# Relativity and Time Travel

Discussion in 'Physics' started by 402DF855, Apr 7, 2013.

1. ### 402DF855 Thread Starter New Member

Feb 9, 2013
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Something has puzzled me for years, perhaps someone can explain it to me.

If your spacecraft flies at some speed close to the speed of light (c), for say a year and returns to Earth to find 1000 years have passed. Maybe 0.99c.

So what if your spacecraft flies at 0.99c but in an orbit identical to Earth's just far enough away from the sun to maintain 0.99c.

How could 1000 years have passed since Earth and the spacecraft circled the sun the same number of times over the course of the flight?

2. ### Wendy Moderator

Mar 24, 2008
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For one thing it would have to be a powered orbit, even after you achieved your target speed you would need continuous thrust just to travel in a circle, gravity would be no help at all.

The earth circles the sun at a set speed. Your space craft also circle the sun at a set speed. These are not the same speeds, so there will be time slippage. You are talking as if they are traveling the same speeds, they are not.

The rate of time flow aboard the space craft would slow down. That is how it works.

3. ### DerStrom8 Well-Known Member

Feb 20, 2011
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Bill, I think this is what he's asking:

The earth orbits the sun at a certain speed.

If you have a ship that orbits the sun at a much faster speed, but is far enough away from it to keep the same orbit as the earth (the earth stays between the ship and the sun), what would happen to the time difference? Would it be non-existent?

4. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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using an atomic clock it has been proven...

Time passes slower and slower compared to empty space, the more intense a gravity field is.

Time goes by faster in orbit(weightlessness) than it does on Earth below.

5. ### 402DF855 Thread Starter New Member

Feb 9, 2013
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Yes, exactly.

6. ### DerStrom8 Well-Known Member

Feb 20, 2011
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A similar, yet fairly unrelated question I've had is this: If you hold, say, a ruler at the end in one hand and pivot it, the opposite end moves a greater distance than the end closest to you in the same amount of time, right? Since speed is distance divided by time, that means the opposite end is moving faster. If that's the case, then in theory, if you have a ruler long enough, couldn't you make the opposite end move at the speed of light by pivoting slightly on the end closest to you?

7. ### #12 Expert

Nov 30, 2010
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Yes, as energy approaches infinity and the strength of the ruler approaches infinity.

8. ### 402DF855 Thread Starter New Member

Feb 9, 2013
9
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I've read a similar question posed with a giant scissor. Regardless, I think the space around the far end of the ruler deforms to limit the velocity to c.

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9. ### Wendy Moderator

Mar 24, 2008
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The speed the earth orbits is dictated by the gravitational field. For you to orbit at 99% you would be so far out you would have to use a rocket just to maintain the orbit (it would not be a true orbit in other words). The outer planets take much longer to complete an orbit, while the inner planets do so much faster. Mars takes two years (give or take), while Venus takes around ½ year.

My assumption from the question is that the hypothetical spaceship would match earths location relative to the sun, for this to happen (and remember, thrust would be needed to curve the trajectory since gravity doesn't work) you would have to be very far away from the sun indeed, in deep space far away from this solar system.

A black hole would allow a hyperfast true orbit, however the gravitational forces would tear you apart.

The experiments showing time slippage from space craft orbiting the earth were not caused by the gravitational field, but by the motion of the space craft relative to the earth. In other words, pretty much what you were asking. This has been verified many times, many ways.

A gravitational field also causes time dialation, such as near a black hole. It is estimated that Mercury has a 2% time dialation compared to earth from our suns gravity. The earth's gravity is far too puny to cause this effect in a noticable way.

One of the proofs of Einstein's theories was the 2% variation, which Neuton's theories could not account for.

10. ### amilton542 Active Member

Nov 13, 2010
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More often than not, I'm plagued by intense vivid dreams on a regular basis.

I'll add to this, even though it's not relevant.

We can never leave the Universe. It's a building we can never leave. The illusions of exit at a distance are open doors that close via shutters. As you approach closer and closer towards the door its shutters oscillate up/down faster and faster; until when you reach this exit the shutters are sealed shut.

Only when you approach the exit at the speed of light can one leave.

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11. ### DerStrom8 Well-Known Member

Feb 20, 2011
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Well guys, I had a little extra time on my hands and decided to do a little bit of math regarding my proposition of an insanely long ruler:

Let's assume the end of the ruler closest to you is attached at the end and acts as a pivot. The ruler is rotated 90* in one second. The distance traveled by any given point on the ruler is (2*pi*r)/4--circumference divided by 4 to get 90*. So by setting this as a rate with respect to time, you could set it up as follows:

(2*pi*r)/4 = c (where c is the speed of light, 3*10^8 m/s).

From there you can calculate the necessary radius:

r = 4c/(2*pi)

r = [4(3*10^8)] / (2*pi)

r = (12*10^8) / (6.28)

r = 1.91*10^8 meters.

The board would have to be 1.91*10^8 meters long.

After working through this it occurred to me that chances are, the thing that would keep the opposite end from reaching the speed of light would be the object's inertia. even if it was weightless, it would have a lot of inertia, which would make it nearly impossible to move the end closest to you 90* in 1 second. That, in turn, would prevent the opposite end from reaching the speed of light.

Sorry to take over the OP's thread like this, just figured that since I mentioned the problem before, I should mention the answer I got as well.

If I made an error or if anyone has any thoughts, please let me know

Cheers,
Matt

12. ### #12 Expert

Nov 30, 2010
17,818
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Spooky Mulder.

We are having difficulty getting to the next PLANET! Next solar system? Next galaxy? The Restaurant at the end of the universe is a very long way. Perhaps it is for people to be comfortable while they wait for the shutters to open.

13. ### amilton542 Active Member

Nov 13, 2010
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What I'm trying to say is, these "wormholes" could exist, but up close they do not.

Only if you approach one at exactly the speed Celeritous could it continue to exist.

A bit ironic, don't you think.

But hey, just a mad-man's ramble.

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14. ### 402DF855 Thread Starter New Member

Feb 9, 2013
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If I understand the Wikipedia article on time dilation (a big if), the speed needed to advance 1000 Earth years in one spaceflight year is: 99.99995% of c.

15. ### SplitInfinity Member

Mar 3, 2013
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The answer to this is...because time would be required to travel the distance away from Earth necessary to travel at .99c...it would be IMPOSSIBLE to obtain such an orbit following Earth orbit of it's sun unless the craft started such an orbit at the vast distance away necessary.

Split Infinity

16. ### 402DF855 Thread Starter New Member

Feb 9, 2013
9
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My analysis is probably wrong, but the distance from the sun required to achieve the necessary orbit is 255x the Pluto-sun distance. A spacecraft capable of flying the requisite velocity can make this distance in 58 days.

17. ### SplitInfinity Member

Mar 3, 2013
369
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You have to take into account that the craft must first travel that distance out to begin the orbit.

The time passage on Earth would be years if the craft were traveling at .99c just to get to the distance for this orbit.

Even if the craft just started at this orbital distance...it could not keep up with the Earth's orbit as Time Dilation would not allow the craft...even at .99c...to keep up with Earth's orbit.

Split Infinity

18. ### 402DF855 Thread Starter New Member

Feb 9, 2013
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1
I think you're right. As the spacecraft maintained the proper velocity it would see the Earth spinning around the sun at a much faster pace.

19. ### BillO Distinguished Member

Nov 24, 2008
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You mean one of those parsec long, infinitely strong, mass-less rulers they sell at Home Depot?

20. ### BillO Distinguished Member

Nov 24, 2008
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Actually, the distance around the sun would look shorter to the folks on the spaceship (Lorentz contraction).