Is relativistic effect of length contraction physically real ?

[Fredrik]

It expands in all frames where its speed is decreasing, and contracs in all frames where its speed is increasing.

Note that a "frame" is another word for "coordinate system" and that a coordinate system is just a function that assigns four numbers (t(p),x(p),y(p),z(p)) to each event p. A hypersurface of constant t is "space at time t" in a particular coordinate system. The union of all such hypersurfaces defined by a coordinate system is spacetime. So a coordinate system defines a way to "slice" spacetime into 3-dimensional spaces representing space at different times. But different inertial coordinate systems slice spacetime in different ways to make sure that the speed of light is the same in all of them. If your velocity relative to me is v in the x direction, my "slices" would intersect yours at an angle arctan v.

The motion of a rocket is represented by a set of curves ("world lines") in spacetime (e.g. one for each atom). What represents the rocket "right now" in a particular coordinate system is the set of points where those curves intersect "space at time t". But my "space at time t" is tilted by an angle arctan v relative to yours. So when we both try to measure the distance "in space" between the world line of the front of the rocket and the world line of the rear of the rocket, we're not measuring the same thing. We're both measuring a distance between the same two world lines, but not between the same two points on those world lines.

 

[Fredrik]

I don't know if granpa was talking to you or me.

I think he was talking to you.

My question is: when the engine is off, and the acceleration is gone, does the ship remain contracted?

Yes. As I (and others) have been saying, the length is a function of the velocity and nothing else.

 

[Austin0]

It is easy to tell which is real: they both are.

HI I hope this is all right, to ask this, but I saw a comment of yours in another thread [which I can't find again ], in which you mentioned electron bunching in accelerators as an indication of the reality of length contraction. I have searched the web and found lots of references to the problem and phenomena, but no real specifics as to how it related to SR or any actual description of the effect.
So any details or referrals to sourse would be appreciated. Thanks

 

[Doc Al]

ZapperZ has discussed how the description of "bunching" of particles in an accelerator must include length contraction (and other relativistic corrections) here: https://www.physicsforums.com/showpost.php?p=1589357&postcount=45

 

[Dale]

HI I hope this is all right, to ask this, but I saw a comment of yours in another thread [which I can't find again ], in which you mentioned electron bunching in accelerators as an indication of the reality of length contraction. I have searched the web and found lots of references to the problem and phenomena, but no real specifics as to how it related to SR or any actual description of the effect.
So any details or referrals to sourse would be appreciated. Thanks

My comment was in https://www.physicsforums.com/showpost.php?p=1758556&postcount=3", but the link Doc Al provided to ZapperZ's comment is more detailed.

 

[phyti]

It expands in all frames where its speed is decreasing, and contracs in all frames where its speed is increasing.

Amy accelerates from Earth to catch up with Bob who is passing Earth at speed v, shuts off rear engine at speed v. Amy and Bob share the same ref. frame. Amy starts front engine, accelerates toward earth, shuts off engine at speed -v (rel to Bob).
According to your statements, for the 2nd leg of her trip, Earth viewer sees Amy-ship decelerating and stretching, while Bob sees Amy-ship accelerating and contracting.
How does this happen?

 

[granpa]

Amy accelerates from Earth to catch up with Bob who is passing Earth at speed v, shuts off rear engine at speed v. Amy and Bob share the same ref. frame. Amy starts front engine, accelerates toward earth, shuts off engine at speed -v (rel to Bob).
According to your statements, for the 2nd leg of her trip, Earth viewer sees Amy-ship decelerating and stretching, while Bob sees Amy-ship accelerating and contracting.
How does this happen?

the answer is 'relativity of simultaneity'. don't feel bad. its where all beginners get lost. 90% of the posts in this forum are due to misunderstanding relativity of simultaneity.

 

[Doc Al]

Amy accelerates from Earth to catch up with Bob who is passing Earth at speed v, shuts off rear engine at speed v. Amy and Bob share the same ref. frame. Amy starts front engine, accelerates toward earth, shuts off engine at speed -v (rel to Bob).

Let's keep it simple. Bob moves to the right with velocity +v with respect to earth. Amy starts at speed zero, then accelerates until she also moves at velocity +v with respect to earth. Then she decides to reverse her speed, accelerating until her velocity is -v with respect to the earth.

According to your statements, for the 2nd leg of her trip, Earth viewer sees Amy-ship decelerating and stretching,

The Earth viewer sees Amy's speed go from v to 0 back to v, so he sees (calculates/measures, really) her ship expand (decontract) to its normal length, then recontract.

while Bob sees Amy-ship accelerating and contracting.

Right. From Bob's view Amy's speed increases so her ship contracts.

How does this happen?

Where's the problem?

 

[granpa]

Where's the problem?

 

[phyti]

Let's keep it simple. Bob moves to the right with velocity +v with respect to earth. Amy starts at speed zero, then accelerates until she also moves at velocity +v with respect to earth. Then she decides to reverse her speed, accelerating until her velocity is -v with respect to the earth.

The Earth viewer sees Amy's speed go from v to 0 back to v, so he sees (calculates/measures, really) her ship expand (decontract) to its normal length, then recontract.

Right. From Bob's view Amy's speed increases so her ship contracts.

Where's the problem?

my post:
"accelerates toward earth, shuts off engine at speed -v (rel to Bob)."

earth sees her go from 0 to v, then to o

 

[Doc Al]

my post:
"accelerates toward earth, shuts off engine at speed -v (rel to Bob)."

earth sees her go from 0 to v, then to o

OK, my bad. So Earth observers see her contract on leg 1 and stretch (decontract) on leg 2.

So?

 

[Fredrik]

Amy accelerates from Earth to catch up with Bob who is passing Earth at speed v, shuts off rear engine at speed v. Amy and Bob share the same ref. frame. Amy starts front engine, accelerates toward earth, shuts off engine at speed -v (rel to Bob).
According to your statements, for the 2nd leg of her trip, Earth viewer sees Amy-ship decelerating and stretching, while Bob sees Amy-ship accelerating and contracting.
How does this happen?

When you understand what I said in #71, you're not going to think that this is strange at all. I don't know what to tell you other than that I think it would really help you to learn to draw spacetime diagrams.

 

[granpa]

the length of an object is the distance between the front and back at one simultaneous moment. they need to make a sticky explaining this.

 

[phyti]

When you understand what I said in #71, you're not going to think that this is strange at all. I don't know what to tell you other than that I think it would really help you to learn to draw spacetime diagrams.

I can draw space-time diagrams.
The example was to show that the ship can't physically stretch and contract simultaneously.

 

[DrGreg]

This has been a long thread and I haven't read every word of every post, but let me add my twopennyworth.

"Length" depends on what procedure you use to measure it. If a rod is stationary relative to you, measuring it is easy: line your ruler up against one end, move to the other end and make your reading. If the rod is moving, you have a problem: once you've lined up your ruler against one end of the rod, by the time you get to the other end, the rod has moved. You need to employ an assistant to read one end simultaneously with you lining up the other end. But in relativity, no one can agree what "simultaneous" really means, and no-one can legitimately claim that their own version of "simultaneous" is better than anyone else's. So the reason different observers measure different lengths for the same object is because they are each using their own definition of simultaneity which disagrees with everyone else's. And if you are speeding up relative to the rod, it is getting shorter according to your changing definition of length. If someone else is slowing down relative to the rod, it is getting longer according to their changing definition of length. And both these things can happen at the same time.

Whether you regard these changes of measured length as "physically real" or not depends on what you understand by the term "physically real". You decide.

 

[Fredrik]

I can draw space-time diagrams.
The example was to show that the ship can't physically stretch and contract simultaneously.

It you draw the spacetime diagram correctly, you should see that the ship is getting longer in one frame and shorter in another.

The word "physically" is much to vague to be useful in discussions like this (as John 8 unintentionally demonstrated earlier). Can you explain what you had in mind without using that word?

 

[Dale]

As vector A is being rotated its projection on vector B can be increasing at the same time that its projection on vector C can be decreasing. In the same way, and for essentially the same reason, a ship undergoing Born-rigid acceleration can really stretch in one frame while it is really contracting in another frame. As Fredrik said, whether you call this "physical" or not is a semantic argument, it is a measurable coordinate-dependent effect.

 

[john 8]

No, that's not the question. This was the question:

Note that he defined what he meant by a "real" contraction. He defined a contraction to be "real" if you have to supply an extra input of energy to make the object shorter, i.e. if you have to supply more energy than you need to accelerate the atoms individually.


It's real, but not in the sense that it satisfies his definition of "real".

You don't have to supply any more energy than what's needed to accelerate the atoms individually. You don't have to perform any work to "squeeze" the object. However, that's not a good reason to say that the contraction isn't "real".

Alright. Tell me if in this "contraction" of an object, does the object actually physically contract?

 

[john 8]

Hello john 8.

Explaining the reality of relativistic contraction is difficult because of the difficulty of defining the meaning of real. I cannot attempt to explain this 'reality' other than saying it is not purely an effect of optical perspective. However it is quite clear from the very basic axioms of relativity that, 'real' or not, for relativistic length contraction no force is required. You must understand that the nature of this contraction is far more fundamental than a mere physical compression or shrinkage.

Matheinste.

Stop trying to skirt the issue. If you want to know what the definition of real is just refer to your dictionary. I am using that definition of real.

What do you mean by far more fundamental than a mere physical compression? Does the object physically compress or not? Simple as that. If you think that an object actually compresses, then I am sure you could show some evidence of this, like a mathematical equation that shows force , visual evidence, something, right?

 

[john 8]

Special Relativity is a set of equations that transforms coordinates, not physical objects!.

RIGHT! Length contraction is not about actual real physical objects physically contracting. It is all a math model.

The issue is length of object (in its frame) vs measured length from another frame.

The issue is if a real physical object physically contracts. You say that length contraction does not have to do with physical objects.

As mentioned by john 8, the object is moving inertial in a state of equilibrium, with no forces acting on it. The only way an object changes form is by non uniform forces.

Consider a rod in its rest frame, and A and B moving past it at different speeds. Each will measure it differently, a result of their relative speed. No forces act on the rod, and if 'real' length contraction, it would have to assume two different lengths simultaneously!

The fact that the 'measured length' changes with the motion of the observer should tell you it's perception (observer dependent).

So, are you saying that physical objects physically contract?

 

[john 8]

No. You need to apply an external force to one end of the rod to accelerate the individual atoms, but you don't have to push "extra hard" or "from both sides" to compress it. The internal forces will make sure that it contracts by a factor of $\gamma$ when it's accelerated. They do this by keeping each infinitesimal segment of the rod a constant length in the inertial frame that's co-moving with that segment, which is exactly what they're doing when the rod isn't accelerating.

In order to compress an object you have to apply force to both ends. When an object is compressed, force is applied to one end and there has to be something to push against, something that pushes back with equal force.

This is only true if there's friction. The discussion is about length contraction, so it's appropriate to consider a scenario where friction can be ignored?

Wrong. See above statement.

Do you really think it's a great idea to go to a physics forum and aggressively claim that special relativity is false without learning what the theory says first?

Yes. S.R has scientific flaws. There are too many outpoints.

You will provide evidence of this. Can you answer these simple questions?

What is time as it is defined or explained by Einstein in S.R. ?

If time dilates then it must be a thing, so what is it?


We all know about Einstein's example of the railway carriage and the embankment when he was trying to show relative motion and frames of reference. He basically stated that an observer on the train would consider himself stationary relative to the embankment, and the embankment would be what is in motion. And conversely he stated that an observer on the embankment would consider himself stationary relative to the train, and the train would be in motion relative to embankment.

We all know that in order for the train to be in motion a force has to be applied to it. We can all agree that force is applied to trains everyday to get them to move. This is no mystery and is done with ease. Here is the problem, just because there is someone on a moving train that happens to be looking out the window, does in no way apply any force to the embankment to get it to move. The embankment never moves relative to the train. The embankment is not in motion at anytime from any frame of reference. In order to move the embankment a force has to be applied to it, just like a force is required to move the train.

If you think that embankments move due to an observer on a moving train, then please provide empirically evidence of this.


Try this one on for size.

Einstein uses a moving train, a stone, and the embankment to show how time dilation is possible.

The example states that an observer on a moving train when he drops a stone will SEE the stone fall down in a straight line from the point of release to the impact on the ground.

An observer on the embankment watching the same stone drop will see the stone travel in a parabolic curve.

Now since the straight line traversed by the stone is shorter then the curve traversed by the same stone and yet both events took the same amount of time, we are supposed to believe that this is due to some magical time dilation that allows the stone to travel the curved path in the same time as the straight path. This whole idea is false. The stone never travels in a straight line from the point of release to the ground. The stone or any object dropped from a moving frame of reference in an environment that contains gravity will always share the same speed as the moving frame of reference and will fall due to gravity. These two motions will cause the dropped object to have two forces acting on it, the motion (or force) derived from the moving frame and the force of gravity. This will cause the object to travel in a parabolic curve as it falls to the ground.

The false data in Einstein's example of time dilation is that the stone actually falls in a straight line when observed by the person one the train.

Question. If you think that I am wrong then explain how a mass will fall in a straight line when dropped from a moving body.

 

[jtbell]

Yes. S.R has scientific flaws. There are too many outpoints.

Please read the sticky post at the top of this forum, IMPORTANT! Read before posting.

 

[Integral]

Length contraction has nothing to do with compression. Remember that the observer moving with the rod measures the same length at all times so nothing is being compressed.

It is more correct to view length contraction as a rotation in space time. Have a friend hold a meter stick some distance way from you perpendicular to the line between the 2 of you. From a distance do a measurement. Now have your friend rotate the meter stick 45deg, measure it again. Now from your view point it is shorter then it was before.

Given that, suppose I have a large block of lead weighing several tons. Now if I strike that lead block with a hammer it will dent. Is not that compression with no opposing force?

I recommend that you back off from your aggressive stance, you are close to getting a infraction.