I will forgo the FTL implications of the former threads and confine the consideration to subluminal functions.

It has been established (claimed) that at relavistic velocities of recession that lorentz contraction results in an actual reduction of spatial distance. That is the faster you receed the closer you get.

Counter intuitive certainly but if we embrace Relativity then we must simply say "that is the way it is"

Understanding that then one must stop and question our observations and conclusions.

We observe that the more remote galaxies are receeding at relavistic velocities in an accelerating expansion of the universe.

Or are they?

From what you have just learned that should mean that they are in fact "Decelerating" causing inverse Lorentz Contraction or Lorentz Expansion.

If the Big Bang propagated the raw energy at relavistic velocities, then the initial dimensions of the universe would be compressed by Lorentz but as matter began to form and gravity started to tug on each other decelerating their motion, space would actually expand.

So the expansion we observe is the material universe actually slowing down its radial velocity. Suggesting there is in fact sufficient matter to stop the expansion and result in a point being achieved where matter will rush back towards its birth place, once again compounding the situation by contracting the space in which it has motion, to cause another horrendous Big Bang.

Isn't Relativity fun? :D

MacM:

You're obviously confusing yourself again. You REALLY need to study up on the basics of reference frames. But then, I've advised you to do that many times before and you've never bothered to take my advice, so I doubt this time will be any different. Ho hum.

It has been established that at relavistic velocities of recession that lorentz contraction results in an actual reduction of spatial distance. That is the faster you receed the closer you get.

You've left out all the qualifications about reference frames here. As a blanket statement, this is untrue and misleading.

We observe that the more remote galaxies are receeding at relavistic velocities in an accelerating expansion of the universe.

Or are they?

From what you have just learned that should mean that they are in fact "Decelerating" causing inverse Lorentz Contraction or Lorentz Expansion.

It wouldn't make a jot of difference if distant galaxies were accelerating, decelerating or doing the hokey pokey. Sitting here on Earth, we'd observe no change at all in the measured distances to these galaxies.

There's no point reading the rest of your post, since it is based on this basic misunderstanding.

James R or others,

Humor me for a moment.

PLease explain in detail (other than some pure mathematical expression), how it is that:

1 - Relative velocity between A and B = X,

2 - Therefore relative velocity between B and A = X

3 - Lorentz Contraction is a function of Relative velocity X

4 - How is it that you propose that A sees spatial contraction but B sees none?

4 - How is it that you propose that A sees spatial contraction but B sees none?

Where did James say this ?

Crisp,

He has actually said it several times but I have only a few minutes to get ready for work (actually therapy for my shoulder - ouch) but if he doesn't comment here on this issue I will look up some references tonight and post them here. Done - See below

I will add that Paul T has made the same assertion and perhaps he will repond to you also.

James R & Crisp,

Actually, I hope you both understand that I am "trolling" a bit here but I think it is useful to readers (including me) to see how such issues are (or are not) resolved.

In that regard it would be more useful to justify your assertions than to simply state that mine are bullshit. :D

MacM, have u heard of the twin paradox and why it actually can't be a paradox? I think this must have something to do with that. Now, Im no expert in relativity so Crisp or James please correct me if Im wrong.

The relative velocities between the twin on earth and the one in the spaceship is the same for both journeys. Time dilation has only happened to one twin though!

MacM, have u heard of the twin paradox and why it actually can't be a paradox? I think this must have something to do with that. Now, Im no expert in relativity so Crisp or James please correct me if Im wrong.

The relative velocities between the twin on earth and the one in the spaceship is the same for both journeys. Time dilation has only happened to one twin though!

Yes, I am familiar with it but no expert. I'll leave further comment to others. But thanks for your participation

Crisp,

To respond to your question as to James R's position on this issue.

Posted by James R in the original "Lorentz Contraction Paradox":


5/30 @ 8:43 PM:"Who sees space contract, according to you? Is it the person in the rocket, or the person on Earth, or both? It's a simple question. Can you answer it, please?"

5/31 @ 1:38 AM: "There are two people: one on Earth, which is taken as stationary, and one in a rocket travelling at 99% of the speed of light relative to Earth. Who sees space contract, according to you?

Is it:

(a) the person in the rocket
(b) the person on Earth
(c) both?

Please choose (a), (b) or (c). "

Posted by MacM:

5/31 @ 9:16 AM:

"C", otherwise it is not truely relative. "

Posted by James R:

5/31 @ 7:15 PM: "I disagree with you, then.

The person on Earth measures all distances the same as normal, regardless of the speed of the rocket. The Earth observer sees no spatial contraction."

If you like I can get several more like statements, also it is the view of Paul T. But I think this should suffice. It is a vey clear statement that relativie velocity is claimed to produce different affect of the two observers that form the relative velocity couple.

I have seen no acceptable justification for it other than "because Relativity says so".

If you like I can get several more like statements, also it is the view of Paul T. But I think this should suffice. It is a vey clear statement that relativie velocity is claimed to produce different affect of the two observers that form the relative velocity couple.

You did not fully understand the situation I am afraid. There are two aspects to it.

1. The effect is mutual in the sense that the earth observer will see slower times and shorter distances for a moving observer. The moving observer, looking back to earth will say that it is not his clock that is running slower, but earth's clock (and earth lengths that contract).

Every statement you make can in that sense be reversed to the other observer.

2. However, if you are moving together with an object (e.g. if you are talking about an object that is at rest with respect to one of the two observers) then the story is quite clear: the observer at rest with the object sees nothing strange: no length contraction and no time dilatation; only the moving observer will claim to see these.


It is not about two measures for the same thing either: if you combine the second statement for two different observers, you get the first (think about that for a whole). That is also the reason why James said that you should get your "frame of reference" thing straight. Time dilatation and length contraction are usually introduced in the sense of statement 2. It is only when you start combining statements about observers that you should take care (and that misconceptions, assumptions about simultanity, ... enter)

You did not fully understand the situation I am afraid. There are two aspects to it.

1. The effect is mutual in the sense that the earth observer will see slower times and shorter distances for a moving observer. The moving observer, looking back to earth will say that it is not his clock that is running slower, but earth's clock (and earth lengths that contract).

Every statement you make can in that sense be reversed to the other observer.

That has been precisely my statement but James R and Paul T disagree and said that the rocket pilot does see the contraction but that the earth observer doesn't.

I do not accept that. Period.

2. However, if you are moving together with an object (e.g. if you are talking about an object that is at rest with respect to one of the two observers) then the story is quite clear: the observer at rest with the object sees nothing strange: no length contraction and no time dilatation; only the moving observer will claim to see these.

Again we are in 100% agreement.

It is not about two measures for the same thing either: if you combine the second statement for two different observers, you get the first (think about that for a whole). That is also the reason why James said that you should get your "frame of reference" thing straight. Time dilatation and length contraction are usually introduced in the sense of statement 2. It is only when you start combining statements about observers that you should take care (and that misconceptions, assumptions about simultanity, ... enter)

My frames of reference appear to be normal. Since we seem to agree, let me suggest you re-read James Post referenced above and see if you don't read it the way I do.

Lesson 153 for MacM on the basics of reference frames.

Let's assume for simplicity that Earth and Alpha Centauri do not move relative to one another. Now, imagine a rocket travelling at 3/5 of the speed of light, on a journey from Earth to Alpha Centauri. Let observer A be a person watching from Earth. Let observer B be a person travelling in the rocket.

The distance from Earth to Alpha Centauri is measured by A to be 4 light years. Because A is stationary with respect to Earth and Alpha Centauri, the 4 light year distance is the rest length distance between Earth and Alpha Centauri, or the "proper" distance, to use a technical term. This is not a "contracted" distance for observer A.

The simple rule is that an observer at rest relative to two objects sees them as separated by their proper distance.

Notice that A's observations are not in any way affected by the existence of the rocket or observer B. I mention this only because you seem to struggle with this point.

Now, look at B's reference frame. To state what should be obvious, B is not at rest with respect to Earth or Alpha Centauri. To B, Earth is moving at 3/5 the speed of light, and so is Alpha Centauri. Hence, B sees the distance between the two to be contracted by a factor of gamma, which in this example has the value 1.25. In other words, B measures the distance to be 3.2 light years. But this is a contracted distance, because Earth and Alpha Centauri are moving.

How long will the trip take?

Both observers write: t = d/v, to calculate the travel time, but they MUST measure all quantities in their own reference frame.

According to A, the rocket travels distance 4 light years at a speed of 3/5 light year per year, so the trip takes 4/(3/5) = 6.67 years.

According to B, Alpha Centauri must travel 3.2 light years at a speed of 3/5 light year per year, which takes 3.2/(3/5) = 5.33 years.

Important notes for MacM

Notice that the situation between A and B is NOT symmetrical.

A is in the rest frame of the Earth-Alpha Centauri system. B is not.
A measures the proper distance between Earth and Alpha Centauri. B does not.
B's proper time for the trip is 5.33 years. This is B's proper time because the clock used to measure it in the same reference frame as B. 5.33 years is NOT the proper time of an Earth clock, because B's clock moves relative to Earth. A does not measure the proper time for B's trip.

I hope this clarifies things for you.

MacM,

I would like to add that what James said is not at all in contradiction with my post (... unless I missed something crucial, but I am pretty confident that I didn't ;)).

If you don't immediatelly see why, then, before you complain, first think two days on why there is no disagreement.

Bye!

Crisp

My frames of reference appear to be normal. Since we seem to agree, let me suggest you re-read James Post referenced above and see if you don't read it the way I do.

I reread them. I agree with James' statements, and if you read my post properly, then you will see that it also agrees with James' statements (because you are in situation TWO for each of the individual observers).

James R., & Crisp,

Great. Now I see what you are saying and I agree 100% that A.C. and Earth cannot see their seperation change because of a moving observer inbetween them. What I envisioned in your statement Crisp was two rockets moving side by side and of course there is no relative velocity or affects between them.

James R, Paul T claimed in the presentation I made which was simply a rocket and the earth that the rocket sees the distance between earth and the rocket as changing but that earth did not.

That is unacceptable and not advocated by Relativity at all.

What is consistant is 'A', 'B' & 'C' where 'B' is in motion between 'A' & 'C':

A & B see contraction between them vice-versa. B & C see contraction between themselves and vice-versa but A & C do not see contraction between themselves.

You cannot have a situation where there is motion relative between 'A' & 'B' and 'A' sees an affect and 'B' does not. I would truely appreciate James R, or Paul T, justify saying there is motion between 'A' and 'B' and that 'A' sees contraction of spatial dimension between them and that 'B' does not.!!!!!

Paul T gave the counter example where he attached a string to earth and floated it out into sapce and said it was ludricrus to claim the string contracted. I would have to agree with that but point out it is HE not I that can't seem to keep frames of reference straight.

What happens (if it happens at all) is that earth would see the rocket as being a a different loction along the string. Space would be contracted between the rocket and erth for both observers.

To clarify this instead attach a tape measure "between" the earth and the rocket.

Interesting votes. :D

Now it would be good if the Relativists post their proof.

Take the most remote object detected to date which is approximately 13 B Ly away and is moving at V = 0.95 c

Gamma = 1/(1-(v/c)^2) = 1/(1 - 0.95^2) - 1/0.0975 = 10.2564.

True distance therefore (if space contracts with relative velocity) would be 13 B Ly * 10.2564 = 133.333 B Lyr !

Do you believe the universe is that big and is contracted by Lorentz to only appear 13 B Lyr?

Of course not. So do you claim space contracts with relative velocity?

Oh, I forgot it is a rachet. Spatial expansion is outside Relativity but Relativity contracts (higher velocity) or expands (lowering velocity) space. Interesting concept this Relativity.

Lets see if it were to decelerate to 0.94 c over a 20 year period, gamma = 2.9310.

From the rest dimension of 133.333 B Lyr /2.9310 = 45.4899 B Lyr!.

WOW the universe has an accelerating expansion. HeHeHe. What a joke.

It is slowing down and we see it speeding up. 45.4899 B Lyr - 13 B Lyr = 32.4899 B Lyr in just 20 years. It slowed down 1% and the universe got bigger by a factor of 2.5!!!

Go ahead challenge the data Relativists. You must either give up your spatial contraction claim or agree what we see as acceleration is actually deceleration. Didn't teach you that did they?

MacM:

Until you do me the courtesy of reading my previous post to this thread, I will not bother addressing you again.

Which part of my post do you have a problem with, if any?

You can't just pretend that I didn't write it.

James R.,

Ok, I read it before and per your request I have read it again. I don't see that anything has changed or that I intrepret anything different in what I read.

Your post is absolutely correct in all respects except one.

Notice that A's observations are not in any way affected by the existence of the rocket or observer B. I mention this only because you seem to struggle with this point.

The effort to make it appear that MacM has ever had any confusion over this situation is absolutely false. I have understood your explanation as you described it for the past 50 years at least.

The problem here James R, is that you are not addressing my case as presented, nor responding to my request to clarify or prove your prior claim that in the case where we only had two observers, the earth and the rocket and I stated that both had to either see spatial contraction or it didn't exist and neither saw it.

You and Paul T both disagreed and stated the rocket saw distance between himself and earth as contracted but that earth did not see space to the rocket contracted.

Paul even backed that up suggesting I tie a string to the earth and float it into space along the rockets path as his arguement of proof. We all see the difference between the string and the rocket. The rocket has relative motion to earth, the string doesn't.

I still say you are wrong and you are not addressing this issue here. I will try once again to get you to respond to the actual issue:

A rocket is accelerating away from earth and sees space between himself and earth contracting. Does earth see space between it and the rocket contract?

Yes_______________

No________________

MacM:

A rocket is accelerating away from earth and sees space between himself and earth contracting. Does earth see space between it and the rocket contract?

Of course not. How could the motion of the rocket possibly affect an observer on Earth in any way? An Earth observer's perception of space is not affected by the motion of any object in space. It is only affected by the Earth observer's own motion. The same applies for the rocket.

Not too long ago, this was your own line of argument, but suddenly you've decided to back flip. I can't for the life of me work out why. Your thought processes are a complete mystery to me.

MacM:

Of course not. How could the motion of the rocket possibly affect an observer on Earth in any way?

Gee, I wonder what happened to the term "Relative Velocity"?

An Earth observer's perception of space is not affected by the motion of any object in space.

We are not talking about space at large but only space inbetween observers with relative velocity.

It is only affected by the Earth observer's own motion. The same applies for the rocket.

Gee again what happened to there is no absolute motion, only relative motion. This motion we are discussing only exists between the earth and the rocket. Not the universe at large. Are you now claiming an absolute rest frame in background space. Just what is our velocity here on earth to this absolute background?

Not too long ago, this was your own line of argument, but suddenly you've decided to back flip.

I do challenge you to show where my position has changed. The only thing that may have changed is my presentation (hence your understanding) of my position.

I can't for the life of me work out why. Your thought processes are a complete mystery to me.

Interesting. It is a mystery that I should suggest that Relativity be consistant. That relative velocity mean relative velocity in all cases and not adopt a selective absolute background when to maintain relative velocity as required to have relavistic affects the results become something other than desired Hmmm. The mystery is on your side I am afraid.

MacM:

Gee, I wonder what happened to the term "Relative Velocity"?

Nothing.

Me: An Earth observer's perception of space is not affected by the motion of any object in space.

You: We are not talking about space at large but only space inbetween observers with relative velocity.

You will note that "space in between observers with relative velocity" is a subset of "space".

Gee again what happened to there is no absolute motion, only relative motion. This motion we are discussing only exists between the earth and the rocket. Not the universe at large. Are you now claiming an absolute rest frame in background space. Just what is our velocity here on earth to this absolute background?

I have not mentioned any kind of absolute reference frame. Nor have I implied one. I am not claiming the Earth needs to be stationary relative to anything, including the rest of the universe.

Look, stop dancing around the issue and address the actual argument, which is this:

If the rocket travels at speed v relative to Earth, which corresponds to a Lorentz factor of gamma, then if Earth measures the distance between Earth and the rocket to be x, then the rocket will measure the distance between Earth and the rocket to be x/gamma.

Which part of this do you disagree with, and why?

I hope you can now clear up this thing which so mystifies you.

James R.,

Nothing.

Good then perhaps we can still apply it uniformaly.

You will note that "space in between observers with relative velocity" is a subset of "space".

So what. If I extend a measuring tape from earth out into space and the rockets moves past it, you argue that the rocket sees contraction of the tape. But that earth obviously doesn't see space contrct since the tape measures normally (which it should since it has no relative velocity to earth).

But lets use a complete set of data for relative velocity shall we?

Lets attach another tape measure to the rocket and let it be dragged past the earth.

Now you can see that each observers view of the others tape is absolutely identical and ALL relavistic functions apply between the observers in relative motion. (That does not include other objects in the universe); except for the rocket which is changing relative to all objects. The earth is changing only to the rocket. So spatial contraction for the rocket is universal while spatial contraction to earth is only in relation to the rocket.

I have not mentioned any kind of absolute reference frame. Nor have I implied one. I am not claiming the Earth needs to be stationary relative to anything, including the rest of the universe.

You simply fail to understand the concequences of your position. By failing to recognize the relative velocity between the rocket and earth means spatial contraction between the rocket and earth and vice-versa you are forcing the earth to assume some absolute reference in relation to the balance of the universe. You simply ignore the relative velocity as viewed by earth.

Look, stop dancing around the issue and address the actual argument, which is this:

If the rocket travels at speed v relative to Earth, which corresponds to a Lorentz factor of gamma, then if Earth measures the distance between Earth and the rocket to be x, then the rocket will measure the distance between Earth and the rocket to be x/gamma.

Not so.

Which part of this do you disagree with, and why?

Done above.

I hope you can now clear up this thing which so mystifies you.

Not mystified by Relativity. Mystified by the lack of understanding of relavistic principles by so many experts. :D

Not good enough, MacM.

I said:

If the rocket travels at speed v relative to Earth, which corresponds to a Lorentz factor of gamma, then if Earth measures the distance between Earth and the rocket to be x, then the rocket will measure the distance between Earth and the rocket to be x/gamma.

If you disagree with this, then tell me what distance the rocket will measure between Earth and itself when the distance measured by somebody on Earth is x.

James R.,

So somehow you see a different gamma for the rocket and earth?????

You have a relative velocity between a rocket and earth of 0.866 c.

Gamma for the earth's view is 4.00
Gamma for the Rockets view is 4.00

Just how is it you propose that they have different relavistic affects and or views of each other?

The rocket is changing its velocity to all objects in the universe including earth and sees universal contraction. But the only thing changing in earth's view is the relative velocity to the rocket and its affects are limited to its view of the rocket. i.e. time rate, mass and distance of the rocket.

I'll say it one last time either they both see the affects or neither see them.

Your "Not Good Enough" is not good enough I am afraid. Claiming the earth sees its rest tape unchanged hence no contraction also applies to the rocket and his tape. He see no change either using that logic. Whatever view you want to impose on observers you must impose like views on the other observer. They will always see the same thing, unless you shift basis for the views.

Please remember that you stated two rockets in space saw the same distance. I then revealed that one rocket was still sitting on the launch pad on earth. Do you now want to claim that that somehow changes the perspective. I would hope not.

MacM:

Please see the "FTL spin off" thread, where I have addressed your question in detail. I have just realised that your problem is that you are mixing two sets of observations which take place at different times as measured by one or the other observer's respective clocks. It's the old simultaneity of relativity chestnut again.

Click http://www.sciforums.com/showthread.php?t=37305]here for the other thread.

James R.,

Thanks I came here to do the same.

It is not possible to unambiguously specify a distance between two comoving objects, unless that distance is zero.

In the current argument, several parties have implied that the distance between Earth and the rocket is unambiguous, and not contracted by the rocket's phenomenal acceleration.

This is incorrect.

To define a spatial separation unambiguously, you must specify either two space-time events, or two objects in the same reference frame.

For example:
Imagine the rocket is passing a planet that is stationary with respect to Earth. We can unambiguously refer to the distance between Earth and that planet. To the rocket, this distance is contracted. To Earth, it isn't.

But!
Now imagine that a planet that is stationary with respect to the rocket is passing Earth. We can unambiguously refer to the distance between that planet and the rocket.
To the rocket, this distance is not contracted. To Earth, it is.

To me, this puts a whole new spin on the situation...
When the rocket accelerates, the distance it measures to Earth does not contract, unless it is measuring a distance in Earth's frame.
If it measures the distance in its own frame, it is not contracted at all!

This is a bit of a revelation for me... I'll have to go and look at the previous threads to see what assumptions I made in my arguments.

Pete,

Your comments seem in order. Thanks. I will note however that in the case of the rocket passing another planet stationary to planet earth and your assumption that it sees them at the same distance is likely not entirely correct.

That would have to do with James R's simultaneity of the event (passing the planet by the rocket). It is not perceived as being at the same time. So earth would see a foreshortened distance based on its clock and conclude that the rocket is not actually passing the planet. That is spatial contraction or a shift in view due to the simultaneity issue.

Relativity is such that one can argue about any specific issue by referencing different frames or clocks and therein lies the confusion.

The bottom line seems to be the consideration that the two rocket example is claimed to see the same distance but if you claim one rocket is then still sitting on the launch pad on earth they want to claim they see different distance. Since it is the exact same situation there should not and cannot be two actually different answers. The answer must be the same in both cases.

The tendancy to look at earth as a stationary reference is the problem. It is not. It is nothing more than another rocket.

MacM,


The tendancy to look at earth as a stationary reference is the problem. It is not. It is nothing more than another rocket.

It is not true. Earth is special as long as we measure length in meter and time in second. Those meter and second are earth standard for condition while earth is at rest.

MacM,

It is not true. Earth is special as long as we measure length in meter and time in second. Those meter and second are earth standard for condition while earth is at rest.

WHAAA?? Perhaps you can explain how not also applying those same standards to one of the rockets in the two rocket case is not the same???

Thanks. I will note however that in the case of the rocket passing another planet stationary to planet earth and your assumption that it sees them at the same distance is likely not entirely correct.

I think it is.
With things happening at the same place, simultaneity is not relative.
Something at a particular place and a particular time is a space-time event.

In this case, imagine the rocket's wing clips the planet... that collision happens unambiguously for all observers.

I think it is.
With things happening at the same place, simultaneity is not relative.
Something at a particular place and a particular time is a space-time event.

In this case, imagine the rocket's wing clips the planet... that collision happens unambiguously for all observers.

My first thought is you have evaded the issue of simultaneity for the earth observer. We all understand when s = 0 that simultaneity is simultneous in a real sense but to earth it would not be simultaneous.

Don't misunderstand what I said. The earth will indeed see the wing clip and passing of the planet as simultaneous events but they will not see them as being at the same instant as they occurred to those present at s = 0.

It is not true. Earth is special as long as we measure length in meter and time in second. Those meter and second are earth standard for condition while earth is at rest.
Have you been drinking, Paul?

Don't misunderstand what I said. The earth will indeed see the wing clip and passing of the planet as simultaneous events but they will not see them as being at the same instant as they occurred to those present at s = 0.
Is that relevant?

The issue immediately at hand is whether the Earth sees the rocket and the planet at the same distance when the rocket passes the planet.

(What are s and o?)

Pete,

(What are s and o?)

's' is distance it is preferred to write ds/dt than dd/dt :D 0 is numerical zero.

As to your question, I refer you to James R's post and my response in "FTL Spin Off".

My first thought is you have evaded the issue of simultaneity for the earth observer. We all understand when s = 0 that simultaneity is simultneous in a real sense but to earth it would not be simultaneous.

Don't misunderstand what I said. The earth will indeed see the wing clip and passing of the planet as simultaneous events but they will not see them as being at the same instant as they occurred to those present at s = 0.

(those darn O's and zeros! :o )

The planet and the Earth are in the same reference frame.
Events that are simultaneous according to the planet will be simultaneous according to Earth.

Also, events that happen at the same time and place in one frame will happen at the same time and place in all frames (this is a spacetime event). This must be so, otherwise observers would disagree about unambiguous events - one observer would say that the rocket hit the planet, another would say that it did not. This kind of disagreement can't happen, because a dented rocket is dented in all frames!

Pete,

I agree on location and the actual crash but the time that each claims the crash occurs I believe is shifted. See FTL Spin Off conclusion.

It does cause me some confusion how this event fits the simultaneity solution in that thread for spatial contraction.

Yes, that is true.
But that's not the point - at the time of the collision (by Earth clocks), the Earth sees the planet and the rocket at the same distance.

Yes?

Yes, that is true.
But that's not the point - at the time of the collision (by Earth clocks), the Earth sees the planet and the rocket at the same distance.

Yes?


Yes and therein the confusion. How does simultaneity resolve this issue? It can't. So how then does one get what is now agreed that both observers are subject to relavistic functions of mass, time and space, with simultaneity considered, and all of a sudden reach a condition where earth's view is no longer contracted?

http://www.sciforums.com/showthread.php?t=37305&page=3

In this string we have (James R., has shown) that both do indeed see space contracted, which had been my point. But that is when the rocket(s) are in space. But pin the tail on the donkey and Wham! contraction disappears.

Very intersting diversion here. Thanks. Anxious to see some explanations.

Let's see how I go... this might be more for my benefit than anyone else's, but it might help.


The distance between reference points in Earth's frame are contracted according to the Rocket, and not contracted according to Earth.

The distance between reference points in the Rocket's frame are contracted according to Earth, and not contracted according to the Rocket.

This is possible because distance is a spacetime interval between simultaneous events. But because simultaneity is relative, when two observers in different frames measures a distance, they are measuring different spacetime intervals.

In the case of measuring the distance between the Earth and the Rocket, there are actually four distinct spacetime intervals that could be measured (only considering the two relevant frames).

Note that spacetime intervals are not relative. All observers will agree on the magnitude (in length units) of any spacetime interval.


Here are some events that define end points of distinct intervals that are valid Earth-Rocket distances:
Spacetime events:
A - Rocket clips object comoving with Earth, and X units from Earth in Earth's frame
B - Earth clips object comoving with Rocket, and X units from Rocket in Rocket's frame
C - Earth at time simultaneous with A in Earth's frame
D - Earth at time simultaneous with A in Rocket's frame
E - Rocket at time simultaneous with B in Rocket's frame
F - Rocket at time simultaneous with B in Earth's frame

Here are the four intervals, and the distances to which they correspond:
Distances:
Earth-Rocket distance in Earth's frame as measured by Earth = interval A-C = X
Earth-Rocket distance in Earth's frame as measured by Rocket = interval A-D = X/gamma
Earth-Rocket distance in Rocket's frame as measured by Rocket = interval B-E = X
Earth-Rocket distance in Rocket's frame as measured by Earth = interval B-F = X/gamma


Appendix - Order of Events
In Earth's frame, events occur in the following order: D, BF (simultaneous), AC (simultaneous), E

In Rocket's frame, events occur in the following order: F, AD (simultaneous), BE (simultaneous), C


Here are two diagrams showing the two reference frames, with the events labelled.

The green lines are the four distances measured.
<img src="/attachment.php?attachmentid=2941&stc=1">
<HR>
<img src="/attachment.php?attachmentid=2942&stc=1">

(technical addendum - The time axes on these diagrams should technically be labelled ct, ie time multiplied by c to get a distance.)

Pete,

I think you have made an excellent presentation. Unfortunately your diagrams are not showing. We have had problems with attachments for some time now.

The block of prime interest to me obviously was this:

Here are the four intervals, and the distances to which they correspond:
Distances:
Earth-Rocket distance in Earth's frame as measured by Earth = interval A-C = X
Earth-Rocket distance in Earth's frame as measured by Rocket = interval A-D = X/gamma
Earth-Rocket distance in Rocket's frame as measured by Rocket = interval B-E = X
Earth-Rocket distance in Rocket's frame as measured by Earth = interval B-F = X/gamma

These statements are precisely my statement. Earth and the Rocket see the same thing. The confusion in my statement is that others took it to mean if the Rocket sees 10 Lyr that Earth sees 10 Lyr at the same time. In a sense they do but the times are shifted. As James R., pointed out my statement and yours require the understanding that respective time are critical to such understanding.


Thanks. I believe your presentation may be helpful to others in sorting out how all this fits together.

READERS:

Now back to the topic. Since each observer does ultimately see the same spatial affects.

How do you suppose we see an accelerating expansion per relativitiy and it not actually be deceleration?

The above votes are interesting especially if you'll note that nobody had voted "Contraction" and these votes include mine. :D

Can our mathematically inclined here explain that so that we may all grasp what is going on here?

Diagram links have now been corrected. Sorry about that.

Note that there is a semantic issue here. To me, *length* contraction (as described by SR) is not *space* contraction. "Length" implies measurement, which may be observer dependent, where "space" implies some objective reality. (I tend to use "space" loosely where I should probably use "spacetime").

And back to the topic... (warning! Slightly edited stream of consciousness approaching!)

How do you suppose we see an accelerating expansion per relativitiy and it not actually be deceleration?
Firstly, I think we measure distances to cosmological objects in our own reference frame, so there is no length contraction.

But it occured to me as I was typing this post that if we measure distance in the CMBR (http://scienceworld.wolfram.com/physics/CosmicBackgroundRadiation.html) frame (a logical thing to do in this context, I think), then there might be an issue... but then I realised that the CMBR frame is not the accelerating frame of the CMBR surface in a particular direction, but the local average CMBR from all directions, which is not an accelerating frame so it's still not an issue.

A "contracting expansion" could only apply for distances measured in the moving from our own frame, and I think that the acceleration required to maintain such contraction would be quite massive (I'll do the maths later).

Furthermore, it occurs to me that we haven't yet mentioned General Relativity in this thread.
Now I hate to open another cannikin of annelids, but I think that GR throws a real spanner in the works.
My understanding is that in GR, two observers in the same inertial reference frame can have a significant apparent relative velocity. In other words, if You and I start out comoving but separated by some distance, then the expansion of space drives us apart without either of us actually accelerating.
And in this situation, I'm not sure if/how the length contraction and time dilation formulas of SR apply - particularly since GR has no problem with You and I having an apparent relative velocity greater than c!


For the purposes of the discussion, I was going to attempt to figure out the details for the most distant galaxy ever observed, but I'm out of time. Here's a start, if anyone wants to follow it through.

Abell 1835 IR1916 (http://www.cnn.com/2004/TECH/space/03/01/farthest.galaxy.ap/) seems to be around 13.23 billion light years away, with a redshift of Z=10.

Pete,

Great. You have taken this right where I was hoping it would go. I am looking to determine why we see expansion as non-relavistic but that Relativity can expand and contract as a function of velocity increase or decrease.

I have a hunch that the galactic expansion is actual expansion and Relavistic contraction etc., is perceptional.

Using:

Omega = 1
Lambda = 0.5
Hubble = 60-80
z = 10

I got a recession velocity of 1.23c for IR 1916 and 0.3 B years after the Big Bang 4c.

I think that cosmological expansion and Lorentz contraction are very very different beasties. I know next to nothing about General Relativity, so I'm reluctant to go further here.

I'm in two minds about the "perception" issue of Length contraction. On one hand, I agree that it is a "point of view" difference, and that there is no fundamental physical change involved.
(Edit - added important "no")

On the other hand, however, the contraction is quite objective in the specified frame. I think the Barn and Pole (http://math.ucr.edu/home/baez/physics/Relativity/SR/barn_pole.html) situation drives this home... in the Barn frame, the pole really is shorter.


I think that it is true to say that if "perception" is defined in such a way that the difference between space and time is purely one of perception (ie they are different perceptions of spacetime), then and only then are length contraction and time dilation only perceptual effects.

I meant to add earlier that I'm having an absolute ball in this discussion.

I've learned things, found errors in my thinking, and raised uncertainties about things I though I had nailed.

I love it!

I meant to add earlier that I'm having an absolute ball in this discussion.

I've learned things, found errors in my thinking, and raised uncertainties about things I though I had nailed.

I love it!

I have enjoyed your participation. It is regrettable that some others have felt offended. As I said the votes here have been enlightening as well. (3) "I don't care's" and (3) "You are driving me nuts". :D You would think those persons would have greater interest in the subject. 2 out of 3 negatives is not a good commentary for the forum.

Once again you and James R., should be commended. Thanks.

Confession -
I voted "You are driving me nuts!" - but I meant it as a positive!

Confession -
I voted "You are driving me nuts!" - but I meant it as a positive!

HeHeHe. Not a problem. :D

But it occured to me as I was typing this post that if we measure distance in the CMBR frame (a logical thing to do in this context, I think), then there might be an issue... but then I realised that the CMBR frame is not the accelerating frame of the CMBR surface in a particular direction, but the local average CMBR from all directions, which is not an accelerating frame so it's still not an issue.

There is no such thing as the CMBR frame of reference. You cannot pinpoint it to one location by another observer, and it goes at the speed of light. As a matter of fact, it is just a bunch of photons floating around in space. We could just aswel try to use the "solar photon frame of reference", which I hope everybody feels is a bit peculiar.

There is no such thing as the CMBR frame of reference. You cannot pinpoint it to one location by another observer, and it goes at the speed of light. As a matter of fact, it is just a bunch of photons floating around in space. We could just aswel try to use the "solar photon frame of reference", which I hope everybody feels is a bit peculiar.

We've had this conversation before. The fact that we can (and have) detected our motion relative to the CMBR does make it possible I think to view it as a reference. although it is dynamic.

I brought up this topic last year here: A “relatively” simple explanation for accelerating cosmic expansion (http://www.sciforums.com/showthread.php?t=19465). I still think the idea is valid. I think space is expanding through uncontraction.

Zanket,

I have just read your piece. We seem to be suggesting the exact same thing.
When you posted 15 months ago you didn't seem to get responses.

The trick is to piss them off so they attack and then slowly work your way around to proving your point. :D

What I am actually looking for in this thread is to see how one justifies the assumption that observed accelerated universal expansion is not relavistic in nature (other than it destroys the v = c limit claimed but not supported by evidence or test data) and still claim that relavistically we expand and contract space due to velocity increase or decrease.

It seems as assumptions are being made for no other reason than observations that violated assumption made in Reltivity must therefore be excluded from being relavistic. Rather than modifying or adjusting Relativity.

My point comes to this. If the observed expansion is not subject to relativity then it would be due to such expansion being on going creation and not mere expansion or contraction where density would change.

There is no such thing as the CMBR frame of reference. You cannot pinpoint it to one location by another observer, and it goes at the speed of light. As a matter of fact, it is just a bunch of photons floating around in space. We could just aswel try to use the "solar photon frame of reference", which I hope everybody feels is a bit peculiar.

Try "the frame in which there is no CMBR dipole".