Are the transformations just observed ones or real ones?

[nitsuj]

SR is about physical phenomena interacting with human observation and experience, and that makes it also a theory of perception.
There are no known physical phenomena that would cause the universe to instantly contract because a spacecraft launched, or a bunch of particles are accelerated to .9c. There are however direct and indirect measurable effects implying td and lc on fast moving objects. It's reasonable to assign the change to the object (observer) as a result of its motion. This would explain why no one else perceives what the moving observer perceives. Compare this to someone on hallucinogenic drugs. Their images are real to them, but no one else experiences them, since they are confined to the mind. This analogy is why I use the term altered perception.

! It's a long thread to read it all to be sure of the context, that said SR isn't (directly) a theory of perception. In the least a theory that introduces time into geometric structure.

The physical phenomena is causation. The whole universe has contracted from this "fast moving object" perspective. It must for the sake of the fast moving object to see the "happenings" within the universe in the same order as everyone else regardless of comparative motion.

If the comparative time interval of 10 seconds for the fast moving object is much much longer then the "stationary" observer's, the fast moving object must see "happenings" across a much much shorter distance then the "stationary" observer. To the very specific point that both would measure c to have the same value.

It is comparatively altered geometry, not comparatively() altered perception.

 

[ghwellsjr]

Sorry for the confusion. I forget that what's easy for me may not be for someone else.
Following the convention S vs S', the primed (') values correspond to those for the observer in S', and the labels refer to both frame and observer. The light gray horizontal and vertical lines are for measurements.
In the left drawing the vertical line at x = 1.00 represents the fixed object location in the S frame.

I thought you said in post #36 that the stick was moving in the left drawing. Isn't that the only object in this scenario? What do you mean by "the fixed object location in the S frame"?

E, R, D, are events according to S. E', R', D', are events according to S'.

Ok, but what are the D' and R' events in the left drawing and what is the R event in the right drawing?

As S' moves along the x axis, he records the distance markers for each local event (E and D) he experiences.

I presume you mean he records 0 for event E and 0.75 for event D. What does he do with those numbers?

The arc is a simpler method than the hyperbola to indicate the time on the S' clock (follow the original values from the example).

Let me see if I got this right. To determine what time is on the S' clock at D in the left diagram, you draw a horizontal line from D to the left and a vertical line from D down to the bottom. Then you draw a segment of a circle from where the horizontal line intersect the S time axis to the right with the center at the origin. You stop when the segment encounters the previously drawn vertical line. From that point you draw another horizontal line to the left and read the value on the S time axis, in this case, 1.00. Correct?

Let's assume S is an absolute rest frame as drawn. That means the perception of S is not affected by time dilation (td) or length contraction (lc). What S observes is basic physical phenomena, including td and lc (via deformed em fields), both due to extended light paths and a constant c.

By "extended light paths", are you referring to the extension of the reflected light from R to D and continuing up to the S time axis at 2.00? If so, how does the observer at x=0 in S determine td and lc?

The right drawing is the S' perspective, with S moving to the left. Only the necessary elements are transformed from the left drawing. The object at x = 1.00 is now at x' = .80,...

This must be a different object than the 0.5 unit stick you originally mentioned. Is this a different 0.25 unit stick going from x=0.75 to x=1 in S and intersecting the t'=0 axis in S' from x'=0.6 to x'=0.8? Why no mention of the 0.5 unit stick? Isn't it shown by the two parallel lines in S and the two vertical lines in S' at x=0 and x=0.5?

...explained as a reciprocal effect of td for S'.

Why td and not lc?

The world of S' outside his frame is smaller by a factor of 1/γ = .80. If .60c was an absolute speed for S', then the magenta path would be the speed of light relative to S', and R the reflection event.

Are you making the point that R cannot be the refection event because the magenta lines are not at c?

Since only the relative speed can be measured, the time and location of R or R' is uncertain.

Are you making the point that this would be true if we didn't have SR?

The SR convention resolves this issue by defining the light paths as equal, resulting in the maximum spatial interval equal to γ*(D'-R interval in the S frame). The S' stick still measures .50 relative to his ruler.

But isn't the point that S measures the stick to be 0.4, just that he can't do it with only his ruler (because it is moving with respect to him) but S' can measure the stick with only his ruler because it is stationary with respect to him?

Thanks for helping me understand a little more but I'm afraid I still have a long way to go.

 

[phyti]

! It's a long thread to read it all to be sure of the context, that said SR isn't (directly) a theory of perception. In the least a theory that introduces time into geometric structure.

It's many things, thus 'also' a theory of perception. Time was revised from a universal value to an observer dependent value.

The physical phenomena is causation. The whole universe has contracted from this "fast moving object" perspective. It must for the sake of the fast moving object to see the "happenings" within the universe in the same order as everyone else regardless of comparative motion.

All moving observers do not see events in the same order.

If the comparative time interval of 10 seconds for the fast moving object is much much longer then the "stationary" observer's, the fast moving object must see "happenings" across a much much shorter distance then the "stationary" observer. To the very specific point that both would measure c to have the same value.

The fast moving observer sees more events in a shorter time interval, in support of his conclusion of the contracted universe.

It is comparatively altered geometry, not comparatively() altered perception.

The observer, including his mental analysis of sensory input, is modified by td and lc to the same extent as the frame he occupies.

 

[nitsuj]

All moving observers do not see events in the same order.

Who cares and what is the physical significance of non casually connected events. "Happenings" is referring to the specific event(s) of a cause and then effect; and not referring to RoS. I wouldn't retort with some point that has no physical significance so thought what I meant would be clear, specifically by not even using the word event, since that could be merely a position in spacetime.

 

[Sugdub]

Let me illustrate the concept of an inertial coordinate system, so that you can see what "velocity" and "length" relative to an inertial coordinate system mean...
Now, right outside the A train, on a parallel track, is another train, the B train...
This length is certainly NOT an illusion of any kind. It's dependent on a convention for synchronizing clocks and for measuring distances, etc. But given those conventions, the length of the B train relative to the frame of the A train is perfectly objective.

Thank you for proposing this thought experiment. I think there are similar ones in the litterature, which presentation is generally not so clear. Because yours is so straightforward it is easier for me to identify where exactly I fail to follow your reasonning.
Whatever the distance between both parallel tracks, "right outside" or far away, light signals are required between both ends of train B and observers located in train A, respectively, in order for your thought experiment looking "feasible".
Can you please elaborate (in simple words as you perfectly did for presenting this example) on the simultaneity criterion required for the reception events of both signals in train A and on the simultaneity criterion required for the emission events of the same signals in train B in order for them to be representative of the length of train B. Can both criteria be met concurrently if both light rays travel the same distance (parallel tracks)?
A positive answer to my question would imply that a potential observer at rest in train B (this is a slight add-on to the experiment you propose) would measure a different value for the length of train B as compared to those located in train A, this difference being due to their relative motion in respect to each other. I do think it is logically impossible that relative motion of its own triggers changes in observed phenomena. As already stated I believe SR is a wonderful theory but my criticisms relate to the way it is presented, which does not match the spirit of the original presentation made by Einstein in his 1905 paper.

 

[harrylin]

[..] I believe SR is a wonderful theory but my criticisms relate to the way it is presented, which does not match the spirit of the original presentation made by Einstein in his 1905 paper. [..].

I agree; and that's why several people here including myself present it similar to the way he did! He stayed away from making any metaphysical claims.
[edit: I should have added: when first presenting Special Relativity. He did engage in somewhat metaphysical discussions from about 1920 onwards, with such titles as "Ether and the Theory of Relativity"].

And note that I showed with my calculation example that just knowing the change in relative motion between objects is of its own not enough to predict the observed phenomena.

 

[stevendaryl]

Whatever the distance between both parallel tracks, "right outside" or far away, light signals are required between both ends of train B and observers located in train A, respectively, in order for your thought experiment looking "feasible".

Well, not necessarily light signals.

Can you please elaborate (in simple words as you perfectly did for presenting this example) on the simultaneity criterion required for the reception events of both signals in train A and on the simultaneity criterion required for the emission events of the same signals in train B in order for them to be representative of the length of train B. Can both criteria be met concurrently if both light rays travel the same distance (parallel tracks)?

Sure. Let e_1 be the emission of a light signal from the left end of the B-train. Let e_1' be the reception of that signal by car number n_1 of the A-train. Let e_2 be the emission of a light signal from the right end of the B-train. Let e_2' be the reception of that signal by car number n_2 of the A-train. The assumption, for the purposes of this thought experiment is that e_1 and e_1' have negligible separations in both space and time, and similarly e_2 and e_2'. So the pairs of events are approximately simultaneous in both frame A and frame B. The thought experiment is assuming that the distance between the tracks is negligible compared with the distance between two cars of either train.

But the simultaneity criterion for e_1 and e_1' is completely unconnected with the simultaneity criterion for e_1' and e_2'. The first depends on e_1 and e_1' being close together in space and time, while the latter depends on clock synchronizations.

A positive answer to my question would imply that a potential observer at rest in train B (this is a slight add-on to the experiment you propose) would measure a different value for the length of train B as compared to those located in train A,

Yes, that's definitely true. Or at least, that's the prediction of Special Relativity.

this difference being due to their relative motion in respect to each other. I do think it is logically impossible that relative motion of its own triggers changes in observed phenomena.

That way of putting things doesn't make any sense. Relative motion can't "trigger" anything, because it's not an event. Events trigger other events. If you want to talk about events causing things to happen, then the relevant event would be the acceleration or deceleration of one of the trains.

So you can imagine that initially both trains are moving in the same direction at the same velocity. Then later, the B-train changes speed (say by braking). A sudden change of speed will cause the cars of B to jerk and strain. You can't brake all points along train B simultaneously. If you tried to, it would be simultaneous according to one frame, but then it wouldn't be simultaneous according to a different frame. But if B is braking, then it is CHANGING frames, so there is no single frame to use. So braking will put stress on the B-train. After the braking stops, the stresses will go away, and the train will re-establish some equilibrium length. But there is absolutely no reason to think that this equilibrium length will be the same (as measured by the frame of the A-train) as it was before braking. SR predicts that it won't be.

But it's not that relative motion triggers a change of length--it's whatever actions put the train into relative motion that triggers a change of length.

As already stated I believe SR is a wonderful theory but my criticisms relate to the way it is presented, which does not match the spirit of the original presentation made by Einstein in his 1905 paper.

That's kind of a ridiculous thing to say. SR has been examined by physicists from more angles and from more different perspectives than just about any other theory of physics. There has been 100 years of thought experiments, paradoxes proposed and resolved, alternative derivations, alternative mathematical formulations, etc. If physicists are unwilling to hear your particular spin on SR criticism, it's because at some point, people have to make a judgment call as to what is worth spending more time on. At this point, SR is about as well-established as Euclidean geometry. Arguing about it is sometimes a good way for a student to learn, but it's not going to be of any benefit to working physicists at this point.

 

[stevendaryl]

I agree; and that's why several people here including myself present it similar to the way he did! He stayed away from making any metaphysical claims.
[edit: I should have added: when first presenting Special Relativity. He did engage in somewhat metaphysical discussions from about 1920 onwards, with such titles as "Ether and the Theory of Relativity"].

And note that I showed with my calculation example that just knowing the change in relative motion between objects is of its own not enough to predict the observed phenomena.

I think that at the time that Einstein wrote his paper on SR, he was under the influence of the positivists, and thought that all concepts of physics should be given operational definitions. He had difficulty maintaining his positivist stance when he turned to GR, because there did not seem to be any nice operational way to define a coordinate system for an observer in the presence of gravity.

 

[phyti]

I thought you said in post #36 that the stick was moving in the left drawing. Isn't that the only object in this scenario? What do you mean by "the fixed object location in the S frame"?

It's a random fixed object located at x = 1.00 in the S frame, for demonstration purposes. It could be a jar of pickles or the distance marker.

Ok, but what are the D' and R' events in the left drawing and what is the R event in the right drawing?

The events to the right are the same events to the left, just different perspectives.
At the left, D' and R' are locations according to S' as determined by his clock. They provide a comparison of where S' thinks he is relative to the locations S assigns for S', D and R.

As S' moves along the x axis, he records the distance markers for each local event (E and D) he experiences.

Yes he records 0 and .75 as x values. He uses the corresponding t' values 0 and 1.00 to calculate the t' value for R', being 1.00/2 = .50. By symmetry x' = ct' = .50.
Now to the .75 distance marker which is fixed in the S frame. By his clock, S' records 1.00 at D when at the .75 marker, yet he calculates distance traveled as .6c*1.00 = .60. How does he reconcile this mismatch? If he cannot detect his frame contraction or his time dilation, since he is also effected by both, he concludes the world outside his frame is length contracted.

Let me see if I got this right. To determine what time is on the S' clock at D in the left diagram, you draw a horizontal line from D to the left and a vertical line from D to the x axis. Then you draw a segment of a circle from where the horizontal line intersect the S time axis to the right with the center at the origin. You stop when the segment encounters the previously drawn vertical line. From that point you draw another horizontal line to the left and read the value on the S time axis, in this case, 1.00. Correct?

Correct (with a minor revision, underlined)j. Make it more specific, as you keep encouraging us to do.

By "extended light paths", are you referring to the extension of the reflected light from R to D and continuing up to the S time axis at 2.00? If so, how does the observer at x=0 in S determine td and lc?

If the stick was at rest in S, light would require 1.00 S time for the round trip. He observes the stick, which has contracted during its acceleration, prior to t = 0. S measures .4 for the stick length using; the radar method shown, simultaneous clock readings on the x axis, or the time for the stick to pass a given position.
Since clocks are frequencies, S and S' observe equal doppler shifts for the other clock.
Extended light paths result from motion of the target object. The light has to compensate for the motion of S'. More time is required on the outbound path, and less time on the inbound path, with the increase always greater than the decrease, i.e. a net increase of time.

I'll finish the response for the rest later (to keep them short).

 

[phyti]

I agree; and that's why several people here including myself present it similar to the way he did! He stayed away from making any metaphysical claims.
[edit: I should have added: when first presenting Special Relativity. He did engage in somewhat metaphysical discussions from about 1920 onwards, with such titles as "Ether and the Theory of Relativity"].

And note that I showed with my calculation example that just knowing the change in relative motion between objects is of its own not enough to predict the observed phenomena.

It's good to find another 'free thinker'. I agree with you, and yes you can demonstrate from a universal fixed frame, that the absolute speed determines lc and td. Despite the limitation on measuring an absolute speed, a relation can be established between relative speed and relative lc and td. It's not magic! In summation, an observer only measures the differences in speed, lc, and td. But that's the principle idea in 'relativity'. In fact all measurement is relative to a standard.

 

[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.

 

[Noyhcat]

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

The rethinking bit.

 

[Windows]

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.

You didn't get it. You define time as what you see, i.e, as the information you get from photons. Time is related to motion, photons do travel, so without time photons would not even travel. And the EM process is just photons, you again define time as the motion of photons which is incorrect.

 

[Dale]

You didn't get it. You define time as what you see, i.e, as the information you get from photons. Time is related to motion, photons do travel, so without time photons would not even travel. And the EM process is just photons, you again define time as the motion of photons which is incorrect.

Again, I didn't define time in this thread, and please don't presume to put words in my mouth, particularly not words that are so completely unrelated to anything I have ever or would ever say. If I were to define time I certainly wouldn't define it as "what I see" nor as "information I get from photons".

The definition I like is "time is the quantity referred to by the variable 't' in the standard physics formulas." This can be practically restated as "time is what a clock measures".

I wouldn't define time as "the motion of photons" because time is part of the strong and weak nuclear forces as well as gravity. Time is not exclusive to the EM force, and time passes even when there are no photons.

 

[Sugdub]

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: ...

Observing and measuring imply dealing with events which affect the observer and his/her measurement devices. Such events are co-located with the observer or with his/her devices. Hence a signal must bring some information there and the physical characteristics of its propagation must be taken into account. There is no such thing in your input, therefore I believe you are not actually dealing with observations and neither with measurements. This thought experiment will become clear once all references to “observers” or “someone” or “view” has been removed.
You are dealing with two theoretical representations of a thought experiment: one description (A) hooked on an inertial reference frame which is at rest in respect to the train; a second description (B) hooked on another inertial reference frame which is in relative motion in respect to the train. You are representing the same three events (emission, reflection and detection of a single light ray) by assigning to each event different coordinates in both reference frames. These are precisely the conditions under which the Lorentz transformation has been formally derived under the SR theory: it enables swapping from the coordinates of an event represented in frame A to the coordinates of the same event represented in frame B. SR deals with providing a continuous range of Lorentz-equivalent representations of the world (or the relevant subset of it) lying in the background of one single experiment. In any of these “representations of the world” time is dilated and lengths are contracted as compared to the “world” attached to frame A.
But these values should not be confused with the outcome of observations or measurements: the propagation of different signals towards an observer at rest in frame A and towards an observer at rest in frame B, respectively, must be applied to the aforementioned values in order to compute their respective “observed” or “measured” values.

I hope you could rework the text below the diagram you presented in this post since it is fully relevant to clarifying what is "real".

 

[harrylin]

You didn't get it. You define time as what you see, i.e, as the information you get from photons. Time is related to motion, photons do travel, so without time photons would not even travel. And the EM process is just photons, you again define time as the motion of photons which is incorrect.

Again, that's putting things upside down. Our time concept is based on motion and measured with clocks - even light clocks are possible. Clock frequency is a result of motion. No motion => no clocks and no concept of time possible.

 

[harrylin]

So my comment was correct? [..].

No. I explained that already in great detail in post #74 - but post #95 is pertinent for understanding that clock readings can depend on motion. And the lightclock illustration in post #79 is most useful to explain the concept.

 

[Noyhcat]

I hope you could rework the text below the diagram you presented in this post since it is fully relevant to clarifying what is "real".

If I get you correctly, I need to work on my terminology, and I don't disagree. I am consciously working to better this as I move forward.

You didn't get it. You define time as what you see, i.e, as the information you get from photons. Time is related to motion, photons do travel, so without time photons would not even travel. And the EM process is just photons, you again define time as the motion of photons which is incorrect.

I wonder if maybe a more real-world example would help...

I think of a GPS satellite, up in space. The engineers building it on Earth must purposely configure it's clock to move faster that what we normally see a clock running at. In other words, in the lab, before it's launched into space the satellite's clock is ticking at a faster rate than the clock on the wall in the same lab. This is by design.

Now they send the satellite up into space, and the clock, relative to us, slows down, as predicted by SR. If the engineers did their calculations right, the clock on the satellite now in orbit ticks at the same rate as the clock on the wall in the lab, relative to lab. In order for us on the ground to directly interact with the satellite now in orbit sensibly, we have to account for the actual time dilation that is going on.

Relative to the satellite, the clock on the wall in the lab is now ticking faster, but sure enough, it's ticking at the same rate as the satellite's clock. This is how we actually build satellites.

Time is not absolute. It is perceived differently by people moving relative to each other, but it behaves the same everywhere. Time does not appear to pass slower on the satellite to people on Earth because the light coming from it hits our eyes slower or later. Time appears to pass slower because it is passing slower, relative to us.

 

[harrylin]

A few little corrections:

[..]
I wonder if maybe a more real-world example would help...

I think of a GPS satellite, up in space. The engineers building it on Earth must purposely configure it's clock to move faster that what we normally see a clock running at. In other words, in the lab, before it's launched into space the satellite's clock is ticking at a faster rate than the clock on the wall in the same lab. This is by design.

Now they send the satellite up into space, and the clock, relative to us, slows down, as predicted by SR.

The clock must be made to tick at a slower rate to compensate for the combined effects of speed and gravitation as predicited by GR. See: https://en.wikipedia.org/wiki/Error...sitioning_System#Calculation_of_time_dilation

If the engineers did their calculations right, the clock on the satellite now in orbit ticks at the same rate as the clock on the wall in the lab, relative to lab. In order for us on the ground to directly interact with the satellite now in orbit sensibly, we have to account for the actual time dilation that is going on.

Relative to the satellite, the clock on the wall in the lab is now ticking faster, but sure enough, it's ticking at the same rate as the satellite's clock. This is how we actually build satellites.

Time is not absolute. It is perceived differently by people moving relative to each other, but it behaves the same everywhere. Time does not appear to pass slower on the satellite to people on Earth because the light coming from it hits our eyes slower or later. [..].

"Relative to the satellite" doesn't mean much: the satellite isn't even nearly in rest in any inertial frame (and I did not copy your last sentence which I could not parse).
[addendum: and the clock on the wall uses the ECI frame]

 

[yuiop]

... "Relative to the satellite" doesn't mean much: the satellite isn't even nearly in rest in any inertial frame ...

I think you are being a little bit picky here. The satellite is moving inertially, in so much as it does not experience proper acceleration and it is moving along a geodesic. The spacetime local to the satellite is almost Minskowkian. However I would agree that the clock on the Earth's surface that it being compared with, is not at rest in a inertial reference frame as it experiences proper acceleration. The difference in altitude between the two clocks in a gravitational field, excludes it from being a purely SR situation.

I think the spirit of the OP is about the physical significance of measurements made between two purely inertial reference frames, where the measurements are exactly symmetrical, so I agree that the satellite example does not fit in very well with that premise.

 

[harrylin]

I think you are being a little bit picky here. The satellite is moving inertially, in so much as it does not experience proper acceleration and it is moving along a geodesic. The spacetime local to the satellite is almost Minskowkian. [..]
I think the spirit of the OP is about the physical significance of measurements made between two purely inertial reference frames, where the measurements are exactly symmetrical, so I agree that the satellite example does not fit in very well with that premise.

My reason for being a bit picky about that is that the comparison is non-local and includes "absolute" SR time dilation per each rotation (just like Einstein's SR clock scenario). With all mentioned caveats and the level of discussion it's perhaps better not to bring GPS in it, or otherwise to leave out all the details and just point out the main result: the clocks are offset before launch in order to tick approximately in synch in the ECI frame after launch; and the total effect can be calculated with the transformation equations (SR+GR).