Therefore, how do we know distant galaxies aren't made of antimatter ?
Maybe there's an obvious answer, but as far as I know, the only thing we can know about these galaxies, we know because of the radiation they emit.
Rgd "anti matter galaxies", we do detect particle cosmic rays on earth that originate from outer space (outside the solar system), though I don't know either whether they might originate from as far as other galaxies.
However, gravity is weak compared to the other forces, so galaxies can be expected to eject fast moving matter in addition to radiation (just like our sun).
So, given my meagre understanding of physics, I wouldn't expect to have seen anti-matter galaxies without also seeing gamma radiation fields spanning the space between a matter and an antimatter galaxy as particles annihilate each other.
Most people believe antimatter attract just like matter, so it's a moot point.
I would see no reason for Newton's law to not hold true around our scale, so repelling means F=Gm1m2/r^2 which only makes sense if one of the masses is actually negative. Just the thought of it looks like fun.
I am also surprised.
While there does seem to be a lot of dark matter out there, if it were actually antimatter and had opposite gravity to normal matter one would imagine there would be a huge discrepancy between cosmologists' models and observation. The high-end supercomputer simulations and regressions cosmologists employ have good predictive power, even though we lack a full explanation for the phenomena we can observe. Cosmologists deal with such far-out concepts to begin with (by definition) that it's not as if they'd be averse to a concept like antigravity if it had predictive utility.
toys idly with desk magnets while thinking about it
Then again, given that photons are massless maybe there could be such a thing as a gravitational dipole...
The first property rules out baryonic antimatter as dark matter, since that would interact electromagnetically. The last two properties rule out anything that has a repulsive gravitational interaction as dark matter.
while we don't know whether it exists at all. Whatever phenomena are explained by DM are also easy explained by molecular hydrogen (i.e H2) which is almost not detectable. [The hydrogen we know about in the space is atomic - H, and there should be orders of magnitude more of molecular one than atomic when you have atomic at such interstellar space conditions]
to put it mildly, it is a very overreaching statement.
>Second, dark matter doesn't seem to behave the same way that baryonic matter behaves (normal matter clumps, dark matter doesn't appear to do so).
double whammy - we couldn't observe dark matter and we couldn't observe its clumps.
Or consider it another way - while it supposedly have gravitational attraction, and have no other strong interactions known, we somehow should suppose that its non-clumping, ie. gaseous/cloudy/spread-around state is still a normal thing.
Or I am wrong, in which case I'll happily pay out. I'm a layman, as if that wasn't obvious, and it will be a much-needed physics lesson. Win-win.
 If in Australia, payout will take the form of alcohol credits at an establishment of winner's choosing. Hey, I want to learn from my superiors :) If not, a crisp USD$100 note will be posted to any mailing address in the world, to be supplied.
I will happily spend more than $100 just on delivery if that is required to make a payout in liquor (local Swedish flavors) across the world. Crisp paper is fully acceptable as well :)
Doubt we will ever settle this though... I have painfully little faith in scientific breakthroughs.
The null hypothesis (and, in the absence of any strong evidence to the contrary, the stronger hypothesis) is that antimatter and matter respond to gravity in exactly the same way.
For example, if antimatter rises, there ought to be a statistical preference for black holes to emit antimatter.
One of the posters on that site said CPT violation would be another great thing to check. Hell yeah!
Don't worry. The design is impractical anyway - regulating the "anti-mass" would be extremely difficult. Just when you got it balanced, the kid would drink a glass of water and the board would sink into the ground.
Due caution would indeed be advisable when selling these gadgets, though. A matter-antimatter collision of 50kg would be equivalent to about 2.1 gigatons of TNT, or about 400 times more powerful than the most powerful hydogen bomb ever developed, so it would indeed caused a "considerable amount of damage".
Nah, you just need a miniature collider inside the board, manufacturing more anti-matter as necessary.
And obviously, it would be powered by annihilating matter/antimatter together, via a small Mr. Fusion at the back.
If a nuke fails, nothing happens. If antimatter containment fails, your entire arsenal of antimatter weapons go up at once.
For a similar reason, nitroglycerin is strangely unpopular for both civilian and military purposes.
It would be heinously expensive and would require the kind of energy input that only gigawatt-grade nuclear power plants could provide. Rather than using a complicated, failure prone and inefficient way to transform nuclear fission into destruction, it would be simpler and more effective to lob a nuke with the same amount of uranium or plutonium.
Hence, for your day-to-day megadeath needs, thermonuclear weapons will remain the tool of choice for the foreseeable future.
I guess thermonuclear weapons are cheaper, though...
We may still see some normal devices which use antimatter in one way or another.
To put things into perspective, 1kg (~2.2lb) of antimater + 1kg of matter = 2* (9×10^16 J/kg) of energy, TNT = 4.2×10^6 J/kg so that works out to 42,000,000,000 kg of TNT or a 100 mega ton bomb which is larger than the largest H bomb ever tested.
PS: The other issue is unlike gas which is fairly stable, any sort of containment failure = detonation.
the pieces of Universe that is more than 13B light years from us are moving with speed faster than light relative to us. The machinery behind it is space expansion. And by definition that is using energy less or equal to the universe's energy. So that is the start. The Alcubierre drive is the idea of how to use the same principle on much smaller scale. It is very doubtful that it would require more energy than moving the whole galaxies [which, let me repeat, are moving faster than light relative to us and to each other]
which have been observed a few times in colliders
How do you cool something at this scale? And how about when it can't collide with any normal matter?
although iirc Dark matter is something does not interact with light.
I'm pretty pessimistic that antimatter will be repulsed by gravity, because you would suddenly have a term that would have to appear for the potential energy of the now-flipped gravity field. It makes much more sense for it to be affected normally. It's an interesting question that we might as well look at, on the off chance that the science will be wrong (which is when it advances, after all), but I wouldn't hold your breath.
- I think all photons fall down in a gravitational field.
- you claim photons are their own antiparticle.
Hence, antiphotons fall down in a gravitational field. How then would antimatter fall upwards ~because~ it is the antiparticle of matter? Am I wrong in 'knowing' that all light bends the same under gravity?
That could finally put an end to the chicken-egg problem. There was an anti-matter chicken in the first egg and it had moved back in time to lay its own egg after it hatched!
So, umm, no.
Though strictly speaking, from a bomb-building point of view, it would be very difficult to design a weapon where matter-antimatter mixing happened more or less completely and simultaneously. Otherwise you "merely" have a series of uncontrolled multi-megatonne explosions in an increasingly large area, instead of a single multi-gigatonne explosion.
Disintegrating the device is a serious problem for nuclear bombs, because when they break themselves up, they leave otherwise good material unused. For an antimatter bomb, splashing the AM uncontrollably would, if anything, speed up the initiation. (After the AM and the normal matter around it is ionized, the particles home for the closest thing to annihilate with. AM basically does all the work of the weapons designer for him.)
"The long term storage of significant amounts of antihydrogen should soon settle the question of whether antimatter falls up or down."
One reason they would like to know if antimatter is repelled by gravity is that it could explain why the Universe is expanding at an accelerating rate.
Would this mean that if I am "holding onto" a chunk of antimatter heaver than myself I will fall up into space?
But yeah, one theory is that if you hold one that's more massive than you are, its repulsion would overpower your attraction, and bang, zoom, straight to the moon!
10g of antimatter yields about 10^15 Joules - i.e. 1 PetaJoule, or the equivalent of a 2+ Megaton nuke.
If you were holding onto a chunk of antimatter large enough to hold onto you'd be missing half of London.
That's what nukes do too. Turns out that dumping absurd amounts of light (various parts of the spectrum, but certainly including visible) into the surrounding area absurdly fast tends to fuck stuff up pretty good.
By contrast, if you were holding a lump of antimatter contained by an electromagnetic field, a power failure would result is the antimatter escaping confinement and annihilating with your body, releasing an unimaginably large amount of energy nearly instantly. This is what you might call a "fail-deadly" system, because a failure results in a decidedly unsafe state.
The civilizations of the galaxy call it... MASS EFFECT.