I was searching these boards for an answer to a friend’s question:
How does gravity get out of a black hole, if the speed limit of c applies to gravity?
One of the responses invoked “virtual particles.” Which I’ve heard of, but never really understood.
So what’s a virtual particle? My impression was that virtual particles were things like gravitons, are only theoretical but can mathematically be treated as particles for results consistent with observations.
But the thread implied that virtual particles behave differently from (real?) particles, having real effects at speeds faster than c.
Maybe if I got VIRTUAL stamped across my driver’s liscence I would never get another speeding ticket?
Start with Heisenberg’s Uncertainty Principle. The quantum nature of the universe says that you can only measure pairs of quantities with so much exactitude. The usual example is that you cannot measure both position and momentum simultaneously to full precision. (No more than h bar, which is, natch, Planck’s constant. It’s very, very small so we never see it in ordinary life. But it’s big compared to single particles.)
But there are actually several pairs of measures that full under this restriction. Another of them is energy and time. A particle with x energy can come into existence from the quantum vacuum purely through sheer quantum fluctuations, and normally the universe doesn’t care as long as it exists for less than y seconds. As long as x times y is less than h bar, that is. You can pack a whole lot of energy into this if the time is short enough. That’s why physicists think that the universe itself started from one such quantum fluctuation.
Of course, it’s a bit more complicated than that. What you usually get are pairs of particles, one the antiparticle of the other so that they almost immediately (within y seconds) come back together and annihilate one another. Modern versions of quantum mechanics and more advanced stuff like supersymmetry and M theories says this happens all the time around all particles, but we can ignore that for the moment.
Now let’s get to that black hole. There is an event horizon around the black hole. Anything that’s inside the black hole will fall into it and be captured forever. On the outside it may not be. So what happens when you get pairs of virtual particles created right at the event horizon? Sometimes one of them will fall in and the other escape, taking a bit of black hole energy away with it. That’s why Stephen Hawking announced that black holes will eventually evaporate.
Does this answer your questions? Not really, because I’ve never heard anyone say any of the things you quote. Virtual particles never move faster than the speed of light. It is not well understood how gravity gets out of a black hole (there’s a recent thread on this but it doesn’t give any answers) but gravitons and not virtual particles are probably involved.
So why did I give you that long buildup first? Because that’s the only connection between virtual particles and black holes that I’m familiar with and I’m assuming that your other thread was just getting confused about the real story.
Physicist Dopers are now welcome to clarify my mistakes and oversimplifications.
Virtual particles aren’t really particles, but sort of like fluctuations that exist beyond a possible measurement.
http://math.ucr.edu/home/baez/physics/ The Usenet Physics FAQ is an incredibly good resource for questions like these. Check it out.
The easiest (if slight oversimplistic way) to think of it is that quantum mechanics stops space from being completely empty as that would mean absolute values, so you get energy fluctuations as described above. Now energy can only exist in quantized packets or particles, so one way to model these flucuations is as particle-anti-particle pairs appearing, travelling short distances before meeting eachother again and anilhating each other.
The virual particles don’t travel faster than c, in the case of black holes, the pair appear above the event horizion, one falls in and the other escapes to become a real particle.