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seen Feb 22 at 9:32

Aug
26
comment Physical meaning of the sign basis in quantum mechanics
@user1247: After an ideal measurement the atom is in the state you measured, that's how an ideal measurement is defined. Before the measurement it probably wasn't.
Aug
25
comment Why can't electromagnetic radiation from magnets be used for apparent perpetual motion?
@orokusaki: If I understand your setup correctly, then yes, that should work. You are moving the magnets orthogonal to the force, therefore you're not doing any work, and thus the magnet should be moving forever. Or in field energy terms: The fielf always looks exactly the same, just at a different position, and since you don't accelerate, there's also no radiation. However, that's no perpetuum mobile, because you don't gain energy; it's no different than the magnet moving with constant speed where there's no other magnet (that is, no magnetic field).
Aug
25
comment Why can't electromagnetic radiation from magnets be used for apparent perpetual motion?
you don't take energy from the other object, but from the field. A nice example where you can see it quite directly is the electric field of a (charged, but not connected) plate capacitor. The two plates attract each other, therefore you need energy to move them apart. There's a homogeneous field in between then, whose energy you can calculate before and after moving the plates. If you do so, you'll find that the total energy in the field has increased exactly by the amount of work needed to move the plate against the field. So the plate's potential energy actually is the energy of the field.
Aug
25
comment Why can't electromagnetic radiation from magnets be used for apparent perpetual motion?
You cannot radiade a force. You radiate waves, or you radiate particles. The magnet is surrounded by a magnetic field, but it doesn't radiate it. Note that also an electromagnet doesn't radiate (except when you switch it on or off, and ignoring the heat radiation because of the wire's resistance). And it is true that you do work (and thus change energy) if you move a magnetic object in the magnetic field, that's something different from radiating energy. When radiating, you lose energy even without objects moving. Also note that when you move objects in the field of another object, …
Apr
15
comment Will a hole cut into a metal disk expand or shrink when the disc is heated?
@Mallik: But it would also try to expand along the ring, and if it moves inward, it has even less space. Therefore it has to move outwards in order to expand (with the outer parts moving outwards more).
Jan
30
comment What makes some laser beams visible and other laser beams invisible?
@Revo: Exactly. So any movie showing laser beams in outer space is simply wrong (unless in the future outer space is full of dust from all the space ships crossing it ;-))
Jan
18
comment Why does the water rise?
I'd question one thing from that answer, though: Nowhere is a vacuum created. There's always air in the glass, and it always fills the whole space not occupied by water. When the air cools down, it doesn't contract by itself, only its pressure goes down (intuitively: Since the molecules get slower, they hammer less onto the water surface). As result the water is pressed more in by the air outside than out by the air inside, and thus flows inside. This rising water compresses the air inside, which causes air density and thus pressure inside to rise again until equilibrium is reached.
Jan
16
comment How is the classical twin paradox resolved?
According to Misner, Thorne, Wheeler, Gravitiation, Chapter 6: "When spacetime is flat, move however one will, special relativity can handle the job."
Jan
16
comment Why is cold fusion considered bogus?
There's a big difference between cold fusion and multiverses: Cold fusion, if it exists, should be experimentally demonstrable (indeed, Pons and Fleischmann claimed to have done exactly that), but would violate current theories. On the other hand, multiple universes are not observable in principle, but don't violate any known law of physics, and can only be proven false by proving false one of the physical theories they are supposed to explain. Therefore the question of cold fusion is a physical one, while the question of multiverses is basically a philosophical one.
Jan
16
comment How do I express the Kepler general orbit $r(\phi)$ in rectangular coordinates?
I don't think you have to do the general calculation; however I think it is important that the reader is aware that the calculation is for a special case, hence my comment.
Jan
16
comment How do I express the Kepler general orbit $r(\phi)$ in rectangular coordinates?
Of course you've assumed here that the perihelion of the orbit is on the x axis. But then, the general case can be obtained just as easily by using the addition formula for $\cos(\phi-\phi_0)$.
Jan
15
comment Why does kinetic energy increase quadratically, not linearly, with speed?
Your argument is wrong: When the machine is standing on the car and trying to accelerate the ball, it not only put a force on the ball, there's also the recoil force onto the machine. The machine is fixed on the car, therefore the recoil force slows the car. That is, the car loses kinetic energy. Where does that kinetic energy go to? Well, it goes to the ball!
Jan
15
comment Why can't General Relativity be written in terms of physical variables?
Wouldn't the connection coefficients (Christoffel symbols) be the proper analog of the vector potential, rather than the metric tensor?