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Time

Postby jimwalton » Mon Jan 07, 2019 12:31 pm

I was musing the other day, and I'm curious to dialogue with others about this subject. I was rolling around a brain teaser: if you are moving backwards through time, are you not also simultaneously moving forward through time, because your backward movement progresses linearly? If I spend 2 minutes going backward in time, am I not also going 2 minutes forward in time as I experience this backward motion? Is it so, then, that time can contradict itself and yet not be a contradiction? I can move forward and backward at the same, uh, time.

I don't know where to go with this thought. Wondering what others think.
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Re: Time

Postby Mike DeHaan » Tue Jan 08, 2019 10:21 am

1. Einstein's theory of Special Relativity allows observers moving in different frames of reference to disagree about the sequence of events. I suppose they might argue that the other was travelling backwards in time. But I think you're not talking about standing on a platform watching a train speed past you.

2/ I think you're saying that you are watching events and you are not moving relative to those events, so Einstein has no say in the matter. You see an event going backward in time, for eg you observe "An already splattered raw egg gathered itself off the floor, and jumped into Joe's hand. And the clock ticked backward 1 second". So your brain spent 1 second recording an external event that took 1 second. You aged one second. Your body is moving forward in its own time frame. The rest of the universe went back 1 second.

But "observing" means photons went from the egg to your eye. From the universe's point of view, the first photons showed a whole egg in Joe's hand, and the last photons showed the mess on the floor. How was it you processed the "first photons" after the "last photons"? Especially if you're not moving at nearly the speed of light, relative the to event you're observing. So the easy answer is, you're describing an impossible situation, so there's nothing to explain.

3/ Let's assume you really make this observation, but: You keep pointing at the egg as it flies from the floor to Joe's hand. And Joe is making a video of you watching him drop the egg. Does he, and does his video, see you point? Does he see you point from up to down, which would make him believe you are aging in his timeline? Or does he see you point at the clean floor when the egg is still in his hand, and then he sees you finish by pointing at his empty hand? If you were showing Joe a running stopwatch, does he see it running backwards?

You and a buddy each have a running stopwatch. Each of you sees your own stopwatch running as expected, but each sees the other person's stopwatch running backward. Somehow you can each communicate this to the other, so you each have reason to believe the other is going backwards in time (and does not have a wonky stopwatch). You each drop an egg and see the other catch an egg that jumps off the floor. My reply: you're each aging "forward in time" in separate frames of reference. No contradiction within your own frame of reference. Not likely that either of us will make these observations.
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Re: Time

Postby jimwalton » Tue Jan 08, 2019 10:21 am

> allows observers moving in different frames of reference to disagree about the sequence of events

That's fascinating. So the sequence of events is variable according to one's perspective? Sequence isn't linearly constant and objective? Boooschssshhh (mind blowing up).

> 2/ I think you're saying...

Yeah, that's exactly what I'm musing about.

> you're describing an impossible situation, so there's nothing to explain.

Ah, yep.

> My reply: you're each aging "forward in time" in separate frames of reference. No contradiction within your own frame of reference. Not likely that either of us will make these observations.

Thank you. My musings are nothing more than sci-fi and impossible in the real world. Thanks for contributing to my clarity.
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Re: Time

Postby Local Horse » Tue Jan 08, 2019 3:20 pm

> So the sequence of events is variable according to one's perspective? Sequence isn't linearly constant and objective? Boooschssshhh (mind blowing up).

The order of two space-like separated—i.e. in no causal connection—events depends on the observer. But causality relations are a purely geometric property of spacetime and independent of the observer.

What Mike DeHaan, "I suppose they might argue that the other was traveling backwards in time," is wrong. The events on the world line of an observer are obviously causally connected.
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Re: Time

Postby jimwalton » Tue Jan 08, 2019 3:23 pm

Thanks. Just rolling it all around in my head. I know in one sense there's no such thing as going backward in time, so my question is completely theoretical, but in another sense time is such an unknown entity, and there are so many speculations about black holes, relativity, and quantum, it's fun to toy with various possibilities that may one day be better understood.
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