Is it possible to determine absolute speed?

[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.