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Gravity and light

  1. Jan 15, 2009 #1
    Few questions:

    1. Is gravity always proportional to the mass of the planet/star? Is there any other factor we consider when we determine the gravity of a distant star?

    2. Does light bend only if it interacts with the gravity of a huge star, or,
    the bend is visible with interaction huge stars, but happens with any small amount of gravity - but unable to detect with the current technology or our maths knowledge.

    3. Can there be a possibility of so huge gravity (or mass) and it causes a halo around the star? (and no light comes out of it and we call it black hole, he...he...)

    4. If gravity can affect photons, is there a possibility of mass for the photon which is so so negligible? Why do we reject any theory based on that (or, is there a mathematical proof that says photon has to be massless?)
  2. jcsd
  3. Jan 15, 2009 #2


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    Distance - the force of gravity is the mass of the object * the distance away - squared

    No any mass bends light. In very senstive astronomical observations we have to calculate the effect of light being bent by the gravity of other planets (especially Jupiter - which is still pretty massive).

    Yes it's called a gravitational lens. We can see halos or multiple images of very distant objects because a galaxy near us is acting as a giant lens.

    General relativity shows us that gravity doesn't act directly on mass as you learnt in high school - it actually bends space and this causes light (which has no mass) to bend.
    There was a famous experiment almost 100years ago when stars were observed to move as their light passed very close to the sun (during an eclipse) which proved this theory.
  4. Jan 15, 2009 #3
    The experiment looks like a proof/observation of bending of light. If there any observable event for the bending of space? Or, is it a mathematically proved thing?
  5. Jan 15, 2009 #4


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    The point of the experiment is that if you say the light is bent by gravity as if it had a mass (from E=mc^2) and work out the bending angle you get half as much as GR's bending of space theory predicts.
    The experiment was to measure how much it bends - the result was for the bending of space.

    Of course philosphically you could say this doesn't prove the theory - it only proves that the real theory should predict the same result as GR. But thats true of the whole of science.
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