Antiparticles move backwards in time?!

[AlP]

Feynman said this of antiparticles.

The backwards-moving electron when viewed with time moving forwards appears the same as an ordinary electron, except it's attracted to normal electrons - we say it has a "positive charge." (Had I included the effects of polarization, it would be apparent why the sign of j for the backwards-moving electron appears reversed, making the charge appear positive.) For this reason it's called a "positron." The positron is a sister particle of the electron, and is an example of an "anti-particle."

This phenomenon is general. Every particle in Nature has an amplitude to move backwards in time, and therefore has an anti-particle. When a particle and its anti-particle collide, they annihilate each other and form other particles. [...]

- QED (1985)

Feynman was known for pulling peoples' leg though. Was he in this case?

One can view this as an electron moving forwards in time colliding with a electron moving backwards in time and forming a photon. Or one can view it as an electron moving forward in time absorbing a photon and becoming a positron right? Causality isn't broken here? Surely, you're joking Mr Feynman!

If Feynman isn't pulling my leg again, then the existence of the backwards moving electron at the point of annihilation is predetermined? Not only its existence, but from the momentum of the original electron and the momentum of the resulting photon, by conservation of momentum, the direction and momentum of the electron from the future is predetermined?

Physics just made my head explode again. Brains splattered everywhere. What a mess.

[Sophus]

I've studied Physics some but have never come across this so I probably won't be of much help at all.

Backwards in time doesn't seem plausible to me. Slower in time, perhaps. But not backwards.

[PipeBox]

It's not actually moving backwards through time, it just has opposite spin and charge.  He's just telling you that an electron would look just like a positron if it were set on "rewind".  This, however, reminds me of a humorous part of Michio Kaku's book Parallel Worlds where he was awoken by some other physicist at some ungodly hour of morning to hear the guy piping "I KNOW WHY ALL ELECTRONS [and positrons] LOOK THE SAME!  THEY'RE ALL THE SAME ELECTRON!"   Problem is, then, that for it to be the same one, every apparent electron must have a corresponding positron.  For every zig in time, there must be a zag.  Since we do not see an equal number of the two, this cannot be the case, unless all the positrons do exist but are outside of our ability to observe them (some part of the universe beyond our light cone).  They would have to be completely separate from normal matter, as matter-antimatter reactions en mass would make their presence obvious.  Incidentally, there's no problem with the the one electron/positron constantly killing itself, either, since these would be different instances of that electron.  Anyway, I'm a lay person, too, so take what I say with a bit of salt but I haven't read any research suggesting positrons are moving backward through time.

[SSY]

One can view this as an electron moving forwards in time colliding with a electron moving backwards in time and forming a photon. Or one can view it as an electron moving forward in time absorbing a photon and becoming a positron right? Causality isn't broken here? Surely, you're joking Mr Feynman!

The second case is impossible, as electric charge is not conserved. The first case does not even really make sense, how can you collide with something moving backwards in time? Pipe has it right about the rewind way of looking at it.

When one draws a feynman diagram, we but the arrows on anti particles backwards, but it does not mean they move backwards in time, we just have the inputs of a reaction on the left, and the outputs on the right.

[AlP]

Yay! The universe still seems absurd but not so absurd that I can't understand how absurd it is. . Wait... Can Pipe or SSY explain the Twins Paradox in a way that doesn't involve multiple universes? I think it has something to do with inertia but the fountain of all knowledge (wikipedia) isn't making sense =(.

[Will]

We must go to the Devron system!

But seriously, I'm pretty sure the answer is "we don't know yet". No, I don't think Richard Feynman was pulling anyone's leg, but I think this was more about the dreaming that those special few physicists are capable of more than demonstrable science.

[SSY]

The twins paradox is only brought about through incorrect application of SR rules.

SR applies to inertial frames only, when the twin on the ship sets off, and then returns, he changes inertial frames (acceleration). Since the other twin is not accelerating during this time, we are ok to specify the traveling twin as the one that undergoes time dilation effects. It is then simple to calculate the degree to which the traveler will have aged less than his twin by.

[iNow]

In regard to the thread title, it's been mostly addressed, but I may be able to put it more simply.  When modeling and measuring particles moving forward in time, they are completely indistinguishable and the same as their antiparticle moving backward in time.  It's a description... a way to show they are similar, just with one marked difference... their time arrow.  It's the math which bears this out.

Have you ever heard the saying, "Six to one, half a dozen to another?"  That's really all that's going on here.

So, the positron moving forward in time looks exactly the same (in our models and measurements) as an electron moving backward in time.
An electron moving forward in time looks exactly the same (in our models and measurements) as a positron moving backward in time.
This applies to all particle/antiparticle pairs.  If one moves forward, it is indistinguishable from it's anti-partner moving backward...

In fact, it very well could be an antiparticle moving backward in time... There's no way to tell the difference, and our choice of forward/backward and particle/antiparticle is somewhat arbitrary.


As for the twin paradox... The passage of time is relative to the observer.  When you fly on a fast plane around the world, you are aging just a tiny bit more slowly than the people back down on the ground.  It's relatively inconsequential at such slow speeds (only a coupla hundred miles per hour), but becomes more important at speeds approaching the speed of light (~286,000 miles per second).  When you travel that quickly, the effects of dilation are more profound.

So... The idea goes that there are two twins on earth.  They are exactly the same age, and in the same reference frame.  One twin gets on a rocket and blasts into space traveling close to the speed of light, and his brother stays home on earth.  The guy in the spaceship doesn't notice any change.  Everything looks good, time passes like normal... that is, from his perspective.  The brother on earth, however, would notice his brother on the rocket aging more slowly.  For the earth twin, 10 years could pass, but for his brother on the rocket it would have only been 20 minutes... Something like that.  They both experience reality as if nothing has changed... It's just that the rate of times passage... when measured relative to the other twin... has changed... and they are both correct because of this idea that there are "no preferred reference frames," nor is there any "universal" or "absolute time."  Time is a function of your reference frame, not the universe as a whole.

Anyway, it's not really a paradox, just a strange outcome of the math... Math which has proven time and again to offer an amazingly accurate description of our universe on large scales.  These two links always helped me to better understand it:

http://faraday.physics.utoronto.ca/PVB/ ... radox.html
http://math.ucr.edu/home/baez/physics/R ... intro.html


If you enjoyed that flash animation above, there a lots of other really great ones available here: http://www.upscale.utoronto.ca/GeneralI ... ison/Flash