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Old Sep 26th 2018, 09:21 AM   #1
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what do I see out the window?

Hi folks,

I'm hoping this will be really easy for you guys. Also that I am posting in the right forum.

If I am in a space ship travelling near the speed of light, time will slow down. I experience time as I would here on earth and only notice the effect of time when I get back to earth and discover that a lot of time has passed. I have travelled into the future.
My question is: if while in my ship where time is very slow, I look out the window and see another moving object, will it appear (almost) stationary, because my time has slowed down so much or will I see it moving at its actual speed since it is outside my environment?
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Old Sep 26th 2018, 09:26 AM   #2
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Originally Posted by uxinox View Post
My question is: if while in my ship where time is very slow, I look out the window and see another moving object, will it appear (almost) stationary, because my time has slowed down so much or will I see it moving at its actual speed since it is outside my environment?
Second option, but you will see it moving at its speed relative to yours.
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Old Sep 26th 2018, 10:38 AM   #3
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Many thanks!
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Old Sep 26th 2018, 07:05 PM   #4
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Originally Posted by benit13 View Post
Second option, but you will see it moving at its speed relative to yours.
I'm not sure that's correct ......

You are in a spaceship that has left earth and is travelling away from Earth at 0.6c ....

Another craft leaves earth traveling at 0.8c ... it's chasing you and will overtake you given time ....

You look out the back window .How fast does it appear to you that the second craft approaching you ???

0.8c - 0.6c = 0.2c ??? No , wrong ..

it appears to you to be travelling at 0.385c ....

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Old Sep 27th 2018, 02:24 AM   #5
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Originally Posted by oz93666 View Post
I'm not sure that's correct ......

You are in a spaceship that has left earth and is travelling away from Earth at 0.6c ....

Another craft leaves earth traveling at 0.8c ... it's chasing you and will overtake you given time ....

You look out the back window .How fast does it appear to you that the second craft approaching you ???

0.8c - 0.6c = 0.2c ??? No , wrong ..

it appears to you to be travelling at 0.385c ....

I don't claim that relative speed should be calculated using subtraction like that. You have to use the Lorentz transformations (in SR). It's still a relative speed.
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Old Sep 27th 2018, 11:19 AM   #6
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Great. Many thanks, Gents, for the detailed answer.
Now, if I were to ask the question, what do I see looking into the window, that is, looking into the ship from a stationary point outside. Would I see people moving slowly inside the ship? Or would the practical difficulty of seeing through such a fast moving window invalidate the question?
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Old Sep 27th 2018, 07:03 PM   #7
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Originally Posted by uxinox View Post
....what do I see looking into the window, that is, looking into the ship from a stationary point outside....

I confess I'm not comfortable with this subject .... I've been watching some Youtube videos ... My understanding is time moves slower on a moving object and it doesn't matter if the object is moving towards , or away from the stationary reference ....

One video talked about einstein's thought experiment , he concluded that if he was travelling away from a clock at close to the speed of light the clock hands would stand still ... time for him time would stand still ...well maybe , I can sort of see the logic in that ...

But if he were moving towards the clock surely the clock would appear to move fast and time would speed up ???? 35 secs in this video


Last edited by oz93666; Sep 27th 2018 at 07:07 PM.
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Old Sep 28th 2018, 02:19 AM   #8
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Originally Posted by uxinox View Post
Great. Many thanks, Gents, for the detailed answer.
Now, if I were to ask the question, what do I see looking into the window, that is, looking into the ship from a stationary point outside. Would I see people moving slowly inside the ship? Or would the practical difficulty of seeing through such a fast moving window invalidate the question?
I think it's both. However, even in an ideal case, it's not straightforward. I think it's best to think of it is this way: "you would observe the motion as obtained using the Lorentz transformations, which will show the effect of time dilation". So if somebody calculates the relative motion using the Lorentz transformations but does not explicitly calculate the time dilation, the answer is still correct because time dilation is a consequence of applying special relativity, not an intrinsically modelled effect.

In all honesty, my relativity is rusty, so I'm not particularly confident in this answer. Perhaps other forum members could weigh in?

Last edited by benit13; Sep 28th 2018 at 03:46 AM. Reason: typo
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Old Sep 28th 2018, 03:44 AM   #9
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many thanks for the video, oz! It actually confirms that I thought. A person in a space ship (or a very fast tram!) doesn't experience time slow down, but when he looks outside he sees the world slow down. This means he doesn't see other objects, like the hands of the clock, at normal speed, but at a reduced speed and if he could get his ship to the speed of light the outside objects would stop. In the ship, time passes as normal.
But this is confusing stuff!
Benit13 could be right!
Would be great if some other guys could 'weigh in'!

Last edited by uxinox; Sep 28th 2018 at 03:46 AM.
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Old Sep 28th 2018, 06:10 AM   #10
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Everything is Relative

You look out of the window and glimpse (as you wiz past) someone looking back out of the window of another space ship.

Who is moving and who is stationary, relative to what "fixed" point.

The other person is moving relative to you,
so (to you) their time will seem slow relative to yours.

You will be moving relative to them
so (to them) your time will seem slow relative to theirs.
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