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

[swansont]

Then they will crash.

 

It's a thought experiment, so no big deal.

 

In reality, they will almost be co-located. In any event, you can send a timing signal to check clock readings. When they are close enough the transmission delay is negligible.

[Robittybob1]

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.

I am beginning to think this is only part of the answer. There is this stage too where they both observe the other's meter as being shorter. But can we explain it in terms of the traveling twins, the same ones who come back from their star journey one younger than the other? Who has length contraction then?

[Dekan]

Couldn't all this be put to a scientific test - ie, by very accurate measurement of physical human bodies?

 

For example - I've read about astronauts who spend 6 months orbiting the Earth, aboard the International Space Station.

Apparently, when they de-orbit and return to Earth, they're permanently 0.07 seconds younger. This is because Time has contracted during their orbiting.

 

Why that should be so, seems far from clear. They orbited at a constant speed, relative to the Earth. So no real acceleration was involved. The astronauts just went steadily in circles round the Earth.

 

However, suppose we accept that during the orbits, time was somehow, permanently changed. Then shouldn't space also have been permanently changed? Space and Time are, after all, thought to be just different aspects of a single unified entity - Spacetime. And if one component of this entity is changed, ie Time - then shouldn't the other component, ie Space, also be affected? By way of reaction, or compensation, I'm not sure which. So as to maintain unity.

 

Therefore, it seems to me, that if astronauts come back with their ages permanently changed - then their spatial bodies must show some corresponding change.

Like getting taller, or shorter. Or with subtle differences in the size of their organs.

 

Have any such changes been measured by the post-flight doctors who examine the astronauts?

Edited December 20, 2014 by Dekan

[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".

But if the traveling twin can complete his journey quicker and hence return to his brother younger because at some stage the distance was shorter I think it can be argued there are leftover effects both from the time dilation and length contraction.

[Janus]

Why that should be so, seems far from clear. They orbited at a constant speed, relative to the Earth. So no real acceleration was involved. The astronauts just went steadily in circles round the Earth.

Circular motion is accelerated motion. Their speed may not change but their velocity does(in that by traveling in a circle the direction of their speed is constantly changing)

[Robittybob1]

Couldn't all this be put to a scientific test - ie, by very accurate measurement of physical human bodies?

 

For example - I've read about astronauts who spend 6 months orbiting the Earth, aboard the International Space Station.

Apparently, when they de-orbit and return to Earth, they're permanently 0.07 seconds younger. This is because Time has contracted during their orbiting.

 

Why that should be so, seems far from clear. They orbited at a constant speed, relative to the Earth. So no real acceleration was involved. The astronauts just went steadily in circles round the Earth.

 

However, suppose we accept that during the orbits, time was somehow, permanently changed. Then shouldn't space also have been permanently changed? Space and Time are, after all, thought to be just different aspects of a single unified entity - Spacetime. And if one component of this entity is changed, ie Time - then shouldn't the other component, ie Space, also be affected? By way of reaction, or compensation, I'm not sure which. So as to maintain unity.

 

Therefore, it seems to me, that if astronauts come back with their ages permanently changed - then their spatial bodies must show some corresponding change.

Like getting taller, or shorter. Or with subtle differences in the size of their organs.

 

Have any such changes been measured by the post-flight doctors who examine the astronauts?

You won't see much difference in a human body in just 7 milliseconds.

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.

If the journey to the stars can be made shorter simply by going faster it does matter a lot. For it seems to suggest star travel becomes possible for the enormous distances and times involved can be made shorter.

[Strange]

You won't see much difference in a human body in just 7 milliseconds.

 

Exactly. But of course, the effect has been measured using atomic clocks. So no one seriously doubts it is real.

[Robittybob1]

 

Exactly. But of course, the effect has been measured using atomic clocks. So no one seriously doubts it is real.

They explain that using words like "asymmetry", for even though there was relative motion affecting both parties only one went through the acceleration which broke the symmetry of the situation.

[Dekan]

Circular motion is accelerated motion. Their speed may not change but their velocity does(in that by traveling in a circle the direction of their speed is constantly changing)

Thanks Janus, But I don't quite get it - I thought "acceleration" meant going faster.

 

What's change of direction got to do with it?

 

I mean, suppose you decide to keep walking in a circle round your house. Will you be constantly accelerating?

Edited December 20, 2014 by Dekan

[Robittybob1]

Thanks Janus, But I don't quite get it - I thought "acceleration" meant going faster.

 

What's change of direction got to do with it?

 

I mean, suppose you walk ten paces to the north, then turn round, and walk ten paces to the south.

Does this change of direction, mean that your walking has accelerated?

 

Or, suppose you decide to keep walking in a circle round your house. Will your walking pace will be constantly accelerating?

You are changing velocity a vector quantity, as opposed to speed or direction when considered separately.

[Janus]

Thanks Janus, But I don't quite get it - I thought "acceleration" meant going faster.

 

What's change of direction got to do with it?

 

I mean, suppose you decide to keep walking in a circle round your house. Will you be constantly accelerating?

Acceleration is a change of velocity, velocity is both speed and direction. Thus a change in direction with no change in speed is still acceleration.

 

When you walk in a circle around your house you are constantly accelerating, the direction of the acceleration will be towards the center of the circle.

If the journey to the stars can be made shorter simply by going faster it does matter a lot. For it seems to suggest star travel becomes possible for the enormous distances and times involved can be made shorter.

What I meant by it doesn't matter is that, as far as length contraction is concerned, it doesn't matter who accelerated, If you accelerate relative to the star and Earth, the distance between star and Earth length contracts as measured by you. If the Earth and star were the ones to accelerate, you would still measure the distance between Earth and star as contracted.

 

With the accumulative time difference between clocks that separate and re-join, it doesn't matter which of the two accelerated to create the separation, but it does matter which the two accelerates in order to bring them back together again.

[Dekan]

Acceleration is a change of velocity, velocity is both speed and direction. Thus a change in direction with no change in speed is still acceleration.

 

When you walk in a circle around your house you are constantly accelerating, the direction of the acceleration will be towards the center of the circle.

Thanks for post Janus, - could you just expand on that please, because it seems to me, that if:

 

1. I walk at a constant speed of 4mph round a circle

2. The circle is a constant 50 yds in diameter;

 

Then both speed and circle stay constant. They don't undergo any "acceleration"

I mean, "acceleration", literally, means ""speeding-up".

 

Where's the "speeding-up" in this example?

Edited December 20, 2014 by Dekan

[imatfaal]

Thanks for post Janus, - could you just expand on that please, because it seems to me, that if:

 

1. I walk at a constant speed of 4mph round a circle

2. The circle is a constant 50 yds in diameter;

 

Then both speed and circle stay constant. They don't undergo any "acceleration"

I mean, "acceleration", literally, means ""speeding-up".

 

Where's the "speeding-up" in this example?

 

Constant motion is in a straight line - to move in a circular motion you constantly change your heading. A change of heading requires an acceleration. Velocity and acceleration are vectors - they have a size and a direction; a change in direction or size in velocity is an acceleration.

 

Speed is a scalar and only has magnitude - your speed does not change.

 

You can think of it as a little way straight and a little way in towards the centre repeated endlessly - if you are walking around in a circle each step with your left foot will fall slightly to the left of where it would if you were walking in a straight line, you pivot slightly swing your body around and place your right slightly to the left, pivot and push inward to the left slightly etc.... In mechanics problems we lower the step size to being continuous. There is a well defined acceleration to the centre of circle

[Strange]

I mean, "acceleration", literally, means ""speeding-up".

 

In physics, that is not what it means.

 

 

The final mathematical quantity discussed in Lesson 1 is acceleration. An often confused quantity, acceleration has a meaning much different than the meaning associated with it by sports announcers and other individuals. The definition of acceleration is:

• Acceleration is a vector quantity that is defined as the rate at which an object changes its velocity. An object is accelerating if it is changing its velocity.

http://www.physicsclassroom.com/class/1DKin/Lesson-1/Acceleration

 

And remember that velocity is defined as speed and direction.

One thing that should make this obvious: you need to apply a force to change the speed but you also need to apply a force to change the direction. And force = mass x acceleration.

[Janus]

Thanks for post Janus, - could you just expand on that please, because it seems to me, that if:

 

1. I walk at a constant speed of 4mph round a circle

2. The circle is a constant 50 yds in diameter;

 

Then both speed and circle stay constant. They don't undergo any "acceleration"

I mean, "acceleration", literally, means ""speeding-up".

 

Where's the "speeding-up" in this example?

Your using the colloquial usage for acceleration.

 

The scientific definition means any change in velocity where velocity is a vector with both magnitude and direction. Even reducing the speed of an object( what we call deceleration in everyday usage) is acceleration in physics.

 

Walking in a circle requires centripetal acceleration, in your example of 4 mph with a radius of 50 yds this works out to an acceleration of 0.0000153 ft/sec^2 or 0.00000156g If you weigh 160 lbs, this means that as you walk you will have to exert an outward force of 0.00028 lbs on the ground to maintain that circular path. It is the force needed to constantly deflect your mass to follow a circular path rather than a straight line.

[Dekan]

Many thanks Imatfaal, Strange, and Janus for your very helpful and informative replies.

Having considered and mulled them over, they've increased my understanding of the physics involved.

 

Especially in the case of Janus's post #40, where he says: "it is the force needed to constantly deflect a mass to follow a circular path rather than a straight line"

My only quibble, is whether confusion is caused by using the word "acceleration" for this force.

 

The word does literally mean something like "speeding-up". Which is bound to seem contradictory. When, for example, we're asked to accept that the Moon, which is in a stable orbit, is actually accelerating round the Earth. That suggests that's its speeding up and about to fly off into outer space!

 

The word "acceleration" doesn't seem right. Couldn't we just use "inertia"?

Edited December 20, 2014 by Dekan

[Robittybob1]

Many thanks Imatfaal, Strange, and Janus for your very helpful and informative replies.

Having considered and mulled them over, they've increased my understanding of the physics involved.

 

Especially in the case of Janus's post #40, where he says: "it is the force needed to constantly deflect a mass to follow a circular path rather than a straight line"

My only quibble, is whether confusion is caused by using the word "acceleration" for this force.

 

The word does literally mean something like "speeding-up". Which is bound to seem contradictory. When, for example, we're asked to accept that the Moon, which is in a stable orbit, is actually accelerating round the Earth. That suggests that's its speeding up and about to fly off into outer space!

 

The word "acceleration" doesn't seem right. Couldn't we just use "inertia"?

I think you are thinking of orbital energy or even instantaneous tangential velocity, that stays the same.

Late last night I watched a youtube video explaining length contraction and I finally could see the reason it has been causing all the trouble it has.

Have a look at this one when you can for my explanation will be referring to it and a similar diagram to it."Special relativity: time dilation and length contraction" by drdwittman @

[swansont]

Many thanks Imatfaal, Strange, and Janus for your very helpful and informative replies.

Having considered and mulled them over, they've increased my understanding of the physics involved.

 

Especially in the case of Janus's post #40, where he says: "it is the force needed to constantly deflect a mass to follow a circular path rather than a straight line"

My only quibble, is whether confusion is caused by using the word "acceleration" for this force.

 

The word does literally mean something like "speeding-up". Which is bound to seem contradictory. When, for example, we're asked to accept that the Moon, which is in a stable orbit, is actually accelerating round the Earth. That suggests that's its speeding up and about to fly off into outer space!

 

The word "acceleration" doesn't seem right. Couldn't we just use "inertia"?

No, it doesn't. Acceleration is defined as a change in velocity per unit time, and it's a vector. You can't use the lay definition in a science application.

[MigL]

You would have no problem using 'acceleration' if you dropped a ball off the roof of your house. It would accelerate to the ground at 10m/s/s.

How is an orbit any different ?

 

Similarily, if you tie a 20 ft rope around a tree and around your waist, and then walk in a straight line , you'll find that there is a 'force' constraining you to walk an orbit around the tree. That 'force', is the rope, and we could call it gravity in case of the Earth or centripetal force in other cases.

That force generates an acceleration towards the center which may not change your speed but changes your direction.

[Robittybob1]

I think you are thinking of orbital energy or even instantaneous tangential velocity, that stays the same.

Late last night I watched a youtube video explaining length contraction and I finally could see the reason it has been causing all the trouble it has.

Have a look at this one when you can for my explanation will be referring to it and a similar diagram to it."Special relativity: time dilation and length contraction" by drdwittman @

My question is do you think length contraction really happens? What I see in the YT clip is that time is dilated and the ship goes further in dilated second .

Could length contraction be better explained better by showing differing relative speeds rather than than saying the distance to a star is altered by relativistic speeds?

Edited December 22, 2014 by Robittybob1

[Robittybob1]

My question is do you think length contraction really happens? What I see in the YT clip is that time is dilated and the ship goes further in dilated second .

Could length contraction be better explained better by showing differing relative speeds rather than than saying the distance to a star is altered by relativistic speeds?

We measure the traveling person moving at "v"/sec but the driver moves V*gamma in one of his dilated seconds, are we really sure he measures his speed at "v"/sec or could it really be "v"* gamma/sec?

 

How could the traveling person check his speed? does he have to take his speed measurement to be the same as ours?

(the section of the YT that I question is from 5:00 to 6:14)

It seems logical to have two things with relative motion to be travelling with the same speed with respect to each other but if each took their own speed measurements using their own clocks would they still come out the same? The hypothesis to check is that there is no such thing as length contraction due to relative motion but only different absolute rates of velocity.

Edited December 22, 2014 by Robittybob1

[swansont]

My question is do you think length contraction really happens? What I see in the YT clip is that time is dilated and the ship goes further in dilated second .

Could length contraction be better explained better by showing differing relative speeds rather than than saying the distance to a star is altered by relativistic speeds?

 

 

 

That's the explanation from the observer's point of view the moving clock is running slow, and that explains what happens from their frame. But every inertial observer can claim s/he is at rest. They have to observe length contraction, otherwise physics breaks.

There is no absolute speed.

[Robittybob1]

 

 

 

 

That's the explanation from the observer's point of view the moving clock is running slow, and that explains what happens from their frame. But every inertial observer can claim s/he is at rest. They have to observe length contraction, otherwise physics breaks.

 

There is no absolute speed.

 

I know that is the standard approach but if length contraction is considered normal why is so difficult to think of speed variation.

As I drive to work I noticed as I allowed time to slow down I would pass more distance in the same amount of time so that means even though my speedo says 100 km/h I would actually be going faster than what the speedo reads rather than having to think the power poles were physically closer together.

 

Can someone really explain if that was the case why physics breaks simply by looking at the situation from a different perspective?

The diagram clearly shows more distance was traveled in a dilated second and there was no logical connection to assume that is solved by length contraction.

(I would be willing to start a thread specifically on this topic, if necessary.)

[swansont]

I know that is the standard approach but if length contraction is considered normal why is so difficult to think of speed variation.

As I drive to work I noticed as I allowed time to slow down I would pass more distance in the same amount of time so that means even though my speedo says 100 km/h I would actually be going faster than what the speedo reads rather than having to think the power poles were physically closer together.

 

Can someone really explain if that was the case why physics breaks simply by looking at the situation from a different perspective?

The diagram clearly shows more distance was traveled in a dilated second and there was no logical connection to assume that is solved by length contraction.

(I would be willing to start a thread specifically on this topic, if necessary.)

 

There is no time variation in the rest frame, so how do you get a speed variation without length contraction?

[Robittybob1]

 

There is no time variation in the rest frame, so how do you get a speed variation without length contraction?

I have not done the calculations as yet but if allowed we will go through them step by step. Here I will propose a thought experiment.

 

It is similar to the Twin Paradox situation. The twins ages are synchronized because they start off on Earth together. To know anything about rates of velocity the space travelers would need to at some stage synchronize their clocks and calibrate their measurements of distance and speed so we would give them both a calibrated radar speed camera recorder before they set off on the long journey to the closest star.

To test out the equipment one twin (Bob) flies out to the Moon and back and on the return pass he buzzes Alice on the ISS at 0.8 the speed of light.

 

Do both of them measure the pass as being at the same speed? Lets see if we can use the speed cameras to find out.