# Thread: Relativity and a resting frame of reference.

1. Sometimes I feel like I have an ok understanding about some of the issues of relativity. Time Dilation, Length Contraction and the sort.

Other times, like right now, I don't feel like that is the case. I'm sure this is answered simply through relativity, but right now it's eluding me. I have a question based on one assumption which we know to be true. The assumption is that the faster you move, the slower time progresses. With that assumption, would it be possible to have one atomic clock in one position, and another atomic clock in another position and determine which atomic clock is slightly faster? Obviously you would have to narrow out other factors that may affect it, such as the gravitational fields in which the atomic fields travel. Also, let's say for the sake of argument we didn't have to travel (or accelerate) to check the atomic clocks, we could check them via wireless signal.

If we could determine that one is moving through space time faster than the other, then couldn't it be said that one is moving slower?

Through numerous experiments, numerous atomic clocks and some nifty triangulation shouldn't it be possible to eventually determine a resting frame of reference? Wouldn't that contradict relativity?

I'm sure it's a newb question, so I apologize in advance.

Also, I'm new here. Hey everyone. A lot of you have impressed me with your knowledge... except a few people in the political section :-D

EDIT: Or is it that they will both read the same time, with the exception of which has been accelerated more, or in which gravitational fields the atomic clocks have traveled? I'm pretty sure now that I think about it a few more moments.

I guess the thread can still have some value, in case others have had the same question.

2.

3. Here's the thing, there is no absolute motion. If you have two clocks with a realtive motion with respect to each other, which one runs slow depends on who is comparing the clocks.

If you are stationary with respect to clock 1, then it is clock 2 that is running slow, but if you are stationary with respect to clock 2, then it is clock 1 that runs slow.

4. hmmm... I think if you are moving through space at a constant speed, you will always observe yourself to be 'resting', so, I don't think you can. I think 'resting' is entirely relative to one's own frame of reference.

edit: and also that any motion is, in your own frame, a form of moving away from you while you are at 'rest'. so, pun not intended, it's all relative.

5. Originally Posted by Janus
Here's the thing, there is no absolute motion. If you have two clocks with a realtive motion with respect to each other, which one runs slow depends on who is comparing the clocks.

If you are stationary with respect to clock 1, then it is clock 2 that is running slow, but if you are stationary with respect to clock 2, then it is clock 1 that runs slow.
So.. pretty much, if you're in the same frame of reference you measure it to be faster.

Makes sense. I wonder if I were exposed to this at an early age (like when we start dealing with 'conventional' time) if it would be easier to comprehend.

Thanks though. It makes sense, and that's pretty much the clarification that I was looking for.

6. Absolute motion does exist. We know that the faster you go, the slower your clock ticks. Therefore, if we synchronise the clocks of two space-ships at rest with respect to eachother and then they start moving at different speeds, when they move back together, there clocks will show different readings which is proof that motion is absolute.

7. Originally Posted by Waveman28
Absolute motion does exist. We know that the faster you go, the slower your clock ticks. Therefore, if we synchronise the clocks of two space-ships at rest with respect to eachother and then they start moving at different speeds, when they move back together, there clocks will show different readings which is proof that motion is absolute.
Nope.

Sometimes I feel like I have an ok understanding about some of the issues of relativity. Time Dilation, Length Contraction and the sort.

Other times, like right now, I don't feel like that is the case. I'm sure this is answered simply through relativity, but right now it's eluding me. I have a question based on one assumption which we know to be true. The assumption is that the faster you move, the slower time progresses. With that assumption, would it be possible to have one atomic clock in one position, and another atomic clock in another position and determine which atomic clock is slightly faster? Obviously you would have to narrow out other factors that may affect it, such as the gravitational fields in which the atomic fields travel. Also, let's say for the sake of argument we didn't have to travel (or accelerate) to check the atomic clocks, we could check them via wireless signal.

If we could determine that one is moving through space time faster than the other, then couldn't it be said that one is moving slower?

You can determine which one has a higher velocity in a given direction by measuring redshift of their signals to each other. But that only tells you their relative speeds to each other, and the value is never higher than C.

Through numerous experiments, numerous atomic clocks and some nifty triangulation shouldn't it be possible to eventually determine a resting frame of reference? Wouldn't that contradict relativity?
It was people attempting to do so and failing that eventually lead to relativity. Some very ingenious methods were employed.

Originally Posted by Janus
Here's the thing, there is no absolute motion. If you have two clocks with a realtive motion with respect to each other, which one runs slow depends on who is comparing the clocks.

If you are stationary with respect to clock 1, then it is clock 2 that is running slow, but if you are stationary with respect to clock 2, then it is clock 1 that runs slow.
So.. pretty much, if you're in the same frame of reference you measure it to be faster.

Makes sense. I wonder if I were exposed to this at an early age (like when we start dealing with 'conventional' time) if it would be easier to comprehend.

Thanks though. It makes sense, and that's pretty much the clarification that I was looking for.
Lorentz contraction seems to be what glues it all together. Space itself seems to compress in the direction of motion, which solves the problem of speeds of objects adding up to more than C.

9. Originally Posted by kojax
Lorentz contraction seems to be what glues it all together. Space itself seems to compress in the direction of motion, which solves the problem of speeds of objects adding up to more than C.
You also have to include The Relativity of Simultaneity.

For instance:

Asuume you have a rod of length l with clocks at each end that are synchonized in the frame of the rod. An object travels from one end of the rod to the other at a speed of v. This means that the object leaves clock 1 when it reads t0 and arrives at clock 2 when it reads t1.

Now imagine the rod is moving in the same direction with repect to an observer at a velocity of u.

According to this observer, the rod is shorter than l, the clocks both run slow and Clock 2 lags behind Clock 1 in time.

This observer also sees the object leaving clock 1 when it reads t0 and arrive at clock 2 when it reads t1.

Taking into account the length of the rod, and how far clock2 travels until it reads t1, and the time this takes according to his own clock, we get how fast the object moves wtih respect to the observer. It will always be less than c. (you can actually derive the velocity addition formula this way)

Sometimes I feel like I have an ok understanding about some of the issues of relativity. Time Dilation, Length Contraction and the sort.

Other times, like right now, I don't feel like that is the case. I'm sure this is answered simply through relativity, but right now it's eluding me. I have a question based on one assumption which we know to be true. The assumption is that the faster you move, the slower time progresses. With that assumption, would it be possible to have one atomic clock in one position, and another atomic clock in another position and determine which atomic clock is slightly faster? Obviously you would have to narrow out other factors that may affect it, such as the gravitational fields in which the atomic fields travel. Also, let's say for the sake of argument we didn't have to travel (or accelerate) to check the atomic clocks, we could check them via wireless signal.

If we could determine that one is moving through space time faster than the other, then couldn't it be said that one is moving slower?

Through numerous experiments, numerous atomic clocks and some nifty triangulation shouldn't it be possible to eventually determine a resting frame of reference? Wouldn't that contradict relativity?

I'm sure it's a newb question, so I apologize in advance.

Also, I'm new here. Hey everyone. A lot of you have impressed me with your knowledge... except a few people in the political section :-D

EDIT: Or is it that they will both read the same time, with the exception of which has been accelerated more, or in which gravitational fields the atomic clocks have traveled? I'm pretty sure now that I think about it a few more moments.

I guess the thread can still have some value, in case others have had the same question.
Special relativity DOES NOT say that "the faster you move the slower time progesses."

What special relativity addresses is the issue of how time and space measurements depend on the reference frame of the observer. It relates to differences seen by two observers in uniform motion with respect to each other. In his own reference frame an observer sees things quite "normally". It is only when two observers in motion with respect to one another try to measure the same event that there is any question.

So, if two observers are in motion each sees the other's clock as "slow" and the other's ruler as "short". Neither one is "short" or "slow" in his own reference frame. So, no you cannot say that one is "slow" or one is "short".

What is says is that the notions of time and space are not absolute concepts, but depend on the reference frame in which they are measured.

You mght consider reading a book on the subject. Introduction to Special Relativity byWolfgang Rindler is a good choice. This Wiki article is not bad either. http://en.wikipedia.org/wiki/Special_relativity

11. I will have to agree with Dr. Rocket The motion from witch any object that moves through space is only different to two or more observers. Time dialation is relative to a point when you refer to the black hole effect or event horizon on a worm hole. You have to see that time is constant no matter how hard you try to dissprove it. Its only the distance that makes us beleive time can be distorted by motion. If you were to be one light from an event lets say a sun dieing then you would see it happen fast since you basicly on the doorstep of the event. However if you were ten light years away it would take that ten years to get to you and it would seem that it took forever to happen its all about the reference to which you think about it. As with anything we all need to be more open minded and discover what works for us. thank you and have a nice day.

12. not quite. The dieing sun would appear to take just as long no matter how far you were from it. We would see it approx. 8 minutes after, but the ammount of time from the initiation of the death to the completion of the death would still be the same, but we see it 8 minutes later than it actually occured rather than 10 years. If, however, you were moving at some high velocity away from the sun (or towards, I'm not sure if it matters) THEN it will appear to take longer. relative motion is what 'alters' time. not so much how far away you are.

13. Thought experiment:
--------------------------------
Suppose you're standing on an asteroid at rest with the most powerful telescope ever built, and off in the far distance, a very long ways away you see 2 space ships traveling perpendicular to your vantage point. So, if you were looking "North-South", they're headings are "East-West".

Suppose these space ships have windows, and you can see clocks inside their windows. Suppose one of them is moving very near the speed of light, and the other is moving much slower.

----------------------------------------------

The clock in the window of the faster moving space ship should appear to be ticking away time more slowly, from your vantage point.

It can be shown using geometry that neither ship is moving toward you or away from you very fast. At a very far distance, perpendicular motion has a negligible effect on the overall distance. So, propagation delay of the light itself can pretty much be ruled out as a cause.

Relativity actually predicts that the events transpiring inside those space ships should be uniformly transpiring at different speeds. It also predicts that the faster moving space ship should appear shorter overall, in length.

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