Posted on 02/28/2003 2:59:02 PM PST by sourcery
Yeah. That sourcery character would be a bit of a drag on me if we were yoked together, I suppose.
It was once thought that black holes did raise problems for entropy and the second law. Does a black hole retain all the entropy of all the stuff that it eats and if so how does it show it? That kind of thing. Even Hawking thought that BHs violated the second law.
Kip Thorne*, after many convolutions, describes the bottom line as follows: A black hole's entropy is the logarithm of the number of ways that the hole could have been made. This entropy does not disappear but is retained in the surface area of the hole's event horizon as the evaporation-radiation particle "atmosphere."
The second law is obeyed. The dim glow of a black hole may be a free lunch to local systems, but only in the same way the sun is to local systems. The entropy of the universe goes up when the black hole eats something. It goes up more as the BH evaporates.
* Black Holes and Time Warps, W. W. Norton, 1994.
Fascinating. So ZPE is not associated with the mechanism for EM wave propagation through a vacuum?
That's not what I said. What I said was that ZPE is not the medium in which EM propogation occurs. ZPE is a consequence of QED/QCD, not the other way around.
I'm not trying to criticize, I'm trying to understand. Can you give me a thumbnail sketch of EM wave propagation?
For the discussion, here are two signficiant zero point energy related articles:
THE CASIMIR FORCE, a 1948 theoretical prediction in which the seemingly desolate "vacuum" creates a tiny force between a pair of conductors, has been precisely measured for the first time. According to quantum mechanics, empty space (the "vacuum") is not truly empty but instead contains fleeting electromagnetic waves and particles that pop into and out of existence. However, when the vacuum is bounded by a pair of conducting surfaces, the only electromagnetic waves that can exist are those with wavelengths shorter than the distance between the surfaces. The exclusion of the longer wavelengths results in a tiny force between the conductors. To measure the Casimir force, Steve Lamoreaux, now at Los Alamos (505-667-5005), employs a torsion pendulum, a twisting horizontal bar suspended by a tungsten wire. The attraction between a gold-plated sphere and a second gold plate causes a small twisting force in the bar. By applying a voltage sufficient to keep the twisting angle of the bar fixed, Lamoreaux determined the force caused by the attraction of the plates. His results agree with theory to a 5% level. (Upcoming paper in Physical Review Letters.) Researchers previously measured the Casimir-Polder force (Update 122), a different but related effect in which the vacuum creates an attraction between a conducting plate and a neutral atom.
ZERO-POINT MOTION IN A BOSE-EINSTEIN CONDENSATE has been quantitatively measured for the first time, allowing researchers, in effect, to study matter at a temperature of absolute zero. According to quantum mechanics, objects cooled to absolute zero do not freeze to a complete standstill; instead they jiggle around by some minimum amount. MIT researchers (Wolfgang Ketterle, 617-253-6815) measured such "zero-point motion" in a sodium BEC, a collection of gas atoms that are collectively in the lowest possible energy state (Update 233). According to Ketterle, "the condensate has no entropy and behaves like matter at absolute zero." The MIT physicists measured the motion (or lack thereof) by taking advantage of the fact that atoms absorb light at slightly lower (higher) frequencies if they are moving away from (towards) the light. To determine these Doppler shifts (100 billion times smaller than those of moving galaxies), the researchers used a technique known as Bragg scattering. In this technique, atoms absorb photons at one energy from a laser beam and are stimulated by a second laser to emit a photon at another energy which can be shifted upward or downward depending on the atoms' motion towards or away from the lasers. Measuring the range in energies of the emitted photons allowed the researchers to determine the range of momentum values in the condensate. Multiplying this measured momentum spread (delta p) by the size of the condensate (delta x) gave an answer of approximately h-bar (Planck's constant divided by 2 pi)--the minimum value allowed by Heisenberg's uncertainty relation and quantum physics. While earlier BECs surely harvested this zero-point motion, previous measurements of BEC momentum spreads were done with exploding condensates having energies hundreds of times larger than the zero-point energy. (J. Stenger et al., Physical Review Letters, 7 June 1999.)
tpaine shows his stupidity. He should stick to his paid spamming on his drug legalization threads. His motto: Ecstasy in every pocket.
It makes sense to the uneducated laymen that has no concept of aerodynamics.
What do you mean: "Ooops"?
OK. Go ask 100 high school drop outs how a propellor gets it's thrust. Then ask them how a rocket gets it's thrust in outer space where their is NO air to push against! You will get 100 shrugged shoulders.
Are you saying momentum is not conserved and that the "pressure gradient" contributes but not in a way that conserves momentum? I think I will have to take issue with that.
Is there a problem using Webster's as an authority?
Oof. That's not what I meant, but I see why you might take it that way. The fault lies entirely with my wording.
An electric field is measured/defined by the intensity of the force (accelerative effect on mass) that is produced by the field. An electric field propagates as a wave. The wave is produced by oscillation (to and fro movement) of an electric charge. So an electric field wave is simply the change over time in the strength of an electric field, produced as a result of the oscillative movement of the charge that produces the field.
Magnetic fields are more complicated, but the analogous distinction exists between magnetic fields and magnetic waves.
How fields themselves work, and their relation to space, time, matter and energy, is the core subject of any unifield theory of physics. For a very good discussion, I recommend Brian Greene's "The Elegant Universe."
Magnetic fields are induced by currents and time-varying electric fields. Waves in an electric field make the field strength vary over time. Time-varying magnetic fields produce electric fields. Waves in the magnetic field make the field strength vary over time. It should be obvious, then, that electric field waves cause magnetic field waves, and magnetic field waves cause electric field waves. It is this process of A causing B causing A by which electromagnetic wave propagation occurs. An electromagnetic wave is both an electric field wave, and also a magnetic field wave. One wave causes the other, recursively.
You can see a graphical depiction here: Animation of the Propagation of Electromagnetic Waves
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