http://cs.astronomy.com/asy/general_discussion/f/27/t/55764.aspx?page=2#507568

Thank you Gerry, for creating this thought experiment. It’s kind of like having a puzzle to solve. Trying to see which pieces fit and which don’t. Even though I might answer as though I might know something I could quite possibly be wrong on some points, so please don’t take offense at me or my ignorance. So here goes.

Two spaceships are traveling, one behind the other, each at the same constant velocity such that their distances apart remain the same. The rear spaceship sends signals to the spaceship in front. The two spaceships notice no doppler affect in the signals because they are in sync with each other time wise. If they are both traveling through space/time in tandem, at the same speed, then their time dilation is also the same, and so both of their clocks are running at the same speed, and the signal would then be in sync with both. That is how I see it, which I think is in agreement with your statements.

“The spaceships in tandem could not use Doppler to detect or distinguish if they are in motion, or at rest, in space.”

Yes, they could not use their own signal to determine motion in regards to each other, as there would not be any doppler affect in the signal.

“So the spaceship observers could say they were at rest, despite the roar of their warp drive engines.”

They could only say that they are at rest “relative to each other,” but this does not mean that they could automatically conclude that they are at complete rest in space/time. To make a determination that they are at rest, or in motion, in regards to other objects in space/time, they would have to take other measurements outside of their frame of reference.

“On a nearby planet there is an observer that can view the two spaceships because they are within the sighting range of his telescope. The planet is at rest relative to the spaceship tandem...it is orbiting, along with the spaceships, around some galactic “zero point”. ... However, the planet observer would view the spaceships flying across the night sky. Thus, he would see the light pulse have to travel by some increased distance, such as L + _ because of the forward velocity of the spaceship tandem.”

I may be missing some aspect of this thought experiment, but from the above quotes, I am getting a conflicting image in my mind. If the planetary observer is at rest relative to the spaceships, then the spaceships would not be moving across the sky. They would appear fixed. If there is no motion between the planetary observer and the spaceships then he would see no doppler affect, and he would conclude that the spaceships were getting neither farther or closer to him.

However let us conclude that the spaceships are in motion compared to the planetary observer.

“...the planet observer would view the spaceships flying across the night sky. Thus, he would see the light pulse have to travel by some increased distance, such as L + _ because of the forward velocity of the spaceship tandem.”

Yes, he would see a doppler affect in their signals.

“The planet observer on the other hand, sees the light pulse travel between the spaceships, as well as their forward motion. All these observers experience different philosophical realities in their own reference frames, which Einstein tries to resolve by Lorentz transformations and bending space-time.”

If the observer is in motion in regards to the spaceships, then the observer will see a doppler affect from any signals coming from the spaceships. If the spaceships could measure signals coming from the planet, then they too would see a doppler affect in the signals from the planet. They would each be experiencing different realities because they are in different reference frames.

Light is known to travel at the same speed for all frames of reference, so what must change then for these different frames of reference? Time flow must change. How is this manifested between different frames of reference in regards to light? Light waves must show a compression or elongation in regards to different frames of reference.

“By Einstein, their units of length measurement have contracted, creating, for them, the illusion of being at rest. I feel this illusion is just a physics trick of compressing and stretching waves, a trick that is elevated to a scientific principle (Relativity).”

This length contraction is not measurable to a single frame of reference, just as a slow down in time flow rate is not detectable to a single frame of reference. The doppler affect of light is not measurable to a single frame of reference, which is why the MM experiment did not work. In a single frame of reference, you only have one time flow rate, and everything within that framework will always appear normal to the observer in that frame.

The only way to notice differences is to compare your frame of reference with a different one. You can call Relativity a physics trick or an illusion, and you may be right. However the formulas do work, and at particle accelerators these affects do seem to actually happen. I will say that I don’t think everything is quite as relative as they say.

In most thought experiments regarding spaceships, the universe itself is pretty much ignored. They don’t look at the stars, galaxies, or the CMB which is everywhere. They pretend that all motion is relative only to other objects. For instance, a spaceship fires it’s rockets and is traveling through the universe. But some say that you could just as easily say that the rocket ship is stationary, and that the universe started moving past the spaceship. I would disagree with that. Why? Stand up and spin around. Did you spin, or did you make the entire universe circle about you? Did you make distant stars travel around you at faster than light? But matter can’t travel at faster than light, so it must have been that it was just you that spun around. Those rocket engines on the spaceship did not move the universe, but only the little ship that owned them. Time dilation therefore cannot also be thrown around with impunity. It affects only objects that are in motion through space/time. Relativity has its limits. I think that finding out what those limits are is what you’re concerned with. Thanks again.