Study finds semiclassical gravity counterintuitive, but on the horizon of testability

May 08, 2013 by Lisa Zyga feature

(Phys.org) —One of the more controversial theories of quantum gravity, which attempts to unify quantum mechanics and general relativity, is semiclassical gravity, which was proposed in the 1960s. As its name suggests, semiclassical gravity involves a combination of quantum and classical components. Specifically, matter obeys the rules of quantum mechanics while gravity and the spacetime structure obey classical laws. Many physicists think that integrating quantum and classical systems in this way creates physical contradictions and mathematical inconsistencies. However, in a new paper, physicists have closely analyzed exactly how classical gravity might affect the quantum properties of macroscopic objects, and found that the effects of semiclassical gravity may be experimentally detectable with state-of-the-art technology.

The physicists, Huan Yang, et al., at the California Institute of Technology in Pasadena, California, and the National Dong Hwa University in Hua-Lien, Taiwan, have published their paper, called "Macroscopic Quantum Mechanics in a Classical Spacetime," in a recent issue of Physical Review Letters.

Most theories of quantum gravity predict that gravity should be quantized. However, as once said, " does not absolutely guarantee that gravity has to be quantized. ... I would like to suggest that it is possible that quantum mechanics fails at large distances and for larger objects." Since concrete, unambiguous experimental signatures of the of gravity still do not exist (other than when assuming the "many-world" interpretation of quantum mechanics), the physicists here thought it would be worthwhile to investigate if a theory of quantum gravity could involve classical gravity.

"Semiclassical gravity is one of the existing models which tries to unify quantum mechanics and general relativity," Yang told Phys.org on behalf of his coauthors. "Instead of trying to quantize space and time (as in string theory, loop quantum gravity, etc.), in this model the spacetime is assumed to be a classical entity, although the matter particles in the spacetime are quantum. Our findings show the imminent possibility of verifying or falsifying the semiclassical gravity experimentally. More importantly, we point out that both self-gravitational effects and quantum effects are important for macroscopic objects—the only regime accessible by table-top experiments so far. This opens up the possibility for testing other models that try to unify quantum mechanics and , as well: gravity decoherence models, stochastic gravity models, emergent gravity models, etc. We don't necessarily believe any of these models, but we believe it is important to test signatures/predictions of these models experimentally and let nature tell us what the true physics is, as we don't have a conclusive theory of quantum gravity yet."

Combining gravity and quantum mechanics

In their paper, the scientists used a non-relativistic version of a semi-classical gravity model, which describes how the quantum state of a system changes over time under the influence of classical gravity, called the Schrödinger-Newton (SN) equation.

Although physicists have extensively used the SN equation to study the quantum states of single particles, in the new paper the physicists use the equation to study the quantum states of macroscopic objects consisting of many particles. They show that the SN equation can be used to describe the quantum evolution of a macroscopic crystal's center of mass, the point at which the object's weight is perfectly balanced. The center-of-mass gives information on the self-gravitational effect that depends on the object's internal structure, which enables physicists to investigate how the object's may be affected by classical gravity.

The physicists' calculations revealed several unique signatures of classical gravity on macroscopic quantum mechanics. Most interestingly, they found that the center-of-mass motion of a crystal is found to deviate slightly from standard , obeying the SN equation where the center-of-mass wave function evolves nonlinearly due to self-gravitating effects.

The calculations also revealed other interesting insights. For instance, the classical gravity of a single macroscopic crystal is much stronger in relation to itself than it is between two separate crystals. The physicists explain that this effect arises because the mass of a macroscopic crystal is concentrated near its lattice sites. Another signature of classical gravity acting on macroscopic quantum objects is that classical gravity cannot be used to transfer quantum uncertainties between two objects. In addition, the scientists discovered a unique signature regarding the evolution frequency of expectation values of position and momentum.

Searching for semiclassical signatures

Although these effects are extremely weak, the physicists predict that one or more effects may induce visible signatures that are detectable with state-of-the-art optomechanics experiments. This kind of experiment could monitor and manipulate a macroscopic object's center-of-mass at quantum levels. Although the individual particles in a macroscopic object cannot be accessed separately, a light beam could probe the average displacement of the atoms in the first few layers of a reflective coating on a macroscopic object. Since the motion of these surface atoms is related to the center-of-mass motion of the entire object, the experiment could potentially probe some of the unique effects hinting at semiclassical gravity.

"If the future experiment sees the effects predicted by the SN equation, this means gravity/spacetime is classical, and previous attempts for quantizing gravity were on the wrong track, which is unlikely but nevertheless possible," Yang said. "However, if the experiment shows null results, it is highly possible that gravity is quantum. The third possibility is that the experiment sees some non-null result which is not consistent with SN prediction either. This may inspire physicists to formulate new theories of quantum gravity, given the experimental results."

Overall, the results show that semiclassical gravity involves effects that are counterintuitive, but not necessarily contradictory. Although experimentally detecting such effects is unlikely, their detection would open up many new opportunities in the search for .

"We talked to several experimental groups, including Nergis Mavalvala's group at MIT, Thomas Corbitt's group at Louisiana State Universiity, Markus Aspelmeyer's group at the University of Vienna, and Michael Tobar's group at Western Australia University," Yang said. "They were very excited about the idea and the possibility of doing the experiment. We will collaborate with experimentalists in performing such an experiment in the future."

Explore further: Simon's algorithm run on quantum computer for the first time—faster than on standard computer

More information: Huan Yang, et al. "Macroscopic Quantum Mechanics in a Classical Spacetime." PRL 110, 170401 (2013). DOI: 10.1103/PhysRevLett.110.170401

Related Stories

Physicists propose test for loop quantum gravity

Jan 03, 2012

(PhysOrg.com) -- As a quantum theory of gravity, loop quantum gravity could potentially solve one of the biggest problems in physics: reconciling general relativity and quantum mechanics. But like all tentative ...

Looking at quantum gravity in a mirror

Mar 18, 2012

Einstein's theory of gravity and quantum physics are expected to merge at the Planck-scale of extremely high energies and on very short distances. At this scale, new phenomena could arise. However, the Planck-scale ...

Why we've got the cosmological constant all wrong

Mar 05, 2012

(PhysOrg.com) -- Some scientists call the cosmological constant the "worst prediction of physics." And when today’s theories give an estimated value that is about 120 orders of magnitude larger than the ...

Recommended for you

Ultra-short X-ray pulses explore the nano world

5 hours ago

Ultra-short and extremely strong X-ray flashes, as produced by free-electron lasers, are opening the door to a hitherto unknown world. Scientists are using these flashes to take "snapshots" of the geometry ...

Physicist pursues superconductivity mysteries

9 hours ago

More than a quarter of a century after its discovery, high-temperature superconductivity still challenges condensed matter physicists. For Binghamton's Pegor Aynajian, the key to unlocking the mystery—which will ultimately ...

User comments : 21

Adjust slider to filter visible comments by rank

Display comments: newest first

ValeriaT
1.4 / 5 (16) May 08, 2013
Combining gravity and quantum mechanics
From AWT perspective one of the biggest misunderstandings of contemporary physics is the conviction, that the quantum gravity is sorta esoteric science, which requires huge ultra-detectors and ultra-sensitive experiments. The quantum gravity combines the observable worlds at the relativity scale and quantum scales - and the human observer scale resides BETWEEN them, not outside of them. The subject of quantum gravity are common, real life phenomena and forces, which we can observe all around us.
vacuum-mechanics
1.3 / 5 (16) May 08, 2013
One of the more controversial theories of quantum gravity, which attempts to unify quantum mechanics and general relativity, is semiclassical gravity, which was proposed in the 1960s. As its name suggests, semiclassical gravity involves a combination of quantum and classical components. Specifically, matter obeys the rules of quantum mechanics while gravity and the spacetime structure obey classical laws. Many physicists think that integrating quantum and classical systems in this way creates physical contradictions and mathematical inconsistencies...

Maybe understanding the working mechanism of both quantum mechanics and gravity (general relativity) could pave the way to unify both theories as follow…
http://www.vacuum...=9〈=en
ValeriaT
1.7 / 5 (12) May 08, 2013
The quantum gravity effects manifest itself with violation of inverse square law for gravity and with violation of constant speed of light. But many common forces don't follow the inverse square law and every refractive or reflective phenomena in material environment violate the constant speed of light. They're serving as an evidence of extradimensions, which the physicists are seeking for. What these scientists are doing is both a good joke, both school of life for those, who are paying it from their taxes:

The story begins at dark ages. A group of theorists seeks for violation of gravitational law at short distances, for example. They indeed find nothing, because their wooden experimental device is not sensitive enough. OK...

The sensitivity of devices improves gradually, until some experimentalist finds the solely unexpected electrostatic force, which no gravity theory considered so far...
ValeriaT
1.7 / 5 (12) May 08, 2013
Next generation of theorists already knows about it - so they arrange their experiments in such a way, the electrostatic force doesn't interfere their gravitometric measurements. And again, they find no violation of gravitational law at short distances...

The sensitivity of devices improves gradually, until some experimentalist finds the solely unexpected Van derWaals dipole force, which no gravity theory considered so far.

Next generation of theorists already knows about it - so they arrange their experiments in such a way, neither electrostatic force, neither dipole forces interfere their sensitive gravitometric measurements. As usually, they find no violation of gravitational law at short distances...

The sensitivity of devices improves gradually, until some experimentalist finds the solely unexpected Casimir force, which no gravity theory considered so far....
ValeriaT
1.7 / 5 (11) May 08, 2013
Next generation of theorists already knows about it - so they arrange their experiments in such a way, neither electrostatic force, neither dipole force, neither Casimir force interferes their extra-sensitive gravitometric measurements. As usually, they found no violation of gravitational law at short distances...

The sensitivity of devices improves gradually, until some experimentalist finds the solely unexpected dynamic Casimir force, which no gravity theory considered so far.

Next generation of theorists already knows about it - so they arrange their experiments with single neutrons in such a way, neither electrostatic force, neither dipole force, neither Casimir force, neither thermal Casimir force (..ffffuuuu...!) interferes their ultra-mega-sensitive gravitometric measurements. As usually, they found no violation of gravitational law at short distances...
ValeriaT
1.7 / 5 (11) May 08, 2013
This story hasn't ended yet. The grants are continuing, experiments are becoming more and more sensitive & expensive gradually, so that money are flowing ... and everyone is happy, because these money are well spent - for science!
no fate
2 / 5 (4) May 08, 2013
Thank you for omitting the usual analogies and just speaking your mind Mr. AWT. You can provide an accurate assessment of reality when you try.
ValeriaT
1 / 5 (6) May 08, 2013
This story is indeed AWT motivated parable as well - it just uses the approach of physicists instead of water surface analogy. We can imagine the low-dimensional worlds of quantum mechanics and general relativity like two mutually intersecting hyperdimensional branes. We are living at their interesection, but the physicists tend to follow one brane geometry deeply inside of another one. Theyse physicists are behaving like the travelers, who are looking for the coastline of Amazonian delta deeply inside of Brazilian forest. For example, the physicists handle the centers of galaxies in similar way, like the vacuum at free space - but from AWT perspective these centers are already forming the surface if not interior of black holes, which are residing there. This is particularly relevant for young gigantic galaxies, which are behaving like the white holes (time inverted black holes) in certain extent.
ValeriaT
1 / 5 (5) May 08, 2013
The atmosphere of planets can be perceived like the layer of classical physics, separating the particles of planetary body, where the energy spreading is driven with laws of quantum mechanics from the empty vacuum, which the energy spreading is driven with laws of general relativity. At the case of more dense stars - like the neutron stars - their atmosphere represents nearly seamless transition between quantum mechanical and relativistic worlds. Such an atmosphere will be more and more formed with transparent "forces" and "fields", like with magnetic fields - rather than particle systems. Currently the physicists are discussing just the question, whether this transition can be fully seeming and transparent or not.
swordsman
1.5 / 5 (8) May 09, 2013
Some 22 years ago, I investigated the properties of the hydrogen atom using classical analysis and was able to develop a model that conforms to the laws of gravity. In a later investigation of electromagnetic radiation, I developed a clasical mathematical model in the with cooeficients of the exponential eigenvector equation (the complex eigenvalues)contain the Einstein/Minkowski relationship of space-time. This shows that the Einstein interpretation is flawed. Electromagnetic waves bend with transverse velocity, which yields the coefficient of the speed of light in the radial direction. The explanation is far more simple than the problem. It is amazing that no one had yet found the solution. I believe that it is too much mathematics involving second-order partial differential equations that are subject to error in the determination of the coefficients. Relativity theory is out the window. Check it out, Valeria.

http://www.science-site.net/
no fate
1.6 / 5 (5) May 09, 2013
Alot of interesting info. on your link swordsman. I agree with some of your assessments.

AWT, back to the analogies you go.
vlaaing peerd
3 / 5 (2) May 10, 2013
Some 22 years ago, I investigated the properties of the hydrogen atom using classical analysis and was able to develop a model that conforms to the laws of gravity. In a later investigation of electromagnetic radiation, I developed a clasical mathematical model in the with cooeficients of the exponential eigenvector equation (the complex eigenvalues)contain the Einstein/Minkowski relationship of space-time. This shows that the Einstein interpretation is flawed.

http://www.science-site.net/


I'm not an expert like you, so forgive my questions, but your site is very interesting;

- If gravity was a electric differential force shouldn't it be repulsive as well?
- How about observable time dialation, can an electric differential force do that?
- oyea, you found the exact orbit of an electron around the nucleus, please tell me more about that too.

It all sounds very intruiging, on what grounds were you refused to publish this article as you state on the page?
Opposed opposition
1 / 5 (4) May 12, 2013
Since most comments appear to be questioning the methods used in the article, even declaring them a waste of time and money, my post with the opinion that one of the most basic assumptions about the universe is flawed, shouldn't stand out like a sore thumb. However I realize that it will, based on the fact that I hold no degrees in mathematics or physics, which will be obvious to those who do. If you are inclined to respond, please do so in words, not formulas or I won't be able to understand you. I'm putting my thoughts out there for the world to make fun of, but I really want the criticism, constructive or destructive. Thanks.
Opposed opposition
1 / 5 (4) May 12, 2013
It is my belief that everything in the universe is made up of pure energy. Energy and mass are interchangeable. There are many, many forms, or frequencies, of energy. The frequencies, or vibrations, take up space, giving the energy volume, or mass. The frequencies of different forms of energy are bound to a certain size and shape as determined by the strength of the energy, or the frequency at which it vibrates. Positive or negative charges give energy the force of attraction, or gravity. Energy, along with it's inherent qualities, can be divided and compounded, but it cannot be created or destroyed, only forced to change forms. One of it's qualities, it's charge, which gives it attraction/repulsion can likewise be divided and compounded, but cannot be created or destroyed, only forced to change forms. Matter, true to this definition of energy, cannot be created or destroyed, only forced to change forms.
Opposed opposition
1 / 5 (4) May 12, 2013
Being uneducated in mathematics and physics hinders my understanding of the equations involved to prove or disprove such an idea, and as of yet, I have found no one interested in taking the time to explain them in terms I can comprehend. I am left with the belief that the universe was created from pure energy by pure energy, which has always existed, and which will always exist.
ValeriaT
1 / 5 (3) May 12, 2013
It is my belief that everything in the universe is made up of pure energy
This is not a scientific idea, until you derive some testable prediction from it. My problem with this concept is, it doesn't provide any geometric insight into situation. The energy in form of invisible many swirling particles provides some clue, but just the plain energy?
one of it's qualities, it's charge, which gives it attraction/repulsion can likewise be divided and compounded, but cannot be created or destroyed, only forced to change forms
the mainstream physics has already better quantitative tools prepared for it: Noether theorems and the laws of lepton number conservation. You've no chance with it here, because the science already uses these concepts at qualitative basis one century - you just don't know about it. You're just like the naive guy, who invents wheel, whereas the planes are already flying above his head.
Opposed opposition
1 / 5 (4) May 12, 2013
I am not schooled enough to provide you with an argument, however Noether's (first) theorem states that any differentiable symmetry of the action of a physical system has a corresponding conservation law. This leads me to the law of Conservation energy-mass, suggesting energy loss is coupled with mass loss. Wherever I look, I find connections with energy and mass which is what led me to my conclusion in the first place. I realize you cannot educate me from my present position to the forefront of physics, I was just looking for some truth, or fact, which would destroy my assumptions.
ValeriaT
1 / 5 (3) May 12, 2013
I was just looking for some truth, or fact, which would destroy my assumptions.
It's not that simple. For example the mass-energy conservation law may lead into phenomena, which would seemingly violate it, when the another dimensions are involved. If you introduce some energy into water, then the water surface may propagate surface waves slower or faster and it will behave like more or less dense/massive depending on whether the energy has been introduced in form of transverse or longitudinal waves into the underwater. The energy of extradimensional scalar waves is positive, but with respect to mass in our space-time it behaves merely like the negative energy and it seemingly violates the mass-energy equivalence. In dense aether model all physical quantities are relative and the energy, mass or gravity can get the negative signs under certain circumstances. I.e. the physical quantities cannot be separated from space-time geometry, in which they do apply.
Silverhill
5 / 5 (3) May 12, 2013
@ValeriaT
But many common forces don't follow the inverse square law...
Many common forces, however, are mediated by things (such as photons) which *do* follow the inverse-square law. Local complications of structure can make it difficult to perceive the inverse-square behavior, but it is still there.

...and every refractive or reflective phenomena in material environment violate the constant speed of light.
No, they do not. The speed of light (in free space [in vacuum]) is truly invariable. The *effective* speed of photons in a medium is lowered by the fact that the photons' paths are made highly nonlinear by collisions and re-emissions. *Between* interactions with matter particles, the photons still travel at c -- but because the total path length in the medium is increased, the total time of transmission is increased.
ValeriaT
1 / 5 (5) May 13, 2013
Local complications of structure can make it difficult to perceive the inverse-square behavior, but it is still there
Why not, but after then you're adjusting the reality to theory, not vice-versa.
The speed of light (in free space [in vacuum]) is truly invariable.
If it would, then the effects like the gravitational lensing would never exist. The refraction means, the speed of light is changing. The general relativity says, it's because the space-time is curved there. But after then we can say, the glass doesn't refract light, just the space-time inside of atoms is curved - why not? We are apparently adjusting the reality to theory again with such application of double standards for description of single refraction phenomena.
ValeriaT
1 / 5 (4) May 13, 2013
I'm usually explaining the (paradox of) gravitational lensing with this simple animation. It implies, that the light around massive objects still travels in constant speed - but along longer path through compacted extradimension of space, which corresponds the time dimension and is represented with density fluctuations of vacuum.

We apparently have two perspectives here: the intrinsic perspective of photons which "believe/think", they're moving in constant speed - and the extrinsic perspective of macroscopic observer, which will see the refraction and changed speed of light.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.