Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Can gravity creating by energy?

  1. Sep 2, 2008 #1
    First please bare with me, I'm not close to being a physicist.

    So if I combine E=m*c^2 and that mass creates gravity, I conclude (probably wrongfully) that with enough energy concentrated at the same point we can generate a noticeable gravity field.

    Now I know that it will require HUGE amounts of energy but my first question is if its theoretically possible or am I missing something?
  2. jcsd
  3. Sep 2, 2008 #2


    User Avatar
    Science Advisor

    No, you are not missing something :) Classically, we are used to think that a pointmass M warps a gravitational field, but energy doesn't; that just simply follows from Newtons formula for gravitational fields:
    U(r) = -\frac{GM}{r}

    However, the equations of general relativity don't use just mass as a source for gravity, but the so-called energy-momentum tensor. This object describes the energy and momentum density of space-time. So for instance, an electromagnetic will create a gravitational field. This gravitational effect will be much, much smaller than the electromagnetic effect. But it's there!

    This is a clear distinction between Newtonian gravity and Einsteinian gravity.
  4. Sep 2, 2008 #3

    Hmmm well this sounds a bit out of my knowledge but if I think I understand you right then by focusing at one point a very strong electromagnetic wave such as a laser beam (or more precisely many lasers beams that will all reach that point in the same positive phase) will create a gravity field in that point.

    If I understood you correctly and the above I wrote is correct then that what I was looking for :)
  5. Sep 2, 2008 #4
    Yes, you are basically correct. If you could pack enough energy in a light-tight box, the box would "weigh" the same as though you had packed an equivalent amount of mass. In that sense, a bound photon with a high enough frequency would appear indistinguishable from an uncharged particle of the same mass, at least as far as gravity is concerned. That is why some had speculated that all mass was made of photons; unfortunately this hueristic cannot account for charge or other forces that mass possesses.
  6. Sep 2, 2008 #5

    Well if so, than we theoretically found a way to create a gravity propulsion engine....not an efficient one...but still :)

    Just think of a similar structure to a fusion reactor where they concentrate many laser beams on a single spot , if you take a spaceship and have a laser array that all will hit in the correct phase in a spot slightly ahead of the spaceship it will be possible to propel it forward.

    Since the beams will be in a "reverse satellite dish" setup the gravity vectors of all the beams will always equal propulsion forward.

    Now...the intersting part is where I thought about all of this....but that's not for this board :)
  7. Sep 2, 2008 #6
    Let me guess - a mouse riding a rabbit by dangling a carrot in front of it?

    I think there are a lot of problems with your idea that you're overlooking. For starters, the energy requirements to generate a gravitational field sufficient to move your ship are magnitudes greater than if you just burned rocket fuel.
    Last edited: Sep 2, 2008
  8. Sep 2, 2008 #7

    First and foremost I did say I'm not an expert in physics and this is just the most general description and thesis to such propulsion.

    Forgive me but I don't see where I break the law of energy conservation? I'm assuming the vehicle will have a power source that will supply energy to the antenna array.

    What I do think you are right is that if there will be such a gravitational field and it will accelerate the vehicle the wave should lose some of its energy and with my limited knowledge besides reducing its frequency and I cant see how this will happen here.

    Anyway...I'm confused lol.
  9. Sep 2, 2008 #8
    LOL not a rabbit, I was actually at a beach with two bi sexual friends (females) of mine, while they were doing their thing a few feet next to me I was starting at the stars, which I haven't seen in a LONG while since I live in a city and started pondering about that lol.

    Anyway, rocket fuel have HUGE problems in terms of high speeds. A nuclear or fusion reactor can supply the complete power requirements of a spaceship including this propulsion while maintaining low weight.

    Obviously I'm not speaking of something with today's technology I'm just talking in far far theory here.
  10. Sep 2, 2008 #9


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    One major problem with your design is that there's no way to power the lasers without some kind of recoil effect (sending energy forward will push the ship back), so forget about the mouse riding the rabbit, this is more like flying by pulling your own hair.
  11. Sep 2, 2008 #10
    Exactly. And that's the conservation of energy problem. If you want to move yourself, you have to expend energy. But if you pack a whole boatload of energy in front of you but it's still part of you, zero energy has come in or out of the system. So you're not going to move. If you put all of your mass energy on one side of the ship, it will cause the ship to shear toward that energy, but, again, the structure as a whole will not shift positions.

    The reason I deleted the C.O.E. point was because I recognize that you're not an experienced physicist so I chose to make a more basic objection instead, which was that even a rocket fuel chemical reaction will give you more bang for the buck than using energy to make a gravitational field.

    Yes, you're right, we didn't need to know...
  12. Nov 27, 2008 #11
    Forgive me if i'm wrong.
    I'm writing a story about time travel using lasers to create a warp in space time, and was so inspired to come up with another method of travel.

    In theory, if you were in Earth's atmosphere, you could attract air particles towards the lasers. Since the law of mutual attraction still applies (i think >=( ) the particles of air would be attracted towards the gravity source equally as much as the craft is towards the air. So, in a device similar to a fan, this object would propel itself from the gravitation of air particles.

    only problem is, what happens to the air? it dosn't have a output valve like a fan, so how would the air get out? I have been contemplating this point, but I haven't come up with a logical solution. can someone with a degree (or just smarter than me) help?
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Can gravity creating by energy?
  1. How is gravity created? (Replies: 18)