Approaching 50 Years of String Theory (math.columbia.edu)
78 points by jjgreen 19 hours ago
gnfargbl 17 hours ago
People often say that the problem with string theory is that it doesn't make any prediction, but that's not quite right: the problem is that it can make almost any prediction you want it to make. It is really less of a "theory" in its own right and more of a mathematical framework for constructing theories.
One day some unusual observation will come along from somewhere, and that will be the loose end that allows someone to start pulling at the whole ball of yarn. Will this happen in our lifetimes? Unlikely, I think.
marcosdumay 7 hours ago
The problem is that once, a long time ago, String Theory was something that made concrete predictions that people just couldn't calculate.
Then people managed to calculate those predictions, and they were wrong. So the people working that theory up relaxed some constraints and tried again, and again, and again. So today it's that framework that you can use to write any theory you want.
That original theory was a good theory. Very compelling and just a small adjustment away from mainstream physics. The current framework is just not a good framework, it's incredibly hard to write any theory in it, understand what somebody else created, and calculate the predictions of the theories you create.
steppi 9 hours ago
I was planning to make a similar comment. Conjecturing that some theory in the string theory landscape [0] gives a theory of quantum gravity consistent with experiments that are possible but beyond what humans may ever be capable of isn't as strong of a claim as it may first appear. The intuition I used to have was that string theory is making ridiculously specific claims about things that may remain always unobservable to humans. But the idea is not that experiments of unimaginable scale and complexity might reveal that the universe is made up of strings or something, it's just that it may turn out that string theory makes up such a rich and flexible family of theories that it could be tuned to the observed physics of some unimaginably advanced civilization. My impression is that string theory is not so flexible that its uninteresting though. There's some interesting theoretical work along these lines around exploring the swampland [1].
tlogan 15 hours ago
I am old enough to remember when string theory was expected to explain and unify all forces and predict everything. Sadly, it failed to deliver on that promise.
And there is no known single real world experiment that can rule out string theory while keeping general relativity and quantum mechanics intact.
More accurately, string theory is not wrong (because it just cannot be wrong). Because it does not predict anything and cannot invalidate anything, it does not help to advance our understanding of how to integrate general relativity and quantum mechanics.
It should not be called theory - maybe set of mathematical tools or whatever.
nrhrjrjrjtntbt 9 hours ago
string boot framework
pfdietz 17 hours ago
Or, that day will never come, because string theory isn't reflective of the actual world, or because there are so many theories possible under the string theory rubric that we can never find the right one, or because the energies involved to see any effect are far beyond what could be reached in experiment.
gnfargbl 17 hours ago
It isn't completely implausible that a future civilisation could perform the experiments to gather that data, somehow; but it is hard to envisage how we do it here on Earth.
Your implicit point is a good one. Is it sensible to have a huge chunk of the entire theoretical physics community working endlessly on a theory that could well end up being basically useless? Probably not.
XorNot 8 hours ago
tpoacher 2 hours ago
> the problem is that it can make almost any prediction you want it to make
In logic this is either the principle of "contradiction elimination" or a "vacuous truth". Depending on how you look at it. i.e. given sufficiently bad premises, you can prove anything.
wayeq 2 hours ago
> less of a "theory" in its own right and more of a mathematical framework for constructing theories.
so it's javascript?
slashdave 6 hours ago
Theorists are real good at bending around experimental data, unusual or not
mkw5053 18 hours ago
I thought Brian Greene did a great job interviewing Edward Witten (Fields Medal winner) on the World Science Festival Youtube channel [1].
I also just really enjoy Brian Greene, his books, and the World Science Festival Youtube channel.
Levitating 18 hours ago
You might not like Angela Collier's video on string theory[1].
niccl 3 hours ago
_totally_ off topic, but what sort of brain must Ms Collier have that she can play a game at the same time as giving a cogent lecture on String Theory with very few hesitations? I could hardly concentrate on the lecture because of the game being shown in an inset window. Truly impressive
mkw5053 18 hours ago
Interesting, thanks for sharing!
I definitely don't walk away from any of Brian Greene's content thinking that String Theory is anything close to a confirmed fact at all.
It's been some times since I read his earlier books, possibly his tone has changed?
I'll also say, I'm far from a professional physicist. I'm reading and watching for fun and intellectual curiosity, not to learn physics with the goal of doing my own research. I always thought of String Theory as being more of a study of math where many people have unsuccessfully tried to apply it to physics. And, that it's lead to some really interesting ideas. I just find him and his work really enjoyable.
michaelcampbell 18 hours ago
Thanks for posting that; as soon as I saw the title here I was going to look that up if no one else had already. Sabine Hossenfelder too, though there's far too much content from her on this to put a list, but anyone interested might like some of her takes.
A few debates between Brian and other notables; Hossenfelder, Eric Weinstein, and Roger Penrose to name a few; have popped up in my youtube feed lately which are typically also engaging.
UncleMeat 18 hours ago
dang 10 hours ago
The OP is about that video, though one wouldn't know it from a web page that won't download. I've put an archive.org link in the toptext now.
greenavocado 18 hours ago
The Planck scale where string theory's distinctive physics should appear is around 10^19 GeV. The LHC operates at about 10^4 GeV. That's a factor of 10^15 which is a million billion times too weak. No foreseeable accelerator technology can bridge this gap. The proposed Future Circular Collider (FCC) would reach maybe 10^5 GeV. Still 14 orders of magnitude short.
PunchyHamster 17 hours ago
So the grift can continue
ecosystem 17 hours ago
Non-Euclidean geometry (geometric axioms in which one postulate is rejected such that the 3 angles of a triangle are not exactly 180 degrees) was considered a meaningless word game and fundamental mistruth.
Later, non-Euclidean geometry was actually essential to modern physics.
It's intellectually sketchy to judge future value by the present.
PunchyHamster 17 hours ago
snapplebobapple 16 hours ago
emil-lp 17 hours ago
constantcrying 9 hours ago
cess11 17 hours ago
greenavocado 17 hours ago
Planck energy: ~10^19 GeV is approx 2 GJ per collision
Energy to vaporize Earth's oceans: ~4 x 10^27 J
For a Planck-scale linear collider at LHC-like collision rates (~10^8/sec):
Beam power requirement: ~2 x 10^17 W
With realistic wall-plug efficiency of ~1%: ~2 x 10^19 W
Annual energy consumption: ~6 x 10^26 J
At 1% efficiency, one year of operation would:
Vaporize about 15% of Earth's oceans
Or vaporize the Mediterranean Sea roughly 50 times
Or boil Lake Superior every 5 hours
Or one complete ocean vaporization every 6-7 years of operation
It's about 1 million times current global power consumption
Or about 50,000 Suns running continuously
Or 170 billion Large Hadron Colliders operating simultaneously
mishaevtikhiev 6 hours ago
I have a PhD in high energy theoretical physics (hep-th for short) and I've written a paper on string theory, so I'd like to comment on some things:
1. I think there are two reasons why string theory is cool (other people may have different opinions). Please note that none of these two reasons are directly related to the extension of Standard Model. 1.1. String theory is the only theory so far that can mix gravity and quantum mechanics, and it can be even used to derive Hawking entropy of a black hole from "first principles" (see paper by Strominger and Vafa). The obvious trouble is that the black hole in question lives in five-dimensional space and is unrelated to the real-world black holes, but this is way better than what one can get from Standard Model physics (which is, no gravitons for you).
1.2. Through AdS/CFT correspondence, string theory can be used to describe quantum field theories that are not related to string theory by themselves. This gives a very strong tool to study these quantum field theories, and the paper by Maldacena that discovered this correspondence is one of the most important papers in the field.
2. It is true string theory is unusable as of now to derive the Standard Model physics (and provide extensions for it). Unfortunately, I would say that hardly any papers in high energy _theoretical_ physics currently address Standard Model physics. Roughly speaking, in late 1970s, after quantum chromodynamics was established and the asymptotic freedom was discovered, it turned out that it is extremely hard to compute many things we are generally interested in. At this point, high energy theoretical physics split in two sub-areas: phenomenology (which tries to extend the Standard Model to derive things like neutrino mass) and theory (which is a more formal theory and tries to answer questions like "how to quantize gravity"). One can argue that this makes hep-th an area of mathematics, and I would agree with that (eg in Cambridge theoretical physicists are in the same department with applied mathematicians).
2.1. The things theoretical physicists study tend to pop up in various places, even if the original motivation is misplaced. Even the string theory itself originated as a way to explain the Regge trajectories (which were explained with quantum chromodynaics afterwards), and not to quantize gravity. For a more practical example, Witten introduced topological quantum field theories long before anyone understood how to apply them to real-world physics.
3. I do not agree that string theory dominates the hep-th field. I would say that its popularity changes with the time, going up and down. While the main conference in the hep-th field is called "Strings", the talks at it are not necessarily related to strings theory, and at the 2025 conference I'd say that only 1/3 of the talks were anyhow related to the strings theory. Moreover, there is no hard division between people working on string theory and people working on other hep-th subjects, so that e.g. Witten made many contributions to hep-th which are not anyhow related to string theory.
3.1. As for the push to do string theory that eg Sabine Hossenfelder alludes to, I'd say that I experienced no such push during my MSc and PhD studies. I've written four papers, and worked on a couple of projects that did not become a paper, and out of those, only one was dedicated to string theory.
3.2. On the other hand, the more fringe theories that can provide alternative to string theory are also more high-risk endeavors (as you are quite likely to fail to produce anything coherent within a typical timeframe you allot to write a paper). Hep-th is strongly underfunded, and I believe, that with greater funding (and less need to publish-or-perish) some people would also pursue the more fringe directions in hep-th.
3.3. A comment on the naming: hep-th is a field which is very hard to name. The name I use is the traditional one (and is used as eg a name for the field on arXiv). However, many things derived by the physicists in the field are not anyhow related to the high energy in the literal meaning of the words "high energy". When talking to people at a party, I say that I studied string theory, because the name is catchy and it rings a bell, but this way of referring to the field is definitely a misnomer.
nsoonhui 2 hours ago
Thanks for the comment that exposes the nuances. As a non physicist who is interested in physics, I would say that the predicament is that we simply don't have enough data to tell how to improve our physics theory, but we have too many physicist working in the field, so ideas just get staled.
Is that a correct assessment?
GMoromisato 5 hours ago
What experiment(s) would we have to run to see deviations from the Standard Model and be able to come up with newer models (maybe String Theory maybe not)?
Is it just about higher energy particle collisions? Or does it involve things like doing experiments next to a black hole?
machinationu an hour ago
we already have experimental data for deviations from the standard model: neutrino mass for example
or dark matter
tim333 10 hours ago
String theory has always seemed intuitively wrong to me. From Wikipedia:
>In theories of particle physics based on string theory, the characteristic length scale of strings is assumed to be on the order of the Planck length, or 10E−35 meters
Yet electrons repel each other over distances of many meters by I think the virtual exchange of photons. How on earth would that work? How does your photo string know to head to an electron string trillions and trillions of times it's length away?
As far as I can tell the field became popular for sociological reasons that you could get grants for it and the like rather than any connection to reality(?)
ajkjk 7 hours ago
whether or not string theory is at this point grifty and weird, the theoretical basis for it is far stronger than you would think based only on reading critics / pop sci explainers. It is not like, missing any obvious physical facts in its foundation. Rather it is trying to say: look, we have this zoo of particles with seemingly random masses and properties; is there same lower-level framework which can produce the zoo that we see according to a simpler list of rules? The obvious choice for this, especially given some of the "hierarchies" of particles that are observed, is that they are in some way resonant modes of some kind of underlying object. Which is where you get the strings from. (Which might sound like a weird justification if you are not aware of all the other aspects of physics which get explained as resonances of fields; this is a standard sort of justification which there's a lot of good reasons to be interested in, at least initially.)
jiggawatts 9 hours ago
You're being somewhat unfairly voted down, it's a legitimate questions because the popular media so grossly misrepresents what string theory is, especially in their visuals.
It's hard to visualise in 3D, but if you cut down the spatial dimensions to just 1D (a line), then theories like string theory just turn the infinitely thin mathematical line into a tube. You can picture a tube that vibrates, or has waves in its cross-section. Don't think of the the "strings" as actual little loops moving around in space, they're a modification of what space is.
You can even do the same kind of line->tube extension of a space with even more extra "loop" dimensions than the number of base dimensions. AFAIK the current theories have 10 total, of which 3 are the usual "large" dimensions of space, the rest are "small" and rolled up like the tube example.
tim333 9 hours ago
Even so I don't get how electrons a meter apart interact through that stuff, as opposed to the electrical fields which spread out through space and so interact with other electrons as featured in quantum field theory which is what physicists use to actually calculate physical results, as opposed to string theory which fails to calculate actual physical results.
"if you cut down the spatial dimensions to just 1D" doesn't sound very physical to me.
I'm maybe being downvoted fairly. I studied physics and it don't think it's a misunderstanding of popularisation or that string theory is untestable, I just think it's straight wrong and not how the physical universe works.
jiggawatts 9 hours ago
ajkjk 6 hours ago
throw4847285 7 hours ago
What would Imre Lakatos say? Is it degenerating or not?
paulorlando 4 hours ago
30 years ago in college, at a physics club talk, I got my first intro to string theory. I didn't understand it but was intrigued enough to go to a small one-off seminar led by Prof. Greene. About 15 people around a table while he spoke. I kept thinking I was too ignorant of the subject matter to be able to ask a question. But nobody asked any questions.
grunder_advice 18 hours ago
I wonder what Ed Witten would have accomplished if he had gone into another field instead of choosing to dedicate his life to mathematical physics.
r721 17 hours ago
You reminded me about this Abstruse Goose comic: https://web.archive.org/web/20230202225744/https://abstruseg...
nrhrjrjrjtntbt 6 hours ago
M for Magic
tome 17 hours ago
Not sure if you're making a joke about this, but Ed Witten tried a number of fields before settling on theoretical physics.
nextworddev 6 hours ago
Basically a framework Ed Witten invented for job security
tuhgdetzhh 19 hours ago
An in half a century we still haven't found a single actually testable prediction.
SiempreViernes 18 hours ago
I think that's not quite right: it is reasonably certain that string theory can produce both the standard model and most extensions people have dreamt up, so the problem is rather that all the obviously "stringy" predictions are currently unavailable, while the string theory derived predictions for achievable experiments look like what we get from other theories we already have.
atakan_gurkan 18 hours ago
To make this valuable, it should produce a limited set including standard model. If you produce pretty much everything one can dream of, that does not carry predictive power.
What does string theory predict that (1) is within experimental reach in, say, 5 years (2) if not found, would prove it wrong. Was there ever anything satisfying these two simultaneously? AFAIK,the answer is "no".
ecosystem 17 hours ago
XorNot 8 hours ago
tejohnso 17 hours ago
But we've discovered a number of useful tools and techniques that are applicable to other areas of research have we not? The billions of dollars spent on string theory hype might have unlocked a strategy or technique that ends up being useful in a civilization changing way that we just don't know about yet. Maybe string theory and the hype it was able to generate was just the catalyst that we needed.
bluGill 17 hours ago
what didn't se develop because those people were working on string theory? That is an unanswerable question. It is also the important question.
mtoner23 17 hours ago
giardini 10 hours ago
Hasn't ChatGPT solved string theory yet?