# Could two past events result in the same happening?

1. Jun 24, 2013

### Gonzalo

Hello Everybody.

Could two past events result in the same happening? For example: We have observed the "incident A" and we have two different stories to explain that incident: The "history 1" and the "history 2". Both stories are equally plausible and equally likely. The question is "which of these stories is true?" Does this question make sense? Are both true stories?

The thing is that I've been thinking about Schrödinger's poor cat and I wondered if, just as there are multiple futures, there may be multiple pasts.

Thank you very much and sorry for my English :)

2. Jun 24, 2013

### Bill_K

Sure, in the double-slit experiment a particle hits the screen. In the past, it might have gone through slit 1 or it might have gone through slit 2.

3. Jun 24, 2013

### Gonzalo

Thanks :)

4. Jun 24, 2013

### BenjaminTR

I am not sure about this answer. In two slit experiments, we have one wave function evolving deterministically until measurement at the screen. According to many QM interpretations, this is just one history: the particle does not follow a determinate path and does not have a determinate position until measurement. Before then the wave function is all.

Using such interpretations, the answer to the original question is still yes. Two wave functions can both collapse to the same position at measurement, for example. Measuring that position would be one event with two possible histories.

5. Jun 24, 2013

### Bill_K

This is the whole point of Feynman's sum over histories. There's an amplitude for the particle to pass through slit 1, and an amplitude for the particle to pass through slit 2, and the probability for the particle to be detected at a certain point on the screen is a coherent combination of the two.

6. Jun 24, 2013

### BenjaminTR

I see. I was thinking of the question as asking whether backward determinism is true in QM even if forward determinism is not. I now realize this interpretation of the question does not make sense given the analogy to Schrodinger's cat, so your answer is better.

7. Jun 24, 2013

### skeptic2

I think this happens everyday in courtrooms. :-)