s an
example of something else we might need?
Maybe a bulk description of the quantum properties
of space-time itself, rather than a holographic boundary description. There
hasn’t been much progress in a long time in getting a better bulk description.
And I think that might be because the answer is of a different kind than
anything we’re used to. That would be my guess.
Are you willing to
speculate about how it would be different?
I really doubt I can say anything useful. I guess I
suspect that there’s an extra layer of abstractness compared to what we’re used
to. I tend to think that there isn’t a precise quantum description of space-time
— except in the types of situations where we know that there is, such as in AdS
space. I tend to think, otherwise, things are a little bit murkier than an
exact quantum description. But I can’t say anything useful.
The other night I was reading an old essay by the
20th-century Princeton physicist John Wheeler. He was a visionary, certainly.
If you take what he says literally, it’s hopelessly vague. And therefore, if I
had read this essay when it came out 30 years ago, which I may have done, I
would have rejected it as being so vague that you couldn’t work on it, even if
he was on the right track.
You’re referring to Information, Physics, Quantum, Wheeler’s 1989 essay
propounding the idea that the physical universe arises from information, which
he dubbed “it from bit.” Why were you reading it?
I’m trying to learn about what people are trying to
say with the phrase “it from qubit.” Wheeler talked about “it from bit,” but
you have to remember that this essay was written probably before the term
“qubit” was coined and certainly before it was in wide currency. Reading it, I
really think he was talking about qubits, not bits, so “it from qubit” is
actually just a modern translation.
I tend to assume that
space-time and everything in it are in some sense emergent.
Don’t expect me to be able to tell you anything
useful about it — about whether he was right. When I was a beginning grad
student, they had a series of lectures by faculty members to the new students
about theoretical research, and one of the people who gave such a lecture was
Wheeler. He drew a picture on the blackboard of
the universe visualized as an eye looking at itself. I had no idea what he was
talking about. It’s obvious to me in hindsight that he was explaining what it
meant to talk about quantum mechanics when the observer is part of the quantum
system. I imagine there is something we don’t understand about that.
Observing a quantum system
irreversibly changes it, creating a distinction between past and future. So the
observer issue seems possibly related to the question of time, which we also
don’t understand. With the AdS/CFT duality, we’ve learned that new spatial
dimensions can pop up like a hologram from quantum information on the boundary.
Do you think time is also emergent — that it arises from a timeless complete
description?
I tend to assume that space-time and everything in
it are in some sense emergent. By the way, you’ll certainly find that that’s
what Wheeler expected in his essay. As you’ll read, he thought the continuum
was wrong in both physics and math. He did not think one’s microscopic
description of space-time should use a continuum of any kind — neither a
continuum of space nor a continuum of time, nor even a continuum of real
numbers. On the space and time, I’m sympathetic to that. On the real numbers,
I’ve got to plead ignorance or agnosticism. It is something I wonder about, but
I’ve tried to imagine what it could mean to not use the continuum of real
numbers, and the one logician I tried discussing it with didn’t help me.
Do you consider Wheeler a
hero?
I wouldn’t call him a hero, necessarily, no. Really
I just became curious what he meant by “it from bit,” and what he was saying.
He definitely had visionary ideas, but they were too far ahead of their
time. I think I was more patient in reading a vague but inspirational essay
than I might have been 20 years ago. He’s also got roughly 100
interesting-sounding references in that essay. If you decided to read them all,
you’d have to spend weeks doing it. I might decide to look at a few of them.
Why do you
have more patience for such things now?
I think when I was younger I always thought the
next thing I did might be the best thing in my life. But at this point in life
I’m less persuaded of that. If I waste a little time reading somebody’s essay,
it doesn’t seem that bad.
Do you ever take your mind
off physics and math?
My favorite pastime is tennis. I am a very average
but enthusiastic tennis player.
In contrast to Wheeler, it
seems like your working style is to come to the insights through the
calculations, rather than chasing a vague vision.
In my career I’ve only been able to take small
jumps. Relatively small jumps. What Wheeler was talking about was an enormous
jump. And he does say at the beginning of the essay that he has no idea if this
will take 10, 100 or 1,000 years.
And he was talking about
explaining how physics arises from information.
Yes. The way he phrases it is broader: He wants to
explain the meaning of existence. That was actually why I thought you were
asking if I wanted to explain the meaning of existence.
I see. Does he have any
hypotheses?
No. He only talks about things you shouldn’t do and
things you should do in trying to arrive at a more fundamental description of
physics.
Do you have any ideas about
the meaning of existence?
No. [Laughs.]
Correction: This article
was updated on Nov. 29, 2017, to clarify that M-theory is the leading candidate
for a unified theory of everything. Other ideas have been proposed that also
claim to unify the fundamental forces.
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Note:The original have photos from Jean Sweep for Quanta Magazine