# Why is the Wikipedia article about Bell's spaceship paradox disputed at all?

• Fredrik
In summary, The Wikipedia article about Bell's spaceship "paradox" is disputed at all because it is claimed that the condition that the spaceships experience the same acceleration implies that their world lines will have the same shape. This implies that the length of the rope will remain constant in the launcher's frame. Think about that for a second. After a while, the rope is moving at a high velocity in the launcher's frame, and must therefore be Lorentz contracted, but it's still the same length! That means that it must have been stretched. If it was already stretched to its maximum length when the acceleration began, it must break.
Fredrik
Staff Emeritus
Gold Member
Why is the Wikipedia article about Bell's spaceship "paradox" disputed at all?

This problem is ridiculously simple. The condition that the spaceships experience the same acceleration implies that their world lines will have the same shape. (The acceleration doesn't have to be constant. It's sufficient that both spaceships accelerate the same way). This implies that the length of the rope will remain constant in the launcher's frame. Think about that for a second. After a while, the rope is moving at a high velocity in the launcher's frame, and must therefore be Lorentz contracted, but it's still the same length! That means that it must have been stretched. If it was already stretched to its maximum length when the acceleration began, it must break. It's as simple as that.

This is all very basic stuff that belongs in an introductory level class about special relativity. So why is this article disputed at all?

Is it because of the common (but silly) misunderstanding that you can't solve a problem involving any kind of acceleration entirely in SR? (It's really weird how many people who have studied SR still believe that you need GR for problems like this).

Or is it because some people who understand that the rope gets stretched are arguing that SR somehow also implies that the rope gets stronger, so it can handle getting stretched?

I can't think of a third reason.

I know that some people here have been working on this article. Perhaps one of you can explain this to me.

Perhaps as one poster said, there is no stretching at all - just a matter of siumultaneity.
To measure length, you need to know two points at the same time.

Bell's spaceships is didputed because Bell was wrong !

You're right about the problem being simple but it is rather important to use special relativity and not Lorentz's pre-1905 theory, which postulates actual physical contraction of solid lengths in the direction of motion. It should be remembered that Einstein's SR is a fundamentally different theory even though it leads to the same Lorentz transformations.

Einstein's theory involves a purely kinematical approach involving no physical "shrinkage" but achieving contracted measurements by means of the relativity of simultaneity. That is to say, the shift in simultaneity causes the front end to be measured first with respect to the rear end a moment later, resulting in a reduced measurement.

What this means is that a rod initially at rest with respect to an observer does not, in SR, change its length with respect to that same observer, as it is accelerated to some fraction of c. What happens is that the length of the rod defined by another observer moving with it will appear to get longer with respect to the "stationary" observer who sees the rear end marked increasingly before the front end, as the moving observer's simultaneity shifts.

Einstein's 1905 paper only concludes that a length defined in K' appears shorter in K by the Lorentz factor, and vice versa, where K' and K are in relative uniform motion. It does not say nor suggest that a body would change its physical length during acceleration.

The idea of a rod "shrinking" as it accelerates is an unfortunate anachronism - a "hang-over" from Lorentz's earlier theory that still lingers on a century later and even finds its way into textbooks now and then.

If you read Bell's original article you will see that he makes it clear throughout that he is using Lorentz's theory in preference to SR. He was a quantum physicist with no track record in relativity and it's not clear whether he disliked Einstein's SR or whether he didn't realize the significant difference between the two approaches.

Boustrophedon, you're wrong. I don't know how you got the idea that everyone who's making correct claims about SR is actually wrong because they're using some pre-SR theory that we've barely heard about, but I can assure you that's not what I'm doing, and it's not what the people in the other thread are doing either.

Last edited:
quantum123 said:
Perhaps as one poster said, there is no stretching at all - just a matter of siumultaneity.
If someone said that, he or she was either talking about Lorentz contraction in general, or hasn't understood SR at all. Lorentz contraction can be said to be "just a matter of simultaneity", but in this case the rope is clearly being forcefully stretched. Otherwise its length wouldn't remain the same when it gets Lorentz contracted.

quantum123 said:
To measure length, you need to know two points at the same time.

The problem has nothing do with whether you choose Einstein or Lorentz. When people here are speaking of actual stretching/compression, they mean actual stretching.
There is confusion about what exactly "accelerating a rod" means. I can think of 4 substantial different scenarios:
1. The rod is being pulled. It will first get stretched, stay so during acceleration, and finally come to "rest" at its initial proper length when the acceleration ceases. There are no internal stresses then.
2. The same with a rod being pushed; just replace "stretched" with "compressed".
3. The rod is being pushed and pulled in a way that the proper accelerations of both ends are the same. It will experience stretching and compression (depending on the position) during acceleration. It will come to "rest" with a greater proper length, actually stretched.
4. Every single point of the rod is being accelerated with a carefully chosen proper acceleration such that no internal stresses occur. Its length, as measured in in a suitable comoving frame, will stay constant. It will come to rest with its original proper length an no stresses.[

OK.
I see the problem here.
There is a rigid rod to the co-moving observers.
But the lab observers see a contracting rod.
So shall I say that the spring constant has also got be relativistically transformed.

5. Every point of the rod is instantaneously (or near instantaneously) boosted to a new velocity, all at the same time in the frame where the rod was at rest before the boost. This stretches the rod to a longer proper length.
6. Every point of the rod is instantaneously (or near instantaneously) boosted to a new velocity, all at the same time in the frame where the rod will be at rest after the boost. This compresses the rod to a shorter proper length.

quantum123 said:
OK.
I see the problem here.
There is a rigid rod to the co-moving observers.
But the lab observers see a contracting rod.
So shall I say that the spring constant has also got be relativistically transformed.

If you measure the rod contracted according to Length Contraction, you know that it has not changed at all. No need to consider spring constants.

I think he's trying to argue that the string/rope/rod in the Bell's spaceship scenario won't break even though it's getting stretched. Any such argument would have to say something about the properties of the material.

Fredrik said:

5. Every point of the rod is instantaneously (or near instantaneously) boosted to a new velocity, all at the same time in the frame where the rod was at rest before the boost. This stretches the rod to a longer proper length.
6. Every point of the rod is instantaneously (or near instantaneously) boosted to a new velocity, all at the same time in the frame where the rod will be at rest after the boost. This compresses the rod to a shorter proper length.

Both 5 and 6 are not possible. Forces travel at finite speed (the speed of sound) in rigid materials. This is a pretty slow speed , so you cannot have "instantaneous (or near instantaneous) velocity boost". This is what Born rigidity is all about.
So, the rear of the rocket , where the engine is, is boosted earlier than the front.
So, the rear of the leading rocket is boosted earlier than the front of the trailing rocket.
So, the rod anchored between the rear of the leading rocket and the front of the trailing rocket gets STRETCHED (if the two rockets motors exhibit the same uniform acceleration) .
If you would like the mathematical tratment that goes with it, you can check wiki on the "Bell's paradox" or I can add a reference to a college course notes on hyperbolic motion/Born rigidity. They show the conditions under which the rod breaks.

Now, why is the wiki article disputed? I am sure CH can explain this a lot better, the short of it is that it takes one kook (Rod Ball in this case) to slap the "NPOV disputed" on any wiki article. If you click on "discussion", you will find the never ending argument with Rod Ball.

Last edited:
nakurusil said:
Both 5 and 6 are not possible. Forces travel at finite speed (the speed of sound) in rigid materials.
They're possible. You just have to find a way to push every atom at the same time.

I'm not saying it's easy. 5 and 6 are not reasonable ways to accelerate objects, but they have a pedagogical value.

Fredrik said:
They're possible. You just have to find a way to push every atom at the same time.

Physics says that the above is not possible. (unless you decide to attach a miniature rocket motor to each atom)

I'm not saying it's easy. 5 and 6 are not reasonable ways to accelerate objects, but they have a pedagogical value.

If you use absurd premises don't be surprised to get absurd conclusions.

nakurusil said:
Physics says that the above is not possible. (unless you decide to attach a miniature rocket motor to each atom)
So it is possible. What's interesting is what's possible in principle, not what's easy.

5 and 6 are interesting mainly because thinking about those only for a few seconds is by far the easiest way to understand the claim that there are no rigid bodies in SR.

Fredrik said:
So it is possible. What's interesting is what's possible in principle, not what's easy.

5 and 6 are interesting mainly because thinking about those only for a few seconds is by far the easiest way to understand the claim that there are no rigid bodies in SR.

Sounds like an attempt to justify why you got the solution wrong at post #1

Are you one of those guys who just tries to deliberately misunderstand everything?

Fredrik said:
Are you one of those guys who just tries to deliberately misunderstand everything?

No, not at all. I tried to help you see your own mistakes. Look at your initial post and at your persistence that all atoms in a rigid object can be accelerated in sync.

Fredrik said:

This problem is ridiculously simple. The condition that the spaceships experience the same acceleration implies that their world lines will have the same shape. (The acceleration doesn't have to be constant. It's sufficient that both spaceships accelerate the same way). This implies that the length of the rope will remain constant in the launcher's frame. Think about that for a second. After a while, the rope is moving at a high velocity in the launcher's frame, and must therefore be Lorentz contracted, but it's still the same length! That means that it must have been stretched. If it was already stretched to its maximum length when the acceleration began, it must break. It's as simple as that.

This is all very basic stuff that belongs in an introductory level class about special relativity. So why is this article disputed at all?

Is it because of the common (but silly) misunderstanding that you can't solve a problem involving any kind of acceleration entirely in SR? (It's really weird how many people who have studied SR still believe that you need GR for problems like this).

Or is it because some people who understand that the rope gets stretched are arguing that SR somehow also implies that the rope gets stronger, so it can handle getting stretched?

I can't think of a third reason.

I know that some people here have been working on this article. Perhaps one of you can explain this to me.

You are assuming that the reason articles get tags is based on logic and an actual substantial objection. It's actually more of a political process.

It's disputed mainly because of Rod Ball, although the detailed history of how it got that tag is a little more complicated. If you're really curious, and have enough time, you can see the history of the evolution of the article, and the talk pages for the article.

I believe that I actually tagged the article with an NPOV tag when it was in a significantly different form - the history page shows my September 2006 edit below as adding the tag.

I don't feel that there are any NPOV problems with the current article either, but as a very involved party in writing the current version of the article, I don't feel it's appropriate for me to remove the NPOV tag. In fact, I'm not quite sure what the wikipedia process for reomving a tag is (if there is one).

From my POV, the biggest open question with some (small) amount of merit was the debate over whether or not a man named Petkov and his opinions should be mentioned. AFAIK Rod Ball is the only one who feels this particular author's contributions are notable.

I'm happy to have what I think is a reasonably good article with a NPOV tag stuck on it, as opposed to having a very bad article with a NPOV tag stuck on it, so I haven't really investigated closely what it would take to get the tag removed.

Both 5 and 6 are not possible. Forces travel at finite speed (the speed of sound) in rigid materials. This is a pretty slow speed , so you cannot have "instantaneous (or near instantaneous) velocity boost". This is what Born rigidity is all about.
How about gravity, are the atoms not accelerated equally (or nearly equally because of longer distances from back to front of the object to the pulling object) The atoms are equally gaining acceleration, but the atoms are organizing themselves so the object will be compressed, but if the atoms did not interact with each other, the object would be stretched in a frame equal to the original frame before the objects starts moving. This is making these points real:
5. Every point of the rod is instantaneously (or near instantaneously) boosted to a new velocity, all at the same time in the frame where the rod was at rest before the boost. This stretches the rod to a longer proper length.
6. Every point of the rod is instantaneously (or near instantaneously) boosted to a new velocity, all at the same time in the frame where the rod will be at rest after the boost. This compresses the rod to a shorter proper length.
The only difference is that it is looked upon at a different frame of movement.

Anyway, I don't see the reason why the rope is not being snapped.

If one spaceship is being pulled from behind by it's rocked engine, it is contracted, and observed as compressed in a rest frame. But this is only if it is accelerated from one point, or plane of the object. If it was pulled from the front, it would also get a compressed form.
If each atom now were accelerated equally with respect to the REST frame the object would indeed be the SAME size in the rest frame, but larger in the moving frame. (Each atom has a greater distance to each other in the moving frame) This makes it true that if an object were pulled by two or more points of the object, the object would appear larger.

Now, if we see the two spaceships as ONE object, it would be true that the object "stretches". Because both ships are simoultaneously accelerated in the rest frame. As the acceleration continue, the distance in the rest frame will appear un-changed (although each spaceship will look contracted). In the moving frame however, the distance will be enlargened.
The rope that is tied together between these ships (assuming that the ships are accelerating the same direction, but the one slightly ahead of the other, with respect to the direction of acceleration) will be accelerated equally in both front and back in the rest frame. As the distance between the ships remain unchanged in the rest frame, the distances will, as said, increase in the moving frame. The same applies for the rope. It is accelerated at the same time equally in the rest frame, which means it will be larger in the moving frame. (If you apply te atoms interaction with each other, they will make the object stretch, but as acceleration continue, the object will inevitably snap, since the atoms cannot hold the stretching. This will happen at the place where the atoms hold on each other least. (a weak spot in the rope for example)

This explanation is only true if the spaceships are NOT accelerating in a directioin showed here:

"S"=spaceship
"-"=rope
^=direction of acceleration
"_"=empty space(no effect on the situation, only added because of to make the direction sign be in the correct position)

____^____
S---------S

It must be something like this, (actually, anything except the situation above)

"S"=spaceship
"."=rope
"-"=rope
"^"=direction of acceleration
"_"=empty space(no effect on the situation, only added because of to make the direction sign be in the correct position)

______^________
S---...
______ ---...
____________---S

I just want to say, that I have given a lot of thought into this, and find it correct. If you believe I am wrong, please elaborate why. But please make sure which frame I am basing my statements on, before you argument over it...
I am relatively new to the concept of general relativity :tongue:

Last edited:
Jarle said:
How about gravity, are the atoms not accelerated equally (or nearly equally because of longer distances from back to front of the object to the pulling object) The atoms are equally gaining acceleration, but the atoms are organizing themselves so the object will be compressed, but if the atoms did not interact with each other, the object would be stretched in a frame equal to the original frame before the objects starts moving. This is making these points real:

The only difference is that it is looked upon at a different frame of movement.

Your post is not understandable, are you asking for the GR treatment of the problem instead of the treatment under SR with accelerated motion? Of course we can treat it using GR but the gravitational field difference between the two ends of the rockets is so small that we can neglect it wrt the much higher effect due to the thrust acceleration. Would you care to repost such that your question is more understandable?

nakurusil said:
Your post is not understandable, are you asking for the GR treatment of the problem instead of the treatment under SR with accelerated motion? Of course we can treat it using GR but the gravitational field difference between the two ends of the rockets is so small that we can neglect it wrt the much higher effect due to the thrust acceleration. Would you care to repost such that your question is more understandable?

First of all, I have read my post through a couple of times, and find it fairly understandable.

What is the difference between the GR and SR treatment of the problem?

And the gravitational field between the objects ARE so small that they are unmeasurable, I didn't state it were significant factors. I only implied the factor excisted. What I wrote did not have anything directly to do with the spaceship situation. I only stated that this:
Both 5 and 6 are not possible. Forces travel at finite speed (the speed of sound) in rigid materials. This is a pretty slow speed , so you cannot have "instantaneous (or near instantaneous) velocity boost". This is what Born rigidity is all about.
Is incorrect. My upper paragraph which is "translated" under, had only one point, to argument that statement.

OK, I can make it more understandable:

How about gravity, are the atoms not accelerated equally (or nearly equally because of longer distances from back to front of the object to the pulling object) The atoms are equally gaining acceleration, but the atoms are organizing themselves so the object will be compressed, but if the atoms did not interact with each other, the object would be stretched in a frame equal to the original frame before the objects starts moving. This is making these points real:
How about gravity? When gravity are interracting with an object, it pulls every atom at the SAME rate. (if we overlook the insignificant difference in the gravity field) In practice, the force between the atoms will make them stay at their normal places. (as if you stretched a string, it wants to go back to normal size, because of the atom interraction, it does not wish to stay stretched (This got nothing to do with the spaceship situation!) ).
So during the acceleration of gravity, the atoms will organaize so it will behave like it was pushed from one point of the object, as it reaches a high velocity(in rest frame!(it will stay the same size in the moving frame)). But that is not the point here. The point is that an object can indeed be accelerated at the same rate for each atom.

Bottom line: In a gravitational pull, the object is not pushed from behind, or pulled at the back, each atom is taking effect directly from the gravitational force. As the distance between them in the objects frame, the atoms will automatically interact with each other, and making the distance between each other, normal for their frame. (If atoms had no force in between themselves, the object would in the objects frame be stretched, and each atom would be further and further away from their neighbour atoms)

Last edited:
Jarle said:
OK, I can make it more understandable:

How about gravity? When gravity are interracting with an object, it pulls every atom at the SAME rate.

Gravity is a force like any other force. It propagates at FINITE speed in VACUUM (c). Inside a body it will propagate at the much lower speed of sound. So, you are wrong, it will not propagate thru a body instantaneously. Therefore, the effect of gravitational force on the two rockets on the acceleration will be very similar (albeit much weaker) than the force due to engine thrust. To recap:

1. The rear of the rocket(s) reaches a higher speed than the front of the rockets during the acceleration phase.

2. Because of that, the rod beween rockets gets stretched.

3. The above is not a kinematic effect, cannot be treated as "length contraction" , either in SR or in LET.

So during the acceleration of gravity, the atoms will organaize so it will behave like it was pushed from one point of the object, as it reaches a high velocity(in rest frame!(it will stay the same size in the moving frame)).

You are trying to treat the problem as a kinematic problem,as frame-dependent "length contraction" , this is wrong, see points 1-3 above.

But that is not the point here. The point is that an object can indeed be accelerated at the same rate for each atom.

I suggest that you read on "Born rigidity" before making such claims. Because your claims are wrong.

nakurusil said:
Gravity is a force like any other force. It propagates at FINITE speed in VACUUM (c). Inside a body it will propagate at the much lower speed of sound. So, you are wrong, it will not propagate thru a body instantaneously.
Did I use the word instantaneously? If I did, I deeply regret it, because it was not what I meant. On the other hand, THAT was not my point at ALL. I think you search for a single thing that is insigninficantly incorrect, and then point it out, and make it sound like everything is wrong. The acutal point was that each atom interract with the gravitational pull, and not just the end of the object, making it push the rest of it with it. (that makes point 5. and 6. correct, which was my main intention. I didn't question wether the gravitational waves had a finite speed or not...
Therefore, the effect of gravitational force on the two rockets on the acceleration will be very similar (albeit much weaker) than the force due to engine thrust. To recap:

1. The rear of the rocket(s) reaches a higher speed than the front of the rockets during the acceleration phase.
[/quote] Well, if the object fell long enough, this diference would be irrelevant.
2. Because of that, the rod beween rockets gets stretched.

3. The above is not a kinematic effect, cannot be treated as "length contraction" , either in SR or in LET.
Why is that, I have been told that gravitational movement is the same as movement by push and pull, just that the force is excerted at each atom, rather than on a point of the object.

You are trying to treat the problem as a kinematic problem,as frame-dependent "length contraction" , this is wrong, see points 1-3 above.

I suggest that you read on "Born rigidity" before making such claims. Because your claims are wrong.
If I got you correctly, the rigidy has insignificant meaning here. The atoms may use some time to reorganize, but this will happen constantly throughout acceleration. You might have misunderstood my meaning.

Last edited:
Jarle said:
If I got you correctly, the rigidy has insignificant meaning here. .

You got it exactly backwards.

http://www.nizkor.org/features/fallacies/

Read through this site and learn all of the axioms of logic and their fallicious counter-parts. This way, the debate can be more structured and you can easily identify the logical constructions and postulates that are being posited.

nakurusil said:
You got it exactly backwards.

And why is that? What does rigidy excactly have to do with the pull of gravity?
I did say that the object IS observed smaller in a rest-frame when it is pulled by gravity. When I said that it wouldn't, that was under the circumstances of no interaction between the atoms.

I believe that we just misunderstand each other...

Anyway, the gravity points has nothing to do with the main point of this thread...

EDIT: Ok, even though I do not understand your point, let's just let it go...

Last edited:
nakurusil said:
Sounds like an attempt to justify why you got the solution wrong at post #1

nakurusil said:
No, not at all. I tried to help you see your own mistakes. Look at your initial post and at your persistence that all atoms in a rigid object can be accelerated in sync.
I didn't get #1 wrong, and what I said about the items 5 and 6 that I added to Ich's list has nothing to do with post #1.

And what's up with that "...all atoms in a rigid object..." comment?! I never said anything like that! So don't say that I did.

Fredrik said:
I didn't get #1 wrong, and what I said about the items 5 and 6 that I added to Ich's list has nothing to do with post #1.

And what's up with that "...all atoms in a rigid object..." comment?! I never said anything like that! So don't say that I did.

Fredrik said:
This problem is ridiculously simple. The condition that the spaceships experience the same acceleration implies that their world lines will have the same shape. (The acceleration doesn't have to be constant. It's sufficient that both spaceships accelerate the same way). This implies that the length of the rope will remain constant in the launcher's frame.After a while, the rope is moving at a high velocity in the launcher's frame, and must therefore be Lorentz contracted, but it's still the same length! That means that it must have been stretched.

Ummm, no, nothing to do with "Lorentz length contraction", sorry.This is not a kinematics problem, this is why I tried (and unfortunately failed) to explain it to you from the point of view of dynamics. If you still don't get it after all the explanations, there is nothing that I can do for you.

Last edited:
I really hope this not yet another troll...

Fredrik asked about Link to the article. I am no longer participating in Wikepedia, but I was principle author of a previous version (see http://en.wikipedia.org/wiki/User:Hillman/Archive) . The reason for the flags is that this article has been the subject of a long running "edit war" between members of WikiProject Physics (a group of Ph.D. physicists and mathematicians to which I formerly belonged) and a single crank who asserts, in the face of all proof to the contrary, that the mainstream analysis is wrong.

Perhaps one of you can explain this to me.

My horrible experience with the article in question was responsible for my abandoning my attempts to add content to the Wikipedia; thereafter I stayed on for a few months to trying to help formulate proposed policy reforms aimed at streamlining cruft eradication and so on, but harrassment from embittered cranks led me to abandon this effort as well. So it should not be necessary for me to explain why I decline to discuss this matter further.

Anyone interested in the actual physics/math is urged to consult my last version (see the link in my archive page to a quartet of closely related articles I wrote on the Bell and Ehrenfest "pardoxes" plus the Rindler and Born coordinates) and then to study the papers cited in these articles, then the papers cited in the review paper by Gron, and so on. A related webpage by Greg Egan (see gregegan.customer.netspace.net.au/SCIENCE/Rindler/RindlerHorizon.html) is also well worth reading. It is sad that this lovely subject has been permanently spoiled for me by my awful experience at Wikipedia.

Last edited by a moderator:
nakurusil said:
Ummm, no, nothing to do with "Lorentz length contraction", sorry.This is not a kinematics problem, this is why I tried (and unfortunately failed) to explain it to you from the point of view of dynamics. If you still don't get it after all the explanations, there is nothing that I can do for you.
What are you talking about? You haven't tried to explain anything to me except the trivial and irrelevant fact that 5 and 6 are practically impossible. (It's trivial in the sense that any moron understands it, and it's irrelevant because it's what's possible in principle that's interesting in a discussion about a theory). You have however invented opinions for me that I don't have, and falsely claimed that I'm wrong about more important things (post #1) without providing a shred of evidence, or anything that resembles an explanation.

Thank you Chris and Pervect for your answers. They are both interesting.

However, I noticed that neither of you answered the question about what kind of objections have been presented against the obvious conclusion. Do they claim a) that GR is needed since acceleration is involved, b) that SR somehow implies that the rope will not break even though it's getting stretched, or c) some other kind of crank nonsense?

Last edited:
Fredrik said:
Thank you Chris and Pervect for your answers. They are both interesting.

However, I noticed that neither of you answered the question about what kind of objections have been presented against the obvious conclusion. Do they claim a) that GR is needed since acceleration is involved,

No, SR handles accelerated motion, you need to understand hyperbolic motion.

b) that SR somehow implies that the rope will not break even though it's getting stretched,

No again, you need to understand that forces applied to an object propagate at finite speed (speed of sound). This is why, when you couple SR hyperbolic motion with Born rigidity you get the CORRECT explanation of the problem . I gave it to you three times, here it is one more time:

1. The rear of the rocket (where the motor is) reaches the cruising speed v BEFORE the front of the rocket (due to ...Born rigidity)

2. Therefore the rear of the leading rocket reaches the cruising speed v BEFORE the front of the trailing rocket.

3. Therefore the rod connecting the rear of the front rocket and the front of the rear rocket stretches

4. All of the above has NOTHING to do with Lorentz contraction, contrary to your repeated claims.

5. All of the above shows that your claims 5-6 are physically impossible, contrary to your insistance to the contrary. You cannot "accelerate all the points in a real rigid object simultaneously" Born rigidity theory precludes this from happening.

Last edited:
Fredrik said:
I didn't get #1 wrong, and what I said about the items 5 and 6 that I added to Ich's list has nothing to do with post #1.

And what's up with that "...all atoms in a rigid object..." comment?! I never said anything like that! So don't say that I did.

Fredrik said:
5. Every point of the rod is instantaneously (or near instantaneously) boosted to a new velocity, all at the same time in the frame where the rod was at rest before the boost. This stretches the rod to a longer proper length.
6. Every point of the rod is instantaneously (or near instantaneously) boosted to a new velocity, all at the same time in the frame where the rod will be at rest after the boost. This compresses the rod to a shorter proper length.

The situations that you list are unphysical, they violate the way forces propagate in a solid.

Last edited:
For any of you that have a copy of the 2nd addition of Spacetime physics, you can see the problem and its explanation on pages 117 - 119. The authority embraces the solution championed by Nakurusil (also Hillman and pervect)

yogi said:
For any of you that have a copy of the 2nd addition of Spacetime physics, you can see the problem and its explanation on pages 117 - 119. The authority embraces the solution championed by Nakurusil (also Hillman and pervect)

Thank you!

• Special and General Relativity
Replies
16
Views
243
• Special and General Relativity
Replies
75
Views
3K
• Special and General Relativity
Replies
24
Views
2K
• Special and General Relativity
Replies
27
Views
2K
• Special and General Relativity
Replies
11
Views
396
• Special and General Relativity
Replies
5
Views
630
• Special and General Relativity
Replies
115
Views
5K
• Special and General Relativity
Replies
11
Views
1K
• Special and General Relativity
Replies
98
Views
2K
• Special and General Relativity
Replies
21
Views
2K