No time paradoxes?

[Dorian Gray]

Before beginning, I freely admit that I can see over the water, have no life preserver, and am getting tired from treading, but am not quite in over my head.

I recently read Brian Greene's The Fabric of the Cosmos. In it, there is a description of an experiment that basically depicts interference, demonstrating the wave-like properties of quanta. It's a beam-splitter experiment, and in it, the photon can travel along one of two paths If this is confusing, I'm sorry. They then put detectors to see which path the photons travel along, but then they know the path, and there is no interference pattern generated.

Anyway, it's supposed to demonstrate that without the detectors, it is necessary to combine all the possible histories in order to reach the 'average' destination of the photon (on a screen, let's say).

What I am suggesting is that since photons apparently have a number of different paths that they can take to reach the same point, then it is possible that historical events themselves have a number of different historical paths they can take to reach the same event.

To use the most common thought experiment (which is going back and killing your grandfather which results in a 'paradox') what I am suggesting is that the events - you going back in time, you being born, your grandfather dying, etc. - each has multiple ways of coming about, and therefore, going back in time and killing your grandfather would not set up a time paradox. You'd simply be born in one of the other possible ways.

If someone needs clarification, I'd be happy to screw you up even more. But hey, people with more physics background than me, is what I am suggesting in the realm of common sense, or what?

 

[athon]

I'm afraid I'm one of the more physics-illiterate people on here, especially when it comes to quantum stuff. I have the basics, but start to get too weird and my brain shrivels.

Firstly - and this might be my ignorance rather than your explanation - how does the beam splitting experiment show that a photon has multiple ways of reaching a single destination? To me, I'm picturing a beam splitter and two different detectors. A photon is released and can go one of two ways, and then arrive at one of two destinations. The relevant detector would tell the path it went to get there. Without the detector, it would be guess work to work out which way it might have gone. But there are still two destinations (unless I missed something).

Secondly, it seems very deterministic. In other words, while I can grasp that multiple causes can provoke the same event, the more variables you introduce the less chance this will happen. So, a single photon arriving at the same destination via different pathways is one thing. Multiple photons all changing their pathways...I find it hard to think that they could all arrive at the same place. It would need some additional law (outside of causality) to control all of the variables.

Just some thoughts.

Athon

 

[BillyJoe]

.....how does the beam splitting experiment show that a photon has multiple ways of reaching a single destination? To me, I'm picturing a beam splitter and two different detectors. A photon is released and can go one of two ways, and then arrive at one of two destinations. The relevant detector would tell the path it went to get there. Without the detector, it would be guess work to work out which way it might have gone. But there are still two destinations (unless I missed something).

When there is no knowledge about what path is taken by the single photon, it is as if the photon has travelled along both paths and interferred with itself producing the interference pattern (it's actually lots of photons released one at a time). When measurements ARE taken and there IS knowledge about what path is taken by the single photon (or multiple photons released singly), no interference pattern is produced (just a random scatter of dots)
In the first instance, light acts as a wave, in the second as a particle.

.....it seems very deterministic.

It is (in the sense that the outcome is predictable). But it's not intuitive.

least that's my understanding,
BillyJoe

 

[SpaceFluffer]

What I am suggesting is that since photons apparently have a number of different paths that they can take to reach the same point, then it is possible that historical events themselves have a number of different historical paths they can take to reach the same event.

I don't see how these things have anything to do with one another. What is true at the subatomic scale becomes less relevant the closer you get to the human scale, let along at the scale of large numbers of people.

One other suggestion in the same vein, however, is the Everett-DeWitt many worlds hypothesis. This is the idea that every time there is a 'decision' to be made at the subatomic level (i.e. some non-deterministic outcome), the universe 'splits' and each outcome is played out in each universe. It's not taken seriously by the overwhelming majority of the field, but seems to be similar to what you're talking about.

 

[TonyL]

The problem with the analogy is that although there are multiple paths that can get to the same location, there are often far more paths that lead to differenct locations.

As for the two-slit (beamsplitter) experiment, instead of thinking of the photon as both a particle and a wave, it's easier to think of the photon as a "quantized" wave. It's a wave in all senses of the word, it spreads out, passes though two slits, interferes with itself, etc. However, there is only one quantum of it, so when it interacts with anythng along its path, it's been spent, and can't interact with anything else. When the detectors are not in place, the photon travels both paths and intereferes. when detectors are placed in each path, the photon spreads out, goes through both paths and has a 50/50 chance of interacting with either of the detectors. When the wave interacts with the detector, it's properties change, and it is now a new quantized wave acting as if it was emitted from the location of the detector. As such, it cannot interfere because there is no wave comming from the location of the other detector. Now keep in mind, this is just a way of thinking about the phenomena that is consistent with the mathematical theory and experimental results, I'm not saying that this is the exact physical mechanism. (interpretations of QM are just that, interpretations, not theories)

 

[LordoftheLeftHand]

I don't see how these things have anything to do with one another. What is true at the subatomic scale becomes less relevant the closer you get to the human scale, let along at the scale of large numbers of people.

One other suggestion in the same vein, however, is the Everett-DeWitt many worlds hypothesis. This is the idea that every time there is a 'decision' to be made at the subatomic level (i.e. some non-deterministic outcome), the universe 'splits' and each outcome is played out in each universe. It's not taken seriously by the overwhelming majority of the field, but seems to be similar to what you're talking about.

I like the many worlds hypothesis, but it doesn't do a very good job of explaining interference. In the slit experiment, a photon is emitted and has a 50% chance to enter either slit. The world splits; it enters the first slit in our world and the second slit in some other world. How does it interfere with its self at the end of the experiment if the "worlds split"?

LLH