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What Is Time?

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Re: What Is Time?

#21

Post by ScottBurson »

I read somewhere once that it's not actually true that special relativity permits time travel into the future. Such time travel would require leaving Earth, accelerating to a relativistic velocity, travelling some distance, then accelerating back toward Earth, again reaching a relativistic velocity, then finally accelerating to match Earth's velocity and position, and landing.

The argument was that although it's true that your time will pass more slowly than time on Earth while you're coasting, you also clearly have to do a lot of accelerating, and acceleration itself also has an effect on the apparent passage of time in your reference frame -- and in this scenario, where you start and end in Earth's reference frame, that effect cancels out the effect of velocity (perhaps even more than cancels it out; I don't recall).

Does anyone here know if this is true?

Re: What Is Time?

#22

Post by p1mrx »

ScottBurson wrote: post #21

I read somewhere once that it's not actually true that special relativity permits time travel into the future. Such time travel would require leaving Earth, accelerating to a relativistic velocity, travelling some distance, then accelerating back toward Earth, again reaching a relativistic velocity, then finally accelerating to match Earth's velocity and position, and landing. The argument was that although it's true…

Without acceleration, the system is symmetrical, so each observer thinks the other's time is passing more slowly. But that also means they can never meet up again to compare ages and see who's "right".

The observer who accelerates is the one who breaks the symmetry, and thus travels into the other's future. I haven't done the math, but Wikipedia has pictures:

http://en.wikipedia.org/wiki/Twin_paradox

Re: What Is Time?

#23

Post by jhrobert »

thisjepisje wrote: post #5

"Time Is What Prevents Everything From Happening At Once.." - John Wheeler

Perfect.

Why?

If effects from causes occurred immediately, versus with a distinct delay, everything would occur at once (or loop forever, that's a bug).

Hence, because we can observe that things happen "in sequence", there is a "propagation delay". That's what time is: "Time: the process whereby effects occur after causes, in a step by step manner".

Re: What Is Time?

#24

Post by leephillips »

ScottBurson wrote: post #21

I read somewhere once that it's not actually true that special relativity permits time travel into the future. Such time travel would require leaving Earth, accelerating to a relativistic velocity, travelling some distance, then accelerating back toward Earth, again reaching a relativistic velocity, then finally accelerating to match Earth's velocity and position, and landing. The argument was that although it's true…

It's not true. In fact, particles "travelling into the future" is routinely measured in high energy physics experiments: a particle's decay time will be lengthened exactly as special relativity predicts. Also, the effect has been measured directly using atomic clocks. The circumferences of your hard drive platters are younger than the part near the spindle.

Re: What Is Time?

#25

Post by JoeNa »

ScottBurson wrote: post #21

I read somewhere once that it's not actually true that special relativity permits time travel into the future. Such time travel would require leaving Earth, accelerating to a relativistic velocity, travelling some distance, then accelerating back toward Earth, again reaching a relativistic velocity, then finally accelerating to match Earth's velocity and position, and landing. The argument was that although it's true…

The fact that Special Relativity has meant different things to different people and that even most relativist professors have made statements that imply contradictory interpretations is well known and well documented. http://worknotes.com/Physics/SpecialRelativity/TwinParadox/a...

Re: What Is Time?

#26

Post by ScottBurson »

leephillips wrote: post #24
ScottBurson wrote: post #21

I read somewhere once that it's not actually true that special relativity permits time travel into the future. Such time travel would require leaving Earth, accelerating to a relativistic velocity, travelling some distance, then accelerating back toward Earth, again reaching a relativistic velocity, then finally accelerating to match Earth's velocity and position, and landing. The argument was that although it's true…

It's not true. In fact, particles "travelling into the future" is routinely measured in high energy physics experiments: a particle's decay time will be lengthened exactly as special relativity predicts. Also, the effect has been measured directly using atomic clocks. The circumferences of your hard drive platters are younger than the part near the spindle.

I'm not questioning the existence of time dilation; I'm asking about the effect of acceleration on that dilation. Perhaps this is a question for General Relativity.

Re: What Is Time?

#27

Post by ScottBurson »

JoeNa wrote: post #25
ScottBurson wrote: post #21

I read somewhere once that it's not actually true that special relativity permits time travel into the future. Such time travel would require leaving Earth, accelerating to a relativistic velocity, travelling some distance, then accelerating back toward Earth, again reaching a relativistic velocity, then finally accelerating to match Earth's velocity and position, and landing. The argument was that although it's true…

The fact that Special Relativity has meant different things to different people and that even most relativist professors have made statements that imply contradictory interpretations is well known and well documented. http://worknotes.com/Physics/SpecialRelativity/TwinParadox/a...

Okay, I read some of the material on that site, and all it says to me is that (a) not everyone who has been exposed to Special Relativity understands it, and (b) some people think that if they don't understand something, it can't be true.

Re: What Is Time?

#28

Post by ScottBurson »

ScottBurson wrote: post #21

I read somewhere once that it's not actually true that special relativity permits time travel into the future. Such time travel would require leaving Earth, accelerating to a relativistic velocity, travelling some distance, then accelerating back toward Earth, again reaching a relativistic velocity, then finally accelerating to match Earth's velocity and position, and landing. The argument was that although it's true…

Downvoted for asking a question. Geez, tough crowd.

I found the Wikipedia page on the twin paradox[0]. It explains the matter very clearly.

[0] http://en.wikipedia.org/wiki/Twin_paradox

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