GBAR Experiment (CERN) - Genève

[Marilyn67]

TL;DR Summary

GBAR experiment at CERN
What consequences in the event of unexpected results ?

Hello,

My question concerns the (improbable but possible...?) results of the GBAR experiment at CERN which should resume in May 2021, according to my latest information. (delay due to Covid 19).

Most scientists (and me too) believe that antimatter behaves the same as matter in a gravity field. (principle of equivalence of General Relativity).

However, today a slight doubt remains, and the experiment aims to eliminate this doubt.

My question is the following :

If the experiment were to reveal that antimatter behaves in an inverse manner to matter in a field of gravity (repulsion), what would be the repercussions in General Relativity in particular, and all physics in general ?

I take the (extreme) example of the black hole.
No (classic) information can escape the black hole, once the horizon has been crossed.
(I leave aside the Hawking radiation which is quantum in nature).

If antimatter actually behaves in the opposite way, it would be “pushed back” by the black hole and could “escape” from it.
(That couldn't even get into it).

One can imagine an observer having crossed the horizon who uses gamma rays projected on a target, to create pairs of particles - antiparticles, in order to send a message below the horizon, which is in contradiction with the fact that no "classic" information won't escape from a black hole. (ejection of antimatter).

We would be dealing with a “half-naked” singularity. (if I may say so).

What would be the consequences on our current knowledge?

1 / At the level of General Relativity ?
Would it become false?

2 / At the level of Physics in general and the principle of causality?
(for an observer who has crossed the horizon, the exterior corresponds to the "past", and it no longer interferes with it) :
Here, he could do it with a "flow" of antimatter particles in the form of bits (0,1,) and violate the principle of causality?

I'm not sure my question is well put.

In advance, thank you for your answers, as usual, very relevant.
Marilyn,

 

[Vanadium 50]

I'm not sure my question is well put.

I'm afraid it's not. It's filled with all sorts of assumptions, most of which are not justified. You don't need to (and shouldn't) go into black holes and causality, and so on.

No experiment is done in a vacuum. (I'm not saying ion diffusion pumps are not very good; I'm saying that there is context to every experiment.) We know GR is an extremely accurate description of nature, and it's effective strength for different materials (antimatter is just a different material) is small - in particular we know that the free-fall of light and matter is the same to better than 1%. Any "signal" larger than that is inconsistent with other results (and certain assumptions, e.g. energy is conserved, G does not vary in time and space etc.)

You would need to place a positive result in a framework that can incorporate all the other gravity tests: Pound-Rebka, Eotvos, the whole PPN suite, gravitational waves, etc. The "least bad" way to do this is to postulate a fifth force that is much weaker than gravity that affects matter and antimatter differently. Note that the most plausible outcome in this case is that antimatter falls slightly faster than matter - but only by a fraction of a percent.

 

[vanhees71]

I'd say if the experiment doesn't lead to what we expect, namely that anti-matter just falls as matter in a gravitational field, that would lead to the need of a really new theory about the gravitational interaction. GR, which up to today has been found to describe gravity with amazing precision, predicts that the gravitational interaction is universal in the sense that the source of gravity are all kinds of energy, momentum, and stress distributions of matter and radiation, described in the Standard Model by all other fields than the gravitational field with one unique coupling constant (expressible in terms of Newton's gravitational constant) no matter which kind of energy, momentum, and stress is involved. That's a direct conclusion from the equivalence principle (or more formally from the gauge-theoretical paradigm applied to general relativity which just makes the Lorentz invariance of special relativity a local symmetry).

Now, of course, GR is tested only with matter, because we can test it only in an astronomical context, where we have large masses (stars, galaxies,...) which consist of matter and no antimatter. So to check how antimatter falls in the gravitational field of the Earth is something which should be done to check whether the prediction of universality of the gravitational interaction holds really for all forms of energy and momentum including also energy and momentum of antimatter.

 

[Marilyn67]

Hello,

Thank you for your answers.

I'd say if the experiment doesn't lead to what we expect, namely that anti-matter just falls as matter in a gravitational field, that would lead to the need of a really new theory about the gravitational interaction.

Yes, I agree precisely.
Is there any work that has already been done, in particular to describe a black hole in this new context ?
I ask this question because the consequences (black hole) would be revolutionary, right ?

So to check how antimatter falls in the gravitational field of the Earth is something which should be done to check whether the prediction of universality of the gravitational interaction holds really for all forms of energy and momentum including also energy and momentum of antimatter.

Yes I agree !

 

[A.T.]

Now, of course, GR is tested only with matter, because we can test it only in an astronomical context, where we have large masses (stars, galaxies,...) which consist of matter and no antimatter.

How do we actually know, that distant galaxies consist of matter and not antimatter?

 

[Marilyn67]

I'm afraid it's not. It's filled with all sorts of assumptions, most of which are not justified.

Okay.
You are right .

Let's take a single hypothesis (which is not excluded for the moment, although it's improbable) :

If tomorrow GBAR shows that antimatter is "falling upwards", how to revise the black hole model ?

 

[PeterDonis]

If tomorrow GBAR shows that antimatter is "falling upwards", how to revise the black hole model ?

It wouldn't be revised at all. A black hole is a vacuum solution; there is no stress-energy anywhere. So finding a new weirdness about a particular kind of stress-energy (antimatter) wouldn't require any change at all in our model of a black hole (or any other vacuum solution).

If antimatter actually behaves in the opposite way, it would be “pushed back” by the black hole and could “escape” from it.

No. "Pushed back" is not the same as "traveling faster than light". The latter is what would be required to escape from a black hole.

 

[Marilyn67]

No. "Pushed back" is not the same as "traveling faster than light". The latter is what would be required to escape from a black hole.

I agree, that's what the current model predicts (release speed), but if antimatter behaves in the opposite way, it wouldn't need to go faster than light, right ?

If antimatter is pushed back by the Earth, a star, a neutron star, then why would it be any different with a black hole?
I don't understand...

 

[PeterDonis]

if antimatter behaves in the opposite way, it wouldn't need to go faster than light, right ?

Wrong. The behavior of antimatter does not change the spacetime geometry, and the fact that you would need to go faster than light to escape from inside a black hole's horizon is a property of the spacetime geometry.

If antimatter is pushed back

Gravity is not a force in GR, and trying to think of it as one will lead to errors. This is an example.

 

[PeterDonis]

If antimatter is pushed back by the Earth, a star, a neutron star, then why would it be any different with a black hole?

It's possible that this effect could prevent antimatter from falling into a black hole, as you suggest in the OP to this thread. But if antimatter were inside a black hole, this effect would not allow it to escape.

 

[Vanadium 50]

If tomorrow GBAR shows that antimatter is "falling upwards", how to revise the black hole model ?

It literally cannot. (And this should not be in this section - it better fits BTSM)

First, GBAR does not measure the gravitational acceleration on antihydrogen. It measures the total acceleration on antihydrogen. By itself, it is insufficient to reject GR. One would also need to exclude fifth forces.

Second, if there is no fifth force, it violates the weak equivalence principle, a prediction of GR. If GR is wrong, it's wrong. You can't go around picking and choosing what predictions to keep and what to ignore.

More directly, if gravity does not come from spacetime geometry, what does it even mean to have a black hole?

Third, if antimatter falls up, energy is not conserved. We know photons fall down (within 1% or better of the same strength as matter) so if antimatter falls up we can build a perpetual motion machine extracting energy from antimatter as it falls up, annihilating it at the top, extracting the energy as the photons fall down, and repeat. So you've not only wiped on Einstein, you've wiped out Newton.

Now, what is allowed? The gravitational difference between matter and antimatter can be no bigger than twice the gravitational difference between matter and light. That's under a percent. A -100% effect is already excluded.

If you say, "yeabut...we've never tested this exact system" I would respond with we've never tested astatine either. Or francium. Maybe there's a rare mineral, called "cavorite". Or a rare polymer, called "flubber".

As an aside, while one cannot make a bulletproof quantitative prediction without a theoretical framework, one expects an effect to appear at loop levels in Eotvos-type experiments. That sets a bound of about 10^-6 of the strength of gravity, or a differential acceleration of 10^-3 cm/s². The argument is quite general: it can depend on N(p), N(e) and N(n), with N considered negative for antimatter (that is, one antiproton is the same as minus one proton). N(p)-N(e) is electric charge, and thus unmeasurable. That leaves N(n) and N(p)+N(e). There are extremely good (better than one part per trillion) measurements of the weak equivalence principle on half a dozen materials. That is sufficient to exclude a "gravitational charge" on the antiproton opposite to that of the proton. If that were the case, we'd be seeing it now.

 

[PeterDonis]

this should not be in this section - it better fits BTSM

If the OP wants to discuss the experiment itself, or its implications for possible changes to the Standard Model, i.e., to our theory of interactions other than gravity, yes, that discussion belongs in the BTSM forum.

But it appears that the OP is asking about implications for GR specifically. I think that discussion is appropriate for this forum.

 

[Vanadium 50]

appears that the OP is asking about implications for GR specifically

Is she? I think she's tossing a grenade at it.

If you toss the WEP, you've pretty much tossed the notion of gravitation as space-time curvature.

If you have two test bodies starting from the same point and moving along different geodesics, is it even a geodesic?

 

[PeterDonis]

If you toss the WEP

Then we are no longer talking about "what would happen if antimatter fell towards a black hole", or more generally, "what would happen if antimatter fell differently from matter in a field of gravity", which is the question asked in the OP of this thread. If we are talking about a "black hole" and a "field of gravity" at all, we are talking about a spacetime geometry modeled by GR. And in that context, any differing behavior of antimatter as compared to matter would have to be due to a "fifth force" of some kind, not a violation of the WEP. And no such "fifth force" would allow any object to escape a black hole.

In other words, the only on-topic discussion that it is possible to have in this thread and this forum is what would happen if antimatter behaved differently to matter due to a "fifth force", with the WEP and GR still being valid. And that question has been answered, at least as far as escaping a black hole is concerned.

If the OP wants to ask about what the implications are if the WEP is violated, you are correct that that discussion belongs in the BTSM forum, not here, since GR would no longer be valid. But then the OP would need to give a reference to whatever alternative model we are to use; otherwise questions about what would happen aren't well defined.

 

[PeterDonis]

I think she's tossing a grenade at it.

I'm not sure the OP realizes fully the implications of the questions being posed. I've tried to clarify them in my previous post just now.

 

[Vanadium 50]

In other words, the only on-topic discussion that it is possible to have in this thread and this forum is what would happen if antimatter behaved differently to matter due to a "fifth force", with the WEP and GR still being valid.

Yes, but I don't think that's what she's asking. The only person who mentioned the fifth force is me. (And now you)

To sum up my position:

• GBAR is not sensitive enough - by far - to falsify GR. Anything it could find is already excluded, often by many more orders of magnitude than GBAR could see.
• GBAR is potentially sensitive to fifth-force type interactions, although it's not very likely. A model that would show up here and not in a torsion balance experiment would need to be very, very contrived.
• No fifth force can prevent an object from falling into a black hole. It's no different than any other force.
• If GBAR could falsify GR (which it can't), one would have no business talking about black holes in that case.

 

[vanhees71]

How do we actually know, that distant galaxies consist of matter and not antimatter?

The usual argument is that, if there were somewhere greater amounts of antimatter, maybe dust or even forming stars, it would annihilate with matter close to it, and we'd see the corresponding electromagnetic radiation.

 

[vanhees71]

Is she? I think she's tossing a grenade at it.

If you toss the WEP, you've pretty much tossed the notion of gravitation as space-time curvature.

If you have two test bodies starting from the same point and moving along different geodesics, is it even a geodesic?

Right, and physics is an empirical science. That's why it is natural to look at the behavior of antimatter concerning its gravitational interaction (in this case the fall of anti-hydrogen atoms in the Earth's gravitational field), which never has been done before. It's another test of GR, or rather its Newtonian approximation. As I said, if you find clearly a deviation from usual free fall (for sure one has to take carefully into account all kinds of other forces acting on the atom), it would be a sensation and then there'd be tons of papers on the arXiv with alternative theories, including a possible 5th force etc. I'd not bet any money on finding such deviations though. For that GR has been confirmed by observations too well yet!

 

[Marilyn67]

Good morning all,

I have carefully read all of your arguments and you have convinced me.

In particular :

There are extremely good (better than one part per trillion) measurements of the weak equivalence principle on half a dozen materials. That is sufficient to exclude a "gravitational charge" on the antiproton opposite to that of the proton. If that were the case, we'd be seeing it now.

I understand my mistake.

I think the GBAR experiment was set up, to verify that there is not a "small gap" between the behavior of matter and antimatter.

I didn't know that the existence of a "fifth force" was postulated.

Indeed, in view of your arguments, I sincerely think that no scientist worthy of the name considers for a single second a "repulsion" of antimatter in a gravitational field !

I am wrong ?

I was the victim (once again) of tempting popular science articles, (typically French !) while the question of a repulsion of antimatter in a field of gravity doesn't arise for scientists ... !

These French science journalists are a "wart"...!

It will not happen.
Ok.

This is not the vocation of the GBAR experiment, the aim of which is to measure a "tiny difference", not any repulsion.

This is not his goal, we know the answer :

Antimatter "falls down".

You convinced me!

Thank you all for your contributions and in particular to @Vanadium 50

Have a nice day to you all,

Cordially,
Marilyn

 

[Vanadium 50]

Right, and physics is an empirical science. That's why it is natural to look at the behavior of antimatter concerning its gravitational interaction (in this case the fall of anti-hydrogen atoms in the Earth's gravitational field), which never has been done before.

Why not astatine? Why not flubber or cavorite? "Never been done before" is nice, but there are lots of experiments that have never been done before. ("Yeah, but it's never been measured on a Tuesday!") and that by itself is not a terribly strong reason. Life is short - as an experimenter I can only measure so many things.

Let me give an example of where "never been done before" worked: the isotope effect in superconductors. "The existence of a small quantity of Hg-198 at the National Bureau of Standards prompted us to investigate its properties as a superconductor." But this was a case where this truly was a new measurement: if isotope effects were searched for in lead, tin and vanadium and found to be absent (and with many orders of magnitude more sensitivity), "hey. let's try this in mercury" would have been at a low priority.

I think the GBAR experiment was set up, to verify that there is not a "small gap" between the behavior of matter and antimatter.

That is correct. They are looking for a 1% difference between matter and anti-matter. A good place to find this out (especially as this is an A-level thread) is the proposal.

Linking the two themes of this message together, at 1% they are still in territory that's been well-explored. But they think with a long run they can get to 0.1%. Things start to get interesting at 10^-3 or a few 10^-4. That's a region that is not strongly excluded by Pound-Rebka and torsion balance type experiments.

 

[vanhees71]

Well, my point is that nobody has ever checked whether antiparticles fall in the same way as particles in the gravitational field of the Earth. That's why I think it's worthwhile to do that experiment, though it's extremely unlikely that the antiparticles don't just behave as expected from GR or Newtonian gravity. So I think it's always worth checking whether there's something unexpected particularly for a case which has never been done before.

It's like with the magnetic moment of the proton, which was a big challenge to measure at the time, and most theorists thought it's not worthwhile the effort, because it's anyway clear that it will have gyrofactor of about 2, because it's just an elementary particle as the electron and thus described by the Dirac equation, which necessarily leads to that value. We all know that this is plain wrong, and Otto Stern was right to do this challenging measurement. A similar story can be told about the discovery of violation of space-reflection symmetry by the weak interaction. Famously Pauli wasn't interested in Wu's experiment, because it's anyway clear that parity must be conserved. We all know...

The example with the isotope effect on the superconductivity phase-transition temperature is of the other kind: In this case there was an effect predicted by BCS theory but challenging to confirm experimentally, but with the quoted experiment it was indeed confirmed. It's one of the empirical evidence that BCS theory is correct.

 

[Vanadium 50]

The example with the isotope effect on the superconductivity phase-transition temperature is of the other kind: In this case there was an effect predicted by BCS theory but challenging to confirm experimentally, but with the quoted experiment it was indeed confirmed.

History is different. The isotope effect was discovered by Maxwell (no, not that Maxwell ) in 1950. BCS was invented 1957.

I'm not saying GBAR should not be done. I am saying that it probes a parameter space already excluded by more sensitive experiments. The real value is in developing a technique that may allow one to probe beyond this region.

 

[Marilyn67]

Hello @Vanadium 50 ,

In fact, the real question is :

How sensitive is the GBAR experience ?
Can we comment on this question ?

Do we have data on the precison of the measurements ?
Can's provide details that we didn't have before?

 

[Vanadium 50]

How sensitive is the GBAR experience ?

That is correct. They are looking for a 1% difference between matter and anti-matter. A good place to find this out (especially as this is an A-level thread) is the proposal.

 

[Marilyn67]

Hello,

I understood that they are looking for a difference of 1% between matter and anti-matter.
My question is the following :
Can the precision of the GBAR experiment be better than 1%? (0,001% or better yet?)

Now I have a second question: I don't understand the correspondence between these two things :

There are extremely good (better than one part per trillion) measurements of the weak equivalence principle on half a dozen materials.

That is sufficient to exclude a "gravitational charge" on the antiproton opposite to that of the proton. If that were the case, we'd be seeing it now.

What are these materials ?
You mean that these materials contain antiprotons whose mass (supposedly negative) would influence the measurement in a noticeable way, which would eliminate this assumption ?

I understood well or not ?

 

[Vanadium 50]

Can the precision of the GBAR experiment be better than 1%? (0,001% or better yet?)

You need to read the proposal.
You posted this at A-level, which means you want a graduate-level answer. A graduate student should be able to find and read the proposal.

You mean that these materials contain antiprotons whose mass (supposedly negative) would influence the measurement in a noticeable way, which would eliminate this assumption ?

Of course not. Where did I say that?

 

[Marilyn67]

Ok, 1% !
Sorry, I had a hard day, no need to be unpleasant !

I don't see the connection between your half-dozen materials (wich ?) and the consequences on the antiproton's gravitational charge, sorry.

Good night, it's late here.

 

[PeterDonis]

I don't see the connection between your half-dozen materials (wich ?) and the consequences on the antiproton's gravitational charge

One way of describing the connection is that, according to quantum field theory, the properties of what we call "matter" depend on the properties of virtual antiparticles as well as on the properties of virtual particles. (Another way of saying this is that, according to quantum field theory, antiparticle properties are not independent of particle properties; the latter determine the former.) The "properties of matter" here include what you are calling "gravitational charge" (which is really a bad term since gravity is not a force in GR; a better term is "obeying the weak equivalence principle"). So when you do an extremely sensitive test of whether a piece of matter obeys the weak equivalence principle, you are testing whether the virtual antiparticles in that piece of matter obey the WEP as well. And if virtual antiparticles obey the WEP, so will real ones, like antiprotons made in an accelerator and subjected to tests like the GBAR experiment.

 

[Vanadium 50]

The proper A-level/graduate answer on how to judge an experiment's sensitivity is to read the proposal. That's where the best information is, written by the people who know it best. Further, it's a skill grad students should learn. If you don't want the grad-level answer, you shouldn't say you do by posting this as A-level thread. That will confuse everyone and won't get you want you want.

However, we're at an impasse. You don't want to read it, and I'm sure not going to type it in.

 

[hmmm27]

Is there a PF link explaining the "basic, medium intermediate, advanced" parameter of a post ? I originally thought it referred to the question, not the questor.

 

[Nugatory]

Is there a PF link explaining the "basic, medium, advanced" parameter of a post ? I originally thought it referred to the question, not the questor (which makes much more sense, if you think about it, of course).

https://www.physicsforums.com/threads/new-thread-level-prefixes-b-i-a.824360/
https://www.physicsforums.com/threads/filters-in-forums-b-i-a.824739/
https://www.physicsforums.com/threads/whats-the-deal-with-the-forced-prefix.827372/

 

[Marilyn67]

Thank you to you @PeterDonis, for your precise and consistent answer !

@Vanadium 50 : Next time, express yourself more clearly and less evasively .
I have nothing more to tell you.

Have a good day,
Marilyn