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New perspective on Time Travel?

by Marco12
Tags: perspective, time, travel
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Pretty Pony
#19
Dec30-10, 04:23 AM
P: 8
So what ramifications does Quantum Mechanics have on this discussion? Wave-function collapse must happen before our eyes catch the affiliated photons. Since we are always looking into the past, does this mean that we can also change it? For example, imagine the double-slit experiment set up one light-year away from Earth. By observing the interference pattern from this distance, and thus forcing the photons out of a super-position, you are essentially changing the behaviour of photons that existed one full year ago. Now isn't that changing the past?
JaredJames
#20
Dec30-10, 06:26 AM
P: 3,387
Quote Quote by Pretty Pony View Post
So what ramifications does Quantum Mechanics have on this discussion? Wave-function collapse must happen before our eyes catch the affiliated photons. Since we are always looking into the past, does this mean that we can also change it? For example, imagine the double-slit experiment set up one light-year away from Earth. By observing the interference pattern from this distance, and thus forcing the photons out of a super-position, you are essentially changing the behaviour of photons that existed one full year ago. Now isn't that changing the past?
To be blunt, no.

The photons exist now when we observe them a light year away. They are one year old.

That's like saying the aircraft that travelled from london to new york hasn't aged during the duration of the trip and interfering with it in new york affects it when it was back in london (i detonate a bomb in ny and it explodes on the runway at heathrow).
Dougggggg
#21
Dec30-10, 08:20 AM
P: 163
Not that I am suggesting it at all possible or anything of the sort, but perhaps some ideas of retrocausality could apply here?
JaredJames
#22
Dec30-10, 08:24 AM
P: 3,387
Quote Quote by Dougggggg View Post
Not that I am suggesting it at all possible or anything of the sort, but perhaps some ideas of retrocausality could apply here?
Why?

What makes the OP scenario different to ZapperZ's snail mail one?
dkotschessaa
#23
Dec30-10, 08:26 AM
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P: 571
Quote Quote by Pretty Pony View Post
So what ramifications does Quantum Mechanics have on this discussion? Wave-function collapse must happen before our eyes catch the affiliated photons. Since we are always looking into the past, does this mean that we can also change it? For example, imagine the double-slit experiment set up one light-year away from Earth. By observing the interference pattern from this distance, and thus forcing the photons out of a super-position, you are essentially changing the behaviour of photons that existed one full year ago. Now isn't that changing the past?
No, but cool idea. :) Could work in sci-fi!

Again this phrase "looking into the past" is a metaphorical one.

There's a sense in which it's true. According to the table here (we know Wikipedia is completely accurate all the time) the light from an object 1 foot away takes 1 nanosecond to arrive.

So if you're looking at an object 10 feet away, you're seeing what happened 10 ns ago. The closer you get, the closer in real time you get to the object, but since the speed of light is finite, you will never really see the *object* in real time.

However, what you're really seeing is not the object, but the *light* from the object when it hits your eye. That is a current, real time event.* What's happening *now* is that the light is hitting your eyes.

I'm just using a basic thought process here (which I think is pretty logical) and not really thinking about QM. I'm not sure how to describe this in terms of the observer effect. If something is a billion light years away, anything you do to change it is going to take (at least) a billion light years to have any effect, and you won't be able to see the result for another billion years. (Somebody correct me if I'm off here).

-DaveKA


* Yeah, it might take a little bit for your brain to process the light information, and to figure out what the object is, but that's another discussion.
Dougggggg
#24
Dec30-10, 08:39 AM
P: 163
Quote Quote by jarednjames View Post
Why?

What makes the OP scenario different to ZapperZ's snail mail one?
I was more just trying to play the role of devil's advocate. I think ZapperZ's example was perfect. I just wanted to make sure we at least covered all possible ideas in this concept.
JaredJames
#25
Dec30-10, 09:10 AM
P: 3,387
Quote Quote by dkotschessaa View Post
I'm just using a basic thought process here (which I think is pretty logical) and not really thinking about QM. I'm not sure how to describe this in terms of the observer effect. If something is a billion light years away, anything you do to change it is going to take (at least) a billion light years to have any effect, and you won't be able to see the result for another billion years. (Somebody correct me if I'm off here).
If you change the object a billion light years away (let's say we destroy a star somehow) then it will take a billion years before we see this change reflected in the sky.

However, if we simply 'change the light' as we see it here, a billion light years away from the source, it doesn't affect the source of the light in any way.
Pretty Pony
#26
Dec30-10, 03:35 PM
P: 8
Quote Quote by jarednjames View Post
To be blunt, no.

The photons exist now when we observe them a light year away. They are one year old.

That's like saying the aircraft that travelled from london to new york hasn't aged during the duration of the trip and interfering with it in new york affects it when it was back in london (i detonate a bomb in ny and it explodes on the runway at heathrow).
It was not those photons that I was concerned about; just the ones with the interference pattern on the screen. Perhaps I should use a less ambiguous scenario:
Now imagine a small sampling of Cesium-134 (134Cs) floating around in space, one light-year away. With a very powerful telescope, one can change the rate of the radioactive decay by observing it, because of the Quantum Zeno effect. One year ago, this Cesium sample suddenly stops decaying because you are observing it now. Blink, and for about 300-400 milliseconds, it continues to decay. This would be changing the past, no? I'm sure there would be some application of the butterfly effect to this thought experiment, but I don't know a great deal about radioactive decay. Suffice to say that these observations may cause the Cesium to reach its first half-life sometime last September, rather than last July. That, surely, is changing the past? Note that in real time, once you start observing the Cesium, it already reached its first half-life in July. As you observe it, you change this date to sometime in September, thus changing events that have already transpired in the past.
JaredJames
#27
Dec30-10, 05:13 PM
P: 3,387
So every time we look up at the stars we extend their life?

I think someone more qualified on the subject should pick things up from here.
apeiron
#28
Dec30-10, 05:48 PM
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P: 2,432
Quote Quote by Pretty Pony View Post
So what ramifications does Quantum Mechanics have on this discussion? Wave-function collapse must happen before our eyes catch the affiliated photons. Since we are always looking into the past, does this mean that we can also change it? For example, imagine the double-slit experiment set up one light-year away from Earth. By observing the interference pattern from this distance, and thus forcing the photons out of a super-position, you are essentially changing the behaviour of photons that existed one full year ago. Now isn't that changing the past?
This is the upshot of delayed choice quantum eraser experiments - probably what you were thinking about here. By looking at ancient star-light, we are indeed collapsing its wave function and so some kind of retrocausality has to be factored into a fuller model of time.

http://en.wikipedia.org/wiki/Wheeler...ice_experiment

The letter analogy works for macroscale classical objects, but QM does say there is more to the story.
Dougggggg
#29
Dec30-10, 06:07 PM
P: 163
Quote Quote by apeiron View Post
This is the upshot of delayed choice quantum eraser experiments - probably what you were thinking about here. By looking at ancient star-light, we are indeed collapsing its wave function and so some kind of retrocausality has to be factored into a fuller model of time.

http://en.wikipedia.org/wiki/Wheeler...ice_experiment

The letter analogy works for macroscale classical objects, but QM does say there is more to the story.
Called it.


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