Physicists discover that gravity can create light

https://phys.org/news/2023-04-physicists-gravity.html

by wglb • 3 years ago

232 105 3 years ago

phys.org

brad0• 3 years ago

How does this not have more upvotes? This is a great find!

yarg• 3 years ago

phys.org can be a bit sensationalistic.

Quanta takes their time getting to the point, but is a bit more reliable in terms of the summary provided.

So if there's a popular physics article, it generally seems to be quanta.

uoaei• 3 years ago

phys.org re-publishes press materials from universities. The writers at Quanta write their own articles.

avmich• 3 years ago

The title is about gravity, but the first line is about wave of gravity. Hmm. We know that gravity alone can create radiation - the Hawking one. We also know that gravitational waves - the spacetime curvature changes with time - carry energy, so can be transformed into light. Do we still know if spacetime alone can create light?.. I'm not sure we know it today. So... we have a great experiment here, which shows something known in a different way - is it correct?

raattgift7• 3 years ago

The preprint of the paper that is the subject of the (not so great) phys.org article at the top is <https://arxiv.org/abs/2205.08767>. An accessible HTML5 version is available at <https://ar5iv.org/abs/2205.08767> (arxiv->ar5iv, the latter expands to a link within ar5iv.labs.arxiv.org).

Hawking radiation is a semiclassical result: the curved spacetime is classical General Relativity and the scalar field (in which Hawking quanta arise near the central black hole) is quantum.

The dynamical spacetime creates -- through the equivalence principle -- an acceleration between past observers and future observers, and this acceleration corresponds with the Unruh effect. The Unruh effect rests on the definition of a vacuum as a state in which an observer sees no particles, and that when an observer accelerates a no particle state may be transformed into a state with particles. Equivalently, differently-accelerated observers will count different numbers of particles in a spacetime-filling quantum field. (A family of observers may count no particles, i.e., it's vacuum.)

The important part here is that a dynamical spacetime ("gravity") and a relativistic quantum field is needed for Hawking radiation.

So, "[can] spacetime alone ... create light?" No. There must be a matter field filling the spacetime. That matter field, if quantum, can look like it has no particles in it to some observers, but not all observers. The dynamical evolution of the spacetime can cause observers' counts of particles to evolve.

gravitational waves ... carry energy, so can be transformed into light

The paper is about how, given:

* a massless quantum field theory proxying for light

* a quantum field theory in which gravitation is mediated by a massless spin-2 boson

* a dense medium with a (light-) refractive index greater than 1

* standing gravitational waves of significant amplitude occur in cases where gravitational radiation from widely separated sources converge within the dense medium and somehow [a] cancel out polariation and [b] are within a wide (compared to the wavelength) patch of flat spacetime

* the non-light massive and massless particles within the medium couple very weakly to the incoming gravitational radiation

* the particles of the refracting medium couple weakly to the "light" field, and generate practically no spacetime curvature even in bulk

then the light-proxying particles may be produced via a process which the authors compare with electron-positron pair production and Cherenkov radiation. (Although they do the latter comparison very very breezily, not delving into the cross section of light-by-light scattering).

There are weaknesses in this list of requirements, some of which the authors admit requires further study.

The key point though is that their mechanism cannot work in vacuum. It absolutely requires that the light travels significantly slower than the gravitational radiation (which in turn is assumed to travel at c, even in the non-vacuum in which light travels slower than that) and that a far-from-negligible momentum is lost by the incoming gravitational radiation as it passes through the refracting medium.

great experiment here

The last paragraph in the Conclusions and Discussion section suggests there may be avenues for experimenting with the ideas in the paper.

twawaaay4• 3 years ago

The key point though is that their mechanism cannot work in vacuum. It absolutely requires that the light travels significantly slower than the gravitational radiation

I am not a physicist, but I understand we are talking about Universe so early after Big Bang that it wasn't yet transparent to light. There simply wasn't vacuum yet if by vacuum you mean electromagnetic waves being able to travel long distances.

adastra224• 3 years ago

We know that gravity alone can create radiation - the Hawking one

To be pedantic, we don't know this. Hawking radiation has never been observed.

PartiallyTyped• 3 years ago

Relevant: https://phys.org/news/2021-02-stationary-hawking-analog-blac...

deciplex• 3 years ago

to be even more pedantic, hawking radiation isn't exactly created by gravity

chasd00• 3 years ago

didn't the LHC create a black hole that immediately evaporated due to Hawking radiation? ..maybe it was only a theory that the LHC could do that but i thought it actually did create one.

mfer• 3 years ago

Not only has Hawking radiation not been observed but the article starts with a "may have" and concludes by pointing out that there aren't conditions to observe the phenomenon described today.

Maybe I'm just not a fan of strong language, making it appear we know something, where we don't.

marginalia_nu• 3 years ago

If gravity would quantize into photons, that would have very weird implications for the standard model. It would threaten to equate gravity with the electromagnetic force, and mass with charge.

pa7x12• 3 years ago

Gravity cannot be exchanged by photons or any other spin 1 particle, for that matter. Spin 1 particles lead to repulsive forces for like charged particles.

This leaves you with spin 0 and spin 2 as the simplest alternatives. Spin 0 doesn't result in light bending and gives a wrong result for Mercury's perihelion precession. Spin 2 gives you General Relativity.

kordlessagain2• 3 years ago

That would imply there could be devices that can create thrust with electricity. Faraday ran experiments to try to test this…in the 1800s.

https://skullsinthestars.com/2009/03/06/michael-faraday-gran...

antonvs3• 3 years ago

We know that gravity alone can create radiation - the Hawking one.

What does "gravity alone" mean here? Hawking radiation depends on a black hole that has energy to radiate. The radiation is not due to "gravity alone".

XorNot3• 3 years ago

Hawking Radiation didn't depend on gravity either, it depends on an event horizon. Any phenomenon which would separate virtual particle pairs would produce it - i.e. the edge of the observable universe would do it too.

gus_massa• 3 years ago

I never thought about Hawking radiation in this way! How is the Feynman diagram??? [Ok, there is no Quantum Gravity theory yet, but is there a good guess?]

mitthrowaway23• 3 years ago

We know that light can create gravity, so it stands to reason that there should be a reciprocal relationship!

cocoa19• 3 years ago

"We know that light can create gravity"

I've never heard of light creating gravity, where did you learn this?

nimish• 3 years ago

Concentrate enough energy in a small area and baby, you got a black hole going!

raattgift7• 3 years ago

In General Relativity, any momentum is encoded in the stress-energy tensor in the Einstein Field Equations, which relates curvature (mainly described by the Einstein tensor) and matter (mainly described by the stress-energy tensor). The vacuum of General Relativity has the stress-energy tensor filled with zeroes in all its components; a wave or beam of light introduces one or more nonzeroes. In suitable coordinates and the flat spacetime of special relativity, this is encoded in the "p" (p for momentum) in e.g. E^2 = (pc)^2 + (mc^2)^2, a fuller version of the famous E = mc^2. See <https://en.wikipedia.org/wiki/Energy%E2%80%93momentum_relati...> for details.

Alternatively, we can consider the active and passive gravitational charges of a given mass, also commonly called the active gravitational mass and the passive gravitational mass. The passive charge describes a mass's response to a known source of gravitation; the active charge describes the strength of the gravitational effects generated by an object. In General Relativity the version of the equivalence principle that says that all objects fall identically no matter what their internal composition is ("universality of free fall") ensures that the passive and active charges are identical for all matter. Moreover, in the approximately three hundred years before General Relativity was first written down, there were many successful tests of the equality of the active and passive gravitational charges for many masses; many of these tests were motivated by the work of Newton.

In both Newtonian gravity and General Relativity, light is deflected around large masses (e.g. light from distant stars, or radio beams from <https://en.wikipedia.org/wiki/MESSENGER> grazing the sun, and also lunar laser ranging experiments), so in both theories light has a passive gravitational charge (or passive gravitational mass). If passive and active gravitational charges are identical or at least totally equivalent, light must also source gravitation.

mitthrowaway23• 3 years ago

It's a direct consequence of general relativity, because photons carry energy and momentum.

Some discussion here: https://physics.stackexchange.com/questions/22876/does-a-pho...

jethkl• 3 years ago

1letterunixname2• 3 years ago

E=mc^2. If you have enough energy, in any form, it is equivalent to mass and should disturb spacetime equivalently.

Light can also be used to push objects. Interestingly, sunlight exerts a pressure of 6.56e-10 [psi = lbs/in^2] or 4.53e-6 [N/m^2] on the Earth. This is roughly 5.75e8 [N] or 46 Space Shuttle SRBs. The gravitational force between the Earth and Sun is 3.52e22 [N], or + ~14 orders-of-magnitude.

Objects on Earth at the rotational equator at MSL weigh 0.2% less than at the rotational poles due to the centrifugal force.

monstertank• 3 years ago

Obviously gravity works on particles to condense into stars, which create light...so I'm guessing it's not talking about that?

jacquesm2• 3 years ago

You added 'particles'.

joadha2• 3 years ago

The authors added "electromagnetic field", which also happens to involve particles.

qikInNdOutReply• 3 years ago

So the trick to creat this light in todays environment would be to make a gravity amplifying device similiar to a https://en.wikipedia.org/wiki/Gravity_laser and overlay the amplitudes? That can be easy. Just have a microscopic black hole particle go back and forth in a field..

soylentcola• 3 years ago

And I just upgraded all of my bulbs to LED...

kgwxd• 3 years ago

This made my fucking day

digitaltrees• 3 years ago

Phenomenal. (Second definition intended)

yosito• 3 years ago

Creating a microscopic black hole sounds like a great filter type of event.

still_grokking• 3 years ago

If Hawking was right microscopic black holes should be harmless. If not, well…

But we only find out if we try, I guess.

badrabbit5• 3 years ago

Isn't gravity just a fundamental force? Could it be matter reacting to gravity? Light is energy but gravity is not energy and has no mass so how can it create something out of nothing? How is hawking radiation even possible unless something else was involved?

habibur2• 3 years ago

Or gravity is just bending of space around a mass. And not a force.

cronix• 3 years ago

Not a physicist, but how do you get something to bend without a force exerted upon it?

joadha2• 3 years ago

I used to share your confusion on this topic, but I think I got it:

General Relativity posits that gravity embodies both of those qualities you mentioned.

It is a force that bends space and dilates time.

narag• 3 years ago

https://www.youtube.com/watch?v=ZHxoM9lvzVA

Maursault3• 3 years ago

Isn't gravity just a fundamental force?

Not really.

     Gravity is most accurately described by the general theory of relativity... which describes gravity not as a force, but as the curvature of spacetime, caused by the uneven distribution of mass, and causing masses to move along geodesic lines.[1]

[1] https://en.wikipedia.org/wiki/Gravity

badrabbit5• 3 years ago

Ok, so how does curvature cause light and radiation on its own?

jerf2• 3 years ago

You have to remember that General Relativity is wrong. We don't know exactly how it's wrong, but it's wrong. In light of that fact, it's not really a proper answer to say that "General Relativity says it isn't so it isn't." It is true that GR says gravity isn't a force, but the implication doesn't follow. We don't know whether a Grand Unified Theory would have gravity as a force or not. Given that a great deal of the answer to "Is gravity a force?" depends heavily on the definition of force being used, we don't even know what the final definition will be in light of a GUT. There may or may not be a meaningful distinction between gravity and other forces in the real universe.

In QM, gravity isn't a force because gravity doesn't exist. Clearly this is wrong as well. Saying "GR says gravity isn't a force" is effectively the same thing as saying "Gravity doesn't exist because QM says it doesn't." This is exactly where both theories break, so we can't use them to answer this question.

computerfriend3• 3 years ago

Electromagnetism is also curvature though.

badrabbit5• 3 years ago

Ok, why all the downvotes even after starting a healthy discussion thread?

And can someone help me understand in layman terms how current physics posits that gravitation is generating light or even hawking radiation?

pa7x12• 3 years ago

Gravity couples with anything that carries energy (in more precise terms with the energy-momentum tensor). Photons carry energy. Therefore there is interaction between photons and gravity. In particular, you can write Feynman diagrams for the interaction between a quanta of gravity (a graviton) and a photon. Some of the simplest ones look like this:

https://www.researchgate.net/figure/Tree-level-Feynman-diagr...

There you can see different ways in which a pair of gravitons can interact leading to a pair of photons. If you want to dig deeper in the math behind it: https://physics.stackexchange.com/questions/335917/how-do-i-...

Regarding Hawking radiation, it's not gravity itself that causes it. It's a consequence of the existence of an event horizon. In fact, you can have a similar effect in flat spacetime (i.e. no gravity), an accelerating observer will see an event horizon and it will feel itself immersed in a thermal bath of particles at certain temperature that has to do with its acceleration. This is called the Unruh effect. Further reading: https://en.wikipedia.org/wiki/Unruh_effect

jordann• 3 years ago

Thomas Pynchon was ahead of his time with his book title https://en.wikipedia.org/wiki/Gravity%27s_Rainbow

blockwriter• 3 years ago

Isn’t that about the arc a rocket makes?

mr_dupes• 3 years ago

He picked a metaphor with many possible interpretations : )

My main take away is something like "gravity" is ~ death / extensional pain of life & the "rainbow" is the beauty that exists/we make under that

stared• 3 years ago

All quantum fields have their vacuum energy fluctuations. When we change the parameters of the space (e.g. stretch or compress it), the current state is no longer the ground state but becomes a so-called squeezed vacuum state.

This effect is used in laser physics to "split photons" in spontaneous parametric downconversion. That is, an intense laser changes the refractive index of a medium periodically. These oscillations generate a squeezed vacuum state.

https://en.wikipedia.org/wiki/Squeezed_coherent_state

westurner4• 3 years ago

Is there a corollary SQG Superfluid Quantum Gravity fluidic description of squeezed coherent states?

And what of a Particle-Wave-Fluid triality?

westurner4• 3 years ago

Noting that I attempted to link to relevant research and cite the source for fluidic corollaries but was prevented from contributing. https://westurner.github.io/hnlog/#comment-35661155

gaze• 3 years ago

That's a nice explanation. Finally some high falutin physics I can understand in terms of paramps.

heelix• 3 years ago

I, on the other hand, looked at this and thought "I know these words". I don't think a cup or to of coffee is going to help.

upsidesinclude• 3 years ago

You seem educated on some matters, so please reply here. My physics education stopped at modern physics (optics, general relativity) so I'm missing the core mathematics of quantum mechanics and deeper dives into particle physics.

In the actual summation, does the calculation of vacuum energy take into account a cross section of neutrinos and extended "particles". I've always found the concept of vacuum energy lacking in that most(even physicists) do not fundamentally conceptualize a permanent neutrino and elementary particle flux in all but the most astoundingly shielded vacuum.

If there is a medium in which to effect the refractive index, then isnt this an interaction of the molecular structures therein?

methods21• 3 years ago

Thought LIGO can detect these waves? https://www.ligo.caltech.edu/

dogma11382• 3 years ago

LIGO detects gravity waves as in when space actually expands and contracts slightly this is about gravity being able to emit photons.

raattgift7• 3 years ago

The paper summarized at the link at the top hypothesizes that in a setup like this:

Bb --- Earth --- molecular cloud --- B'b'

where Bb and B'b' are very similar (from the molecular cloud's perspective) inspiralling binary black hole pairs, and Earth is where LIGO (and Virgo and other detectors) are, then we will see characteristic bright flashes from within the molecular cloud and a change in the B'b' detected waveform because it will have lost some energy to the production of the flashes of light.

(The light flash will trail behind the dampened B'b' waveform detection because this hypothesized mechanism only works when a refractive medium slows light from c (its speed in vacuum) but does not slow gravitational waves from c. Although the authors do not touch on the matter, I suspect that we would also be interested in <https://en.wikipedia.org/wiki/Light_echo>s.)

The paper focuses on an analysis of water as the dominant molecule. There are known astrophysical water megamasers (see second paragraph at <https://en.wikipedia.org/wiki/Megamaser>), so this is far from ridiculous.

Trying to match detected flashes of light (including gamma rays) and neutrinos with gravitational wave detections is part of multimessenger astronomy <https://en.wikipedia.org/wiki/Multi-messenger_astronomy>.

harveywi• 3 years ago

All this time I thought that gravity creates heavy, not light.

parlortricks• 3 years ago

I see dad found the Internet again.

slewis• 3 years ago

How dare you! This is a serious place. ^

1letterunixname2• 3 years ago

Burn them. With a gravity-assisted optical laser.

layer8• 3 years ago

It’s the light side of the gravitational force, bringing balance to the universe.

SebJansen• 3 years ago

you underestimate the power of the dark side

sinuhe69• 3 years ago

And God said, "Let there be light," and there was light Interesting coincidence, huh ;)

kazinator• 3 years ago

Gravity makes heavy; gravity makes light.

Physicists discover that gravity can create light

Related Stories