Tuesday, December 30, 2014

A new proposal for a fifth force experiment

Milla Jovovich in “The Fifth Element”
I still find it amazing that all I see around me is made up of only some dozens particles and four interactions. For all we know. But maybe this isn’t all there is? Physicists have been speculating for a while now that our universe needs a fifth force to maintain the observed expansion rate, but this has turned out to be very difficult to test. A new paper by Burrage, Copeland and Hinds from the UK now proposes a test based on measuring the gravitational attraction felt by single atoms.
    Probing Dark Energy with Atom Interferometry
    Clare Burrage, Edmund J. Copeland, E. A. Hinds

Dark energy is often portrayed as mysterious stuff that fills the universe and pushes it apart, but stuff and forces aren’t separate things. Stuff can be a force carrier that communicates an interaction between other particles. In its simplest form, dark energy is an unspecified smooth, inert, and unchanging constant, the “cosmological constant”. But for many theorists such a constant is unsatisfactory because its origin is left unexplained. A more satisfactory explanation would be a dark-energy-field that fills the universe and has the desired effect of accelerating the expansion by modifying the gravitational interaction on long, super-galactic, distances.

The problem with using fields to modify the gravitational interaction on long distances and to thus explain the observations is that one quickly runs into problems at shorter distances. The same field that needs to be present between galaxies to push them apart should not be present within the galaxies, or within solar systems, because we should have noticed that already.

About a decade ago, Weltman and Khoury pointed out that a dark energy field would not affect gravity on short distances if it was suppressed by the density of matter (arXiv:astro-ph/0309411). The higher the density of matter, the smaller the value of the dark energy field, and the less it would affect the gravitational attraction. Such a field thus would be very weak within our galaxies, and only make itself noticeable between galaxies where the matter density is very low. They called this type of dark energy field the “chameleon field” because it seems to hide itself and merges into the background.

The very same property that makes the chameleon field such an appealing explanation for dark energy is also what makes it so hard to test. Fifth force experiments in the laboratory measure the gravitational interaction with very high precision, and they have so far reproduced standard gravity with ever increasing precision. These experiments are however not sensitive to the chameleon field, at least not in the parameter range in which it might explain dark energy. That is because the existing fifth force experiments measure the gravitational force between two macroscopic probes, for example two metallic plates, and the high density of the probes themselves suppresses the field one is trying to measure.

In their new paper, Burrage et al show that one does not run into this problem if one uses a different setting. To begin with they say the experiment should be done in a vacuum chamber as to get the background density to be as small as possible, and the value of the chameleon field as high as possible. The authors then show that the value of the field inside the chamber depends on the size of the chamber and the quality of the vacuum and that the field increases towards the middle of the chamber.

They calculate the force between a very small, for example atomic, sample and a larger sample, and show that the atom is too small to cause a large suppression of the chameleon field. The gravitational attraction between two atoms is too feeble to be measureable, so one still needs one macroscopic body. But when one looks at the numbers, replacing one macroscopic probe with a microscopic one would be enough to make the experiment sensitive to find out whether dark energy is a chameleon field, or at least some of it.

One way to realize such an experiment would be by using atom interferometry which has previously been demonstrated to be sensitive to the gravitational force. In these experiments, an atom beam is split in two, one half of it is subjected to some field, and then the beams are combined again. From the resulting interference pattern one can extract the force that acted on the beams. A similar setting could be used to test the chameleon field.

Holger Müller from the University of California at Berkeley, an experimentalist who works on atom interferometry, thinks it is possible to do the experiment. “It’s amazing to see how an experiment that is very realistic with current technology is able to probe dark energy. The technology should even allow surpassing the sensitivity expected by Burrage et al.,” he said.

I find this a very interesting paper, and also a hopeful one. It shows that while sending satellites into orbit and building multi-billion dollar colliders are promising ways to search for new physics, they are not the only ways. New physics can also hide in high precision measurements in your university lab, just ask the theorists. Who knows, there might be a chameleon hidden in your vacuum chamber.

This post first appeared on Starts with a Bang as "The Chameleon in the Vacuum Chamber".


  1. "But for many theorists such a constant is unsatisfactory because its origin is left unexplained."

    One often hears this argument, but is it a good argument? Look at the Einstein equations. There is G and there is lambda. We can no more "explain" G than we can explain lambda. (OK, one could say that the evidence that G is non-zero is somewhat firmer, but that is irrelevant to the point that I am trying to make.)

    Yes, there are unproven theories which try to explain G and/or lambda from a particle-physics point of view. But keep in mind that GR per se has nothing to do with particle physics. So, either just accept that lambda is non-zero, just like G, with no further explanation. (This will be discussed here the week after next where there is a session entitled "Why not just Λ?") Or, if you think that the cosmological constant has to be "explained", then provide an "explanation" for G as well.

  2. If there is such a field, how would it vary as a function of time in the history of universe? Surely it just did not come up only recently when there are people capable of observing it as CC.

  3. Hello Bee,
    is this relavant in the context of
    this blog:
    Excuse if my post is too "layish"

  4. My favorite movie.
    We try to impose some ordered interpretation of abstract concepts that could be different than what may be absolute in some hierarchy of discovery.
    Gravity and dark physics can be one unified concept.
    The question if one or the other is not needed is today's foundational question. Loop cosmology asks if on the next level there is uniform color space. String asserts such an explanation is not needed above the symmetry and its breaking as black and white (duality of good and evil in the movie if you will).
    If there were a supreme being born speaking an ancient language could we not take cross sections of her highly compact DNA?
    But for more than token questions to our boarding pass to give existence by love to this thus vulnerable being this generation does not ask permission.
    It cannot plan and lets machines clean up the mess.
    All is green. The higher levels beyond color could be best described as inscrutable., irrational, chaotic and the paranormal as not existing. The gods at best tricksters. That outside the scope of these force issues as science.

  5. Theory cannot correct what it wrongly assumes. Empirical reality cannot be chiral, for absolutely discontinuous symmetry chirality is not Noetherian. Emprical reality is chiral. Test for vacuum chiral asymmetry toward massed enantiomorphs: Single crystal test masses in the 11 pairs of enantiomorphic space groups; racemic 4-oxa-D_3-trishomocubane microwave rotation temperatures.

    "Perhaps Looking-glass milk isn't good to drink," Alice said to her cat. Through the Looking-Glass, and What Alice Found There (1871). Perhaps dark matter is Milgrom acceleration, so sourced. Look at feet not chameleons.


  6. "If there is such a field, how would it vary as a function of time in the history of universe? Surely it just did not come up only recently when there are people capable of observing it as CC."

    That would indeed be strange, but no-one claims that this is the case. Actually, something like the reverse is true. Some people are puzzled by the fact that the energy densities due to matter and the cosmological constant are approximately equal, but were not in the early universe (matter dominated (and before that, radiation), nor in the distant future (when the cosmological constant will dominate). I am not.* These something-other-than-a-true-cosmological-constant theories generally tend to try to tie the two densities together, so that their near equivalence is not a "coincidence". In other words, it is the cosmological constant whose importance increases with time (it is constant per volume, but as the universe expands, its total energy increases while that of matter stays constant, diluted in density by the expansion).

    *This "coincidence" is essentially equivalent to, and no more puzzling than, the question "If the universe will last forever (as the values of the cosmological parameters indicate), then why are we living near the beginning?

  7. Back in the 1970's I came up with a model of the universe that seemed to make sense (I was soaking in the tub, like Archimedes). And what interests me these days is that my model actually turns out to be consistent with what is now being called "dark energy."

    One feature of my model sees our universe (universe A) as not simply expanding, but expanding into a black hole. On the "other side" of this black hole is another universe (universe B), which I call "Tiny Alice," because it is smaller than the Planck limit. It is also inside-out, which means that it contains everything our universe contains, including galaxies, but the largest things in Tiny Alice are particles, while the smallest things are galaxies. If you find this confusing, just think: extra dimensions.

    According to this model (NOT yet a theory, just a model), the galaxies of universe A are accelerating away from one another due to the gravitational pull of universe B. Which means that universe B is not only at the center of universe A, but also at its periphery. (Again, think extra dimensions.)

    So if you really hate "dark energy" or the "cosmological constant," you might want to look into the model I concocted while in the tub, back in the magical 70's, when anything was possible. For more details, see http://fqxi.org/data/forum-attachments/Is_the_Universe_Expanding_Into_a_Black_Hole.pdf

  8. "According to this model (NOT yet a theory, just a model), the galaxies of universe A are accelerating away from one another due to the gravitational pull of universe B."

    Leaving aside the issue of Occam's razor, your model cannot quantitatively explain the observed accelerated expansion.

  9. Phillip Helbig: "your model cannot quantitatively explain the observed accelerated expansion." If you mean my model is quantitatively inconsistent with what is known about the expansion, then it would be falsified, yes. Have you actually done the math, or are you guessing?

  10. Hi Bee,

    First of,all, happy New Year.
    When people talk about four forces I guess they mean...
    1) The strong force, which has the gs coupling in the SM Lagrangian,
    2 and 3) The electromagetic and weak interactions with the couplings g and g' in the SM, with the appropriate mixing.
    And 4) gravity, which as seen as an interaction has its own coupling, say G.

    But there is another coupling in the SM which is the Higgs self coupling lambda, which already makes a fifth force. We have not seen Higgs self interactions directly yet, of course, but with this counting, I guess what you are talking about would be a sixth force, wouldn't it?

    Of course, this is just a matter of names, but I am somewhat surprised that people are not counting yet the Higgs inteaction as THE fifth force.

    Once again, my best wishes for 2015 and congratulations for the blog


  11. Alice,

    Everyone is startled at first.
    Startled at what?
    When sound breaks silence.

    Then the eventual realization
    both are necessary and sufficient.

    The ever ascending tone stairway is an acoustic illusion.

    Then the eventual realization the illusion comes from an endless finite loop.

    Another great blog and comments.

    Need a force or field?
    Unifying and constant?
    Without illusion?
    Physicists aren't modest.

    Beware of Bob*
    *Beware of Dog.

  12. @Phillip Helbig - In Einstein's equations G is dimensional so it can be set to 1 with suitable choice of unit of mass. (In fact, as conventionally written, there is already a choice of unit of length/time, so that c is set to 1. Otherwise c would also appear.)

    Having set G to 1 there are no other dimensions that can be adjusted, and the cosmological constant is a parameter with real physical significance. Depending on its value the universe has quite different behaviors.

  13. Phillip:

    Is it a good argument? That's a question of opinion. Honestly, I don't think so, but then people need to justify their work with something. A good argument is if such a dynamical field has a mechanism to also explain the observed value of the CC. And that is much easier to achieve with a dynamical field than with a fixed constant. Best,


  14. kashyap,

    Depends on the type of field. I'm not an expert on different dark energy fields, there's a whole class of them all with slightly different time-dependence, and all with slightly wacky names like chameleon field, phantom field and quintessence and so on. They're looking for any time dependence in dark energy now, and if they find something this can be used to sort out the models. Best,


  15. JRPS: Yes, what you say is correct, but language doesn't follow definitions.

  16. In M-theory, brane configurations begin in a tachyonic phase and undergo tachyon condensation, leading to the creation of matter excitations and symmetry breaking (a Higgs mechanism). A cosmological model with tachyon contribution is thus quite natural from a brane perspective. See astro-ph/0212198.

  17. Frank Wilczek: I named them after a laundry detergent, since they clean up a long standing problem in theoretical physics.”

    Hi Sabine,

    Maybe cleaning up the questions with regard to a chameleon force would have a basis in understanding an issue in theoretical physics?


  18. The conversion from an axion to X-ray through helioscope would relinquish photon to be interchangeable from massless to mass?

    Speculating here.

  19. X-ray and gamma ray would refract with matter

  20. From massless to mass means refractory gives matter identification signature? Thinking, speed of light in vacuum converts axion to X-ray gamma ray and interaction with mass gives angular spin?

    Please feel free to correct.....layman speculating.


  21. Sorry Bee you can delete comments if not felt appropriate....the axion represents the fifth force to me and to suggest experiment using interferometer method seeks to identify minimum mass is to identify the signature......so you use that to manipulate observed dilation affects as an unseen force?
    Photonics molecules are a synthetic form of matter in which photons bind together to form "molecules". According to Mikhail Lukin, individual (massless) photons "interact with each other so strongly that they act as though they have mass". The effect is analogous to refraction. The light enters another medium, transferring part of its energy to the medium. Inside the medium, it exists as coupled light and matter, but it exits as light.[1]

  22. "Is it a good argument? That's a question of opinion. Honestly, I don't think so"

    I agree, Sabine.

    It's fun... Usually I'm never agree with people...


  23. I think this paper is a red herring myself. If you read the original material by Einstein I think you get a clear idea of what dark energy is. But I'm confident now that very few people have read the original material by Einstein. Particularly particle physicists. They come out with things that absolutely don't square with what Einstein said. Cosmologists too.

  24. I don't believe it necessarily helps understanding of physics to ask if dark energy between galaxies is a fifth force. In all of science there is a need for taxonomy, the naming of things. It is a shorthand to give reference to what you are talking about. But it does not give understanding of the underlying structure of things. Worse, people use taxonomy to convince themselves and others that they know more than they do.

    Physicists need to understand the interrelationships of things and the commonalities. It may be too early to ask the question of how dark energy is related to the other forces and if there is something in common to all the forces, but that is the ultimate question that needs to be asked and answered.

    Everyone knows people who don't know what they don't know. This kind of naming just encourages those blowhards.

  25. Eric,
    I like your points, especially from a linguistic view.
    A periodic table of language has proven difficult to frame from a particular model.
    Even where we do not know what we do not know time (force like) moves linearly in directed increments.(unless time stops people age in our dreams. )
    So if we know we do not know something but cannot say what, is that not like a simplified and maybe not needed concept of dark physics or cosmology?
    Light itself has a simple purpose but. Noether does not say, tachyons maybe besides the physical landscape, what toward or around what. (singularities? )

  26. Uncle AI
    The fire Burns brighter this side of the looking glass. Your proposed experiment and this one could both in theory work, but from Alice's view somewhere in passing thru and on the mirror, as far as observations even on either side the experiments cancell each other out. (But Don, measurement and the proper worked out math is still possible.)
    Some have already connected Higgs concepts to dark concepts BTW, but they have not shown its place as initiator or terminator, origins, as part of the Big Picture.

  27. This comment has been removed by the author.

  28. I'm very happy to read these difficulties. It means that a solution has still to be found and that can be found.

  29. The Solor Corona problem together with QGP are grounds through which axion understanding can and are being developed.

  30. David,

    Electric, magnetic, space, time is a four way deal. Particles if of the standard model could be seen as quantized with respect to a Higgs field where it could indeed be imagined a fifth force the four others seem continuously embedded within.
    The unity of such forces on this primitive movable cleft of origins or residual imagined inflation's or minor traces of centripetal levels of mass is more than the adjacent inter dimensional fermion bosom differences as Abellian central facing inversions or not as four fold squares in the 24 lattice permutations in group theory extending self dualities up to at least 5 or 8 natural dimensions.

  31. David: Please read our comment rules, no links for the sake of self-promotion other than to published papers. You're welcome to repost your comment without the URL.

  32. Bee, sorry about violating the rules. I'll repost the comment, minus the URL, after correcting a spelling error.

  33. Hopefully it’s OK to toss in my own two cents on the nature of dark energy. It’s speculative, but here’s an idea that weaves a common thread between dark energy, the stability of atoms, and claims of tiny acceleration signals from superconductors.

    In hundreds of experimental runs, between 2003 and 2006, a group at the Austrian Research Center, led by Martin Tajmar, reported 100 to 277 micro-g signals from a niobium ring subjected to tangential accelerations of 7.33 g’s at the ring’s periphery. While 277 micro-g’s is miniscule, by everyday standards, it is orders of magnitude larger than can be accounted for by known physics.

    Electron orbital stability stems from the fact that all fundamental particles exhibit wavelike behavior. With an integral number of the electron’s intrinsic wavelength wrapped around the orbital circumference a stable orbit is assured. But exactly ‘what’ is ‘waving’ in a matter wave has never been satisfactorily explained.

    The speculation is that matter-waves are a manifestation of a new field - an analogue of the electromagnetic (EM) field, in which the electric and magnetic variables are replaced by length and time variables. Its intrinsic strength would match the EM field, but its range would be microscopic – only about 10^-19 meters (TeV scale). Neglecting, for the moment, the confined range of this field, an observer moving along a length-time wave would observe the metric to alternate between positive and negative states of the vacuum.

    Therefore, it’s speculated that an electron, in stable, standing-wave orbit, must be entrained at a maxima node of such a wave, such that the negative vacuum state faces the nucleus and the positive vacuum state faces away from the nucleus. This is the recipe for an Alcubierre warp metric, albeit microscopic. The net result - the electron will be in a state of free-fall within its reference frame, and thus not radiate electromagnetic energy.

    But if the atom is subjected to a small acceleration the micro-warp field would try to compensate. Not being instantaneous, presumably negative-energy gravitons would momentarily evolve from the negative vacuum side of this warp field. Perhaps these small fluxes of neg. energy gravitons drive inflation once they spread out into the larger Universe.

  34. This looks like a very promising experiment for detecting the Chameleon field. From reading their paper, they expect to have a sensitivity of one millionth of a g, with systematic errors being only a thousandth of that. It will be fascinating to see what they find.

  35. /* an atom beam is split in two, one half of it is subjected to some field, and then the beams are combined again */

    How they want to eliminate the dark energy/chameleon field in one beam?

  36. This puzzles me. I was taught that Einstein's lambda term implies a universal expansion at every scale. Supposedly, there is an increasing distance between the sun and its nearest neighbor, the earth and the moon, and even from one end of a measuring stick to the other. This seemed to imply that electrons were closer to protons in old atoms or perhaps more distant, depending on how one measured things.

    This fifth force seems to get around this by only operating in the absence of matter, almost Einstein's curvature equation inverted, rather than a simple negative constant term. I suppose we'll have to see what we can see.


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