The God Photon

**gonegahgah** · Jun17-09, 10:31 AM

I've read somewhere that a single photon is traveling backward and forward in time and that this is supposed to be every photon that exists?
1) Did I read that wrong?
2) Is this part of current science lore?
3) Is it just a proposed idea?

May I just let you know why I am asking too. (Curiosity is a terrible beast; especially when you are only an outside observer).
I was wondering, if it were so, how we would not be awash in photons filling every conceivable place.
I don't want to ask to try be a hijacker or anything so I thought I should explain why I was asking.

**gonegahgah** · Jun17-09, 10:45 AM

Sorry, that might have been electron I just remembered. Correct me please.
There was supposed to be something going backward and forward through time.

**HallsofIvy** · Jun17-09, 10:55 AM

"Photon"? The way I heard a similar concept, due to Feynman, in regard to the electron and positron: Draw a chart using the horizontal axis as an "x-axis" and the vertical axis as "time". Draw a slanting line segment, say from lower left to upper right, representing an electron moving to the the right. Draw a second line, from lower right to upper left, connected to that first line segment, representing a positron moving to the left. The point where both lines terminate is where the electron and positron annihilate each other, giving off a burst of energy. Of course, that positron had to "orginate" somewhere- the creation of an electron-positron pair. So from the lower-right end of that positron line segment, draw a new line segment going to the upper right, representing the electron that was created along with the positron. We can repeat that as often as we like.

If we move a horizontal straight edge up the paper, each point where a line segment crosses our straight edge is an electron or positron at that point, at that time. But we really have a "broken" line across the two dimensional page. We can think of that as representing the motion of a single particle, the "right-up" segment, an electron moving forward in time, the "left-up"= "right-down", a positron moving forward in time= that same particle moving backward in time.

I'm not sure how serious Feynman was about that but it does explain why all non-charge properties of electrons and positrons are the same. It even explains why all electrons are identical- there is really only one electron! The idea that there exist one electron "bouncing" back and forth in time does NOT imply that we would "be awash" in electrons.

Since I know no case of two photons annihilating one another, I don't know how that might be applied to photons.

**Mute** · Jun17-09, 10:56 AM

You're probably thinking of an electron. As far as I know, no one really takes the idea seriously, as I've only ever heard of it in one place, a book about or by Feynman (I don't remember if it was one of Feynman's, or if it was in Gleick's "Genius").

As I recall, Feynman's advisor, Wheeler, propsed the idea that there was just one electron travelling back and forth through spacetime, and since in some sense electrons moving backwards in time are positrons, it was both a positron and electron. I think the idea was, and I'm going to use my own words to explain it here: suppose you're in a room and you see a fly buzzing all over it and you can see the path the fly leaves behind. Choose one of the dimensions of the room to represent time, and then take a sheet of paper in the way of one of the paths the fly has travelled. Its path will intersect the sheet of paper at numerous points - those points where it's going "forward" in time are regarded as electrons, and those where it's going "backwards" in time are positrons. This is how one electron could be viewed as many electrons and positrons. And since they're point particles, with a lot of room in spacetime to travel, they will not necessarily have covered all of space... I suppose.

Anyways, I don't think the idea is to be taken seriously. I don't remember why it was abandoned; perhaps because it would seem to imply an equal number of electrons and positrons, which isn't the case.

Edit: beaten!

**A.T.** · Jun17-09, 11:26 AM

I think that was just a joke by Feynman.

If I remember correctly the idea is that, particles and the corresponding anti-particles are advancing trough time in opposite directions. But it actually means their proper-time not the observers time, so they are not duplicates of the same particle. Going back in proper-time has nothing to do with "Back to the Future" duplicating Martin McFly, and is far less spectacular:

If you think of proper-time as a dimension orthogonal to space, you have:
- light advancing only trough space (no proper time)
- matter advancing trough proper-time in one direction
- anti-matter advancing trough proper-time in the opposite direction

This picture fits well with the fact that anti-matter behaves like mirrored matter (right hand rule for matter becomes left hand rule for anti-matter): Anti-matter is matter mirrored along the proper-time dimension.

If you look on a space proper-time diagram:
http://www.adamtoons.de/physics/relativity.swf
you can easily see how length contraction is a result of projection onto the space dimension. If you set the speed to 1c (red slider) the projection has 0 length. Now imagine you could change the direction in space proper-time beyond this point, so the the rocket would be advancing in the negative proper-time dimension: The length of the rocket in space would increase again, but it would be mirrored.

You cannot do that in reality, but when particles were created they could go either way in proper-time. Besides being mirrored in space, the direction in proper-time doesn't have much effect. Unless you hit someone going in the opposite direction in proper-time. Then you both end up as light not moving in proper-time anymore.

**gonegahgah** · Jun17-09, 11:59 AM

Thank for those answers. I'll take it as not true then from that.
If anyone has grounds to disagree let me know so I get it right.

**Max™** · Jun17-09, 12:15 PM

I assumed he was speaking of advanced and retarded wave solutions.

Feynman diagrams admit both solutions, with the difference between advanced waves move upstream towards the past, retarded waves move downstream towards the future.

So you could imagine a photon traveling across your room and striking your retina, which then sends an advanced photon upstream into the past to tell itself where it went.

How literally you take it is up to you, it is simply a valid description permitted by Quantum Theory, which Wheeler noted could be applied to Electron/Positrons, and Feynman absolutely loved the idea of it so much he mentioned it at his Nobel ceremony.

**gonegahgah** · Jun18-09, 06:53 PM

Just another question out of curiosity?

Electrons are all identical it has been explained here.
Are the other particles different or the same to their own? Protons, neutrons, photons?

**HallsofIvy** · Jun19-09, 07:32 AM

I don't think anyone is ever quite clear on when Feynman was joking!

**Buckethead** · Jun24-09, 02:43 AM

Originally Posted by gonegahgah

Just another question out of curiosity?

Electrons are all identical it has been explained here.
Are the other particles different or the same to their own? Protons, neutrons, photons?

This is interesting because an electron going back in time, then forward in time but in a different location would appear as the same electron being in different places at the same time. Is this the same electron? What if the same thing happened to a human. Would you say that there were two of the same person, or one person being in two different places at the same time?

**Buckethead** · Jun24-09, 02:46 AM

Originally Posted by Max™

I assumed he was speaking of advanced and retarded wave solutions.

Feynman diagrams admit both solutions, with the difference between advanced waves move upstream towards the past, retarded waves move downstream towards the future.

So you could imagine a photon traveling across your room and striking your retina, which then sends an advanced photon upstream into the past to tell itself where it went.

How literally you take it is up to you, it is simply a valid description permitted by Quantum Theory, which Wheeler noted could be applied to Electron/Positrons, and Feynman absolutely loved the idea of it so much he mentioned it at his Nobel ceremony.

I've read about this "photon going upstream" idea before. I think it was used as an explanation for some of the paradoxes that show up in single photon dual slit experiments, especially the ones involving delayed decisions. It seems like a sensible theory.

**Tac-Tics** · Jun26-09, 02:38 PM

Does it really matter how many there are? What interesting, yet to be discovered properties would this allow? It's not as if by capturing a single photon, you could steal all the light from the rest of the universe!

**A.T.** · Jun26-09, 05:05 PM

Originally Posted by Buckethead

This is interesting because an electron going back in time, then forward in time but in a different location would appear as the same electron being in different places at the same time.

No it wouldn't. Read my post #5. It would become a positron when going in the other direction in time.

**diazona** · Jun27-09, 09:15 PM

Originally Posted by Mute

You're probably thinking of an electron. As far as I know, no one really takes the idea seriously, as I've only ever heard of it in one place, a book about or by Feynman (I don't remember if it was one of Feynman's, or if it was in Gleick's "Genius").

As I recall, Feynman's advisor, Wheeler, propsed the idea that there was just one electron travelling back and forth through spacetime, and since in some sense electrons moving backwards in time are positrons, it was both a positron and electron. I think the idea was, and I'm going to use my own words to explain it here: suppose you're in a room and you see a fly buzzing all over it and you can see the path the fly leaves behind. Choose one of the dimensions of the room to represent time, and then take a sheet of paper in the way of one of the paths the fly has travelled. Its path will intersect the sheet of paper at numerous points - those points where it's going "forward" in time are regarded as electrons, and those where it's going "backwards" in time are positrons. This is how one electron could be viewed as many electrons and positrons. And since they're point particles, with a lot of room in spacetime to travel, they will not necessarily have covered all of space... I suppose.

Anyways, I don't think the idea is to be taken seriously. I don't remember why it was abandoned; perhaps because it would seem to imply an equal number of electrons and positrons, which isn't the case.

Edit: beaten!

Yes, that's exactly the story I'm familiar with as well.

Unless somebody discovers a universe-sized blob of antimatter, it's just a story, not to be taken too seriously.