Why speed of light is measured same regardless of their speed?

[Michamus]

Can you quote what I said that led you to believe that I was applying a Universal FOR?

I'll quote the statement you made following this.

Since the speed of light is less than 300,000 km/s, I would have to say your explanation involving objects traveling at 1 million km/s and 500,000 km/s is gibberish as well as this statement:

In this statement, you're applying a Universal FOR, in that you are stating C is some sort of speed limit. It isn't that way at all, because regardless your velocity, C is always C. You can be traveling at 1 million km/s and light will still travel at C. However, time dilation (Lorentz Factor) will ensure that you never arrive to a location any faster than (or the same speed as) C to any other observer. So, the statement "you can't travel faster than light" is like saying "you can't travel faster than your nose".

Maybe you should start using the very common method of specifying speeds as a fraction of c to avoid making such obvious blunders. There is even a special symbol, β, to refer to speeds as a fraction of c.

Again, as I stated, that equation is dedicated to the observer's frame. If two observers have two different frames, they can use that equation to calculate a third frame as being .99C while traveling at different velocities in excess of 300,000 km/s. As such, an observer "C" would see object "A" and "B" traveling at .99c even if object "A" were traveling 1 LY/s and object "B" were traveling .5 LY/s. The Lorentz Factor would dictate that Object "A" would experience 1 second for every 1 year of Object "C" and Object "B" would experience 2 seconds for every 1 year of Object "C".

 

[vela]

In this statement, you're applying a Universal FOR, in that you are stating C is some sort of speed limit. It isn't that way at all, because regardless your velocity, C is always C. You can be traveling at 1 million km/s and light will still travel at C. However, time dilation (Lorentz Factor) will ensure that you never arrive to a location any faster than (or the same speed as) C to any other observer. So, the statement "you can't travel faster than light" is like saying "you can't travel faster than your nose".

So if you pass me going at 1 million km/s at the same time I send out a flash of light in the same direction you're moving, you're claiming that the light will reach a point 1 million km away in less than the second it would take you to get there? How is that supposed to work?

 

[Michamus]

So if you pass me going at 1 million km/s at the same time I send out a flash of light in the same direction you're moving, you're claiming that the light will reach a point 1 million km away in less than the second it would take you to get there? How is that supposed to work?

From your FOR nothing can exceed light-speed (c), but let's suppose an object is traveling in the same trajectory as your intended light-beam.

You would see the object traveling near c in the same trajectory as your light-beam. As such, the object would appear to you to be slowly outpaced by the light-beam. Now, here's where things get interesting; From the FOR of the object, that same light-beam would propagate ahead of it at c. This means that from your FOR the object would be nearly at pace with the light-beam, but from the object's FOR, the light-beam would propagate ahead of it at c, regardless the object's velocity.

SR debunked the myth of a Universal FOR and replaced it with a relative FOR that assumes you are always at rest and everything else is moving.

 

[WannabeNewton]

SR debunked the myth of a Universal FOR and replaced it with a relative FOR that assumes you are always at rest and everything else is moving.

This has nothing to do with SR; I have no idea where this crazy misconception originated. This concept is taken advantage of over and over when solving various first year mechanics problems (e.g. sphere rolling down accelerating incline) and if you're talking about SR then surely you've seen this in Newtonian mechanics tons of times.

 

[vela]

From your FOR nothing can exceed light-speed (c), but let's suppose an object is traveling in the same trajectory as your intended light-beam.

So how can you be traveling at 1 million km/s? Your claim is that you're moving at 1 million km/s. That has to be relative to something and it might as well be me. So which is it? You can or can't move faster than $c$?

One of ghwellsjr's points is that as soon as you try to use an example where you claim to be moving faster than $c$ right from the start, it's pointless to go on because you're talking about a physically impossible situation.

 

[Michamus]

This has nothing to do with SR; I have no idea where this crazy misconception originated. This concept is taken advantage of over and over when solving various first year mechanics problems (e.g. sphere rolling down accelerating incline) and if you're talking about SR then surely you've seen this in Newtonian mechanics tons of times.

Again, you're applying a Universal FOR, in that you're assuming there's some Universal viewer that decrees when something is approaching light-speed. This simply isn't so. You'll never see anything move faster than light. However points in the Universe can move to any object at any velocity. That is, Alpha Centauri can move to me in 4 seconds, if I were able to withstand the acceleration and supply the energy required to do such a thing. However, my 4 seconds of flight would appear to have taken 4 years to any bystander on Earth. This is why time dilation (Lorentz Factor) exists.

So how can you be traveling at 1 million km/s? Your claim is that you're moving at 1 million km/s. That has to be relative to something and it might as well be me. So which is it? You can or can't move faster than $c$?

The representation was to show that two objects can be traveling at two different velocities altogether, yet still appear to be .99c to a third observer. I even simplified my statement as follows:
Object A's destination is moving toward it at 1 LY/s
Object B's destination is moving toward it at 2 LY/s
Object C sees both Object A and Object B at traveling .99c, due to Lorentz factor we can calculate that Object A experiences 1 second for every 1 year experienced by Object C. Object B experiences 2 seconds for every 1 year experienced by Object C.

One of ghwellsjr's points is that as soon as you try to use an example where you claim to be moving faster than $c$ right from the start, it's pointless to go on because you're talking about a physically impossible situation.

No object can move faster than c, because c is always c. Regardless how quickly the Universe appears to be moving, light will always propagate at c.

In fact, using your faulty conception of relativity, I could achieve superior speeds using fixed FORs. That is, I could have a spacecraft traveling at 299,000 km/s that launches a whole new spacecraft . This spacecraft can then accelerate to 299,000 km/s relative to it's mother craft. The mother craft would see itself at rest, with it's child craft traveling at 299,000 km/s. To a third observer, both crafts would be traveling at .99c.

 

[WannabeNewton]

Again, you're applying a Universal FOR, in that you're assuming there's some Universal viewer that decrees when something is approaching light-speed. This simply isn't so. You'll never see anything move faster than light. However points in the Universe can move to any object at any velocity. That is, Alpha Centauri can move to me in 4 seconds, if I were able to withstand the acceleration and supply the energy required to do such a thing. However, my 4 seconds of flight would appear to have taken 4 years to any bystander on Earth. This is why time dilation (Lorentz Factor) exists.

What in the world does this have to do with your extremely erroneous statement that the notion of relative motion was only introduced in SR? At this point, all I see is a random mess of physics terms.

 

[rbj]

Mich, once you start saying something concrete, then there are concrete problems that can be pointed out.

However points in the Universe can move to any object at any velocity.

hypothetical "points" are sort of meaningless. sure, i can imagine a hypothetical point right next to me and then i can imagine translating that hypothetical point 4 LY away in 4 seconds, but it's just a abstract coordinate. it is nothing of physical consequence. heck, then i'll imagine translating that point to the Andromeda galaxy. big deal.

That is, Alpha Centauri can move to me in 4 seconds, if I were able to withstand the acceleration and supply the energy required to do such a thing.

that statement is concrete and is false.

 

[Doc Al]

Again, you're applying a Universal FOR, in that you're assuming there's some Universal viewer that decrees when something is approaching light-speed. This simply isn't so. You'll never see anything move faster than light. However points in the Universe can move to any object at any velocity. That is, Alpha Centauri can move to me in 4 seconds, if I were able to withstand the acceleration and supply the energy required to do such a thing. However, my 4 seconds of flight would appear to have taken 4 years to any bystander on Earth. This is why time dilation (Lorentz Factor) exists.

You are confused. What I assume you meant to say is that a spaceship going fast enough can cover the distance from Alpha Centauri to Earth in 4 seconds as measured by that ship's clock. That's true. If you were in the ship, the travel time would be 4 seconds; if you were on Earth, the travel time would be around 4 years. Nothing here is traveling faster than light speed, unless you foolishly combine distance and time measurements from different frames. (A common freshman error.)

In fact, using your faulty conception of relativity, I could achieve superior speeds using fixed FORs. That is, I could have a spacecraft traveling at 299,000 km/s that launches a whole new spacecraft . This spacecraft can then accelerate to 299,000 km/s relative to it's mother craft. The mother craft would see itself at rest, with it's child craft traveling at 299,000 km/s. To a third observer, both crafts would be traveling at .99c.

Please explain how that process, which is perfectly legitimate as a thought experiment, leads to "superior speeds" (by which I assume you mean speeds greater than c). It will not.

 

[Michamus]

You are confused. What I assume you meant to say is that a spaceship going fast enough can cover the distance from Alpha Centauri to Earth in 4 seconds as measured by that ship's clock. That's true. If you were in the ship, the travel time would be 4 seconds; if you were on Earth, the travel time would be around 4 years. Nothing here is traveling faster than light speed, unless you foolishly combine distance and time measurements from different frames. (A common freshman error.)


Please explain how that process, which is perfectly legitimate as a thought experiment, leads to "superior speeds" (by which I assume you mean speeds greater than c). It will not.

I don't know how much more often I can state "nothing travels faster than c to any other frame". The part I made bold is almost identical to this statement I made: "However, my 4 seconds of flight would appear to have taken 4 years to any bystander on Earth.".

I've not once claimed that anything would ever occur faster than c to any other frame.

that statement is concrete and is false.

There is absolutely no reason I can't do what I said. Sure, my clock will read 4 seconds, but to someone on Earth, it will take me 4 years. There's really no limit as to the velocities you can achieve in your own FOR, because you're not really moving at all in your FOR, everything else is, which is why light is c regardless your frame.

 

[Doc Al]

I don't know how much more often I can state "nothing travels faster than c to any other frame". The part I made bold is almost identical to this statement I made: "However, my 4 seconds of flight would appear to have taken 4 years to any bystander on Earth.".

I've not once claimed that anything would ever occur faster than c to any other frame.

What do you mean by any other frame? There is no frame in which anything travels faster than c.

There is absolutely no reason I can't do what I said. Sure, my clock will read 4 seconds, but to someone on Earth, it will take me 4 years. There's really no limit as to the velocities you can achieve in your own FOR, because you're not really moving at all in your FOR, everything else is, which is why light is c regardless your frame.

With respect to yourself, your speed is of course zero. With respect to you, the speed of anything else never exceeds c.

Realize that when you are in that spaceship traveling at near light speed (with respect to the Earth) from Alpha Centauri to Earth, that the distance between the two is much shorter in your frame of reference. That's why you can cover the distance in only 4 seconds, even though your speed is less than light speed.

 

[vela]

I've not once claimed that anything would ever occur faster than c to any other frame.

Sure you have:

For your experiment though, let's suppose a spacecraft is orbiting the sun at 1 million km/s at a distance roughly 150 million km.

There is absolutely no reason I can't do what I said. Sure, my clock will read 4 seconds, but to someone on Earth, it will take me 4 years. There's really no limit as to the velocities you can achieve in your own FOR, because you're not really moving at all in your FOR, everything else is, which is why light is c regardless your frame.

Is the following your logic? An observer at rest on Earth would say the distance to Alpha Centauri is about 4 ly. You, moving near the speed of light relative to said observer to Alpha Centauri, find it takes you 4 seconds to reach your destination from Earth. Are you claiming you moved at 1 ly/s because you went 4 ly in 4 second? If so, you're making the "common freshman error" Doc Al mentioned a few posts earlier. Dividing the distance measured by the observer at rest on Earth by the time measured by you, the moving observer, is meaningless.

You should keep in mind that most people who have posted in this thread are well versed in the basics of special relativity. We know how time dilation, length contraction, and the velocity-addition work. You now have four of these people pointing out that your statements are misleading or just plain wrong. You might want to entertain the notion that perhaps the misconception lies with you, not everyone else.

 

[ghwellsjr]

Realize that when you are in that spaceship traveling at near light speed (with respect to the Earth) from Alpha Centauri to Earth, that the distance between the two is much shorter in your frame of reference. That's why you can cover the distance in only 4 seconds, even though your speed is less than light speed.

I already pointed out that the distance between the Sun and the Earth is 1/7 in the rest frame of a pilot traveling at 0.99c with respect to the Sun-Earth rest frame:

So the distance between the Sun and the Earth is different in the rest frame of a traveler than it is in the rest frame of the Sun-Earth. In fact it is Length Contracted by the factor of 1/gamma which in this case is about 1/7 and we can see that our answer of 70.6 seconds is about 1/7 of 6 minutes and 19 seconds.

and he called it:

A long rant of gibberish

Why do you think he's going to believe you?

 

[Michamus]

What do you mean by any other frame? There is no frame in which anything travels faster than c.


With respect to yourself, your speed is of course zero. With respect to you, the speed of anything else never exceeds c.

Realize that when you are in that spaceship traveling at near light speed (with respect to the Earth) from Alpha Centauri to Earth, that the distance between the two is much shorter in your frame of reference. That's why you can cover the distance in only 4 seconds, even though your speed is less than light speed.

That's a better way of wording it than I've done so far.

 

[Boy@n]

So the distance between the Sun and the Earth is different in the rest frame of a traveler than it is in the rest frame of the Sun-Earth. In fact it is Length Contracted by the factor of 1/gamma which in this case is about 1/7 and we can see that our answer of 70.6 seconds is about 1/7 of 6 minutes and 19 seconds.

I thank you very much for taking the time to answer me with all the details. (especially post #22)

I am still trying to understand it all, so pardon my ignorance if/as it arises.

What I don't get is how can light from Sun to Earth take 499 seconds or 70.6 seconds depending on FOR, when the physical distance is always the same (if we ignore it orbits the Sun) and light travels at constant C speed?

Space (distance) between Sun and Earth doesn't physically contract for real, right? So, what happens is that time dilates for traveller and to him it just appears that light from Sun to Earth took 70.6s because his clock runs slower than the clock on Earth?

On the other hand, if distance for traveler really shortens to 1/7 when he travels at 0.99c it appears as if Universe changes for him... Plus, if he were to slow down and travel that path again the distance would increase by 7 times for him, right?

So, are there infinite number of distances between Sun and Earth depending on FOR?

The faster you travel through space the slower your clock ticks relative to observer on Earth, right? And at the speed of light the clock stops ticking and the distance between any objects (from photon viewpoint) becomes zero?

 

[ghwellsjr]

So the distance between the Sun and the Earth is different in the rest frame of a traveler than it is in the rest frame of the Sun-Earth. In fact it is Length Contracted by the factor of 1/gamma which in this case is about 1/7 and we can see that our answer of 70.6 seconds is about 1/7 of 6 minutes and 19 seconds.

I thank you very much for taking the time to answer me with all the details. (especially post #22)

I am still trying to understand it all, so pardon my ignorance if/as it arises.

What I don't get is how can light from Sun to Earth take 499 seconds or 70.6 seconds depending on FOR, when the physical distance is always the same (if we ignore it orbits the Sun) and light travels at constant C speed?

Even if we assume that the physical distance between the Sun and Earth is always the same, how do we know what that physical distance is? Prior to Einstein and his postulate that light propagates at c in all directions in any IRF, scientists had concluded that the physical distance between the Sun and Earth was already contracted because they assumed that the solar system itself must be traveling with respect to some presumed absolute IRF and how could you prove them wrong? Doesn't it make sense, if you want to declare that there is only one correct constant distance between the Sun and Earth that you should make your best assessment as to the motion of the solar system?

Space (distance) between Sun and Earth doesn't physically contract for real, right?

Well, your spaceship traveling at 0.99c from the Sun to the Earth does physically contract in the Sun-Earth rest frame compared to its length before and after the trip. So if we can think about the formation of the solar system and imagine that it got thrust away from some starting point in which it was at rest, then we would have to say that along its direction of motion, it is physically contracted.

So, what happens is that time dilates for traveller and to him it just appears that light from Sun to Earth took 70.6s because his clock runs slower than the clock on Earth?

Let's suppose that the solar system is traveling at 0.99c from a prior state of formation so that it is experiencing time dilation as well as the spaceship prior to its trip and then the spaceship starts traveling in the opposite direction at 0.99c so that it is now at rest in that prior state, wouldn't you have to say that its clock is running faster than the clock on the Earth?

On the other hand, if distance for traveler really shortens to 1/7 when he travels at 0.99c it appears as if Universe changes for him...

Assuming the previous supposition, we would have to say that the length of the spaceship and the Sun-Earth distance were already 1/7th and then during the trip, the spaceship goes back to normal. (Remember, we are talking about the IRF prior to the formation of the solar system.)

Plus, if he were to slow down and travel that path again the distance would increase by 7 times for him, right?

In the supposition that we are now considering, when he stops, his length goes back to 1/7 just like before he left.

So, are there infinite number of distances between Sun and Earth depending on FOR?

I hope you are seeing that the problem is that we cannot identify a physically real distance between the Sun and Earth nor a physically real rate of time. Or to put it another way, nature won't reveal to us the answer to that problem.

So Einstein's brilliant idea was that if nature won't do it for us, we'll do it our self. If nature won't disclose to us the state in which light propagates at c in all directions, we'll make up our own answer. And that answer is, we will merely assume that light propagates at c in any IRF we choose and we'll use that to define distances and times throughout that IRF. But, we can only use those definitions in one IRF. When we do this for the Sun-Earth rest frame, we don't care if it had a prior history of motion due to its formation. We could also do it for that prior state of rest in which case we wouldn't care about it current state of motion. It's important to stick to anyone IRF of our choosing and not to mix definitions from multiple IRFs. We can always use the Lorentz Transformation process to see what those definitions look like in another IRF but we don't want to say that the universe physically changes dimensions every time we change our chosen IRF. But we do want to say that when objects/observers/clocks change their motion, their dimensions really do change according to the definitions assigned by the chosen IRF and thus their measurements of things moving with respect to them will change as a result.

The faster you travel through space the slower your clock ticks relative to observer on Earth, right?

Not quite right: the faster you travel relative to the IRF in which the Earth is at rest, not relative to an observer. Inertial observers can make measurements consistent with the assumption of the constant speed of light propagation relative to them and derive the same Time Dilation and Length Contraction that is defined by the IRF, but they cannot do this in real time. That's what the radar measurement does as I described in post #22.

And at the speed of light the clock stops ticking and the distance between any objects (from photon viewpoint) becomes zero?

No clock can go the speed of light so it doesn't make sense to say what happens to a clock at the speed of light. No object can go the speed of light so it doesn't make sense to talk about what happens to objects at the speed of light. Photons cannot have a viewpoint so it doesn't make sense to talk about a photon viewpoint. However, we can get as close as we want to the speed of light but the numbers get very difficult to handle (so many nine's).

 

[Boy@n]

No clock can go the speed of light so it doesn't make sense to say what happens to a clock at the speed of light. No object can go the speed of light so it doesn't make sense to talk about what happens to objects at the speed of light. Photons cannot have a viewpoint so it doesn't make sense to talk about a photon viewpoint. However, we can get as close as we want to the speed of light but the numbers get very difficult to handle (so many nine's).

Thanks for another informative reply.

Not sure yet how to imagine 'physical reality' with keeping in mind that physical 3D space changes depending on own speed relative to speed of C.

As it's my bed time just one quick comment/question: but it is possible for a particle, say electron, to move at C speed or faster, if it starts moving at that speed, right? (Probably we didn't observe that and maybe never will, but theoretically it is possible?)

What I meant with the clock and photon is of course not that I think a photon can carry a clock ;-)... But that at speed of C, time for photon doesn't exist, and is thus everlasting. But it also seems to mean that photon 'knows' no distances?

Good night.

 

[ghwellsjr]

No clock can go the speed of light so it doesn't make sense to say what happens to a clock at the speed of light. No object can go the speed of light so it doesn't make sense to talk about what happens to objects at the speed of light. Photons cannot have a viewpoint so it doesn't make sense to talk about a photon viewpoint. However, we can get as close as we want to the speed of light but the numbers get very difficult to handle (so many nine's).

Thanks for another informative reply.

You're very welcome.

Not sure yet how to imagine 'physical reality' with keeping in mind that physical 3D space changes depending on own speed relative to speed of C.

I never said that 3D space changes depending on own speed or that it is relative to the speed of c. Nothing is relative to the speed of light.

Let me say it again. We define the speed of light to be c relative to any arbitrarily chosen IRF. We defined time and distances according to that same IRF using the speed of the propagation of light in the one IRF. We aren't changing space or time. We are defining our coordinates so that we can make meaningful assessment and measurements of 3D space and time according to our definition. Meanings come from definitions. That's all this is about. I did also say that objects/observers/clocks physically change when they change their motion and they can make measurements that are consistent with the IRF in which the speed of light is c by also assuming that it is c when they make their measurements.

As it's my bed time just one quick comment/question: but it is possible for a particle, say electron, to move at C speed or faster, if it starts moving at that speed, right? (Probably we didn't observe that and maybe never will, but theoretically it is possible?)

No, an electron is an object so it cannot go at c or faster. It cannot start moving at c. We cannot transform an IRF to a second IRF moving at c or faster. Theoretically it is not possible.

What I meant with the clock and photon is of course not that I think a photon can carry a clock ;-)... But that at speed of C, time for photon doesn't exist, and is thus everlasting. But it also seems to mean that photon 'knows' no distances?

Good night.

It's redundant to say "at speed of c" when talking about a photon--it is defined to travel at c but you are correct, the proper way to say it is that time doesn't exist for a photon but it is not correct to say that it is everlasting--that is just another statement about time for a photon which you said doesn't exist for a photon.

Same thing for distances for a photon--distance doesn't apply for a photon and it's not because it doesn't "know" anything.

 

[Popper]

Can someone please explain me why speed of light is measured same regardless of their speed?
Will not a person moving with 0.6c measure speed of light as 0.4c?

As explained above, no. I'd like to phrase this in my own way in addition to how it was explained above.

Maxwell's equations describe the laws of electrodynamics. They are postulates, i.e. laws of nature. That means that we assume that they are true at all times and in all places. They have been verified by (i.e. are consistent with) countless experiments. They describe light as waves/disturbances in the EM field as moving at a finite speed, c = 1/sqrt(epsilon_0*mu_0). The first postulate of the special theory of relativity states that the laws of nature (including Maxwell's equations) are valid in all inertial frames of reference, i.e. are covariant. The second postulate states that the speed of light is the same in all inertial frames of reference. So Maxwell's equations postulate a finite speed and relativity postulates that it’s invariant with respect to a Lorentz transformations. This means that the speed of light has the same speed in all inertial frames of reference. In non-inertial frames, i.e. in the presence of a gravitational field, the speed is a function of the gravitational potentials.

 

[WannabeNewton]

George's post #12 was spot on, especially the 2nd paragraph. Maybe you should learn how Newtonian mechanics works before you start commenting on special relativity and start disgustingly insulting other members. Learn to put your money where your mouth is.

 

[WannabeNewton]

his or her measurements come out the same because his or her physics are the same because each, being in their own inertial frame of reference, are in indistinguishable situations. they are, in their own unaccelerated position, operationally at rest and it's the other observer who is moving.

(Bolded by me). You assert that the following statement "An inertial observer can claim he is at rest and that all other inertial observers are moving relative to him" IMPLIES "Laws of physics are the same in all inertial reference frames in special relativity". Since you were so adamant about it, prove, using just that first statement, that if Maxwell's equations hold in one inertial frame in special relativity then they hold in all inertial frames in special relativity.

 

[Boy@n]

Even if we assume that the physical distance between the Sun and Earth is always the same, how do we know what that physical distance is? Prior to Einstein and his postulate that light propagates at c in all directions in any IRF, scientists had concluded that the physical distance between the Sun and Earth was already contracted because they assumed that the solar system itself must be traveling with respect to some presumed absolute IRF and how could you prove them wrong?

So if we had some absolute frame of reference (so to say a viewpoint outside of Universe - please allow me this hypothetical situation, and let's say we have a rod of 1 meter for measuring distances anywhere in Universe), the physical space of our solar system would be different depending on two things:

1. Motion (e.g. if whole solar system moved at .1c or .5c the the distances between Sun and planets would be different? Smaller at .5c. So, 1 meter on Earth would always be 1 meter, no matter of solar system speed, but if measured with that absolute meter the distances would be different at different speeds, right?)

2. Gravitation (e.g. if we had a Sun 100 times more massive than our Sun today the distances between Sun and planets would be different? Smaller with more massive Sun?)

Doesn't it make sense, if you want to declare that there is only one correct constant distance between the Sun and Earth that you should make your best assessment as to the motion of the solar system?

It does, but the whole truth is then hidden to ignorant people as myself.

That distance is only constant if we observe it from planet Earth, right?

And it changes depending on own frame of reference?

I hope you are seeing that the problem is that we cannot identify a physically real distance between the Sun and Earth nor a physically real rate of time. Or to put it another way, nature won't reveal to us the answer to that problem.

So our understanding of our reality (distances, speeds, time) are all based on our (human) assumptions and conventions?

So Einstein's brilliant idea was that if nature won't do it for us, we'll do it our self. If nature won't disclose to us the state in which light propagates at c in all directions, we'll make up our own answer. And that answer is, we will merely assume that light propagates at c in any IRF we choose and we'll use that to define distances and times throughout that IRF.

But, we can only use those definitions in one IRF. When we do this for the Sun-Earth rest frame, we don't care if it had a prior history of motion due to its formation. We could also do it for that prior state of rest in which case we wouldn't care about it current state of motion. It's important to stick to anyone IRF of our choosing and not to mix definitions from multiple IRFs. We can always use the Lorentz Transformation process to see what those definitions look like in another IRF but we don't want to say that the universe physically changes dimensions every time we change our chosen IRF.

But we do want to say that when objects/observers/clocks change their motion, their dimensions really do change according to the definitions assigned by the chosen IRF and thus their measurements of things moving with respect to them will change as a result.

But what changes in truth? Distances? Time? Both? Neither, something else maybe?

Or nothing at all changes except our model of reality, or better to say, we adjust parameters (time, distances, speeds) in our model, so we can logically describe what is happening?

If that's the case (which I suspect it is), how do we know we have the right, or say, best possible, model (GR & SR)? OK, the model covers our experiments, observations, calculations and predictions (let's assume again) to perfection, and that's it?

We don't care to (or maybe we cannot?) find how the nature really functions, is it enough we have models which are practically useful? Maybe is.

In other words... We made models so nature fits into them nicely, we didn't discover models which really describe how nature functions?

I guess I am asking too much ;-) ...since, if we really knew how nature functions we could make 'new nature' ourselves (e.g. create a little universe in a lab.)

 

[Boy@n]

Let me say it again. We define the speed of light to be c relative to any arbitrarily chosen IRF. We defined time and distances according to that same IRF using the speed of the propagation of light in the one IRF. We aren't changing space or time. We are defining our coordinates so that we can make meaningful assessment and measurements of 3D space and time according to our definition. Meanings come from definitions. That's all this is about. I did also say that objects/observers/clocks physically change when they change their motion and they can make measurements that are consistent with the IRF in which the speed of light is c by also assuming that it is c when they make their measurements.

I am getting confused now. Is C phsically maximal possible speed for anything in motion in Universe or is it just our definition/convention?

No, an electron is an object so it cannot go at c or faster. It cannot start moving at c. We cannot transform an IRF to a second IRF moving at c or faster. Theoretically it is not possible.

You can not accelerate a massive object to C, but theory doesn't prevent possibility that it can start at C or at higher speed? (Not my idea, I read it somewhere long ago, just checking it out now.)

It's redundant to say "at speed of c" when talking about a photon--it is defined to travel at c but you are correct, the proper way to say it is that time doesn't exist for a photon but it is not correct to say that it is everlasting--that is just another statement about time for a photon which you said doesn't exist for a photon.

A photon can travel at different speeds, depending on a medium (vacuum, air, water, diamond), no? Thus, since time doesn't exist for a photon it also means it never ages, thus is everlasting, with which I mean, even if our Universe suffers a cold end photons will still go on and on for ever... well, while space-time exists, even if no matter would exist in it anymore, right?

Same thing for distances for a photon--distance doesn't apply for a photon and it's not because it doesn't "know" anything.

I like it how you worded it, distance and time doesn't apply for a photon. But that also kinda sounds strange, as if they are out of our reality of space, matter and energy where distances and time apply.

 

[Naty1]

I am getting confused now. Is C phsically maximal possible speed for anything in motion in Universe or is it just our definition/convention?

it IS the maximum possible vacuum speed...of energy,, of information, of waves, however you
want to state it...

You can not accelerate a massive object to C, but theory doesn't prevent possibility that it can start at C or at higher speed? (Not my idea, I read it somewhere long ago, just checking it out now.)

no such theory. An electron can never get to 'c'. Not even AT the big bang AFAIK.

A photon can travel at different speeds, depending on a medium (vacuum, air, water, diamond), no? Thus, since time doesn't exist for a photon it also means it never ages, thus is everlasting, with which I mean, even if our Universe suffers a cold end photons will still go on and on for ever... well, while space-time exists, even if no matter would exist in it anymore, right?

There is no reference frame applicable to a photon...no inertial frame to move with it. The interval of a photon is called a null interval because it is inherently different than either a timelike or a spacelike spacetime interval. We can neither measure it with either a clock or a ruler, neither can we characterize as either timelike or spacelike.

I saved this from a FAQ here:

One of the key axioms of special relativity is that light moves at c in all reference frames. The rest frame of a photon would require the photon to be at rest and moving at c . That of course is contradictory. In other words, the concept doesn't make sense.

I like it how you worded it, distance and time doesn't apply for a photon. But that also kinda sounds strange, as if they are out of our reality of space, matter and energy where distances and time apply.

It's a different reality than we are accustomed:

from another discussion in these forums:

https://www.physicsforums.com/showthread.php?p=4248714#post4248714

PeterDonis:
photon worldlines contain multiple events. You can't use proper time to label the events, but you can use other affine parameters; and the fact that you can't use proper time to label the events does *not* mean that "they all happen at the same time".

 

[Fredrik]

George's post #12 was spot on, especially the 2nd paragraph.

I have to disagree with this.

Relative to the light.

This should be "relative to the light source". But even if ghwellsjr had said that, it wouldn't have added anything to the discussion. Rbj didn't ask "relative to what?" because he wanted to know. He asked to remind the OP that he shouldn't have left that information out.

You're missing the whole point of the Principle of Relativity. It's not that each observer is at rest and so his measurements come out the same--it's that even when an observer is not at rest but traveling at some high rate of speed, his measurements still come out the same.

There's nothing wrong with the part of rbj's post that got this response. All ghwellsjr did here was to make a personal attack on rbj. This description of what the principle of relativity is about isn't any more accurate than what rbj said. I like rbj's version better than this one.

 

[WannabeNewton]

There's nothing wrong with the part of rbj's post that got this response.

Granted that exact paragraph written by rbj that was quoted was not saying anything wrong but rbj was making the mistake of saying relative motion was something special to SR / novel and that the bare concept of relative motion for inertial observers is what implies the laws of physics are the same in all reference frames which is certainly not true; there are more conditions that need to be assumed, otherwise Galilean relativity would leave Maxwell's equations invariant as well under Galilean boosts.

 

[ghwellsjr]

George's post #12 was spot on, especially the 2nd paragraph.

I have to disagree with this.

And I have to disagree with your disagreement and I'll show you why.

Relative to the light.

This should be "relative to the light source". But even if ghwellsjr had said that, it wouldn't have added anything to the discussion. Rbj didn't ask "relative to what?" because he wanted to know. He asked to remind the OP that he shouldn't have left that information out.

It was obvious to me and I was attempting to point out to rbj what adjacent meant in his OP:

Can someone please explain me why speed of light is measured same regardless of their speed?
Will not a person moving with 0.6c measure speed of light as 0.4c?

Isn't it obvious that he is considering the person to be moving at 0.6c relative to the light which is moving at 1.0c and wondering why the difference of 0.4c isn't correct?

You're missing the whole point of the Principle of Relativity. It's not that each observer is at rest and so his measurements come out the same--it's that even when an observer is not at rest but traveling at some high rate of speed, his measurements still come out the same.

There's nothing wrong with the part of rbj's post that got this response. All ghwellsjr did here was to make a personal attack on rbj. This description of what the principle of relativity is about isn't any more accurate than what rbj said. I like rbj's version better than this one.

Yes there is something wrong with rbj's post. He expressed over and over again that having an acceleration of zero leads to a velocity of zero which is not true. Instead it leads to any constant velocity of which zero is just one of an infinite number of valid answers. The integral of acceleration is velocity and like all integrals, there's always that "plus any arbitrary constant value (+C)" tacked on.

Here's a spacetime diagram illustrating how a person that is at rest in an Inertial Reference Frame measures the speed of light to be c (defined for our purpose here to be 1 foot per nanosecond):

The "person" is shown in blue with each nanosecond of time marked off with dots. He has placed a mirror a measured six feet away shown in red. At his time of 4 nanoseconds, he sends off a flash of green light which travels at c toward the mirror and reflects off of it at coordinate time of 10 nanoseconds and arrives back at the "person" at his time of 16 nanoseconds. The "person" measures that the round-trip time for the light took 12 nanoseconds and so he calculates the speed of light to be one-half of 12 nanoseconds divided into 6 feet or 1 foot per nanosecond.

Now we use the Lorentz Transformation process to create an IRF moving at -0.6c to see the scenario that adjacent described in his OP. Now the "person" is traveling at 0.6c toward the flash of light which is traveling at 1.0c. He's wondering why the "person" won't measure the speed of the light to be traveling at 0.4c.

Now you can see that the difference between his speed and the speed of the light as it is moving away from him is 0.4c but he can't measure that by itself because he can't know when the light hits the mirror. Instead, he does a round-trip measurement that turns out to be identical as in the first IRF. And that illustrates what adjacent was asking about.

 

[Fredrik]

Granted that exact paragraph written by rbj that was quoted was not saying anything wrong but rbj was making the mistake of saying relative motion was something special to SR / novel and that the bare concept of relative motion for inertial observers is what implies the laws of physics are the same in all reference frames which is certainly not true; there are more conditions that need to be assumed, otherwise Galilean relativity would leave Maxwell's equations invariant as well under Galilean boosts.

I can't interpret what rbj said in that way.

his or her measurements come out the same because his or her physics are the same because each, being in their own inertial frame of reference, are in indistinguishable situations. they are, in their own unaccelerated position, operationally at rest and it's the other observer who is moving.

You assert that the following statement "An inertial observer can claim he is at rest and that all other inertial observers are moving relative to him" IMPLIES "Laws of physics are the same in all inertial reference frames in special relativity".

He says that each since all inertial observers are in indistinguishable situations, their measurements will come out the same. To say that the measurements will come out the same, is just another way of saying that their situations are indistinguishable. So the statement can be shortened to "all inertial observers are in indistinguishable situations". This is just a statement of the principle of relativity, not a statement of some crazy implication.

Even your paraphrased version of the statement, which was meant to look crazy, just looks like "PoR $\Rightarrow$ PoR" when I think about what it means. Even a non-inertial observer can claim to be at rest, so I would assume that the reason you're mentioning inertial observers is that they have a good reason to claim that they're at rest. The only good reason I can think of is that they will all be indistinguishable in some sense. So even in this case, the antecedent of the implication seems to be the principle relativity.

 

[WannabeNewton]

He says the laws of physics / measurements are the same in all inertial reference frames because anyone inertial observer can claim he/she is at rest and that the other is moving. How does the latter imply the former? In Newtonian mechanics, I can claim the exact same thing i.e. that I am at rest and the other observers are moving but this obviously does not immediately imply the laws of physics are the same in all inertial reference frames.

 

[Fredrik]

Isn't it obvious that he is considering the person to be moving at 0.6c relative to the light which is moving at 1.0c and wondering why the difference of 0.4c isn't correct?

OK, I see that this interpretation is possible too. But until now I thought it was obvious that the OP was talking about a person moving at 0.6c relative to the light source (in the same direction as the light) and wondering why this person will not assign velocity 1-0.6=0.4 to the light. This interpretation makes a lot more sense, since "relative to the light" is nonsense in SR. (I understand that the OP may not know that).

Yes there is something wrong with rbj's post. He expressed over and over again that having an acceleration of zero leads to a velocity of zero which is not true. Instead it leads to any constant velocity of which zero is just one of an infinite number of valid answers.

An acceleration of zero makes the observer an inertial observer, and an inertial observer has velocity 0 in the inertial coordinate system associated with his own motion. That's what rbj pointed out, and then you came along and pointed out that an inertial observer has a non-zero velocity in other inertial coordinate systems. Since this doesn't in any way disagree with what rbj said, it was a very bad time to claim that he was "missing the whole point of the principle of relativity". rbj definitely went too far with his insults later in the thread, but I have to agree with him that you misrepresented his position.

Here's a spacetime diagram illustrating how a person that is at rest in an Inertial Reference Frame measures the speed of light to be c (defined for our purpose here to be 1 foot per nanosecond):

I haven't participated in the discussion about the OP's question, and I don't think I will. I don't doubt that you are competent enough to handle that.

 

[Fredrik]

He says the laws of physics / measurements are the same in all inertial reference frames because anyone inertial observer can claim he/she is at rest and that the other is moving. How does the latter imply the former? In Newtonian mechanics, I can claim the exact same thing i.e. that I am at rest and the other observers are moving but this obviously does not immediately imply the laws of physics are the same in all inertial reference frames.

As I said, even your distorted version of his statement can be interpreted as "PoR $\Rightarrow$ PoR". But it's clearer in the original statement. He says that the indistinguishability of the situations implies that the physics is the same, and that this implies that measurements are the same.
$$\text{Indistinguishability}\Rightarrow\text{Physics}\Rightarrow\text{Measurements}.$$ The implications holds because all three statements mean the same thing.

 

[WannabeNewton]

What definition of implication is being used here? To use the same example, I can take two inertial observers in Newtonian physics, related by a Galilean transformation. One will say the other is moving and he is at rest and the other will say he is at rest with the first moving but Maxwell's equations don't transform covariantly under Galilean boosts so how does the implication follow? If, on the other hand, you are saying it should imply it as a matter of physical principle then I can agree with that but that just leads us down the road of replacing Galilean transformations with Lorentz transformations; here we are accepting on physical grounds that the situation should imply the laws of physics are the same - we aren't taking the situation and actually showing it implies the laws of physics are the same from first principles.

 

[HomogenousCow]

It is one of the fundamental postulates of special relativity, like the axioms in quantum mechanics.
One could alternatively start with the postulate that the metric of spacetime is the minkowski metric in vaccum, then we may declare the equivalency of all inertial frames and derive the expressions which show us the asymptotic behavior of particles as they approach the speed of light.
For the electromagnetic field, it is apparent in the general solution to Maxwell's equation for prescribed sources, all times are replaced with "retarded times".

 

[WannabeNewton]

It is one of the fundamental postulates of special relativity...

Yes exactly, we are accepting the implication as a basic truth; we aren't proving it from first principles.

 

[Boy@n]

Here's a spacetime diagram illustrating how a person that is at rest in an Inertial Reference Frame measures the speed of light to be c (defined for our purpose here to be 1 foot per nanosecond):

The "person" is shown in blue with each nanosecond of time marked off with dots. He has placed a mirror a measured six feet away shown in red. At his time of 4 nanoseconds, he sends off a flash of green light which travels at c toward the mirror and reflects off of it at coordinate time of 10 nanoseconds and arrives back at the "person" at his time of 16 nanoseconds. The "person" measures that the round-trip time for the light took 12 nanoseconds and so he calculates the speed of light to be one-half of 12 nanoseconds divided into 6 feet or 1 foot per nanosecond.

Now we use the Lorentz Transformation process to create an IRF moving at -0.6c to see the scenario that adjacent described in his OP. Now the "person" is traveling at 0.6c toward the flash of light which is traveling at 1.0c. He's wondering why the "person" won't measure the speed of the light to be traveling at 0.4c.

Now you can see that the difference between his speed and the speed of the light as it is moving away from him is 0.4c but he can't measure that by itself because he can't know when the light hits the mirror. Instead, he does a round-trip measurement that turns out to be identical as in the first IRF. And that illustrates what adjacent was asking about.

Excelent diagrams.