# That damn relativity stuff again

I know you’ve been through this a million times, and I really tried to understand it on my own. I did a search and read this thread, this thread and this article, and I still don’t get it.

I understand the Michelson-Morley experiments and what they meant, and I understand that a clock on a spaceship will be slightly behind a clock kept on Earth (not that I understand why, but I know that it is so and accept it), but I still don’t get the explanation. It’s the “no absolute time” business that gets to me.

One way I had special relativity explained to me is this: You’re in a spaceship some distance from a black hole. You send out a probe towards the black hole. On the probe is a lamp that blinks once every 30 seconds. You sit there looking at the probe and the lamp (it’s a slow day).

Now, in the beginning you see a flash of light every 30 seconds. However, as the probe comes closer and closer to the black hole, its gravity starts slowing down the flashes of light, which means you see them further apart than 30 seconds. The intervals get longer and longer, even though the lamp on the probe still sends out a flash of light every 30 seconds. Finally, the probe disappears into the black hole and we see no more flashes.

This is simple and easy to understand. But how do we get from this to deducing that there is no absolute time? The blinking still happens every 30 seconds, we just see them further apart than that. That we see something doesn’t make it true. If you have a chain attached to the bottom of a bathtub, hold the chain straight and fill the bathtub up with water, it will look like as if the chain bends on the point where it crosses the surface. That doesn’t mean the chain isn’t straight, it is. Our eyes are fooled into thinking it isn’t.

So, could anyone please take a brave stab at explaining this so the other nerds don’t laugh at me anymore?

The first one I had read (and linked to) but the second one was new, and it actually did it for me. The light clock with one stationary and one moving observer was the example that made the penny drop. Thanks a lot. I still don’t get why it is so (does anyone?) but I can see that it is.

The spaceship-probe thingee is still a bunch of bulls**t, though.

Old models assumed light speed was just like anything else. Einstein goes a different route. We assume the speed of light is constant. This leads to paradoxes in thought experiments. Paradoxes can be resolved by assuming time is not constant. Lo and behold, the math turns out very neat and better yet, makes predictions that turn out to be true. We decide the theory is a good one.

As for why – that’s a very loaded question that can be explained on many different levels. For me, I think it’s safest to say there is no why, it just is.

Actually, if you sent a clock to a black hole, along with your buddy, and were able to get them back after a while, your buddy would report that the clock ticked normally, and you would report that the clock and him seemed to slow way down.

Then, when you retrieve the clock and your buddy, the clock is behind your time. Your buddy would tell you that your were in fast forward mode, but to you he was in slow mo.

So, think of it as a clock you can get back.

Actually, this was done here by flying a clock above the earth. Upon returning, the clock was ahead of another clock (not literally ‘clocks’ in our tradition)

Someone can provide more info on the clock thing, and that should make it a bit clearer.

the clock experiments:

http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/airtim.html

“During October, 1971, four cesium atomic beam clocks were flown on regularly scheduled commercial jet flights around the world twice, once eastward and once westward, to test Einstein’s theory of relativity with macroscopic clocks…”