Validating String Theory: A Debate Among Physicists

[David Neves]

The following article debates whether string theory is testable.

https://www.quantamagazine.org/20151216-physicists-and-philosophers-debate-the-boundaries-of-science

Even though they mention philosophy, this is not merely a philosophical debate. It a serious question for physicists as to how they plan to validate string theory.

 

[DaveC426913]

Has anything changed in the decades since it was first asked whether string theory was testable?

 

[David Neves]

We know that quantum mechanics and general relativity explain what we observe within their prospective ranges of applicability, but they are inconsistent with each other, so we know that there has to be some more fundamental theory that they are limiting cases of. String theory is still a work in progress but it's the most successful theory we have so far in terms of attempting to unify quantum mechanics and general relativity. There are other theories such as loop quantum gravity. It is not entirely true that they are not testable. We might not be able to build an accelerator that can reach those energies but they have to be consistent with everything we currently observe, as well as being internally mathematically consistent, which would be an impressive feat. I could give a flippant answer that the successful prediction of string theory is that if you climb on the roof of your house, and jump off, you hit the ground, in other words the existence of gravity, which is not predicted by any other quantum field theory. Many apparent coincidences can be explained by the anthropic principle which requires the multiverse. Lastly, when you say "how do you know a theory is true?", it depends on what you mean by "true", Every theory to have ever existed in the product of the human mind, based on our limited experience, and is thus not the same as the "real" universe that actually exists external to ourselves. In that sense, no theory invented by humans can be literally "true". The real question is does it explain what you observe. String theory, although still a work in progress, appears to be the best chance we have in deriving a theory that can explain everything that we observe.

 

[DaveC426913]

Unfortunately, all of the above logic can also be ascribed to other equally untestable, unfalsifiable theories, such as a ... supernatural one.

 

[bapowell]

It is not entirely true that they are not testable. We might not be able to build an accelerator that can reach those energies but they have to be consistent with everything we currently observe, as well as being internally mathematically consistent, which would be an impressive feat. I could give a flippant answer that the successful prediction of string theory is that if you climb on the roof of your house, and jump off, you hit the ground, in other words the existence of gravity, which is not predicted by any other quantum field theory.

There are an infinite number of such theories: being consistent is not the same thing as being predictive. This is one of the reasons that Popper introduced the criterion of falsifiability as a means of distinguishing scientific proposals from others.

 

[RJLiberator]

This is a lovely and fascinating topic.

Can String theory be considered true if it is untestable? In my opinion, no, science depends on developing testable predictions.

I have to hope that string theory will be tested in the future, or at least parts of it.

Thank you for the impressive link. This site also has a link to various theories of the past 40 years in the article as well.

 

[DaveC426913]

Can String theory be considered true if it is untestable?

Well, in science, we don't determine if things are true, simply that they are a good (or best) model.

I think the question being raised by string theory is: even if not falsifiable, does it still provide productive avenues of exploration into understanding the universe?

 

[haushofer]

Well, yes: it offered a first explicit realization of holography.

 

[DaveC426913]

Well, yes: it offered a first explicit realization of holography.

Are you referring to the Holographic Principle? Or run-of-the-mill holography?

 

[haushofer]

The first ;)

 

[lpetrich]

There are three main possibilities for string theory, as I see it:

1. The theory makes unique low-energy predictions which agree with what we observe.
2. The theory agrees with what we observe, but it has numerous other possible low-energy limits.
3. The theory is falsified by some observations.

So far, it seems like we are at (2), with as many as 10⁵⁰⁰ possible string vacua or ground states, the "string landscape". I must say that I don't see much progress toward either (1) or (3), though someone here may know more about this issue than I do.

 

[Dr.AbeNikIanEdL]

1. The theory agrees with what we observe, but it has numerous other possible low-energy limits.

So far, it seems like we are at (2), with as many as 10⁵⁰⁰ possible string vacua or ground states, the "string landscape".

Is that actually the case, i.e. is there a string theory which would have precisely the standard model at least as a possible low energy limit?

 

[haushofer]

String theory is a framework like qft, so the question whether it is testable is perhaps ill-posed. Is qft testable?

 

[Paulibus]

Quantum Field Theory agrees with observation to an extent not quantitatively matched by any other human mental construct. Doesn't this answer your question in the affirmative, Haushofer?

 

[lpetrich]

Is that actually the case, i.e. is there a string theory which would have precisely the standard model at least as a possible low energy limit?

I don't recall anyone claiming to get the entire Standard Model from some string-theory state, parameter values and all, but it is possible to get much of the Standard Model from it. It requires compactification and some complicated symmetry breaking, however. Something like this: [hep-th/0512177] The Exact MSSM Spectrum from String Theory:

We show the existence of realistic vacua in string theory whose observable sector has exactly the matter content of the MSSM. This is achieved by compactifying the E_8 x E_8 heterotic superstring on a smooth Calabi-Yau threefold with an SU(4) gauge instanton and a Z_3 x Z_3 Wilson line. Specifically, the observable sector is N=1 supersymmetric with gauge group SU(3)_C x SU(2)_L x U(1)_Y x U(1)_{B-L}, three families of quarks and leptons, each family with a right-handed neutrino, and one Higgs-Higgs conjugate pair. Importantly, there are no extra vector-like pairs and no exotic matter in the zero mode spectrum. There are, in addition, 6 geometric moduli and 13 gauge instanton moduli in the observable sector. The holomorphic SU(4) vector bundle of the observable sector is slope-stable.

It starts out with the HE heterotic superstring and then breaks one of the two gauge E8's into SO(10)*SO(4). The SO(10) then breaks down into the Standard-Model gauge symmetry SU(3)*SU(2)*U(1) with an extra U(1).

 

[haushofer]

Quantum Field Theory agrees with observation to an extent not quantitatively matched by any other human mental construct. Doesn't this answer your question in the affirmative, Haushofer?

No, the standard model is, which is an explicit realization of a qft with a specific gauge group, number of families etc. Qft is a math.framework, and the SM provides a physical model of it.

 

[MacRudi]

We have right now a good instrument, thanks to Maldacena, with AdS/CFT to downgrade to the phenomenology in our reality. This is right now the best instrument to see, if Stringtheory has something to do with our world.
We have right now with the mathematic of String theory the best instrument to describe condensed matter superconductor physics and quasi particles which are white wholes in QT and cannot be described with phenomenology of QT
But there are some more hints, which can help us to understand physics of exotic phenomens only with mathematics of stringtheory. The best framework we have right now.

 

[MacRudi]

the discussion goes on with new papers about the theme of the conference.

http://arxiv.org/abs/1601.06145

Why trust a theory? Some further remarks (part 1)
Joseph Polchinski
(Submitted on 22 Jan 2016)
I expand on some ideas from my recent review "String theory to the rescue," I discuss my use of Bayesian reasoning. I argue that it can be useful but that it is very far from the central point of the discussion. I then review my own personal history with the multiverse. Finally I respond to some criticisms of string theory and the multiverse.

 

[haushofer]

I'm not sure if these kind of papers, with all those personal notes, add that much to the arXiv, and I'm saying this with the conviction that physicist should pay more attention to this kind of questions. Especially that last section about Woit makes me feel a bit uncomfortable.

 

[MacRudi]

I'm not sure if these kind of papers, with all those personal notes, add that much to the arXiv, and I'm saying this with the conviction that physicist should pay more attention to this kind of questions. Especially that last section about Woit makes me feel a bit uncomfortable.

It's an open discussion now and if people write on blogs such mess like Woit then it shows on what level this discussion is really. The conference has shown that there is a lot of unspoken things. We can see it here in this forum also.