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Homework Help: Light Bending

  1. Apr 6, 2005 #1
    Hi, this may seem like a really easy simple question, but I've not yet managed to figure it out. Yesterday I was looking through the encarta and I came across 'Relativity' so I started reading about it, on one of the demos it said;
    'When Sara is standing still in space she switches on a torch and the beam is straight'
    'When Sara is accelerating in a space shuttle up and she shines a torch the beam bends'

    Now im thinking, if the speed of light is somewhat 300mill m/s or something, how would u be able to see the light bending?
    If any of you play games you may come across something called FPS, hypothetically speaking if you set your fps on 200 and u play this certain game then change it to 250 you wont be able to see any differance (unless the game has certain smoothing) so therefore wouldn't this be the same idea of light bending due to its speed being so fast the eye cannot process this and therefore your unable to notice a bending effect; or am I wrong?

    Thanks In advance
  2. jcsd
  3. Apr 6, 2005 #2
    Well imagine light as several rows of particles. Let say she is moving and she lights up a torch. The first row will move perpendicularly to the direction of the ship. Since light does have a length-wave (its magnitude depends of the circumstances), and thus a period, the second row takes a certain time to quit the torch. Meanwhile, the ship has moved. For this reason, row 2 will not be completely aligned with row 1. So for row, row 4, etc. I do not think the bending was meant in the way your eye sees it but physically. ;)
    Last edited: Apr 6, 2005
  4. Apr 6, 2005 #3


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    The article did not SAY she could SEE the light bending! It just said the light WOULD bend. That can be observed, for example, with light that passes very near the sun.
    Last edited by a moderator: Apr 6, 2005
  5. Apr 6, 2005 #4


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    I'm going to take a stab at this question, although I'm sure that people after me will give a much better explanation.

    The passage you are reading is giving an example of one of the main effects of General Relativity (also known as Einstein's theory of gravity). In GR, Einstein showed two things that have to do with that passage. First, acceleration is the same as gravity. Suppose that your in an elevator with no windows. If the elevator is on the earth, then if you drop a ball it will accelerate downards at g=9.8m/s/s. But what if you were in empty space accelerating at g? If you dropped the ball it would still accelerate down at 9.8m/s/s. So how do you know the difference? More specifically: How do you know the difference between gravity and an accelerating body?

    One of the more interesting (at least in my current opinion) that comes out of this equivalency between gravity and acceleration is the bending of space-time. The statement is "Matter tells space how to curve and space tells matter how to move." When space-time comes to a mass, it curves around, much like a piece of tissue curves when you put a weight in the middle. Draw a bunch of parallel lines on a piece of paper. Then hold out that piece of paper and place a coin in the centre. Notice how all the lines start curving? That's what happens.

    Now as for seeing it, it's a lot simpler than you think (and it's used a hell of a lot in astronomy). The closest big mass to the Earth is the Sun (and you can use Galaxies instead from what I've heard), so it should bend light a bit. During an eclipse astronomers pointed their telescopes into the sky and took pictures of the sky. When compared with pictures that you would normally see, they were able to see a star that should be covered by the Sun. What was actually happening, was the Sun was bending (or lensing) the light from the distant star around it and onto Earth.

    Since Einstein showed that gravity is the same as acceleration, the same would be for Sara in her space shuttle. When she accelerates, the light beam actually bends.

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