# Time arrow of Antiparticle

1. Feb 24, 2010

### luxiaolei

Hi all, I have just learnt antiparticle today, my lecturer explained as put negative sign in front of time and also energy will give the result, he describes at microscopic level, time is not distinguishable, hence, the time arrow of particles can be forward or backward..

I am wondering, how? can anyone explain it abit more? thanks in advance!

2. Feb 24, 2010

### Frame Dragger

I don't know that anyone can explain that in physically realizable forms EXCEPT... for Unruh and Hawking Radiation.

Frankly, I think the best answers you'd get would be to do a bit of reading on those subjects; especially Hawking Radiation. It may or may not be a realistic representation of nature, but it might be if HR exists...

Hawking Radiation, if you think about it, is the production of a real photon from a 'virtual' pair at the Event Horizon of a Black Hole, right? OK, so one part of the 'pair' is lost, and the other may eventually 'escape'. As you say, all of that can be thought of as ONE particle with a single worldline extending from the EH (and presumably inside it), and we observe the portion of its history that we can.

From there, the rest follows pretty simply, and HR then Unruh really helps to understand the process in a 'toy' system.

3. Feb 24, 2010

### luxiaolei

@Frame Dragger, Thanks for replay, great explanation although I dont really understand it, too deep for me..but thanks:)

4. Feb 24, 2010

### Frame Dragger

Thank you, but don't sell yourself short, I'm not very good at this. It may be that one of the members here can give you the mathematical treatment, which seems to be the best way to understand it.

Anyway, we all start somewhere, right? Do a little research on The Unruh Effect, and Hawking Radiation (remembering that they are both conjectured, not proven), and you'll be on your way to getting those deeper concepts. It's how we all learn!

5. Feb 24, 2010

### cepheus

One of the most fascinating implications of this proposed by Wheeler was that there were only one electron in existence travelling forwards in time until it is converted to a positron travelling backwards in time and so forth, providing the illusion of many particles. If this was true however we should see as much antimatter as matter and is not treated seriously? A clever idea though. I am a great fan of Wheeler's ideas.

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

6. Feb 24, 2010

### Frame Dragger

Like Dyson the ideas are interesting enough to be worth the read, whether or not they have a physical reality. Mental calisthenics!

As for the matter/anti-matter, the current imablance isn't well explained at all, so I don't know if this would have implications for this theory. Maybe at this period in the universe's expansion/evolution matter dominates antimatter, but that may not always be the case? If you believe in a cyclical universe then it could just be a 50-50 odds thing, and some universes end up being what we call anti-matter, and others not.

Then again, maybe nature sides with 'normal' matter for reasons not yet understood, and this 'preference' (partially described in QCD) could extend to a single evolving particle that Wheeler describes. I think.

7. Feb 24, 2010

### luxiaolei

@Frame Dragger. To be honest, I found myself quite hard to accept the idea that antimatter travel back in time....especially thinks of relativistic time equation and also entropy increasing stuff etc, just does not work in my mind.

8. Feb 24, 2010

### Glen Bartusch

An interesting fact about the mathematics of physical reality for the author of the thread to consider is this: There is no natural law that would prohibit 'time' from 'going' backwards. If we lived in a world where time does indeed go 'backwards,' we'd have an enormous memory stored about what 'happened' in our future, but as time passes, we'd experience past events as they come.

9. Feb 25, 2010

### cepheus

Wouldn't we be able to predict the future if this was true?

10. Feb 25, 2010

### Frame Dragger

Keep in mind that life as we know it uses ordered energy and reduces it to a state of lesser order (heat usually, and poop of course! ) so the notion of a time-reversed life only makes sense in:

1.) A Deterministic universe
Or
2.) Time passes normally and a 'switch' is thrown, 'rewininding' the tape.

Either way is not physically realizable, and life that creates order from disorder wouldn't be life as we recognize it.

In short, beyond the toy model, there are no people in a time-reversed universe (from our frame of reference).

11. Feb 25, 2010

### Glen Bartusch

Predict the future?! You would have already lived the future! It's the past that's in front of you; you contain memories about the future only. The past is what is unknown.

12. Feb 28, 2010

### vanesch

Staff Emeritus
Indeed, fun ideas. In fact, what you write is applicable to a large part of ideas in modern physics.

As to anti-particles, in modern quantum field theory, in fact, the "particles of negative energy" which are proposed by Wheeler to be "particles with positive energy going back in time", are re-interpreted again as "anti-particles with positive energy going forward in time".

So instead of having a negative-energy electron going forward in time, or a positive-energy electron going backward in time, one (in a rather dull way) considers them to be NEW particles, with positive energy, going forward in time, and one calls them anti-particles.

Mathematically, the 3 views are equivalent.

13. Feb 28, 2010

### Frame Dragger

Well, I think people tend to forget that these theories are the result of models in which what you said is designed to be true so the models work! It must be, or something is terribly wrong with the theory. It's interesting, if not scientific, to consider what the physical reality of a mathematical construction might be. It may not be a meaningful concept to consider, but you have to love the notion of a single anti-particle with positive energy with advancing and retarded temporal action. That is what the TI is essentially for $$\Psi$$.