Does Antimatter Fall Up? 255
New submitter Doug Otto sends word that researchers working on the ALPHA experiment at CERN are trying to figure out whether antimatter interacts with gravity in the same way that normal matter does. The ALPHA experiment wasn't designed to test for this, but they realized part of it — an antihydrogen trap — is suitable to collect some data. Their preliminary results: uncertain, but they can't rule it out. From the article:
"Antihydrogen provides a particularly useful means of testing gravitational effects on antimatter, as it's electrically neutral. Gravity is by far the weakest force in nature, so it's very easy for its effects to be swamped by other interactions. Even with neutral particles or atoms, the antimatter must be moving slowly enough to perform measurements. And slow rates of motion increase the likelihood of encountering matter particles, leading to mutual annihilation and an end to the experiment. However, it's a challenge to maintain any antihydrogen long enough to perform meaningful experiments on it, regardless of its speed. ... The authors of the current study realized that [antiatoms trapped in ALPHA] eventually escaped or were released from this magnetic trap. At that point, they were momentarily in free-fall, experiencing no force other than gravity. The detectors on the outside of ALPHA could then determine if the antihydrogen was rising or falling under gravity's influence, and whether the magnitude of the force was equivalent to the effect on matter."
Re:What am I missing? (Score:4, Informative)
The problem is there's something like a 1000. Total.
Actually measuring them accurately is a challenge, although no one in the physics community really expects the answer to be "they fall up" at this point. It would be a huge upset if they did.
Re:What am I missing? (Score:2, Informative)
No, hydrogen atoms in a vacuum are going to be more effected by nearby weak magnetic fields then by gravity. And even more so by what direction they where traveling. So you'd first need to slow them down which is hard to do when all you can use is magnetic force. Of course you could try to accurately measure there trajectory at two or more points and then figure out how gravity effected the atom. But it's hard to get real good measurements of atomic trajectories with strong magnetic fields. If we could make enough of and contain them; a simple balance scale would do.
Re:H goes up, anti H goes up, unless anti-N is pre (Score:5, Informative)
3 questions.
1. Hydrogen rises in gravity because it is less dense than air(mostly Nitrogen), So if there was no air/vacuum then hydrogen would fall towards the earth.?
I can't say anything to the other two questions, but this question is easily answered by something my high school physics teacher said to me. It has stayed with me since then as it is as eye-opening as it is obvious (in hindsight):
"The first mistake is to assume that helium rises. The truth is that it falls down towards the earth just like any other object. The reason for what you see is much simpler: It does not rise; it's just that everything else simply falls harder." (Freely translated from memory and German)
Helium only rises over the air, because regular air has the stronger draw to be below it. This explains why, in the absence of gravity; there is no lift. In the absence of a pull, the air has no impulse to displace the helium.
More generally, the same is true for liquid mixtures like oil/water. In gravity, the oil will rise above the water. In (close-to-)zero gravity, the oil and water will separate but stay where they are. That is because the water can't displace the oil without gravity pulling it more strongly down.
The same is true for solids. In meteorites with too little gravity, no submersion of the "heavier" elements like iron happen. This is why Earth has an iron core, but iron-rich asteroids have it distributed all over their volume.
Re:What am I missing? (Score:2, Informative)
When you put a "normal" gas in a container, room temperature imparts speeds on the atoms that would take several kilometers to reach the top of a parabolic trajectory assuming no other interaction with container walls or other gas molecules. In a distance of one meter the difference of speeds would only be about 40 ppm at best for such molecules, and the mean free path is much shorter.
If you put any gas though into a container such that the mean free path is longer than the distance it takes for significant action of gravity, it will not fill the container. This can be done by some combination of slowing the molecules down (making it colder) so it takes less time for a significant change in velocity, or making a container large enough and the pressure low enough that they have room to travel without interaction.
Re:I must be stupid (Score:5, Informative)
Looking at just the object's reaction to EM fields, you couldn't. But you could also observe its effects on charged bodies of known mass and charge, which would provide you with enough data to deduce the reality.
Incidentally, it's not a question of "inetia agains X fields" - it's simply that there are two quantities we call mass - inertial mass, which resists acceleration by *any* force, and gravitational mass, which creates and reacts to a gravitational field ("gravitational charge"). There are theoretical reasons why inertial mass will be constant regardles of the nature of the force, but no accepted explanation for why gravitational and inertial mass maintain a fixed ratio in all observed phenomena.It is *presumed* that antimatter has positive gravitational mass because the existence of negative g-mass particles would have some really wierd consequences, the possibility of perpetually accelerating machines not the least of them. A positive g-mass with a different ratio to inertial mass would be unexplained by current theory, but wouldn't really break things - we still don't actually have a very good theory as to the source of gravitaional mass effects.
Re:I must be stupid (Score:5, Informative)
However, this is not how we have traditionally defined anti-matter; the original definition was actually due to the fact that the universe has significantly less mass than it should, and "anti-matter" was hypothesized as an explanation.
Actually, the original modern definition of anti-matter was "Dirac's relativistic equation for the wave function of the electron had negative energy states as well as positive energy states, which was a bit weird, so it was proposed that all the negative energy states were filled, and if you knocked an electron out of one of the low-energy states, a "hole" would be left behind, and that hole behaved like an electron, except that it has a positive charge". It was later seen in the real world (particles moved in a magnetic field as if they had the mass of an electron and a +1 electrical charge). See, for example, the Wikipedia article about the positron [wikipedia.org].
Background (Score:5, Informative)
The question of whether anti-matter experiences anti-gravity goes back as far as I can personally remember (1970's) and probably some decades before that.
For most of the past 300 years in physics, experiment has led theory. We measure something, it leads to a theory, and then experiments are done to check the theory. Examples abound of theories that explain previous observations, and also predict something new - probably the most famous is relativity predicting the precession of Mars, but there are lots of others. (Newton predicting elliptical orbits based on the inverse square law of gravity comes to mind.)
Since about 1970 the situation is reversed - theory has led experiment. We have a satchel of theories which attempt to explain questions in physics which have no discriminatory evidence. Theories such as "Super Symmetry", "Loop Quantum Gravity", and "String Theory". I'm reading a book right now [amazon.com] which claims 10^500 different string theories (yes, that's 10 with 500 zeroes after it), and lamenting the fact that few of these actually make testable predictions.
Relativity predicts that anti-matter should have positive gravity, but this has never been tested.
Until recently, the only antimatter we had access to has been charged particles: anti-protons and anti-electrons. Measuring the gravitational force on a charged particle is nigh impossible because the EM force is so large (relative to the gravitational force) that any EM effects swamp the readings. You can't just see if the particle falls in the container, because it's essentially impossible to shield a container well enough. It's like trying to measure the mass of a cork floating in a tornado.
Anti-hydrogen would work, but until recently we had none to test. Antiparticles tend to have high velocities when produced - they have to escape their anti-nemesis which is also produced - so they have to be slowed down enough to "pair" to make the neutral antimatter particle.
The vacuum used for the experiments has a big effect also. Depending on the level of vacuum used, any particle has a "mean free path [wikipedia.org]" before it will impinge on another particle. You have to get your anti-particles to slow down, form antimatter, and conduct the experiment before another particle comes in and annihilates it. This requires insanely good vacuum which is both hard to achieve and highly expensive.
The ALPHA [web.cern.ch] experiment at CERN now produces antimatter, so the referenced paper asks the question: what is the ratio "F" between the inertial mass and the gravitational mass of antihydrogen? For normal matter it's 1 and for "antigravity matter" it would be -1.
The paper reports that they have measurements within specific confidence levels that F < 110 almost certainly, and F < 75 at the 95% confidence level.
If the experiments outlined in the paper are continued (and perhaps refined), over time they can statistically narrow the results and ultimately settle the question by experiment.
I think that this would be a good thing, it would confirm (or contradict) by experiment something that is predicted by theory.
Re: AntiGravity (Score:5, Informative)
Re:I must be stupid (Score:4, Informative)
Inertial mass and gravitational mass are observed - for normal matter - to be exactly equivalent. There's no actual reason they should be though, since they're the product of very different interactions
Well, if you believe General Relativity, they darn well better be equivalent. In fact, Einstein took the Equivalence Principle as one if his starting points when developing GR. If the Equivalence Principle fails (which it must if anti-matter falls up), then they will have disproven Einstein's theory, which would be very big news, indeed.
Re:What am I missing? (Score:5, Informative)
I'm an ion trapper, and though I don't work on this experiment, I've heard their group leader speak on exactly this topic a year or so ago, so hopefully I can do it justice from memory.
There are a couple challenges. One is "letting go". The atoms are trapped by very strong magnetic fields, and those have to be turned off rapidly to "let go" of the atoms. They turn off the superconducting magnet coils by heating them above their critical temperatures to make them normal-conducting and dumping all that energy into heat ("quenching" the magnet). Then the atoms are free to move around, but they weren't just sitting perfectly still in the traps, they had some thermal motion, which could fling them in any direction, including up. They've had trouble getting the atoms as cold as they had planned. They hoped they would be around 3 K, but I think they were stuck at 10 or 20 K for some unknown reason. So they aren't really just "dropping" the atoms. More atoms will go down than up if they are affected by gravity as expected, but it isn't remotely universal. Additionally their current trap is horizontal because the beam comes in from that direction, so there are only a few vertical cm in which to build up that bias.
Perhaps the bigger issue is actually knowing which way the atoms went. Their current trap was designed to do laser spectroscopy of atoms sitting in the trap, not tracking atoms as they fly around the beamline. What they do is wait until an anti-atom hits a surface and annihilates with a normal atom, and detect the radiation that is released from the annihilation. The radiation flies off in every direction though, so it takes some doing to build a radiation detection array that can reconstruct where in the apparatus the annihilations actually take place. As I mentioned, the current trap was not optimized for this particular study, so the reconstruction ability is pretty weak.
They are working on building the next generation of the experiment that will include a vertical trap, better detection arrays, and colder atoms, so that should be able to get to a better detection.
Re:Maybe our universe is a 'matter bubble' (Score:5, Informative)
The defining character of mass is not gravity, as that is merely a potential which exists when you have two or more massive objects or a massive object and a photon. The defining character of mass is momentum. As such, in order for antimatter to fall up, it must inherently have mass, but that mass reacts to the potential of gravity by being repelled.
Sure, for many calculations, using a negative mass number will make the vector equations work out correctly for Newtonian dynamics and Galilean translations involving matter / antimatter gravitational interactions.
Relativistic mass adjustments will need to use the E^2 - (pc)^2 = (mc^2)^2 equation or simply be redefined as the magnitude of mass (minor notational changes really).