(Phys.org)—Although quantum theory can explain three of the four forces in nature, scientists currently rely on general relativity to explain the fourth force, gravity. However, no one is quite sure of how gravity works at very short distances, in particular the shortest distance of all: the Planck length, or 10^{-35} m. So far, the smallest distance accessible in experiments is about 10^{-19} m at the LHC.

Now in a new paper published in *Physical Review Letters*, physicist Vahagn Gharibyan of Deutsches Elektronen-Synchrotron (DESY) in Hamburg, Germany, has proposed a test of quantum gravity that can reach a sensitivity of 10^{-31} m down to the Planck length, depending on the energy of the particle accelerator.

As Gharibyan explains, several models of quantum gravity predict that empty space near the Planck length may behave like a crystal in the sense that the space is refractive (light is bent due to "gravitons," the hypothetical particles that mediate gravity) and has birefringence/chirality (the light's bending degree also depends on the light's polarization).

In quantum gravity, both refractivity and birefringence are energy-dependent: the higher the photon energy, the stronger the photon-graviton interaction and the more bending. This correlation is the opposite of what happens when photons interact with electromagnetic fields or matter, where these effects are suppressed by photon energy. The predicted correlation also differs from what happens according to Newtonian gravity and Einstein's general relativity, where any bending of light is independent of the light's energy.

"If one describes gravity at the quantum level, the bending of light by gravitation becomes energy-dependent – unlike in Newtonian gravity or Einstein's general relativity," Gharibyan told *Phys.org*. "The higher the energy of the photons, the larger the bending, or the stronger the photon-graviton interaction should be."

Gharibyan suggests that this bending of light according to quantum gravity models may be studied using high-energy accelerator beams that probe the vacuum symmetry of empty space at small scales. Accelerators could use high-energy Compton scattering, in which a photon that scatters off another moving particle acquires energy, causing a change in its momentum. The proposed experiments could detect how the effects of quantum gravity change the photon's energy-momentum relation compared with what would be expected on a normal scale.

For these experiments, the beam energy is vital in determining the sensitivity to small-scale effects. Gharibyan estimates that a 6 GeV energy lepton accelerator, such as PETRA-III at DESY, could test space birefringence down to 10^{-31} m. Future accelerators that could achieve energies of up to 250 GeV, such as the proposed International Linear Collider (ILC), could test birefringence all the way down to the Planck length. For probing refractivity, Gharibyan estimates that a 6 GeV machine would have a sensitivity down to 10^{-27} m, while a 250 GeV machine could reach about 10^{-31} m.

As Gharibyan explains, probing Planck-scale gravity in this way is somewhat similar to investigating nanoscale crystal structures.

"Conventional crystals have cell sizes around tens of nanometers and are transparent to, or do not interact with, photons with much larger (m or mm) wavelengths," Gharibyan said. "In order to investigate crystal cells/structures, one needs photons with compatible nm wavelength: X-rays. However, visible light with wavelengths 1000 times more than the crystal cell can still feel the averaged influence of the cells: the light could be reflected singly or doubly. Comparing this to the Planck-length crystal, we don't have photons with a Planck wavelength or that huge energy. Instead, we are able to feel the averaged effects of Planck crystal cells – or space grains – by using much [relatively] lower-energy photons."

In fact, as Gharibyan has found, there are already experimental hints of gravitons.

"This work presents evidence for quantum gravity interactions by applying the developed method to gamma rays faster than light, which I found earlier in data from the largest US and German electron accelerators," he said. "The absence of any starlight deflection in the cosmic vacuum hints that Earth's gravitons should be considered responsible for the observed bending of the accelerators' gamma rays."

Gharibyan found that data from the now-closed 26.5 GeV Hadron-Electron Ring Accelerator (HERA) at DESY measured a Planck cell size of 2.6x10^{-28} m, and data from the mothballed 45.6 GeV Stanford Linear Collider (SLC) at Stanford University in the US measured a space grain size of 3.5x10^{-30} m. While these results provide some hints of Planck-scale gravity, neither of these experiments was designed as a tool to specifically test gravity, so Gharibyan warns that uncontrolled pieces of setups could mimic observed effects.

If Gharibyan's newly proposed experiments are performed, they would provide the first direct measurements of space near or even at the Planck scale, and by doing so, offer a closer glimpse of gravity in this enigmatic regime.

**Explore further:**
Gravity eases its pull

**More information:**
Vahagn Gharibyan. "Testing Planck-Scale Gravity with Accelerators." *Physical Review Letters* 109, 141103 (2012). DOI: 10.1103/PhysRevLett.109.141103

Vahagn Gharibyan. "Possible observation of photon speed energy dependence." *Physics Letters B* 611 231-238 (2005). DOI: 10.1016/j.physletb.2005.02.053

## ValeriaT

## ValeriaT

## ValeriaT

## ValeriaT

## ValeriaT

## ValeriaT

## ValeriaT

The second way is minimalistic and it considers the quantum gravity as a model, which deals with dimensional/energy density scale inside of the thin zones around "pure relativity" and around "pure quantum mechanics", which still cannot be described with classical physics.

## Nanowill

## thermodynamics

While that is true for the convenience of observations,both theories are applicable to any scale. Planck didn't say small and Einstein didn't say large. It is just that to see the effects we have to look at where they show up. However, you have to apply relativity to atomic behavior just as you have to apply quantum mechanics to lens coatings at a macro scale. The fact that we haven't been able to make them work together in a single theory does not mean their realms don't overlap.

## ValeriaT

## ValeriaT

## ValeriaT

## vacuum-mechanics

Actually, conventionally general relativity does not explain how gravitation MECHANISM works, what it say is just gravity was created by curve space-time, with NO reason behind! May be this physical view could help to visualize how gravity works!

http://www.vacuum...18〈=en

## Jitterbewegung

## Torbjorn_Larsson_OM

Add the Backreaction analysis that ValeriaT points to. It seems legit, Sabine is herself a "quantum gravity" backer IIRC.

The problem with quantum gravity is that photon dispersal observations of supernova photon timing (and polarity, if supersymmetry is correct), probes well beyond Planck scales already, and space is perfectly smooth there as predicted by relativity.

## JIMBO

http://arxiv.org/abs/1207.7297

## Torbjorn_Larsson_OM

We know that QM can never be predicted by classical mechanics, and that is of course the whole point of introducing it in the first place. No hidden variables.

So your speculations are amusing but unfactual.

Also, magnetism is an everyday relativistic low energy effect of electricity, the Lorentz force can be derived by applying relativity to classical E fields. Similarly quantum effects like entanglement can be applied to macroscale objects. It is fuzzy lines.

@ Nanowill:

Gravity can't be predicted by EM theory, it is its own fundamental (so far) interaction.

@ vacuum-mechanics:

GR is a mechanistic theory. You are asking for more detail, which is what the article is about.

You are linking to a crackpot non-peer reviewed "theory of everything". Sorry, no Nobel prize has been awarded to those, and peer review has rejected all of them to date ( mostly because most proposals are crackpot).

## El_Nose

## AmritSorli

energy density of quantum vacuum in empty space is: (mp x c2) Vp

by an elementary particle energy density of quantum vacuum diminishes: (mp x c2)- E(energy of the particle) / Vp.

Diminished energy density caused inertial mass and gravitational mass of an elementary particle.

## Standing Bear

## jsdarkdestruction

## antialias_physorg

## Jitterbewegung

I thought that inertial mass was caused by the Higgs mechanism?

## allotrope6

## Leavingmymind

## Leavingmymind

## Leavingmymind

## antialias_physorg

It's sort of funny what people who have never made an experiment or even ever looked at (much less tried to work with) any math/physics believe.

## ValeriaT

## Leavingmymind

Its funny how those that keep their head in the math never see. :D

Eyes open, the world is around you.

## antialias_physorg

That's the whole point. You open your eyes. make a theory based on what you see. AND do the math (and physics). Then you extrapolate from that math and look where you haven't looked before to see if what your prediction expects is actually there.

What you re doing i brainfarts without looking at anything. That's not 'looking around you'. That's just living in lala-land.

## Leavingmymind

Looking around refers to observation. Thats where this comes from. Stop being arrogant, and pose a reason why this doesn't work or shut up.

## randybeemen

The problem is that, without the math, what you are proposing is just a random collection of words that could easily be substituted with any others and have just as much of a chance at being correct. It is the same with every alternate "theory" posed in the comments section on this site, something outlandish is proposed and then it is expected that others learned in the art should have to stop what they are doing and fill in the numbers. Sorry, science doesn't work that way. If you wish you argue a point, YOU have to back it up, with all the hard work and withering peer review that this entails. Anything less is mere sophistry.

Scientific discovery is neither easy nor sanguine, nor should it be.

## Bowler_4007

you don't need to know maths just make some proper observations and someone else can help with the maths

Einstein was a theoretical physicist not an experimental physicist

## ValeriaT

## ValeriaT

My point therefore is, no quantitative description of Nature cannot be complete without it's qualitative understanding and vice-versa: the qualitative explanation cannot be complete without quantitative predictions. Everything else is just a biased politics of various lobbyist groups in physics.

## A2G

spot on AAPO. You can't just have an idea, or even just an idea and the maths to back it. You have to physically prove that your concept is correct. Ideas in your head can be totally wrong, no matter who you are. Even Einstein. So physical proof is needed to support your idea in the end. Not just the math.

It is not up to the rest of it to disprove your idea if you are too lazy to do the math AND provide the physical proof. It is up to you to prove that you are correct. The rest of us have a lot more to do than work on some unproven idea on a blog or comment section.

In the end the math helps, but you have to have physical proof (physics) or the idea can be total BS.

Math does not always lead to reality.

## Leavingmymind

Thanks. I'm talking about a proton in a nucleus.

## Leavingmymind

This has little to do, in my mind, with asking someone to disprove it. The idea is posed to ponder on by people who wish to do so and know about the physics. I ponder on things creatively and logically, and, as a hobby, I like to research quantum and put the workings into motion in my head. I'm in no way capable of doing atom scale physics experiments in my garage. So take it for what it is worth and ponder on it, if you so choose, or scrap it. I care very little.

Blah, blah, blah,... blah spot on Jeeves! Neither of you should step on the mouth of creativity, as it serves about as much purpose as you having read the entry in the first place... Troll on and I will see your mom in college. :D

## Leavingmymind

Scientific discovery is neither easy nor sanguine, nor should it be.

Actually, I disagree with this completely. Why shouldn't it be easy? It should be as easy as the passion you are willing to apply to the problem. The fact that it isn't doesn't mean it should be. Think without the box.

## antialias_physorg

The point is that you put out ideas based on unfounded assumptions. No one can disprove unfounded assumptions unless they do an experiment.

Example:

I say: "Gods make electrons spin - prove me wrong"

Now you would immediately say: "Onus of proof is on you. I will not accept this (or do any work on this) unless you show some experiment that corroborates this."

It's just the same with your brainfarts. If you want to ponder your own ideas: fine.

But don't expect others to take the time (we have our own ideas - and much better ones at that)

## Leavingmymind

You've taken that the wrong way, I posed an idea to think about; not to disprove. Someone wanted to shoot it down based on nothing but "Oh, do your own work". Such a thing is a musing of possible mechanics, and someone who is not a douche will probably give it some thought. Also, if you had a better idea, maybe it would be solved. Otherwise, you're still sitting with an equally lame idea and your d*ck in your hand, sweetheart.

## Bowler_4007

well next time check your spelling

## Leavingmymind

Next time I'll check your grandma on my d.