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Is time travel really possible? I would think not.

The reason being that in the space-time continuum, we would always be at any given time at a particular space. In other words we would always travel both in space and time.

Therefore, I would think that if you travel back in time, you would travel in space as well. Hence, even if we found a way to travel back in time, we would be at some space - may not be on Earth, may not even be in Solar System.

Is my reasoning right?

Thanks.

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On 2003-02-27 22:24, SiriMurthy wrote:
Is my reasoning right?
SiriMurthy, you are exactly right. Moving through time would certainly put the time traveler some place not on Earth. Consider...the Earth is rotating at about 1000 mph. Taking that into account, if our time traveler moved 1 hour through time he would find himself 1000 miles from his starting point. The Earth is also revolving around the sun...if he traveled 6 months through time, he would be 2 AU's away from his starting point! Now consider the Sun is also moving around the center of the Milky Way...and the Milky Way is moving through space... and the whole Universe is expanding. So moving just a couple of years (or even a couple of minutes) through time would put you smack dab in the middle of empty space.

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<font size=-1>[ This Message was edited by: R.A.F. on 2003-02-27 23:18 ]</font>

3. On 2003-02-27 23:03, R.A.F. wrote:
SiriMurthy, you are exactly right. Moving through time would certainly put the time traveler some place not on Earth.
Well, sure, but you have to deal with Galilean relativity. If you jump up in the x coordinate, you still move in the y and z. So, if you jump up in the t coordinate, why wouldn't the x, y, and z continue to move?

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Kilopi, I'm not exactly sure what you mean in your post. I certainly could be wrong. Could you elaborate?

5. Well, I meant it in a lighter vein, but there's nothing really wrong with it.

It looks like you're assuming that, if you start at (x,y,z,t) and jump five minutes into the future, you'd think you'd land at (x,y,z,t+300)--and of course, the Earth would no longer be anywhere near (x,y,z)

But isn't that like assuming that you're at (x,y,z,t) on a moving train, and you jump up 1 meter in one second, that you'll arrive at (x+1, y, z, t+1)? Instead, your y and z change because of the train's motion--Galilean relativity, in other words.

The real tough part about time travel is the distance involved. We all know how tough it is to get to the moon. The "conversion factor" between distance and time is obviously c, so a jump of one second into the past would be like a jump of 300,000,000 meters. That's gonna take some energy. It's probably not even impossible--just expensive. Maybe things are easier in space. [img]/phpBB/images/smiles/icon_smile.gif[/img]

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imagine you are jumping up and down on the bed of your buddy's truck. while you jumped up, he turned the wheel and accelerated... then you landed with a mouthful of asphalt...

the motion of the earth is, or the sun, or the galaxy is not in a straight line

7. On 2003-02-28 07:59, Resu wrote:
the motion of the earth is, or the sun, or the galaxy is not in a straight line
It's along a geodesic in spacetime, though, the curved-spacetime analogue to a straight line, so the principle is the same, just a bit more involved.

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I see the point. However, the expansion of universe is accelerating. Hence even if we know how to be at the same {x, y z} coordinates and just change the {t} dimension, I still think that we would land at some unknown (well, unknown is a relative term as much of universal motion can be calculated) {x, y, z} space.

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Piers Anthony used this idea in his novel Ghost. In it, the crew travelled in time to travel in space.

10. On 2003-02-28 11:25, SiriMurthy wrote:
I see the point. However, the expansion of universe is accelerating. Hence even if we know how to be at the same {x, y z} coordinates and just change the {t} dimension, I still think that we would land at some unknown (well, unknown is a relative term as much of universal motion can be calculated) {x, y, z} space.
But I'd think that if we ever got to the point where we could do that, we'd be able to handle whatever corrections we'd have to make.

But I'm speculating. [img]/phpBB/images/smiles/icon_smile.gif[/img]

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So what would your ultimate reference point be? Where is Universal x=0, y=0, z=0? To calculate your space-time manuever, you'd have to know exactly where things are now and where they'll be.

And what does that do to conservation of momentum? In the Juping-in-the-back-of-a-truck analogy, it is your momentum that keeps you in place with respect to the truck. But if you suddenly accelerate in time, you'd have to accelerate and then decelerate your mass as well by the c factor.

Just thinking out loud. [img]/phpBB/images/smiles/icon_smile.gif[/img]

12. On 2003-02-28 14:10, RafaelAustin wrote:
So what would your ultimate reference point be? Where is Universal x=0, y=0, z=0? To calculate your space-time manuever, you'd have to know exactly where things are now and where they'll be.
Yeah, that's basically the reason that I (and my friend over here with the white fuzzy hair) think it's all relative. No ultimate reference frame at all.

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On 2003-02-28 01:44, kilopi wrote:

It looks like you're assuming that, if you start at (x,y,z,t) and jump five minutes into the future, you'd think you'd land at (x,y,z,t+300)--and of course, the Earth would no longer be anywhere near (x,y,z)
Heinlein did pretty well with this in "The Number of the Beast" - not a favourite, but interesting in that the "time travel" system had several verniers - controlling different aspects of displacement. Not too bad, really. Careful treatement was given to how things moved, etc.

If you want to hurt your head concerning time travel, try "Time Travel in Einstein's Universe" by Richard Gott.

I found it kinda hard not to hurl the book into the past, really. But there are some worthwhile things in there.

Jack

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It actually reminds of this quote that i heard quite a while ago from a friend, "if there is something there is nothing if there is nothing there is something but what is there if there's everything" (correct me if wrong) i don't know if it's related but that's what popped into my head when i read about time travel.

15. On 2003-02-28 14:10, RafaelAustin wrote:
So what would your ultimate reference point be?
[snip]
Just thinking out loud. [img]/phpBB/images/smiles/icon_smile.gif[/img]
Just thinking out loud, too.

What if you used your point now as your starting point. Then in addition to whatever your time calculations are, calculate the earth's rotation, the earth's rotation around the sun, the sun's rotation around the center of the galaxy, the galaxies whatever-movement-it-is-doing, the expansion of the universe, . . . ?

(Assuming, of course, you are capable of all these measurements needed.)

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Although no reference frame is preferred, everyone has a reference frame. Therefore, you can't really say that there is no way to measure the difference... you just measure the difference between event P and your reference frame.

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For \$25.00 I'll register your reference frame with The Library of Congress and name it after you.

18. Quote:imagine you are jumping up and down on the bed of your buddy's truck. while you jumped up, he turned the wheel and accelerated... then you landed with a mouthful of asphalt...

well, in theory, yes, but if you move through one, the others will move at the same rate. difficult to explain. but then, i may well be wrong.

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On 2003-03-02 12:26, Reacher wrote:
Quote:imagine you are jumping up and down on the bed of your buddy's truck. while you jumped up, he turned the wheel and accelerated... then you landed with a mouthful of asphalt...

well, in theory, yes, but if you move through one, the others will move at the same rate. difficult to explain. but then, i may well be wrong.
Welcome to the board, Reacher.

It works as long as there is no acceleration and the movement is linear with constant velocity.

Otherwise, if there is acceleration, or angular momentum, you will certainly fall out of your buddy's truck.

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On 2003-02-27 22:24, SiriMurthy wrote:
Is time travel really possible? I would think not.

The reason being that in the space-time continuum, we would always be at any given time at a particular space. In other words we would always travel both in space and time.

Therefore, I would think that if you travel back in time, you would travel in space as well. Hence, even if we found a way to travel back in time, we would be at some space - may not be on Earth, may not even be in Solar System.

Is my reasoning right?

Thanks.
Tipler wrote a paper quite a while back on infinitely long super-massive rapidly rotating cylinders. He showed that if such an entity existed, that there were closed time-like loops around the cylinder. In other words, there were orbits that would take you backwards in time to the same region in space. This has worked its way into several SF stories (including at least one by Niven).

As long as we're on the subject, Niven wrote an essay on The Theory and Practice of Time Travel. In it, he proved that a time machine would never be built. The proof has its holes, but it's worth going through.

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....

As long as we're on the subject, Niven wrote an essay on The Theory and Practice of Time Travel. In it, he proved that a time machine would never be built. The proof has its holes, but it's worth going through.
Interesting. Is that essay or an exerpt of it online? Could you point me to it? I am afraid I may not understand all the mathematics behind it, but I will try.

Thanks.

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On 2003-03-03 15:19, SiriMurthy wrote:
....

As long as we're on the subject, Niven wrote an essay on The Theory and Practice of Time Travel. In it, he proved that a time machine would never be built. The proof has its holes, but it's worth going through.
Interesting. Is that essay or an exerpt of it online? Could you point me to it? I am afraid I may not understand all the mathematics behind it, but I will try.

Thanks.
It's in ALL THE MYRIAD WAYS. I don't know if it's online (i didn't see it at the Niven site). There are no mathematics in it. Niven's story based on the Tipler paper is "Rotating Cylinders and the Possibility of Global Causality Violation". I haven't checked, but i think that's likely the name of the Tipler paper as well.

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On 2003-03-02 12:26, Reacher wrote:
Quote:imagine you are jumping up and down on the bed of your buddy's truck. while you jumped up, he turned the wheel and accelerated... then you landed with a mouthful of asphalt...

well, in theory, yes, but if you move through one, the others will move at the same rate. difficult to explain. but then, i may well be wrong.
Reacher...I've sorta done the scene postulated above. Kids, don't ever straddle the gate of a pickup truck as it hits an icy wood plank bridge. The back end of the truck bounces you up, and when the truck's back end slides, it pulls itself out from under you. Result -- you end up rolling down a dirt road in Upper Michigan. Don't stick a hand out to stop rolling -- you WILL find the only sharp rock in the general vicinity with the palm of your hand. And, you will wind up with a mouthful of sand.

'Nuff said...(ptooey...).

24. Ok, I certainly hope im not dragging this out too long, but should we also take into consideration that time slows down when we go faster? We travel through x,y,z, and t moves to compensate for this change of rate, by slowing down. Would x,y,z return the favour for t?
By the way, thanks for the warm welcome, guys. i appreciate it.

25. On 2003-03-05 12:06, Reacher wrote:
Ok, I certainly hope im not dragging this out too long, but should we also take into consideration that time slows down when we go faster? We travel through x,y,z, and t moves to compensate for this change of rate, by slowing down. Would x,y,z return the favour for t?
I would say yes. The space dimensions change as well, according to special relativity. Is that what you were thinking of?
By the way, thanks for the warm welcome, guys. i appreciate it.

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Hmmm.....all this talk of time travel, it just makes me giddy. Anyway, this is a topic that has been debated by some of the best minds of science throughout time. When Einstein proposed his theory of general relativity, one of the first things people derived from it was the twins paradox.

According to Einstein, the faster you go, the slower time goes for you relative to an observer, say here on earth. So if there are two twins on earth and one of them blasts off and goes 80 percent of the speed of light. The twin on earth sees his twin go at 80 percent of the speed of light, he goes to Alpha Centari (or beta if you really want to be specific) 4 light years away and the twin on earth sees him take about 5 years to do it. However, the twin on the space ship realizes that as he goes at 80 percent the speed of light, the distance between him and Alpha Centari has shortened and it only takes him maybe a year to get to Alph Centari. On the return trip, by general relativity, the same thing happens.

So when the twins are united back on earth 10 years has passed for the earth bound twin, while only a year or so has passed for the space bound twin.

Time travel? Maybe, maybe not, it does seem plausible by general relativity, but there are schools of though that say perhaps general relativity is wrong in this sense. But most of the books I've read seem to agree that time travel to the future is more likely than to the past.

Stephen Hawking has a theory that time travel to the past is made impossible by the fact that time and nature has its ways of preventing paradoxes (like going back in time and killing your grandfather). In fact if I remember correctly he has a bet with Kip Thorn in the amount of 20 dollars, that time travel to the past will never be possible.

For more about this time travel stuff, I suggest you read <u>A Brief History of Time</u> and <u>The Universe in a Nutshell</u> by Stephen Hawking, <u>The Future of Spacetime</u> by Stephen Hawking, Kip Thorne, Igor Novikov, Timothy Ferris, Alan Lightman, and Richard Price, and <u>Time Travel in Einstein's Universe</u> by J. Richard Gott.

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Stephen Hawking has a theory that time travel to the past is made impossible by the fact that time and nature has its ways of preventing paradoxes (like going back in time and killing your grandfather). In fact if I remember correctly he has a bet with Kip Thorn in the amount of 20 dollars, that time travel to the past will never be possible.
That's a safe bet. If they invent time travel he can go back and talk himself out of making the wager.

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