I am an amateur in the realm of relativity, but I think I grasp the basics. Here is my question: If you are travelling in a space ship away from earth near the speed of light, how would you in the space ship perceive your clock to be moving? I believe that since in your frame of reference, the Earth is moving away from you, so if you could look at a clock on Earth from your space ship it would slow down, therefore making your clock faster by comparison. Is this correct reasoning? On the other hand an observer on Earth would see you and your spaceship moving from his/her frame of reference and see the spaceship clock moving slower. My ultimate question is will the observer in the spaceship really see his clock move faster with respect to the Earth clocks? Thanks for your help. I have a dinner bet on this so don't let me down!

This is correct. The observer on the spaceship would say the Earth-bound clock is running more slowly than the spaceship clock. The observer on Earth would say the spaceship clock is running more slowly than the Earth-bound clock.

Of course, it's not just the "clocks" -- the observers would say that time itself is flowing more slowly at the other point.

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SeanF

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I may have missunderstood the question but I believe you have it backwards. For the person in the ship time would slow down WRT time on Earth so the Earth clock would appear to be very fast. If the traveller came back to Earth, all of the people who stayed behind would have aged more than the traveller.

Now from the travellers own point of view time would pass normally. Earth bound people would see the traveller's clock seem to slow and traveller's aging would slow also.

This is sometimes refered to as the "Twins Paradox". It is not actually a paradox but..... what the heck? [img]/phpBB/images/smiles/icon_wink.gif[/img]

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No, not for the usual formulation of the problem.

That's true, though.

Also true.

Russ, check GrapesOfWrath's post above mine here. The ship observer sees the Earth-bound clock as running more slowly than his own while he's moving away from Earth and while he's moving back towards Earth. What he sees happening back on Earth at the time he stops and turns around has been the subject of much discussion here on this board, and comes into play when determining why more time has passed on Earth and not less.

In other words, it is a bit of a "common sense" paradox, but the math does actually work out . . .

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SeanF

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Howdy folks,

I just wanna add a little to what SeanF said. Whether the spaceship is moving towards Earth or away from Earth, our intrepid traveller will see Earth's clock ticking at the same slower rate. The amount of time dilation is a function of the square of velocity, so regardless of whether you're coming or going, it's the same. (For those who know vector calculus and want to get picky, it's actually a function of velocity dotted with itself.)

If he could see the Earth clock from his ship, the Earth clock would be moving slower than his spaceship clock as he approaches the speed of light. If he just looked at his clock inside the ship, it would appear to be running at the "normal" speed all the time, and not speeding up. His spaceship clock runs faster in relation to the reference frame of the Earthly clock as he approaches the speed of light.

<font size=-1>[ This Message was edited by: Chip on 2002-02-15 11:29 ]</font>

I believe that we are mis-interpreting statements such as the one above.

When the observer looks back on Earth, I believe that from his perspective ALL movements on Earth will seem faster (like watching a video on fast forward).

As I read statements such as the one quoted above I am given the impression that the opposite was intended, but I find it difficult to believe that somehow I have got this thing backwards (if observer on the spacecraft actually saw movements slowing down on Earth this would make no sense at all).

To make myself clear I offer a simple thought experiment. Let us assume that the observer on board the spacecraft completes his journey in just 6 hours (Earth time). If we now assume that the clock on board the spacecraft has advanced just 3 hours, then the hands on the clocks on Earth must have been moving twice as fast as those on the spacecraft.

Now if he was watching the clocks on Earth and the hands appeared to be moving slower throughout the trip, then how did the clocks on Earth end up showing times which were ahead of the times denoted by the clocks on board the spacecraft ?

Jeff


<font size=-1>[ This Message was edited by: Phobos on 2002-02-15 21:46 ]</font>

That's why it's called a paradox, although it really isn't. Just seems counter-intuitive, I guess.

Twin paradox is my attempt at explaining it. And this, and this.

Your examples are well presented explanations of certains specifics about the twins paradox, but you have not addressed the example I gave of the 6 hour trip.

In this example there is only one possibility and that is that the observer on the spacecraft will see clocks on Earth move faster than clocks on board his spacecraft.

Phobos

Just substitute "hours" for "years" at that first link. Then Ann, on Earth, ages 10 hours, while Bob ages 8 hours. And yet, while traveling, they both see each others clocks as slow.

Another quick question.

A fellow brought up the following point when we were discussing time dilation. If you travel near the speed of light to a planet 100 light years away, the spaceship would get there in say 20 years. Now only 20 years have passed for our traveller, but he has reached a planet that should take a 100 years at the speed of light. He has no reference (assuming the planet has no one to greet him). It was brought to my attention that the traveller would be convinced that he must have travelled faster than the speed of light since only 20 years have elapsed for him. Weird. Am I missing something? Oh, and thaks for all your help on the previous post!

Yes, you are missing a possibility. The guy in the space ship, during the turn around, sees the universe in the direction of his acceleration speed up. In other words, he all of a sudden is under the influence of a gravity-like force, except that this force has no mass associated with it. The universe in the direction opposite this "nonNewtonian" force has a rate of time that is speeded up relative to his own. The further that portion of the universe is, the bigger it speeds up.

Just repeat after me. An inertial frame is one where the laws of physics are simplest. An accelerated frame is one where the laws of physics are a bit complicated. The time dilation (the slow down) only has to apply to the inertial frame.

Yep, you're missing something. Relativity not only involves a temporal "distortion," but also a spatial "distortion." While moving, the traveler would measure the distance between the two planets as being a little less than 20 light-years. Therefore, since it took him 20 years to travel less than 20 light-years, he would measure his own velocity as less than the speed of light.

When he's done and stopped, he could look at Earth 100 light years away and say, "Gee, only 20 years passed since I left there," but he would at no time during the journey have measured his own velocity as exceeding c.

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Phobos, in order to get back to Earth, the traveller needs to stop and turn around. He would "see" weird things happening with the clocks on Earth while doing so.

However, he would see the Earth clock as running more slowly than his own clock during the entire time he is simply moving relative to Earth.

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SeanF

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What if when he turns around in a large curve without slowing down? by the time he reaches earth there would be a huge discrepancy between the earth clock as he observes it and the actual time on the clock.The clocks must be syncronised when he stops so when and how does the clock as the shipboard observer sees it catch up to the clock as it realy is?
I have been thinking about this and would like to present a thought experiment which gives a different view on things:
On earth is a clock E1e;on a space ship are three clocks, S1 shows ship time, E1s shows earth time via an instantanious link to E1e,and E2 which is timed by a powerful laser pulsing at 1 second intervals fired along the course the ship will travel. The ship flies off into space and returns 10 years later at which time E1s shows 10 years and S1 shows 8 years.
Let us now look at the journey by freezing the action at certain points.(The figures are illustrative not exact)On the way out: 1 light year out, E1e reads 20 months E1s (because of its instantanious link) reads 20 months, S1 reads 16 months(80% earth time)and E2 reads 8 months(E1s-12months because of distance). 2 light years out, E1e reads 40 months E1s reads 40 months, S1 reads 32months and E2 reads 16 months(40-24) So far the earth clock E2 appears to be running slow but what about the eventual return trip? 2 light years out, E1e reads 80 months E1s reads 80 months, S1 reads 64months and E2 reads 56 months(80-24); 1 light year out, E1e reads 100 months E1s reads 100 months, S1 reads 80 months and E2 reads 88 months(100-12). In both directions regardless of speed E1e and E1s agree,On the way out clock E2 appears to run slow buton the way back clock E2 appears to speed up until when it finaly arrives back on earth E1e and E2 once again read the same.

Sarnian,

All your numbers seem to be right, and there's no discrepancy in the times the clocks are reading at the end, so I'm not sure what your question is.

Something to think about, though -- ignore the E1s clock (since under current theory instantaneous communication is impossible) and figure out how a person on the spacecraft would determine the time of E1e based on what he's reading on E2.

Keep two things in mind: First, that as far as he's concerned he's sitting still and the Earth is moving; Secondly, that the speed of light and his own speed should remain constant for him.

Specific example: at the first point you mentioned (S1 reads "16 months") Earth's been moving away from him at 0.6c for 16 months (by his measurements), and he's receiving a signal from E1e that says "8 months". How long ago (S1 time) did that "8 months" signal leave Earth, and where was Earth at the time? What was the difference between S1 and E1e at that time?

Figure this out for various points on the outgoing trip, getting further from Earth each time. Then, figure this out for various points on the return trip (keeping in mind that Earth is moving towards the spaceship now).

What happens to the spaceship's measurement of Earth time close to the turn-around point?

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SeanF

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Excuse me but- I haven't heard anyone mention that there is a difference between the time dilation and our perception of the time dilation. Specifically, the question was "what would the observer see" that means we are asking for the latter, not the former. The reason I say this is because the signals which are travelling back and forth between the Earth and the ship will appear red shifted, or stretched. This causes the perception, on both ends, that the other end is running slow. Isn't that correct? And by extension that means on the return trip both sets of signals would be blue shifted, or in other words, speeded up.That means both ends would see the other's time as moving faster.

<font size=-1>[ This Message was edited by: grumium on 2002-02-24 12:12 ]</font>

If so, then how would it work out for the return journey? As you point out, it seems to reverse the effect--so that's a clue.

<a name="20020225.5:09"> page 20020225.5:09 aka on Impulse
On 2002-02-25 04:54, GrapesOfWrath wrote: To: 5:10 A.M. PST

On 2002-02-24 11:31, grumium wrote:


LOCKTHRD = HUb' Ba INDX
6:43
5:10
1:33 to convert PST to BaBBpt add one HOUR 33 min?

<font size=-1>[ This Message was edited by: HUb' on 2002-02-25 06:47 ]</font>

Are you saying the signals would not be blue shifted on the return? Or that they would not appear speeded up? Or something else?