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Time Travel is Impossible

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thetabularasa posted on Wed, Nov 28 2012 3:28 PM

The quantum physics discussion in my Holiday Dinner Table thread got me thinking about time and space, what is bendable, what isn't, whether General Relativity is a viable theory and so forth, and naturally I started considering the possibility of time travel. Here's how I know it is impossible:

Time doesn't exist. It is a manifestation of the human imagination. Things change; the world changes, we change and everything seems to be in flux somehow. Even if an object takes millenia to destruct and end, it inevitably does, similar to entropy, I suppose, in the sense that there is a systematic degradation involved in all things. Nevertheless, my point is that things are always changing, and of course distances between objects exist, but time itself does not exist. It is merely a subjective measurement system.

Prove me wrong if you must!

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eliotn replied on Wed, Nov 28 2012 3:43 PM

www.achrongame.com - Proves that time travel will exist in about ~200 years.  Its just that only achronal beings remember what happened in the previous iterations.

/sarcasm

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I think general relativity does in fact prove that time does exist, as we can measure different rates of the passage of time using satellites and gps, etc.

If you want to travel in time, simply wait a few seconds :P

But that's not really what we mean by time travel as a scifi concept, we mean somehow transcending the natural flow of time either fowards or backwards.

Time travel into the future is certainly possible, it's the backwards travel that is impossible.

If you want to travel into the future, do a single close orbit near to a black-hole's event horizon. You'll come out of that orbit, depending on how close it is, far into the future, perhaps hundreds or thousands or millions of years, yet for you only a few hours or days will have seemed to pass.

Yes, in that sense, time travel is perfectly possible, but only into the future.

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Well if cryo technology is ever perfected, that would be, essentially, time travel into the future (Futurama, anyone?). But yeah, just ask Walt Disney when he wakes up.

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Anenome:

I think general relativity does in fact prove that time does exist, as we can measure different rates of the passage of time using satellites and gps, etc.

If you want to travel in time, simply wait a few seconds :P

But that's not really what we mean by time travel as a scifi concept, we mean somehow transcending the natural flow of time either fowards or backwards.

Time travel into the future is certainly possible, it's the backwards travel that is impossible.

If you want to travel into the future, do a single close orbit near to a black-hole's event horizon. You'll come out of that orbit, depending on how close it is, far into the future, perhaps hundreds or thousands or millions of years, yet for you only a few hours or days will have seemed to pass.

Yes, in that sense, time travel is perfectly possible, but only into the future.

Thanks for the info, Anenome. But is there any way to actually get that close to a black hole? Also, how is it determined that you can, indeed, time travel when it's never been done? And a black hole of all things? Why exactly?

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hashem replied on Wed, Nov 28 2012 8:29 PM

thetabularasa:
Time doesn't exist.

This was my thought coming into this thread, and I'm glad you pointed it out.

Time, past, present, future are figures of speech; they're concepts. When we speak of "the past" or "the future", we're speaking figuratively about imaginary arrangements of matter. Matter exists in its current arrangement. That's self-evident but sufficiently mindblowing to merit pondering. The capacity to think in terms of a concept so abstract as time, and to act upon predictions derived from those thoughts, and to communicate that data probably requires a brain so amazing I feel humbled to posses it.

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hashem:

thetabularasa:
Time doesn't exist.

This was my thought coming into this thread, and I'm glad you pointed it out.

Time, past, present, future are figures of speech; they're concepts. When we speak of "the past" or "the present", we're speaking of arrangements of matter which don't exist. Matter exists in its current arrangement. That's self-evident but sufficiently mindblowing to merit pondering. The capacity to think in terms of a concept so abstract as time, and to act upon predictions derived from those thoughts, and to communicate that data probably requires a brain so amazing I feel humbled to posses it.

 

I'd say you're partially correct. Time is indeed a concept that has a physical reality. Consider special relativity, radioactivity, etc.

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thetabularasa:

Thanks for the info, Anenome. But is there any way to actually get that close to a black hole?

Yes, actually. We could do it now actually. Basically you take a nuclear power source, combine it with an ion engine with a hydrogen scoop, and begin picking up momentum towards the black hole. The crazy thing is that, the faster you go the slower time moves for you, another property of special relativity (although how the universe knows you're moving faster than other things I have no idea unless there's an actual universal frame of reference, which to this point we don't think there should be).

So, half way to the black hole you're picking up speed, and you could get to a pretty significant fraction of light-speed, say .99999% or so. At that speed and higher, you'd have considerable time-dilation effects--time moving slower for you than outside your frame of reference, meaning the universe would seem to be moving at fast-forward.

Then, once halfway there, you flip the ship around and begin decelerating, until at your desired black-hole orbital speed. Aim for a precise orbit around it, at a precise speed, and skim the event horizon. Do that, and you could watch thousands of years pass before your eyes in a matter of seconds. You'd see the stars dance and skip, see galaxies crash together, and stars wink out one by one or explode as stellar fireworks. You might even witness the heat-death of the universe (perhaps, if you strayed inside the event horizon. And if your black hole didn't explode.).

Anyway, even doing all that, it would still take something like 1,000 years to reach the nearest black hole, as perceived from inside your ship.

You could do it a lot faster if you could figure out a way to use a black hole on the other end to slow you down, so you didn't have to start braking at halway. Hmm, maybe like the reverse of a gravitational slingshot. Don't think I've ever heard anyone propose that maneuver before. It may not really be possible. No way to get directly oppositional gravity to your line of travel, only tangent to it, which would only accelerate you further.

Now, if we manage to someday figure out how to create artificial gravity, then we may be able to to create a ship that travels faster than the speed of light, or at least to greatly improve acceleration to that speed. I actually think we may indeed be able to do it eventually.

thetabularasa:
Also, how is it determined that you can, indeed, time travel when it's never been done? And a black hole of all things? Why exactly?

Read up on this link and you'll get it. It's called gravitational time dilation, and is a real thing that's verified and all that.

There was a pretty cool episode of Star Trek Voyager where the ship entered a planet with a massive time dilation effect around it. While the ship simply tried to enter the atmosphere, the inhabitants below called it a star or a moon. For a thousand of their years, it hung there in the sky :P

 
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Blargg replied on Wed, Nov 28 2012 10:04 PM

When we talk of moving through a physical dimension, it's in reference to time. At time t, we're at x, at t+1 we're at y, at t+2 we're at z. What does it mean to move through time? At t we're at t1, at t+1 we're at t2, huh? What does it even mean? Subjectively, it might mean that your experience of time is that you were at t1, then at t2, but physically there's no movement in time, just a frozen 4D universe.

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AJ replied on Wed, Nov 28 2012 10:05 PM
OP is correct. All conceptions of time boil down to a comparison of two distances. The distance between two locations of a clock hand compared with the distance between two locations of a car on a racetrack; the distance between two locations of the constituents of a cesium atom in an atomic clock compared with the distance between two locations of a ball in the air; the distance between two locations of the Sun compared with the distance between two sets of positions of molecules in a wilting flower.

Comparison is an action, hence for the purposes of physics (rather than the loose sense of everyday talk), "time" is a verb. One can travel through an object or space, but one cannot travel through a comparison! Perhaps something else is meant by "travel through time," but that awaits coherent definition.

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Blargg:

When we talk of moving through a physical dimension, it's in reference to time. At time t, we're at x, at t+1 we're at y, at t+2 we're at z. What does it mean to move through time? At t we're at t1, at t+1 we're at t2, huh? What does it even mean? Subjectively, it might mean that your experience of time is that you were at t1, then at t2, but physically there's no movement in time, just a frozen 4D universe.

 

Special relativity proves the interconnectedness of space and time. Time is not meaningless.

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hashem replied on Wed, Nov 28 2012 10:45 PM

I don't know much about special relativity and a connection between space and a concept—time. But I don't think anyone's saying time is a meaningless concept. I support OP and AJ and Blargg, perhaps in my ignorance.

However the idea that you can manipulate a concept (e.g. time, a la Anenome) as though it weren't just an abstract thought in your brain seems silly, and I'll be the first to say that's probably due in part to my ignorance on the subject. But I'm willing to guess that behind all the definitions what's really being described is just laws of physics, not the ability to experience arrangements that don't exist.

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Anenome replied on Wed, Nov 28 2012 11:07 PM

I would disagree with the characterization of what I said as "maniupulating time." Experiencing a slower time reference via strong gravity or acceleration is just as possible as hearing different octaves of sounds via the doppler effect and speed differentials.

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hashem replied on Wed, Nov 28 2012 11:10 PM

Oh ok I thought you were talking about seeing into the future lol.

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That's my point. Time is not merely an abstract thought. An abstract thought is the number two. You may see a collection of indistinguishable objects and develop this idea of "two." But that's in no way connected to physical reality in a causal sense. Looking at special relativity, you can see and ultimately observe that time is a parameter of physical reality.

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