Monday, August 26, 2013

Special Relativity finally makes sense

Preface: I studied physics in college, yet there were parts of it that didn't click with me. Special relativity with all its "a flashlight on a train" examples, that's counterintuitive and hard. Yesterday in Surfaces & Essences, the authors explain Einstein's thought process when he came up with it, and it makes sense this way. It's also an interesting example about how genius can happen when we let go of what we know.

You know how when you're on a plane, travelling smoothly at 600 mph, you can pour water the same way as when you're standing still? If you throw a ball straight up, it falls back down into your hand.

This is called "Galilean Relativity." It says that while you're traveling at a constant velocity, mechanical experiments behave the same as at any other velocity (including standing still). It's only changes in velocity -- speeding up, slowing down, bumps and curves -- that affect the motion of objects you have with you. This means there is no mechanical experiment you can perform to find out whether you're flying at 600 mph or sitting on the tarmac. The water and the ball behave the same.

Einstein said, why should electromagnetic experiments be any different? Why should a lens, a magnet, or an electron care about your velocity? Your computer works the same in the air as on the ground, and there are a lot of electrons moving around in there.

The behavior of light passing through a lens or electricity moving through your processor is closely related to the constant speed of light. For your glasses and your computer to behave exactly the same in a moving plane, the speed of light inside the plane must be the same as the speed of light on the ground. The electrons in your computer move the same way, relative to you, at 600 mph or at a stop.

Speed is distance divided by time. The distance that water traveled between the pitcher and the cup looks different to you and to the watchtower, since the watchtower sees the plane moving horizontally. Same with the light from your monitor: it travels a different distance from the tower's perspective than yours. If the distances are different, how can the speed be the same? Speed can be the same if time is different too.

Therefore time is stretched or squashed depending on velocity, so that the speed of light is the same from every perspective. No experiment can tell the difference, and electrons in your computer don't care how fast you're going.

Next time you sip a drink on your way across the Atlantic, appreciate time dilation. You may not notice the fraction of a second this trip adds to your life, but reliable electronics are important at every speed.

In this story, Einstein takes one belief about the world (Galilean Relativity) and broadens it. In doing so, he narrows  another belief about the world (consistent time). The result is a new, self-consistent worldview. This worldview has proven useful.
It pays to be specific about how we know something is true, and the range of experience we can be sure about.

29 comments:

  1. I'm not sure what you're trying to say here. I confess that I have a degree in physics and think of special relativity in terms of certain equations, symmetries, and invariance. I believe that at an intuitive level, "spacetime physics" is the best way to visualize, geometrically, the paradoxes of special relativity.

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    1. I have a degree in physics too. (just a bachelor's.) Yet, I never understood why time has to be different. I didn't get the importance of that.

      Also it fascinates me the way letting go of "time is a constant" allowed the much more appealing "everything is consistent regardless of velocity."

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    2. Yes, that leap of letting go of one assumption was basically the genius of Einstein. Lorentz had already gotten important equations proposed, and Poincaré had put in his work, but Einstein instead of arguing for an interpretation in terms of deformations of the ether or of matter, simply gave up the concept of universal time from Newton. (And later, he did the same with giving up Euclidean geometry, whereas Poincaré famously rejected the non-Euclidean geometry interpretation of gravity.)

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  2. What made relativity click for me was a line in the book "Relativity Visualized" by Lewis Carroll Epstein:

    "The reason you can't go faster than the speed of light is that you can't go slower. There is only one speed. Everything, including you, is always moving at the speed of light."

    This let me make the leap between formal description and intuitive appreciation.

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