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Quantum particles feel the influence of gravitational fields they never touch (sciencenews.org)
77 points by pseudolus 4 days ago | hide | past | favorite | 61 comments

Sounds more like a demonstration of an ability to detect the influence of gravitational fields at scales previously unavailable. Specifically, in testing something called the "Aharonov-Bohm influence/[effect]" previously demonstrated within EM fields and now for gravity.

Quote: """

The study not only reveals a famed physics effect in a new context, but also showcases the potential to study subtle effects in gravitational systems. For example, researchers aim to use this type of technique to better measure Newton’s gravitational constant, G, which reveals the strength of gravity, and is currently known less precisely than other fundamental constants of nature (SN: 8/29/18).


The wavelength of an atom (DeBroglie Wavelength) is much lower than that of a photon of visible light, so it follows that anything that makes the path of one beam even slightly longer, such as putting a mass near it, would be far more detectable in this manner.

There might be a small, very hard to detect optical interference shift if the experiment were re-run with a coherent laser beam.

I'm not sure how it's possible to "never touch" a gravitational field given that they extend throughout the universe and are impossible to be shielded.

As is unfortunately common in pop science writeups, the article gives a garbled description of what the Aharonov-Bohm effect actually is. The key point is not that "the field never touches the particles". The key point is that the potential, rather than the field, has an observable effect. In classical gauge theories, the potential itself is not considered to be observable; only the field (the gradient of the potential) is. However, in QM, the potential itself can have observable effects. That's what's going on here, but with the gravitational potential instead of the EM potential (as in the ordinary Aharonov-Bohm effect).

The paper itself is here:


It's paywalled and unfortunately I have not been able to find a preprint on arxiv.org.

"The key point is that the potential, rather than the field, has an observable effect. In classical gauge theories, the potential itself is not considered to be observable; only the field (the gradient of the potential) is. However, in QM, the potential itself can have observable effects."

That comment is short, succinct and understandable, it's the best summary of the physics involved that I've heard in so few words.

It seems to me that we don't put sufficient emphasis into considering potentials. Viewing things from the aspect of potentials I reckon changes one's perspective, and I think the Aharonov-Bohm effect is a case in point, without doing so we easily come aground.

However, the Aharonov-Bohm effect still perplexes me more than I'd like - in QM, why exactly does potential have observable effects? Why is it that a phase shift occurs in the wave function of a charged particle in the near vicinity of a solenoid even though both magnetic and electric fields are negligible? (I'm either not convinced by explanations that I've heard to date or thst I don't fully understand them.)

Whilst my understanding of the Aharonov-Bohm effect is deficient in this regard, it nevertheless seems to me that if the effect can also be detected in a gravitational field context then we've likely found a profound and deeply relevant connection between EM fields/relatively and gravity. If verified, then we'd have to consider it a breakthrough in our understanding of what up until now has been an intractable problem.

The key point is that we've now experimental evidence for such a connection and that's really good news.

> if the effect can also be detected in a gravitational field context then we've likely found a profound and deeply relevant connection between EM fields/relatively and gravity.

I'm not sure I'd say it's a breakthrough since theorists have expected it all along. And current treatments of this all have a key limitation, that they are using non-relativistic QM. Which in practice works fine since all the experiments we can currently do in this area are well within the non-relativistic domain.

A real breakthrough would be a relativistic quantum theory that included gravity as well as the other interactions. But that's probably still some way off.

"real breakthrough would be a relativistic quantum theory that included gravity..."

Yeah, sometimes one has to be optimistic. I'm aware of the limitations of using non-relativistic QM but it seemed to me that this experimental connection, if confirmed, would encourage or even force relativistic work to center stage.

Essentially, both theoretical and experimental practitioners could now say 'we've now a concrete stating point'. On reflection, I wouldn't expect a full GR explanation to come easy. Still, it's a start.

> in QM, why exactly does potential have observable effects?

The simple answer is that it's right there in the Hamiltonian, and the Hamiltonian is the central operator in QM, the one that determines time evolution. The fact that the EM potential appears there has been known almost as long as QM itself. Much of the recent QM experimentation in gravitational fields has been making use of recent technological advances to verify, what theorists have expected all along, that the gravitational potential acts just like the EM potential in the Hamiltonian.

I suppose I fell foul of AE's warning about oversimplification but it's sometimes difficult to avoid in HN posts. Also, I am not a professional physicist which means I don't have the depths of understanding at the edges of the subject that I wish I had, this essentially limits my comments to the accepted orthodox understanding of the subject.

It's a while since I last looked at Aharonov and Bohm's '59 paper but if I recall your point about the Hamiltonian is covered there. My understanding is that in this paper the key difference from the earlier work to which you also refer is that their new solution to the Hamiltonian now involves a phase factor.

The point I should have made was that I wasn't thinking so much about the mathematical explanation of the Aharonov-Bohm effect but in more general terms where perhaps this new experimental work may reinvigorate interest in the subject and in related areas.

Whilst QFT provides us with an exquisitely accurate mathematical account for the purposes of calculation, it says little about the underlying physics per se. Thus, it seems to me that we stil have a limited understanding about the nature of say virtual particles, ZPE, etc. and essentially no understanding of why the electric, magnetic and fine structure constants and others are the values they are.

Research into the Aharonov-Bohm effect, could eventually lead to a deeper more fundamental understanding of the subject although, given past history, I fully accept that coming up with a major breakthrough in the near future is probably unlikely. (It's even more unlikely that we'll resolve the constants problem anytime soon, if at all.)

It seems to me that the most significant aspect of this work is that we now have more than just a theoretical framework that connects EM and gravity/the gravitational field at a QM level (or seemgly so). Tenuous it may be but it seems like a good start.

For my part, I still worry about why, say, the electric and magnetic constants have the value they do or why our understanding of ZPE is seemingly at odds with reality given the ludicrous value of the calculated zero-point radiation of the vacuum. But then, this is more about philosophy than it is about physics.

You're right, most descriptions of Aharonov-Bohm fail to mention Electromagnetic Potential, and just describe it as quantum woo. I'd love to have an actual quantum detector, like a SQUID, to do some physics experiments with.

You'll recall that Feynman whinged about the lack of potentials when he was trained (there bring too much emphasis on fields and almost nothing on potentials). And I reckon he was right to do so for the same reasons, as it took me a considerable amount of time to shift my thinking around to thinking potentials and not fields.

Re SQUIDs, likewise, I could amuse myself for ages if I had several to play with.

> description

Importantly, it's "famed" and "eerie."

They touch on that in the article:

> Notably, the particles weren’t in a gravitational field–free zone. Instead, the experiment was designed so that the researchers could filter out the effects of gravitational forces, laying bare the eerie Aharonov-Bohm influence.

I'm no physicist, but I think they calculated the expected influence and compared that with a measurement.

The "filter out the effects of gravitational forces" means that the measured particles were in free fall; in free fall there is no gravitational force.

Is that the common way you'd express that they were measured in free fall -- that the effects were "filtered" out? That seems like an odd way of saying it, but I'm not a physicist.

I don't think it's a very good way of expressing that the objects were in free fall, no. Nor, as far as I can tell, does the actual paper (as opposed to the pop science article) use such an expression.

They measured a bigger gravitational effect on the particle, because the superpositional pair of the particle flew closer to a mass than the actually measured particle.

Regarding the "never touch", gravity decreases with distance squared, so it diminishes quickly with distance. There is a big difference in being near the mass, as opposed to feeling the dimished effect of it from far.

> They measured a bigger gravitational effect on the particle

No, they didn't. They measured a phase shift in the particle's wave function. There is no "gravitational force" in free fall, and the particles were in free fall.

> gravity decreases with distance squared

The Newtonian gravitational force does, but the Newtonian gravitational force is irrelevant for an experiment conducted in free fall, as this one was. The gravitational potential is the key thing being measured, and it's not the potential due to the Earth, it's the potential due to a 1-kg "source mass".

> There is a big difference in being near the mass, as opposed to feeling the dimished effect of it from far.

The particles were near the 1-kg source mass.

"The gravitational potential is the key thing being measured, and it's not the potential due to the Earth, it's the potential due to a 1-kg "source mass"."

Right, it seems that for many the 'potential' worldview is hard to grasp (it was for me too until it drawned on me that it's important).

I blame this on poor training and poor textbooks, they don't emphasize the importance of potentials. Also, we seem to grow up with a 'fields' perspective, electric fields and so on.

Maxwell was on the 'right' path with his original formulation of his equations where potentials were involved. However, when Heaviside reformulated them to the 'vector' view we quickly lost the 'potential' one.

No doubt, Heaviside's formulation is incredibly useful in electrical engineering and eleconics and as you'd know that's how they usually appear in textbooks. Trouble is, outside advanced physics texts the 'potential' view is usually omitted. Educators really need to fix this.

Another problem is that the description of a potential is not up to scratch. All too often we seem to be stuck with highschool physics descriptions - those that involve pith balls. The concept that we never measure absolute energy, but only differences often gets lost when describing potentials.

You'd reckon that after Feynman's well documented whingeing about the fact he wasn't taught about potentials early enough that you'd think by now educators would have had sufficient time to have rewritten their notes but apparently they've not.

>Each of those two sets of atoms were split into superpositions, with one path traveling closer to the mass than the other, separated by about 25 centimeters

One path of the particle in superposition was closer to the 1.25Kg mass than the other path, and they did measure a difference when doing that.

I don't know if you are trying to be pedantic, or just want to contradict. I know what you are saying, but the the expression "not touching the field" makes perfect sense to me. Try plotting the 25cm distance difference for the 1.25Kg mass, and see if it makes a difference or not...

> they did measure a difference

They measured a phase shift in the wave function, as I said. They did not measure any direct difference in "gravitational effect" on the particles, as for example a difference in bending of their trajectories due to the source mass would be.

> the expression "not touching the field" makes perfect sense to me

The problem with it, as several commenters have pointed out, is that you can't shield anything from gravity. The "not touching the field" comes from electromagnetism, where you can shield things from the field. So the "not touching the field" interpretation, while it works for EM, does not work for gravity.

> There is no "gravitational force" in free fall, and the particles were in free fall.

what is free fall in a reference frame is a particle subject to a force in another reference frame.

No, it isn't. Free fall is invariant: attach an accelerometer to the object and it reads zero. That is true regardless of your choice of reference frame.

One, in theory could shield of them, by having the same field apply from the opposing direction and distance. So if one could create such a "mirror" copy of all attractors - one would have a L1 Lagrange point. The forces still interact, but chancel each other out.

While this is of course possible in theory, it's not at all what was being done in this experiment.

Changes in the gravitational field are only propagated at the speed of light, so in principle you could say something "never touched" a gravitational field if it just hasn't been reached by it yet.

"The two theories that underlie this experiment, general relativity and quantum mechanics, don’t work well together."

The Aharonov-Bohm effect alone is perplexing enough and there isn't full unanimity about the theory that underpins it. That we're seeing a similar effect with gravity is truly exiting as I reckon it will attract a great deal more research in this area.

It seems to me that Aharonov-Bohm effect now has a gravitational parallel tells us that we're honing in closer and closer to having a quantum understanding of gravity.

Excellent stuff.

I don't see the new. All this tells me is that entangled particles react to things that impact some but not all of their various superpositions/entanglements. Sounds like basic double-slit stuff to me. The particle that goes through the slit A reacts to its partners going through slit B. The particle that passed through gravity field A reacted to its partners going through field B.

Quantum mechanics news hasn't been giving us anything new for decades. Every modern article is about a new speculation, or at best an or experiment that reconfirms what we already knew. Yet, articles will never cease to use click-bait to make you think a revolution has occurred.

The closer you can get the words "quantum" and "gravity" together in the title the better.

There was an article about China reaching quantum supremacy some time ago, so I suppose we achieved something.

(Per the article): Is it correct to describe superposition as the probability of, e.g., an electron being in one of two places? In such an example, is it instead correct to say that superposition is a probability distribution of infinitely many possible locations?

It depends. In the general case a superposition is a probability field with infinitely many options once the waveform collapses, but the specifics of the experiment might discretize that practically into two options.

Completely misleading and eye ball gripping “article’ …

Quite possibly, but a comment like this is only helpful if it explains how the article is misleading—i.e. if it provides more correct information on the topic. Otherwise it's just another low-quality dismissal, which we're trying to avoid here.


I know this is a trope that comes up in half of the posts involving a journal article, but scientific journal pricing is so confusing to me.

I can pay $30 for digital access to a 4 page article (it says Vol 375 pg 226-229). Or I can pay $15 and get the entire issue of Vol 375 in print? Or I can pay $80/yr to join AAAS and get "50 issues of Science", but which issues? I assume they mean the upcoming year of issues, but I still want to read issue 375...

I consider buying or joining every once in a while when a cool article like this links back to a paper I can't find on Arxiv. But then I remember how expensive individual articles are and how confusing it is to sign up.

Your confusion is warranted, the system has twisted incentives. I am a scientist working at a rich university. I still mostly use scihub unless the article is on arxiv, even for articles which have my name in the byline. All this to say, I consider pirating these papers the moral highground and you should not feel bad doing it. Thankfully, I have the job security and funding to demand my work be open access (a whole other can of worms).

i think a possible startup idea would be to start a "Github for science" (replace "science" with any academic or interest area really). Encourage folks to post their papers in latex or markdown. Post their data, code, mechanical and electrical drawings, videos and pictures of their experimental setup, videos explaining their research, videos performing the experiment?, and so on. Allow others to comment on research with questions, concerns, suggestions for enhancement, or link it to their own supporting work, etc. Allow other researchers to fork people's research (but all forks link back to the original to give full attribution). The core focus is open up access to academic papers and research, foster better communication between people, and set a higher bar for communication of research.

Going the non-profit route: I think it would be cool for the site to be federated/distributed so universities & countries could host replicated nodes. It would be best if somehow the software system development and hosting costs could be underwritten by governments and universities.

Going the for-profit route: The site could have an inexpensive PRO account to help support the site and software development. The site could be add supported, but zero tracker, with adds that would be small and content-based purely on the content of the paper topic.

Add Examples: Reading an article on Julia numerical programming... maybe show an add for the upcoming JuliaCon or Julia Computing

Reading a metallurgy analysis paper... maybe show a small add for an x-ray fluorescent system for metal composition analysis or advertise an upcoming metallurgy conference

... the idea being that these adds are (1) not targeted at users, just targeted at content, (2)small, dismiss-able, and not annoying (3) hopefully actually useful and interesting to the reader.

The site could also sell "advanced API access" to interested parties for knowledge management/search on fast trending research and topical areas, research community graph analysis (maybe for recruiting?)... kind of the LinkedIn layer of the site.

The problems with this are overcoming the startup inertia and coming to a common agreement about how to implement it.

In my view, the latter is by far the most difficult and failing to come up with a common objective or view is likely to either scuttle the project or render it ineffective or inefficient.

Let me give you a real world example: Linux. We've so many distros that it's hard for anyone of them to get real traction. As we've seen for decades, Linux has never managed to take on Windows on the desktop for the reason that there are so many disparate views about how tackle the problem. Both businesses and ordinary users want certainty not constant variation.

We see the same problem in other areas of the net, for instance copyright reform. The only way to tackle that is on a worldwide basis - as the forces against it are aldo already very well organized on a worldwide basis and have been so for some 140 years (since the original Berne Convention).

I've often thought that we need a worldwide union of computer users to tackle these issues and take on Big Tech, etc. but how does one start something as big and significant as this let alone hold it altogether for any length of time?

I wish I knew.

Just use sci-hub. It always works, and it's very easy. There's no good reason to give the journal your money to read an article they had no hand in writing or funding.

article goes on to state that gravitational pull effects surrounding objects without touching them, you know, like how we already know every fucking planet works?

quantum physics: the biggest waste of time since philosophy

You're about a hundred years late to be making this stand. I've never actually seen anyone claim that quantum physics in general is hooey. What do you even mean by that? That in fact, all of modern physics observations, as well as cosmology, materials science, etc, can actually be explained by classical physics? Or do you mean that you think there is some underlying classical/deterministic process which drives nature, and quantum mechanics is our attempt to predict outcomes based on an incomplete understanding? That is a defensible position, but it still wouldn't make sense to call quantum physics a waste of time.

Quantum physics is worse than a waste of time. It actually impedes real progress.



OK, Mr Troll. That site is like I had a seizure while playing with Adobe Dreamweaver.

I was expecting Time Cube, but close enough I guess.

What about the use of quantum physics in things like semiconductors?

semiconductors don't "use" quantum physics, quantum physics just tries to explain how they work. semiconductors still exist without requiring quantum physicists to come in and try and steal credit for something they had no influence on whatsoever.

They used to but manufacturers are starting to have to deal with quantum effects lately https://semiengineering.com/quantum-effects-at-7-5nm/

How is explaining/figuring out how quantum physics effects semiconductors "stealing credit"? Learning about how quantum tunneling effects impact super small transistors is extremely useful since chip designers can use this info to design chips that mitigate this effect as we scale down.

Also in the material sciences metamaterials have special properties precisely because of how the molecular structure manipulates quantum forces. https://www.nature.com/articles/s41467-019-09939-8

Nothing in the universe »uses« quantum physics. Or classical physics. Or any physics. Or any science. Or anything made by man. Or aliens. All a waste of time and resources.

This is sort of a profoundly different way of looking at it. The creation of the first semiconductors was closely tied to the development of quantum theory around electrons in metal. Bell labs hired up Shockley, Bardeen and a bunch of other solid state physicists (when it started to become obvious that the US needed to build computing devices that were faster and more rugged than vacuum tubes) and it was their knowledge of quantum physics that enabled them to solve key problems in the development of the transistor.

QM is still the best theory for semiconductors and a lot of semiconductor improvement happens by applying quantum physics.

The important part to recognize is that this is a part of QM that doesn't involve entanglement or wave function collapse, but definitely relies heavily on quantum tunnelling. All of this is well documented by the primary literature in the field.

It may be that is how it started, but I'll never forget the day my Advanced Chemistry teacher at Rose-Hulman used standard chemistry (I think it was the Nerst equation, 40 years ago!) to explain how a diode works.

Then he continued and explained how a junction transistor worked with the same equations!

No QM required.

The nernst equation is https://en.wikipedia.org/wiki/Nernst_equation (redox). THere's also https://en.wikipedia.org/wiki/Nernst%E2%80%93Planck_equation which also isn't exactly for diode modelling.

I'm sure you can find classical equations that model some aspects of p-n junctions but you're ultimately going to see that p-n junction physics is literally quantum physics of tunneling electrons in atomic solids.

It's not tunneling, it's conduction. There are liquid electrolytic rectifiers, they suck, but they exist.

Oh, I see what you're saying now. Your teacher showed you the equations explaining a classical (pre-semiconductor) diode, then showed those equations predict some aspects of semiconductor diodes.

Yeah, that doesn't mean that diodes don't work in a fundamentally quantum way. There are a number of details about diodes (for example, the emitted frequency of light in an LED) that are very specifically due to quantum energy transitions of electrons in outer shells. It doesn't get any more quantum physics than that.

This was the equations for a doped semiconducting device, not a vacuum tube. All the equations we had previously been using to describe buffered solutions, etc. also happened to work perfectly well for semiconductors.

LEDs weren't discussed that day.

Indeed, or superfluids, Fermi-Dirac and Bose-Einstein condensates, quantum computers, lasers, quantum dot displays, quantum key distribution, MRI scanners, that trick with 3 polarising filters letting though more light than just 2, antimatter, and electron microscopes.

Oh, and Casimir cavities.

Not for nothing but Shockley, Bardeen, and Brattain were physicists awarded a Nobel "for their researches on semiconductors and their discovery of the transistor effect". Bardeen also won a separate Nobel for a theory of superconductivity. Claiming they're not quantum physicists or had no influence on semiconductors is misinformed.

> quantum physics: the biggest waste of time since philosophy

Sure. After all, natural sciences from physics to biology used to be called natural philosophy until not so long ago.

It says:

> subatomic particles can feel the influence of this warping even if they aren’t subject to gravitational forces

So I guess they are talking about particles without mass.

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