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Thread: Let's Talk About Relativity

  1. #1

    Default Let's Talk About Relativity

    I've had this stewing in my brain for a couple of years, and I'd like to wrap my head around some things.

    According to classical physics -- the natural laws discovered by Isaac Newton and various other scientists -- position and motion are relative. For example, a baseball player running at 5 m/s throws a baseball directly ahead. This thrower sees the ball leave his hand at 15 m/s, but a player standing nearby sees the same ball hurtle through the air at 20 m/s. (5 + 15 = 20) This is a super important concept to understanding physics, and it makes just about all of our machines work properly.

    But apparently -- for reasons I don't yet understand -- the above concept doesn't apply to light. Light travels at 300 million m/s, absolutely. For example, the starship Enterprise is cruising through space at 100 million m/s when Captain Kirk turns on the ship's high beams. He and his crew see light leaving the Enterprise at 300 million m/s, and the people on Earth see the light traveling at the very same speed. (100 million + 300 million = 300 million) WFT, Einstein?!

    Anyhow, this quirk of light (aka Special Relativity) creates a few weird results which I also don't understand:
    1. Two events happening simultaneously for one person may not happen simultaneously for another person.
    2. Moving clocks tick more slowly than stationary clocks.
    3. Objects get heavier, as well as shorter, the faster they move.
    4. E = mc^2 (I.e., mass and energy are fundamentally the same, and can be converted back and forth. See: the A-bomb.)
    5. Nothing can ever ever EVER travel faster than the speed of light. (300 million m/s)

    This means that 1) the Enterprise can approach the speed of light, but can never actually break the warp 1 barrier, 2) the people on Earth will have aged faster than Kirk and his crew every time they go adventuring and then come back home, and 3) the Enterprise might go from say 500 tons and 800 meters long at rest to 1,000 tons and 400 meters long during travel.

    When a professor first told this to me, I thought "Yeah right, pull the other one!" because everything from 'light travels at the same speed no matter who's watching' onward is apparently absurd. What bugs me in particular is: How can position and motion be relative, but time absolute? In other words, every time the Enterprise leaves the Earth, it's just as valid to say that the Earth leaves the Enterprise. And every time the Enterprise returns to the Earth, it's equally valid to say that the Earth returns to the Enterprise. So why does time slow down for one but not the other? If it weren't for the fact that GPS satellites apparently use Einstein's theories to operate properly, I'd chalk all of this up to 'Well most geniuses have a crackpot idea or two...'

    Oh, and there's also the theory of General Relativity, in which Einstein says incomprehensible things about gravity, curves, and spacetime to draw a few more weird conclusions:
    1. Clocks tick more slowly in high gravity. (I.e., "Don't go down to the water planet, Matthew McConaughey!")
    2. Orbits precess in ways that classical physics can't explain. (Click here for an explanation and helpful pic of precession.)
    3. Rotating masses like planets 'drag' spacetime along with them. (Whatever 'spacetime' is, I dunno.)
    4. Gravity bends rays of light. (See: black holes)
    5. The universe is expanding faster than the speed of light. (WTH Einstein, now you're just trolling us!)


  2. #2
    & Badger, Ratty and Toad Array Mole's Avatar
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    Mar 2008


    How can we falsify Relativity?

    Relativity says there is gravity well between us an the Sun. If this is so, then as we loose energy we would spiral slowly into the Sun. But in fact, as we loose energy we are spiralling out from the Sun.
    There is NOTHING--absolute nothing--half so much worth doing as simply messing about in boats. Simply messing.
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  3. #3


    Well I'm reading wikipedia, and I haven't gotten to gravity wells yet.

    I have however learned that spacetime (aka 'the space-time continuuuuuum!') is just shorthand for 'every event happens at a particular time and place.' It sounds much more arcane and heady when it's used on the sci-fi channel.

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