String theory can now describe a universe that has dark energy? | Hacker News Reader

String theory can now describe a universe that has dark energy?

quantamagazine.org

59 pointsnsoonhui67 comments

I foolishly sat in 8.821 [0] while at MIT thinking I could make sense out of quantum gravity. Most of the math went over my head, but the way I understand this paper, it’s basically a cosmic engineering fix for a geometry problem. Please correct me if necessary.

String theory usually prefers universes that want to crunch inwards (Anti-de Sitter space). Our universe, however, is accelerating outwards (Dark Energy).

To fix this, the authors are essentially creating a force balance. They have magnetic flux pushing the universe's extra dimensions outward (like inflating a tire), and they use the Casimir effect (quantum vacuum pressure) to pull them back inward.

When you balance those two opposing pressures, you get a stable system with a tiny bit of leftover energy. That "leftover" is the Dark Energy we observe.

You start with 11 dimensions (M-theory) and roll up 6 of them to get this 5D model. It sounds abstract, but for my engineer brain, it's helpful to think of that extra 5th dimension not as a "place" you can visit, but as a hidden control loop. The forces fighting it out inside that 5th dimension are what generate the energy potential we perceive as Dark Energy in our 4D world. The authors stop at 5D here, but getting that control loop stable is the hardest part

The big observatiom here is that this balance isn't static -- it suggests Dark Energy gets weaker over time ("quintessence"). If the recent DESI data holds up, this specific string theory solution might actually fit the observational curve better than the standard model.

[0] https://ocw.mit.edu/courses/8-821-string-theory-and-holograp...

> we perceive as Dark Energy in our 4D world

This is a bit of a technicality, but we don't live in a 4D world, we live in a 3+1D world - the 3 spacial dimensions are interchangeable, but the 1 time-related dimension is not interchangeable with the other three (the metric is not commutative).

I'm bringing this up because a lot of people seem to think that time and space are completely unified in modern physics, and this is very much not the case.

To expand on this a little for those interested, time has properties space doesn't. For example, you can turn left to swap your forward direction for sideways in space. You cannot turn though, in a way that swaps your forward (as it were) direction in space for a backward direction in time.

Equally, cause always precedes effect. If time were exactly like space, you could bypass a cause to get to an effect, which would break the fundamental laws of physics as we know them.

There's obviously a lot more, but that's a couple of examples to hopefully help someone.

I had always thought that the fundamental forces were largely the same regardless of whether time was reversed or not.

Not really. Even the electric force is not purely time symmetric - you have to flip the sign of the charge if you want to flip the direction between forwards vs backward in time.

Even worse, the weak force breaks another symmetry as well, parity symmetry (which basically means that moving backward in time, weak force particles "look" like their mirror image, instead of looking the same).

Theoretically this holds true, but in practice it never happens.

Why is a major question, but any understanding of our universe must assume this fact.

How do you test that behavior if you can't make time go backwards?

How is the difference between them characterised in physics?

It seems like it would be hard to distinguish from the point of view of a 4D unit vector XYZT if T was massively larger. Is it distinguished because it's special or is it just distinguished just because the ratio to the other values is large.

Imagine if at the big bang there was stuff that went off in Z and XY and T were tiny in comparison? What would that look like? Part of me says relativity would say there's no difference, but I only have a slightly clever layman's grasp of relativity.

The difference is this: in regular 4D space, the distance between two points, (X1 Y1 Z1 T1) and (X2 Y2 Z2 T2) is (X1-X2)^2 + (Y1-Y2)^2 + (Z1-Z2)^2 + (T1-T2)^2), similar to 3D distances you may be more familiar with.

However, this is NOT the case in Special Relativity (or in QM or QFT). Instead, the distance between two points ("events") is (cT1-cT2)^2 - (X1-X2)^2 - (Y1-Y2)^2 - (Z1-Z2)^2. Note that this means that the distance between two different events can be positive, negative, or 0. These are typically called "time-like separated" (for example, two events with the same X,Y,Z coordinates but different T coordinates, such as events happening in the same place on different days); "space-like separated" (for example, two events with the same T coordinate but different X,Y,Z coordinates, such as events happening at the same time in two different places on Earth); or light-like separated (for example, if (cT1-cT2) = (X1 - X2), and Y, Z are the same; these are events that could be connected by a light beam). Here c is the maximum speed limit, what we typically call the speed of light.

This difference in metric has many mathematical consequences in how different points can interact, compared to a regular 4D space. But even beyond those, it makes it very clear that walking to the left or right is not the same as walking forwards or backwards in time.

Edit to add a small note: what I called "the distance" is not exactly that - it's a measure of the vector that connects the two points (specifically, it is the result of its scalar product with itself, v . v). Distance would be the square root of that, with special handling for the negative cases in 3+1D space, but I didn't want to go into these complications.

Greg Egan is famous for examining various other variations of this physics.

e.g. Dichronauts examines the 2+2D case which turns out to be very different from 4D or 3+1D.

https://www.gregegan.net/DICHRONAUTS/00/DPDM.html

Can you expand on this? I’m guessing that it’s something to do with preservation of mass & energy? Like mass doesn’t have to be preserved over a spatial dimension (eg rotating an object) but does over time.

I explained in another comment, but it's more fundamental than that.

In pure mathematical terms, the vector space used in special relativity (and in theories compatible with it, such as QM/QFT), while being 4 dimensional, is not R^4, it's not a 4D cartesian vector space.

Specifically, the scalar product of two vectors in R^4 (4D space) is [x1,y1,z1,h1] dot [x2,y2,z2,h2] = x1x2 + y1y2 + z1z2 + h1h2. You can order the coordinates however you like - you could replace x with h in the above and nothing would change.

However, SR space-time is quite different. The scalar product is defined as [x1,y1,z1,t1] dot [x2,y2,z2,t2] = c^2 * t1t2 - x1x2 - y1y2 - z1z2. You can still replace x with y without any change with the result; but you can't replace x with t in the same way. This makes it clear from the base math itself that the time dimension is of a different nature than the 3 space dimensions in this representation. This has a significant impact on how distances are calculated, and how operations like rotations work in this geometry.

I don't know who wrote the title for this submission, but adding a question mark that is not in the linked article seems like a terrible editorial decision.

Agree, its editorialising and not allowed under the guidelines here (unless it was in the original and that was changed), but given the uselessness of the field you could argue that any "String Theory" claim in any title should have an automatic question mark (or perhaps several) attached afterwards.

> given the uselessness of the field

It's not useless, though. String theory can be a fad (or "difficult to prove", per Witten) but some of the mathematics used in its research or "trying to prove it" have been used in other fields.

Side note, there’s been a few recent publications showing that dark energy may not be needed to explain what we are seeing.

1. Inhomogeneity backreaction (Moffat 2025) Large-scale cosmic inhomogeneities such as voids and dense regions can create an effective expansion history that mimics evolving dark energy when averaged using standard homogeneous assumptions. https://arxiv.org/abs/2503.20912

2. Timescape cosmology (Wiltshire) Because cosmic voids expand faster than dense regions and dominate volume at late times, observers may infer acceleration from redshift data even if the universe is not globally accelerating. https://www.livescience.com/physics-mathematics/dark-energy/...

3. Local giant void hypothesis If the Milky Way resides inside a large underdense region, locally measured redshifts and distances can bias expansion measurements and partially explain apparent acceleration and Hubble tension. https://www.livescience.com/space/cosmology/echoes-from-the-...

4. Void universe models (LTB cosmologies) Placing the observer near the center of a large cosmic void can reproduce supernova redshift–distance relations without dark energy, though such models struggle with other cosmological constraints. https://arxiv.org/abs/0807.1443

5. Structure formation and virialisation effects The growth of cosmic structure and entropy production alters averaged expansion rates, potentially generating an apparent dark-energy-like signal without introducing a new energy component. https://www.aanda.org/articles/aa/full_html/2024/09/aa50818-...

6. Redshift drift as a discriminator Measuring how cosmological redshifts change over time can distinguish true cosmic acceleration from redshift effects caused by voids or inhomogeneous expansion. https://arxiv.org/abs/1010.0091

The prediction is just 105 orders of magnitude (and an extra dimension) away, but ok.

Only about 2 orders of orders of magnitude - not bad for string theory.

Here is the latest and in my opinion the best interview with Ed Witten [1]

Things he talks about go mostly over my head. What disappointed me a little bit is that he seems to be a materialist. But that is pretty common position among physicists anyway, so not that surprising.

[1] - https://www.youtube.com/watch?v=sAbP0magTVY

If materialists disappoint you, then you should check out Deepak Chopra, for all your self affirming quantum woo needs and desires. He will make your dreams come true! Just buy lots of his books, and you both will be very happy.

I'm sad to discover that wisdomofchopra is no more.

We have llms now so no need for another bullshit generator.

Robots will need to become fat and human before they can ever generate Deepak Chopra Quality Woo!

https://www.youtube.com/watch?v=aO2dPIdEaR4

"But as Deepak Chopra taught us, quantum physics means anything can happen at any time for no reason. Also, eat plenty of oatmeal, and animals never had a war! Who's the real animals?" -Professor Hubert Farnsworth

It probably ran in 500kb and cost nothing.

You are clearly very philosophically inclined.

Hm, string theory can describe a lot of things, but it's not testable with current technology. I'm pretty sure that other mathematical constructs exist that could also describe a similar set of properties, but we just happened to stumble upon string theory first, and got enamored with some of the nice properties it had initially.

Sounds like overfitting.

Yeah I am awaiting the Copernican Revolution of quantum mechanics before I care about this stuff.

I personally have no practical application, so it does me no good to learn this stuff that will be obsolete sooner or later.

Just few extra dimensions and you can handle anything.

String theory does not work with de-Sitter spaces, only with anti de-Sitter spaces. Science has proved we are living in a de-Sitter space. String theory cannot be true.

That is literally what this article is about.

> “What they have found is a 5D de Sitter solution, and we don’t live in 5D,” said > Antonio Padilla(opens a new tab) of the University of Nottingham.

> Still, the work is expected to launch a new era in matching the mathematical > elegance of string theory to the actual world we live in."

yeah, sounds real promising. string theory all over. nice maths but who cares if it doesnt map to reality, its nice maths!

Which would be fine if they were calling themselves mathematicians, we can debate if their ideas are more/less worthy of funding vs all the other mathematicians working on interesting math that might or might not be useful. However when they call themselves physicists we demand they prove they are creating useful physics. There are other areas of study in Physics that are producing results and thus seem more worthy of funding.

Remember resources are limited. We cannot fund everyone who wants it. Society needs to make choices, we are generally okay with a bit of "interesting but unlikely to produce anything important", but most of what we fund needs a return on investment.

Science never proves a positive.

You can only disprove.

The only way to prove a positive if there is a finite number of possibilities and you have disproven all but one. But even then, someone could conceivably come up with an alternate description that preserves the current understanding but makes additional predictions or is a simpler model making the same.

As Feyman said: "We can never know if we are right, we can only be certain if we are wrong".

I don't think that's true regardless of whether you or Feynman or anyone else says it.

For example:

Every continuous symmetry of action in a physical system with conservative forces has a corresponding conservation law. (Noether's Theorem)

There must be two antipodal points on Earth with exactly the same temperature and barometric pressure (as a result of the Borsuk-Ulam Theorem)

As far as I know these are absolutely proved positively because they are mathematical consequences of the properties of continuous functions etc. I'm not a scientist, but there are thousands of things like this where we are definitely absolutely certain we are right because of the possibility of a mathematical direct proof.

You can prove Noether's Theorem in a mathematical sense, but you cannot conclusively prove that a specific physical force is conservative or that a specific physical symmetry of action is continuous.

Likewise, we assume at an operational level that temperature and barometric pressure are continuous functions (as assumed in Borsuk-Ulam), but it's not something you can conclusively prove aobut reality.

Sure but that doesn’t matter for my examples. The parent of my comment said “science never proves a positive” and I gave a couple of examples of proving implications. Proving “If A then certainly B” is definitively proving a positive whether or not we can prove A.

I guess this comes down to what you mean by "science". Some would say that science is the process of testing hypotheses about reality. Mathematical facts exist in an abstract sense apart from reality, and so mathematics is not really science to those people.

There's an argument that you are still doing science if you construct a logical proof showing that "if the world is like X, then it will behave like Y". A lot of theoretical physics is like this, and people call that science. But I think there's truth to what OP is saying in that science does not conclusively positively prove things about reality.

This is just sleight of hand. It's true that science can never be certain about anything, not to the same level as mathematics.

But otherwise, there is nothing special about positive or negative statements. You can express any positive statement as the negation of a negative statement, so to the extent that science can "disprove negatives", it can equivalently "prove positives".

but you can use that exact logic to say the inverse; that nothing can be disproved, because you can never be sure youve disproved every alternative

They had to keep the theory alive somehow.

Only in universe with 5 dimensions. Shouldn't string theory be given up on at this point? This theory has existed for over 50 years and hasn't produced any results. Even the predictions made by it such as e.g. supersymmetry have not been confirmed despite searching for them at particle colliders.

It has produced some good math though. That's something.

At what opportunity cost?

If you want to bash badly-spent potential look at people doing cutting edge ad research and optimization, or HFT. This is at least good base research that others can build on.

Fair point, but waste in one domain should not be used to excuse waste elsewhere. Unless your argument is that it's generally hard for human societies to know where to best invest their scientific talent without the benefit of hindsight.

I agree plus ST takes a person who would have researched somewhere else. The Googler or Jane Street or guy who decides to travel the world in the canoe have different reasons and probably would need way more persuading to be in academia.

human societies don't decide where to invest their talent; individuals do

I've worked in academia. How grants are won and research is received and encouraged is not an individual decision.

> don't decide

shouldn't* decide

Think of it as a playground for the exercise and training of a pool of minds that will one day either make the glove fit or kick the sand castle over replacing it with a better mousetrap.

Too many metaphors? Hmmm, maybe fold in some dimensional reduction somehow.

String theory has generated a lot of hype over the years, but never delivered anything. Looks to me like they are all the negatives you hate about ad research.

Advertising has been a net negative and the mathematics produced by string theory research is amazing.

But HFT is good! It allows better pricing of equities and increases liquidity.

Low. It's too expensive to send all of humanity across the stars at present.

100% unknown. That's always the problem with science and why political control and direction always backfires and is stupid.

As I understand it, it's still our best candidate for a unified theory of everything. Not for lack of effort in researching alternatives, either.

  Shouldn't string theory be given up on at this point?

Has anti string theory propaganda taken over HN? Sabine Hossenfelder succeeded?

Anyone who is anti string theory actually qualified to make statements saying string theory is wrong or not worth more investment from researchers?

Are these anti string theory posts on HN mostly just laymen hearing how string theory can’t be tested and we wasted a lot of resources on it so it needs to be repeated on every string theory post here?

Susskind himself said its dead in the water. We dont live in an anti de-Sitter space, the basis of string theory.

Source?

Here is a lengthy interview where he discusses this at length (with a timestamp where he says exactly that String Theory, the precise mathematical model, doesn't describe the real world):

https://www.youtube.com/watch?v=2p_Hlm6aCok&t=10m15s

This also references this podcast discussion between Leonard Susskind and Lawrence Krauss, where they discuss the same thing:

https://www.youtube.com/watch?v=qhszd_wqAgQ

Note that he still thinks that there is a way to produce some kind of similar theory, "a string theory" as opposed to "String Theory", could be the best answer.

https://www.reddit.com/r/AskPhysics/comments/1kolwxm/last_ye...

According to this Reddit thread, he doesn't say it's "dead in the water" at all. It's just a version of string theory.

This is what I'm afraid of. People who aren't qualified spreading fake news on string theory.

I don't claim to be qualified. I just want to call out HN people who are extremely confident that string theory is dead but has no background in physics.

You can listen to the podcast yourself. I explained my own understanding of his position: that String Theory, the specific mathematical structure, is categorically not a good model of the real world. He does believe that it is possible to create a different theory inspired from String Theory, "a slightly expanded version of String Theory", that could be correct. But, he also says that no version of a string theory that exists today fulfills this role.

Now, if by "string theory is dead in the water" someone means that "working on a generalization of string theory is a bad idea", then they are wrong, Leonard Susskind doesn't believe that.

But if by "string theory is dead in the water" they mean "there is no point in studying String Theory deeper, with its general mathematical properties, maybe with a slight tweaks, as it is right now it can't describe the real world", then this is quite clearly professor Susskind's position.

The issue isn't string theory yes or no (there are reasons physicists went this way and other alternatives aren't so much better), but the difficulty in getting data and testable predictions. It's very likely the most effective way to help particle physics is getting way more data at high energies, not a new theory.

Yes. Like Python can describe any turing machine.

The string theory hype machine will never die

https://news.ycombinator.com/item?id=46621518

Reply on news.ycombinator.com