Quantum particles can feel the influence of gravitational fields | Hacker News Reader

Quantum particles can feel the influence of gravitational fields

sciencenews.org

109 pointsgalaxyLogic45 comments

The article's description of the Aharonov–Bohm effect seems kinda misleading to me. It's not that particles are being affected by a field that's not there, it's that the particles are affected by the electromagnetic potential, which can be non-zero even though the field is zero (the two are related through some simple equations).

So to be simple you're saying the field is the slope (or actually gradient) of the potential. So if it's constant (but high) there is no slope, but significant potential, like being on a mesa. And that difference in potential rather than slope affects the paired quantum particles?

It's not the electric field; it's the magnetic field. There is still a potential, but the potential is a vector.

My ability to picture what is going on has never extended to the magnetic vector potential, so I have no intuition about how that plays in here...

There is an electrical variant of the effect, it just hasn't been well tested experimentally.

Right, but pairs of particles aren't relevant. The effect occurs for any particle that interacts electromagnetically. And the magnetic field is the curl of its potential.

Kinda. The magnetic field is the curl of the vector potential. If the vector potential is nonzero but curl-less then you get zero magnetic field, but the particles’ wavefunctions still feel the vector potential.

(Specifically, the phase of the wavefunction is affected; obviously nothing observable about a single wavefunction could be affected in the absence of a field, via the correspondence principle. But when you have two electrons, that phase difference does show up in their interference pattern.)

I think I understand the Aharonov–Bohm effect for magnetism, but I don't understand how you can have the branches at different gravitational potentials without the packets experiencing a field somewhere when they branch/join.

I'm trying to figure out how gravity has curl?

Some argue that it's neither a non-local interaction (the test particle being affected despite no field in its region) nor that the interaction is caused by the four-potential, which would then be physical and more fundamental than the field tensor. On the contrary, it may just be an artefact of the semi-classical treatment that is normally done: classical theory for the fields, quantum one for the test particle. See this, for example: https://arxiv.org/abs/1110.6169

The title here leaves off "they never touch", which is important to show evidence for for the Aharonov-Bohm effect. Having quantum particles feel the influence of gravitational fields isn't exactly news.

I could point out several that are doing that very thing right now.

Including particles that have no rest mass. E.g. photons: light follows the curvatures in space/time that are linked to gravity.

Pardon the stupid question but since most experiments and observations are done on earth, how do scientists know that fundamental forces like electromagnetism and the strong force are not being influenced by gravity? What if chemical/atomic bonds and molecular structures don't form the same way or require more/less energy depending on gravitational influence?

As a layman, I would think that gravity pulls down sub-atomic particles, wouldn't that slow them down compared to say outside the heliosphere and oort cloud? Would electrons spin faster if they are located at an inter-galactic void?

More insane is the measurement of 'c', is all using electromagnetism, but if that in itself is being slowed down by gravity, well I can't even begin to comprehend the implications.

If the electromagnetic force changed in some way depending on the gravitational influence then atoms would produce different spectral lines. However, we can observe the spectral lines produced by atoms and molecules in the interstellar medium where the net gravitational force is much weaker. There is no difference between these lines and what we observe in the lab. (Except for certain expected effects from the reduced pressure like reduced Doppler broadening and the appearance of "forbidden" lines.)

But if the atoms produced different spectral lines, wouldn't observers on earth just end up wrongly believing they are seeing a different type of an element? For example you might think a star is made up of hydrogen when it is made up of helium?

I have no knowledge of how spectal lines work so pardon my ignorance once more, but is it possible for gravity or other forces to produce spectral lights of similar elements than the original one or is it a very unique fingerprint not similar to close by elements?

Also, if gravity can bend light (blackholes), how come it cannot attenuate it? Light is on the EM spectrum and the speed of light is different in different mediums so I am presuming it is similar to how electrons attenuate on copper wire.

> wouldn't observers on earth just end up wrongly believing they are seeing a different type of an element?

It would be very very very unlikely. The spectrum of an ionized atom is not just a single line that you could easily mistake for another: there is a sequence of lines for each electronic level. There's also a lot more structure when you have molecules: in this case you also have lines associated to changes in the rotational and vibrational state of the molecule, and these also combine with electronic transitions to produce even more lines with characteristic spacings. For example, the simplest molecule, H2, has such a rich spectrum some people spent decades in the 50s to fully characterize it.

> is it possible for gravity or other forces to produce spectral lights of similar elements than the original one or is it a very unique fingerprint not similar to close by elements?

Yes, there's the gravitational redshift: it affects light emitted near a massive object such a black hole and it has been observed. The effect is very small, but I guess it could potentially shift the frequency of a line of element X to near the nominal frequency of a line of element Y. However, the redshift applies to all lines, so it's very easy to recognize you're looking at a shifted spectrum if you see more that a single line.

> Also, if gravity can bend light (blackholes), how come it cannot attenuate it?

To attenuate light you necessarily need some kind of dissipative process, so light has to be at least partially absorbed by something and its energy converted to something else. In a vacuum this is really not possible. Maybe with the inclusion of some very high energy process like γγ → e⁺e⁻ or the gravitational equivalent of pair production, but these are really way out of the energy ranges of molecular and atomic spectra.

First, we have extensive observational data from elsewhere in the universe that tells us that the fundamental interactions work the same everywhere. For example, we see light coming from regions that have very different gravity, but it still behaves the same.

Second, on the scale of atoms, or even on the scale of ordinary macroscopic objects, gravity is so extremely weak that its effects on things like chemical bonds or the structure of nuclei, atoms, and molecules is negligible. If you have a very massive object like a star (or a white dwarf or neutron star), then of course you have different states of matter possible (degenerate matter in stellar cores, white dwarfs, and neutron stars), but even those states of matter still have all the fundamental interactions working the same way. The different states of matter are due to the extreme density and pressure, not due to any change in the fundamental interactions.

Third, what we usually call "the speed of light in vacuum" is actually a property of spacetime, not specifically of light. So the idea of this speed being "slowed down by gravity" is based on a confusion. That property of spacetime (a better name for it would be local Lorentz invariance) is the same everywhere no matter how weak or strong gravity is.

Isn't gravity a property of spacetime? (curvature)

You can describe it that way. But different parts of spacetime can have different curvature, so you get different impact of gravity. No matter how you describe it, gravity is different on Earth, Moon or in deep space and your theory has to model that no matter if it uses forces, curvature of spacetime or something else.

More precisely, tidal gravity is spacetime curvature.

What most people think of as "gravity"--for example, feeling weight when you are standing on the surface of the Earth--is actually non-gravitational forces, such as the Earth's surface pushing up on you, that are keeping you from freely falling. If you are in free fall, the "force of gravity" as people normally think of it disappears. (This was the key insight that started Einstein on the path to General Relativity.) What is left over is tidal gravity--freely falling objects converging or diverging because of the geometry of spacetime.

> For example, we see light coming from regions that have very different gravity, but it still behaves the same.

Blackholes bend light for example, isn't that light behaving differently due to gravity? Couldn't pulsars and quasars do the same?

> That property of spacetime (a better name for it would be local Lorentz invariance) is the same everywhere no matter how weak or strong gravity is.

Isn't gravity a product of spacetime curvature? Normal objects' "speed" changes when they curve, can observational evidence froma distance, especially considering relativistic effects be enough to tell observers on earth thay even during curvatures of spacetime, the speed of light in vaccume locally at that point won't be affected?

> Blackholes bend light for example, isn't that light behaving differently due to gravity?

No. Light "bending" due to gravity isn't a change in the behavior of light. The light is still traveling on the straightest possible path and obeying the same Maxwell's Equations. It's just doing it in a curved spacetime instead of a flat one.

> Isn't gravity a product of spacetime curvature?

See my response to rkagerer downthread.

> Normal objects' "speed" changes when they curve

The fact that you put "speed" in scare-quotes shows that, even if you don't realize it, you intuitively grasp that the concept you are trying to use doesn't work the way you are saying it does.

Normal objects in free fall, i.e., moving only under the influence of "gravity", move on straight lines in spacetime, just like light does. (They are different straight lines because normal objects have mass while light is massless. But they're still straight lines in both cases.) The apparent "curvature" comes from the curvature of spacetime, not any change in the behavior of the objects. The objects have no way of telling locally whether the spacetime they are in is flat or curved.

> the speed of light in vaccume locally at that point won't be affected?

No. You can't tell anything about spacetime curvature or gravity by local measurements of the speed of light in vacuum.

That all makes much more sense now, thanks for taking the time to patiently explain it all.

> most experiments and observations are done on earth

Yes, but all astronomy built on same foundations. For example just cup of years, exists literally observation of Black hole, but few decades before their behavior observed only by indirect measurements, which are absolutely confirmed theory.

Also last years launched few large scale experiments with satellites, and few others planned.

For example, LISA Pathfinder was launched to orbit 500000km..800000km, which is not much for astronomic scales, but definitely is not on Earth surface (Moon is less then half million kilometers from Earth) :)

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

First very far automatic chemical lab, probably was Mariner series on Mars. Later was Galileo, Cassini–Huygens (list included on bottom of page).

https://en.wikipedia.org/wiki/Cassini%E2%80%93Huygens

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

Must admit, I cannot remember any space mission, with rich chemical lab , considered to work at open space (Vega has chemical lab on descent craft, Halley comet part was just radar/imaging).

But chemical rocket engines work same, and all known electronics also work same as on Earth surface. I'm few decades waiting for any evidence of chemical reactions depending on gravitational influence, but nothing happen.

I mean, they have to, no? To the best of my understanding, gravity isn't a force, it's the bending of space around us caused by mass. Everything is forever moving in a perfect straight line from its perspective, but that line is curved depending on the mass of nearby objects, which affects the speed of movement (time). We're all moving around the Earth, which moves around the sun, which moves around the galaxy, which moves around the universe. This includes all particles which makes up everything. The idea that the gravitational field has no effect on them makes no sense. The question is simply "how much" of an effect.

I struggle with this too. Is gravity a force or a geometry? (or both?)

I have a physics degree, and struggle with it too. Granted, my degree was 30 years ago and I've been working in industry, so I haven't maintained my theory chops.

The way it was explained in our graduate QM class was that certain operations in QM theory only work in Cartesian space. This is probably an over simplified statement of a Hard Problem, which is that QM and gravitation are both known to be exquisitely accurate theories, but mutually incompatible.

So if I am reading this correctly this is basically an observation that the Aharonov-Bohm effect can be done by gravity on entangled particles ?

I’m guessing this won’t provide much insight on theories for quantum gravity?

Well, you never know... The current explanation of this (and of the original) effect uses the classical interpretation of the field involved.

Observation of a gravitational Aharonov-Bohm effect https://www.science.org/doi/10.1126/science.abl7152

I thought all gravity from everything touched everything.

I wonder if you could use these as a gravity detector. Like detect if you’re in a gravity field?

As I understand these matters, you could know, if quantum entangled particle enter gravity field. Must admit, I don't know how this experiment will be organized, but looking possible.

It’s not about fields. Everybody knows fields affect particles. This is about the gravitational potential.

But without a field there is no potential, is there?

Should put 2022 in the title.

Now, how do they know? Have they interviewed any quantum particles?

No need (and who wants to be lied to, anyway?), the behavior observed was quite telling.

What the hell are "quantum particles"?

I guess, this is a shorthand for "particles studied in quantum physics."

In this case, they mean particles they have hit with a laser to split into superposition, so "quantum particles" = "particles in superposition".

objects described by wavefunctions

Just a layman but whenever I read that, I interpret it as "particles exhibiting quantum phenomena", like superposition or entanglement.

This would be unlike classical particles which does not exhibit these effects.

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