Are the transformations just observed ones or real ones?

[Dale]

I do think it is logically impossible that relative motion of its own triggers changes in observed phenomena.

It is not a change in an observed phenomenon, it is a disagreement about whether or not the observed phenomena constitute a length. Your objection is not pertinent to the topic.

If in my frame I measure that the back of the train is at x=0 and the front of the train is at x=1, both at t=0, then I will say that the length of the train is 1. However, someone moving at v=.6 relative to me will say that my measurement of the front of the train was at x=1.25 and at t=0.75 (in units where c=1). So they will disagree that my measurement constituted a measurement of the length.

Again, length contraction isn't about changes in length, it is about disagreement between frames.

 

[choran]

Hope I'm OK with asking this in this thread--if not, feel free to ignore. I wonder if there is a difference between the length contraction under discussion with the phenomenon involved wherein the light from a moving object reaching the detector (eye, CCD, whatever) of necessity originates at different points along the object, and thus has traveled different distances to the detector from various points on the object, and different times in its path of travel. Hope that's clear enough. I believe it's called, or related to Penrose-Terrell. Question is, is length contraction the same as, different from, in addition to or...?

 

[yuiop]

I wonder if there is a difference between the length contraction under discussion with the phenomenon involved wherein the light from a moving object reaching the detector (eye, CCD, whatever) of necessity originates at different points along the object, and thus has traveled different distances to the detector from various points on the object, and different times in its path of travel. Hope that's clear enough. I believe it's called, or related to Penrose-Terrell. Question is, is length contraction the same as, different from, in addition to or...?

Penrose-Terrell rotation is a purely optical effect. A sphere does not appear length contracted due to the rotation effect and quite a lot people give this as a proof that length contraction is not a physical phenomena. However the length contraction of a long rectangular object is not totally obscured by the PT rotation and can in principle be photographed.

If each photon that lands on the film of a Penrose-Terrell camera had a time stamp with its time of emission, it would be seen that the pixels that make up the photograph would have a wide variety of time stamps. If we took a series of photographs and used a computer to assemble an image made of pixels with exactly the same time stamp, then we would have an image complete with length contraction (and no rotation).

Essentially, length contraction is a mental picture of where all the parts of a moving object are at a given simultaneous instant of time. This assembled picture takes into account any delays due to light travel and removes those delays, so that the resulting calculation has more physical significance than just optical appearance.

I think the best demonstration of the physicality of length contraction is in the Ehrenfest paradox, where the length contraction of the outside edges of a rotating object causes real stresses that would eventually tear the the object apart if the radius was not permitted to alter as the rotational speed varied. The next best demonstration is Bell's rockets paradox, where a string of fixed length (in one reference frame) breaks due to length contraction, but a lot of people don't get the solution to that paradox.

 

[choran]

Would be correct that no Penrose-Terrell photos have been taken, but that the "images" are simply mathematically derived by applying a non-relativistic formula based upon the speed of arrival of light from different portions of the object as it moves through space, as you explain above, by calculating the wide variety of "time stamps"? Is it also correct to state that the Penrose effect or procedure would not capture a length contraction, and is that simply because by definition the length contraction posited in relativity theory is not the one described and measured by the Penrose situation/procedure? Thanks again for your help.

 

[yuiop]

Would be correct that no Penrose-Terrell photos have been taken, but that the "images" are simply mathematically derived by applying a non-relativistic formula based upon the speed of arrival of light from different portions of the object as it moves through space, as you explain above, by calculating the wide variety of "time stamps"?

Every time an ordinary photo is taken of a moving object, it is effectively a Penrose-Terell type image. It is just that the velocities of common objects are usually too low for any relativistic rotation or length contraction effects to visibly noticeable. A hypothetical PT camera has additional sophistications such as curved back to equalise the light path from the lens to the film and an extremely fast shutter. The mathematical calculations of Penrose-Terrell rotation are relativistic, because they take into account the effect of length contraction and then factor in the light delays to calculate what image would be produced on a camera film.

Is it also correct to state that the Penrose effect or procedure would not capture a length contraction,

No. It would capture the length contraction of a long thin rod moving parallel to its long axis. The apparent length of the rod would be changing in successive images, but the one when both ends of the rod are exactly the same distance from the camera lens would show the length contracted length. For the exceptional case of a sphere, the length contraction is hidden by the apparent rotation.

and is that simply because by definition the length contraction posited in relativity theory is not the one described and measured by the Penrose situation/procedure? Thanks again for your help.

Yes, they are two different things. If the leading end of the rod is opposite the lens when the photograph is taken, the light from the trailing edge of the rod must have left much earlier and this makes the rod appear longer on the image.

If the trailing edge of the rod is directly opposite the lens when the photo is taken, then the light from the leading edge must have left much earlier and gives the optical impression of the rod being much shorter.

Length contraction on the other hand is the calculated difference between the positions of the leading and trailing edge, when they are measured simultaneously. This length is constant (for constant velocity) independent of whether the rod is approaching or receding from the observer.

 

[choran]

Last question: Are you saying that Penrose describes a type of relativistic effect, but not the one normally alluded to when people speak of "length contraction"?
Thanks so much.

 

[yuiop]

Last question: Are you saying that Penrose describes a type of relativistic effect, but not the one normally alluded to when people speak of "length contraction"?
Thanks so much.

Yes. Your welcome ;)

P.S. I should probably add that the examples of 'physical' length contraction I gave in post #63 both involve acceleration. The Lorentz transformations usually relate to observers and objects moving with purely inertial motion and then the observed measurements are observer dependent and reciprocal and no tangible physical effects occur purely as a result of transforming reference frames.

 

[harrylin]

It is not a change in an observed phenomenon[..]
Again, length contraction isn't about changes in length, it is about disagreement between frames.

The term "length contraction" has two different meanings; one meaning relates to a reduction in a moving object's or system's equilibrium length according to a system in which that object or system was in rest before. This was also how Einstein used it in 1905: "let a constant velocity v be imparted in the direction of the increasing x of the other stationary system".
I illustrated that with my calculation example and Yuiop next illustrated it as follows:

[..] I think the best demonstration of the physicality of length contraction is in the Ehrenfest paradox, where the length contraction of the outside edges of a rotating object causes real stresses that would eventually tear the the object apart if the radius was not permitted to alter as the rotational speed varied. The next best demonstration is Bell's rockets paradox, where a string of fixed length (in one reference frame) breaks due to length contraction[..].

Just two side notes:
- Ehrenfest: real stresses would not tear a rotating object apart due to length contraction (=inward) but due to inertia (=outward).
- Bell: the change of stress-free length plays a role according to all inertial reference systems .

 

[ghwellsjr]

Penrose-Terrell rotation is a purely optical effect. A sphere does not appear length contracted due to the rotation effect and quite a lot people give this as a proof that length contraction is not a physical phenomena. However the length contraction of a long rectangular object is not totally obscured by the PT rotation and can in principle be photographed.

If each photon that lands on the film of a Penrose-Terrell camera had a time stamp with its time of emission, it would be seen that the pixels that make up the photograph would have a wide variety of time stamps. If we took a series of photographs and used a computer to assemble an image made of pixels with exactly the same time stamp, then we would have an image complete with length contraction (and no rotation).

If these time stamps originate from the moving object, which I presume you mean by "time of emission", then I would assume that they have been synchronized according to the rest frame of that moving object which will result in measurements of the Proper Time and Proper Length of the object.

If you want to be able to measure Length Contraction, then you might be able to do this with a strobe lamp with time stamped photons that is colocated with the camera. The camera would then record reflections with the time stamps for the round-trip timings of the light. This will employ the radar method of establishing relativistic distances to points on moving objects and from this you can determine the lengths of objects according to the frame of the strobe/camera. I'm sure this would work for inline motions and I think it will work for lateral motions.

 

[stevendaryl]

The term "length contraction" has two different meanings; one meaning relates to a reduction in a moving object's or system's equilibrium length according to a system in which that object or system was in rest before. This was also how Einstein used it in 1905: "let a constant velocity v be imparted in the direction of the increasing x of the other stationary system".

Yeah, there are two "length contraction" effects, one having to do with the changes in the measured equilibrium length of an object that is set in motion, and the second having to do with a comparison of distances in two different inertial coordinate systems.

There are similarly two "time dilation" effects: the changes in the measured rate of a clock that is set in motion, and the second having to do with a comparison of elapsed times in two different inertial coordinate systems.

Of course, these pairs of effects are closely related:

• From the assumption that clocks and rods undergo time dilation and length contraction when set into motion, one can show that a coordinate system based on those moving clocks and rods will be related to the original coordinate system through the Lorentz transformations.
• From the assumption that the forces governing rates of clocks and lengths of objects are Lorentz-invariant, one can derive that they must undergo time dilation and length contraction.

 

[Windows]

Sorry for my long absence but here is what I mean: I was just asking if I was moving at a great velocity, if someone sees me holding a clock, he will see it go slowly, but from my reference frame, the clocks is not slowed down and that is just a consequence of the electrodynamics of moving objects taking in consideration the constant speed of light.
So time has nothing to do with velocity just as length, they are just 'measured' transforms because the speed of light is constant, i.e, that my time is the same as yours even if my v=0.999c but you just observe me having a slower time because of the constant speed of light.

 

[Ibix]

I'm not sure. Try this.

I set up and synchronise two clocks, one here and the other one light second away. In practice, I will see that the distant clock is 1 second behind the near one. This is because it takes the light 1 second to reach me.

It is conventional to subtract out any distance-related effects like these, because they just confuse the issue. They also depend on where the observer is, which means adding more information to the maths - it's not worth it.

Now, a spaceship passes me and my clock, moving at 0.6c towards the distant clock. At the instant it passes me its on-board clock and my clock read zero. The distant clock also reads zero (although I'll see -1s because the light showing me it reading zero hasn't reached me yet).

About 1.67s later, the ship passes the distant clock. My clock reads 1.67s. The distant clock also reads 1.67s. However the ship's clock will read 1.33s. Again, it'll be another second before I see the two clocks next to each other with different times - but they do show different times due to time dilation.

To summarise:

Distant stationary clocks appear to be behind due to the travel time of light. The amount behind depends on distance but is constant over time.

Moving clocks appear to run fast as they approach you and slow as they go away from you. This is due to the finite speed of light and is called the Doppler effect.

Conventionally, relativity questions are presented with these two effects removed - the observers are smart enough to correct for them.

After that correction, clocks stationary with respect to one another stay in sync. Clocks moving with respect to one another drift out of sync.

 

[Noyhcat]

Hello!
Are the transformations such as time dilation, length contraction and relativistic mass just observed ones or real ones?
Thank you.

Sorry for my long absence but here is what I mean: I was just asking if I was moving at a great velocity, if someone sees me holding a clock, he will see it go slowly, but from my reference frame, the clocks is not slowed down and that is just a consequence of the electrodynamics of moving objects taking in consideration the constant speed of light.
So time has nothing to do with velocity just as length, they are just 'measured' transforms because the speed of light is constant, i.e, that my time is the same as yours even if my v=0.999c but you just observe me having a slower time because of the constant speed of light.

The transformations are real, in that they are not illusions of light but are a physical consequence of the nature of our universe.

If your v=0.999c, I will observe your clock moving slower, because it actually is, relative to me.

There is no "true" speed of the clock. The speed of your clock physically slows down as its velocity increases, relative to me. When I measure your clock, I am not measuring a distorted clock. I am measuring the real actual thing, and it runs slower, and it is correct.

This is to say that movement causes something to occur such that objects moving at different speeds physically differ from one another in such a way that needs to be compensated for should they wish to interact with each other in a productive way.

 

[harrylin]

Sorry for my long absence but here is what I mean: I was just asking if I was moving at a great velocity, if someone sees me holding a clock, he will see it go slowly, but from my reference frame, the clocks is not slowed down and that is just a consequence of the electrodynamics of moving objects taking in consideration the constant speed of light.
So time has nothing to do with velocity just as length, they are just 'measured' transforms because the speed of light is constant, i.e, that my time is the same as yours even if my v=0.999c but you just observe me having a slower time because of the constant speed of light.

Not quite: measurements of speed, length and time are not transforms. If you check out for example post #10 (the answer on "Are the transformations such as time dilation, length contraction and relativistic mass just observed ones or real ones", is No!), as well as #25 and the last part of #63, then you may notice that it's not just a matter of measuring, there are physical changes when you changed velocity.
See also my post here: https://www.physicsforums.com/showthread.php?p=4518770. Clocks may really end up with different time readings. I don't think that your way of putting it can explain such physical realities.

 

[Windows]

The transformations are real, in that they are not illusions of light but are a physical consequence of the nature of our universe.

If your v=0.999c, I will observe your clock moving slower, because it actually is, relative to me.

There is no "true" speed of the clock. The speed of your clock physically slows down as its velocity increases, relative to me. When I measure your clock, I am not measuring a distorted clock. I am measuring the real actual thing, and it runs slower, and it is correct.

This is to say that movement causes something to occur such that objects moving at different speeds physically differ from one another in such a way that needs to be compensated for should they wish to interact with each other in a productive way.

That's the picture I was sticking to before reading things such as the "Twin Paradox".
And also, how can time run slower according to you?

 

[phinds]

That's the picture I was sticking to before reading things such as the "Twin Paradox".

Since there is nothing in noyhcat's discussion that is in any way inconsistent with the twin paradox, why do you think there is?

And also, how can time run slower according to you?

This has been asked and answered.

 

[Windows]

This has been asked and answered.

I was asking why time is related to velocity.

 

[Dale]

I was asking why time is related to velocity.

Because the speed of light is frame invariant. (And because the laws of physics are the same in all inertial frames).

 

[Noyhcat]

I was asking why time is related to velocity.

Because the speed of light is frame invariant. (And because the laws of physics are the same in all inertial frames).

Right and if you check out the common thought experiment whereby a flashlight is pointed up toward the ceiling of a moving train, you will see why time is affected by velocity.

Because light travels at the same speed for all observers, and because the light coming from a flashlight that is sitting on the floor of and that is pointed at the ceiling of a moving train has to travel a farther distance when viewed by someone who is not on the train, AND because the universe behaves the same no matter what you're doing... time must dilate.

It takes just as long for light to hit the ceiling when viewed by observer A in the train as it does when viewed by observer B outside the train.

I took this from the internet via http://www.copyright.gov/fls/fl102.html special powers:

 

[yuiop]

It takes just as long for light to hit the ceiling when viewed by observer A in the train as it does when viewed by observer B outside the train.

Actually it takes longer for the light to hit the ceiling according to observer B relative to the time observed by A. This is because the speed of light is the same according to both observers, but B sees the light travel a longer diagonal path, so it it must take longer.

 

[Windows]

Because the speed of light is frame invariant. (And because the laws of physics are the same in all inertial frames).

So you define time in terms of light? But that's obviously wrong, without time, photons would not even move.

 

[harrylin]

So you define time in terms of light? But that's obviously wrong, without time, photons would not even move.

Photons cannot be in rest. "Time" is an abstract concept from our minds while light exists without our minds (at least most of us assume that the universe exists without us!). (and what about post #74?)

 

[Noyhcat]

Actually it takes longer for the light to hit the ceiling according to observer B relative to the time observed by A. This is because the speed of light is the same according to both observers, but B sees the light travel a longer diagonal path, so it it must take longer.

Right! My mistake... sorry about that. And if i got it right, the reason for the slowing of time is because from light's reference frame, it can't take two different times to get somewhere at the same time.

 

[phinds]

Right! My mistake... sorry about that. And if i got it right, the reason for the slowing of time is because from light's reference frame, it can't take two different times to get somewhere at the same time.

There is no such thing as "the light's reference frame" so you'll need to rethink this.

see the cosmology FAQ's --- https://www.physicsforums.com/forumdisplay.php?f=206

 

[Noyhcat]

There is no such thing as "the light's reference frame" so you'll need to rethink this.

Say there's no time dilation. Then observer B would need to see light travel faster than c,as it would travel a longer distance but take just as long as it does for observer A, which was also pointed out in #80.

This violates the 2nd postulate of SR. The dilation occurs because c is the same across all reference frames. :P

 

[phinds]

Say there's no time dilation. Then observer B would need to see light travel faster than c,as it would travel a longer distance but take just as long as it does for observer A, which was also pointed out in #80.

This violates the 2nd postulate of SR. The dilation occurs because c is the same across all reference frames. :P

What does any of that have to do with my post?

 

[Nugatory]

There is no such thing as "the light's reference frame" so you'll need to rethink this.

Say there's no time dilation. Then observer B would need to see light travel faster than c,as it would travel a longer distance but take just as long as it does for observer A, which was also pointed out in #80.

This violates the 2nd postulate of SR. The dilation occurs because c is the same across all reference frames. :P

Right, but there is still no such thing as the reference frame of light. In this light-clock exercise, we're comparing the reference frames of two observers, one of whom is at rest relative to the light-clock and one of whom is moving relative to the light-clock.

 

[Windows]

Right, but there is still no such thing as the reference frame of light. In this light-clock exercise, we're comparing the reference frames of two observers, one of whom is at rest relative to the light-clock and one of whom is moving relative to the light-clock.

So my comment was correct?
And it has nothing to do with "there is no such thing as the reference frame of light.".

 

[Dale]

So you define time in terms of light? But that's obviously wrong, without time, photons would not even move.

First, I didn't define time at all. I answered your question about why time was related to velocity. That question already presupposes that time is well defined elsewhere.

Second, it is not obviously wrong, especially not for the reason you gave. Currently, the best definition of a unit of time, the SI second, is based on atomic transitions (hyperfine splitting of cesium). That is fundamentally an EM process, so it is reasonable to say that time is defined in terms of light, from an experimental standpoint, and the definition is far from obviously wrong. In the case of the second, the motion of the light is not important, just it's frequency.

 

[Noyhcat]

Right, but there is still no such thing as the reference frame of light. In this light-clock exercise, we're comparing the reference frames of two observers, one of whom is at rest relative to the light-clock and one of whom is moving relative to the light-clock.

I agree. Bad choice of words on my part.