The Relativity of Simultaneity

[Motor Daddy]

You reveal the exact opposite in your next statement:

So that would be a "no," you won't be helping James? That's what I figured, because if you could have helped him you would have jumped all over the chance. Give it the old college try, just for the fun of it, eh? Humor me...literally.

You're not presenting a challenge for relativity then. You're simply denying it. What you say above is exactly what everyone believed in the nineteenth century and before. It's what everyone was probably born believing.

So the question is, why should everyone believe you, when you say something that we all used to believe but have since had good reason to abandon? Why should everyone believe you when you say distance and time are absolute?

So everyone was probably born believing they could measure an absolute velocity of a box in space, from within the box, with no external references? Can you show me some of the old calculations of the past to back up your assertion that people used to believe what I now believe? Did they have actual real world measurements to support their beliefs? What makes all the previous evidence false now, and what makes you think that your current evidence will remain intact unlike the previous evidence?

Answering this seems a bit pointless. At t = 1 in the observer's rest frame, the light is 299,792,358 meters in front of him. These qualifiers are necessary because in SR, where distance and time are explicitly not absolute, what the source-riding observer calls t = 1 isn't the same as what a different observer would call t = 1, and what the source-riding observer calls 299,792,358 meters isn't what a different observer would call 299,792,358 meters.

Boring even, according to James, and yet he's now so bored that he ignores the questions, not unlike yourself. How far away from the point of emission is the source and observer at t=1?

If distance and time were absolute then you'd be right and what I said above would be wrong. But there's the problem: when you say distance and time are absolute, nobody has any reason to believe you just because you said so.

Distance and time are absolute because light travels a specific distance in space in a specific duration of time. It never changes, it is absolute, regardless of what an observer says!

Which is why answering the question was pointless. If you understood that distance and time might not necessarily be absolute then you should have been specific about whose metres and seconds you were using to measure things. If you thought that distance and time must be absolute then you should have explained why we should believe that, when we have good evidence to the contrary, instead of asking a question that presupposes it. So either way your question was pointless and only demonstrates your misunderstanding of SR.

There is not a "whose meters," there are meters, as defined by the constant speed of light. The speed of light is a constant so therefore the meter is a constant. There is not "whose meters," there are meters, as defined by light travel time!

 

[przyk]

MD, I tried to approach the question from the standpoint that time dilation and length contraction must have physical causes and have encouraged a discussion along those lines once or twice with no response.

The short answer is that it's due to the laws of physics possessing a symmetry.

To elaborate, imagine salt as an example. As everyone learns in highschool chemistry class, salt is composed of sodium and chloride ions which arrange themselves in a lattice structure like this:

The point is, ions can't just arrange themselves in any old way. There are physical laws like electromagnetism and quantum physics governing their behaviour which predict that some arrangements of ions are stable and others aren't. For sodium chloride, the structure is stable if the ions are 282 picometres apart, but anything else isn't a solution to the physical laws governing the structure of sodium chloride crystals.

While the physical laws governing the structure of matter can get rather complicated, one of their nice features is that they have a lot of symmetries. For example, all the known laws of physics are invariant under translations. That means that if you know a certain arrangement of atoms behaves a certain way, you can move the whole arrangement somewhere else and it will keep behaving in the same way. For example, if you know that the laws of physics allow sodium and chloride ions to form a salt crystal here, then you automatically know that they would allow the ions to form an identical crystal, only 3 metres to the left. Translational invariance in physics is the reason you can take your best shirt on holiday with you and you don't have to worry about it crumbling apart (beyond the normal stresses of traveling). It's not trivial that this is true. You could imagine physical laws being such that arrangements of matter that are stable in one place aren't stable somewhere else.

Another symmetry you know about from everyday experience is rotational invariance. If salt crystals are possible, then identical salt crystals rotated at 45 degrees are also possible.

Relativity is about another symmetry in physics called Lorentz invariance. Basically it says that if some physical system can behave a certain way, then a Lorentz transformation of it is also allowed by the laws of physics. That means they allow for the existence of an identical system, except length contracted, time dilated, and moving instead of at rest. You can think of a Lorentz transformation as a sort of rotation-like transformation in space-time. Again, it's not trivial that there even is a symmetry of this kind at all. You could imagine that the laws of nature were such that arrangements of matter that were stable at rest were no longer stable and crumbled apart when in uniform motion. It's not like they should necessarily remain stable or maintain the same length "by default".

Someone correct me if I'm wrong when I say that time dilation and length contraction are attributed to the relativistic Doppler effect.

No. To some extent they're independent of one another: there would be a Doppler effect even if SR were wrong. But you need to take SR into account in order to work out the actual Doppler shift factor. Length contraction and time dilation don't change that there is a Doppler effect but they do change its magnitude.

 

[przyk]

So that would be a "no," you won't be helping James?

Why? I just pointed the real problem out: you're assuming relativity is wrong a priori.

Give it the old college try, just for the fun of it, eh? Humor me...literally.

You think I've got nothing better to do? James can handle himself just fine.

So everyone was probably born believing they could measure an absolute velocity of a box in space, from within the box, with no external references?

No, and that's not what I said. I said people naturally believed that distance and time were absolute. People would probably have believed you could feel how fast you were moving in a box in space in ancient times. Then Newtonian mechanics suggested you couldn't. Then electrodynamics in the nineteenth century suggested maybe you could if you measured the speed of light inside the box. Then it was found experimentally that even that wouldn't work.

Can you show me some of the old calculations of the past to back up your assertion that people used to believe what I now believe?

Why the hell do you think the Michaelson-Morley experiment was ever performed? People back then were hoping to use light to measure the "absolute" velocity of the Earth in space, because they believed that time and distance were absolute.

Did they have actual real world measurements to support their beliefs? What makes all the previous evidence false now

Their experiments and experiences involved speeds much less than the speed of light, where relativistic effects are generally negligible.

and what makes you think that your current evidence will remain intact unlike the previous evidence?

Because generally if we do the same experiment twice, we get the same results. It's possible that new experiments might come along that disprove relativity, but existing experiments already put limits to how wrong relativity can be. Relativity will always at worst be approximately true, and more accurate than the idea of absolute space and time.

Boring even, according to James, and yet he's now so bored that he ignores the questions, not unlike yourself. How far away from the point of emission is the source and observer at t=1?

What's the "point of emission"?

Distance and time are absolute because light travels a specific distance in space in a specific duration of time. It never changes, it is absolute, regardless of what an observer says!

So, basically, distance and time are absolute because you *say so*. Didn't I just tell you that would never fly?

There is not a "whose meters," there are meters, as defined by the constant speed of light. The speed of light is a constant so therefore the meter is a constant. There is not "whose meters," there are meters, as defined by light travel time!

Non-sequitur. If light travels 1 light-second (= 299,792,358 metres) in 1 second, what's the speed of light? If from a different observer's point of view the same light travelled 1.732 metres in 1.732 seconds, what's the speed of light? Invariance of c doesn't require distance and time intervals to be absolute. It just requires them to scale by the same factor along the light cone.

 

[Motor Daddy]

Why? I just pointed the real problem out: you're assuming relativity is wrong a priori.

Then show me the error of my ways and help James out at the same time. Tell me how YOU explain the diagram James posted? Do you have an answer? If so post it. If not, say so.

You think I've got nothing better to do? James can handle himself just fine.

Oh, the old, "I don't have time for that nonsense" routine, and yet you continue posting nonsense and continue to waste your time. If your time is so precious, why are you trying to convince me I'm wrong? If you want to convince me I'm wrong, start with explaining the diagram James posted.

No, and that's not what I said. I said people naturally believed that distance and time were absolute. People would probably have believed you could feel how fast you were moving in a box in space in ancient times. Then Newtonian mechanics suggested you couldn't. Then electrodynamics in the nineteenth century suggested maybe you could if you measured the speed of light inside the box. Then it was found experimentally that even that wouldn't work.

You said,

What you say above is exactly what everyone believed in the nineteenth century and before. It's what everyone was probably born believing.

I don't believe you can feel velocity. I believe you can feel acceleration, the rate of change of velocity.

I asked you before to show me the calculations of measuring the absolute velocity of the box, and now you blindly assert that it was tried in the nineteenth century and failed. Oh darn it, you really got me there!

Nobody has ever used my method, so don't try to claim its been tried before and it failed. Prove your claim that it was EVER suggested how to determine the velocity of the box from within the box using light.

Why the hell do you think the Michaelson-Morley experiment was ever performed? People back then were hoping to use light to measure the "absolute" velocity of the Earth in space, because they believed that time and distance were absolute.

Oh, the old MMX routine, eh? The one where they never measured the one way times of light travel in each direction? As a matter of fact, can you show an experiment that proves SR that uses the one-way times?


I have to go now, I'll finish the response later...

 

[przyk]

Oh, the old, "I don't have time for that nonsense" routine, and yet you continue posting nonsense and continue to waste your time. If your time is so precious, why are you trying to convince me I'm wrong?

I have the right to spend my time any way I want, you know, and I am under no obligation whatsoever to explain someone else's post if it doesn't interest me. Why are you so determined I divert my attention to James R's diagram? Are you that frightened of the points I raise in my own posts?

Here's a thought: why don't you ask James R about his diagram, and answer me the simple question I asked you: why should everyone believe distance and time intervals are absolute?

I don't believe you can feel velocity. I believe you can feel acceleration, the rate of change of velocity.

Given none of this is being contensted, why are you bringing this up?

I asked you before to show me the calculations of measuring the absolute velocity of the box, and now you blindly assert that it was tried in the nineteenth century and failed. Oh darn it, you really got me there!

Nobody has ever used my method, so don't try to claim its been tried before and it failed. Prove your claim that it was EVER suggested how to determine the velocity of the box from within the box using light.

Oh, nobody's used your method. How terrible, that invalidates everyone else's experiments! Oh darn it, you really got the last century's worth of physicists there!

Oh, the old MMX routine, eh? The one where they never measured the one way times of light travel in each direction? As a matter of fact, can you show an experiment that proves SR that uses the one-way times?

This took me half a minute:

http://www.desy.de/user/projects/Physics/Relativity/SR/experiments.html#one-way_tests

So much for your ability to type things into Google.

 

[arfa brane]

We can see the foolishness of the notion of treating light as a physical phenomenon that "you don't have to measure". Actually treating any physical quantity this way is completely crazy, or "not quite right in the head"--think Father Jack, here.

The supporting evidence for a non-requirement of measurements (from MD's lexicon), is that light is defined (nowadays) in terms of metres and seconds, both of which are defined in terms of the 'speed' of light.

So if you want to know how long a metre is, well, it's defined in terms of light speed which "you don't have to measure". So you use metres which you "don't have to measure" the length of, because they are defined by a pre-existing "absolute" reference.

In other words, in Motor Daddy's uncritical universe, everything has an absolute length you can measure with metres, which are defined in terms of the speed of light, which you don't measure because it's predefined.

What's wrong with this picture, and why wouldn't someone see the problem with it?
Selective blindness? An aphasic condition?

What?

 

[James R]

Motor Daddy:

Sorry for the delay in replying. I have been busy with other things over the past couple of days.

Let's discuss one aspect of the diagrams at a time, so that we can identify each problem separately.

In your diagram on the left, Einstein's embankment frame, you show that the embankment observer thinks light traveled .5 seconds to travel from the source at the midpoint of the train to the receiver attached to the side of the train, which is .5 light seconds from the source.

No. My diagrams for that frame do not show the light reaching the y detector.

The embankment frame using Einstein works exactly the same way that your embankment or "space" frame works. Light takes longer than .5 seconds to reach the y detector because the y detector is moving along in the x direction in the embankment frame.

It is the train frame that we disagree about. The diagrams for your train frame and for Einstein's are quite different, whereas the diagrams for your embankment frame and Einstein's are the same. That's because of your different speed-of-light postulate.

The embankment observer will see that the receivers on the sides of the train have moved to the right in the diagram, as the receivers on the sides of the train are affixed to the train, and the train has traveled to the right in your diagram. So, in the bottom pic of your left diagram, the light has not yet reached the side receivers at the midpoint of the length of the sides of the train. In other words, in Einstein's embankment frame, the light can't possibly reach the side receivers in .5 seconds, because light has to travel a greater distance than .5 light seconds to reach them. The radius lines of the light sphere from the center of the sphere to the point the receivers are attached at the midpoint of the length of the train will be at an angle to the right, which means the radius lines are longer than .5 light seconds in length.

Please explain that before we continue.

I agree with everything you have said here. So would Einstein, I'm sure.

I've attached a pic to show the distance the light has to travel to reach the side receivers, of which the light has clearly not reached yet. Also note, that if the light sphere needs more time to expand to reach the side receiver, the train is also traveling to the right, so the time is even greater, as the angle increases. The light sphere will grow outside the train's front before the light reaches the sides.

What you say is possible. It depends on the particular speed of the box relative to the embankment. Recall that in Einstein's universe the box is length contracted by different amounts in the embankment frame depending on the speed of the box. If the box is going fast enough, then the light will hit the front of the box before it hits the y detectors on the sides. If the box is not going so fast, the light will hit the y detectors on the sides before it hits the front of the box. It just depends on the speed of the box, which determines the amount of length contraction.

 

[Motor Daddy]

I have the right to spend my time any way I want, you know, and I am under no obligation whatsoever to explain someone else's post if it doesn't interest me.

Are you saying the mistake James shows in his diagram is not interesting? Certainly you have interest in a possible inconsistency in SR, don't you?

Why are you so determined I divert my attention to James R's diagram? Are you that frightened of the points I raise in my own posts?

Because it's an inconstancy in SR which you can't explain, which is precisely the reason you aren't interested in it.

Yes, you terrify me.

Here's a thought: why don't you ask James R about his diagram, and answer me the simple question I asked you: why should everyone believe distance and time intervals are absolute?

I've already asked James and he hasn't responded. If you're scared say you're scared, don't pretend it doesn't interest you.

Given none of this is being contensted, why are you bringing this up?

You mentioned it earlier, and made reference to my beliefs dating back to ancient times. You've still not backed up your claim that my method was used in the 1900's and has long since proven wrong. Show me!

Oh, nobody's used your method. How terrible, that invalidates everyone else's experiments! Oh darn it, you really got the last century's worth of physicists there!

It was your claim that my method was used in the 1900's and long since been proven wrong. So show me who used my method, and show me the calculations they had of my method, since you claim people used to believe that.

Do you have anything productive to add to the conversation, or are you just here to heckle?

 

[Motor Daddy]

No. My diagrams for that frame do not show the light reaching the y detector.

I know. Are you trying to say the embankment observer doesn't know when the light reaches the y receiver?

The embankment frame using Einstein works exactly the same way that your embankment or "space" frame works.

No, you're wrong, James. The observer in my embankment doesn't see length contraction or time dilation. The observer in my frame knows the absolute velocity of the embankment, he doesn't ASSUME the embankment to be at rest, he measures the absolute velocity of the embankment. Einstein always assume one or the other frames is at rest and the other has all the motion. I don't assume one is at rest. It could be that the embankment has a 3,000 m/s velocity and the train has a 2,000 meter velocity in the same direction, or opposite direction. There is a HUGE difference in my embankment and Einstein's embankment. It just so happens that in Einstein's Chapter 9, the embankment is at a zero velocity, because of the way he designed the scenario. It has to be at a zero velocity, and the train has to be in motion.

Light takes longer than .5 seconds to reach the y detector because the y detector is moving along in the x direction in the embankment frame.

Correct, so you have a dilemma on your hands now. The embankment observer sees that it takes longer than .5 seconds for the light to reach the y receiver, but the train observer says it takes .5 seconds in the train frame, which clearly it doesn't. Are you saying there is some sort of time dilation or length contraction going on in the length between the source on the train and the y receiver on the train? What does the train observer think the length is between the source and the receiver, .5 light seconds? How much time does he think it took for light to travel from the source to the y receiver?

It is the train frame that we disagree about. The diagrams for your train frame and for Einstein's are quite different, whereas the diagrams for your embankment frame and Einstein's are the same. That's because of your different speed-of-light postulate.

That is a separate issue. We'll get to all the problems, and there are many.

What you say is possible. It depends on the particular speed of the box relative to the embankment. Recall that in Einstein's universe the box is length contracted by different amounts in the embankment frame depending on the speed of the box. If the box is going fast enough, then the light will hit the front of the box before it hits the y detectors on the sides. If the box is not going so fast, the light will hit the y detectors on the sides before it hits the front of the box. It just depends on the speed of the box, which determines the amount of length contraction.

So you've just shown that the box observer can't measure the speed of light to be c in all directions in the box.

 

[Motor Daddy]

This took me half a minute:

http://www.desy.de/user/projects/Physics/Relativity/SR/experiments.html#one-way_tests

So much for your ability to type things into Google.

I happened to notice this in the link you provided:

7. Tests of Length Contraction

At this time there are no direct tests of length contraction, as measuring the length of a moving object to the precision required has not been feasible.

So there are no direct tests of length contraction, and yet, you claim it to be a fact. What do you base your facts on?

 

[arfa brane]

So there are no direct tests of length contraction, and yet, you claim it to be a fact. What do you base your facts on?

There are no direct tests of changes in the speed of light. Yet, you claim the changing speed of light is a fact. What do you base your facts on?

You also claim that light propagating at c in a vacuum is a fact. What do you base this fact on?
It's not "I can write it on a piece of paper" is it?

 

[James R]

Motor Daddy:

Are you trying to say the embankment observer doesn't know when the light reaches the y receiver?

No.

Give me the speed of the box and I'll calculate for you when light reaches the y detector.

Alternatively, tell me when light reaches the y detector and I'll tell you the speed of the box relative to the embankment.

The embankment frame using Einstein works exactly the same way that your embankment or "space" frame works.

No, you're wrong, James. The observer in my embankment doesn't see length contraction or time dilation.

Yeah, ok. That's a difference, but it makes no difference to the calculation of the time it takes to for light to reach the y detector. There's no length contraction in that calculation, since the y detector is a point in space, not a length. It is irrelevant that the whole side of the cube the y detector sits on is length contracted, for the purposes of that calculation.

Time dilation doesn't enter the problem either, because we're only working in one frame - the embankment frame. You, Motor Daddy, never do any calculations in the train frame. All your calculations are always in the embankment frame, so there's no need to even apply relativity. The only time we need relativity is if we want to translate our times and distances and stuff into the train frame. And you never seem particularly interested in doing that.

The observer in my frame knows the absolute velocity of the embankment, he doesn't ASSUME the embankment to be at rest, he measures the absolute velocity of the embankment.

For the purposes of this discussion, I am taking the embankment to be absolutely at rest when I discuss the Motor Daddy universe. In that universe, if the embankment was not absolutely at rest, then things would be more complicated. I don't think we need to introduce extra complications such as the embankment moving relative to "space" and box moving relative to embankment. Two frames is enough; we don't need to discuss three. You have enough trouble even discussing one.

Einstein always assume one or the other frames is at rest and the other has all the motion. I don't assume one is at rest.

Yes, yes. I know.

It just so happens that in Einstein's Chapter 9, the embankment is at a zero velocity, because of the way he designed the scenario. It has to be at a zero velocity, and the train has to be in motion.

No. Einstein's universe doesn't have absolute velocities at all. All he uses in chapter 9 is the relative velocity of the train to the embankment. He never mentions absolute velocities at all. Read it and see!

Light takes longer than .5 seconds to reach the y detector because the y detector is moving along in the x direction in the embankment frame.

Correct, so you have a dilemma on your hands now. The embankment observer sees that it takes longer than .5 seconds for the light to reach the y receiver, but the train observer says it takes .5 seconds in the train frame, which clearly it doesn't. Are you saying there is some sort of time dilation or length contraction going on in the length between the source on the train and the y receiver on the train?

In Einstein's universe, train clocks do not run at the same rate as embankment clocks. So, yes, there is time dilation. The only length contraction is the contraction of the box in its direction of motion, as shown in my diagrams for the embankment frame.

What does the train observer think the length is between the source and the receiver, .5 light seconds? How much time does he think it took for light to travel from the source to the y receiver?

According to the train observer, the distance from source to y receiver is 0.5 light seconds. It takes light 0.5 seconds to travel that distance, as measured by the train clocks.

So you've just shown that the box observer can't measure the speed of light to be c in all directions in the box.

Einstein's 2nd postulate specifies that the speed of light is measured to be the same in all directions in the box. That's the starting point.

It's you who says that the speed of light will be measured to be different in different directions in the train frame.

 

[James R]

Show me that regardless of the acceleration of the cube, the light always takes .5 seconds to reach each receiver at the center of each wall, and that light reaches each receiver simultaneously.

Special relativity doesn't deal with acceleration, so don't go introducing that now. We're dealing with a situation where the box moves at constant velocity with respect to the embankment.

Measuring bars can be used. It's Einstein that has trouble with measuring bars, not me. At any point in time, I can take a .5 light second bar and fit it perfectly between the source and each side, and also between the source and the front and rear.

In Einstein's universe that is true in the train frame but not in the embankment frame.

What [Einstein] fails to account for is that if the distance from the source to the side remains .5 light seconds in width, regardless of the velocity, that light will take more time to reach the side receivers centered on the length of the train, or cube as the velocity increases. So, there is NO WAY that light can reach the receivers on the sides in .5 seconds at any velocity greater than 0 m/s.

How do you explain that?

You're not keeping your frames straight. You're mixing them up.

In the embankment frame, your above statement is true. In the train frame it is not.

[James's] representation is accurate according to SR. SR is incorrect, and it shows in his representation. There is no fixing that slight little problem without having to admit SR is inconsistent.

You have shown no inconsistency of SR, here or anywhere. Often you refuse to work with Einstein's postulates when you attempt to discuss SR, and that refusal leads you to make errors. The problem is not that SR is inconsistent; it's that you never apply it properly. In fact, you refuse to learn how to apply it. And that's why you really don't understand it at all. You're not imaginative enough to explore an alternate kind of mathematical or conceptual universe. You are stuck in the "common-sense" one that you have devised, to the point where you deny reality.

I don't know what you are trying to say.

The time it takes light to reach the side receiver from the source is affected by the train's absolute velocity.

Not absolute velocity. Velocity in whatever frame you're measuring the time.

Which statement best describes your statement?

1. A light is emitted from a moving source at t=0. At t=1 the light will be 299,792,458 meters from the point in space the source emitted the light, and if the source was moving at .5 c during that 1 second, the source will be 149,896,229 meters away from the point of emission, and the light will be 149,896,229 meters in front of the source.

This is true in the embankment frame, which sees the source moving.

2. A light is emitted from a moving source at t=0. At t=1 the light will be 299,792,458 meters in front of the moving source, and if at t=1 the moving source is 149,896,229 meters away from the point in space the light was emitted, then the light will be 449,688,687 meters away from the point in space the light was emitted.

This is wrong, because you're mixing the frame in which the source moves with the frame in which the source doesn't move.

In the cube, light has to travel a distance to reach the receiver. If the receiver "runs away" from the light, the light has to travel a greater distance and time to catch it. That doesn't mean the speed of light changes, it means the distance and time are greater to reach the receiver, which means the measured speed of light is not c due to the motion of the receiver and source.

Both the distance and time increase. The ratio of the two remains the same: the constant speed of light.

So you think that regardless of the motion of a source, the light will always be 299,792,458 meters in front of the source 1 second later.

Only in a frame where the source is stationary (such as the train frame).

If a train is in motion going down the tracks, and a light is emitted at t=0 at the rear of the train towards the front of the train, and the light reaches the front of the train at t=1, how long is the train??

In which frame are the times being measured? Are we using embankment clocks or train clocks? You haven't specified.

So everyone was probably born believing they could measure an absolute velocity of a box in space, from within the box, with no external references? Can you show me some of the old calculations of the past to back up your assertion that people used to believe what I now believe? Did they have actual real world measurements to support their beliefs? What makes all the previous evidence false now, and what makes you think that your current evidence will remain intact unlike the previous evidence?

Galileo, back in the 1600s, had a theory essentially the same as yours, with no time dilation, no length contraction, and a speed of light that varied in different frames in a "common sense" way. The only claim he didn't make is that there is an absolute reference frame.

Distance and time are absolute because light travels a specific distance in space in a specific duration of time. It never changes, it is absolute, regardless of what an observer says!

Using your theory, light travels different distances in the train frame in the same amount of time, depending on the train's "absolute" speed. So, what you say is wrong by your own theory.

Nobody has ever used my method, so don't try to claim its been tried before and it failed.

Anybody can use your method on the embankment to determine the speed of the train relative to the embankment. They use embankment rulers and clocks, and they're done.

Don't imagine for a moment that this method gives you any kind of absolute speed though.

 

[James R]

So there are no direct tests of length contraction, and yet, you claim it to be a fact. What do you base your facts on?

On the indirect tests, of course!

Relativity makes many many predictions about all sorts of experimental situations. Each one that is confirmed with real-world evidence supports the entire, self-consistent theory.

Thousands of real-world experiments that confirm Einstein's relativity are done every day.

 

[Motor Daddy]

Motor Daddy:
No.

Give me the speed of the box and I'll calculate for you when light reaches the y detector.

Alternatively, tell me when light reaches the y detector and I'll tell you the speed of the box relative to the embankment.

Maybe you don't understand, James. The only way to know the times is to measure them on the train. There is an x and y receiver on the train centered on the front wall and the side wall respectively. A light source remains fixed at the midpoint of the train. The motion of the train is unknown. You send a light from the source at t=0. The light reaches the y receiver in .65 seconds, and the y receiver at 1.384930 seconds. The measurements were taken on the train. As far as the train observer can tell, there is no outside world or embankment. Light travels at c in space. The train can also travel in space during the same time. What is the velocity of the train in space, relative to the point in space the light was emitted?

You say if I give you the times you can tell me the speed of the box relative to the embankment. No, James, you can't, because the times I give you will be as measured on the train. The only thing you know from that is the train's absolute velocity. You have no clue what the embankment's velocity is. You need to measure the embankment's velocity by measuring its times in the embankment frame before you can know the embankment's velocity. You can not tell me the relative velocity between the embankment and the train by just knowing one time or the other, unless you ASSUME one to be at rest.

If you don't understand that, I will make an example of a train with an absolute velocity greater than zero, and an embankment that has an absolute velocity greater than zero. Then I will give you the train's times. You claim you can tell me the relative velocity between the train and the embankment from just those times? Negatory good buddy! No, you can't, because for all you know, you can calculate the train's velocity as say 1,200 m/s in one direction, but without knowing the embankment's 1,500 m/s velocity in the opposite direction, you will NEVER give me the correct relative velocity between the train and the embankment. The ONLY time you would be correct is if the embankment had an absolute zero velocity in space!

 

[arfa brane]

But, in the real universe, the light emitted from the midpoint of the train reaches both receivers at the same time.

When you say "the measurements were taken on the train", you aren't just writing down some formulas that fit a diagram you drew on paper, rather than making actual physical measurements are you? You aren't inventing numbers, rather than measuring them, surely?

Oh, sorry, you are. Well, that's sort of interesting, but not really much use in the real world, is it?

Funny though, that you say this:

The only way to know the times is to measure them on the train.

But, you don't measure them, you make them up. You pull them out of a hat (or your ass) and say they're measurements, when they aren't, they're just numbers you invented.

 

[Motor Daddy]

Yeah, ok. That's a difference, but it makes no difference to the calculation of the time it takes to for light to reach the y detector. There's no length contraction in that calculation, since the y detector is a point in space, not a length. It is irrelevant that the whole side of the cube the y detector sits on is length contracted, for the purposes of that calculation.

The y detector is a length just as the x detector is a length. The y detector is a distance away from the source at any given point in time, just as the x detector is a distance away from the source at any point in time.

You just don't seem to get the fact that if you're riding in the train that has a .9c velocity, you can take a .5 light second bar and place it perfectly in between the source and the front wall, and the source and the side wall. The .9c come from the fact that the source (which remains fixed at the center of the train) is traveling away from the point in space that it emitted light at t=0. The walls are traveling with the source, remaining the same distance away from the source in the train frame. The source alone can be said to posses the velocity, and since all the receivers are an equal distance away from the source at all times, they too posses the velocity in the same direction.

 

[Motor Daddy]

For the purposes of this discussion, I am taking the embankment to be absolutely at rest when I discuss the Motor Daddy universe. In that universe, if the embankment was not absolutely at rest, then things would be more complicated. I don't think we need to introduce extra complications such as the embankment moving relative to "space" and box moving relative to embankment. Two frames is enough; we don't need to discuss three. You have enough trouble even discussing one.

Good, it does make it easier to have the embankment at an absolute zero velocity, because then you can't come back and say it was the embankment that had the velocity and the train can be considered at rest. You can't have your cake and eat it too. If the embankment is at an absolute zero velocity, then it is quite clear the train is in motion, as light always travels at c in space.

 

[Motor Daddy]

No. Einstein's universe doesn't have absolute velocities at all. All he uses in chapter 9 is the relative velocity of the train to the embankment. He never mentions absolute velocities at all. Read it and see!

Einstein doesn't even realize it himself, and neither do you, but the embankment in his scenario has an absolute zero velocity. It must have been at an absolute zero velocity for things to happen the way they did in his example. Like I said before, I can construct a similar example, where the train AND embankment each have an absolute velocity greater than zero, and you just knowing the train's times would not allow you to calculate the relative velocity between the train and the embankment unless you knew the embankment's absolute velocity. That's a fact! Can you tell the relative velocity between you and the car in front of you going down the highway if you only know your velocity? You think you can? You are going 60 MPH down the highway. At t=0 the car is 50 feet in front of you. How far is the car in front of you at t=1?

 

[Motor Daddy]

In Einstein's universe, train clocks do not run at the same rate as embankment clocks. So, yes, there is time dilation. The only length contraction is the contraction of the box in its direction of motion, as shown in my diagrams for the embankment frame.

So the y clocks are dilated, but the length isn't contracted?