Is it possible to determine absolute speed?

[talk2glenn]

No, I pursue it because I see a contradiction in my understanding and in what others say.

What contradiction? Be specific.

Am I the one moving or is that other object moving past me.

You've answered your own question. You cannot describe the motion of the object without the "past me". It has only relative motion; there is no such thing as absolute motion. Forget the calculus for a moment - just as a thought experiment try and describe the motion of an object through space, without an observer the object moves relative to.

This has been understood in some form since Galileo posited the man in the ships hold experiment, and predates both relativity and calculus.

 

[DaveC426913]

bkelly, I ask you again:

if you took the entire experiment - trolley, yardstick, Sally and her friend - and accelerated it to .5c and took the measurements again, do you expect to see any differences in the outcome?

 

[JesseM]

Please explain the term: the x-coordinate dx. I understand two and three dimensional Cartesian coordinates and understand that dx stands for delta in x or a difference. But I don't know what delta you are referring to.

Remember, I referred to two events: the first event was the light striking next to the left side of the meter-stick, the second event was the light striking next to the right side. If the first event had position coordinate x₀ in the cartesian coordinate system where the wall is at rest, and the second event had position coordinate x₁ in this same coordinate system, then dx in this coordinate system is x₁ - x₀, that's why I said "the difference in x-coordinate dx" (i.e. dx is the difference between the x-coordinates of the two events, you took this out of context when you just quoted the second part "x coordinate dx"). Likewise, if the first event had time coordinate t₀ in the rest frame of the wall, and the second event had time coordinate t₁, then dt=t₁ - t₀ (so if the events occurred at the same moment in this frame, dt=0 in this frame)

 

[Janus]

To All,
A problem that I see in open threads like this is that the thread gets fragmented and there are too many points to address at one time. I am not able to keep track of all these concepts simultaneously. I don’t want to short anyone a deserved reply, but I need to ask about some fundaments for which I don’t understand the importance.

My basic problem is this simultaneity concept. From post 23 by Dave:



I chose that post as it is a good simple place to begin.

From this response I conclude that Dave believes the photons arrive at the fence at sufficiently different times to spoil the test. It is perfectly reasonable to state: It is a given that the light source is stationary wrt to Sally, and wrt to the fence, and the distance from the light source to both ends of the meterstick is identical (at the time the image is created). If you please, let's also presume that all calculations have been done in advance and the light strobes (flashes) at just the right time so that the photons will reach the meterstick and wall and the right time to create the shadow image. Remember I did say that the meter stick is only an atom's width in thickness and only an atom's width distance from the wall.

I say the photons will arrive at both ends of the meter stick as the same time. If Dave says not, then I must ask a few questions.

Before asking "why," I want to stick to my knitting so to speak. I ask for your indulgence and skip the why for the moment and concentrate on the results.
How much difference will there be in the timing of the photons hitting the fence?
I expect the answer to that will provide the answer to “How much difference will it make to my test?”

I will be reading other posts and see what I can learn from them. In the meantime, I don’t want to short any post, I just need to concentrate on a few narrow topics at one time. Thanks for your patience.

I think a major problem here is that there is a bit of "talking past each other" going on here. I have a feeling that when we say that there are "simultaneity issues", you are thinking of something different from what we mean. It is a "concept" problem and not a "How much of a difference will it make?" problem.

So, before we deal with the "how much of a difference it will make" issue, let's make sure that we are on the same page conceptually first.

The concept that you aren't getting is called the Relativity of Simultaneity. One of the postulates of Relativity is that the speed of light is the same for all inertial frames.

Using that, we can show that two observers moving relative to each other will disagree at to whether two events are simultaneous or not.

Consider the following scenario:

You have an observer standing beside a railroad track. Two flashes of light occur to either side of him, both an equal distance from him. He see the flashes at the same instant, and knowing that they originated equal distances from him, he knows that they originated simultaneously.

At the same instant that he sees the flashes, a second observer riding on a railway car passes him. The second observer also sees the flashes at the same time.

The following animation show events as they occur according to the embankment observer.

[URL]http://home.earthlink.net/~jparvey/sitebuildercontent/sitebuilderpictures/train1.gif[/URL]

Now let's consider things according to the rail car observer. We already know that he sees that both flashes at the same time, but what does this tell him about when they originated. Remember, the speed of light is a constant for him. He cannot measure the light light of one flash as traveling faster than the other relative to himself. He also knows that between the time the flashes originated his relative distance from the origins has changed. since he was an equal distance from the origins when he saw the flashes, it stands to reason that he was when the flashes originated, he was closer to one than the other. But, if the flashes originated simultaneously, and he was closer to one than the other when that happened, given a constant speed for both light flashes, he should have seen one flash before the other. Instead, he saw both flashes at the same time. The only way that this could be is if the flashes originated at different times.

this Animation shows the same events as the first animation but according to the railway observer.

[URL]http://home.earthlink.net/~jparvey/sitebuildercontent/sitebuilderpictures/train2.gif[/URL]

He sees the flashes simultaneously, but determines that they originated at different times.

The two observer will disagree as to whether or not the flashes originated at the same time or not.

Are you with me so far?

 

[bkelly]

I think a major problem here is that there is a bit of "talking past each other" going on here. I have a feeling that when we say that there are "simultaneity issues", you are thinking of something different from what we mean. It is a "concept" problem and not a "How much of a difference will it make?" problem.

That may well be.

The two observer will disagree as to whether or not the flashes originated at the same time or not.
Are you with me so far?

I don't think I am with you as I don't see anything to do with relativity. Two lights separated by some distance flash. At some point between the two lights the the light waves cross. Anyone that is in the location will see both lights at the same time. It does not matter if they were standing there when the lights flashed, or if they were standing somewhere else and moved there in time to be there when the light waves intersected each other.

However, I presume that you will follow this up and will wait for that post.

Thanks for taking the time to post.

 

[DaveC426913]

bkelly, I ask you again:

Hrmf. You said you would answer me.

I think you're avoiding answering questions that don't jive with what you want to be true.

I think you know that, the moment you tried to answer my question, you'd realize your experiment will fall apart (i.e. by not falling apart).

Accelerating the entire experiment to relativistic speeds and then taking the measurements again will have absolutely no effect on the results. If "being stationary" and "moving at .5c" results in no change to the experiment, one need go no further than that to conclude that there cannot be an absolute velocity.

QED.

 

[bkelly]

Remember, I referred to two events: the first event was the light striking next to the left side of the meter-stick, the second event was the light striking next to the right side. If the first event had position coordinate x₀ in the cartesian coordinate system where the wall is at rest, and the second event had position coordinate x₁ in this same coordinate system, then dx in this coordinate system is x₁ - x₀, that's why I said "the difference in x-coordinate dx" (i.e. dx is the difference between the x-coordinates of the two events, you took this out of context when you just quoted the second part "x coordinate dx"). Likewise, if the first event had time coordinate t₀ in the rest frame of the wall, and the second event had time coordinate t₁, then dt=t₁ - t₀ (so if the events occurred at the same moment in this frame, dt=0 in this frame)

Lets simplify a little bit and go one step at a time. Change the scenario and have the meterstick hovering next to the fence, not moving relative to the fence. The light flashes. The distance between the light and each end of the meterstick is identical. Will the photons at each end of the meterstick reach the fence at the same time?

I say yes.

The photons get to each end of the meterstick at the same time and go by it to hit the fence, or get stopped by it. The stationary meterstick cannot change the distance the photons travel and cannot change their velocity. The photons don't care if the meter stick is moving. It still cannot change the photon's travel distance or velocity.

As I understand you, you are saying that is not the case. Where in the above scenario do we have a disagreement?

 

[bkelly]

Hrmf. You said you would answer me.

I think you're avoiding answering questions that don't jive with what you want to be true.

I think you know that, the moment you tried to answer my question, you'd realize your experiment will fall apart (i.e. by not falling apart).

Accelerating the entire experiment to relativistic speeds and then taking the measurements again will have absolutely no effect on the results. If "being stationary" and "moving at .5c" results in no change to the experiment, one need go no further than that to conclude that there cannot be an absolute velocity.

QED.

My apologies, but to create an analogy: I am on the bottom rung of this ladder and you are several steps up. I need to simplify things and understand this bottom rung before I move up. I'll let my recent two posts ride a bit before I continue.
Thanks for your patience.

 

[Janus]

That may well be.
I don't think I am with you as I don't see anything to do with relativity. Two lights separated by some distance flash. At some point between the two lights the the light waves cross. Anyone that is in the location will see both lights at the same time. It does not matter if they were standing there when the lights flashed, or if they were standing somewhere else and moved there in time to be there when the light waves intersected each other.

However, I presume that you will follow this up and will wait for that post.

Thanks for taking the time to post.

The fact that they see the flashes at the same moment in only a part of the example. (In fact, the example is deliberately set up so they do. The issue revolves around when the events that caused the flash they saw occurred.

The first observer was always halfway between the points where these events occurred. Since the light, traveling at a constant speed, had to take an equal amount of time to reach him from each event, he correctly concludes that these events occurred simultaneously.

The second observer is different. He sees the flashes at the same time, and he knows that he is halfway between the two points when he sees the flashes, however, he also knows that the point where the flashes originated are moving with respect to himself. To him, the light originates at the source and expands as a sphere from a point that maintains a constant distance from himself while the source moves on. (Second postulate of relativity: The speed of light is a constant relative to any inertial frame of reference.) Since the event that caused the flash had to happen sometime before he reached the midpoint between the sources, he had to be closer to one source than the other when they occured. Thus, if the events that caused the flashes happened simultaneously in his frame of reference, he would have to see one flash before the other. But we've already established that he saw them at the same time. The only way this can happen and still hold to the constant speed of light postulate, is for the events the created the flashes to occur at different times from each other as shown in the second animation.

If you can't visualize the above example, here's another:

You have two clock that you want to synchronize. You decide to carefully measure out the distance between them and set off a flash of light from a spot exactly halfway between the two. The clocks are initially set to zero and are designed to start the instant the light hits them:

[URL]http://home.earthlink.net/~parvey/sitebuildercontent/sitebuilderpictures/synch1.gif[/URL]

Notice how the expanding light(shown by the circle) moves out from the point between the two clocks, reaching them at the same time and starting them simultaneously. From that point on they run at the same rate and show the same time.

Now, however, we add a second observer in a reference frame that is moving with respect to the two clocks.

He also sees the flash start at a point exactly halfway between the two clocks and expand out as a sphere. However, the clocks do not maintain their positions with respect to him or the expanding light. The clocks are moving to the right (in our example). This means that one clock is rushing towards the part of the light heading in its direction, and the other clock is running away from the light headed in its direction:

[URL]http://home.earthlink.net/~parvey/sitebuildercontent/sitebuilderpictures/synch2.gif[/URL]

As a result, one clock is struck by light before the other and starts ticking before the other. Once both clocks are running, they run at the same rate, but are always offset from each other.

Remember, this are the exact same clocks and the exact same light, just according to two different frames with a relative motion with respect to each other.

And this is the simultaneity issue that we have been talking about. Events that are simultaneous in one frame will not be simultaneous in all frames.

 

[Bussani]

[PLAIN]http://home.earthlink.net/~parvey/sitebuildercontent/sitebuilderpictures/synch2.gif[/QUOTE]


I have to ask a side question here: with a setup like this, couldn't you conclude that you're moving at, say, 0.9c, even without something else to immediately compare yourself to?

 

[Janus]

I have to ask a side question here: with a setup like this, couldn't you conclude that you're moving at, say, 0.9c, even without something else to immediately compare yourself to?

No. Remember this animation shows what happens according to someone who has a relative velocity to the clocks. It doesn't matter whether you consider that its him or the clocks that are moving.

According someone stationary with respect to the clocks events would occur as shown in the first animation. This is the point. Someone moving with the clocks says that they start simultaneously, while someone watching the clocks move past them says that they don't, and they are both equally correct in making their claim.

 

[Bussani]

No. Remember this animation shows what happens according to someone who has a relative velocity to the clocks. It doesn't matter whether you consider that its him or the clocks that are moving.

According someone stationary with respect to the clocks events would occur as shown in the first animation. This is the point. Someone moving with the clocks says that they start simultaneously, while someone watching the clocks move past them says that they don't, and they are both equally correct in making their claim.

Ah, I see. I guess I should say that what I find it hard to wrap my head around is why you can't use a light emitter and two light sensors at equal distances from it to detect motion, since even with the effects of time dilation, the light shouldn't be able to move at a different speed to reach the one moving away from it at the same time as the one moving towards it. It seems that this is the heart of simultaneity, but I find it hard to picture why it's so. Is it something to do with length contraction? Or should I just accept that light is always constant for the observer and leave it at that?

Edit: Sorry for hijacking the thread a bit.

 

[JesseM]

Lets simplify a little bit and go one step at a time. Change the scenario and have the meterstick hovering next to the fence, not moving relative to the fence. The light flashes. The distance between the light and each end of the meterstick is identical. Will the photons at each end of the meterstick reach the fence at the same time?

Yes, in the frame where the fence and meterstick are at rest. But we are free to analyze this situation from a different frame where they are both in motion, and in this frame the photons don't hit every point on the fence at the same time, instead they strafe across, hitting different points on the fence in succession.

As I understand you, you are saying that is not the case. Where in the above scenario do we have a disagreement?

If you're just talking about analyzing things from the perspective of the frame where the fence is at rest, then what you're saying is correct, the photons do hit both sides of the stick simultaneously in this frame. But in other frames this is not true.

 

[JesseM]

I don't think I am with you as I don't see anything to do with relativity. Two lights separated by some distance flash. At some point between the two lights the the light waves cross. Anyone that is in the location will see both lights at the same time. It does not matter if they were standing there when the lights flashed, or if they were standing somewhere else and moved there in time to be there when the light waves intersected each other.

But it does matter if observers moving at different speeds both assume that the two light beams traveled at the same speed relative to themselves. Then if one observer sees that both flashes occurred at the same distance from himself, and he receives the light from each at the same time, he must conclude that since both flashes were covering the same distance at the same speed, they must have occurred at the same time. That would be the embankment observer (yellow dot on side of tracks) in this animation:

[URL]http://home.earthlink.net/~jparvey/sitebuildercontent/sitebuilderpictures/train1.gif[/URL]

On the other hand, another observer might see that one flash occurred nearer to himself than the other (he could judge the distance by visual parallax for example, or he might actually be carrying a long ruler at rest relative to himself and observe which marking on his ruler each flash occurred next to). In that case, the only way to explain why the light from each flash reached him at the same time, consistent with the assumption that both beams traveled at the same speed (so if the light from the left flash had less distance to travel, there must have been a shorter time between the light being emitted by the left flash and the light from the left flash reaching his eyes), is that the two flashes actually occurred at different times in his frame, the farther flash happening earlier than the closer flash. That's what's illustrated by this animation where the train observer (yellow dot on the train) is at rest while the tracks are in motion):

[URL]http://home.earthlink.net/~jparvey/sitebuildercontent/sitebuilderpictures/train2.gif[/URL]

 

[Janus]

Ah, I see. I guess I should say that what I find it hard to wrap my head around is why you can't use a light emitter and two light sensors at equal distances from it to detect motion, since even with the effects of time dilation, the light shouldn't be able to move at a different speed to reach the one moving away from it at the same time as the one moving towards it. It seems that this is the heart of simultaneity, but I find it hard to picture why it's so. Is it something to do with length contraction? Or should I just accept that light is always constant for the observer and leave it at that?

Edit: Sorry for hijacking the thread a bit.

In Relativity, the constant speed of light is a postulate; something taken to be true, and everything else,(time dilation, length contraction, the relativity of simultaneity) fall out as a result.

That's not to say that he just pulled the idea out of his hat though. Maxwell's equations describing electromagnetism already predicted that the speed of light would be independent on the velocity of the source. Einstein took this idea and Galileo's principle of relativity and carried it out to the logical conclusion.

Length contraction and the relativity of simultaneity can be seen as running hand in hand.

Consider Einstein's train example:

Like above, it starts with an observer on the side of the tracks and an observer in the train.

But unlike my earlier example, this time the flashes of light originate when the two observers are even with each other:

[URL]http://home.earthlink.net/~jparvey/sitebuildercontent/sitebuilderpictures/trainsimul1.gif[/URL]

Note that according to the embankment observer, not only is the other observer even with him when the flashes originate at the red dots, but each end of the train is next to a red dot when they originate. Also, it needs to be pointed out that since the train is moving relative to the observer and dots, it is length contracted, and it is this contracted length that fits between the dots. Also notice how the train observer meets up with one flash before the other.

Now let's switch to the frame of the train. In this frame, the train is not contracted. In fact since it is the embankment that is moving, it is the embankment that is contracted. Now the distance between the red dots is shorter than the length of the train. Both ends of the train cannot therefore be next to the red dots at the same time. Since we established that the flashes originated when each end of the train was next to a red dot (for the sake of argument we will assume that it was this event that caused the flash), It is obvious that the flashes have to originate at different times according anyone on the train.

[URL]http://home.earthlink.net/~jparvey/sitebuildercontent/sitebuilderpictures/trainsimul2.gif[/URL]

Note how our train observer will see the flashes at different times, just like he does according to the embankment observer. Not only that, but he sees each flash in the second animation in the exact same spot relative to the embankment and he does in the first animation. For example, the leftmost flash reaches him just as he is passing the right red dot in both.

 

[bkelly]

bkelly, I ask you again:

if you took the entire experiment - trolley, yardstick, Sally and her friend - and accelerated it to .5c and took the measurements again, do you expect to see any differences in the outcome?

I don't know. I am reading all these posts, not understanding many things, trying to find the bits I do understand, and use those bits to leverage an understanding of other bits. I'm just not ready to say anything yet. Please interpret my silence as taking time to think and re-think.

I'll be back soon.

 

[Bussani]

Thanks for the reply, Janus. That's a great explanation; it really made sense to me. I guess I've always had it in my head somewhere that time dilation, length contraction and whatever else came together to make the speed of light constant for everyone, but I see now that it's a more fundamental truth than that. I still have some thoughts I'd like to clarify, but I'll probably make a new thread rather than hijacking this one further.

 

[Austin0]

In Relativity, the constant speed of light is a postulate; something taken to be true, and everything else,(time dilation, length contraction, the relativity of simultaneity) fall out as a result.

That's not to say that he just pulled the idea out of his hat though. Maxwell's equations describing electromagnetism already predicted that the speed of light would be independent on the velocity of the source. Einstein took this idea and Galileo's principle of relativity and carried it out to the logical conclusion.

Length contraction and the relativity of simultaneity can be seen as running hand in hand.

Consider Einstein's train example:

Like above, it starts with an observer on the side of the tracks and an observer in the train.

But unlike my earlier example, this time the flashes of light originate when the two observers are even with each other:

[URL]http://home.earthlink.net/~jparvey/sitebuildercontent/sitebuilderpictures/trainsimul1.gif[/URL]

Note that according to the embankment observer, not only is the other observer even with him when the flashes originate at the red dots, but each end of the train is next to a red dot when they originate. Also, it needs to be pointed out that since the train is moving relative to the observer and dots, it is length contracted, and it is this contracted length that fits between the dots. Also notice how the train observer meets up with one flash before the other.

Now let's switch to the frame of the train. In this frame, the train is not contracted. In fact since it is the embankment that is moving, it is the embankment that is contracted. Now the distance between the red dots is shorter than the length of the train. Both ends of the train cannot therefore be next to the red dots at the same time. Since we established that the flashes originated when each end of the train was next to a red dot (for the sake of argument we will assume that it was this event that caused the flash), It is obvious that the flashes have to originate at different times according anyone on the train.

[URL]http://home.earthlink.net/~jparvey/sitebuildercontent/sitebuilderpictures/trainsimul2.gif[/URL]

Note how our train observer will see the flashes at different times, just like he does according to the embankment observer. Not only that, but he sees each flash in the second animation in the exact same spot relative to the embankment and he does in the first animation. For example, the leftmost flash reaches him just as he is passing the right red dot in both.

Great graphic , its just too bad it goes by so quickly. Somebody should run it through After Effects or Final CUt and slow it down so we could really see it.
Thanks

 

[Janus]

Great graphic , its just too bad it goes by so quickly. Somebody should run it through After Effects or Final CUt and slow it down so we could really see it.
Thanks

There are two ways I could slow it down. One is to decrease the frame rate, but that would make it run choppy. The other would be to increase the number of frames. This would allow it to run slower and still run smooth. The drawbacks with that are that it would require that I re-render the animation and that it would increase the file size considerably. Since I have limited amount of server space through my ISP, and I have other uses for it too, I'm not inclined to devote that much resources to this animation.

 

[bkelly]

Hello Janus,

Just in case, this is a response to posts 54 and 59 of this thread where you posted your animations:

To him, the light originates at the source and expands as a sphere from a point that maintains a constant distance from himself while the source moves on. (Second postulate of relativity: The speed of light is a constant relative to any inertial frame of reference.) Since the event that caused the flash had to happen sometime before he reached the midpoint between the sources, he had to be closer to one source than the other when they occured.

I don’t see anything relationship with relativity. All four examples could be conducted with sound in our atmosphere and obtain the same results.

I have been roundly criticized via the simultaneity problem. But the simultaneity concept applies to these animations. According to several responders, these lights cannot be guaranteed to flash at the same time, they are two separate events. Why do the readers allow these animation go by without a peep when I have a one meter long trolley and the two markers cannot be allowed to make a mark at the same time?

 

[Bussani]

I am also looking at animations of post 54 and see a problem there. I have been roundly criticized via the simultaneity problem. But the simultaneity concept applies to these animations. According to several responders, these lights cannot be guaranteed to flash at the same time, they are two separate events. Why do the readers allow these animation go by without a peep when I have a one meter long trolley and the two markers cannot be allowed to make a mark at the same time?

The lights flashing in the animation are the same events seen from different perspectives. That's the point, if I understand right. If you make the marks at the same time from the perspective of the moving trolly, someone on the ground will see the marks get made at different times. They can't agree.

 

[Dale]

I have been roundly criticized via the simultaneity problem. But the simultaneity concept applies to these animations. According to several responders, these lights cannot be guaranteed to flash at the same time, they are two separate events. Why do the readers allow these animation go by without a peep when I have a one meter long trolley and the two markers cannot be allowed to make a mark at the same time?

The animations all come in pairs showing the same situation in two different frames. In each pair the light flashes or clocks are synchronized in one frame and not synchronized the other frame. This is correct.

Your error is assuming that they could be synchronized in both frames, which is not possible. The marks can be synchronized in a single frame, and in all other frames they will occur at different times. The distance between the marks will then only be related to the length in that single frame, as I showed earlier.

 

[Bussani]

I just want to make sure I've got all this right. The trolley passes by a fence at relativistic speeds. As it does, devices at the front and back are set to make marks on the fence simultaneously. From the perspective of the trolley, the fence appears length contracted as it passes it, but it nonetheless makes its two simultaneous marks. When the trolley then stops and returns to the fence, the fence will obviously not be contracted any more, so the marks should ultimately be a distance apart greater than the rest length of the trolley.

However, from the point of view of the fence in this very same experiment, it was the trolley that was length contracted as it passed. For the marks to end up in the same place (i.e. farther apart than the rest length of the trolley), someone by the fence would see the contracted trolley leave its marks at different times, rather than simultaneously. It would leave the one from the back of the trolley first, travel farther along, and then leave the one from the front. Like before, when the trolley slows down and returns to the fence, the marks would be farther apart than the rest length.

"I made the marks simultaneously, and they ended up this far apart because the fence’s length was contracted," says the man from the trolley.

"You’re wrong," says the man next to the fence. "It was the trolley that was contracted. The reason the marks are so far apart is that you did not make them simultaneously."

So both agree where the marks are, but disagree when it comes to when the marks were made. Is this accurate, or do I have it wrong?

 

[Mentz114]

Why do the readers allow these animation go by without a peep when I have a one meter long trolley and the two markers cannot be allowed to make a mark at the same time?

As everyone is telling you, you are missing the point that 'at the same time' is frame dependent. If two things appear simultaneous in one frame ( the trolley) they will not be simultaneous in other frames ( the fence).

 

[Janus]

Hello Janus,

Just in case, this is a response to posts 54 and 59 of this thread where you posted your animations:I don’t see anything relationship with relativity. All four examples could be conducted with sound in our atmosphere and obtain the same results.

No, they would not produce the same results. Sound has a constant speed relative to the medium through which it propagates. Since the train observer has a relative motion with respect to the the atmosphere, he will measure the speed of the sound relative to himself as being equal to the sum of the velocity of the sound relative to the air and his relative velocity with respect to the atmosphere. As a result, for him, the sound coming from behind him travels slower than the sound coming from in front. this cancels out the fact that he is closer to the rear sound when it was made and would end up with him determining that both sound were made simultaneously, just like the embankment observer does.

Light, on the other, as shown in the animations has the same speed relative to the observer regardless of whether it is the embankment observer or train observer. The embankment observer measures the light as traveling at 299,792,458 m/s relative to himself from both directions, and the train observer measures the same light as traveling at 299,792,458 m/s relative to himself from both directions.

The second animation done for sound instead of light would look completely different.

I have been roundly criticized via the simultaneity problem. But the simultaneity concept applies to these animations. According to several responders, these lights cannot be guaranteed to flash at the same time, they are two separate events. Why do the readers allow these animation go by without a peep when I have a one meter long trolley and the two markers cannot be allowed to make a mark at the same time?

Again, we are writing one thing and you are reading another. We are not saying that you cannot make two marks at the same time according to any given frame. We are saying that another frame will not agree with the fact that you made them simultaneously. And this is where things fall apart for you. In order for your test to provide evidence of absolute motion, both Sally and Tom would have to agree that he made his marks simultaneously, and while this would be true for Tom, it would not be so for Sally.

 

[bkelly]

There obviously is something I am missing. Let's try this.

Two markers are one meter apart. The make a mark at the same time. Let's not worry about the mechanics or electronics, or anything about how they are made. Let's presume I cause the marks to be made at the same instance. How can anyone else declare they were made at differing times?

I am not referring to when someone detects them being made as that can vary. How can they not be made at the same time in all references.

 

[atyy]

There obviously is something I am missing. Let's try this.

Two markers are one meter apart. The make a mark at the same time. Let's not worry about the mechanics or electronics, or anything about how they are made. Let's presume I cause the marks to be made at the same instance. How can anyone else declare they were made at differing times?

I am not referring to when someone detects them being made as that can vary. How can they not be made at the same time in all references.

We restrict ourselves to inertial reference frames. An inertial reference frame is a division of spacetime into space and time such that Maxwell's equations look simple. There are many such reference frames.

The different reference frames will not agree that they are 1 metre apart.
This is because what is space for one reference frame is a mixture of space and time for another reference frame.

The different reference frames will not agree that the marks are made at the same time.
This is because what is time for one reference frame is a mixture of space and time for another reference frame.

 

[Janus]

There obviously is something I am missing. Let's try this.

Two markers are one meter apart. The make a mark at the same time. Let's not worry about the mechanics or electronics, or anything about how they are made. Let's presume I cause the marks to be made at the same instance. How can anyone else declare they were made at differing times?

I am not referring to when someone detects them being made as that can vary. How can they not be made at the same time in all references.

I refer you to the image in the attachment, it shows one frame from both animations.

The top is from the embankment frame. The relative velocity difference between train and embankment is 0.5c . As a result, the train is contracted by a factor of 0.866. if the train is 100m long in its rest frame, then it is 86.6 m long in the embankment frame. The red dots are 86.6 m apart and the train just fits between them. Thus, as the train moves along the track, there will be a moment when the ends of the train and the red dots line up, and in this instance, the rear and front of he train lines up with their respective dots simultaneously.

The bottom shows the same train from the train frame. Here, the train is at its proper length of 100m, and it is the embankment that is length contracted. The distance between the red dots has contracted to 75 m (0.866 of 86.6m). It is obvious that the train is longer than the distance between the red dots. Thus it is impossible for the two ends of the train to meet up with their respective dots at the same time. The front of the train will reach and pass its dot long before the rear of the train reaches its dot.

Thus we have the same events according to two different frame. In one they are simultaneous, and in the other they are not.

 

[Bussani]

How can they not be made at the same time in all references.

How could they be? From the trolley the fence would be contracted, so the marks would end up farther apart when un-contracted. From the fence the trolley would be contracted, so the marks would end up closer together than the (rest) length of the trolley. It can't be both at the same time, which is what would happen if both frames of reference agreed on the marks being made simultaneously.

 

[Mentz114]

How can anyone else declare they were made at differing times?

I am not referring to when someone detects them being made as that can vary. How can they not be made at the same time in all references.

Now you're asking the right questions, and they've been answered perfectly in the above posts.

 

[bkelly]

How could they be? From the trolley the fence would be contracted, so the marks would end up farther apart when un-contracted. From the fence the trolley would be contracted, so the marks would end up closer together than the (rest) length of the trolley. It can't be both at the same time, which is what would happen if both frames of reference agreed on the marks being made simultaneously.

One little bit at a time if you please.

From the trolley the fence would be contracted, so the marks would end up farther apart when un-contracted.

Only if the fence were moving and the trolley not moving.

From the fence the trolley would be contracted, so the marks would end up closer together than the (rest) length of the trolley.

Only if the trolley were moving and the fence is not.

Regardless of which is moving, I see no reason why the marks cannot be made at the same time. If one or the other were moving then that one would be length contracted and the other not. If both are moving, both might be length contracted. Still, the marks can have been made at the same time from either reference point.

Lets keep the questions and answer simple with this: If the trolley is moving and the fence is not, and the marks made simultaneously from the perspective of the trolley, then at what times are the marks made with respect to the fence. Let us declare that the fence sees the left mark of the trolley made at exactly 1:00 PM, with an accuracy of better than one millionth of a femtosecond. What time does the fence see the mark for the right side made? And why do you say that?

 

[Dale]

If the trolley is moving and the fence is not, and the marks made simultaneously from the perspective of the trolley, then at what times are the marks made with respect to the fence. Let us declare that the fence sees the left mark of the trolley made at exactly 1:00 PM, with an accuracy of better than one millionth of a femtosecond. What time does the fence see the mark for the right side made? And why do you say that?

From the http://en.wikipedia.org/wiki/Lorentz_transformation" :

t' = \frac{1}{ \sqrt{1 - { \frac{v^2}{c^2}}}} \left( t - v x/c^{2} \right)

So for t = 0, v = .5 c, and x = 1 m, we get t' = 2 ns

 

[JesseM]

From the trolley the fence would be contracted, so the marks would end up farther apart when un-contracted

Only if the fence were moving and the trolley not moving.

From the fence the trolley would be contracted, so the marks would end up closer together than the (rest) length of the trolley.

Only if the trolley were moving and the fence is not.

In relativity there is no frame-independent truth about who "is moving" and who "is not". If two ships A and B are in relative motion, then in the rest frame of ship A it is ship B whose length is contracted, while in the rest frame of ship B it is ship A whose length is contracted.

 

[Janus]

One little bit at a time if you please.


Only if the fence were moving and the trolley not moving.


Only if the trolley were moving and the fence is not.

Regardless of which is moving, I see no reason why the marks cannot be made at the same time. If one or the other were moving then that one would be length contracted and the other not. If both are moving, both might be length contracted. Still, the marks can have been made at the same time from either reference point.

It doesn't work that way. There is no "one is moving and the other isn't", there is only a relative velocity difference between the two. You cannot tell which is "really moving". In fact, "really moving" has no meaning. From the fence the trolley has a relative velocity and is contracted and from the trolley the fence has a relative velocity and is contracted. Which one is contracted only depends upon the frame in which the measurement is made

Lets keep the questions and answer simple with this: If the trolley is moving and the fence is not, and the marks made simultaneously from the perspective of the trolley, then at what times are the marks made with respect to the fence. Let us declare that the fence sees the left mark of the trolley made at exactly 1:00 PM, with an accuracy of better than one millionth of a femtosecond. What time does the fence see the mark for the right side made? And why do you say that?

It does no good to give a numerical answer to this question when you are still laboring under misconceptions about what is going on. (besides, someone already told you how to get this answer several posts back.)

It comes down to this: Your whole test for absolute motion rests on the assumption that there is such a thing as absolute motion. You then interpret Relativity on the idea that it involves absolute motion, and create a thought experiment based on that interpretation, and surprise, surprise, you get a result that says you can detect absolute motion.

Actual Relativity, on the other hand, disavows the whole concept of absolute motion, and says that you can only measure relative velocity. Actual Relativity behaves differently from your interpretation of it and gives a totally different result for your thought experiment. One that does not allow you to measure absolute motion.


Every real experiment done to date agrees with the Relativity of the last paragraph.

 

[Mentz114]

Only if the fence were moving and the trolley not moving.
Only if the trolley were moving and the fence is not.

You can choose which one is your rest frame, but there's no physical reason to choose one over the other.

All motion is relative, until you understand that you're lost.

 

[bkelly]

Too much to grasp right now

There are too many things that don't make sense to me to discuss them all and take up that kind of bandwidth on a forum such as this. I have the Sam Lilley's book "Discovering Relativity for yourself" and am reading that.

The problem is I don't take well to someone saying this is true and believe it 'cause I said so. Over and over there are things I question and don't have anyone to ask. I think I should let this thread go for now and work on my reading. If anyone has a preferred book let me know and I will check it out.

I wish to express one point before closing out.

Suppose I go up to my fence with my one meter long trolley and markers. I travel along the fence at 1/2 c and make my marks. When I make them, while moving, they look to be 1 meter apart. When I stop and go back I see that they are 0.866 meters apart. That's understandable.

Then I sit still and move the fence past me at 1/2 c and make the marks. If I can see the marks as they are made and instantly take a measure, then will appear to be 1 meter apart. When I bring the fence back to put the marks in front of me, and the fence is stationary again, I believe the marks will be 1 / 0.866 meters apart or about 1.15 meters apart. All well and good.

However, that tells me that I can determine if the fence was moving or if I was moving. That contradicts my understand that many are telling me that everything is relative and I cannot determine if I am moving or if you (or the fence) is moving.

 

[Doc Al]

I wish to express one point before closing out.

Suppose I go up to my fence with my one meter long trolley and markers. I travel along the fence at 1/2 c and make my marks. When I make them, while moving, they look to be 1 meter apart. When I stop and go back I see that they are 0.866 meters apart. That's understandable.

No. When you stop you'll find the marks are 1/0.866 meters apart.

Then I sit still and move the fence past me at 1/2 c and make the marks. If I can see the marks as they are made and instantly take a measure, then will appear to be 1 meter apart. When I bring the fence back to put the marks in front of me, and the fence is stationary again, I believe the marks will be 1 / 0.866 meters apart or about 1.15 meters apart. All well and good.

OK.

However, that tells me that I can determine if the fence was moving or if I was moving. That contradicts my understand that many are telling me that everything is relative and I cannot determine if I am moving or if you (or the fence) is moving.

Your example doesn't work the way you think. They give the same results!

For some reason, you accept length contraction (but not the relativity of simultaneity) which is based on not being able to detect absolute motion.

 

[bkelly]

Suppose I go up to my fence with my one meter long trolley and markers. I travel along the fence at 1/2 c and make my marks. When I make them, while moving, they look to be 1 meter apart. When I stop and go back I see that they are 0.866 meters apart. That's understandable.

No. When you stop you'll find the marks are 1/0.866 meters apart.

I think something is in error there. If I stand at the fence and move the trolley past me and the fence at 1/2 C, then I should see the trolley be length contacted (while it is moving) and the marks being made 0.866 meters apart. If I hop on the trolley while it wizzes past the fence, the trolley would always appear to be 1 meter long and the marks would appear to be 1 meter apart. But when the trolley and I stop moving, as compared to the fence, and return to the marks, then they appear 0.866 meters apart.

Which statement is incorrect?

 

[Doc Al]

If I stand at the fence and move the trolley past me and the fence at 1/2 C, then I should see the trolley be length contacted (while it is moving) and the marks being made 0.866 meters apart.

This is a different statement from what you made earlier, since the trolley is now moving with respect to you. Will you see the trolley length contracted? Sure. Will you see the marks being made 0.866 meters apart? No. Once again, you ignore the fact that you and the moving trolley will disagree that the marks were made at the same time.

If I hop on the trolley while it wizzes past the fence, the trolley would always appear to be 1 meter long and the marks would appear to be 1 meter apart.

OK.

But when the trolley and I stop moving, as compared to the fence, and return to the marks, then they appear 0.866 meters apart.

Nope. As you just said in your previous sentence, the marks would appear to be 1 m apart. And things that move with respect to you are length contracted, so when you stop you see the marks at their 'rest length' of 1/0.866 m.

 

[bkelly]

You missread what I wrote, then did it again.
I will stop pursuing this thread and go to my reading.
Thanks to everyone for taking the time to reply.

 

[Bussani]

I think something is in error there. If I stand at the fence and move the trolley past me and the fence at 1/2 C, then I should see the trolley be length contacted (while it is moving) and the marks being made 0.866 meters apart. If I hop on the trolley while it wizzes past the fence, the trolley would always appear to be 1 meter long and the marks would appear to be 1 meter apart. But when the trolley and I stop moving, as compared to the fence, and return to the marks, then they appear 0.866 meters apart.

Which statement is incorrect?

If this is all one experiment, isn't the incorrect part still the idea that the two frames of reference would agree on the marks being made simultaneously?

If I stand at the fence and move the trolley past me and the fence at 1/2 C, then I should see the trolley be length contacted (while it is moving) and the marks being made 0.866 meters apart.

If the marks were made simultaneously according to someone standing beside the fence, then okay.

If I hop on the trolley while it wizzes past the fence, the trolley would always appear to be 1 meter long and the marks would appear to be 1 meter apart.

The trolley would appear to be the right length, but the fence would appear to be contracted. Assuming this is the exact same experiment and a person by the fence would still see the marks made simultaneously, the marks will not be made simultaneously according to the person riding the trolley, and they'd end up very close together on the length contracted fence instead of 1 meter apart.

But when the trolley and I stop moving, as compared to the fence, and return to the marks, then they appear 0.866 meters apart.

Assuming I understood the experiment, that's right. So the incorrect part is still the idea that the marks would be made simultaneously according to both frames, and thus the idea that the marks would appear to be 1 meter apart from the trolley. If the fence is contracted and the marks have to end up 0.866 meters apart, then they'd be even closer together than that before uncontracting the fence, wouldn't they?

 

[JesseM]

There are too many things that don't make sense to me to discuss them all and take up that kind of bandwidth on a forum such as this. I have the Sam Lilley's book "Discovering Relativity for yourself" and am reading that.

You might also get something out of this free online introduction to relativity, "Relativity for the Questioning Mind":

http://www.oberlin.edu/physics/dstyer/Einstein/SRBook.pdf

This one's pretty good too:

http://en.wikibooks.org/wiki/Special_Relativity

 

[Janus]

I think something is in error there. If I stand at the fence and move the trolley past me and the fence at 1/2 C, then I should see the trolley be length contacted (while it is moving) and the marks being made 0.866 meters apart. If I hop on the trolley while it wizzes past the fence, the trolley would always appear to be 1 meter long and the marks would appear to be 1 meter apart. But when the trolley and I stop moving, as compared to the fence, and return to the marks, then they appear 0.866 meters apart.

Which statement is incorrect?

The last statement. What you keep missing is that length contraction is reciprocal. It doesn't matter whether you say that the trolley is moving past the fence or the fence is moving past the trolley. The trolley will always measure the fence as length contracted as long as there is a difference between their velocities. The same is true for the trolley as measured from the fence, it will always be length contracted as measured from the fence.

You seem to think that if the Trolley is "moving", then it will be length contracted, and thus from its perspective, the fence will be stretched out. This doesn't happen.

And its not just a matter of my saying "because I say so", it is backed up by particle accelerators every day.

Consider that these accelerators routinely get particles that travel at near c speeds. Remember that these accelerators are traveling with the Earth as it orbits the Sun. So some of these particles would traveling in the same direction as the Earth orbits and some in the opposite direction.

If there were absolute motion, this would mean that particles that have the same speeds relative to the lab would have different absolute speeds, and behave differently. Time would slow more for particles moving in the direction of the Earth's orbit than those going in the opposite direction. We would see a pattern of these particles having longer half-lives than the other particles.

We do not see this however, the only thing that effects the particles' time dilation is their relative speed with respect to the lab. No matter what direction they travel with respect to the Earth's orbit, you get the same result.

 

[Dale]

The problem is I don't take well to someone saying this is true and believe it 'cause I said so.

Overall that is a mischaracterization of this thread. You have had many detailed explanations, supporting math, and even animations. I don't know why you would make such a statement.

Suppose I go up to my fence with my one meter long trolley and markers. I travel along the fence at 1/2 c and make my marks. When I make them, while moving, they look to be 1 meter apart. When I stop and go back I see that they are 0.866 meters apart.

In which frame are the marks made simultaneously, the trolley frame or the fence frame?

Then I sit still and move the fence past me at 1/2 c and make the marks. If I can see the marks as they are made and instantly take a measure, then will appear to be 1 meter apart. When I bring the fence back to put the marks in front of me, and the fence is stationary again, I believe the marks will be 1 / 0.866 meters apart or about 1.15 meters apart.

In which frame are the marks made simultaneously, your frame or the fence frame?

 

[Doc Al]

You missread what I wrote, then did it again.

I don't think so. I did make the assumption that the marks are made simultaneously in the frame of the trolley. You must make some assumption about that--you cannot have those marks made simultaneously in every frame. That's key to understanding the reciprocal nature of length contraction and time dilation.

I have the Sam Lilley's book "Discovering Relativity for yourself" and am reading that.

That's a fun book, but I would choose one more focused on special relativity. In addition to what JesseM suggested in post #92 (Dan Styers excellent--and free--book), I recommend that you consider "It's About Time" by N. David Mermin.