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## Relativity Question

I have thought about this for a very long time. Lets say that you are traveling on a spaceship that is going to the Andromeda galaxy at a speed of 99.99999999% the speed of light, so that time dilation effects become enormous. Will the extremely high speed enable the space travelers to get to the Andromeda galaxy faster, or will it still take the 2 million years? My question is does traveling at speeds very close to light actually make time flow slower for you, or does time go at the same speed for you and increase for others?

Thanks for the responses.

2. ## Re: Relativity Question

I have thought about this for a very long time. Lets say that you are traveling on a spaceship that is going to the Andromeda galaxy at a speed of 99.99999999% the speed of light, so that time dilation effects become enormous. Will the extremely high speed enable the space travelers to get to the Andromeda galaxy faster, or will it still take the 2 million years? My question is does traveling at speeds very close to light actually make time flow slower for you, or does time go at the same speed for you and increase for others?

Thanks for the responses.
Both

All Reference Frames are Equal!

Repeat that Three Times.

3. To add to Zaphod's post, a couple question to think about after the mantra:

How will that trip look to a person in the spaceship (distance travelled/time taken)?

How will that trip look to a person watching the spaceship from Earth(distance travelled/time taken)?

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## Re: Relativity Question

I have thought about this for a very long time. Lets say that you are traveling on a spaceship that is going to the Andromeda galaxy at a speed of 99.99999999% the speed of light, so that time dilation effects become enormous. Will the extremely high speed enable the space travelers to get to the Andromeda galaxy faster, or will it still take the 2 million years? My question is does traveling at speeds very close to light actually make time flow slower for you, or does time go at the same speed for you and increase for others?

Thanks for the responses.
The answer is: it depends on who is counting the seconds.

From the perspective of Earth, the ship takes just less that 2 million years to reach Andromeda (but they wouldn't see the ship arrive for another 2million years because of the speed of light). From the perspective of the people on the ship, the travel time would be in the order of months.

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great question! the short answer: time is relative, as is motion. To the folks on the ship, time seems to be going by at the "normal" rate, but they notice the distance to the Androm. galaxy is quite a bit shorter than the 2 million ly's they saw it at when in Earth's frame of reference. Hense, they are able to get there in days/weeks/months (there's a formula that'll tell you exactly how much time it takes them) rather than millions of years (although as mentioned, millions of years will have past back on earth during their trip). The great thing about relativity is, time doesn't just "seem" to pass at different rates for those travelling at relativistic speeds compared to another frame of reference. Time in fact does pass at different rates!

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So even if we could reach a speed very close to light, the only people who would ever know would be the space travelers themselves; even radio communication would take millions of years. :x

7. Originally Posted by Brady Yoon
So even if we could reach a speed very close to light, the only people who would ever know would be the space travelers themselves
Know what?

We accelerate particles to high percentages of the speed of light now.

8. ## Re: Relativity Question

... does travelling at speeds very close to light actually make time flow slower for you ...
Don't you mean, faster? Almost three million years crammed into a few weeks?

Actually, in your frame, you'll experience no change in the rate at which time flows (as other posters have pointed out).

9. ## Re: Relativity Question

Originally Posted by Eroica
... does travelling at speeds very close to light actually make time flow slower for you ...
Don't you mean, faster? Almost three million years crammed into a few weeks?
Slower, in the sense that they would be aging slower.
Actually, in your frame, you'll experience no change in the rate at which time flows (as other posters have pointed out).
Asking about a difference in rate implies that there is a comparison between two different frames. They'll experience a slower rate, in that frame, than someone in the other frame. It will seem normal, I know that is what you mean, but the comparison is valid.

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So even if we could reach a speed very close to light, the only people who would ever know would be the space travelers themselves; even radio communication would take millions of years. :x
Millions of years for who? Keep in mind, when you have 2 frames of references travelling at relativistic speeds compared to each other, "time" kind of becomes a meaningless measurement (well, loosely speaking anyways). You're better off grasping things if you start looking at "when" something happens as opposed to the time it takes. If the guys on the ship could somehow "see" the radio transmission being made back on Earth, they would completely disagree with the Earth people on "when" the message was sent. Cool thing is, both would be complete correct in their perception of the "when"! It is all completely relative!

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This may sound against the mainstream, but here goes:

No human being has ever experienced time dilation they could actually perceive. I'm talking so fast you experience a day in a week or something.

How do we know the effect applies to human conciousness? We know the decay rate of subatomic particles dilates, but will our perception of time? In order to answer that question we would need either to experience significant time dilation or have a model of how conciousness works, and we have neither.

Assuming conciousness is just a series of chemical reactions, then yes you will experience less time at higher speeds because those chemical reactions will be slower. In fact, with a constant acceleration of 1G you could explore the entire universe in a human life time (or so I'm told).

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No human being has ever experienced time dilation they could actually perceive.
While that is a true statement the reason it is true is because time dialation cannot be perceived, regardless of how little or much.

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Originally Posted by Evan
While that is a true statement the reason it is true is because time dialation cannot be perceived, regardless of how little or much.
You can look out the window. Or you can get back to Earth, find a weeks past and think "bloody hell, it felt like a day to me"

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We are looking out the window and we do see red shifted and blue shifted objects. Who is moving, us or them? Are they time dilated or are we?

15. Originally Posted by Evan
We are looking out the window and we do see red shifted and blue shifted objects. Who is moving, us or them? Are they time dilated or are we?
I think Damburger meant, looking out the window of a spaceship.

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You cannot make any measurement within your moving frame of reference that will seem any different than if you were not moving relative to something else. You can look out the window and infer from effects like redshift that there is relative motion occuring but you still cannot perceive an effect on yourself or your moving frame of reference.

17. Originally Posted by Evan
You cannot make any measurement within your moving frame of reference that will seem any different than if you were not moving relative to something else. You can look out the window and infer from effects like redshift that there is relative motion occuring but you still cannot perceive an effect on yourself or your moving frame of reference.
But what happens when Brady's spaceship returns to Earth?

What is perceived looking out the spaceship's window just prior to the return?

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Upon return you will find you have aged less than those who have stayed behind. You can draw two conclusions, both seeming equally valid: They have experienced accelerated time in your absence or you have experienced decelerated time. There is no way to determine which is true.

Even when someone observes you going past them at near lightspeed you will not appear contracted to them, likewise they will not appear elongated. The only effects to be observed are frequency shifting effects such as redshift which may be caused by motion of either frame relative to the other.

See here:

http://math.ucr.edu/home/baez/physic...R/penrose.html

19. Originally Posted by Evan
Upon return you will find you have aged less than those who have stayed behind. You can draw two conclusions, both seeming equally valid: They have experienced accelerated time in your absence or you have experienced decelerated time. There is no way to determine which is true.
Good point. However, the traveler might notice how all other clocks in the universe seem to match the Earth clocks and no longer his. One could argue that the entire universe accelerated with Earth at the time our traveler hit the "fire super-duper rocket engines" button. Logically, most would assume the rocketeer did something to change his time.

That's surprising. Thanks.

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However, the traveler might notice how all other clocks in the universe seem to match the Earth clocks and no longer his.
Nope. You can't compare two clocks in different places.

21. Originally Posted by Evan
However, the traveler might notice how all other clocks in the universe seem to match the Earth clocks and no longer his.
Nope. You can't compare two clocks in different places.
I meant after he returned to Earth.

22. How are you going to read other clocks in the universe from earth? Even if you had a super telescope that could do it, you would see that each had a much different time than the time on earth.

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Originally Posted by George
Originally Posted by Evan
However, the traveler might notice how all other clocks in the universe seem to match the Earth clocks and no longer his.
Nope. You can't compare two clocks in different places.
I meant after he returned to Earth.
I think either clock's atomic-mechanical oscillations would be observed to redshift-outgoing, but blueshift-incoming, observationally speaking, so the net effect upon bringing the clocks back together again should be where their times are once again equal. This may not be exactly the same as clocks slowing in gravitational fields, however... but I could be wrong. [-X :-?

24. Originally Posted by Saluki
How are you going to read other clocks in the universe from earth? Even if you had a super telescope that could do it, you would see that each had a much different time than the time on earth.
You couldn't read them instantaneously even if you could see them. But, you could still calculate their time based on prior information. Your time and my time, for that matter, are different, but we can offset the time difference with our knowledge of spacetime. Of course, the difference between us is insignificant.

It is believed that even though space is expanding, our galactic clock is the same as the other galactic clocks due to symmetry.

What will be noticed is the time Brady's traveler sees once he/she has returned home. Both times are relative as Evan has said but the difference is real and the majority of the universe would say he/she is younger than "normal".

25. Originally Posted by Lunatik
Originally Posted by George
Originally Posted by Evan
However, the traveler might notice how all other clocks in the universe seem to match the Earth clocks and no longer his.
Nope. You can't compare two clocks in different places.
I meant after he returned to Earth.
I think either clock's atomic-mechanical oscillations would be observed to redshift-outgoing, but blueshift-incoming, observationally speaking, so the net effect upon bringing the clocks back together again should be where their times are once again equal. This may not be exactly the same as clocks slowing in gravitational fields, however... but I could be wrong. [-X :-?
Logical, but not correct. They don't balance out.

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It is a basic principle of Relativity that you cannot be in two places at the same time. Therefor there is no way to compare two clocks in different places. If you bring them together so that they are infinitesimally separated then you may compare them. In order to bring them together one or both must be moved. This takes time and affects time. This is inherent in the concept of simultaneity. It is not possible to communicate instantly so absolute comparisons cannot be made. The concept of "the same time as here" has no actual meaning.

This has been considered at great length by many. See here for an in depth discussion.

When we look at the distant galaxies with high redshifts we say that they are moving away from us. But, if they look at us they say we are moving away from them. Which is it?

27. Originally Posted by Evan
It is a basic principle of Relativity that you cannot be in two places at the same time. Therefor there is no way to compare two clocks in different places. If you bring them together so that they are infinitesimally separated then you may compare them. In order to bring them together one or both must be moved. This takes time and affects time. This is inherent in the concept of simultaneity. It is not possible to communicate instantly so absolute comparisons cannot be made. The concept of "the same time as here" has no actual meaning.
I don't disagree with this. However, if you travel to a star of, say, exactly 10 light years distance, then radio back to us your time, we would know, 10 years later from the time you sent us your time, what time it was for you 10 years ago. If someone steals your rocket and comes here, his time will be less than "normal" relative to your time and ours. Of course, you will have to wait 10 years to learn this but you might be glad we at least have your ship.

This has been considered at great length by many. See here for an in depth discussion.
I was hoping not to get this deep. :-?

When we look at the distant galaxies with high redshifts we say that they are moving away from us. But, if they look at us they say we are moving away from them. Which is it?
[added: oops, got in hurry and missed this.] Both, but the expansion does not alter the rate of time between us for the same reason. How could one galaxy be slower or faster than the rest?

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I'm not quite sure I understand your point George. My point is simply the old saw that everything is relative to your point of view and frame of reference.

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Originally Posted by Evan
Upon return you will find you have aged less than those who have stayed behind. You can draw two conclusions, both seeming equally valid: They have experienced accelerated time in your absence or you have experienced decelerated time. There is no way to determine which is true.
I know that generally, this would be true such as looking at a galaxy that is moving away at a high rate of speed. You would not be able to tell who is really moving away from who, etc. But in his example, isn't it obvious what happened?

If you leave in a spaceship that you know will travel at near light speed, and then upon your return to Earth you find that you have aged less than those who remained behind, isn't it safe to say that you experienced decelarated time? In his example, I thought it was a given that the ship was accelerated to near light speeds while the Earth's speed remained constant.

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Imagine that the person taking the trip knows nothing about relativity theory. They then experience the time dilation effect. However, there is no way to determine before, during or after that this will be the case. It is not possible for them to say or prove what happened. In their frame of reference no change to any clock or measuring instrument appeared to occur. This is the case.

That being the case it is just as likely to assume that something happened to the rest of the universe since it it "obvious" that nothing unusual happened to the traveler, from the travelers point of view.

This is the "naive" interpretation of the "Twin Paradox". In actual fact if the traveler is not naive and does have knowledge of Special Relativity then she will know that in this case there are not just two frames of reference to deal with but three.

That is what makes it actually possible as some of you have been maintaining to know (not perceive) the difference. This is why one person ages less than the other. It is still not possible to determine this on the travelers ship but there is a resolution to the problem.

See here

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