The Relativity of Simultaneity

[Neddy Bate]

If you are in a reference frame where you see the cube moving relative to the expanding sphere of light, you can easily calculate the speed of the cube. All you have to do is use your own clocks to measure the light transmission times, and use your own measuring rods to measure the size of the cube. Then you can calculate how fast the cube is moving. You don't even need to know SR. This is what MD has been doing.

But if you are in the reference frame of the cube, the cube is not moving. The expanding sphere of light is still a sphere, expanding relative to the center of the cube. Now, if you use your own clocks to measure time, and you use your own measuring rods to measure distance, you can't figure out how fast the cube is moving. All experiments tell you that the cube is at rest.

Is that how you understand it?

Yes, the first paragraph is MD and the second is SR.

Well, not exactly. What MD does is he takes the first paragraph and claims that is what the guy inside the cube would find. But SR says the guy inside the cube actually finds the second paragraph.

I'm behind on my espresso, lol.

Enjoy your caffeine!

 

[quantum_wave]

Well, not exactly. What MD does is he takes the first paragraph and claims that is what the guy inside the cube would find. But SR says the guy inside the cube actually finds the second paragraph.
...

I understand. Now what we have to do is test at relativistic speeds relative to a cooincident point of emission in space to see if the light sphere moves with the source or from a point of emission.

 

[Neddy Bate]

Now what we have to do is test at relativistic speeds relative to a cooincident point of emission in space to see if the light sphere moves with the source or from a point of emission.

Tests on the surface of the earth find the speed of light is constant, regardless of whether the light source moves or not. That tells you that light acts as an expanding sphere on earth.

So now do you want to conclude that the moving source would find the light to be "lob-sided" instead of a sphere? If that were the case, then why isn't it lob-sided relative to earth? Surely earth is not at absolute rest.

 

[quantum_wave]

Tests on the surface of the earth find the speed of light is constant, regardless of whether the light source moves or not. That tells you that light acts as an expanding sphere on earth.

So now do you want to conclude that the moving source would find the light to be "lob-sided" instead of a sphere? If that were the case, then why isn't it lob-sided relative to earth? Surely earth is not at absolute rest.

You are still trying to put the earth in the box aren't you. In the thought experiment the earth is not in the box.

In addition, space is empty except for a single flash of light.

That flash expands spherically as we agree, either from the point of emission or from the source of emission. It doesn't make a difference if the source and the point of emission are in the same frame. But if there is a coincident point in space in two frames, whether we consider them inertial frames or define one as the rest frame relative to the other, the single light flash at that coincident point in space will act one way or the other depending on if MD is right or SR is right. If MD is right the flash will expand from a fixed point that was coincident to both frames at the instant of emission and the moving frame will notice that the light does not look like a sphere emanating from the source that is moving with the frame.

At least that is how I interpert MD's postulate, .

 

[arfa brane]

I don't know why I'm catching up here, but maybe everyone missed this one:

Light doesn't travel at c relative to the source, it travels at c relative to space.

Light travels at c from the point of emission.

There are at least two problems with this description. 1) Nothing can travel relative to space, because space is nothing. You can't measure the position of space, so there is no frame of reference that light travels "relative to"; it's nonsensical. 2) If light doesn't travel at c relative to the source, how come it travels at c "from" the source? That is a contradiction.

MD seems to be incapable of putting a logical description together, never mind proving SR is wrong.

If you're thinking, well ok, the source can be moving, so it can travel relative to the expanding sphere of light. No dice, unless you can see the light and the source 'traveling' (with superphysical vision?).

Furthermore, the point of emission is not "fixed", except by a proper time of emission. Time doesn't hang around and label coordinates for remote observers, which is why observers see (their own) different times and distances, because light always has the same closing speed in space with observers at arbitrary locations.

Observers in motion relative to each other wil not agree on the time of emission or the distance to the event, the event isn't fixed with an absolute location or time by some helpful agency (the green guys don't hand you a diagram).

 

[Motor Daddy]

1) Nothing can travel relative to space, because space is nothing. You can't measure the position of space, so there is no frame of reference that light travels "relative to"; it's nonsensical. 2) If light doesn't travel at c relative to the source, how come it travels at c "from" the source? That is a contradiction.

It doesn't travel at c from the source, unless the source is at an absolute zero velocity and remains at the point in space the light was emitted.

Do you really think light travels at c from the source? Say you turn on a light bulb and the light travels away from the bulb. Then you throw the light bulb. Do you really believe that because you threw the light bulb, that the outer light sphere traveled with the bulb, and continued to expand at the same time??? You don't honestly believe that do you?

MD seems to be incapable of putting a logical description together, never mind proving SR is wrong.

My description is perfectly logical. It's you that has a hard time understanding a perfectly logical description. You are the problem, I'm not.

Heck, you still don't understand acceleration.

 

[quantum_wave]

I don't know why I'm catching up here, but maybe everyone missed this one:
There are at least two problems with this description. 1) Nothing can travel relative to space, because space is nothing. You can't measure the position of space, so there is no frame of reference that light travels "relative to"; it's nonsensical. 2) If light doesn't travel at c relative to the source, how come it travels at c "from" the source? That is a contradiction.

MD seems to be incapable of putting a logical description together, never mind proving SR is wrong.

If you're thinking, well ok, the source can be moving, so it can travel relative to the expanding sphere of light. No dice, unless you can see the light and the source 'traveling' (with superphysical vision?).

Furthermore, the point of emission is not "fixed", except by a proper time of emission. Time doesn't hang around and label coordinates for remote observers, which is why observers see (their own) different times and distances, because light always has the same closing speed in space with observers at arbitrary locations.

Observers in motion relative to each other wil not agree on the time of emission or the distance to the event, the event isn't fixed with an absolute location or time by some helpful agency (the green guys don't hand you a diagram).

No, arfa, you are letting your enthusiasm to falsify MD fog your memory of the repeated phrasing that should be impressed on your memory by now. You must realize that his statement should read, “Light doesn’t travel at c relative to the source; it travels at c relative to a point of emission in space”. He makes that clear when he says “Light travels at c from the point of emission.”

I know you understand why he makes the distinction. It has to do with the patch of space in which the expansion of the light sphere is taking place. The distinction is clear between the point of expansion being a point in space or being the moving source if you followed Ned’s last few posts. Either the light expands from the point in space where it is emitted, or it expands from the source. This is where the discussion of the moving source comes in. In SR the light expands spherically from the source even though the source can be considered to be in relative motion.

That is the whole bone of contention between MD and SR and you shouldn't be losing track of that and letting the phrase you quoted obscure everything up to this point.

 

[Neddy Bate]

...the single light flash at that coincident point in space will act one way or the other depending on if MD is right or SR is right.

No, it is not "one way or the other". SR says the light can be a sphere in ALL reference frames. So you cannot disprove SR by saying , "Hey, I saw a sphere of light, and there was an object moving relative to it, therefore SR is false."

On the other hand, MD says the light can only be a sphere in one frame, and that frame is the absolute rest frame. So you can disprove MD easily. Just find a light sphere whose origin point is not at absolute rest. We have them on earth.

If MD is right ... the moving frame will notice that the light does not look like a sphere emanating from the source that is moving with the frame.

And since that never happens, MD is wrong.

 

[Motor Daddy]

No, it is not "one way or the other". SR says the light can be a sphere in ALL reference frames. So you cannot disprove SR by saying , "Hey, I saw a sphere of light, and there was an object moving relative to it, therefore SR is false."

You're hallucinating if you think a light sphere can travel with a source, and expand at the same time. You are saying that the speed of light is dependent on the source. You are out of your mind, Ned! The speed of light is not dependent on the source. The speed of light is independent of all objects.

 

[quantum_wave]

No, it is not "one way or the other". SR says the light can be a sphere in ALL reference frames. So you cannot disprove SR by saying , "Hey, I saw a sphere of light, and there was an object moving relative to it, therefore SR is false."

On the other hand, MD says the light can only be a sphere in one frame, and that frame is the absolute rest frame. So you can disprove MD easily. Just find a light sphere whose origin point is not at absolute rest. We have them on earth.




And since that never happens, MD is wrong.

I keep saying that the earth is not in the box, space is empty except for the one light sphere, and the light sphere expands spherically at c.

There is one sphere so if you observe that sphere from a moving frame it does not appear to be expanding spherically from the moving source, hence it said, "If MD is right ... the moving frame will notice that the light does not look like a sphere emanating from the source that is moving with the frame."

We are talking about testing that postulate at relativistic speeds with a point in space that is coincident to both frames. The flash is emitted from that coincident point in space. The light expands spherically from that point regardless of the motion of the source.

 

[James R]

Motor Daddy:

I'm not ignoring your train frame diagrams, you haven't answered the question, how do you explain that the light has not yet reached the y receiver in your Einstein embankment diagram?

Because I chose not to draw the diagram for the instant where the light reached the y detector. I drew diagrams for when it reached the front and back of the train/box. I expected you could fill in the diagram for light hitting the y detector yourself. After all, in the embankment frame my diagrams are the same as your diagrams apart from the length contraction of the box.

I know you would like to move past that point because you have no answer and it is an inconsistency in SR.

If you asked a coherent question I could answer you. Half the time, I can't tell what you're asking because you don't express yourself clearly enough. I have to keep guessing at what you might mean.

Tell me what time the light reached the y receiver?

To calculate that in the embankment frame I need to know the speed of the box in that frame.

Or we can do it the other way around: you tell me the speed of the box and I calculate the light travel time.

This is your example. It's up to you to provide enough information so that it is possible to give an answer.

How do you explain that the length between the x receiver and the source is contracted, but not contracted between the y receiver and the source, and all the clocks are dilated? That causes an inconsistency in the measured speed of light.

All distances in the x direction are contracted in the embankment frame. Light travelling from the source to the y receiver travels in a diagonal path, partly in the x direction and partly in the y direction. y direction lengths are not contracted; x direction lengths are.

All clocks in the box move with the box. All clocks on the embankment move with the embankment. So, all clocks in the box are synchronised with each other; if one runs slow, all run slow. And all clocks on the embankment are synchronised with each other; if one runs slow, all run slow. Clocks on the embankment are not synchronised with clocks on the train, because such a thing is impossible in Einstein's relativity.

Again you fail to grasp the concept of a ruler in motion and the amount of time it takes for light to traverse the ruler.

Not at all.

Take a meter stick. Measure the time it takes light to traverse the stick one-way on a train when the train is going 60 MPH.

You mean, using clocks on the train, or using clocks on the embankment? Because depending on which clocks you use you'll get different times.

Now increase the speed of the train to 200 MPH and measure the time again. Does it take light the same amount of time to traverse the stick at the two different speeds? NO, James, it does not!!!

In the train frame, or the embankment frame?

Again, you're correct in the embankment frame, but not in the train frame.

You seem unwilling to address both frames. You always want to work only in the embankment frame. Until you can come to grips with the fact that the train frame is different, you're unlikely to make any progress in understanding.

If you think it does, then you are saying it always takes light the same amount of time to reach you if you run away from a lamp post when the light is emitted, when running at two different speeds, 60 MPH and 200 MPH. It simply is not possible for light to reach you in the same amount of time, because light has to travel a greater distance to catch you when you are running away at 200 MPH.

All true in the embankment frame. False in the train frame (using Einstein's relativity).

Better yet, measure the time it takes light to travel the length of the stick one-way when the stick is pointing rear to front of the train. Now rotate the stick 90 degrees and measure the one-way time again. You say all the clocks are dilated on the train, and the length is contracted in the x direction, but not the y direction. Are you saying it takes the same amount of time for light to traverse the stick, from rear to front and side to side? How is that possible, James? The stick is shorter in the rear to front direction than it is side to side, and the clocks are dilated the same each way??????

In the train frame, the stick has a constant length, since it moves with the train. That applies no matter which way it is oriented. So light always takes the same time to traverse the stick in the train frame.

In the embankment frame, the stick changes length as you rotate it, due to length contraction in the x direction (direction of the train's motion relative to the embankment). So, it takes light a different amount of time to traverse the stick when it is lined up along the y axis than it takes when the stick is aligned along the x axis. If it is somewhere in between, so is the light travel time.

The distance from the y receiver to the x receiver is the hypotenuse. To find the length of the hypotenuse you use the Pythagorean Theorem, which is a^2+b^2=c^2. The distance between the y receiver and the x receiver is .7071 light seconds.

That's true in the train frame, but not in the embankment frame, since in the embankment frame lengths in the x direction are contracted according to Einstein's relativity.

 

[James R]

Motor Daddy:

Wrong! A light sphere has a center point, and that point is not an object, nor is it capable of motion. The center point is a point in space from which the light expands. The light sphere doesn't travel as an object does in space, it expands in space away from the center point in all directions. Until you understand that concept you will never understand how distance and time are measured using light.

In Einstein's relativity, that centre point you speak of exists in every frame. That is, in any given frame, light expands in a sphere around a stationary centre point.

And that includes the train frame as well as the embankment frame.

In the Motor Daddy universe, in the train frame the centre point moves backwards, because the centre point must remain stationary in Motor Daddy "space" as the train moves forward.

You know why SR can't do acceleration? Because the numbers don't add up, because SR is BS!! My method works flawlessly with acceleration, the numbers add up perfect! Not one little hiccup.

You haven't done any calculation that requires acceleration so far.

There is math behind distance and time, and you can in fact draw math on a paper. What SR draws on a piece of paper is a bunch of objects all claiming to be at rest. Yes, that's right, SR says that every object in the universe is at rest, just ask them, they'll tell you.

Even in the Motor Daddy universe, all objects are at rest in their own reference frames. That follows from what we mean by a reference frame. The difference is that in the Motor Daddy universe, all objects also have an absolute speed in the "space" reference frame, whereas there is no such frame in Einstein's relativity.

Ask any observer in the SR universe and they will say they are at rest. That means all observers are at rest in SR, so can you explain the observed motion in the universe while at the same time saying all observers are at rest?

All observers are at rest in their own reference frames. In other frames, they are moving. The train is at rest in the train frame. The embankment is at rest in the embankment frame. The train is moving in the embankment frame. The embankment is moving in the train's frame.

See?

So there are two SR observers in relative motion. They both demand that they are at rest and the other observer is the one in motion. Did they ever consider that they might both be in motion, or has that thought never occurred to them?

It occurs to them that they can both be moving in some third reference frame. Each is at rest in his own frame. Each moves in the other's frame.

Without reference to another object, you tell me what you mean by the term "at rest."

Can't be done. That's why it's called "relativity".

You are in a box in space. You assume the box is "at rest." What do you mean by "at rest."

It means if you're sitting in the box you don't see the walls moving.

So in a box without reference to another external object, you don't know if you are at rest or in motion?

Correct. Galileo knew that back in the 1600s.

..and, are you trying to tell me that my motion is dependent on another object's motion?

Only if you're interested in how the other object sees you moving.

So say I'm on the highway and another car is in front of me and the distance is remaining the same. All of a sudden the distance starts to increase. Which car accelerated?

The one in which the passengers felt the acceleration.

BULL! I've already showed you how to determine the absolute velocity of a box from within the box.

Yes, but only in the Motor Daddy universe where it is assumed that clocks everywhere in all frames are synchronised and that the speed of light is not the same in all frames but only has its standard value in the embankment frame.

There's nothing wrong with your calculations in the Motor Daddy universe. Your calculations are only wrong in the real world - the Einstein universe.

What you mean to say is that SR has no way of determining the velocity of the box, so it pretends the box is at rest.

No. Step 1 is to choose a reference frame. Step 2 is to determine whether the box is at rest in that frame. There's no "pretend" about it. Either an object is at rest in a frame or it isn't. That's as true in the Motor Daddy universe as it is in the Einstein universe.

Since it bases all it measurements on its pretend "at rest", all its measurements are wrong, unless the box really is at rest, of which it has no way to determine.

In Einstein's universe, there is no "really at rest" in the sense of an absolute rest frame. You don't seem to get it. Your absolute "space" frame simply does not exist in the real world.

Light doesn't travel at c relative to the source, it travels at c relative to space.

...in the Motor Daddy universe.

In Einstein's universe it travels at c in all reference frames, as per Einstein's second postulate.

Light travels at c from the point of emission. The source was at the point of emission when the light was emitted, but that doesn't mean it is there 1 second later.

Yes. That's true in both universes, but not in every frame. In particular, the source never moves in the source's own frame. That's true in both universes, too.

SR makes it sound like the light sphere travels with the cube as it expands. That is not possible!

Correct in the embankment frame in both universes. True in the Motor Daddy universe in the train frame. And in Einstein's universe in the cube frame the cube never moves, so the issue doesn't arise.

Again, if a cube is in space, and you say regardless of the motion of the cube the light always hits all the walls simultaneously, then what you are saying is that the light sphere is expanding in the cube while it is traveling with the cube.

In the cube frame, the cube never moves, so of course the light expands outwards at c in all directions evenly. That's in the Einstein universe, of course, in which the speed of light is c in every frame. In the Motor Daddy universe, the speed of light varies in different directions inside the cube due to the cube's motion through "space", so light won't hit the walls simultaneously in the Motor Daddy universe.

So for instance, say a train car is a cube traveling down the tracks at 60 MPH. You say the light hits all the receivers in the same time in the cube. Now redo the test, but this time increase the speed of the train to 200 MPH. Again, you say the light sphere expands and hits all the receivers in the same amount of time.

In the train frame, yes. In the embankment frame, no.

So, you either agree that the sphere expands as it travels with the train, or you agree that the sphere expands independent of the train. Which is it. Don't pretend that if I studied relativity I would understand it. If you pretend to understand relativity, explain your point. Which is it??

It's both. In Einstein's relativity, the speed of light is c in all frames. So, pick a point of emission in any frame you like. Light will spread out in all directions from that point at speed c in whichever frame you've chosen.

This is not, of course, true in the Motor Daddy universe.

It doesn't travel at c from the source, unless the source is at an absolute zero velocity and remains at the point in space the light was emitted.

True in the MD universe. False in Einstein's.

Do you really think light travels at c from the source? Say you turn on a light bulb and the light travels away from the bulb. Then you throw the light bulb. Do you really believe that because you threw the light bulb, that the outer light sphere traveled with the bulb, and continued to expand at the same time??? You don't honestly believe that do you?

No. Of course not, because the bulb moves relative to the person who threw it. You're working in the thrower's frame, not the bulb's frame.

My description is perfectly logical. It's you that has a hard time understanding a perfectly logical description. You are the problem, I'm not.

Yes. Your description is perfectly logical!

Your description has no logical flaws that I can see, provided that your postulates are correct. In other words, for your descriptions to be true there must be an absolute frame, light only travels at 299792458 m/s in that single frame and no other, etc.

Unfortunately, real-world experiments prove that your postulates are wrong. So, garbage in, garbage out.

You're just working in an imaginary universe, which is fine as far as it goes. Your problem is that you want to pretend you're working in the real universe rather than the fantasy one you created.

 

[AlexG]

Now what we have to do is test at relativistic speeds relative to a cooincident point of emission in space to see if the light sphere moves with the source or from a point of emission.

Why bother? If you don't like the results, you'll simply dismiss them.

idot

 

[arfa brane]

The light sphere doesn't travel in space, it expands in space.

So now expanding a sphere of light isn't the same as light 'traveling' in space. Ok.

It doesn't travel at c from the source, unless the source is at an absolute zero velocity and remains at the point in space the light was emitted.

The source of light isn't the same as the point light is emitted 'from'. Ok

Light travels at c from the point of emission.

But where is the point of emission, if it isn't the source?

Do you really think light travels at c from the source? Say you turn on a light bulb and the light travels away from the bulb. Then you throw the light bulb.

I think the bulb's gone out.

 

[James R]

arfa brane:

Maybe I should explain Motor Daddy's universe to you.

In the MD universe, light only travels at c in one frame, which Motor Daddy calls "space". "Space" is the absolute-zero-velocity reference frame.

Suppose light is emitted from a flash gun that is moving relative to "space". Then, the light will spread out from the point in "space" at which it was emitted at speed c in all directions (but only in the "space" frame). Meanwhile, the actual physical source, the flash gun, continues to move along in "space".

So, some time t after the light has been emitted in the Motor Daddy universe, the flash of light is located on a sphere at distance d = ct from the point in "space" where the light was emitted. If the flashgun is moving at speed v through "space", then the light sphere will not be centred on the location of the flashgun after t=0 since the flashgun will have moved away from the point of emission in "space".

So, when MD uses the term "source", he means the flashgun that emitted the light. When he talks about the "point in space the light was emitted", he means the location in the "space" reference frame at which the light was emitted, which is only the location of the moving "source" at the instant of emission.

Of course, exactly the same thing applies in Einstein's universe in any frame in which the source (flashgun) is moving.

The difference between the MD universe and Einstein is seen when we move into the frame of the moving source (the flashgun). In Einstein's universe, in the source frame, the light still spreads out at c in all directions from the source, but the source remains stationary. So in this frame, the "point of emission of the light" and the "location of the source" remain the same at all times, according to Einstein.

In the MD universe in the source (flashgun) frame, the flashgun remains stationary, but the "point of emission of the light" moves backwards, because the point of emission is only stationary in "space" and not in the source frame. The speed of light is c in "space", but the speed of light varies in different directions relative to the source (flashgun). So, light forms a sphere around the "point of emission", but not around the "source" in the Motor Daddy universe. In the MD universe, the light still spreads out in a sphere in the source frame, but the centre of that sphere ("point of emission") progressively moves away from the source (flashgun) over time.

 

[AlexG]

MD's universe is counter-reality.

How much longer do we have to pretend that MD isn't a wacko?

 

[arfa brane]

Space is not a frame of reference, you can't tell if a source is moving through space. You can tell if an object is in motion relative to another object, but a single object can't refer to space, space is empty.

Since you can't tell if a source remains "at the point of emission", you can't measure absolute velocity relative to space, which is nothing. An object cannot have a velocity relative to nothing.

Motor Daddy might not realise it, but his logic is not affine. It relies on the AND relation: the source is stationary AND the light sphere expands from a point of emission. The source AND the point of emission are different variables.

In his logic, A AND B give a result, C, but then A and B both vanish from the scene.
It should be A OR B to get C. If A is "a point of emission has absolute velocity relative to empty space", I'd go with the latter operation. A is false, but B is true.

B therefore corresponds to a Lorentz frame.

 

[James R]

AlexG:

MD's universe is counter-reality.

Yes.

How much longer do we have to pretend that MD isn't a wacko?

He's not a wacko. He's just unwilling to make any effort to actually understand Einstein's theory. Nor is he interested in seeing any real-world data that might invalidate his own theory.


arfa brane:

Space is not a frame of reference, you can't tell if a source is moving through space. You can tell if an object is in motion relative to another object, but a single object can't refer to space, space is empty.

In the MD universe you can tell. You just follow MD's procedure for finding the absolute velocity of the object.

Of course, MD's procedure doesn't work in the real world.

Motor Daddy might not realise it, but his logic is not affine. It relies on the AND relation: the source is stationary AND the light sphere expands from a point of emission. The source AND the point of emission are different variables.

In the MD universe, the point of emission never moves in "space". Only sources can move.

 

[przyk]

Maybe I should explain Motor Daddy's universe to you.

In the MD universe, light only travels at c in one frame, which Motor Daddy calls "space". "Space" is the absolute-zero-velocity reference frame.

Do length contraction and time dilation occur in Motor Daddy's universe?

Because if not he really has taken a giant leap back to the pre-Lorentz ether theory days. And in that case I've never seen such a blank denial of relativity.

Has he even tried to explain the results of Michelson-Morley experiments, beyond just complaining that they're only sensitive to round trip times?

 

[Motor Daddy]

arfa brane:

Maybe I should explain Motor Daddy's universe to you.

In the MD universe, light only travels at c in one frame, which Motor Daddy calls "space". "Space" is the absolute-zero-velocity reference frame.

Suppose light is emitted from a flash gun that is moving relative to "space". Then, the light will spread out from the point in "space" at which it was emitted at speed c in all directions (but only in the "space" frame). Meanwhile, the actual physical source, the flash gun, continues to move along in "space".

So, some time t after the light has been emitted in the Motor Daddy universe, the flash of light is located on a sphere at distance d = ct from the point in "space" where the light was emitted. If the flashgun is moving at speed v through "space", then the light sphere will not be centred on the location of the flashgun after t=0 since the flashgun will have moved away from the point of emission in "space".

So, when MD uses the term "source", he means the flashgun that emitted the light. When he talks about the "point in space the light was emitted", he means the location in the "space" reference frame at which the light was emitted, which is only the location of the moving "source" at the instant of emission.

Of course, exactly the same thing applies in Einstein's universe in any frame in which the source (flashgun) is moving.

The difference between the MD universe and Einstein is seen when we move into the frame of the moving source (the flashgun). In Einstein's universe, in the source frame, the light still spreads out at c in all directions from the source, but the source remains stationary. So in this frame, the "point of emission of the light" and the "location of the source" remain the same at all times, according to Einstein.

In the MD universe in the source (flashgun) frame, the flashgun remains stationary, but the "point of emission of the light" moves backwards, because the point of emission is only stationary in "space" and not in the source frame. The speed of light is c in "space", but the speed of light varies in different directions relative to the source (flashgun). So, light forms a sphere around the "point of emission", but not around the "source" in the Motor Daddy universe. In the MD universe, the light still spreads out in a sphere in the source frame, but the centre of that sphere ("point of emission") progressively moves away from the source (flashgun) over time.

James, that's a good explanation except for one point. The point of emission doesn't move away from the source, the source moves away from the point of emission. The point of emission is not capable of motion in space, nor is the expanding light sphere as a whole capable of traveling in space. The light sphere simply expands and the source travels away from the point of emission.

There is no saying that it can be said that the source is stationary and the light sphere is traveling away, as Einstein would say. That is not possible. The source is in motion, the light sphere is not, other than expanding its radius from the point of emission at the rate of c.