Why isn't length contraction permanent even though time dilation is?

[Ganesh Ujwal]

It's my understanding that when something is going near the speed of light in reference to an observer, time dilation occurs and time goes slower for that fast-moving object. However, when that object goes back to "rest", it has genuinely aged compared to the observer. It's not like time goes slow for a while, and then speeds back to "normal," so that the age of the observer once again matches the object. The time dilation is permanent. Why wouldn't the same thing happen with length contraction? Since the two are so related, you'd think if one is permanent, the other would be also. And from everything I've read so far, length contraction is not permanent. An object will be at rest touching an observer, go far away near light speed, return to touching the observer, and be the same length it was at the beginning. It shortens, and then grows long again, as if its shrinkage was an illusion the whole time. Did I just not read the right things or what? Were my facts gathered incorrectly?

Edited December 19, 2014 by Ganesh Ujwal

[Janus]

Time dilation is a comparison of rates. When an object is moving fast with respect to you, it's clock rate is slow, and when it comes to rest with respect to you its clock rate returns to normal. The time difference between the two clocks at this time is due to the accumulation due to these different time rates. That is the leftover effect of the time dilation but not the time dilation itself.

 

Length contraction, like time dilation, exists when there is relative motion and goes away when there is no relative motion, but there isn't any "accumulation" with length contraction, so there is nothing to be "left over".

[Robittybob1]

Time dilation is a comparison of rates. When an object is moving fast with respect to you, it's clock rate is slow, and when it comes to rest with respect to you its clock rate returns to normal. The time difference between the two clocks at this time is due to the accumulation due to these different time rates. That is the leftover effect of the time dilation but not the time dilation itself.

 

Length contraction, like time dilation, exists when there is relative motion and goes away when there is no relative motion, but there isn't any "accumulation" with length contraction, so there is nothing to be "left over".

I can see why it is easier to explain this when you think of time having three dimensions.

Length contraction is only one dimensional

Edited December 19, 2014 by Robittybob1

[Delta1212]

Length contraction remains in exactly the same way that time dilation does.

 

When you sync back up with the given rest frame, the rate your clock is ticking at syncs up as well, but you only travelled through time (see: aged) as far as you did while your clock was ticking at the slower rate. You don't suddenly age a bunch to match up with what you would have aged if your clock had been in sync the whole time. We all agree on that.

 

So, with length contraction, I see a star 10 light years away. I speed up to the point that length contraction causes to the star to only be 5 light years away. I travel to the star and then decrease my speed to sync up to the rest frame I started in. The distance is now 10 ly, but my 5 ly journey still covered the full distance. I'm not dragged backward 5 ly to where I would have been if length wasn't contracted during my journey. Maintaining the distance you've travelled is the equivalent of maintaining the amount you've aged.

 

Something remaining length contracted after speeding back up isn't the equivalent of something maintaining its age. It's the equivalent of maintaining it's clock speed. Something remaining length contracted after accelerating back to your rest frame would be the equivalent of it remaining time dilated in the same circumstances; that is, continuing to have its clock tick at a slower rate.

 

The lasting effect of time dilation isn't the time dilation itself, it's the discrepancy between how much the traveling object aged and how much time the resting observer measures to have passed. The lasting effect if length contraction isn't the shortening of the object, it's the discrepancy between how far the object measures itself as having travelled, and the distance as measured by the resting observer.

[Robittybob1]

Length contraction remains in exactly the same way that time dilation does. ....

 

So, with length contraction, I see a star 10 light years away. I speed up to the point that length contraction causes to the star to only be 5 light years away. I travel to the star and then decrease my speed to sync up to the rest frame I started in. The distance is now 10 ly, but my 5 ly journey still covered the full distance. .....

I just want to focus on that bit. You speed up and notice length contraction; I don't believe you've got that right.

[Janus]

I just want to focus on that bit. You speed up and notice length contraction; I don't believe you've got that right.

No, he's got it right. He was next to the Earth and the star is 10 ly away, then he accelerates to 0.866c relative to the Earth and star. Assuming he accelerates fast enough that he does not cover a significant distance during acceleration, when he stops accelerating, he will now measure the star as being 5 ly away. It will take him 5.774 yrs to reach that star at 0.866c by his clock. Meanwhile, for an observer on Earth, he(the traveler) will take 11.548 years to reach the star, aging at a rate of 0.5 due to time dilation and will have aged 5.774 years during the trip.

[Robittybob1]

No, he's got it right. He was next to the Earth and the star is 10 ly away, then he accelerates to 0.866c relative to the Earth and star. Assuming he accelerates fast enough that he does not cover a significant distance during acceleration, when he stops accelerating, he will now measure the star as being 5 ly away. It will take him 5.774 yrs to reach that star at 0.866c by his clock. Meanwhile, for an observer on Earth, he(the traveler) will take 11.548 years to reach the star, aging at a rate of 0.5 due to time dilation and will have aged 5.774 years during the trip.

How can he measure that the star is only 5 ly away? Is the star the same distance away no matter in what direction he is traveling?

When we measure distance it is at a point in time, we don't make allowances for velocity.

Does he measure the length of his spacecraft is also shorter?

Edited December 19, 2014 by Robittybob1

[studiot]

 

How can he measure that the star is only 5 ly away? Is the star the same distance away no matter in what direction he is traveling?

When we measure distance it is at a point in time, we don't make allowances for velocity.

 

 

If speeds are sufficinet then you need to make allowance for length contraction.

 

The space ship is travelling at approx 0.9c directly towards the distant star.

 

So the relative velocity is 0.9c

 

So distance in that direction is subject to the Lorenz contraction, as measured by the traveller.

 

An observer sitting warming his bum on the star can be considered 'at rest' as he watches the traveller approach so he will measure time dilation.

 

The is then the same situation as the extended life of the muon as it approaches Earth.

 

http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/muon.html

[Robittybob1]

 

If speeds are sufficinet then you need to make allowance for length contraction.

 

The space ship is travelling at approx 0.9c directly towards the distant star.

 

So the relative velocity is 0.9c

 

So distance in that direction is subject to the Lorenz contraction, as measured by the traveller.

 

An observer sitting warming his bum on the star can be considered 'at rest' as he watches the traveller approach so he will measure time dilation.

 

The is then the same situation as the extended life of the muon as it approaches Earth.

 

http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/muon.html

How does the person on the craft measure the distance to another star?

[studiot]

Voodoo

 

This is a thought experiment, remember.

 

Or formally she sees all distances measured in the direction of relative motion as contracted by the Lorenz contraction.

 

http://en.wikipedia.org/wiki/Length_contraction

Edited December 19, 2014 by studiot

[Robittybob1]

Voodoo

 

This is a thought experiment, remember.

Which means you have to also think of the answer. You can't just say "he will now measure the star as being 5 ly away" without a way of doing it.

[Janus]

Which means you have to also think of the answer. You can't just say "he will now measure the star as being 5 ly away" without a way of doing it.[/size]

Just imagine that there is a string of buoys spaced 1 meter apart from the Earth to the star, the Ship has a meter stick attached to its side, and the occupants compare the buoys to the meter stick as they pass them. For the ship the buoys will be only 1/2 meter apart,( as one buoy passes the end of the meter stick there will be a buoy passing both the other end and the midpoint of the stick). The ship counts the same number of buoy between earth and star as the Earth does, and with the the buoys only being 1/2 meter apart, determines the distance as being half the distance measured by the Earth (for which the distance between buoys is 1 meter.)

 

Whether or not the buoys actually exist or not doesn't matter, just that it would be possible to measure the distance from Earth to Star by this method by the ship.

[Robittybob1]

Just imagine that there is a string of buoys spaced 1 meter apart from the Earth to the star, the Ship has a meter stick attached to its side, and the occupants compare the buoys to the meter stick as they pass them. For the ship the buoys will be only 1/2 meter apart,( as one buoy passes the end of the meter stick there will be a buoy passing both the other end and the midpoint of the stick). The ship counts the same number of buoy between earth and star as the Earth does, and with the the buoys only being 1/2 meter apart, determines the distance as being half the distance measured by the Earth (for which the distance between buoys is 1 meter.)

 

Whether or not the buoys actually exist or not doesn't matter, just that it would be possible to measure the distance from Earth to Star by this method by the ship.

I think you are wrong about this, a meter ruler will always measure a meter.

Edited December 19, 2014 by Robittybob1

[swansont]

I think you are wrong about this, a meter ruler will always measure a meter.

 

 

Not when there is motion between the ruler and the observer.

[Janus]

I think you are wrong about this, a meter ruler will always measure a meter.

It doesn't matter what you think, reality and experimental evidence says otherwise. Quit trying to force the universe to behave like you think it should and start accepting how it actually behaves.

[Robittybob1]

It doesn't matter what you think, reality and experimental evidence says otherwise. Quit trying to force the universe to behave like you think it should and start accepting how it actually behaves.

I'm not denying length contraction but a situation where a meter ruler will measure 2m as a meter. You see if you can find a respected scientist person to agree with you.

 

 

Not when there is motion between the ruler and the observer.

That is because the ruler is actually (physically) shorter, to the observer it is no longer a meter long and therefore it doesn't have a meter in length any more. But from the perspective of the traveling scientist the ruler still measures a meter for everything has contracted by the same proportion.

Edited December 19, 2014 by Robittybob1

[Strange]

How does the person on the craft measure the distance to another star?

 

I don't think there is any (easy) way to directly measure length contraction. However, the complementary effect of time dilation is very easy to measure, and what is seen by one party as time dilation will be explained as a length contraction by another.

 

For example, muons from cosmic rays would normally decay before they reach the Earth but (from our perspective) they are time dilated and so we detect them. But from their perspective, the distance to the Earth is reduced and so they arrive before they decay. Similarly for electric vs magnetic fields. Or the clocks on GPS satellites versus the triangulation of location. And so on and on.

 

Furthermore, length contraction is part of the theory and the theory is incredibly well verified. So ...

 

 

You see if you can find a respected scientist person to agree with you.

 

I don't think you will find one who disagrees.

[Janus]

I'm not denying length contraction but a situation where a meter ruler will measure 2m as a meter. You see if you can find a respected scientist person to agree with you.

 

That is because the ruler is actually (physically) shorter, to the observer it is no longer a meter long and therefore it doesn't have a meter in length any more. But from the perspective of the traveling scientist the ruler still measures a meter for everything has contracted by the same proportion.

You are trying to treat length contraction as a physical compression rather than what it is, a difference in how each frame measures distance. For instance, when the "traveling" scientist compares his meter stick to the Observer, he will measure that it is the observer's meter stick that will be the shorter of the two. By your description, he should measure the Observer's meter stick as being longer than his.

 

You are still trying to make Relativity fit your terms rather than accepting it on its own.

[Robittybob1]

You are trying to treat length contraction as a physical compression rather than what it is, a difference in how each frame measures distance. For instance, when the "traveling" scientist compares his meter stick to the Observer, he will measure that it is the observer's meter stick that will be the shorter of the two. By your description, he should measure the Observer's meter stick as being longer than his.

 

You are still trying to make Relativity fit your terms rather than accepting it on its own.

You know I have been looking into this from a new perspective lately and it is to do with time. When they have relative velocity they are traveling with different coordinates of time and hence they view their mates rulers as shorter than their own (possibly but they don't know how long they were to begin with) for it is only in cases like the Twin Paradox where the scientists were together to begin with, that they have had a chance of calibrating their rulers in the first place and then to get the speed difference between them one or both of them have to accelerate. When they then come back past each other they will know who has accelerated.

When that happens I might understand it a bit more.

Edited December 19, 2014 by Robittybob1

[studiot]

Did you not find the hyperphysics explanation I linked to in post#8 helpful in illuminating this?

[Janus]

You know I have been looking into this from a new perspective lately and it is to do with time. When they have relative velocity they are traveling with different coordinates of time and hence they view their mates rulers as shorter than their own (possibly but they don't know how long they were to begin with) for it is only in cases like the Twin Paradox where the scientists were together to begin with, that they have had a chance of calibrating their rulers in the first place and then to get the speed difference between them one or both of them have to accelerate. When they then come back past each other they will know who has accelerated.

When that happens I might understand it a bit more.

It doesn't matter which of the two accelerated to create the speed difference between them when they separate, it only matters which one accelerated in order to bring them back together again when it comes to comparing their clocks before and after. And as far as length contraction goes, it doesn't matter at all.

[Bel Gio]

Special Relativity tells us there is no preferred reference frame.

Each of the two observers moving relative to each other considers himself at rest and the other as 'moving'.

Hence reciprocity of time dilation and length contraction.

For observer A the one meter stick of observer B is shorter than his own one meter stick.

For observer B the one meter stick of observer A is shorter than his own one meter stick.

[michel123456]

Special Relativity tells us there is no preferred reference frame.

Each of the two observers moving relative to each other considers himself at rest and the other as 'moving'.

Hence reciprocity of time dilation and length contraction.

For observer A the one meter stick of observer B is shorter than his own one meter stick.

For observer B the one meter stick of observer A is shorter than his own one meter stick.

That counts when they move.

the question is what happen when they meet. For the meeting they must have accelerated (decelerated) in order to be in the same FOR.

[swansont]

That counts when they move.

the question is what happen when they meet. For the meeting they must have accelerated (decelerated) in order to be in the same FOR.

 

You don't have to be in the same FoR to be co-located, i.e. meet. They just won't stay co-located.

[michel123456]

 

You don't have to be in the same FoR to be co-located, i.e. meet. They just won't stay co-located.

Then they will crash.