Does Antimatter Fall Up Or Down?

KentuckyFC writes "There are enough loopholes in the general theory of relativity to allow antimatter to fall up rather than down in a gravitational field. We've never been able to make enough of the stuff to do the experiment. But at the European particle physics laboratory at CERN, where scientists have been refining the technique for making antihydrogen, researchers are designing an experiment called AEGIS that will finally settle the matter. The idea is simple — fire a beam of antihydrogen atoms and watch which way they fall — but the details are fiendish (abstract). The answer should help solve a number of important conundrums such as why there is so little antimatter in our part of the universe and what the value of the cosmological constant is."

Comments

Confused

[wasted]

"There are enough loopholes in the general theory of relativity to allow antimatter to fall up rather than down in a gravitational field. We've never been able to make enough of the stuff to do the experiment. But at the European particle physics laboratory at CERN, where scientists have been refining the technique for making antihydrogen, researchers are designing an experiment called AEGIS that will finally settle the matter.

Or will it settle (or unsettle) the anti-matter?

Re:Confused

[Anonymous Coward]

how fast things can change:

Gravity affects matter and antimatter the same way because gravity is not a charged property and a matter particle has the same mass as its antiparticle.

So the above is no longer believed to be true?

Re:Confused

[complete loony]

There's a big difference between a belief that something is most likely true, and an experiment that removes all doubt.

Re:Confused

[K. S. Kyosuke]

Until now I thought that ouside the real of mathematics (where things can be proven and no further revision is possible, save for attacking the logic of the proof), there is no such thing as "an experiment that removes all doubt"?

Re:Confused

[dnwq]

An experiment that reduces doubt. Does that work for you?

Re:Confused

[glas_gow]

Yes, absolutely. Thanks! :-) Can you prove that it works for you mathematically?

Re:Confused

[smilindog2000]

While this is a reasonable guess, it's about the same as guessing heavier objects fall faster. Consider electron-hole pairs in a silicon lattice. They act very much like electron-positron pairs. However, electrons fall down, and holes fall up. To me, it would seem odd if anti-matter fell down.

Why is there so much matter around, and no anti-matter? Perhaps because they repel each other? There is some evidence that nearby galaxies are made of matter and not anti-matter, but the universe is very big, and time could be effected differently by anti-matter gravity (speeding up). Why are galaxy clusters accelerating in their separation from each other? Could anti-matter still be present somewhere, causing the acceleration? Why is matter in the universe so clumped together, and not more uniformly spread out? Could there be clumps of antimatter between the clumps of matter?

Evidence suggests that there simply is no anti-matter left in the universe, but it's fun to speculate upon implications of anti-matter falling up.

Re:Confused

[courtarro]

Consider electron-hole pairs in a silicon lattice. They act very much like electron-positron pairs. However, electrons fall down, and holes fall up. To me, it would seem odd if anti-matter fell down.

Holes are a virtual particle with no mass, whereas anti-matter has mass. Electrons actually exist, but holes are simply a place where an electron can fit. Take the example of a helium balloon in a car: if you slam on the breaks, it flies to the back of the car because all the air is rushing forward, and the balloon's relative vacuum gets pushed backward. A hole behaves similarly. Antimatter, on the other hand, has mass just like its matter counterpart and therefore isn't directly comparable to a hole.

Re:Confused

[mshannon78660]

Uh, no, actually, they don't. Because F=Gmm'/r (m being the mass of one object, and m' the other, G being the gravitational constant and r being the distance between them), and a=F/m (acceleration equals force divided by mass). When you substitute one for the other, you get a=Gm'/r - which is completely independent of the mass of the object being observed. Now, you could argue that (as you seem to) that the earth is also accelerating towards the other mass, leading to what seems to be a higher rate of falling - but (contrary beliefs about relativity aside), you can't really treat all of the acceleration as relative in that manner - that simplification of relativity only works for uniform motion - it is possible to detect acceleration (although you can also replace it with a gravitational field, but that would then invalidate our original formulas - can't change horses in mid-stream like that).

Re:Confused

[SilentBob0727]

The point of gp is that when you apply maxwell's equations in four dimensions, an antiproton is indistinguishable from a proton moving backwards in time.

If we maintain that causality only travels in the forward direction (not an unreasonable assumption to make), then you could actually solve this problem by saying the antiproton was, from its own frame of reference, annihilated, at the same time that it was created from your frame of reference, and vice versa.

Even more interestingly, when you consider that we travel through spacetime at the speed of light, you can think of the creation of a PP- pair as an antiproton "bouncing in time" off a burst of energy, one that is exactly equal in magnitude to the energy required to reverse the direction of a proton traveling at the speed of light.

Then, when you consider a recently-generated PP- pair that re-self-annihilates, releasing their combined energy, you can think of the same "bounce" in reverse, at which point, you have a single proton bouncing around in a game of nanoscopic temporal ping-pong!

It will fall down

[little1973]

According to GR, gravity is the curvature of Space-Time. As the anti-matter moves through space it has to follow this curvature. If it does not, that means GR is wrong (which may be the case, but I doubt this experiment will disprove GR).

Re:It will fall down

[thue]

Yes, Silly physicist PhDs doing unneccesary experiments. They could have been told the result of the experiment just by asking a random commenter on Slashdot. :)

Because our understanding of physics is so consistent that it is a waste of time to test the cornercases of our theories *cough*quantum gravity*cough*dark matter*cough*dark energy*cough*

:P

Re:It will fall down

[Anonymous Coward]

bollocks. having a faster-than-escape velocity doesn't make things fall upwards. it merely means they don't come down fast enough to hit the earth

Re:It will fall down

[something_wicked_thi]

Oh, dear. I think you should write these guys a letter before they waste all this time and money on an experiment with such an obvious flaw!

Re:It will fall down

[that_itch_kid]

GR is bunk. I find your lack of faith disturbing.

Re:It will fall down

[fireboy1919]

Didn't you have to take high school chemistry? What you're saying has quite a few problems:

1) That determines the rate of effusion. Molecules don't just go straight up. They bounce around. What actually happens in reality is that the force holding those molecules to the earth isn't actually enough to force it to happen. But the upward movement is going to happen slowly. You can still measure the effect that gravity has while this upward movement happens.

2) Also because it has to do with effusion, a *beam* of antiparticles in a vaccuum won't be affected by it. They're not going to bounce around and have effusion effects happen; it's going to be more like a batting practice machine - balls come out and curve, and are done the moment they hit something. This is obviously what they're going to do since antimatter is quickly eliminated in the presence of matter.

3) Even if that was a problem, it's not actually a problem at all temperatures and pressures. If you wanted to do an experiment where *normal* hydrogen didn't rise, just lower the temperature.

Re:It will fall down

[Anonymous Coward]

I'm a physics PhD and this is definitely one of the experiments where there is no reason to expect things to behave differently from the theory. Any reasonable theory already allows us to put low limits on the difference in gravitational behaviour between matter and anti matter and there certainly is no theory of gravity that I know of where antimatter "falls up". There are some where it might fall differently.

There also is some direct evidence that if you have differences they are not due to gravity:

Reference e.g.:

http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/antimatter_fall.html

"The only direct experimental result on antimatter and gravity comes from Supernova 1987A. This supernova in the Large Magellanic Cloud emitted both neutrinos and antineutrinos, some of which were eventually detected on Earth. Those neutrinos and antineutrinos took 160,000 years to reach Earth, and while travelling were bent from a "straight line" path by the gravity from our own galaxy. The bending with gravity changed the time needed to reach Earth by about 5 months, yet both the neutrinos and the antineutrinos reached Earth at roughly the same time (within the same 12 second interval). This shows that the neutrinos and antineutrinos "fell" similarly, to a very high level of precision (about 1 part in a million). [4] and [5] provide some background information on this."

Re:It will fall down

[jandersen]

An alternative explanation is of course that the anti-particles are distracted and therefore miss the Earth, as demonstrated in the well-known experiments performed by Arthur Dent.

Re:It will fall down

[Hankapobe]

Yes, Silly physicist PhDs doing unneccesary experiments. They could have been told the result of the experiment just by asking a random commenter on Slashdot. :)

Why not? This is where I get my IP legal advice!

Re:It will fall down

[geekgirlandrea]

But how does antimatter react to curved spacetime (could it 'roll uphill')...

That's what the experiment in the article is testing. Does antimatter react the same way to an external gravitational field as normal matter, or oppositely?

It'd be Big News if it turned out to be oppositely, though. General relativity describes gravitation in terms of space-time curvature; particles under the influence of gravity alone move along geodesics which only depend on their initial position and velocity. There isn't any way to accomodate different particles feeling gravitation differently in that framework. There are generalizations like Einstein-Cartan gravity to accomodate spin, but that just allows the connection to have an antisymmetric part, and doesn't change the fact that there's only one curvature for every particle to feel. The key axiom of GR is the equivalence principle, which states that, locally, there is no observable difference between gravity an accelerated reference frame. This requires that gravity accelerate every particle by the same amount, independent of any other particle-specific variables.

Put briefly, this has never been tested before, but it'd be a very big surprise if antimatter behaved any differently from normal matter, and would throw most current theories of gravitation out the window. It'd be like a modern-day Michelson-Morley experiment.

and how does antimatter (with mass) curve spacetime? (could it 'outdent' rather than 'indent' it)

That's a different question, and one that would be far more difficult to test. You'd need to gain a few dozen orders of magnitude of precision in measuring these things, or assemble a macroscopic chunk of antimatter somehow.

It'd also be a big surprise for a different reason. This is essentially treating antimatter as having negative mass and thus producing a repulsive gravitational effect. There's no deep reason why this would be mathematically inconsistent with GR, although it would have wacky consequences like perhaps the possibility of stable wormholes and FTL. In technical terms, it violates the weak energy condition. It's also unlikely for a different reason: conservation of momentum in GR requires inertial mass and gravitational mass to be equal, so for antimatter to produce a repulsive gravitational field like this would also require it to have negative intertial mass. It would respond oppositely to ordinary, non-gravitational forces, a positronium atom would have *negative* net mass (the electron and positron masses cancel, and the binding energy makes it negative), and a whole host of other consequences that would be readily observable but haven't been seen. Further, in quantum field theory having negative mass particles would create problems with vacuum stability.

So, both of these are possible in the sense that the experiment hasn't been done yet, so we don't know for sure they aren't true, but either one would invalidate huge swaths of physics and definitely qualify as Big News.

Obvious?

[neokushan]

It doesn't-matter.

Re:I hate "news" like this.

[Travis Mansbridge]

Brought to you by: Molten Boron
Nobody doesn't like Molten Boron!

Re:If light is affected normally by gravity...

[Tom]

And science is all about the difference between "I think..." and "I've tested..."

If it behaves exactly as predicted, you can make another mark and continue. If not, you've found something potentially very important.

Oooh... I can answer this one!

[njcoder]

Does Antimatter Fall Up Or Down? Yes!

Re:Oooh... I can answer this one!

[Idaho]

Does Antimatter Fall Up Or Down?

Yes!

But what if it turns out it falls sideways?

Re:Oooh... I can answer this one!

[teslar]

You may onto something. Perhaps the well-docmumented cases of people falling sideways are due to antimatter build-up within them. We should investigate.

Re:If light is affected normally by gravity...

[Urkki]

Antimatter *could* be different because the mathematics of GR allow it, and we haven't actually done the experiment before. I wouldn't put much faith in human intuition in these matters, considering how counter-intuitive entire GR is...

I mean, we see water falling off edges of waterfalls etc. Why should the edge of the world be any different? ;-)

Ah, so little imagination...

[meringuoid]

There are more options in particle physics than merely up and down.

Antimatter falls strange.

Re:Gravitons

[jandersen]

Gravitons are like photons: simply distortions in the underlying field. When two masses move relative to each other, the change in position corresponds to a change in the force between the two, but this change isn't communicated instantaneously. Instead the change travels as a distortion in the force-field - ie. a graviton (or several, as the case might be). This is what it means, intuitively at least, when they say that "the graviton mediates the force of gravitation"; and the same goes for the other mediators of force: photon, gluon and W- and Z boson. The perceived conflict is an artifact of limitations in the viewpoint of quantum mechanics.

The gravitational field as a scalar field surperposed on a flat space-time is just another way of describing gravitation - the curved geometry of general relativity is a better model, although it is more difficult to get a handle on. Perhaps it would be worth trying to tackle the other forces in the same way, as geometry in some sort of space-time. Perhaps we can even derive quantum mechanics as a special case of such a model; mathematics has certainly come a long way since the time of Einstein and Bohr, and it isn't unreasonable to hope that we are now approaching a situation where we can solve those old problems, that neither had the tools for.

Re:I wish it fell upwards

[pla]

If that were true, while it would have no practical use in the near future

Not necessarily - Merely opening that particular conceptual door would lead to a massive influx of funding and revisited anomalous past results.

Interesting thing about experimentation, even the most honest of researchers tends to throw away "bad" results (in the sense of not publishing them, not in the academically-dishonest sense of omitting them from the data). If the scientific community suddenly accepted the possibility of spooky-effect-X, you can bet that dozens or even hundreds of research groups would dredge up their past efforts to see if effect-X explains their results.

Case in point, l'Acedemie des Sciences and meteorites. Up to the turn of the 19th century, only idiots would dare claim that rocks could fall from space... Until the scientific community decided they could, at which point a huge body of past evidence appeared practically overnight supporting the existance of such falling objects.

Re:I wish it fell upwards

[krnpimpsta]

I wish the results were that antimatter falls upwards. If that were true, while it would have no practical use in the near future, it would be a hole in physics that our far descendants could exploit. Dude, please don't exploit holes in physics. I don't want my access to the universe revoked just because God banned us all from our reality for hax. All it takes is one noob hax0r particle physicist to ruin it for us all.

Antimatter powered airships, hurrah!

[itsdapead]

Just a few technical details to sort out, first :-)

...and, of course, if your antimatter-powered airship crashes, the phrase "Oh, the Humanity!" is going to be even more applicable. Maybe without the "the".

Tested at SLAC with positrons years ago

[franknagy]

As I rememeber, this has already been tested by drifting positrons down the length of the SLAC accelerator tube and measuring the beam deflection due to gravity (at least 20+ years ago).

Yes, anti-matter does fall down just like matter.

Re:Gravitons

[jandersen]

It is my distinct impression that what they mostly do is start from a classical physical model, which is then "quantified" by putting it through the magical transformation, where the classical Hamiltonian is turned into a differntial equation. I have no idea why this is done, nor have I ever met anybody who could explain it convincingly; but it seems to work. It is of course by no means a intellectually satifying method, which is what makes me wonder why nobody seems to seriously do anything about it. It is also, in my opinion, one of the reasons why quantum mechanics has always been plagued by quasi-religious mumbo jumbo like the Copenhagen interpretation.

No Loopholes in General Relativity

[mbone]

There are enough loopholes in the general theory of relativity to allow antimatter to fall up rather than down in a gravitational field.

Uh, no there are not. Gravity (or geometry, same thing in the theory) depends on mass energy in General Relativity. Stuff (with mass energy) follows the metric (the local geodesic). Even photons (which are their own anti-particles) follow the geodesic - and that has certainly been tested. Equivalence principle tests also show that different sorts of nuclear matter (including neutrons) individually follow the geodesic. Anti-matter certainly has mass energy, and (with matter) can be converted to photons and is no different in the theory. In other words "there is only one type of geodesics and there are no antigeodesics for antimatter."

The original article talks about "flavors" of General Relativity. Ain't so such beasties. Period. If you go to the real original article, you find a proposal for a 1% test of the equivalence principle for antimatter, and no such claims of flavors. Now, the equivalence principle has been tested to better than parts per trillion, and part of the mass energy in ordinary matter is made up of antimatter (in virtual particle pairs), so (based on the experimental evidence) I would claim that this test will be negative and is not actually that interesting as new physics. (The articles say that these older tests are "model dependent," but they are not model dependent enough to matter for this.)

That doesn't mean that this shouldn't be done (everything should be tested in physics, and different tests are always useful), but the prediction of General Relativity is clear : if anti-matter has anti-gravity, then General Relativity is wrong. The experimental evidence is also clear : this isn't going to be accurate enough to matter. Will make for some good public relations, though.

Re:I hate "news" like this.

[Anonymous Coward]

You mean apart from all of the air in-between you and the target?

Re:I hate "news" like this.

[SQLGuru]

Publicity is especially important in quantum physics because we don't know if they are working or not working until they are observed.

Layne

Re:Gravitons

[rumith]

I think that the parent means that neutral bosons have no antiparticles. While neutron is a barion and thus has an antiparticle composed of (~u ~u ~d) quarks, neutral bosons (such as photon and Z0) do not have corresponding antiparticles.

Re:The cosmological constant,

[Lijemo]

my relativity teacher told his class, is a function of time: At first, it was non-zero, then people said it was zero, then it might be non-zero after all.

I saw a paper in the Journal of Irreproducible Results advancing the theory that the age of the universe is a nonlinear function of time.

They plotted on a graph the age people (or rather, western civilization) thought that the age of the universe was at various points in history--when the 19th century geologists said it had to be at least hundreds of thousands of years old, when the 20th century astronomers said that it had to be even older than that-- and plotted the points on a graph. They formed a smooth curve demonstrating (I think) a geometric increase.

So their theory was that, assuming all the age-of-the-universe estimates were correct, that means the beginning of the universe is moving backwards in time, away from us. In 1000ad, the universe really was 6000 years old, and now it really is 14.5 billion years old, and in another century, it will probably be in the trillions of years old

(I love the Journal of Irreproducible Results!)

Re:I wish it fell upwards

[smaddox]

Yeah, like that time this Jesus guy found an overflow in the kill process routine, and was able to resurrect himself after 3 days.

I heard God banned him for 3,000 years.