Penrose-Terrell rotation and the 3 great special relativistic myths

[danR]

After searching, I found only one post-title on the Penrose and/or Terrell effect/rotation.

It is odd that innumerable popular discussions on PF and elsewhere omit this 50+ year modification of externally measured length-'contraction'. There is neither contraction, nor measured contraction. There is a 4-space geometrical/optical 'rotation' of a rod on an observer's retina/photographic plate. But if not the same thing, this is hardly more a mysterious 'foreshortening' than the optical rotating a non-moving rod or stick in space. Hold a pen at arm's length and rotate its long axis away from normal to your line of vision. Big deal.

There is not even an optical change of a relativistic sphere. It does not appear flattened--an oblate spheroid--at all. It remains resolutely spheroidal to the observer. The matter comes not too far from Einstein's description actually being wrong.

Relativistic length-contraction, myth number 1, debunked.
____________

It is odd that relativistic mass-increase still is still promulgated, although on this one PF folk are trying to make the correction. There is an increase in energy, but not mass. And it would be too close to equivocation to say m and e are equivalent. If Alice' real antimatter propellant mass were increasing to Bob's measurement system, he would be confident she could eventually get an infinite return in her investment in higher energy photon exhaust, because, hey m $\equiv$ e, right?

If what is increasing, according the observer's reference frame, is only energy, that's not so mysterious, after all is it? It's not the same, but also not too much stranger than our everyday experience where KE increases with velocity.

Relativistic mass-increase, myth number 2, debunked.
____________

Time-dilation. Now this is the only mystery remaining. However, it's also fair to say that 'time' itself is one of the great mysteries to everyone, and much confusion over several concepts of 'time' that need to be separated:

Time as a dimension of space-time. Time as a dimension cannot be 'slower' appearing. That has no meaning.
Time as a vectored entity.
Time as a psychological perception.
Time as entropy.

We cannot talk about time-'dilation' as mysterious, and sweep under the carpet the fact that time itself is mysterious, and possible insolubly so.

Relativistic time-dilation, myth number 3, true, but trivial.
_______________

Now, I expect this post to quickly drop into oblivion, and the endless stream of

'do spaceships really contract...?'
'why would mass increase...?'
'I can prove Einstein was wrong...suppose you have a train...'

to return in a few hours, and be addressed by explanations that really need modification.

Now, there are certainly going some conceptual nuances in my own discussion above, and some things that are just wrong. But the gist of it stands, I think, as long as it's confined to SR.

 

[pervect]

Terrel rotation is a good relativistic description of what you actually directly see, just as the doppler shift is.

It's hard to be sure, but I get the feeling you MAY be drawing more conclusions from the Terrel rotation than are warranted.

For instance, Terrel rotation has nothing to say about the notion that simultaneity is relative, because it does not go beyond what one directly sees to consider issues such as when events are simultaneous.

 

[PeterDonis]

You might want to check out this Usenet Physics FAQ entry:

http://math.ucr.edu/home/baez/physics/Relativity/SR/penrose.html

This entry makes clear a few things:

(1) Using the word "rotation" to describe how an object moving past you at relativistic speed appears has issues as well; it's not *just* a rotation, any more than it's *just* length contraction. The FAQ entry notes:

For example, consider a train carriage that moves with constant speed along a straight track, past us who stand on the platform watching it. Light that leaves the back of the carriage will arrive at our eyes coincident with light from the side of the carriage; and as well, the carriage is Lorentz contracted. At first glance this seems to ensure that the carriage will look rotated, because we are used to seeing just such an effect from rotations in everyday (nonrelativistic) life. But clearly, on second glance, we will realize the carriage is not rotated, because we notice that its wheels are still attached to the track, which has not changed its aspect (since it's at rest relative to us). So there will be a psychological effect of the carriage looking somewhat rotated, but we will quickly notice that this is only an imperfect optical illusion.

(2) To say that "there is not even an optical change of a sphere" is not true. It still looks like a sphere, but features on the sphere are not in the same places if it's moving at relativistic speeds, compared to when it's at rest relative to the observer.

(3) How an object looks to you, if it's moving relative to you, is not necessarily a good indication of all of its physical properties. The FAQ entry notes:

...the contraction can be measured, but the measurement is frame dependent. Whether that makes it "real" or not has more to do with your choice of words than the physics.

When Einstein talked about "length contraction", he was talking about those frame-dependent measurements that you can make, not about how it appears to you. He drew this distinction because, as the FAQ entry notes, when you look at an object at a given instant (or take a "snapshot" of it using a very fast film, which is the scenario the FAQ entry discusses),

...you capture the light rays that hit the film at one instant (in the reference frame of the film). These rays may have left the object at different instants; if the object is moving with respect to the film, then the photograph may give a distorted picture.

 

[danR]

Terrel rotation is a good relativistic description of what you actually directly see, just as the doppler shift is.

Yes, it is.

It's hard to be sure, but I get the feeling you MAY be drawing more conclusions from the Terrel rotation than are warranted.

For instance, Terrel rotation has nothing to say about the notion that simultaneity is relative, because it does not go beyond what one directly sees to consider issues such as when events are simultaneous.

People are drawing too few conclusions from P-T that are being inputted into discussions. That is not hard to be sure. I remember a SA article depicting colliding gold ions as oblate spheroids. They must be flattened, right? Or perhaps gold ions are not spherical anyway. They are oblate spheroids, and are always aligned in an accelerator with their minor axes parallel to their direction.

I wonder if that has implications for the geometric and spectral energy distribution of the detected collision debris.

 

[PAllen]

Also note the following points (which have been discussed recently on threads here):

1) 100 meter pole moving very close to c can be momentarily trapped in a 10 meter barn. Both barn doors can be momentarily closed with the pole completely inside.

2) Further, an observer at the center of the barn (despite any rotated aspect) will see the pole completely contained in the barn. Note that even allowing for diagonal orientation, there is no way the pole should be able to fit in the barn.

You can choose how to interpret these things, but you should be aware of them. (The differences between the pole's perspective and the barn perspective all boil down to very different perceptions of the ordering of events - opening and closing of doors - due to simultaneity differences).

Terrell rotation is an interesting visual phenomenon, but not much of a real change to prior understanding of SR.

 

[PAllen]

People are drawing too few conclusions from P-T that are being inputted into discussions. That is not hard to be sure. I remember a SA article depicting colliding gold ions as oblate spheroids. They must be flattened, right? Or perhaps gold ions are not spherical anyway. They are oblate spheroids, and are always aligned in an accelerator with their minor axes parallel to their direction.

Note that to describe rotated image, you have to model light paths coming from a moving oblate spheroid. If you assume the object itself is spherical and moving fast, you will get the *wrong* image.

 

[PAllen]

An additional point is that if you model head on collisions of nuclei, in center of mass frame, both must be treated as squashed to get the right predictions. You will see references to pancakes in the literature on nuclear collisions - graphically highlighting this necessity.

For example:

http://www2.ku.edu/~nucphys/NuclearHome.html

 

[Meir Achuz]

After searching, I found only one post-title on the Penrose and/or Terrell effect/rotation.

It is odd that innumerable popular discussions on PF and elsewhere omit this 50+ year modification of externally measured length-'contraction'. There is neither contraction, nor measured contraction. There is a 4-space geometrical/optical 'rotation' of a rod on an observer's retina/photographic plate. But if not the same thing, this is hardly more a mysterious 'foreshortening' than the optical rotating a non-moving rod or stick in space. Hold a pen at arm's length and rotate its long axis away from normal to your line of vision. Big deal.

There is not even an optical change of a relativistic sphere. It does not appear flattened--an oblate spheroid--at all. It remains resolutely spheroidal to the observer. The matter comes not too far from Einstein's description actually being wrong.

Relativistic length-contraction, myth number 1, debunked.

You may want to look at http://arxiv.org/abs/0906.1919

 

[PAllen]

You may want to look at http://arxiv.org/abs/0906.1919

The points in this paper are, in my opinion, a bit overstated. Rest length of a 'rigid' object may be taken to be similar to proper lifetime of e.g. a muon. Both can be given invariant interval definitions. However, proper lifetime of a muon is not particularly useful for analyzing muon beams in an accelerator. Nor is rest diameter useful for analyzing collisions of nuclei in accelerators. Nor is it very meaningful to say that the magnetic field produced by a moving charge is 'fictitious' because it is only coulomb in the charge's rest frame.

 

[yuiop]

After searching, I found only one post-title on the Penrose and/or Terrell effect/rotation.

It is odd that innumerable popular discussions on PF and elsewhere omit this 50+ year modification of externally measured length-'contraction'. There is neither contraction, nor measured contraction.

One reason for that is that it is a confusing distraction relating to optical effects rather than real physical effects. Generally speaking in relativity discussions we like to concentrate on physical measurements and it is taken as a given that purely optical effects are factored out.

There is a 4-space geometrical/optical 'rotation' of a rod on an observer's retina/photographic plate. But if not the same thing, this is hardly more a mysterious 'foreshortening' than the optical rotating a non-moving rod or stick in space. Hold a pen at arm's length and rotate its long axis away from normal to your line of vision. Big deal.

Here you appear to be confused. You are leaping to the conclusion that because Penrose Terrell rotation is an optical effect, that length contraction must also be an optical effect, but they are two different things. If you hold a pen 6 inches from your eyes and then hold it at arms length it appears to get shorter, but that is just an optical effect, like Penrose Terrell rotation. If you compress the pen then that is a real physically shortening like length contraction. Two different things.

There is not even an optical change of a relativistic sphere. It does not appear flattened--an oblate spheroid--at all. It remains resolutely spheroidal to the observer.

Yet more confusion. Because it is reported that the sphere does not appear optically flattened when it moving relative tot he observer, people jump to the conclusion that length contraction is not real because "you cannot even see it!". This is misleading. It only applies to special case of an exact sphere with no surface markings, wherein the optical effect appears to cancel out the physical shortening. For any other object, length contraction can be visible, but the optical effect can make things appear longer or shorter than the rest length of the object depending upon whether the object is approaching or receding and at what angle.

The matter comes not too far from Einstein's description actually being wrong.

You are either right or wrong. Einstein is right and you are wrong.

Relativistic length-contraction, myth number 1, debunked.

Not even close.

 

[danR]

Originally Posted by danR
The matter comes not too far from Einstein's description actually being wrong.
______
You are either right or wrong. Einstein is right and you are wrong.
_____

Einstein used 'measurement' and 'appearance' equivalently in the original paper. If he was right in 1905:

"A rigid body that has a spherical shape when measured in the state of rest thus in the state of motion - viewed from the stationary system - has the shape of an ellipsoid of revolution...
Thus, whereas the Y and Z dimensions of the sphere (and therefore also of every rigid body, whatever its shape) do not appear modified by the motion, the X-dimension appears to be contracted in the ratio...viewed from the "stationary" system - shrink into plane figures."​

then Penrose and Terrell (and Lampa) were wrong.


I read two papers where the authors point out that appearance and measure should be carefully distinguished. Unfortunately, neither carefully defined what they meant by 'measured'. I gather than an informal way of measurement is to use infinite speed particles, which apparently cancels out all optical effects. A Wikipedia article, unchallenged, on Lorenz contraction was likewise not abundantly clear on the procedure of formal measurement, and referred back to the Michaelson-Morley experiment itself.

 

[danR]

The points in this paper are, in my opinion, a bit overstated. Rest length of a 'rigid' object may be taken to be similar to proper lifetime of e.g. a muon. Both can be given invariant interval definitions. However, proper lifetime of a muon is not particularly useful for analyzing muon beams in an accelerator. Nor is rest diameter useful for analyzing collisions of nuclei in accelerators. Nor is it very meaningful to say that the magnetic field produced by a moving charge is 'fictitious' because it is only coulomb in the charge's rest frame.

What disappoints me in the paper is the elusive promise of a 'measurement' method, as opposed to a mere appearance. Einstein apparently didn't recognize the difference, but I'm not concerned about that.

If Lucy holds the measurement football, I don't want her to keep pulling it away when I'm about to kick it. Too many papers have done that to me already. The Wikipedia article on Lorentz promised 4 references to the 'real' effects of Lorentz contraction in heavy nuclei collisions. I examined all 4. Total nonsense.

 

[danR]

An additional point is that if you model head on collisions of nuclei, in center of mass frame, both must be treated as squashed to get the right predictions. You will see references to pancakes in the literature on nuclear collisions - graphically highlighting this necessity.

For example:

http://www2.ku.edu/~nucphys/NuclearHome.html

'At the speeds that particles achieve at RHIC, they become “flattened” in the direction of motion according to Einstein’s special theory of relativity. When two of these “nuclear pancakes” moving in opposite directions collide, they end up passing through each other. However, during the instant of overlap the mass of both nuclei occupy the same spatial volume for a fleeting instant.'​

Neither this core reference, nor the rest of the article (at all, I'd say), have anything to do with some necessary difference in energy distribution, geometrically or spectrally, from uncorrected (not-flattened) nuclei. It merely describes their necessary geometry from relativity itself. Let's not confuse the instant of completed merger with their appearance: they would merge 'fleetingly' as spheres or as pancakes.

If the authors are predicting or reporting a difference, it is elsewhere in their article.

 

[danR]

Note that to describe rotated image, you have to model light paths coming from a moving oblate spheroid. If you assume the object itself is spherical and moving fast, you will get the *wrong* image.

I was being a bit facetious. The nuclei are substantially spherical, and continue to appear so under Terrell rotation. Whether they are somehow 'measurably' (relativistic-formally) flattened is my question.

 

[danR]

Also note the following points (which have been discussed recently on threads here):

1) 100 meter pole moving very close to c can be momentarily trapped in a 10 meter barn. Both barn doors can be momentarily closed with the pole completely inside.

2) Further, an observer at the center of the barn (despite any rotated aspect) will see the pole completely contained in the barn. Note that even allowing for diagonal orientation, there is no way the pole should be able to fit in the barn.

You can choose how to interpret these things, but you should be aware of them. (The differences between the pole's perspective and the barn perspective all boil down to very different perceptions of the ordering of events - opening and closing of doors - due to simultaneity differences).

Terrell rotation is an interesting visual phenomenon, but not much of a real change to prior understanding of SR.

The pole in a barn is a well-known accessible example of what I'm asking for as a formalized method of 'measurement'. It is in the class of gendankenexperimenten, however. Can we see a real-world equivalent?

 

[danR]

Incidentally, the colliding nuclei are depicted from a 'resting' observer's viewpoint. Each nucleus regards itself as spherical ensemble of nucleons, and the other, supposedly, as flattened. If there are predictable consequences of this arrangement, differing from both as spherical, or both as 'flattened' from the resting detectors' point of view (the distribution of debris and spectra) we should be able to measure it. This again would be confirmation of measured (not apparent) contraction.

 

[PAllen]

What disappoints me in the paper is the elusive promise of a 'measurement' method, as opposed to a mere appearance. Einstein apparently didn't recognize the difference, but I'm not concerned about that.

If Lucy holds the measurement football, I don't want her to keep pulling it away when I'm about to kick it. Too many papers have done that to me already. The Wikipedia article on Lorentz promised 4 references to the 'real' effects of Lorentz contraction in heavy nuclei collisions. I examined all 4. Total nonsense.

Write ups by experts in the field (from laboratories doing the measurements and computations) and a peer reviewed paper covering relativistic ion collisions are nonsense according to danR. This suggests no actual desire to understand, instead a desire to dispute without foundation. What exactly do you find nonsensical, besides that they disagree with your belief that length contraction is a myth?

As to measurements, there are actually no disputes at all about what would be measured by different procedures. You may attach different emphasis to different measurements. Many of these are necessarily conceptual measurements due to practical difficulties - you can no more photograph a baseball moving .99c than you can measure a ruler moving .99c. However, no one disputes the predictions:

1) A photo of the ball would look round; however, to derive this you *assume* that the ball is actually flattened; otherwise you get the wrong predictions.

2) If the ball is 10 cm in its rest frame, and passes two laser beams 5 cm. apart (directed orthogonal to its flight path), it will not interrupt both at once.

Unless you dispute mathematics and all of SR, you can't dispute either of these predictions. Different measurement produce different results.

 

[PAllen]

An observation about why you are not likely to see macroscopic tests of length contraction: Besides the question of how to accelerate a macroscopic object to relativistic speed, is the question of stopping it: a 1 gram object (about 1/3 of a dime) moving at .86 c will release the energy of the Nagasaki atom bomb on being stopped. (20 kilotons)

A baseball moving at .99c would release about the energy of the largest H-bomb ever exploded (20 megatons).