A Table-Top Test of General Relativity?


Even Albert Einstein might have been impressed. His theory of general relativity, which describes how the gravity of a massive object, such as a star, can curve space and time, has been used to predict small shifts in the orbit of Mercury, gravitational lensing by galaxies and black holes, and the existence of gravitational waves.  Now, new research shows it may soon be possible to study the effects of general relativity in bench-top laboratory experiments.

Xiang Zhang, a faculty scientist with the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and professor at the University of California Berkeley, lead a study that shows the interactions of light and matter with spacetime, as predicted by general relativity, can be studied using the new breed of artificial optical materials that feature extraordinary abilities to bend light and other forms of electromagnetic radiation.

“We propose a link between the newly emerged field of artificial optical materials to that of celestial mechanics, thus opening a new possibility to investigate astronomical phenomena in a table-top laboratory setting,” says Zhang. “We have introduced a new class of specially designed optical media that can mimic the periodic, quasi-periodic and chaotic motions observed in celestial objects that have been subjected to complex gravitational fields.”

Zhang, a principal investigator with Berkeley Lab’s Materials Sciences Division and director of UC Berkeley’s Nano-scale Science and Engineering Center, has been one of the pioneers in the creation of artificial optical materials. Last year, he and his research group made headlines when they fashioned unique metamaterials – composites of metals and dielectrics – that were able to bend light backwards, a property known as a negative refraction that is unprecedented in nature. More recently, he and his group fashioned a “carpet cloak” from nanostructured silicon that concealed the presence of objects placed under it from optical detection. These efforts not only suggested that true invisibility materials are within reach, Zhang said, but also represented a major step towards transformation optics that would “open the door to manipulating light at will.”

Now he and his research group have demonstrated that a new class of metamaterials called “continuous-index photon traps” or CIPTs can serve as broadband and radiation-free “perfect” optical cavities. CIPTs can control, slow and trap light in a manner similar to such celestial phenomena as black holes and gravitational lenses. This equivalence between the motion of the stars in curved spacetime and propagation of the light in optical metamaterials engineered in a laboratory is referred to as the “optical-mechanical analogy.”

Zhang says that such specially designed metamaterials can be valuable tools for studying the motion of massive celestial bodies in gravitational potentials under a controlled laboratory environment. Observations of such celestial phenomena by astronomers are often impractical because of the long time scales of the interactions on a astronomical scale.

“If we twist our optical metamaterial space into new coordinates, the light that travels in straight lines in real space will be curved in the twisted space of our transformational optics,” says Zhang. “This is very similar to what happens to starlight when it moves through a gravitational potential and experiences curved spacetime. This analogue between classic electromagnetism and general relativity, may enable us to use optical metamaterials to study relativity phenomena such as gravitational lens.”

In their demonstration studies, the team used a composite structure of air and the semiconductor Gallium Indium Arsenide Phosphide (GaInAsP). This material provided operation at the infrared spectral range and featured a high refractive index with low absorption.

In their paper, Zhang and his coauthors cite as a particularly intriguing prospect for applying artificial optical materials to the optical-mechanical analogy the study of the phenomenon known as chaos. The onset of chaos in dynamic systems is one of the most fascinating problems in science and is observed in areas as diverse as molecular motion, population dynamics and optics. In particular, a planet around a star can undergo chaotic motion if a perturbation, such as another large planet, is present. However, because of the large spatial distances between the celestial bodies, and the long periods involved in the study of their dynamics, the direct observation of chaotic planetary motion has been a challenge. The use of the optical-mechanical analogy may enable such studies to be accomplished on demand in a bench-top laboratory setting.

“Unlike astronomers, we will not have to wait 100 years to get experimental results,” Zhang says.

The paper titled “Mimicking Celestial Mechanics in Metamaterials” is now available on-line in the journal Nature Physics.

Source: Lawrence Berkeley National Lab

16 Replies to “A Table-Top Test of General Relativity?”

  1. This really is extraordinary science, quite a breakthrough indeed. I suspect this meta-material might just open the doorway to further many other technologies.

  2. Amazing how so at a first look different things can come together in physics to give clues about what’s going on.

    Those meta-materials are strange. A negative refraction index seems so illogical. I wonder when those things get taught in university or school.
    I would like to know more about them, because I have really no idea what is going on in them. 😉

  3. Very nice. Particularly interesting is the potential for the experimental study of chaotic dynamics of orbits etc.

    They say that the path light takes through these materials is analogous to how light moves through curved spacetimes. I wonder how the maths looks – whether the equations take roughly the same forms as their GR counterparts or not.

    I’ll have to have a look through the paper when I get two seconds…

  4. This is a sort of optical emulation of general relativity. In two dimensions the meta-material with a negative index of refraction is treated as a conformal transformation on the complex plane. The solution to the Helmholtz equation is then mapped into these strange solutions where light rays are deflected around objects. This technology is considered to a possible “cloaking device,” where EM radiation propagates around the body without distortion or scatter.

    General relativity with respect to the “time-time” part of the metric is similar to an index of refraction. As a result EM radiation propagates as if in a lens with an index of refraction that depends on the radius as ~1/r.

    Lwrence B. Crowell

  5. Ah, an immediate reply to today’s xkcd:

    – Physicists suck at group sex; they can’t solve the three-body problem. [~ xkcd]
    – It isn’t the oral presentation; it’s how you experiment. [phys]

    As I just learned about the feasibility of “interplanetary highways” (gravitational Poincaré tubes) and gravitational resonance jumps, but the computational difficulties are kind of obvious, I believe I see why this application is interesting.

    “A negative refraction index seems so illogical.”

    This is awkward. I really should know more about the theory behind this even if it isn’t near what I do for a living today. As you say, it’s existence at least will be among basics.

    But FWIW, this is my phenomenological take:

    At a basic level, a negative refraction index isn’t any more perplexing than a negative (differential) resistance. The underlying Maxwell equations permits complex permittivity and permeability to account for diverse (passive and active) material systems.

    At a more specific level, it is indeed perplexing. These metamaterials behave like LC circuits that impose an odd constraint: the phase velocity is antiparallel to the Poynting energy flow. This is presumably how they extinguish the field within. (But the group velocity is still going in the Poynting direction, so all is well with causality and energy.)

    So AFAIU by the LC action these materials conspires to act like a phased antenna, sending the energy where you normally don’t expect it.

  6. I’m amused that they compare these materials to what General Relativity suggests, that gravity curves space and light follows the curvature of space.

    These are actual materials, not empty space.

    For some time now, Scientists have been able to manipulate light (slow it down, bend it, and so forth).

    Not that anything should be taken away from this material science, all done in the lab. This work is excellent and truly remarkable.

    But in a misguided effort to make it more understandable to average folks, they have regrettably compared special materials with unique properties to empty space with no known properties (something can’t act on nothing).

  7. An infinitesimal distance in spacetime with a gravitating body is

    ds^2 = c^2Adt^2 + (1/A)dr^2 + r^2d?^2

    where ? refers to angular stuff. If we divide this through by the speed of light c^2 you get the time interval along a path in spacetime. For a spherical gravity field the germ A = 1 – 2GM/rc^2. For a light ray ds = 0 and there is a path in space or spacetime with an effective index of refraction given by sqrt{A}. This where the optical analogue with general relativity can be directly seen.

  8. My mistake:

    In the theoretical framework of General Relativity, mass supposedly causes the curvature of space and that causes gravity.

    Crowell, with all due respect, your above comment is nothing but mathematical jabberwalky.

    What we have here is a material substance that bends light.

    In the GR framework space is curved and light follows the curvature of space.

    But what is space?

    Answer: the absense of material substance or nothing.

    It is totally misleading to compare a material substance to “nothing”.

    Actually, this report is another example of disproving GR because it demonstrates “one more way to bend light that doesn’t depend on supposed principles of GR.

    And that’s what has been happening in recent yours: More and more ways to bend light, slow, light, or what not, that don’t rely on GR’s principle that light is bent as a consequence of the curvature of space.

    Today, Science know many different ways to bend and manipulate light.

    As example, the light going around the Sun doesn’t bend as the result of GR, but as the result of the magnetized plasma layer wrapped around the Sun, which bends light much like light is bend by water in a bucket.

    That observations & measurements of light bending was a result of GR principles was the claim made by GR proponents.

    Today, we find that simply isn’t true.

    That is the real back-story behind the article.

    One more nail in the coffin of GR.

  9. You really are a nasty piece of work, aren’t you Anaconda?

    In the “Ejected Black Holes Drag Clusters of Stars With Them” UT story comments thread, you also stated this, two days ago.

    the light going around the Sun doesn’t bend as the result of GR, but as the result of the magnetized plasma layer wrapped around the Sun, which bends light much like light is bend by water in a bucket.

    That observations & measurements of light bending was a result of GR principles was the claim made by GR proponents.

    On that thread your comment was challenged (the bending of electromagnetic radiation, in GR, is achromatic; the refraction caused by the same radiation through a plasma, magentised or not, is frequency dependent), sources cited wrt observations that are inconsistent with your claims, and you were invited to cite material in support of your claims.

    Did you respond to any of that?


    What did you do instead?

    Why repeat the claim here, in a different UT story thread!

    Fair question to you Anaconda: is your behaviour consistent with following the scientific evidence where it leads? with giving primacy to observation and measurement?

    Or is it, perhaps, consistent with the behaviour of a crackpot? or a troll?

    I really would like you to address my questions.

  10. All I can say to Anaconda is study some relativity and gravitation. Your statements indicate less some serious objection to general relativity than deep ignorance of it.

    Einstein lensing is observed on a number of scales, including how large elliptical galaxies will lense distant galaxies or quasars. This is clearly not an EM effect.

    If this were due to an EM effect we would expect the lensing to have “shimmers,” just as stars twinkle or mirages fluctuate. The plasma near the solar surface is not constant and uniform.

  11. It also dawned on me another reason why optical lensing around the sun can’t be due to a dielectric constant of plasmas. Plasmas, as with virtually all material media, have dielectric constants that depend on frequency. This means if this were the source of solar lensing there would be optical dispersion. Yet none is detected, as is predicted by the Einstein equivalence principle.

    Lawrence B. Crowell

  12. @ Lawrence B. Crowell

    Your last idea has been proposed to Anaconda at least twice before. He always babbles about “experiments proof everything” – but he should add this is only true as long as they favor him.

    Btw: Your comment that the plasma near the sun is not steady reminds me of Anaconda’s “idea” that the CMB could be a result of processes in the heliosphere/heliopause……

  13. @LBC: I think that’s one of the things I said (well, tried to say) …

    There are other differences to; for example, wrt ‘lensing’ the magnification and shear, as a function of impact parameter, are quite different (GR vs some dispersive/refractive medium).

  14. @LBC, DrFlimmer: what’s also curious is that (radio, microwave) scintillation due to the IPM and ISM has been well-observed for decades, for point sources (pulsars, quasars), and is used as a means to study the ISM and quasars on angular scales below that which is attainable with VBLI (and even space-based VBLI).

  15. He! That was funny, if EU believers joined the thread I would have assumed they would take the ‘equivalence’ between EM effecting metamaterials and gravitation as ‘evidence’.

Comments are closed.