# I QFT, event amplitudes and reversed time...

1. Mar 25, 2017

### asimov42

Hi all,

I've recently been reading a paper by Richard Muller and Shaun Maguire (which is not the exact topic of this post). In that work, the authors mention:

"We note that in quantum field theory, very small, localized and rapid events contain amplitudes that can be interpreted as taking place in reversed time. Such reversed time is not, however, directly observable. "

I'm somewhat confused by this - specifically, what interpretation the authors are referring to? Although the event may contain some amplitudes taking place in reversed time, the overall result is fully causal, correct? Also, what defines such as event as 'rapid'?

2. Mar 25, 2017

### MathematicalPhysicist

Anti-particles in Feynman diagrams their direction of momenta in these diagrams is opposite to ordinary particles; so it seems they are going backward in time, but it's just a mathematical construct. No one knows if anti particles actaully do go backward in time.

If I were to radically postulate, anti-particles are particles which go backward in time; as in positron is actually an electron that goes backwards in time by some mechanism that changes its charge. That can possibly make sense if we only knew how to change the sign of particles' charge.

3. Mar 25, 2017

### vanhees71

To the contrary of many popular-science source, the real thing, i.e., QFT is just using the opposite idea of being causal, i.e., there's nothing running backwards in time. The trick is to write creation operators in front of the negative-frequency modes in the mode decomposition of the quantum field, i.e., instead of something annihilating with negative energy you create something with positive energy, and everything is moving forward in time. The arrows on lines of charged particles in Feynman diagrams, i.e., for particles which are not their own antiparticles, indicates the flow of charge, not momentum, i.e., a Dirac-fermion line pointing out can either mean a particle going out (asymptotic free final state) or an antiparicle moving in (asymptotic initial state). The meaning of these diagrams are just mathematical expressions allowing you to calculate S-matrix elements describing transition probability rates for processes leading from a given asymptotic free initial to another given asymptotic free final state and thus allow to calculate cross sections that can be measured in the real world and compared to the theory.

4. Mar 25, 2017

### Staff: Mentor

We have no way of telling unless we can read the paper and see the context.

5. Mar 25, 2017

### Staff: Mentor

There is no such mechanism in QFT. Please review the PF rules on personal speculation.

6. Mar 26, 2017

### MathematicalPhysicist

At least quote my post in its entirety.

Some might think that I argued that QFT posits what you quoted, where I undeniably wrote:"If I were to radically postulate...."

So we aren't entitled to our opinions?

7. Mar 26, 2017

### Staff: Mentor

I only quoted the part that was problematic.

I didn't. Nor did I say so. If you were, your post would not be personal speculation, it would just be wrong.

Yes, and that is personal speculation. Unless you can reference a textbook or peer-reviewed paper.

You are, but that doesn't mean they are necessarily allowed topics for discussion here at PF. Again, please review the PF rules.