Is relativistic effect of length contraction physically real ?

[phyti]

for feynmann;

Time dilation is an experimentally verified fact. Search the internet.

for makep;

Weight is a measure of force, weight=mass x acceleration. A mass gravitationally accelerates toward the center of the earth, but the ground resists it. Put a scale between the ground and the mass, then read the force/weight indicated by the scale.

fredrik;

Assume the ship accelerates into space, physically contracts, and the rear engine is shut off, the ship is in a state of equilibrium.
Next the front engine is on, accelerates the ship in the opposite direction, with a corresponding contraction. The ship returns to Earth (after one more acceleration/deceleration) shorter than when it left!
That's the question; What does the theory really say?

for anyone;

When the observer measures the moving object, he is simultaneously detecting an event from the near end (which occurred at an earlier time) and an event from the far end (which occurred before the near event). He is measuring two non-simultaneous events, which do not represent anything physical, thus he is not measuring the physical (spatial) length of the object, as measured in it's own reference frame.

 

[granpa]

the measured contraction depends only on the relative velocity of the observer and the measured object. both objects see the other one as contracted. this is possible because of loss of simultaneity. the measured length of an object being the distance between the from and the back at one simultaneous (for the observer) moment.

 

[Fredrik]

Assume the ship accelerates into space, physically contracts, and the rear engine is shut off, the ship is in a state of equilibrium.
Next the front engine is on, accelerates the ship in the opposite direction, with a corresponding contraction. The ship returns to Earth (after one more acceleration/deceleration) shorter than when it left!
That's the question; What does the theory really say?

That its length in Earth's rest frame only depends on its speed in Earth's rest frame.

In Earth's rest frame, the ship gets longer as it begins to accelerate towards Earth (because its speed is decreasing). It keeps getting longer until its velocity in Earth's frame is 0 and then it starts getting shorter again. This will continue until it shuts off the front engine and turns on the rear engine in order to slow down. Then it starts getting longer again until its velocity is 0.

If you instead use a frame in which the rocket is stationary when it has just shut off its rear engine the first time, the description would be different. E.g. when the front engine is switched on, the rocket starts getting shorter in this frame.

When the observer measures the moving object, he is simultaneously detecting an event from the near end (which occurred at an earlier time) and an event from the far end (which occurred before the near event). He is measuring two non-simultaneous events, which do not represent anything physical, thus he is not measuring the physical (spatial) length of the object, as measured in it's own reference frame.

There's more than one way to measure the length of a moving object. This may not be the most practical way, but you could e.g. use a line of rulers with a computer and a detection device at each mark on the rulers, 1 mm apart. Assume that the computers have synchronized clocks. (This is a valid assumption if they're not accelerating and there's no gravity). Now suppose that every computer makes a note of the times when the front and rear of the rocket passed the mark in front of it. You could examine the data after the fact and you would find two computers that have recorded (almost) the same time for the events when the front of the rocket passed the second computer and when the rear of the rocket passed the first computer. To find the length of the moving ship in the computers' rest frame, just compute the difference between the spatial coordinates of those two computers.

 

[Makep]

for feynmann;

Time dilation is an experimentally verified fact. Search the internet.

for makep;

Weight is a measure of force, weight=mass x acceleration. A mass gravitationally accelerates toward the center of the earth, but the ground resists it. Put a scale between the ground and the mass, then read the force/weight indicated by the scale.

fredrik;

Assume the ship accelerates into space, physically contracts, and the rear engine is shut off, the ship is in a state of equilibrium.
Next the front engine is on, accelerates the ship in the opposite direction, with a corresponding contraction. The ship returns to Earth (after one more acceleration/deceleration) shorter than when it left!
That's the question; What does the theory really say?

for anyone;

When the observer measures the moving object, he is simultaneously detecting an event from the near end (which occurred at an earlier time) and an event from the far end (which occurred before the near event). He is measuring two non-simultaneous events, which do not represent anything physical, thus he is not measuring the physical (spatial) length of the object, as measured in it's own reference frame.

Then what is F = ma and F = mg? What I am syaing here is that such a body is generating own weight. Not mass. Otherwise, what happened to the initial acceleration(s) it required to assume a state of constant velocity. Surely, every moving thing must originally have been at rest before they assumed their respective final velocities. And it would have been acceleration that made that possible.

 

[feynmann]

for feynmann;

Time dilation is an experimentally verified fact. Search the internet.

I know it's experimentally verified fact. I wanted to know what you know and think

Special Relativity is a set of equations that transforms coordinates, not physical objects!
The issue is length of object (in its frame) vs measured length from another frame.

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

I don't understand why time dilation is real, but length contraction is just "perception"
Why did you call it "perception" and just coordinate transformation?

 

[phyti]

That its length in Earth's rest frame only depends on its speed in Earth's rest frame.

In Earth's rest frame, the ship gets longer as it begins to accelerate towards Earth (because its speed is decreasing). It keeps getting longer until its velocity in Earth's frame is 0 and then it starts getting shorter again. This will continue until it shuts off the front engine and turns on the rear engine in order to slow down. Then it starts getting longer again until its velocity is 0.

In both the outward and inward acceleration, the ship is accelerated from one end only.
How would it know it's moving to or from earth? What if it was in remote space, and you fired one engine at random, would it contract or stretch?

 

[Doc Al]

In both the outward and inward acceleration, the ship is accelerated from one end only.

So? As long as you accelerate the ship gently enough so that internal forces can maintain equilibrium, it doesn't matter.

How would it know it's moving to or from earth?

Why does it have to "know" anything?

What if it was in remote space, and you fired one engine at random, would it contract or stretch?

Just like time dilation, length contraction depends on the speed with respect to the observer.

 

[Fredrik]

In both the outward and inward acceleration, the ship is accelerated from one end only.
How would it know it's moving to or from earth? What if it was in remote space, and you fired one engine at random, would it contract or stretch?

What Doc Al said. (Edit: And what granpa said in the post after this one).

Just to make Al's last point more explicit: If the rest length of the ship is L, then the result we get when we measure its length is L/\gamma=L\sqrt{1-\vec v^2}, where \vec v is the ship's velocity in our rest frame (in units such that c=1).

 

[granpa]

you seem to think that the acceleration compresses the ship. that's not the case at all. the ship is not a spring being compressed but the force of acceleration. length contraction as simply a property of spacetime.

 

[phyti]

What Doc Al said. (Edit: And what granpa said in the post after this one).

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

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

 

[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?