Is there such a thing as anti gravity to work with anti matter?
We’re almost certain it falls down.
The only way you’d get antigravity, or things which accelerate away from concentrations of mass and energy, is if you had what we call exotic matter, which we’ve never observed. We know antimatter exists, but we’re not sure about exotic matter.
The problem is that we’ve never had enough antimatter at the same place and time to run conclusive experiments. But, as I said, we’re mostly sure it would fall down at the same speed as normal matter.
Nobody’s ever been able to do the direct experiment of dropping a lump of antimatter and seeing which way it falls. The problem is that antimatter is made in such small quantities that forces other than gravity are almost always much larger than gravity, so the signal gets lost in the noise. But we have done indirect gravitational experiments with antimatter, all of which have come out exactly in the way they would with normal matter.
And I would go further than to say that we don’t know if exotic matter exists. If it does (or can) exist, physicists would be extremely surprised, since it would lead to a very large variety of very weird observable results that would be very noticeable, and yet which have never been observed.
All matter contains virtual antimatter. Einstein’s theory of gravity is based on the “equivalence principle” which posits that the quantity that gives rise to the gravitational force (gravitational mass) is equivalent to the quantity that gives rise to inertia (inertial mass). If antimatter behaved differently than normal matter, the equivalence principle would be broken. Experiments have verified the equivalence principle to less than one part in 10[sup]13[/sup], which is many orders of magnitude more sensitivity than needed to rule out antimatter with negative gravitational mass.
Photons act gravitationally as if they had positive mass. They either have no antiparticle equivalent or are their own antiparticles depending on how you want to look at it so it would be indeed strange if antimatter had a negative gravitational mass.
Not precisely. Photons behave as if they have zero mass. It’s just that gravity acts on massless particles, too (though not quite precisely in the same way).
True but needs more detail on about the indirect experiments. Most of our knowledge comes from testing various nuclear reactions that spit out antiparticles (and tests of that can range from Sophomore physics lab to the LHC).
Originally antimatter was a hypothesis to explain what looked like energy and momentum violations. Found actual antiparticles in later experiments.
An example is beta decay. Proton turns into neutron, electron, & anti-neutrino. Early experiments couldn’t detect the neutrino. The tiny difference in mass between p and n/e/neut becomes kinetic energy. Since we only saw protons, neutrons, and electrons back then, it seemed to violate conservation of energy and momentum (why are these particles moving slower than we can explain? Why is a stationary decay have two particles go off adding to non-zero momentum?) To make the math work out, the antiparticles have to have positive mass.
Antimatter: same mass, and still “positive mass”, opposite electric charge, and a tiny difference in antimatter vs matter’s response to the Weak force. Momentum, kinetic energy, gravity all work on antimatter just fine.
“antimatter has antimass” is an error that some current SF writers still have trouble, but I think the examples I most remember were by Heinlein. “We also walk dogs”?
(Purposely leaving out details on neutral antiparticles, possible differences between inertial and gravitational mass. Too complicated.)
All I meant was that photons are attracted to a mass as in gravitational lensing and not repelled. That’s true isn’t it?
They’re apparently testing this at CERN now.
Chopsticks, they’ve done more sophisticated experiments than that, too. For instance, send a beam of antineutrons through a two-slit experiment, then recombine the two paths. If one slit is higher than the other, then there will be a gravitational potential difference between the paths, which would lead to a phase difference and hence interference after the paths recombine.
OldGuy, yes, photons behave around masses in a way that’s better described as attraction than repulsion. But a better description yet would be to say that they’re following geodesics through curved space, a description that only becomes available when you’re using general relativity, not just a Newtonian approximation of gravity as a force.
Thanks for the great replies and elegant discussion. Mostly over my head but it does satisfy my curiosity about anti matter and gravity. I wish I could turn back the clock about 60 years and gotten some education on this.
An interesting question is whether matter and antimatter have the same gravitational attraction to each other as matter-to-matter or antimatter-to-antimatter does. Some speculative “fifth force” theories predict that there might be such an asymmetry.
From my understanding science does not know why they are mostly found on opposite sides of the universe from one another. I would imagine once they figure that out we will know a lot more.
What if the universe matter split off in roughly equal random amounts trillions of years ago. The process of annihilation has mostly settled down and what is left on the two sides is the normal distribution variances. Maybe 98% of the matter is long gone.
??? They have energy, so they have mass (don’t they?) They also have inertial mass, as in when an atom emits a photon, the atom recoils slightly in the opposite direction.
You’re not allowed to talk that way on the Straightdope. The modern convention is to only talk about rest mass. Relativistic mass (E/c[sup]2[/sup]) is verboten. Old geezers like me, however, still like the concept.
Relativistic mass is confusing for precisely this reason: What do you call a particle with no rest mass? “Massless” is convenient and extremely intuitive, but you need to drag in a lot more conceptual machinery to say "it has mass, but not the normal mass, because it only has mass when it’s moving, but it never stops moving… " and it sounds like a hand-wavy mess unless they already understand four-vectors, in which case they wonder why you didn’t just say “massless” and move on.
Remember: The people who grew up with “relativistic mass” were the ones who moved to abandon the concept in the first place!
Unfortunately for this hypothesis, there’s nowhere in the observable universe that appears to have NO mass whatsoever. Even intergalactic voids have some extremely rarefied gas. Which means that there should be some point of contact, and the process of annihilation wouldn’t have settled down enough that we wouldn’t be detecting it. In particular the 511 KeV signature of electron-positron annihilation would be readily observable.
Here’s a post from a past thread in which I quote posts from past threads with some discussion of this. To add:
In fact only 1 part in ten billion remains. That is, the processes in the early universe led to a tiny 0.1 ppb excess of matter over antimatter, and this tiny residual is all that remains today, making up everything(*) in the the observable universe. Why there should be an asymmetry of even this tiny magnitude is a major open question (as linked from “list of unsolved problems in physics”).
sub “Brace for nitpicks!”[/sub]
We finally have some experimental evidence to answer the question. It turns out that anti-matter is affected by gravity in the same way that ordinary matter is. It falls down rather than up.
Observation of the effect of gravity on the motion of antimatter | Nature.
ETA: I’m not sure why the link looks like that rather than the typical link to an article. Is there a way of correcting that?