Whey they say that particles come out of nowhere or call them virtual (as distinguished from actual or real?),
does this mean they are referring to another dimension that precipitates particles into our dimensions? Ie., they would seem to come out of nowhere, just like if you touched Flatland (two dimensional world) with a finger, the inhabitants would see a smudge or circle appear “out of nowhere.”
2) In any case, how can they say particles come out of nowhere when there is the Law of Conservation? Whether they come literally out of nowhere or just out of another dimension beyond our ken, they wouldn’t be part of the whole conservation of energy thing that we have in our dimensions!
3) And on top of all that, what about entropy! I heard that some “day” in the future the matter of the universe from the Big Bang will be all spread out into Uniformity of Total Entropy, so thin that it will approach nonBeing Itself. Then the next day Non-Being Itself will appear and there will be nothing. How does entropy in other words accord with the Law of Conservation of Energy or will particles constantly be appearing everywhere out of that Next Dimension as space gets stretched out?
Well, that would be the big question, wouldn’t it? If the various Laws of Conservation say that you don’t get something from nothing, it kind of begs the question of where everything came from, and if it came from somewhere, can more stuff come from there, say, tomorrow morning?
Conservation only conserves certain parameters. The particles appear in postive/negative pairs, such that the value of the parameters offsets each other. Thus, the parameters are correctly conserved.
Example (not really what happens at all, just to illustrate): An electron and proton appear. Total charge is conserved, since you have a +1 and -1, which equals zero added to the total charge of the universe.
No, the quick explanation is that particles are mealy concentrated energy. Energy concentrations are commonly modelled with wave functions that describe amongst other things thier probabilistic nature. One of the benefits of this model is that it helps explain ‘virtual particles’. They are basically statistical anomalies in the wave functions of the surrounding energy concentrations.
Conservation of energy is maintained the surrounding energy concentrations briefly drop their energy levels which is matched by the increase in energy to form the ‘virtual particles’ which then collapse (self annihilate) liberating the energy back to whence it came.
The law of entropy states, that, all energy tends towards a ground state (approximately 2.7 degrees K). Basically it represents an even distribution of energy in the universe rather than the small parcels of concentrated energy we have now. The same amount of energy jus distributed differently.
There are a couple of caveats here though. This assumes a non-contracting universe, which from what I last read seems a growing certainty. It also assumes no outside energy source (i.e. God or trans-universal phenomena). Oh and that black holes are not in fact acting as a permanent energy sink.
**
No, I don’t believe so. If there were other dimensions, certain of the conservation laws, especially conservation of energy, wouldn’t be true. In other words, it would be possible for, say, energy just to appear and disappear just like your finger in Flatland. For what it’s worth, string theory does predict that there are several other dimensions but that they are “curled up” very tightly.
**
Even though they can have effects in the “real” world virtual particles don’t violate any conservation laws or the uncertainty principle. In fact, it is these very laws that “allow” them to exist.
Didn’t notice the entropy part before.
You might be confusing “energy” with “useful energy”. In a state of total entropy, there is still just as much energy as ever. But because it is perfectly evenly distributed energy, it’s kinda useless.
Confused? Consider the towel energy (courtesy of Richard Feynman). Energy is like druness. When you go swimming, you change from dry to wet. You use a dry towel to dry yourself. Now you are basically dry, and the towel is basically wet. Actually, if you discount things like air conduction and sunshine and such, and just look at you and the towel, you are both equally dry. Total dryness is conserved. You are basically dry, but not totally. And you can’t use this towel to dry you anymore, because it’s just as wet. So you get a different towel to dry yourself more.
OK, I’ve murdered this analogy, but the point is, total entropy is like a universe full of nothing but slightly wet towels. There’s just as much wetness and dryness as ever. But you can’t dry anything anymore, because all towels are equally wet. Dryness is the same as ever, but usable drying ability is gone.
To amplify on this a little, a common explanation of why virtual particles are consistent with energy conservation invokes a form of the Uncertainty Principle. You may have heard something about how the latter prevents anybody measuring both the position of a particle and its speed (technically momentum) very precisely, at the same time. In fact, this is just one example of the principle and the more relevant example limits your ability to measure both energy and time. Because of this, nobody can experimentally exclude small violations of energy conservation, provided these last only a very short time. Virtual particles are just such a small, transient violation of energy conservation. No extra dimensions involved.
It can also be argued that virtual particles don’t exist at all; they’re just an “accounting device” that help in doing quantum calculations. While including them in the theories leads to effects that can be experimentally confirmed, we can’t really say anything about the virtual particles themselves. Whether they’re violating energy conservation or not is therefore somewhat moot - we can’t see them doing so if they are. What’s important is that the non-virtual particles, which we can observe, do respect it. (Most physicists find the idea of virtual particles far too useful as an intuitive explantion of things to be quite so “positivistic”.)
Bonzer please excuse me as this is a real nitpick, and I’m sure you were only simplifying
It’s not just that we can’t measure these conjugate values very precisely, the system actually doesn’t possess these values precisely. In other words if you know the momentum exactly then the particle doesn’t have a position.
Good point, Ring!
Now we get into the Wacky Weirdness of quantum mechanics. I would say that the particle does “precisely” possess these values, it’s just that our macroscopic view of “position” and “momentum” is a gross oversimplification.
Wave functions are often thought of as probability distributions. This is an extremely useful tool, but it doesn’t really describe what’s happening. In QM, momentum and position are expressed by wave functions that do accurately describe where the particle is and what its momentum is. It’s just that the particle is, in fact, “spread out” over a volume of space and has a range of momentum vectors. It’s not just that we can’t measure the position, it’s that the particle is simultaneously present throughout a volume of space. When you take a measurement of say, position, you collapse the wave function which actually “makes” the particle be in a smaller volume of space than it was before, with, of course, a corresponding increase in the momentum uncertainty.
This is probably more than the OP wanted to know. Nonetheless, since he mentioned “Flatland,” let me recommend an oft overlooked classic, “Mr. Tomkins in Wonderland” by George Gamow. This book (which is actually for children) is an extremely readable introduction to quantum mechanics. The book explores what the world would be like if Planck’s constant were very large and we had to deal with QM in our everyday lives.
You’re duly excused. I had hoped that use of the phrasing “you may have heard something about how …” was enough to indicate that some vagueness was in order and so forestall any pedentry well beyond the spirit of the OP. In vain, in vain … 
Thanks for the replies to my whey they say particles come out of nowhere, very interesting. I got something out of it. I’ve heard of wave functions but don’t know what they are, nor what collapsing them means. The only thing I know about functions are if you have x=2y then you claim that y is a function of x. But x is also a function of y, and for all I know each of them must be a function of 2, and 2 is a function of x and also a function of y. What I don’t get is what is the point of saying something is a function in the first place unless it means YOU CAN JUST SUBSTITUTE WHAT YOU SAY IS A FUNCTION FOR WHAT YOU SAY IT IS A FUNCTION OF?
However, that is an autohijack that I may resurrect in a new thread, so back to particles.Someone said that the virtual particles are only there for a short time, so we don’t have to worry about whether they abridge any laws against things coming out of nowhere. But this is like being just a little pregnant, isn’t it?
My distinct impression was that the term “virtual particle” was used for those resonances and such that were so short-lived as to be “instantaneous” on a quantum scale. I.e., they form and within a femtosecond they have broken down into other particles.
I once used the term “virtual subdivision” to define the transfer of land from one lot to an adjoining lot. I.e., lot A has a piece cut off it which is then deeded and consolidated into lot B. It may be a piece of property too small for the local zoning laws to allow as a lot, but it has no lasting existence as a separate lot, existing as a legal entity simply long enough for the two deeds to be filed. Does the analogy help?
Thank you for your forgiveness. The only reason I said anything at all is because I get so tired of hearing that the HUP is just an artifact of imprecise or observer altered measurement.
A lot of popular science books explain it this way and I think the authors should be shot. It clouds the fact that uncertainty is an inherent and crucial part of the way nature operates.
If I’m correctly remembering A Brief History of Time, the event horizon of a black hole can literally/physically/actually split he virtual particles apart so that they don’t annihilate each other. At that point, they’re more than mere accounting anomaly, right?
Virtual particles aren’t just an accounting anomaly. They have real effects in the real world. The process you describe, black hole evaporation (aka Hawking radiation) is just one and occurs when a pair of virtual particles form and one of the particles is sucked inside the event horizon. If the other particle escapes into free space, it appears to have been emitted from the black hole. The black hole decreases in mass by an amount corresponding the mass of the emitted particle.
If you think that’s weird, go check out the Casmir effect!
Polycarp
Virtual particles don’t turn into other particles. I think you’re thinking of some of the real, high energy particles that have very short half-lives. It’s true, however that virtual particle pairs exist for a very short time and that the amount of time any virtual particle pair can exist is inversely proprortional to its energy.
Don
We do have to worry if they abridge any natural laws. The point is, that they don’t! Conservation laws don’t prohibit virtual particles. Rather, it’s the conservation laws combined with the uncertainty principle that allow them to exist. These principles control the amount of time these particles can exist at which energies. It’s really an incredibly elegant (and surprisingly simple) system. If I had to sum it up in one sentence, I’d say, “Virtual particles exist because there’s no reason why they can’t.”
Since I was the one to use the phrase “accounting device”, let me repeat: while including them in the theories leads to effects that can be experimentally confirmed [and the Casimir effect is the best known example], we can’t really say anything about the virtual particles themselves. Literally so; their properties are never observables. What one then believes about them partially depends on one’s philosophical prejudices. As an extreme example of what can happen when people start arguing about the philosophical status of virtual particles, see the pair of papers by Weingard (who argues they’re ficticious) and Harre (who disagrees) in Philosophical Foundations of Quantum Field Theory (Oxford, 1988), ed. by Brown and Harre. Like most physicist I suspect, I find Weingard’s argument uncomfortable in its implications, i.e. can’t you extend it to quarks. The suggestiveness of talking about virtual particles as real things - as in Hawkings’ argument - is also enormously powerful in practice. And that’s pretty much Harre’s rebuttal. But ultimately Weingard’s view is very close to the widespread “all we can do is calculate the amplitudes and don’t worry what it means” position.