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Is there anything such as antigravity?

  1. Oct 2, 2004 #1

    Aki

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    Is there anything such as antigravity?
     
  2. jcsd
  3. Oct 2, 2004 #2

    Nereid

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    Welcome to Physics Forums Aki!

    It rather depends on what you mean by 'anti-gravity'.

    In one sense, bouyancy could be thought of as anti-gravity.

    If you are thinking of an analogy with electrical charges (like charges repel, unlike charge attract), or a bar magnet, then the answer is no; there is no experimental or observational result which suggests that there are forms of mass which can repel each other.

    You might think that atoms of anti-matter would show anti-gravity? (for example, anti-hydrogen, made of an anti-proton and a positron, would 'fall up' in a vaccuum tube here on Earth) Well, no; although this hasn't been observed yet (anti-hydrogen is very difficult to make!), the limited results on the gravitational effect of anti-matter are consistent with the theory - anti-matter 'falls down'.
     
  4. Oct 2, 2004 #3
    Special Relativity predicts a kind of anti-gravity. Special Relativity requires that the vaccuum have a negative pressure equation of state, given this, a positive cosmological constant will act to cause a large scale repulsion.
     
  5. Oct 2, 2004 #4
    I think you mean General Relativity.
     
  6. Oct 2, 2004 #5
    oops - yes!
     
  7. Oct 2, 2004 #6

    Mk

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    An exotic form of matter would, in the spaceitme-is-a-sheet analogy be on top of a hill instead of in a little hole. How is there negative pressure in a vacuum?
     
  8. Oct 4, 2004 #7

    Aki

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    what is negative pressure? I've always had trouble understanding it
     
  9. Oct 4, 2004 #8

    Nereid

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    You might find this page, from Ned Wright's cosmology tutorial, helpful. If, after reading it, you still have questions (I hope that you do!), please come back and ask. :smile:
     
  10. Oct 5, 2004 #9

    Aki

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    thanks for ths site. Um...I already have a question on the first sentence; "Recently two different groups have measured the apparent brightness of supernovae with redshifts near z = 1." What is that "z"?
     
  11. Oct 5, 2004 #10

    Nereid

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    redshift; z = (observed wavelength - 'source' wavelength)/('source' wavelength).
    a webpage with a lengthier definition
     
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