# I The velocity of a moving frame of reference

#### RossBlenkinsop

there may be difficulty devising an experiment to measure where that point is but never the less
u cant have yr cake and eat it to

#### Ibix

if a light strobe at a point X1 Y1 simple maths tells us that 3 photons in the wave front will always triangulate back to the same point in space
Please state this maths. How is it different from what I laid out in #46, and which Janus presented diagramatically in #49? Remember to carry out the maths in two different frames and relate the locations in the two frames. If you do this correctly you will realise that "the same point in space" means different things to different frames. It is not an absolute.
the speed of light not invariant or do the photons triangulate to a set point in space ?
They "triangulate back" to different points according to different frames; the point one frame calls "stationary at the centre of the spherical pulse" other frames call "moving and not at the centre". I have stated this multiple times, both verbally and mathematically, and correct diagrams have been presented to you supporting this. That "a fixed point in space" is a frame-dependent notion, not an absolute one, dates back to Galileo and all of physics assumes the notion.

#### RossBlenkinsop

can you explain how a single strobing light with invariant speed photon can triangulate back to multiple centre points ?

#### Ibix

can you explain how a single strobing light with invariant speed photon can triangulate back to multiple centre points ?
I already did. See post #46.

#### RossBlenkinsop

so it is impossible to conclude that the speed of light is invariant then
ok

#### Ibix

so it is impossible to conclude that the speed of light is invariant then
ok
I used the invariance of $c$ in #46...

#### Ibix

I repeat: please state the "simple maths" that supports your assertion that there is an absolute frame-invariant point from which the light radiates. You claimed there was some in #50. I know, having done the maths myself and presented a sketch above, that this claim is wrong - so I want to see your maths. Obviously there's a mistake in it somewhere.

#### RossBlenkinsop

so what you are saying is if there is a single strobing light that strobes only once and 100 different ppl in 100 different frames, they all agree there is only one light and that it strobed once ,

but 3 photons on that wave front will map back to 100 different points

so what should they conclude from the results of that experiment ?

they cannot conclude there was 100 lights cos they all saw one and they can only conclude it strobed once

they would have to conclude there was 1 light simultaneously in 100 different locations
ok

#### RossBlenkinsop

or the other thing they could conclude is that there was one light at one location and there is something wrong with the ,measuring technique

Now

If they conclude there is one light at one location , then all roads lead to Rome

#### Ibix

so what should they conclude from the results of that experiment ?
Let's define your experiment better: one person is at rest with respect to the strobe. The other 99 are all in motion with respect to the first and to each other, and all pass the first person simultaneously. At the instant they pass him, he triggers the strobe. All one hundred people agree that the light flash happened at their location. Therefore when they subsequently back-trace the light rays they will always lead back to themselves because they each define themselves to be stationary. But they all agree that they are no longer in the same place. So they all disagree about what "where the flash happened" means.

There's nothing wrong with their measurement procedure. It's just ambiguous what "the same place" means, because there's no absolute standard of rest to define it. I don't really understand why you can't grasp this - it's been said enough times in enough different ways, and your own thought experiments lead to it if you analyse them correctly.

I'm still waiting for the "simple maths" that proves this wrong. It will never be forthcoming, of course, because there is no such thing.

#### Ibix

Also, a minor point, but you are required to write in proper English on this site, using capitals, punctuation, and actual words - "people" does not have three letters. Poor writing just makes it harder to understand an already complicated topic.

#### RossBlenkinsop

Let's define your experiment better
no lets not

lets say we have 100 000 people splattered everywhere in 100 000 different frames when the one light strobes once

now you have a single light in 100 000 different locations simultaneously

#### Ibix

Same answer. They all say the light flash happened where they are. They all agree that the others are not where they are. Therefore they all disagree about what "where the flash happened" means at any time except when it happened.

Adding zeros doesn't change anything. You are simply wrong 100,000 times instead of 100 times.

#### RossBlenkinsop

I repeat: please state the "simple maths" that supports your assertion that there is an absolute frame-invariant point from which the light radiates.
so are you saying if there is a single strobe light in my lounge room that strobes once, and photons radiate out from the strobe in straight lines at a set speed

there are multiple center points to that wave front of photons ?

if that is the case the speed of light is not invariant as that is the only way those photons can map back to multiple points

I dont need simple math as your friends animation shows the photons radiate outwards in straight lines in all directions at an invariant speed from a central point... but apparently if time is reversed they map back to multiple points thus negating the entire assertion they are speed invariant

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#### Ibix

Why do you keep asking the same question? Do you think I'll change my mind if you ask it 23 times? Or 24?

Yes. Every frame has a different interpretation of what "centre point" means in this context. This is perfectly consistent with an invariant speed of light and I don't understand why you think otherwise.

I've already shown you the maths to support my claim (post #46 - note the presence of the same $c$ in both frames' calculations). Show me the maths you claim to have that says otherwise. Until you do so, you are simply waving your hands and refusing to believe that the world doesn't work like you think it ought to.

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#### Nugatory

Mentor
so which is it is the speed of light not invariant or do the photons triangulate to a set point in space ?
The speed of light is invariant and the light does not triangulate back to a set point in space.
It does triangulate back to the same point in spacetime (although the correct word for a point in spacetime is “event”, not “point”), the one at which the light was emitted.

You will continue to be confused until you understand the difference between a point in space and a point in spacetime. A point in space is a line (called a “worldline”) in spacetime; googling for “Minkowski diagram worldline” will bring up many explanations and examples.

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#### RossBlenkinsop

Same answer. They all say the light flash happened where they are. They all agree that the others are not where they are. Therefore they all disagree about what "where the flash happened" means at any time except when it happened.

Adding zeros doesn't change anything. You are simply wrong 100,000 times instead of 100 times.

sorry i simply do not understand this reply

#### Nugatory

Mentor
sorry i simply do not understand this reply
He’s saying that if it makes sense for two observers moving relative to one another it will also make sense for ten, or a hundred, or 100,000.... so you should focus on understanding the two-observer case.

#### RossBlenkinsop

as explained before there is a train in a large building, 2 light clocks in the large building (LB) to, 1 in the train the other at rest wrt the building

the building and everything in it is whizzing thru space at some velocity V in some direction V

there is a strobe light in the roof

the train heads off along the tracks. Purely by chance the tracks are parallel to the direction of travel of the LB

The train heads off and attains a steady velocity. Purely by chance the velocity of the train is exactly the same as that of the LB , but in the opposite direction

we end up with the situation as depicted.

Purely by chance the strobe light in the roof strobes at the instant the train is directly below the light

once a photon enters a light tube it is reflected up and down

an observer in the frame will see the train whizzing away from them and the light clock in the train will be perceived by the observer to be ticking slower than the clock at rest wrt the observer

as before pulses are sent back along the train lines

will the train line pulses tick faster than the clock at rest wrt the observer

I have drawn the successive events one under another

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#### Nugatory

Mentor
can you tell me what is wrong with my logic
This picture shows the light propagating at speed $c$ using the frame in which the railcar is at rest, but not using the frame in which the strobe light and other stuff is at rest. Take another careful look at post #29 of this thread by @Janus and also try drawing a Minkowski diagram instead of trying to represent the passage of time as a series of still pictures.

I also think that you may be overlooking the relativity of simultaneity here. That’s another good google search.

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#### Janus

Staff Emeritus
Gold Member
as explained before there is a train in a large building, 2 light clocks in the large building (LB) to, 1 in the train the other at rest wrt the building

the building and everything in it is whizzing thru space at some velocity V in some direction V

there is a strobe light in the roof

the train heads off along the tracks. Purely by chance the tracks are parallel to the direction of travel of the LB

The train heads off and attains a steady velocity. Purely by chance the velocity of the train is exactly the same as that of the LB , but in the opposite direction

we end up with the situation as depicted.

Purely by chance the strobe light in the roof strobes at the instant the train is directly below the light

once a photon enters a light tube it is reflected up and down

an observer in the frame will see the train whizzing away from them and the light clock in the train will be perceived by the observer to be ticking slower than the clock at rest wrt the observer

as before pulses are sent back along the train lines

will the train line pulses tick faster than the clock at rest wrt the observer

I have drawn the successive events one under another
I think a large problem is that you seem to think that "the building and everything in it is whizzing thru space at some velocity V in some direction V", has meaning in a absolute sense; that there is an absolute state of rest that the building can be said moving with respect to. This is not the case. There is no absolute space against which we can measure motion. We can only measure velocity differences between frames of reference.

So, if we assume that we have three light clocks, A B and C, Instead of a large building we will use a long railway car with a flat bed. On this railway car is an automobile. Clock A is affixed to the tracks. Clock B is affixed to the railway car, and clock C is affixed to the automobile.

The railway car is moving right at v relative to the tracks, and the automobile is moving as v to the right as measured from the railway car.

Ergo, Clocks A and C are at rest with respect to each other.

As measured from the tracks, Clocks A and C run at the same rate and clock B runs slow.
As measured from the automobile, Clocks A and C run at the same rate and clock B runs slow.
As measured from the Railway car, Clocks A and C run at the same rate, but run slower than Clock B.

#### RossBlenkinsop

I think a large problem is that you seem to think that "the building and everything in it is whizzing thru space at some velocity V in some direction V", has meaning in a absolute sense; that there is an absolute state of rest that the building can be said moving with respect to. This is not the case. There is no absolute space against which we can measure motion. We can only measure velocity differences between frames of reference.
can you say it is not moving ? and if you were to say it is not moving, would that not mean it is stationary in an absolute sense ?

It appears I am at liberty to assume it may be moving ?

#### RossBlenkinsop

When one is explaining how a light can appear to be in a different position in a moving frame of reference to a person at rest wrt that frame the typical explanation is "well the photons enter your eye at a glancing angle but because your eye is moving at a velocity equal to the cosine of the angle of the photon with your eye, through space, the light appears to be in front of you when in fact it might not"

the above explanation assumes "movement" which, according to your last post, must mean movement wrt to something aka absolute movement doesn't it ?

#### RossBlenkinsop

a person is on a train heading east. Another person is on a train heading west . Person 1 sees the train whiz by, person 2 sees the other train whiz by

i assume we are expected to conclude neither train is moving ?

#### jbriggs444

Homework Helper
can you say it is not moving ? and if you were to say it is not moving, would that not mean it is stationary in an absolute sense ?

It appears I am at liberty to assume it may be moving ?
The point is that an insistence either way is unnecessary. Only relative motion has meaning. There is no meaningful sense of either absolute rest or of absolute motion.

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