How can eternalism be wrong?

[etcetcetc00]

The Twin Paradox seems to suggest that presentism can't be valid. How can an objective present time be established in these circumstances?

[Sayonara]

Define "presentism".

[etcetcetc00]

The philosophy of time that only the present moment exists, versus the eternalist belief that all points in time exist equally in different points in time.

[Sayonara]

Then isn't a better question whether or not an objective present time is a necessity?

[etcetcetc00]

I don't understand what you mean by that

[Sayonara]

I mean, quite simply, does there need to be an objective present time?

[etcetcetc00]

Still confusing. My question pertains to the philosophical concepts of presentism vs. eternalism. Presentism contends that the only objects that exist in the universe exist now, and that all past systems no longer exist, and that future systems do not yet exist. Eternalism argues that past systems still exist, and future systems exist now too. Like in the movie Back To The Future, Marty's parents still existed in their past states when Marty went back to 1955, and his kids existed when he went to 2015.

It seems to me that, if the twin were traveling at light speed and came back at a time that is ahead of what his normal time would have been, there would have to have been that future for him to access.

[Sayonara]

Are you taking the Twin Paradox to be representative of relativity, or attempting to consider that one scenario in isolation?

[iNow]

etcetcet... The present is frame dependent. Yours and mine are not equal. That about sums it up. Also, the comparison of presentism and eternalism is a false dichotomy. There are, in fact, other possibilities.

[etcetcetc00]

I think I'm trying to define time travel. If twin A stays on Earth, and Twin B travels at light speed on a tour of the galaxy and comes back, he will be younger than Twin A. The amount of time it took Twin B to make the trip in his relative frame of reference was less than the time his trip took within the relative frame of reference of Twin A. I don't understand the math behind this, these are just hypothetical numbers, but let's say Twin B was gone a month in his time, and 50 years in Twin A's time. The 1 month point for Twin A is in the past of the 1 month point for Twin B. Their relative "presents" are at different points in time. How could there be only one present under these circumstances?

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What other possibilities?

[iNow]

You're making the classic mistake of thinking of time as some absolute. They are both at the same present when they reunite, they just traveled different paths to get there.

 

It's about the rate at which time passes in their own reference frame.

There is no absolute time.

 

The problem I think you're having is trying to treat the time experienced by Twin A as universal. It's not.

 

Maybe these will help. Check 'em out:

 

http://faraday.physics.utoronto.ca/PVB/Harrison/SpecRel/Flash/TimeDilation.html

http://faraday.physics.utoronto.ca/PVB/Harrison/SpecRel/Flash/LengthContract.html

http://faraday.physics.utoronto.ca/PVB/Harrison/SpecRel/Flash/TwinParadox.html

[Sisyphus]

By the same token, how can eternalism be right? In the same sense that my present is not your present, my future is not your future, either. The truth is quite a bit messier than that, as there is an overall past and future of the universe, just a lot of “fuzziness” caused by weaving world lines.

 

There’s also the problem of quantum uncertainty, which is probably more pertinent to the question. The degree of true randomness which is introduced casts serious doubt on the idea that the future could be said to exist already, as there is nothing embedded in the present that determines the future. That is, unless you believe in multiple universes, in which case every time a particle could zag or zag it actually does both, and two parallel futures are created…

[tvp45]

Don't take the "Twins Paradox" too literally. It only seems a paradox if you ignore parts of the situation.

[etcetcetc00]

If my future is not your future, how does that work outside of the context of MWI? Wouldn't a single Universe have a single time line?

[Sayonara]

...

 

But what does "single time line" mean to the universe? That's just a name we imagined up for a particularly long sequence of events; it doesn't say anything at all about relativistic effects within that sequence.

[etcetcetc00]

Ok, let's put some context into it, then. In 12 hours, I'll be in bed. If someone were to travel in time to that point and enter my room, would they see me asleep? Then, if twelve hours had passed for me, and only a few minutes for them, we would have different present times at the same world time. (1:15AM EDT) I know this is impossible now, but physicists do talk about theoretical time travel. My question is: when they talk about this time travel, do they expect past or future systems to be accessible? Is there too much quantum uncertainty involved to tell? Have they even gotten that far in their discussions?

[Sisyphus]

The point is that there is no such thing as "world time," only time in a given reference frame, or time as elapsed for a particular person or thing. The other point with regards to quantum uncertainty is that what you'll be doing twelve hours from now is not yet determined. I could certainly travel to that time, and in fact I fully expect to. ETA is almost exactly 12 hours from now, my time.

[etcetcetc00]

what if you were to go back 12 hours? I was in bed then. Could you theoretically access that system? I guess the best way to put it is this: We can travel across the first three dimensions. If we could travel in time, would there be a path to travel on? If you could see time flowing backwards, would it look like a film running in reverse?