Is it possible to determine absolute speed?

[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.