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This article doesn't seem fair to physics. For instance, it says: "How can something move, and keep moving forever, without expending energy? It seemed an absu
by tedsanders 14y ago
This article doesn't seem fair to physics. For instance, it says:
"How can something move, and keep moving forever, without expending energy? It seemed an absurd idea — a major break from the accepted laws of physics."
I don't think that something moving forever is a major break from the laws of physics. Consider the following:
(1) An asteroid flies through space forever (doesn't violate laws of physics)
(2) A current persists in a superconductor forever (again, no violation)
(3) Heck, currents can ever persist in non-superconductors (http://en.wikipedia.org/wiki/Persistent_current http://en.wikipedia.org/wiki/Persistent_current)
(4) The motion of the Earth around the sun (the two-body problem doesn't violate physics)
(5) Even the motion of an electron around a nucleus is perpetual motion in a sense
(6) The fact that things have temperature means that their molecules are always moving!
(7) Etc.
Anyway, there are many examples of perpetual motion in physics. The key point is that you cannot extract infinite energy from them, just like you can't extract infinite energy from a time crystal. So why does the article act like time crystals are a big deal in this respect?
-A grumpy physicist
- ivybridge 14y agoI think you're confusing forever with a really long time. (1) Asteroid's velocity is eventually overcome by external forces. (2, 3) Structure decays. (4) Bodies converge or diverge. (5) Radioactive decay. (6) You are observing a high energy system. (7) Etc.
- tedsanders 14y agoI don't think I'm confusing forever with a really long time; I'm just assuming they're effectively the same. (Aside: nothing exists forever - even time crystals - because eventually the universe will be a bunch of infinitely far apart black holes or something. So I'm not sure how that assumption is relevant.)
- kaybe 14y agoThe distinction still makes sense, I think. There are particles with a decay time so long they're practically stable (e.g. bound neutrons) and then there are stable particles. The examples you gave above are forever; I just started assuming things like eternity and I can go on to assume no external forces/stuff in the way. (So not disagreeing with your initial statement, just adding.)
- prof_hobart 14y agoFor me, the difference is whether these things a) have an external force being applied to them, or b) are slowing down and are only going on "forever" because the rest of the universe will collapse before they come to a standstill. For example, even ignoring the fact that the sun is going to explode before the universe does, the earth's rotation around the sun is gradually getting slower and slower. If the sun, and the universe, did last forever then the earth would eventually come to a halt. In the case of these crystals, my impression is that even in that hypothetical never-ending universe, the movement in these crystals would never slow down.
- waps 14y agoActually there is no such thing as a stable orbit in Eindsteinian mechanics (except for 2 point masses orbiting eachother. There is no such thing as a point mass in reality). So the energy conversion from speed to distance and back is not 100% efficient and an asteroid (or the Earth, for that matter) is doomed to eventually fall into the sun or fly away from it forever. It cannot keep circling forever, even in a theoretically perfect situation. In addition to that, the tidal forces will compress and decompress the mass of both objects, which effectively converts potential energy (the distance between the two objectds) into heat. Even though the moon is on a trajectory to eventually leave earth orbit, every tide that comes in and leaves lowers the rotation speed of the moon. While the moon has no liquids on it's surface, which makes the effect nearly invisible, it does have tides (slight variations in the angle and magnitude of gravity at it's surface) which move objects around and the energy for that does indeed come from orbital decay. Similarly, the earth loses orbital energy to the sun.
- bencollier49 14y agoYes. F=ma. There's no v in there.
- solistice 14y agoyes there is, it's right in the v'(t)
- bencollier49 13y agoMy point is that acceleration requires force, but stable velocity requires no acceleration, hence no force.