Of Particular Significance

A Solar System Test of String Theory?!

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 01/13/2014

Baloney.  Hogwash.  Garbage.

That’s what’s to be found in the phys.org news article claiming that “Scientists at Towson University in Towson, Maryland, have identified a practical, yet overlooked, test of string theory based on the motions of planets, moons and asteroids, reminiscent of Galileo’s famed test of gravity by dropping balls from the Tower of Pisa.”

Sounds too good to be true, right?  And it is.

What the scientists have done (or at least claim to have done, and I’ll be happy to take their claims at face value, since I can’t easily check them) is carry out  a technique to test the equivalence principle, a foundation stone of Einstein’s theory of gravity, which implies that all objects, no matter what material they are made of and no matter how heavy they are, will be pulled by gravity in the same way… with the same acceleration.  This principle, in Einstein’s theory, lies behind why all objects on earth fall with the same acceleration (when air resistance can be neglected), and behind why astronauts float in their space stations.

By looking at the precisely measured orbits of different objects in the solar system, which are made from different materials, the authors (James Overduin, Jack Mitcham and Zoey Warecki of little-known Towson University) claim in their July 2013 paper to have provided new tests that the equivalence principle applies to different materials.  That’s very nice work.  The principle works to the precision reached by their tests — which aren’t as precise as some other types of tests, but do explore some domains that haven’t previously been explored.

But what’s that got to do with string theory?  If you read their paper, you will notice that the word “String Theory” appears in only one obscure sentence in the introduction, referring to a very specific form of string theory [with an extremely light spin-zero field, called the dilaton], implying that their work might be relevant for string theory if we lived in a stringy universe that had such a field.  Not even the conclusion, much less the bulk of the paper, mentions strings or string theory.  That’s because the paper has nothing to do with testing string theory; it is merely testing Einstein’s theory of gravity. 

The reason it can’t test string theory is

  1. String theory doesn’t make a precise prediction for how the equivalence principle will be modified, and among the many possible universes string theory can lead to, many have no measurable modification of the equivalence principle;
  2. Even if a violation of the equivalence principle had been detected, or is detected in the future, it isn’t necessarily due to string theory.  It might be due to some other modification of Einstein’s gravity — in fact, the authors consider one such modification in their paper!

So here we have a nice little paper that tests Einstein’s theory of gravity and puts constraints on various alternatives to it — though none of those alternatives is unique to string theory nor is uniquely predicted by string theory.  How did this get billed as a practical test of string theory?

You’ll have to ask the author of the phys.org article, which appears to be a Towson University press release.  [“Provided by Towson University”, says the last line of the phys.org article.] Or you’ll have to ask the scientists involved (unless one of them is the author) — who ought to be pretty darned embarrassed that their work was billed in this way.  I hope they didn’t do this on purpose.  It’s certainly great free advertising for Towson University; who cares if the article’s right if people are willing to read it?  But a willingness to distort the facts to impress and mislead the public is not a worthy attribute.

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40 Responses

  1. Please wake me up when string theory predicts anything definite. Its “Theory of Everything” explains everything except the standard model, that is, everything except our Universe. Many stringers I met said they now do mathematics, but they do this on physics resources. When do we stop this random walking in Hilbert space and go back to physics: physical answers to physical questions?

    1. I have spent some significant fraction of my time — not half, but not a tenth either — using string theory to get “physical answers to physical questions.” Instead of sleeping, perhaps you might enjoy reading more of my papers… since it would appear that there are applications of string theory to real-world questions of which you are unaware.

      Meanwhile, your remark “Its `Theory of Everything’ explains everything except the standard model, that is, everything except our universe” is, to my ear, bizarre. Currently, string theory doesn’t explain anything at all, so what are you talking about? String theory is very useful as a tool for answering important quantum field theory questions for which there are no other methods, but it doesn’t *explain* anything. What do you in fact mean by this strange remark? Or is it merely political grousing?

  2. The scale of the solar system is too small to resolve these issues.
    So too, most probably, is the local galaxy.

    An article in the January 4th edition of “New Scientist” suggested that it might be possible use observations of Gamma-Ray Burst GRB 130427A to test models of spacetime, incuding Special Relativity, String theory and Loop Quantum Gravity.

    A record-breaking Burst from this source was observed by the Fermi-Lat space telescope last April and has raised serious theoretical questions.

    None of the current models for GRB production can explain the behaviour of the emissions during the first pulse of this burst, or the fact that photons in the GeV scale were still being counted 9 hours later.

    Giovanni Amelino-Camelia and his collaborators wrote a paper in May 2013, which suggested that the observations might be evidence of a quantum spacetime which deviates from the predictions of Special Relativity.

    This assumes that all the gamma rays were initially emitted simultaneously.

    But, as the New Scientist article by Stuart Clark points out, Amelino_Carmelia “has banned his team from investigating which, if any, ot these theories is closest to the mark”
    It concludes that further observations are needed, particularly of the behaviour of extra-galactic neutrinos.

  3. I’m not even sure that it tests general relativity. The equivalence principle applies to an “infinitesimal” region, a region of zero extent, which means it doesn’t really apply. A real gravitational field can be likened to accelerating through space, but it just isn’t the same. Have a read of P M Brown’s essay on http://arxiv.org/abs/physics/0204044 and note these quotes by Synge and Ray :

    “…the Principle of Equivalence performed the essential office of midwife at the birth of general relativity, but, as Einstein remarked, the infant would never have gone beyond its long clothes had it not been for Minkowski’s concept [of space-time geometry]. I suggest that the midwife be buried with appropriate honours…”

    “…The first thing to note about the 1911 version of the principle of equivalence is that what in 1911 is called a uniform gravitational field ends up in general relativity not to be a gravitational field at all – The Riemann tensor is here identically zero…”

    I say this as somebody who “roots for relativity”, but who sees the principle of equivalence as an enabling principle only.

    1. John,

      You note: “I’m not even sure that it tests general relativity.”

      The fact that the equivalence principle holds over a tiny area of space is a technicality based on the simple fact that the gravitational field lines produced by a sphere (like the Earth) diverge with distance, so that two objects dropped in an elevator at rest on the Earth will hit the floor slightly closer together than two objects dropped in a uniformly accelerating elevator in deep space.

      However, that misses the point: As Matt noted: “the equivalence principle, a foundation stone of Einstein’s theory of gravity, which implies that all objects, no matter what material they are made of and no matter how heavy they are, will be pulled by gravity in the same way… with the same acceleration.”

      Thus testing the Equivalence Principle is a perfectly valid test of General Relativity.

      1. Sorry to be tardy replying, Dino.

        We’ll have to agree to differ on this I’m afraid. I see the equivalence principle as something like the scaffolding used to build GR rather than a foundation stone.

  4. Matt: You probably know about Gordon Kane’s statement that he and his students predicted Higgs mass from String Theory within a few percent. What is your opinion on this claim? Are there many arbitrary parameters in this calculation?

  5. I agree it doesn’t violate CPT; I originally misunderstood what you were saying and deleted my comment before you replied.


    Sorry I didn’t know you deleted comment.

  6. Matt Strassler commented on A Solar System Test of String Theory?!.

    in response to Bob Zannelli:

    You are not alone. For various reasons many people with more than a passing interest in physics are under the impression that these direct experiments on antimatter were actually performed – they were not! I was under the impression this experiment was done with anti neutrons with a null result. I guess my impression is […]

    No, it would be evidence *Against* supersymmetry. It would violate CPT, which Supersymmetry preserves.


    Actually I don’t think this violates CPT. As long at the magitude of the Gravivector force between matter and matter is the same as the Gravivector force magitude between anti matter and anti matter , given the same charge product ( B-L??) CPT is preserved . Gravitensor and Graviscalar forces involve attraction between like charges while the Gravivector force , as for all forces mediated by vector bosons , involve attraction between unlike charges and repulsion between like charges. It’s this reversed action ( mostly) that breaks the equivalence principle.

  7. You are not alone. For various reasons many people with more than a passing interest in physics are under the impression that these direct experiments on antimatter were actually performed – they were not!

    I was under the impression this experiment was done with anti neutrons with a null result. I guess my impression is wrong, thanks. It will be interesting to see what the CERN experiments show. This would be evidence for SUSY too given a positive result.

  8. I guess, people are putting cart before the horse. First let the experimentalists find departure from equivalence principle. Then it will be appropriate to ask which theory, a particular brand of string theory or some other model gives the most natural explanation.

    1. Well, if the theorists could come up with a definite prediction (i.e. “string theory, in all its possible realizations in nature, predicts a violation of the equivalence principle of one part in 10^10”) then you could try to falsify it. Such a prediction is not going to be possible, unfortunately.

  9. correction

    Experiments looking for the breakdown of the equivalence principle using anti matter have been done in an attempt to test super gravity, so far with null results. Supergravity predicts the existence of Graviscalar and Gravivector interactions which would be highly suppressed by the expected masses of the Graviscalar and Gravivector bosons.The Gravivector force would generate attraction between matter and anti matter and repulsion between matter and matter ( and anti matter and anti matter) causing a breakdown of the equivalence principle at very short range. Interestingly morphing the color force symmetries from SU(3) to U(3) produces a singlet gluon that looks like a Gravivector boson.

    1. Bob,

      The AEgIS group at CERN is preparing the first experiment to directly measure the gravitational acceleration of antihydrogen (likely in 2015).

      Back in the late 1960’s there was a proposal to measure the gravitational acceleration of positrons. The experimental apparatus was built at Stanford University and initially tested using electrons – but the experimental results were controversial and it was never run using positrons. Then, in the late 1980’s there was at CERN proposal to construct a device similar to that used in the 60’s – this time to measure the gravitational acceleration of antiprotons. However the project died for lack of funding and the experiment was never performed.

      You are not alone. For various reasons many people with more than a passing interest in physics are under the impression that these direct experiments on antimatter were actually performed – they were not!

  10. I hope this is not too far off topic (just delete if yes)but do you expect problems with General Relativity, under extreme conditions ?


    “This triple star system gives us the best-ever cosmic laboratory for learning how such three-body systems work, and potentially for detecting problems with General Relativity, which some physicists expect to see under such extreme conditions,” s

    Read more at: http://phys.org/news/2014-01-gravity-lab-pulsar-unique-triple.html#jCp

    1. I haven’t studied this yet, so I’m not precisely sure what it’s best uses are; but yes, it’s potentially a very interesting natural laboratory for asking some questions that weren’t previously accessible. Again, a good place for testing Einstein’s gravity, not string theory.

  11. I am reminded of the “Pioneer anomaly”, where a very tiny, unexpected component of the spacecraft’s deceleration was attributed by some to “new type of physics that contradicted Einstein’s general theory of relativity”.

    Many years were needed to figure out the puzzle, it turned out to be subtle radiation pressure from an asymmetry in heat radiation by the spacecraft.

    1. As always, it’s most likely that an explanation for something that seems extraordinary is going to turn out to be something subtle and mundane. That’s why finding something truly extraordinary is both so rare and so difficult — because you have to rule out all other possible causes, unless you can find independent methods for checking the existence of the effect.

  12. Prof. Strassler,
    This reminded me of something similar, but not as preposterous: the measurement of the”local value” of the Hubble constant by solar system tests, independently of cosmological observations (Sergei Kopeikin, arXiv 1207.3873). I would be interested to know what you make of this!

    1. I can’t evaluate it without study; sounds implausible, because the solar system sits inside a galaxy which in turn is inside a big halo of dark matter, and the whole thing is isolated from the Hubble expansion. Maybe others can comment.

  13. Possibly related; ESA has proposed a space mission to test the equivalence principal.

    Also a paper;

    all related to the Flyby anomaly;
    ” The flyby anomaly is an unexpected energy increase during Earth-flybys of spacecraft. This anomaly has been observed as shifts in the S-Band and X-Band Doppler and ranging telemetry. Taken together it causes a significant unaccounted velocity increase of over 13 mm/s during flybys ”
    Carry on.

    1. Already being constructed is an experiment by the esa to directly test the equivalence principle to 100 times better than the current limit. MICROSCOPE is currently scheduled for launch in 2016.

    1. Weird, isn’t it? As though phys.org just took the press release and put it out without even considering whether it might be accurate or asking anyone about it. If that’s what they did, they deserve some public criticism.

  14. The problem is that there are a huge variety of different string “theories” out there. Actually this is true of all ToEs right now, be it SuSE, superstrings, “foamy” space-time, whatever. Astronomy *can* be useful in testing these ideas, and I’ve engaged in this game myself, but there are so many different ideas out there right now, that the best you can really say is that such-and-such model was tested, not string theory or quantum gravity itself.

    1. That’s not really correct. There’s only one string theory. However, there are a huge variety of string theory vacua, and thus possible types of universes that can arise within string theory. I will explain this more carefully within two weeks.

      1. That’s a bit of a fine point, but I’ll defer to you on this issue, as I’m not a theorist. From my perspective the issue of what different types of universe can arise within string theory is a very important one, but I guess from an epistemological point of view when the idea is not worked out completely — or at least not enough to have tests that are feasible — it becomes difficult for it to make the important transition from an interesting theoretical construct and mathematical work to a true working model for reality.

        About the only more general tests are those which actually look at whether the extra, compactified dimensions required for string theory or some other model for quantum gravity, actually exist. I may be biased because that is the part of this game I’ve actually done some work on 😉 but I have to say I’ve been disappointed in what I’ve seen from the theorist side on this issue, which seems to dodge the issue of whether light from a very distant object would probe these dimensions.

        1. You can show that these extra dimensions, in unfavorable situations [i.e. in string vacua which don’t like to reveal themselves to humans], can have extremely weak effects. They would not affect light from a very distant object in that case. In principle, extra dimensions can have interesting effects that we could observe, but the problem is that if they’re as small as they are able to be (10^{-30} centimeters or so), then they don’t do anything that effective quantum field theory doesn’t already capture… so you could get the same effects from things other than extra dimensions, and they’re far too small to measure anytime soon anyway.

          1. Yes, but if the path length from the object is large (i.e., the object is at cosmological distances), there may be an observable effect, like a “seeing disk”. Anyway …. the data are not decisive on this.

  15. This has happened again and again. Scientists have to take some responsibility in proper wording while talking to journalists or the people who write articles such as the one on phys.org. After all , journalists cannot have technical expertise in everything. Moreover, there is always a human competitive urge of being first, which is understandable. One of the authors must have used the word string theory in his conversations. So the blame lies squarely on the authors.

  16. What strikes me about this is how immune string theory is from any observational -experiental testing. It may be that nature has played a cruel trick, that the correct theory of everything is actually a theory of anything. Or perhaps the lack of observational and experimental grounding has led us astray and the complex mathematical artifact that is string- M theory, really has nothing to do with our universe. Where will we be if SUSY doesn’t show up at CERN? Will that kill interest in string-M theory?

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