Lorentz contraction of box filled with gas

[matheinste]

Hello granpa

I have given the problem more thought and still believe you to be wrong.

You have set a universal time for all clocks using instantaneous signal transmission. OK

You have arbitrarily picked a frame, ( in the case of the twins, the stay at home twin's frame. He knows he is the stay at home twin because he experiences no acceleration ) which you relate everything else to. This is not an absolutely at rest ( or absolutely moving ) frame of reference.

In the case of the twins, the traveling twin ( he knows who he is because he experiences acceleration ) will return with his clock showing the same time as the stay at home twin because we have synchronized all clocks. He will however have aged less than the stay at home twin because he has traveled a different spacetime interval which has less proper time. To do this he has needed to accelerate and decelerate to get out of the initial inertial path and back to it again. This has nothing to do with an absoliute non-moving frame of reference for the non traveling twin.

I have nothing more to say because these are the facts as i see them and i can, unfortunately, express myself no better than i have already done and so i dio not expect what i have said to make you change your mind. Perhaps someone with more skill than me
( if of course they agrre with me ) may be able to convince you. Or if they feel it necessary to point out to me, explicitly!, my errors.

Goodbye. enjoy your day.

Matheinste.

 

[granpa]

i never said the nonmoving twin of the twin paradox had to be at absolute rest.

all observers, moving and nonmoving will indeed SEE the same thing. my point was that they see it for different reasons. the nonmoving twin SEES the other twin as contracted because he really is length contracted. the moving twin on the other hand SEES the other twin as contracted because of loss of simultaneity. but this is an illusion which is lost the minute he stops. the illusion would also be lost if he had ftl communication.

while he was moving the line of clocks seemed to him to be out of synch and based on that he calculated the other twins clock to be a certain value but the very instant he stops suddenly the whole line of clocks are synchronized again and he now calculates the other twin clock to say something completely different. clearly he was seeing an illusion.

but without ftl communication it is absolutely impossible to tell which is moving and which is not and which is seeing an illusion and which is seeing reality. if indeed either of them is. both may be moving.

why should you accept, if only for the sake of a thought experiment, that absolute time can exist but reject that absolute velocity could exist? one follows directly and easily from the other.

and yes the moving twin ages less. i certainly never said otherwise. in fact that was my point. an object that is really moving really does become length contracted and time dilated.

 

[matheinste]

Hello granpa.

One does not follow directly from the other.

Show me.

Matheinste.

 

[granpa]

if they can communicate instantly then they can tell which one is aging less. the one that is aging less is the one moving faster.

its as simple as that. lol.

 

[matheinste]

Hello again.

That is incorrect.

Faster than what.

Until you have an absolute frame of rest, the existence of which you have not demonstrated, faster means nothing.

Before relativity, time was thought to be absolute but it was recognized that velocity was not. Hence the form of the Gallilean transformation. One does not follow from the other.

Matheinste.

 

[granpa]

then why is the one aging faster? remember that in this thought experiment they can communicate instantly so they know exactly how fast they are aging relative to one another.

remember that even though they each see exactly the same thing, they see it for different reasons. in that sense the symmetry is broken no matter what you believe.

 

[matheinste]

Hello granpa.

Before you tried to re-introduce the concept of absolute time we knew one aged more than the other and we knew which it was. The accelerated one. Having an absolute time does not make any difference.Nothing you have said changes this fact. We knew it already. I repeat that absolute motion ( which is non-existent in SR ) has nothing to do with it. It is the acceleration that is absolute in SR.

If you, by saying, " why is the one ageing faster than the other " as a genuine question rather than trying to make a point, then any textbook on relativity and countless threads in this forum will tell you the same.

Mateinste.

 

[granpa]

you're missing the point. the point is that there is a limit. you can't have less than zero velocity. someone somewhere in the universe must be aging at the highest rate. that person would be stationary.

remember that relativity does not say that there in no absolute time, distance, or velocity. it just says that you can't detect it if it does exist.

 

[matheinste]

Hello granpa.

That is precisely the point. You cannot detect it. But without detecting it, or what is the same thing, a knowledge of which frame it is, then you cannot assign velocity values to anything. All you can say is that something is moving relative to something else.

However if you choose a frame and decide that you will reference all other frames to it then you can say something is at rest relative to that particular frame or is moving faster or slower than a third frame which is moving relative to the frame you have selected as a reference. We are perfectly entitled to do this but we must always remember it is an artificial, albeit useful choice made by us and is not a frame absolutely at rest.

If I had time i would gladly try to explain this more easily and explicitly. But all roads lead to the fact that you cannot exhibit an absolute frame of rest.

Matheinste

 

[granpa]

have i said otherwise? i have repeatedly said that we can't tell which is moving and which isnt. the thought experiment we have been discussing involving assuming that they can communicate instantly. that would make it possible to determine their absolute time if nothing else. but as i have shown it would also allow them to determine their absolute velocity.

but of course we don't have ftl communication so its just a thought experiment.

i can't understand why your willing to accept, for this thought experiment, absolute time but totally unwilling to accept absolute velocity. seems contradictory to me.

i also note that you didnt address my point.

 

[matheinste]

Hello granpa.

You have not showm how this makes it possible to determine absolute velocity.

Please show me how.

Matheinste.

 

[granpa]

surely you are messing with me. I've answered that q enough times. i won't answer it again.

i would point out that you haven't actually refuted anything i have said. you just keep repeating that i am wrong and that i haven't proven anything when in fact i have proven my case. in particular, you haven't addressed my point that i made down below.

 

[matheinste]

Hello granpa.

Then we must agree to differ or ask for an expert opinion.

Matheinste.

 

[granpa]

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

 

[matheinste]

Oh well. I guess i'll never know so i will just have to live with my ignorance.

No hard feelings of course.

Matheinste.

 

[yuiop]

Hello granpa.

Then we must agree to differ or ask for an expert opinion.

Matheinste.

I am not an expert, but I would like to add the following thought experiment to the discussion that might help clarify the issues.

Let there be a special clock that can transmit instantaneous signals. Any clock in the universe can be compared with the time signal from the special clock to directly determine if it is synchronised with the special clock. Likewise any observer would easily determine their velocity with respect to the reference frame of the special clock. Is the reference frame of the special clock the absolute reference frame? I think not. If the special clock was accelerated to a new velocity, only clocks in the new reference frame would now consider themeselves to be absolute rest and clocks in the old reference fraem of the special clock would now consider themselves to have absolute motion (even though they never accelerated). The only way some sort of absolute reference frame would be determined is if the special clock had the additional special property that it was immune to time dilation when moving. With that additional special property, it would not require the special property of being able to transmit ftl or instantaneous signals to be able to determine an absolute reference frame or absolute motion.

[EDIT] Perhaps I should add incase it was not clear, that there is nothing to suggest that such a special clock with these special properties could actually exist.

 

[AVentura]

[EDIT] Perhaps I should add incase it was not clear, that there is nothing to suggest that such a special clock with these special properties could actually exist.

If A could cause B instantly from a distance, then some observers could claim B happened before A.

 

[Aachenmann20]

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."

So we all agree that the distance between the two spaceships should stay constant, as viewed from S. Now the question is: would the distance between the gas particles change due to Lorentz-contraction as viewed from S? If yes, why it changes but the distance between the two spaceships should stay constant?

 

[AVentura]

I gave an answer on page 9. basically the gas is part of a system that is interacting. the ships are not (if you ignore gravity between themselves and the universe).

 

[AVentura]

Also, look at some pictures of electric fields of moving charged particles. They are compressed in the direction of motion. If these fields are what keeps the gas particles apart, then they aren't going to do such a great job at it anymore in that direction.

Any relativistic quantum mechanics experts here? Do the wave functions compress in the direction of motion?

 

[granpa]

I am not an expert, but I would like to add the following thought experiment to the discussion that might help clarify the issues.

Let there be a special clock that can transmit instantaneous signals. Any clock in the universe can be compared with the time signal from the special clock to directly determine if it is synchronised with the special clock. Likewise any observer would easily determine their velocity with respect to the reference frame of the special clock. Is the reference frame of the special clock the absolute reference frame? I think not. If the special clock was accelerated to a new velocity, only clocks in the new reference frame would now consider themeselves to be absolute rest and clocks in the old reference fraem of the special clock would now consider themselves to have absolute motion (even though they never accelerated). The only way some sort of absolute reference frame would be determined is if the special clock had the additional special property that it was immune to time dilation when moving. With that additional special property, it would not require the special property of being able to transmit ftl or instantaneous signals to be able to determine an absolute reference frame or absolute motion.

[EDIT] Perhaps I should add incase it was not clear, that there is nothing to suggest that such a special clock with these special properties could actually exist.

no the ability to transmit its signal would not make it be at absolute rest. but it would allow every clock in the universe to compare its time dilation with it and that would allow one to know whether it was moving faster or slower that that clock. now what would happen of every clock could transmit its signal to every other clock?

 

[yuiop]

no the ability to transmit its signal would not make it be at absolute rest. but it would allow every clock in the universe to compare its time dilation with it and that would allow one to know whether it was moving faster or slower that that clock. now what would happen of every clock could transmit its signal to every other clock?

hmmm.. actually.. thinking about it some more.. I guess even with one special specail clock able to transmit its time signal instantaneously anywhere, some clocks would be running running slower than the specail clock and some would be running faster. Finding the absolute rest frame would then be a case of finding the frame that clocks run fastest in relative to the special clock. Does that seem rght?

 

[granpa]

it does to me.

 

[blackwing1]

hmmm

Consider what happens when we accelerate a box filled with gas. We have to expend a certain amount of energy to accelerate the box, In Newtonian mechanics, this energy goes into the kinetic energy of the box: as its speed increases so does its kinetic energy.
This happens in relativity too, of course, but in addition, Do we have to spend some extra energy because the box contracts and its pressure goes up? How does the box know it's moving?

well, no the gas wouldn't get extra pressure. because it's not the box that contracts, it's space itself. THe gas is in the box so both would undergo a lorentz contraction because both of them are in that local inertial frame.

 

[yuiop]

So we all agree that the distance between the two spaceships should stay constant, as viewed from S. Now the question is: would the distance between the gas particles change due to Lorentz-contraction as viewed from S? If yes, why it changes but the distance between the two spaceships should stay constant?

The gas particles are pushed together by the physical box they are enclosed within as the box length contracts. Each particle changes from a sphere to a thin ellipsesoid and so they still fit comfortable within the box and still have as much freedom of movement as they had before the box accelerated and to an observer co-moving with the box everything seems normal.

In the case of the spaceships in Bell's paradox the spaceships are not physically connected and each has its own motor. If only one spaceship was powered and they were connected by a tough rod then the two spaceships would get closer together as the connecting rod length contracted.

A pilot on one spaceship sending bouncing radar signals off the other (unconnected) spaceship would measure the gap between them to be getting progressivley bigger than when they were at rest in frame S. An observer that ramained in frame S would consider the separation to remain constant. In that sense the space between the 2 spaceships is length contracted because S measures a smaller separation distance between the two spacships than the separation distance measured by a pilot onboard one of the accelerated spaceships.

 

[AVentura]

well, no the gas wouldn't get extra pressure. because it's not the box that contracts, it's space itself. THe gas is in the box so both would undergo a lorentz contraction because both of them are in that local inertial frame.

is the space moving?

if not why would it contract?

if so what accelerated it?
how much force did that take?

 

[AVentura]

I have question for the people discussing the synchronization of clocks. What does this have to do with instantaneous transmission? it is easy to synchronize clocks over space. you measure the distance (d) from the master clock, and you subtract d/c from zero when you get your light pulse from it. the ability to lay synchronized clocks over space is a given.

am I missing something? obviously.

 

[granpa]

when an observer is moving he experiences a loss of simultaneity. this is simply because he is composed of atoms that are themselves interacting at the speed of light. as a result he perceives light pulses that are passing him by to be moving at c rather that c+-v.
because of this if he attempts to do as you say and synchronize clocks he will get a different result than a stationary observer. but of course as the twin paradox shows it is just an illusion that vanishes as soon as he stops.

 

[yuiop]

I have question for the people discussing the synchronization of clocks. What does this have to do with instantaneous transmission? it is easy to synchronize clocks over space. you measure the distance (d) from the master clock, and you subtract d/c from zero when you get your light pulse from it. the ability to lay synchronized clocks over space is a given.

am I missing something? obviously.

THe method you derscribe is the classic and accepted way to synchronise clocks in a frame and nothing is wrong with that. However, if all the clock in frame A are sychronised with each other and all the cloccks in frame B are synchronised with each other, then an obser in farm A would say the clocks in frame B are not synchronised and vice versa. Observers in frames A and B claim to be stationary and claim the other frame frame is moving, so in Special Relativity there is no notion of an absolute rest frame or absolute motion. Granpa was just making the point that if instantaneous transmission of information was possible (which it isn't) then an absolute rest frame could be determined.

 

[yuiop]

is the space moving?

if not why would it contract?

if so what accelerated it?
how much force did that take?

The space is not moving.

 

[AVentura]

when an observer is moving he experiences a loss of simultaneity. this is simply because he is composed of atoms that are themselves interacting at the speed of light. as a result he perceives light pulses that are passing him by to be moving at c rather that c+-v.
because of this if he attempts to do as you say and synchronize clocks he will get a different result than a stationary observer. but of course as the twin paradox shows it is just an illusion that vanishes as soon as he stops.

can the mover not see how fast she is moving relative to the master clock (by bouncing photons off it) and account for that?

 

[granpa]

what master clock? earlier we assumed the ability to communicate instantly and this led to the conclusion that we would have a single master clock somewhere. but in reality we can't communicate instantly.

in reality, all observers consider themselves to be at rest and no experiment can show which is really moving and which isnt.

he can determine his motion relative to another object. but which is moving? him or the master clock? its impossible to tell.

 

[yuiop]

when an observer is moving he experiences a loss of simultaneity. this is simply because he is composed of atoms that are themselves interacting at the speed of light. as a result he perceives light pulses that are passing him by to be moving at c rather that c+-v.
because of this if he attempts to do as you say and synchronize clocks he will get a different result than a stationary observer. but of course as the twin paradox shows it is just an illusion that vanishes as soon as he stops.

The twins paradox ceases to be an illusion when the twins come together at the end the age difference is real.

 

[granpa]

The twins paradox ceases to be an illusion when the twins come together at the end the age difference is real.

it ceases to be an illusion when he stops and begins to turn around.
remember the line of synchronized clocks? when he stops all the clocks are synchronized again. at that point he knows what time the stationary twins clocks says.

 

[AVentura]

what master clock? earlier we assumed the ability to communicate instantly and this led to the conclusion that we would have a single master clock somewhere. but in reality we can't communicate instantly.

in reality, all observers consider themselves to be at rest and no experiment can show which is really moving and which isnt.

I'm not saying that it is absolute time (or rest frame).

I am just saying that you can synchronize all clocks over space, moving or not. It's easy.

Some moving ones may need to run slower or faster than this arbitrary clock to stay in sync. But they can easily see by how much. And knowing zero time is even easier (zero-d/c)

 

[granpa]

yes. if we select one clock and call it the master clock then theoretically all clocks can be synchronized to it. but then clocks within your own frame will not be synchronized. which would make them rather pointless.

when people talk about clocks they are usually referring to the rate at which observers are aging.

 

[yuiop]

I'm not saying that it is absolute time (or rest frame).

I am just saying that you can synchronize all clocks over space, moving or not. It's easy.

Some moving ones may need to run slower or faster than this arbitrary clock to stay in sync. But they can easily see by how much. And knowing zero time is even easier (zero-d/c)

Hi AVentura,

there are ways to synchronise clocks that differ from Einstein's method (see this thread https://www.physicsforums.com/showthread.php?t=212171 ) but you usually end up with a system where clocks do not appear synchronised in your own frame or the speed of light is different in different in different frames or the one way speed of light is not isotropic and the maths generally gets more complicated.

Also, without instantaneous information transmission all clocks moving relative to the arbitary clock would initially appear to be running slower and have to be speeded up. (none would appear to be running faster).

 

[granpa]

there is only one way to synchronize the clocks within your own frame.

 

[AVentura]

when people talk about clocks they are usually referring to the rate at which observers are aging.

then you can't synchronize them for moving observers period.

 

[AVentura]

there is only one way to synchronize the clocks within your own frame.

no, you can use light, or snails.

you subtract d/c for light,

or d/s for snails (s=speed of snails)

 

[granpa]

then you can't synchronize them for moving observers period.

no you can't change the rate of aging

 

[granpa]

no, you can use light, or snails.

you subtract d/c for light,

or d/s for snails (s=speed of snails)

the results will be the same

 

[AVentura]

the results will be the same

the same results was the objective, i can think up more. they can all depart from this arbitrary master clock and add however much time they lost from dilation.

 

[granpa]

we were talking about synchronizing clocks within your own frame. not synchronizing all clocks moving or nonmoving to a master clock. post #188

 

[AVentura]

the results will be the same

we were talking about synchronizing clocks within your own frame. not synchronizing all clocks moving or nonmoving to a master clock. post #188

ok, I think see your point. to synchronize clocks in your own frame some signal must originate from the arbitrary master clock. is this what you meant?

 

[AVentura]

when an observer is moving he experiences a loss of simultaneity. this is simply because he is composed of atoms that are themselves interacting at the speed of light. as a result he perceives light pulses that are passing him by to be moving at c rather that c+-v.
because of this if he attempts to do as you say and synchronize clocks he will get a different result than a stationary observer. but of course as the twin paradox shows it is just an illusion that vanishes as soon as he stops.

you said "when an observer is moving"


now why must it be the same frame?

 

[granpa]

someone said something about there being different ways of defining synchronous. i simply pointed out that within anyone frame, at least, there is only one 'simultaneous'.

 

[granpa]

the confusion seems to be because some people are using clocks to refer to the rate at which natural process occur and others are using them to refer to arbitrary temporal measuring sticks.

how about using 'atomic clocks' to make it clear we are referring to rate of aging or rather the rate of atomic vibration.

 

[granpa]

ok, I think see your point. to synchronize clocks in your own frame some signal must originate from the arbitrary master clock. is this what you meant?

my point is that 2 events within a frame either are or are not synchronous. if 2 events occur simultaneously in china and new york the clocks in china may read differently than they do in new york but the events are still simultaneous.

 

[Mentz114]

granpa,

if 2 events occur simultaneously in china and new york the clocks in china may read differently than they do in new york but the events are still simultaneous.

You seem to be implying some kind of absolute simultaneity. Your statement above does not make sense unless you define 'simultaneous', and your definition will require some sort of clock synchronization, after which only some observers, not all, will agree that the events in NY and China were simultaneous.