Lorentz contraction of box filled with gas

[Xeinstein]

You have received the answers several times already : "no" and "no"

Do you realize you are probably the only person in this thread/forum claim length does Not contract in the observer frame in which the box is moving? If that's the case, then you are seriously mistaken. For the 5-th time

In the following quote, kev explains why you are mistaken:

If by \Delta V=0 you are saying that change in volume due to relative motion is zero, then that implies that change in length due to relative motion is also zero and you are seriously mistaken.

If change in length (length contraction) is imaginary then change in clock rate (time dilation) is also imaginary, because they go hand in hand. There is plenty of experimental evidence that time dilation is not imaginary.

You might have noticed that Einstein and Lorentz state that L = L_o \sqrt(1-v^2/c^2) and not L=L_o which is what \Delta V=0 and \Delta L=0 implies.

Why does Special Relativity have all those those complicated transformation formulas if no real physical transformations occur? Why does anybody bother if they are all imaginary and have no consequences? Kind of makes relativity pointless.

What's Lorentz-contraction? Is it an illusion or is it real?
First, we need to know how to measure the length of a moving object? It's not straightforward as you might think. To properly measure the length of a moving object, we must measure the position of both ends at the same time in our inertial frame. However, an observer at rest on the moving object would not agree that the measurements were made at the same time. The observer at rest with respect to the moving object, using her own clocks, would say that the position of the front end was measured at an earlier time than the position of the back end. So both agree that a measurement of length of a moving rod yields a shorter length than the measurement made in the frame of the rod. This is called the Lorentz contraction. This contraction is real in any sense of the word you can think of.

 

[AVentura]

If I accelerated a train to a relativistic speed I would observe a contraction of its length.

But if I accelerate two electrons (or atoms) independently at the same rate I do not observe the distance between them to reduce at all from what it was when they were moving slowly. (Picture two separate Linacs aligned on the same axis, their electrons will not affect one another, otherwise Linacs in different hospitals would be pulling each other's electrons out of their guides!)

Why the two different results? Because the atoms in the train are in a lattice. If you try to accelerate the front of the train independently of the back you will rip it in half. But an observer who is already moving the same direction as the train would claim the front started first, and that's why it ripped in half; no paradox. But the train will rip in half.

 

[granpa]

when an objects moves it really does contract or at least it occupies less space (same thing?). but to that object the rest of the universe appears to be moving and therefore contracted. did the universe contract just because this one object moved? of course not.

the length of an object is the distance between the front and back at one instantaneous moment. imagine that the object is moving past a line of stationary, perfectly synchronized clocks. since, to the moving object, the clocks APPEAR (even after compensating for light travel time) to be out of synch, the line of clocks will also appear to be shorter.

so length contraction is both real and illusory. the moving object really contracts. while everything else only appears to the moving object to contract.

this is similar to the twin paradox where only one of the twin actually ages less even though it appears as though both are time dilated.

 

[matheinste]

Hello granpa.

--------- contraction is both real and illusory. the moving object really contracts. while everything else only appears to the moving object to contract.---------


Length contraction, like time dilation in SR is symmetric. Both observers in relative motion see length contract and time dilate in the other frame. In their own frame of course they see nothing change.

Matheinste.



-----so length contraction is both real and illusory. the moving object really contracts. while everything else only appears to the moving object to contract.-----

 

[granpa]

thats what i said

 

[matheinste]

Hello granpa.

I messed up the last post. The bottom line should not be there i was just pointing out that you were wrong by quoting your words.

This is how it was meant to read. Sorry for the mix up.

To quote your words!

-----so length contraction is both real and illusory. the moving object really contracts. while everything else only appears to the moving object to contract.-----

That is incorrect

My reply!

Length contraction, like time dilation in SR is symmetric. Both observers in relative motion see length contract and time dilate in the other frame. In their own frame of course they see nothing change.

Matheinste.

 

[granpa]

there is no contradiction. both see length contraction and time dilation

one sees a real contraction and the other sees an illusory contraction due to loss of simultaneity.

it is of course absolutely impossible to know which is which.

 

[matheinste]

Hello granpa.

They cannot be real and illusory. Both are real.

Matheinste.

 

[granpa]

care to point out the flaw in my reasoning?

 

[AVentura]

when an objects moves it really does contract or at least it occupies less space (same thing?)

what if you don't let it. do you think it would be ripped apart?

 

[granpa]

what difference does it make?

 

[matheinste]

Hello granpa.

You say ----the moving object really contracts.----

True. But bear in mind that each observer regards the other as moving relative to him so the words moving and stationary only have a relative meaning in his sense.

You say ---everything else only appears to the moving object to contract. ----

Everything else ( moving relative to other observer, or object ) not only appears to contract, it too really contracts. But, again bear in mind, that to each observer the other is regarded as moving.

Matheinste

 

[granpa]

relativity does not say that there is no absolute velocity. it just says that you can't determine what it is without ftl travel.

 

[matheinste]

Hello granpa.

----relativity does not say that there is no absolute velocity. it just says that you can't determine what it is without ftl travel.------

Complete nonsense. Buy a book and read it.

Matheinste.

 

[Mentz114]

relativity does not say that there is no absolute velocity. it just says that you can't determine what it is without ftl travel.

Can you expand on this ? Are you saying that absolute motion could be detected if you had ftl travel ?

 

[granpa]

well suppose you could communicate instantly with any part of the universe or at least with arbitrarily small delay. that would allow you to know the absolute time.

if you know absolute time then you should be able to determine absolute velocity simply by looking at your time dilation.

https://www.physicsforums.com/showthread.php?t=235621

of course, there is no way to do either. some people do suggest that quantum entanglement might allow ftl communication but the consensus seems to be that it cant.

 

[yuiop]

Hello granpa.

----relativity does not say that there is no absolute velocity. it just says that you can't determine what it is without ftl travel.------

Complete nonsense. Buy a book and read it.

Matheinste.

Grandpa is "sort of right". Relativity does not rule out an absolute reference frame of the type proposed by Lorentz (where clocks moving relative to the absolute vacuum time dilate and rulers moving relative to the absolute vacuum length contract), it just says it can not be detected and that there "is no need of one".

 

[AVentura]

what difference does it make?

to what were you referring?

 

[yuiop]

when an objects moves it really does contract or at least it occupies less space (same thing?).

what if you don't let it. do you think it would be ripped apart?

Bell's spaceships paradox says yes, it will be ripped apart.

 

[AVentura]

Bell's spaceships paradox says yes, it will be ripped apart.

thanks, I have been thinking about this for years but didn't know what it was called. (they didn't cover this in my master's program)

but I don't think most Physicist would agree with Bell (none of my professors would have). I got in lots of arguments.

The wikipedia entry on Bell's spaceships paradox says that CERN put together a think tank that disagreed with Bell. What was their argument?

 

[yuiop]

thanks, I have been thinking about this for years but didn't know what it was called. (they didn't cover this in my master's program)

but I don't think most Physicist would agree with Bell (none of my professors would have). I got in lots of arguments.

The wikipedia entry on Bell's spaceships paradox says that CERN put together a think tank that disagreed with Bell. What was there argument?

The wikipedia article goes on to quote Bell as saying "Of course, many people who get the wrong answer at first get the right answer on further reflection".

I am not sure what the argument the CERN scientist's had for assuming the string would not break besides the counter intuitive notion that a physical object between the spaceships length contracts while the space between them does not. That is quite shocking to some people that view length contraction as a mathematical entity rather than a physical one.

The wikipedia article also states a paper by Matsuda and Kins-hita contradicting Bell's conclusion met with much criticism.

Here is my simple explanation of Bell's spaceship paradox if you are interested.

Say we have 2 stationary rockets a distance x apart. A ruler is attached to one rocket and extends to touch the other rocket but is not attached to the second rocket. The 2 rockets launch simultaneously from A's frame with constant and equal proper acceleration and maintaining the same separation in A's frame. (A does not accelerate). Now let's say they stop accelerating simultaneously in A's frame at a final and equal velocity of v=0.8. The distance between the 2 rockets is x according to A and the ruler is now length contacted to 0.6x. If the ruler is now stretched in an attempt to connect it to both rockets its going break, right?

To an observer that was always moving at 0.8c relative to observer A, the rockets do not take off simultaneously but the front rocket appears to take off before the rear rocket. With the rear rocket stationary and the front rocket accelerating it is obvious that if the ruler was attached to both rockets it would be under a lot of strain and will eventually snap.

The arguments for why an object connecting both rockets will eventually snap are so simple and clear, it is difficult to imagine why anyone would disagree.

 

[granpa]

aventura:

i presume what you are asking me is whether i believe that length contraction is real or just an illusion. it is real for an object that is really moving.

 

[Mentz114]

granpa,

well suppose you could communicate instantly with any part of the universe or at least with arbitrarily small delay. that would allow you to know the absolute time.

if you know absolute time then you should be able to determine absolute velocity simply by looking at your time dilation.

https://www.physicsforums.com/showthread.php?t=235621

of course, there is no way to do either. some people do suggest that quantum entanglement might allow ftl communication but the consensus seems to be that it can't.

Thanks for pointing that out. But it only needs arbitrarily fast communcation, not ftl.

What do you mean by 'really moving' in your post above ?

 

[granpa]

arbitrarily fast communcation, not ftl?

arbitrarily fast is a way of saying virtually infinitely fast. how is that not ftl?

in other words, be able to communicate with any part of the universe with arbitrarily small delay (ie. virtually zero delay).

 

[Mentz114]

granpa,
yes, sorry, I meant to say not instantaneous ( it's sunday afternoon ! ).

But you didn't answer my question. I'm not challenging you, you've pointed out some interesting things so far.

 

[granpa]

https://www.physicsforums.com/showthread.php?t=235621

both twins perceive the other to be time dilated but in reality it is only the one that is really moving that is time dilated. once he stops he realizes this. same with length contraction.

that was weird. when i first viewed it my post was blinking solid black. never seen that before.

 

[Mentz114]

granpa,
OK. I suppose you could say 'the one who is off the free-fall geodesic', or 'has his engines on'.
This getting off-topic, so I'll leave it there.

M

 

[Qeinstein]

The wikipedia article goes on to quote Bell as saying "Of course, many people who get the wrong answer at first get the right answer on further reflection".

I am not sure what the argument the CERN scientist's had for assuming the string would not break besides the counter intuitive notion that a physical object between the spaceships length contracts while the space between them does not. That is quite shocking to some people that view length contraction as a mathematical entity rather than a physical one.

The wikipedia article also states a paper by Matsuda and Kins-hita contradicting Bell's conclusion met with much criticism.

Here is my simple explanation of Bell's spaceship paradox if you are interested.

Say we have 2 stationary rockets a distance x apart. A ruler is attached to one rocket and extends to touch the other rocket but is not attached to the second rocket. The 2 rockets launch simultaneously from A's frame with constant and equal proper acceleration and maintaining the same separation in A's frame. (A does not accelerate). Now let's say they stop accelerating simultaneously in A's frame at a final and equal velocity of v=0.8. The distance between the 2 rockets is x according to A and the ruler is now length contacted to 0.6x. If the ruler is now stretched in an attempt to connect it to both rockets its going break, right?

To an observer that was always moving at 0.8c relative to observer A, the rockets do not take off simultaneously but the front rocket appears to take off before the rear rocket. With the rear rocket stationary and the front rocket accelerating it is obvious that if the ruler was attached to both rockets it would be under a lot of strain and will eventually snap.

The arguments for why an object connecting both rockets will eventually snap are so simple and clear, it is difficult to imagine why anyone would disagree.

So the ruler is now length contacted while the distance between the 2 rockets does not change.
How about the gas in the box? Would the distance between gas particles change or not while the box is length contacted? In other words, would the box compress the gas in it?

 

[granpa]

the box occupies less space but isn't compressed. to the box nothing appears to have changed.

 

[AVentura]

So the ruler is now length contacted while the distance between the 2 rockets does not change.
How about the gas in the box? Would the distance between gas particles change or not while the box is length contacted? In other words, would the box compress the gas in it?

just to the initial observer. to that person, the particles would be happy to get closer together without any extra force. their fields are compressed in that direction.

 

[granpa]

thats right. and that brings up the interesting fact that electric and magnetic fields themselves possesses velocity.

 

[AVentura]

it's also important to remember that some observers in different frames can see the box stretched (perhaps they see it getting closer to their velocity) They would see a different set of fields that make sense with this.

 

[AVentura]

The wikipedia article also states a paper by Matsuda and Kins-hita contradicting Bell's conclusion met with much criticism.

no, they agree with Bell (it is linked from wikipedia). they leave out the string and just ask if the ships contract (no they say)
so they and Bell were met with criticism.

but i would tell the critics this:
I work in a radiation therapy department with 2 linear accelerators that are 1 meter in length but are 20 meters apart (co-linear acceleration). The amount of length contraction expected at these speeds can be significant (0.5). That would put the electrons in the hall where I'm standing! Imagine physicists in Cal Tech and MIT that just happened to accelerate electrons in the same direction at the same time. Would they contract to Nebraska?

 

[Qeinstein]

just to the initial observer. to that person, the particles would be happy to get closer together without any extra force. their fields are compressed in that direction.

Can you tell me why the distance between the gas particles would change but the distance between the rocket ships won't change?

 

[AVentura]

the gas particles are pressing off one another, but are bound by the box. the gas particles don't mind getting closer together in the direction of acceleration. at the same time the box is contracting in the direction of acceleration. so it's a nice point that you have made. contraction can occur whenever particles are in some sort of interaction, not just a solid state lattice.

are the ships in some sort of interaction? well, there is gravity. something must be keeping them apart for them to float in space like that. perhaps they are bouncing photons off one another to stay separated. that system may contract under acceleration. i really don't understand gravity honestly. it seems like such a system really includes everything in the universe, and would have to be solved as such.

but the electrons in my linacs are not interacting (or I am overcoming it). and I think that is what Bell intended. even if there is an attraction it is accounted for by some system that keeps the ships with the same acceleration relative to a stationary ruler at rest alongside the ships.

in other words, ships don't contract when we don't let them. seems like a real stupid statement, and it is.

but if you did this, and had a string between them, half the scientific world would be in for a shock, and the other half would make a toast. because this is what has never been done so far. someday, someday.

 

[yuiop]

no, they agree with Bell (it is linked from wikipedia). they leave out the string and just ask if the ships contract (no they say)
so they and Bell were met with criticism.

Your right, the Japenese scientists were agreeing with Bell. More precisely they ask "Does the space (distance) between two ships contract?" and later state "One can conclude that the length of each spaceship may contract according to Eq. (1) but that the distance between the two spaceships should stay constant, as viewed from S."

At the time they seemed to get even more criticism than Bell which made me assume they were disagreeing with Bell. The summary of their paper says:

"After our Japanese papers and a few papers criticizing our argument
appeared in the Japanese physics journal “Parity,” we
discovered that a very similar argument was discussed by Bell,
and that it was met with similar criticism that we are [1]. This
means that, unfortunately, many physicists did not, have not
and still do not understand the real meaning of the Lorentz
contraction even after almost 100 years of the introduction of
special relativity by Einstein."

That is a bit worrying... Are modern scientists so steeped in abstract concepts that they find simple physical concepts difficult to relate to?

 

[AVentura]

That is a bit worrying... Are modern scientists so steeped in abstract concepts that they find simple physical concepts difficult to relate to?

my university experience says yes.

 

[yuiop]

my university experience says yes.

I probably shouldn't appeal to authority, but it may be worth mentioning that the John Bell the spaceship paradox was named after is the same John bell that came up with Bell's inequalities which has been described as one one of the most profound discoveries of science.

 

[matheinste]

Hello again granpa.

Quote:-
----relativity does not say that there is no absolute velocity. it just says that you can't determine what it is without ftl travel.----

So relativity does not deny the existence of absolute velocity we just need FTL travel. This can only be information travel as material and EM FTL is not allowed in SR

Absolute time be determined if we have FTL transmission of information.

So knowing this absolute time, by some method we determine absolute velocity.

OK.

So with instantaneous transmission of information ( it has to be instantaneous or we are back to some finite velocity such as c ) we can synchronize all the clocks in our own frame without need for allowing for transmission time or worrying about the anistropy of c. So can everybody else in their own frame. How does this single out a unique absolute rest frame. ( i say rest frame when i should really say frame with absolute velocity zero ).

If we still have the requirement, which SR has, that c is the same for all observers we are still back to where we started. Every observer in relative inertial motion will observe time dilation in the other's frame. We still cannot tell who is " really " moving. This is a consequence of c being the same for all inertial observers.

If you have a valid theory you will need to spell it out in detail for me to understand as i may be making invalid assumptions about your reasoning or my understanding of basic SR may be at fault.

Thankyou in advance for your more detailed explanation. If you can convince me i will happily concede that i am wrong. It won't be the first time.

Matheinste.

 

[matheinste]

Hello again granpa.

Quote:-
----relativity does not say that there is no absolute velocity. it just says that you can't determine what it is without ftl travel.----

So relativity does not deny the existence of absolute velocity we just need FTL travel. This can only be information travel as material and EM FTL is not allowed in SR

Absolute time be determined if we have FTL transmission of information.

So knowing this absolute time, by some method we determine absolute velocity.

OK.

So with instantaneous transmission of information ( it has to be instantaneous or we are back to some finite velocity such as c ) we can synchronize all the clocks in our own frame without need for allowing for transmission time or worrying about the anistropy of c. So can everybody else in their own frame. How does this single out a unique absolute rest frame. ( i say rest frame when i should really say frame with absolute velocity zero ).

If we still have the requirement, which SR has, that c is the same for all observers we are still back to where we started. Every observer in relative inertial motion will observe time dilation in the other's frame. We still cannot tell who is " really " moving. This is a consequence of c being the same for all inertial observers.

If you have a valid theory you will need to spell it out in detail for me to understand as i may be making invalid assumptions about your reasoning or my understanding of basic SR may be at fault.

Thankyou in advance for your more detailed explanation. If you can convince me i will happily concede that i am wrong. It won't be the first time.

Matheinste.

 

[granpa]

well suppose you could communicate instantly with any part of the universe or at least with arbitrarily small delay. that would allow you to know the absolute time.

if you know absolute time then you should be able to determine absolute velocity simply by looking at your time dilation.

https://www.physicsforums.com/showthread.php?t=235621

of course, there is no way to do either. some people do suggest that quantum entanglement might allow ftl communication but the consensus seems to be that it cant.

 

[matheinste]

Hello granpa.

Quote:-
---if you know absolute time then you should be able to determine absolute velocity simply by looking at your time dilation.----

If you know absolute time, from instantaneous information transmission, then all clocks in all frames would presumably be synchronized to this time and remain so. In this case there would be no difference in anyones clocks whatever their relative motion ( presumably they would instantaneously be updated ) and hence no observable time dilation between frames. So how would this help to determine an absolute velocity or special frame.

You have merely quoted what you said before with no more helpful information. To help anyone understand what you are claiming please give a more detailed reasoning.

Matheinste

 

[granpa]

sorry, your q makes no sense.
of course there is time dilation. look at the twin paradox here:

https://www.physicsforums.com/showthread.php?t=235621

the ability to communicate instantly with any part of the universe would make no difference to time dilation. the reason the moving twin sees the nonmoving twins clock ticking slower is because of loss of simultaneity. but if he can communicate instantly then he can see that all the line of clocks are in fact synchronized and running at twice his own rate (not 1/2 as he would otherwise believe). from that he can determine that he is time dilated. the nonmoving twin will see something completely different.

i edited my post after first posting it. i figured you simply didnt read the new version.

 

[Mentz114]

You could choose one clock, and all other clocks would take their time from it. This would give 'an' absolute time. All clocks would look to all observers to have the same time and rate. Paradoxical ?

 

[granpa]

the clocks might run at the same rate but will the natural processes in each frame run at the same rate? no.

all will indeed look to some central clock to determine what time it really it. proper time i guess. but each frame will still be time dilated. atoms will vibrate at lower rates, ect...

 

[Mentz114]

granpa;

the clocks might run at the same rate but will the natural processes in each frame run at the same rate? no.

all will indeed look to some central clock to determine what time it really it. proper time i guess. but each frame will still be time dilated. atoms will vibrate at lower rates, etc..

Exactly, hence the paradox.

It's late, my computer is going to sleep now.

M

 

[granpa]

thats not what the twin paradox is about.

and time dilation is not a paradox anyway.

 

[matheinste]

Hello again.

How do you single out a frame that is special and thus determine absolute velocity, or rest. Picking out one frame at random and synchronizing all clocks to that frame ( which are alreadt synchronizes anyway )proves or achieves nothing.

Matheinste

 

[granpa]

i just answered that. only the moving twin is actually time dilated. its true that he sees the nonmoving twin as being time dilated but that because of loss of simultaneity. if he can communicate instantly with any part of the universe then loss of simultaneity will not prevent him from seeing that the nonmoving twins clock is actually ticking at twice his own rate rather than 1/2 as he would otherwise believe.

 

[matheinste]

Hello again.

I'm sorry i'll have to give it more thought. It's 2.00AM here and I've got work tomorrow.

Goodnight.

Matheinste.