# Thread: special relativity

1. ## special relativity

I suppose I am not thinking completely clearly,as it seems to me that since travelling away from the observer would slow your time (relative to the observer) then travelling towards should speed up your time(but this is not the case?).
Also since all motion is relative,why does only the spaceship time slow down when both bodies are moving apart relative to each other?
Can a real physicist help me here?

2. Direction of travel is the real question. You say "traveling away?" Do you mean the distance between two objects is increasing with time? If so, how do you determine direction of travel? For instance, two cars can be going down the same road in the same direction, and the distance between them is increasing, or they could be traveling in opposite directions of travel and the distance increases.

Or consider this, a ball is thrown. Is it decelerated or accelerated? Say a ball is thrown towards the rear of a bus from the front of the bus while the bus is in motion. Which direction of travel does the ball travel? Does it travel in the direction opposite the bus, or the same direction at a lower velocity?

Also, and most importantly, time doesn't slow down or speed up, an object's velocity increases or decreases.

3. Originally Posted by Motor Daddy
Direction of travel is the real question. You say "traveling away?" Do you mean the distance between two objects is increasing with time? If so, how do you determine direction of travel? For instance, two cars can be going down the same road in the same direction, and the distance between them is increasing, or they could be traveling in opposite directions of travel and the distance increases.

Or consider this, a ball is thrown. Is it decelerated or accelerated? Say a ball is thrown towards the rear of a bus from the front of the bus while the bus is in motion. Which direction of travel does the ball travel? Does it travel in the direction opposite the bus, or the same direction at a lower velocity?

Also, and most importantly, time doesn't slow down or speed up, an object's velocity increases or decreases.
I really do hope the OP does not listen to you. Things like

''time doesn't slow down or speed up, an object's velocity increases or decreases.''

Is incredibly deceiving - from someone who obviously doesn't understand the complications of relativity, and this is said in the most polite way.

4. Originally Posted by roscogre
I suppose I am not thinking completely clearly,as it seems to me that since travelling away from the observer would slow your time (relative to the observer) then travelling towards should speed up your time(but this is not the case?).
Time dilation does not rely on the direction of travel. It doesn't matter if the object is travel away from or towards the observer, he will still determine that time runs more slowly for it. You might be confusing Doppler shift for time dilation. Doppler shift is the apparent change in clock rate seen by the observer due to the changing distance between himself and the object and the resulting change in the time it takes for the light to travel that distance. Time dilation is what is left over after you account for this.

Also since all motion is relative,why does only the spaceship time slow down when both bodies are moving apart relative to each other?
Can a real physicist help me here?
When two bodies are moving with respect to each other, there isn't any absolute way to say which one's clock is running slower; according to each, it is the other clock that is running slow. You have to bring the clocks together again to compare which one has accumulated less time. In order to do this, one of the bodies has to change it's velocity, this change in its velocity breaks the symmetry between the two bodies.

You also have to consider that time dilation alone is not enough to analyze this problem. You also have to use length contraction and the Relativity of Simultaneity.

For example, if you send a spaceship to a planet 7 light years from Earth, (as measured from the Earth), at 0.99c, it will take a little over 7 years for the Ship to get there by the Earth's clock. The Earth observer will also determine that the ships clock will run at 1/7 speed and only a little over 1 yr will pass on it. Due to length contraction, the ship will also have shrunk to 1/7 its rest length.

For the ship, its clock runs normally, however since the Earth and distant planet are moving at 0.99c relative to them, due to length contraction their lengths and the distance between them will have shrunk by a factor of 7 to 1 ly. Thus at a relative velocity of 0.99c, it only takes a little over a year for the distance between the Earth and Planet to pass by.

This same thing happens for the return trip. Thus the Earth says that 14 yrs has passed for it 2 yrs for the ship, and the ship says that 2 yrs has passed for itself.

This leaves what happens to the Earth clock according to the Ship clock.

While heading out and coming back, the ship will determine that the Earth clock runs at 1/7 speed and that a total of 2/7 of a year pass on Earth during these legs of the journey. So where does the rest of the 6 &5/7 years that pass on Earth come from according to the ship? This happens during the period when the ship slows to a stop and then accelerates back towards Earth and is due to the Relativity of Simultaneity.

The Relativity of Simultaneity means that clocks that are simultaneous in one frame will not be so in another. Thus if I have two clocks at rest with respect to each separated by a distance and that are synchronized to each other according to each other, they will not be synchronized according to someone traveling from one clock to the other. The clock he is heading for will be ahead of the clock he is leaving.( again, this determination is made after he factors out light travel time.)

So if we were to put synchronized clocks on the planet and Earth, according to the Ship, the planet clock will be ahead of the Earth clock on the outbound trip, and behind the Earth clock on the Return trip. What this means for the ship is that during the turnaround, Earth clock will advance 6 & 5/7 years. So for him, the Earth ages 1/7 yr on the outbound trip, 6 & 5/7 yr during turn around, and 1/7 yr on the return trip for a total of 7 yr to the one that passes for him.

5. Originally Posted by roscogre
I suppose I am not thinking completely clearly,as it seems to me that since travelling away from the observer would slow your time (relative to the observer) then travelling towards should speed up your time(but this is not the case?).
When you say "the observer" in special relativity it usually means not just one point. Think of lots of lots of observers that fill space, each having a clear idea of what the distances between them are and each carrying a watch that is synchronized with each other. So there is not really "traveling away from" or "towards" collection of things that fill the entire space.

Originally Posted by roscogre
Also since all motion is relative,why does only the spaceship time slow down when both bodies are moving apart relative to each other?
Each will slow down relative to the other. Think of "time flow of a watch" like velocity of an object, so for instance an object A at rest on a moving train will have a nonzero velocity relative to an observer B standing at the station. If the observer B is holding a bag, then this bag will be at rest to him, but an observer sitting in the train will see that the bag has nonzero velocity. Just like it does not make sense to talk about velocity without telling the reference point with respect to with the velocity is measured, it does not make sense to talk about time without telling the reference frame. This way there is no contradiction as to the time on the ship slows down when measured with respect to an Earth- frame, and the time on Earth slows down when measured from an inertial frame that "follows" the ship.

6. Originally Posted by Motor Daddy
Direction of travel is the real question. You say "traveling away?" Do you mean the distance between two objects is increasing with time? If so, how do you determine direction of travel? For instance, two cars can be going down the same road in the same direction, and the distance between them is increasing, or they could be traveling in opposite directions of travel and the distance increases.

Or consider this, a ball is thrown. Is it decelerated or accelerated? Say a ball is thrown towards the rear of a bus from the front of the bus while the bus is in motion. Which direction of travel does the ball travel? Does it travel in the direction opposite the bus, or the same direction at a lower velocity?

Also, and most importantly, time doesn't slow down or speed up, an object's velocity increases or decreases.
wrong

7. Originally Posted by Motor Daddy
Direction of travel is the real question. You say "traveling away?" Do you mean the distance between two objects is increasing with time? If so, how do you determine direction of travel? For instance, two cars can be going down the same road in the same direction, and the distance between them is increasing, or they could be traveling in opposite directions of travel and the distance increases.

Or consider this, a ball is thrown. Is it decelerated or accelerated? Say a ball is thrown towards the rear of a bus from the front of the bus while the bus is in motion. Which direction of travel does the ball travel? Does it travel in the direction opposite the bus, or the same direction at a lower velocity?

Also, and most importantly, time doesn't slow down or speed up, an object's velocity increases or decreases.
Originally Posted by temur
When you say "the observer" in special relativity it usually means not just one point. Think of lots of lots of observers that fill space, each having a clear idea of what the distances between them are and each carrying a watch that is synchronized with each other. So there is not really "traveling away from" or "towards" collection of things that fill the entire space.

Each will slow down relative to the other. Think of "time flow of a watch" like velocity of an object, so for instance an object A at rest on a moving train will have a nonzero velocity relative to an observer B standing at the station. If the observer B is holding a bag, then this bag will be at rest to him, but an observer sitting in the train will see that the bag has nonzero velocity. Just like it does not make sense to talk about velocity without telling the reference point with respect to with the velocity is measured, it does not make sense to talk about time without telling the reference frame. This way there is no contradiction as to the time on the ship slows down when measured with respect to an Earth- frame, and the time on Earth slows down when measured from an inertial frame that "follows" the ship.
wrong

8. Originally Posted by Janus58
Time dilation does not rely on the direction of travel. It doesn't matter if the object is travel away from or towards the observer, he will still determine that time runs more slowly for it. You might be confusing Doppler shift for time dilation. Doppler shift is the apparent change in clock rate seen by the observer due to the changing distance between himself and the object and the resulting change in the time it takes for the light to travel that distance. Time dilation is what is left over after you account for this.

When two bodies are moving with respect to each other, there isn't any absolute way to say which one's clock is running slower; according to each, it is the other clock that is running slow. You have to bring the clocks together again to compare which one has accumulated less time. In order to do this, one of the bodies has to change it's velocity, this change in its velocity breaks the symmetry between the two bodies.

You also have to consider that time dilation alone is not enough to analyze this problem. You also have to use length contraction and the Relativity of Simultaneity.

For example, if you send a spaceship to a planet 7 light years from Earth, (as measured from the Earth), at 0.99c, it will take a little over 7 years for the Ship to get there by the Earth's clock. The Earth observer will also determine that the ships clock will run at 1/7 speed and only a little over 1 yr will pass on it. Due to length contraction, the ship will also have shrunk to 1/7 its rest length.

For the ship, its clock runs normally, however since the Earth and distant planet are moving at 0.99c relative to them, due to length contraction their lengths and the distance between them will have shrunk by a factor of 7 to 1 ly. Thus at a relative velocity of 0.99c, it only takes a little over a year for the distance between the Earth and Planet to pass by.

This same thing happens for the return trip. Thus the Earth says that 14 yrs has passed for it 2 yrs for the ship, and the ship says that 2 yrs has passed for itself.

This leaves what happens to the Earth clock according to the Ship clock.

While heading out and coming back, the ship will determine that the Earth clock runs at 1/7 speed and that a total of 2/7 of a year pass on Earth during these legs of the journey. So where does the rest of the 6 &5/7 years that pass on Earth come from according to the ship? This happens during the period when the ship slows to a stop and then accelerates back towards Earth and is due to the Relativity of Simultaneity.

The Relativity of Simultaneity means that clocks that are simultaneous in one frame will not be so in another. Thus if I have two clocks at rest with respect to each separated by a distance and that are synchronized to each other according to each other, they will not be synchronized according to someone traveling from one clock to the other. The clock he is heading for will be ahead of the clock he is leaving.( again, this determination is made after he factors out light travel time.)

So if we were to put synchronized clocks on the planet and Earth, according to the Ship, the planet clock will be ahead of the Earth clock on the outbound trip, and behind the Earth clock on the Return trip. What this means for the ship is that during the turnaround, Earth clock will advance 6 & 5/7 years. So for him, the Earth ages 1/7 yr on the outbound trip, 6 & 5/7 yr during turn around, and 1/7 yr on the return trip for a total of 7 yr to the one that passes for him.
Thank you Janus.
I was not confusing time dilation with doppler shift,but otherwise you understood my question and your answer helped me very much.
Much of my confusion I believe comes from trying to understand Einstein conceptually without understanding the math.
Thanks again yours was the only helpful insightful (and correct)answer.

9. Originally Posted by Green Destiny
I really do hope the OP does not listen to you. Things like

''time doesn't slow down or speed up, an object's velocity increases or decreases.''

Is incredibly deceiving - from someone who obviously doesn't understand the complications of relativity, and this is said in the most polite way.
Thank you Green,amazing how many people always try to answer that which they do not know.

10. Originally Posted by Green Destiny
I really do hope the OP does not listen to you. Things like

''time doesn't slow down or speed up, an object's velocity increases or decreases.''

Is incredibly deceiving - from someone who obviously doesn't understand the complications of relativity, and this is said in the most polite way.
Originally Posted by temur
When you say "the observer" in special relativity it usually means not just one point. Think of lots of lots of observers that fill space, each having a clear idea of what the distances between them are and each carrying a watch that is synchronized with each other. So there is not really "traveling away from" or "towards" collection of things that fill the entire space.

Each will slow down relative to the other. Think of "time flow of a watch" like velocity of an object, so for instance an object A at rest on a moving train will have a nonzero velocity relative to an observer B standing at the station. If the observer B is holding a bag, then this bag will be at rest to him, but an observer sitting in the train will see that the bag has nonzero velocity. Just like it does not make sense to talk about velocity without telling the reference point with respect to with the velocity is measured, it does not make sense to talk about time without telling the reference frame. This way there is no contradiction as to the time on the ship slows down when measured with respect to an Earth- frame, and the time on Earth slows down when measured from an inertial frame that "follows" the ship.
wrong again

11. What is wrong in my answer? I would like to learn from you.

12. Originally Posted by roscogre
wrong again
Oh contraire, my statement was not against Temur. My statement qouted someone else entirely.

13. For instance I will be clearer. In relativity, time slowing down for an observer is a stated fact. And relatively-speaking, using two observers are frames of reference, it is more than clear that the faster a frame of reference moves at, the slower time will be perceieved. Saying time does not truely slow down, is obviously a lack of understanding the true terminology of relativity.

14. wait,no.Time does not slow down for the observer.
Time slows down on the observed fast moving spaceship in relation to the observer.
Time always seems the same for you subjectively if you are on the spaceship or as an observer,because your heart and brain slow down with the clock.

15. the observer does mean one point.hence "the observer",singular.

16. yes I understand green your comment was about motor's inaccurate and misleading statements.

17. Originally Posted by roscogre
wait,no.Time does not slow down for the observer.
Time slows down on the observed fast moving spaceship in relation to the observer.
Time always seems the same for you subjectively if you are on the spaceship or as an observer,because your heart and brain slow down with the clock.
Well, yes, of course. You require two frames of reference to compare the difference in time - time does slow in a frame of reference, when compared with another frame of reference at rest.

18. Originally Posted by roscogre
the observer does mean one point.hence "the observer",singular.
I don't want to argue with you, but no, it is NOT one point. You have to talk about inertial frame.

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