Some thoughts on where we are
Daniel Harlow
MIT

June 29, 2018

1

What is this conference about?

2What is this conference about?

One way to decide is to look at the data:2

What is this conference about?

One way to decide is to look at the data:
String theory? (9 talks, more than usual)2What is this conference about?

One way to decide is to look at the data:
String theory? (9 talks, more than usual)
Quantum gravity? (12 talks)2What is this conference about?

One way to decide is to look at the data:
String theory? (9 talks, more than usual)
Quantum gravity? (12 talks)
Quantum field theory? (21 talks)2What is this conference about?

One way to decide is to look at the data:
String theory? (9 talks, more than usual)
Quantum gravity? (12 talks)
Quantum field theory? (21 talks)
Physical mathematics? (9 talks)2What is this conference about?

One way to decide is to look at the data:
String theory? (9 talks, more than usual)
Quantum gravity? (12 talks)
Quantum field theory? (21 talks)
Physical mathematics? (9 talks)
Information theory/computer science? (6 talks)2What is this conference about?

One way to decide is to look at the data:
String theory? (9 talks, more than usual)
Quantum gravity? (12 talks)
Quantum field theory? (21 talks)
Physical mathematics? (9 talks)
Information theory/computer science? (6 talks)
Condensed matter physics? (3 talks, fewer than in previous strings)

2

What is this conference about?

One way to decide is to look at the data:
String theory? (9 talks, more than usual)
Quantum gravity? (12 talks)
Quantum field theory? (21 talks)
Physical mathematics? (9 talks)
Information theory/computer science? (6 talks)
Condensed matter physics? (3 talks, fewer than in previous strings)
One might look at this list and ask:2

What is this conference about?

One way to decide is to look at the data:
String theory? (9 talks, more than usual)
Quantum gravity? (12 talks)
Quantum field theory? (21 talks)
Physical mathematics? (9 talks)
Information theory/computer science? (6 talks)
Condensed matter physics? (3 talks, fewer than in previous strings)
One might look at this list and ask:
Should we change the name?

2What is this conference about?

One way to decide is to look at the data:
String theory? (9 talks, more than usual)
Quantum gravity? (12 talks)
Quantum field theory? (21 talks)
Physical mathematics? (9 talks)
Information theory/computer science? (6 talks)
Condensed matter physics? (3 talks, fewer than in previous strings)
One might look at this list and ask:
Should we change the name?
Should we go our separate ways?

2

I don’t know what we should call the conference, but what I do know
is that I found almost all of the talks to be quite interesting.

3

I don’t know what we should call the conference, but what I do know
is that I found almost all of the talks to be quite interesting.
(And not just becuase I was on the program committee...)3

I don’t know what we should call the conference, but what I do know
is that I found almost all of the talks to be quite interesting.
(And not just becuase I was on the program committee...)
It would be a pity to lose this rather unique opportunity to hear them
all in the same place.

3

I don’t know what we should call the conference, but what I do know
is that I found almost all of the talks to be quite interesting.
(And not just becuase I was on the program committee...)
It would be a pity to lose this rather unique opportunity to hear them
all in the same place.
Indeed I think lists like the one I just presented are rather
wrongheaded: they miss the remarkable interconnectedness of all ofthese topics.

3

I don’t know what we should call the conference, but what I do know
is that I found almost all of the talks to be quite interesting.
(And not just becuase I was on the program committee...)
It would be a pity to lose this rather unique opportunity to hear them
all in the same place.
Indeed I think lists like the one I just presented are rather
wrongheaded: they miss the remarkable interconnectedness of all ofthese topics.
It is almost like one of those jokes: what do a bootstrapper, ageometer, and an entangler all have in common?

3

I don’t know what we should call the conference, but what I do know
is that I found almost all of the talks to be quite interesting.
(And not just becuase I was on the program committee...)
It would be a pity to lose this rather unique opportunity to hear them
all in the same place.
Indeed I think lists like the one I just presented are rather
wrongheaded: they miss the remarkable interconnectedness of all ofthese topics.
It is almost like one of those jokes: what do a bootstrapper, ageometer, and an entangler all have in common?
One answer is that they all come to the strings conference!

3

In recent years this interconnectedness has extended well beyond
traditional “high-energy physics”.

4

In recent years this interconnectedness has extended well beyond
traditional “high-energy physics”. Three examples we saw:4

In recent years this interconnectedness has extended well beyond
traditional “high-energy physics”. Three examples we saw:
The rapid progress on non-susy boson-fermion dualities in 2 + 1dimensions (see Benini, Seiberg, Komargodski) has developed inparallel with a large body of work by condensed matter theorists
studying topological phases of interesting materials, in some cases
resolving puzzles in the literature which were decades old.

4

In recent years this interconnectedness has extended well beyond
traditional “high-energy physics”. Three examples we saw:
The rapid progress on non-susy boson-fermion dualities in 2 + 1
dimensions (see Benini, Seiberg, Komargodski) has developed inparallel with a large body of work by condensed matter theorists
studying topological phases of interesting materials, in some cases
resolving puzzles in the literature which were decades old.
The study of scrambling in black holes has led to new diagnostics forquantum chaos (see Stanford), which are currently being studied
intensively in the many-body physics community.

4

In recent years this interconnectedness has extended well beyond
traditional “high-energy physics”. Three examples we saw:
The rapid progress on non-susy boson-fermion dualities in 2 + 1
dimensions (see Benini, Seiberg, Komargodski) has developed inparallel with a large body of work by condensed matter theorists
studying topological phases of interesting materials, in some cases
resolving puzzles in the literature which were decades old.
The study of scrambling in black holes has led to new diagnostics forquantum chaos (see Stanford), which are currently being studied
intensively in the many-body physics community.
The reformulation of AdS/CFT as a quantum error-correcting code
(see Dong) has given a new perspective on such codes, which has
generated a lot of new activity among quantum information scientists,
and which may well lead “engineering applications” of holography.

4In recent years this interconnectedness has extended well beyond
traditional “high-energy physics”. Three examples we saw:
The rapid progress on non-susy boson-fermion dualities in 2 + 1
dimensions (see Benini, Seiberg, Komargodski) has developed inparallel with a large body of work by condensed matter theorists
studying topological phases of interesting materials, in some cases
resolving puzzles in the literature which were decades old.
The study of scrambling in black holes has led to new diagnostics forquantum chaos (see Stanford), which are currently being studied
intensively in the many-body physics community.
The reformulation of AdS/CFT as a quantum error-correcting code
(see Dong) has given a new perspective on such codes, which hasgenerated a lot of new activity among quantum information scientists,and which may well lead “engineering applications” of holography.
(even DoD is interested!?)4

In recent years this interconnectedness has extended well beyond
traditional “high-energy physics”. Three examples we saw:
The rapid progress on non-susy boson-fermion dualities in 2 + 1
dimensions (see Benini, Seiberg, Komargodski) has developed inparallel with a large body of work by condensed matter theorists
studying topological phases of interesting materials, in some cases
resolving puzzles in the literature which were decades old.
The study of scrambling in black holes has led to new diagnostics forquantum chaos (see Stanford), which are currently being studied
intensively in the many-body physics community.
The reformulation of AdS/CFT as a quantum error-correcting code
(see Dong) has given a new perspective on such codes, which hasgenerated a lot of new activity among quantum information scientists,
and which may well lead “engineering applications” of holography.
(even DoD is interested!?)All of these developments came at least in part out of attempts tounderstand quantum gravity and black holes in AdS!
4

I do not however want this talk to degenerate into unqualified
self-celebration.

5

I do not however want this talk to degenerate into unqualified
self-celebration. In particular, there is a question I would like to ask:5

I do not however want this talk to degenerate into unqualified
self-celebration. In particular, there is a question I would like to ask:
Do we still think that string theory is a plausible or useful candidate
for a theory of high energy physics in our world?

5

I do not however want this talk to degenerate into unqualified
self-celebration. In particular, there is a question I would like to ask:
Do we still think that string theory is a plausible or useful candidate
for a theory of high energy physics in our world?
As we have just discussed, the answer to this question does not need to be
yes in order for work on string theory to be an important part of science.

5

I do not however want this talk to degenerate into unqualified
self-celebration. In particular, there is a question I would like to ask:Do we still think that string theory is a plausible or useful candidate
for a theory of high energy physics in our world?
As we have just discussed, the answer to this question does not need to be
yes in order for work on string theory to be an important part of science.
But we still need to answer it!

5

I do not however want this talk to degenerate into unqualified
self-celebration. In particular, there is a question I would like to ask:
Do we still think that string theory is a plausible or useful candidate
for a theory of high energy physics in our world?
As we have just discussed, the answer to this question does not need to be
yes in order for work on string theory to be an important part of science.
But we still need to answer it!
If the answer is yes, and personally I think it is, then there is a follow-up
question:5

I do not however want this talk to degenerate into unqualified
self-celebration. In particular, there is a question I would like to ask:
Do we still think that string theory is a plausible or useful candidate
for a theory of high energy physics in our world?
As we have just discussed, the answer to this question does not need to be
yes in order for work on string theory to be an important part of science.
But we still need to answer it!
If the answer is yes, and personally I think it is, then there is a follow-up
question:
How are we going to test it?

5I do not however want this talk to degenerate into unqualified
self-celebration. In particular, there is a question I would like to ask:
Do we still think that string theory is a plausible or useful candidate
for a theory of high energy physics in our world?
As we have just discussed, the answer to this question does not need to be
yes in order for work on string theory to be an important part of science.
But we still need to answer it!
If the answer is yes, and personally I think it is, then there is a follow-up
question:
How are we going to test it?
At the moment it might not be the right time to confront these questions
directly, but sooner or later I think we need to.

5I do not however want this talk to degenerate into unqualified
self-celebration. In particular, there is a question I would like to ask:
Do we still think that string theory is a plausible or useful candidate
for a theory of high energy physics in our world?
As we have just discussed, the answer to this question does not need to be
yes in order for work on string theory to be an important part of science.
But we still need to answer it!
If the answer is yes, and personally I think it is, then there is a follow-up
question:
How are we going to test it?
At the moment it might not be the right time to confront these questions
directly, but sooner or later I think we need to.
Personally, I think that cosmology gives us the best hope: it is only therethat we can look for a dynamical explanation of the low-energy effective
field theory we see around us, and most of my work is ultimately aimed inthis direction.
5

To put this in historical context, I want to close by quoting from one of
my favorite papers, “On the stability of the motion of Saturn’s rings”,
Maxwell, 1859.

6

To put this in historical context, I want to close by quoting from one ofmy favorite papers, “On the stability of the motion of Saturn’s rings”,
Maxwell, 1859.

In this paper, Maxwell used theoretical consistency to argue that the ringsof Saturn must be made out of small individually-orbiting objects, aprediction which was not decisively confirmed until the size distribution of
the ring consituents was measured by the Voyager 1 mission in 1980.
6

“There are some questions in Astronomy to which we are attracted ratheron account of their peculiarity, as the possible illustration of some
unknown principle, than from any direct advantage which their solution
would afford to mankind.

7

“There are some questions in Astronomy to which we are attracted rather
on account of their peculiarity, as the possible illustration of some
unknown principle, than from any direct advantage which their solution
would afford to mankind. I am not aware that any practical use has been
made of Saturn’s rings, either in Astronomy or in Navigation. They are
too distant, and too insignificant in mass, to produce any effect on themotion of the other parts of the Solar system; and for this very reason it isdifficult to determine those elements of their motion which we obtain soaccurately in the case of bodies of greater mechanical importance.

7

“There are some questions in Astronomy to which we are attracted rather
on account of their peculiarity, as the possible illustration of some
unknown principle, than from any direct advantage which their solution
would afford to mankind. I am not aware that any practical use has been
made of Saturn’s rings, either in Astronomy or in Navigation. They are
too distant, and too insignificant in mass, to produce any effect on themotion of the other parts of the Solar system; and for this very reason it isdifficult to determine those elements of their motion which we obtain soaccurately in the case of bodies of greater mechanical importance.
But when we contemplate the Rings from a purely scientific point of view,
they become the most remarkable bodies in the heavens, except, perhaps,
those still less useful bodies- the spiral nebula.

7

“There are some questions in Astronomy to which we are attracted rather
on account of their peculiarity, as the possible illustration of someunknown principle, than from any direct advantage which their solution
would afford to mankind. I am not aware that any practical use has been
made of Saturn’s rings, either in Astronomy or in Navigation. They are
too distant, and too insignificant in mass, to produce any effect on themotion of the other parts of the Solar system; and for this very reason it isdifficult to determine those elements of their motion which we obtain soaccurately in the case of bodies of greater mechanical importance.
But when we contemplate the Rings from a purely scientific point of view,
they become the most remarkable bodies in the heavens, except, perhaps,
those still less useful bodies- the spiral nebula. When we have actually seenthat great arch swung over the equator of the planet without any visible
connection, we cannot bring our minds to rest. We cannot simply admitthat such is the case, and describe it as one of the observed facts in nature,
not admitting or requiring explanation.

7

“There are some questions in Astronomy to which we are attracted ratheron account of their peculiarity, as the possible illustration of some
unknown principle, than from any direct advantage which their solution
would afford to mankind. I am not aware that any practical use has been
made of Saturn’s rings, either in Astronomy or in Navigation. They are
too distant, and too insignificant in mass, to produce any effect on themotion of the other parts of the Solar system; and for this very reason it isdifficult to determine those elements of their motion which we obtain soaccurately in the case of bodies of greater mechanical importance.
But when we contemplate the Rings from a purely scientific point of view,
they become the most remarkable bodies in the heavens, except, perhaps,
those still less useful bodies- the spiral nebula. When we have actually seenthat great arch swung over the equator of the planet without any visible
connection, we cannot bring our minds to rest. We cannot simply admitthat such is the case, and describe it as one of the observed facts in nature,
not admitting or requiring explanation. We must either explain its motion
on the principles of mechanics, or admit that, in the Saturnian realms,
there can be motion regulated by laws which we are unable to explain.”7

In other words, Maxwell was a string theorist!

8In other words, Maxwell was a string theorist!
We may not be as successful as Maxwell, but we are part of the samequest, we are proceeding using similar methods, and I think I am right insaying that we are having lots of fun doing so.

8In other words, Maxwell was a string theorist!
We may not be as successful as Maxwell, but we are part of the samequest, we are proceeding using similar methods, and I think I am right insaying that we are having lots of fun doing so.

8In other words, Maxwell was a string theorist!
We may not be as successful as Maxwell, but we are part of the samequest, we are proceeding using similar methods, and I think I am right insaying that we are having lots of fun doing so.

8