# Is a matter-antimatter annihilation just a particle turning around in time?

1. Aug 26, 2010

### gk007

Is a matter-antimatter annihilation just a particle turning around in time, or is it a particle from the present being hit by a partcle from the future (because antimatter is matter going back in time)?

2. Aug 26, 2010

### Astronuc

Staff Emeritus
No, usually matter-antimatter annhilation involves transformations. For example, an annihilation of an electron with a positron results in two photons. In the case of a proton-antiproton, the reaction can produce various particles, e.g., a cascade of mesons (pions), which themselves decay to muons (and neutrinos or anti-neutrinos), which decay to electrons/positrons (and anti-neutrinos or neutrions).

3. Aug 27, 2010

### gk007

But how would that work using the theory that antimatter is just matter going back in time?

4. Aug 27, 2010

### fatra2

Hi there,

I don't think I understand where you get this from. From what I know, positron (anti-electron) has the same properties as an electron, just with a positive electric charge. It is not going back anywhere.

Cheers

5. Aug 27, 2010

### gk007

It was somthing I read in a book, I think it was by Michio Kaku

6. Aug 27, 2010

### nonequilibrium

gk, I've also read that somewhere, although I can't remember my source.

I'm actually not sure what it actually means for a particle "to go back in time", as time actually has no intrinsic direction (?) so "going back" doesn't seem to have a meaning. Maybe I'm missing something.

EDIT: this might be of interest: https://www.physicsforums.com/archive/index.php/t-127621.html

Last edited: Aug 27, 2010
7. Aug 27, 2010

### gk007

I found where I read it (Physics of the Impossible, Michio Kaku).

It says that there is an "Advanced Wave" solution to Maxwell's equation of light, which corresponds to a beam of light coming from the future and going to the past. Apparently it was Feynman who discovered that this meant anitmatter was normal back going back in time.

(on a side note, it is the "Retarded Wave" solution that represents normal light motion).

Here is a wikipedia article: http://en.wikipedia.org/wiki/Wheeler–Feynman_absorber_theory

8. Aug 27, 2010

### Bob S

The only time an electron-positron pair can be considered to be an electron going both forward and backwards in time is in the vacuum-polarization diagram as shown in the Feynman "bubble" diagram. Electrons and positrons are "created" at one vertex, and "annihilated" (to a single photon) at the other. The Feynman diagram (see arrows) is shown in

http://en.wikipedia.org/wiki/Vacuum_polarization

Bob S

9. Aug 27, 2010

### Naty1

Some mathematics suggests antiparticles move in the reverse time direction. If you consider d = vt, and plot in on a simple graph, all you have to do to get something moving "back in time" is to start picking negative t's....the question physical meaning does that have....

I do not think there is any experimental evidence for such time reversal; there IS experimental evidence for Astronuc's description. Our mathematics is generally time symmetric, yet that is NOT how our universe works.....the past appears different than the future. Just another example of how little we understand about the basics suchas time, space,matter, etc.

Still LOTS of cool stuff to discover...

10. Aug 27, 2010

### gk007

Thanks for clearing that up :)

I hope so...

On a side note, why does antimatter react with matter so spectacularly?

11. Aug 27, 2010

### humanino

The basic idea actually comes from Wheeler back in the days when Feynman was his student. Although it is mostly "cute", there is something really fundamental about it, as Wilczek puts it, he asked many of his colleagues "what is the most important lesson one can learn from quantum field theory which one does not know with quantum mechanics and classical relativist field theory separately ?". Apparently, only Dyson came up instantaneously with the answer "Why, but that all electrons are the same of course !".

In quantum field theory, all electrons are created as excitations of the same field. We can even arrange things so that a single electron going back and forth in time beyond our observable "laboratory" makes up all the electrons and positrons we see. We do not claim there is only one electron, but we do not claim either that it is not the case.

12. Aug 28, 2010

Staff Emeritus
Several people have said something of the form:

This is untrue. (Well, I guess it's only untrue if you mean "correct mathematics")

It has been known since 1964 that this is not the case: there is an experimentally measurable difference between matter moving backward in time and antimatter moving forward in time. This work got the Nobel prize, so one would think Prof. Kaku and other popularizers would be aware of it.

13. Aug 28, 2010

### nonequilibrium

Can somebody enlighten me on what is actually meant by "going back in time"? I can't make any sense out of it... Is there such a notion in Newtonian Mechanics? Something analogous? Of course the following idea can't be the correct interpretation (?): let an electron do its thing, stop the tape, rewind, and it has changed parity. If not that, then what is meant by it?

Thank you.

14. Aug 28, 2010

### humanino

You are strictly right, but I think this is not helpful in the context. Your argument applies to weak interactions. What we are talking about here applies to just free relativistic particle particles, even before one begins the chapter on QED.

15. Aug 28, 2010

Staff Emeritus
OK, let's turn it around. Why is it helpful to think of antimatter as matter going back in time? If it's a convenient fiction, what is convenient about it? Is there any phenomenon that is easy to explain in this way?

16. Aug 28, 2010

### nonequilibrium

Vanadium what do you mean with "going back in time"?

17. Aug 28, 2010

### humanino

Because it is the physical meaning of CPT invariance. It is the interpretation people make (after Wheeler suggested it to Feynman) for the necessity of anti-particles. For instance, we can understand in this manner dualities between :
• Compton effect and pair annihilation
• Moeller scattering and Bhabha scattering
• Bremsstrahlung and pair creation
• $e^+e^-\rightarrow \mu^+\mu^-$ and $e^-\mu^-\rightarrow e^-\mu^-$
They can be related (with analytic continuation) through crossing symmetries. Crossing symmetry is also valid at arbitrary order in perturbation.

18. Aug 28, 2010

Staff Emeritus
But it's not CPT - it's either T or CT, depending on how you want to look at it.

19. Aug 28, 2010

### humanino

Your original criticism, namely that we have not been very mathematical here, certainly holds. In particular, it has not been explicit that one must also reverse parity. If one does not reverse parity, then one may get a(n anti)particle which do not exist (such as a right handed neutrino, at least in the approximation that neutrinos are massless), or a process which does not occur, or occurs at a rate different from what one would get using crossing symmetry.

Anyway, I do not know whether it is really helpful to think in this manner, yet I think it makes it more clear what is essential (CPT symmetry) than Dirac's "hole theory" which is still used in some introductory lectures.

I mentionned Wheeler several times, to be quite honest it appears that the first to find that interpretation was Stückelberg. He was quite an interesting physicist, and Baron, his full name was :
Johann Melchior Ernst Karl Gerlach Stückelberg von Breidenbach zu Breidenstein und Melsbach
In his popular book "Facts and mysteries in elementary particle physics", Veltman describes

20. Aug 30, 2010