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Curious Question: Relationship between energy and graviy?

  1. Jan 19, 2009 #1
    Simple Yes or No Question but how would you prove?

    If gravity is a property of matter, when it gets converted to energy (for instance light) does energy also have the property of gravity? If so how could it be proven as a very large amount of energy would be required to equal a small amount of matter, and a small amount of matter has a small amount of gravity? Energy (for instance light) can be affected by gravity.

    If not what happens to the gravity property of matter when it gets converted to energy?

    Just a curious question from watching a show about Einstein last night.

    Thank You,
  2. jcsd
  3. Jan 19, 2009 #2
    matter is NOT 'converted' to energy and vice versa
    They are EQUIVALENT.

    When you annihilate 1Kg of matter + 1kg antimatter for example, you have an enormous explosion, and popular sources say 'matter is converted into energy'

    However, the total mass of all that light emitted during an explosion is exactly... guess what... 2Kg!
  4. Jan 19, 2009 #3
    While gravity IS a property of matter,as you state, gravity is also a property of energy and pressure. All three forms have gravitational effects. Assuming conservation of energy in a system, the conversion of a given amount of mass to energy does not change it's gravitational potential...although the shape of the gravitational field may change.
  5. Jan 19, 2009 #4
    Can you explain what you mean by 'pressure'
  6. Jan 19, 2009 #5
    Maybe should be rephrased to "If matter is a property of gravity, ". Probably not entirely true but more so than the other way round.
  7. Jan 19, 2009 #6
    A closed jack in the box (A toy with compressed spring) exerting a positive pressure has more energy and hence weighs just a tiny,tiny bit more than an open box...with an uncompressed spring. The energy is the result of the work exerted to close the box and compress the spring. Such a compressed box has ever so slightly more gravitational force.

    There are also situations in which the pressure in a region can be negative and such negative pressure exerts repulsive gravity...gravity is NOT always an attractive force! While immesureably tiny, such negative pressure becomes strong over vast cosmological distances...this is Einsteins (repulsive) cosmological constant!! dark energy manifests itself as negative pressure. It is what powers the expansion of the entire cosmos.
    Last edited: Jan 19, 2009
  8. Jan 19, 2009 #7
    I just quickly skimmed Wikipedia hoping it would discuss negative pressure but it does not. Nor did I find it under General Relativity nor stress energy tensor...but pressure is contained with it.
    Alan Guth of inflationary theory fame based his proposal on the fact that a supercooled HIGGS field provides the underlying negative pressure that caused inflationary exmpansion just after the big bang...those wishing the check further can also try that approach. But such a HIGGS field (in a termporary and very unstable high energy plateau) would act like a huge cosmological constant for only an ever so brief period....the existing cosmological constant exerts a negative pressure pressure steadily and constantly in all space.

    A non technical description of gravitational pressure of roughly 15 pages can be found in Biran Greene's FABRIC OF THE COSMOS, chapter 10.

    It's one of the many reasons I like to think that "space IS something". Others here, who may know a lot more than I, maintain space is nothing but a mathematical construct.
    Last edited: Jan 19, 2009
  9. Jan 19, 2009 #8
    When the spring is compressed heat energy will be released to the environment equivalent to the energy required to shut the lid. When the spring is released the same amount of energy will be retrieved from the environment so maybe no extra weight while compressed.
  10. Jan 19, 2009 #9
  11. Jan 19, 2009 #10
    I am unsure if the second part is accurate but I like that kind of thinking!: maybe gravity IS fundamental and after it's emergence from the high energy unstable early universe in turn "decays" in part to mass, energy and pressure...I had not thought of it that way before.
    Physics today usually talks of the four forces being initially unified (combined into one super force)...

    A very interesting question for me is what may have popped out from the initial big bang first....what is fundamental and what is subsequently emergent? Today we have time,mass,energy, four forces,space, etc...but I don't think physics knows which comes "first" nor why!
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