B 2 clocks -- Using orthogonal light path detectors in a space ship

[russ_watters]

If that is the case how does the tube know it is in a moving spaceship so that it knows to contract ?

It appears the tube does know so what factor is the tube measuring in order for it to know it is in a moving frame?

If the tube knows then surely it is possible to know?

I guess it was a poorly designed question. I was trying to get you to think about the geometry, not the material.

The point is that the same piece of paper is both 8.5" in one direction and 11" in another direction. The paper does not need to "know" anything to change from 8.5" to 11". Nothing changes about the paper, it only depends on which measurement you choose to make. Both are true.

Last week I was in a boring meeting and my mind wandered a bit. The projector sitting on the table was angled-up, making the image look like a trapezoid on the screen. This annoyed me and I came up with an invention sure to make me rich: install a camera on the projector, which would then measure the shape of the image and enable the projector to auto-correct it (many include manual adjustments, but people never use them).

But then I realized that the *screen* looked flat/square to me and the *picture* angled/trapezoidal, but to the projector the *picture* looked flat/square and the *screen* looked angled/trapezoidal. So much for my invention...but who is right?

Both are right because both are reporting only what they themselves see and not making any claim about a universal reality that both must agree on. As others have said, that's the hard part of Relativity: recognizing that things we have considered to be universal realities all our lives - in most contexts* - really aren't.

Then I started thinking about laser rangefinders and I'm not going to say anything more until I patent it...

*This is actually less true than most people believe it to be and they know it even if they never make the connection. It clicks for some people when they start traveling long distances in an airplane, with the Earth rotating underneath them and start wondering "how far did I *really* travel?" The fact that there is no universal answer to that question is the "secret sauce" of Relativity...and Galileo knew that long before Einstein added his spin to it.

 

[jartsa]

another thing I don't understand is they say the laws of physics are the same in all frames. But I would have difficulty explaining a self contracting tube?

does it affect the density of the material that the tube is constructed of?

A moving tube is a short tube, a dense tube, and what else ... a slowly aging tube.So let's say we want to pump all the water on Earth through a tube whose diameter is 0.1 m in one second. We can do that because the very fast moving water becomes very dense water, right?Of course there are always some observers saying that the water in the tube is not moving. Those observers are also saying that the density of the water is the normal density of water, 1000 kg/m³

 

[jartsa]

Does that mean if I am standing on a train station with train carriages whizzing by with all the doors open and I repetitively stick my hand in and pull it out of the moving carriages , my hand will shrink and elongate ?

The train whizzes by and is contracted. While the hand is not whizzing by and is not contracted. This is very straightforward stuff

And as always there are some observers that say that the hand is whizzing by, and those observers say the hand that is whizzing by is contracted.

 

[Khashishi]

Ross B, I suggest you study the Michelson-Morley experiment, which is basically the same as your thought experiment.

Whenever you specify the velocity of something, you have to also specify what your frame of reference is, if it is not implied. It's incomplete to say that the train is moving at 100 km/hr. You have to say, the train is moving at 100 km/hr relative to the Earth's surface. If I am on the train, then I am also moving 100 km/hr relative to the Earth's surface. But I am moving at 0 km/hr relative to the train. And the train is moving at 0 km/hr relative to me. And the Earth's surface is moving at -100 km/hr relative to me. I am moving at 0 km/hr relative to me.

The length of something depends on the frame of reference. Since I am moving at 0 km/hr relative to me, I do not see myself as contracted. The train is moving at 0 km/hr, so it is also not contracted. The Earth is moving at -100 km/hr so it is contracted slightly in the direction of motion. If the Earth stops moving (because the train applies its brakes), the Earth becomes longer.

Does that mean if I am standing on a train station with train carriages whizzing by with all the doors open and I repetitively stick my hand in and pull it out of the moving carriages , my hand will shrink and elongate ?

No. You seem to think of a frame of reference as something that you can move your hand into and out of. A frame of reference isn't a physical thing, but a choice of coordinates. You can be inside the train and measure your motion relative to the Earth, or to the train, or to yourself, or to any point you choose.