Are the transformations just observed ones or real ones?

[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).

 

[Sugdub]

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.

Don't take me wrong. I "hope" you will clarify whether the thought experiment you presented actually deals with observers attached to different observation frames or with the representation of three events related to one light ray in two reference frames irrespective of any "observation" or "measurement" being performed.
Also, and if you are dealing with observations, there is no point in mentionning someone outside the train as opposed to on-board since the relevant criterion for SR is the relative motion of the observer in respect to the train (along the x axis), irrespective of the location.

 

[kaplan]

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.

Take a meter stick, and hold it so you're viewing it like this --. It's got a length of one meter (this way --), a height of maybe one cm (this way |), and a width of a few cm (into the page). Now rotate it 90 degrees so you're viewing it like this |. Now it's got a length of 1 cm, a height of one meter, and a width (into the page) of a few cm.

Did the meter stick "really" change? I'd say no, you're just describing it with respect to a new set of orientations. That's almost exactly analogous to time dilation and length contraction - they tell you how time and length transform when you use coordinates that are moving with respect to the original ones.

Of course these transformations have real consequences when two things interact - getting hit by a meter stick will hurt more or less depending on its orientation relative to you, and the traveling twin comes back younger.

 

[WannabeNewton]

The spacetime local to the satellite is almost Minskowkian.

Just for clarification, this has nothing to do with the satellite being locally inertial.

 

[yuiop]

Just for clarification, this has nothing to do with the satellite being locally inertial.

I guess so, because the spacetime is locally Minkowskian even for non inertial objects. Thanks for the pointer. For further clarification, would you agree that the satellite is locally inertial and the clock on the ground is not?

 

[WannabeNewton]

Certainly yes I would agree with that.