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Water Flows Uphill
An anonymous reader writes "The BBC are reporting James Dyson's new garden feature, a waterfall with water flowing uphill. Apparently, he wanted to recreate an Escher drawing."
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(-:Stephonovich:-)
"Who needs reincarnation when we've got parallel universes?" -Me
Since the 70's, there has been a cave on Tom Sawyer Island in Disneyworld in which water appears to flow uphill.
The Imagineers did it cleverly with a slanted room and no point of reference. Not as geeky, but a really cool effect nonetheless which amazed me back in the day.
"The pie shall be cut in half and each man shall receive.....death. I'll eat the pie."
are at the World of Escher. The man was a genius.
I'm not Seth.
If only antimatter was repelled by gravity. Antimatter is just normal matter with reverse charge and spin, so it obeys all normal physical laws. So-called "negative matter" would be repelled by gravity, but we don't know if it even exists or can be made.
The troubles here are two:
1) We have no experimental evidence as to how antimatter reacts to gravity (beond a couple of small ones where the externally-caused experimental error bars render the results statistically meaningless)
2) We don't know how gravity works. In GR, yes, antimatter has normal mass and reacts normally to gravity. But GR is not the last and final word on how gravity works, and several models otherwise fully consistent with known experimental data allow for anitmatter to be affected to a greater or lesser extent than normal matter by gravity, even to the point of sign reversal.
Since we have no experimental evidence and several potentially correct theories that give different answers, the only conclusion is that we don't know. The general opinion is that animatter is affected by gravity as normal matter, but we don't know that it is.
We don't have experimental proof yet, but we have overwhelming reason to believe antimatter fall down just like matter. You can work it out based on hysical constants and conservation of energy in a matter/antimatter annihilation. It is explained in this physics FAQ.
If antimatter is repelled by gravity then you either have a violation of conservation of energy, or physics constants are not constant.
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- - You can't take something off the Internet! That's like trying to take pee out of a swimming pool.
There can be no doubt, for solid physical reasons, that antiparticles behave identically to regular particles when it comes to gravity.
First of all, the only relevant physical quantity to determine how something is affected by gravity is its mass (and equivalently, in relativity, energy). That's practically the definition of gravity -- the force one body exerts on another by virtue of its mass. In physicist speak, the gravitational field "couples" to mass/energy. Any force having an origin in some other physical quantity is by definition not gravity.
Now we have plenty of experimental evidence -- eg from particle accelerators that antimatter has positive mass, just like regular matter. Indeed, antiparticles have IDENTICAL masses to their corresponding real particles. Therefore they must be affected in the same way as regular matter by gravity.
Secondly, in both relativistic and quantum frameworks, gravity can only be understood if it is always attractive. In other words, mass can only be positive. In quantum terms, this comes out of the fact that gravity must be "spin 2" field. (There's a nice book by Feynman on his attempts to come up with a quantum theory of gravity that explains why it has to be spin 2).
Thirdly, according to quantum field theory the vacuum is filled with "virtual" particles and antiparticles -- that's the zero-point energy of the vacuum. Now the whole point about the vacuum is that it's the lowest possible energy state. If anti particles had negative mass-energy, they'd be in a lower energy state than the vacuum, which means that they'd be stable compared to the vacuum and would not decay back into the vacuum.
If that were true, the universe would long ago have filled up with antiparticles...
The author of this post asserts his moral rights.