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Physics professor here. As others have said, this is a really cute idea in physics that touches on some neat properties of the universe (like fundamentally iden

by Steuard 13y ago

Physics professor here. As others have said, this is a really cute idea in physics that touches on some neat properties of the universe (like fundamentally identical particles, conservation of lepton number, and the effects of time reversal on particle properties), but: 1. It's more akin to philosophy than to science: this suggestion either has no testable consequences (or if it does, its predictions look obviously false: see below). 2. In all of our observations of the universe there seem to be many more electrons than anti-electrons, but this concept would seem to imply that the number should be exactly equal. You can try to get around that, but anything you do is a stretch.[1] 3. The formalism of quantum field theory naturally includes plenty of situations where electrons and anti-electrons form "closed loops" in time. (The classic example is a something like a photon giving rise to a virtual e-/e+ pair that immediately annihilates back into a photon, but they can get a lot more complicated.) Those closed loops would not be connected to the hypothesized "one electron" bouncing back and forth through all of time, so this idea would fail to explain why they also look identical to the rest. So in my book, the beauty of this idea lies entirely in the fact that it could be suggested at all. It doesn't actually match reality, but it does give some striking intuition about how particle physics works. [1] You could suggest that there is some other (unobservably distant?) region of the universe where antimatter dominates, but there's absolutely no evidence to support that suggestion. As another idea, the quote in the link suggests that "maybe the extra anti-electrons are hiding in the protons or something". But anything of the sort would seem to eliminate the whole point of saying "it's all one electron" (which always has the same properties, etc.).

seiji 13y ago
situations where electrons and anti-electrons form "closed loops" in time. A Quantum Loooooop! https://www.youtube.com/watch?v=2rjbtsX7twc&t=1m23s https://www.youtube.com/watch?v=2rjbtsX7twc&t=1m23s
Apocryphon 13y ago
"You could suggest that there is some other (unobservably distant?) region of the universe where antimatter dominates, but there's absolutely no evidence to support that suggestion." That would validate the modern day myths crafted by science fiction and comic book writers. Future generations will look back and ask, "How did they know?"
- Steuard 13y ago
  Ah, they got a lot of their ideas from us, and honestly, we got ideas from them, too. (Alcubierre's "warp drive" metric in general relativity was evidently inspired by Star Trek, for instance. And I seem to recall that Arthur C. Clarke invented the concepts of geosynchronous communication satellites and the "gravitational slingshot" maneuver that's routine in spaceflight today.) The cross-pollination between science and sci-fi is something that I quite enjoy.
rismay 13y ago
"It's more akin to philosophy than to science" Exactly. I'm sure Feynman had a beautiful mind and we should rebel in his thoughts. However, let's not mistake that for science. On a similar note, I had an idea where telescopes looking at very distant galaxies were just different states of the same galaxy - the milky way - as it evolved through time and through different space-time / gravity lenses.
- sillysaurus2 13y ago
  I'm sure Feynman had a beautiful mind and we should rebel in his thoughts. However, let's not mistake that for science. ... What? Feynman wasn't advocating this theory at all. "But, Professor", I said, "there aren't as many positrons as electrons." "Well, maybe they are hidden in the protons or something", he said. I would be shocked if there's a single thing Feynman has ever said in a scientific context that you could reasonably say, "Let's not mistake that for science."
VladRussian2 13y ago
> a photon giving rise to a virtual e-/e+ pair that immediately annihilates back into a photon i'm been wondering about validity of virtual particle trick - while it is preserves charge, etc... doesn't it violates entropy principle of the 2nd law, ie. a pair of e-/e+ seems to have less entropy than the original/resulting photon, thus e-/e+ always happy to produce a photon, while photon would never produce e-/e+ pair without "help".
- tgb 13y ago
  There's actually two photons involved, not just one: http://en.wikipedia.org/wiki/Electron%E2%80%93positron_annihilation http://en.wikipedia.org/wiki/Electron%E2%80%93positron_annih... (If there were just one produced by annihilation, you'd have great difficultly satisfying conservation of momentum and having that photon move at the speed of c (when looking in the center-of-mass frame).)
- Steuard 13y ago
  The glib answer here is that there's really no such thing as the 2nd law when you're talking about a small number of individual fundamental particles. The statement "entropy never decreases" is actually a purely statistical claim, and it only rises to its usual level of essentially unbreakable validity when you're dealing with macroscopic numbers of particles (the number 10^23 gets thrown around a lot, though it becomes an awfully compelling "law" even with just a few hundred particles in the mix). But as for your actual example, it's important to distinguish between virtual pair production and longer-lived pair production. A single photon can momentarily "decay" into an e-/e+ pair which then annihilates back into a photon, and in fact quantum field theory tells us that this sort of process is an intrinsic part of how photons travel through space. (Similar things happen for all other particles as well.) But conservation of energy and momentum during this process leads to weird effects like violations of the usual mass/energy/momentum relation of relativity, which is essentially why we call the e-/e+ "virtual" particles (or "off-shell"[1]). For a "real" annihilation between an electron and a positron, you'll always wind up with two photons in the final state (necessary to conserve momentum and energy). By the same token, to produce an e-/e+ pair you'd need two photons, not just one, and those photons would need to have enough energy to provide the mass of both particles and enough kinetic energy to satisfy momentum conservation. I wouldn't invoke the 2nd law here (as I said above), but the point you're making is still valid: because so much of the e-/e+ energy is "used up" in their mass, there is less "phase space" available (fewer possible momentum states) in that configuration than as two photons, so the probability of winding up in that state is correspondingly lower. (Also, an electron and positron are electrically attracted to each other and will naturally tend to meet up to annihilate, but two photons have no such tendency.) I'm not sure that any of that was interesting or useful, but there you go. [1] https://en.wikipedia.org/wiki/On_shell_and_off_shell https://en.wikipedia.org/wiki/On_shell_and_off_shell

by Steuard 13y ago

seiji 13y ago

situations where electrons and anti-electrons form "closed loops" in time. A Quantum Loooooop! https://www.youtube.com/watch?v=2rjbtsX7twc&t=1m23s https://www.youtube.com/watch?v=2rjbtsX7twc&t=1m23s

Apocryphon 13y ago

"You could suggest that there is some other (unobservably distant?) region of the universe where antimatter dominates, but there's absolutely no evidence to support that suggestion." That would validate the modern day myths crafted by science fiction and comic book writers. Future generations will look back and ask, "How did they know?"

Steuard 13y ago
Ah, they got a lot of their ideas from us, and honestly, we got ideas from them, too. (Alcubierre's "warp drive" metric in general relativity was evidently inspired by Star Trek, for instance. And I seem to recall that Arthur C. Clarke invented the concepts of geosynchronous communication satellites and the "gravitational slingshot" maneuver that's routine in spaceflight today.) The cross-pollination between science and sci-fi is something that I quite enjoy.

rismay 13y ago

"It's more akin to philosophy than to science" Exactly. I'm sure Feynman had a beautiful mind and we should rebel in his thoughts. However, let's not mistake that for science. On a similar note, I had an idea where telescopes looking at very distant galaxies were just different states of the same galaxy - the milky way - as it evolved through time and through different space-time / gravity lenses.

sillysaurus2 13y ago
I'm sure Feynman had a beautiful mind and we should rebel in his thoughts. However, let's not mistake that for science. ... What? Feynman wasn't advocating this theory at all. "But, Professor", I said, "there aren't as many positrons as electrons." "Well, maybe they are hidden in the protons or something", he said. I would be shocked if there's a single thing Feynman has ever said in a scientific context that you could reasonably say, "Let's not mistake that for science."

VladRussian2 13y ago

> a photon giving rise to a virtual e-/e+ pair that immediately annihilates back into a photon i'm been wondering about validity of virtual particle trick - while it is preserves charge, etc... doesn't it violates entropy principle of the 2nd law, ie. a pair of e-/e+ seems to have less entropy than the original/resulting photon, thus e-/e+ always happy to produce a photon, while photon would never produce e-/e+ pair without "help".

tgb 13y ago
There's actually two photons involved, not just one: http://en.wikipedia.org/wiki/Electron%E2%80%93positron_annihilation http://en.wikipedia.org/wiki/Electron%E2%80%93positron_annih... (If there were just one produced by annihilation, you'd have great difficultly satisfying conservation of momentum and having that photon move at the speed of c (when looking in the center-of-mass frame).)
Steuard 13y ago
The glib answer here is that there's really no such thing as the 2nd law when you're talking about a small number of individual fundamental particles. The statement "entropy never decreases" is actually a purely statistical claim, and it only rises to its usual level of essentially unbreakable validity when you're dealing with macroscopic numbers of particles (the number 10^23 gets thrown around a lot, though it becomes an awfully compelling "law" even with just a few hundred particles in the mix). But as for your actual example, it's important to distinguish between virtual pair production and longer-lived pair production. A single photon can momentarily "decay" into an e-/e+ pair which then annihilates back into a photon, and in fact quantum field theory tells us that this sort of process is an intrinsic part of how photons travel through space. (Similar things happen for all other particles as well.) But conservation of energy and momentum during this process leads to weird effects like violations of the usual mass/energy/momentum relation of relativity, which is essentially why we call the e-/e+ "virtual" particles (or "off-shell"[1]). For a "real" annihilation between an electron and a positron, you'll always wind up with two photons in the final state (necessary to conserve momentum and energy). By the same token, to produce an e-/e+ pair you'd need two photons, not just one, and those photons would need to have enough energy to provide the mass of both particles and enough kinetic energy to satisfy momentum conservation. I wouldn't invoke the 2nd law here (as I said above), but the point you're making is still valid: because so much of the e-/e+ energy is "used up" in their mass, there is less "phase space" available (fewer possible momentum states) in that configuration than as two photons, so the probability of winding up in that state is correspondingly lower. (Also, an electron and positron are electrically attracted to each other and will naturally tend to meet up to annihilate, but two photons have no such tendency.) I'm not sure that any of that was interesting or useful, but there you go. [1] https://en.wikipedia.org/wiki/On_shell_and_off_shell https://en.wikipedia.org/wiki/On_shell_and_off_shell