Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: distimpson on 01/05/2013 16:35:02
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ok, my very limited understanding of general relativity suggests gravity is described by curved space time so that all objects "fall" the same regardless of mass, including light with no mass. Since anti-matter does not have negative mass, why would this make sense?
http://www.laboratoryequipment.com/news/2013/05/antimatter-might-fall?et_cid=3227861&et_rid=54652410&linkid=http%3a%2f%2fwww.laboratoryequipment.com%2fnews%2f2013%2f05%2fantimatter-might-fall (http://www.laboratoryequipment.com/news/2013/05/antimatter-might-fall?et_cid=3227861&et_rid=54652410&linkid=http%3a%2f%2fwww.laboratoryequipment.com%2fnews%2f2013%2f05%2fantimatter-might-fall)
I'm certainly not saying don't make the measurement but this seems to be a bit off the deep end.
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There are a few reasons to hope it might be different. For example there apparently no large matter/antimatter collisions in space. I.E. no known anti-galaxies, anti-stars, or anti-planets.
Zero or negative gravitational interactions would be a potential explanation for this.
And, of course, the hope to find some kind of true anti-gravity particle.
However, there is increasing evidence that antiparticles have the same mass and gravitational interactions as normal particles.
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No, anti matter, if you by that mean anti particles, are of a same type of mass as particles. So it would then assume that mass have two directions relative gravity. I've seen some people wondering about it before but to me it makes as much sense as assuming that space exchange place with time inside a (non rotating) black hole.
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How would a pair production annihilate? If one of the particles took a quick look around saying "Ahaaa, I'm a anti, I'll go t h a t w a y" I can see his poor brother in vain trying to catch up with our runaway friend. Because he has too :) Which makes it quite weird.
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One theory from many years ago suggested that anti-matter might experience 2/3 the gravitational acceleration of "normal" matter. Unfortunately, the error bars on the Alpha experiment are still far too large to rule out this theory.
From what I have heard, most physicists today expect that antimatter should experience the same gravitational acceleration as normal matter, in the same direction. We must wait for more or better data.
It seems that the major uncertainties in this experiment come from the way they are trapping the neutral anti-atoms in a strong magnetic field (which takes a long time to decay), and has a large uncertainty in the particles's initial position and velocity. Perhaps they could:
- Use antimatter particles like antiprotons instead of antihydrogen. Antiprotons are much easier to produce than antihydrogen, so the number of experiments is larger, and error bars are smaller. You just need to ensure there are no stray electric or magnetic fields in the experiment, as these fields will apply more force to a charged particle than gravity does.
- With a charged particle, you can use a voltage trap. You can remove the voltage rapidly - you just need to ensure that the magnetic field from removing the voltage does not accelerate the protons...
- Use laser traps - the laser could be turned off almost instantly.
- Use a larger experimental chamber, so it can fall for more time.
- They already mentioned laser cooling to reduce uncertainty in the initial velocity
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The AEGIS experiment now being developed at CERN is expected to test the rate of fall of a beam of low-energy antiprotons.
I'd like to point out that there is no way to determine which particles are matter and which are antimatter. There is a convention which is followed where certain particles are defined as "matter" and the rest follow from that arbitrary selection as regards to what is called matter and what's called antimatter.
Why people would think that antimatter would behave differently in a gravitational field is beyond me. Why would I think that an electron would fall at different rate than an positron? They have the same inertial mass but opposite charge. Even the spin is the same.
That said, Ohanian and Ruffini have a paragraph on this topic in their text Gravitation and Spacetime - Third Edition on page 28. It reads, in part
It was pointed out by Schiff (1959) that it positrons had a negative gravitational mass ("fall upward"), then we would expect the nucleus to have anomalous ratios of mG/mI. The reason for this is that a nucleus is surrounded by a fluctuating cloud of virtual electron-positron pairs, a condition known as vacuum polarization. The number of pairs depends on the charge distribution of the nucleus, and if the gravitational mass of positrons were negative, nuclei surrounded by more pairs would appear to have lower gravitational mass. This effect would lead to a difference in mG/mI between platinum and aluminum amounting to a few parts in 107, in clear contradiction to Eotvos's experimental results.