Is gravity a force

  • #1

Main Question or Discussion Point

hi

if gravity is a force of attraction ,a natural question arises ,why it can't be repulsive?as there is a great symmetry in the structure of universe,i think we cannot take the case of attraction for granted.And with this reasoning, for surely gravity must not be a force but something else.GR gives an exact alternative by defining gravity as the path followed by a body in the spacetime curvature caused by a massive object.even though the case is like this why we still treat gravity as one of fundamental forces?why we still try to make a new theory by combining relativity and quantum theory?
and one more question ,by relativity theory,nothing can exceed speed of light,then how long will take for earth
to know the absence of sun,from the moment sun is removed from its position?

thanks.
 

Answers and Replies

  • #2
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I have read that some physicist do not consider gravity to be a force like you have pointed out, but then I have read that others do. When you incorporate quantum physics with GR gravity is a force because it works on the transmission of gravitons.

There is a type of anti gravitational force in the universe, and that is dark energy, it is causing the expansion of the universe to accelerate.

According to GR we would not feel the effects of the suns removal until 8.3 minutes had passed. This is because gravity travels at the speed as light as well.
 
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  • #3
I have read that some physicist do not consider gravity to be a force like you have pointed out, but then I have read that others do. When you incorporate quantum physics with GR gravity is a force because it works on the transmission of gravitons.

There is a type of anti gravitational force in the universe, and that is dark energy, it is causing the expansion of the universe to accelerate.

According to GR we would not feel the effects of the suns removal until 8.3 minutes had passed. This is because gravity travels at the speed as light as well.
is there any experimental evidence to the presence of dark energy or dark matter?or just it is only a mathematical outcome?
 
  • #4
WannabeNewton
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hi

if gravity is a force of attraction ,a natural question arises ,why it can't be repulsive?as there is a great symmetry in the structure of universe,i think we cannot take the case of attraction for granted.And with this reasoning, for surely gravity must not be a force but something else.GR gives an exact alternative by defining gravity as the path followed by a body in the spacetime curvature caused by a massive object.even though the case is like this why we still treat gravity as one of fundamental forces?why we still try to make a new theory by combining relativity and quantum theory?
and one more question ,by relativity theory,nothing can exceed speed of light,then how long will take for earth
to know the absence of sun,from the moment sun is removed from its position?

thanks.
Well since light from the sun takes about 7 minutes to get to earth we would know if the sun suddenly disappears once the last of the light from the sun reaches the earth. If you mean in terms of earth's orbit gravitational waves will propagate due to the disturbance the sun's disappearance creates in the geometry and this propagation is at the speed of light.
 
  • #5
there may be dark energy and dark matter,but not antigravity:wink:
 
  • #6
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Force is simply an acceleration upon a mass. Since accelerations are observed, it is a force. The fact that it is a relativistically neutral and less than a quantum process, makes it no less a force.
The graviton is a boson (force) because it acts by an action-reaction exchange. However the graviton is not emitted from any particular quantum particle. Therefore, we might say it is not a quantum force, where quantum means waving, but the force of energy itself. (Curviture=Energy Density)
 
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  • #7
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Question1: Gravity can't be repulsive because energy can't be inside out. Like curviture can't be inside out. However it seems charges can.

Question2a&b: Gravity is a force, and we are trying to combine relativity and quantum mechanics; they just don't fit well together at all yet.

Question3: The earth will notice the sun is gone when light and gravity (moving at the speed of light) cease to arrive at earth.
 
  • #8
pervect
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I mentally added a question mark to the statements above to turn them into questions- they aren't really questions as written - I imagine that this was the intent, though.

question 1. "Energy being inside out" doesn't make any sense. But we can rephrase this so it does make sense, hopefully it's along the lines of what you had in mind. "Gravity isn't known to be repulsive because all known forms of matter and energy have a positive energy density".

However, it is possible to have both positive and negative curvatures in principle.

Question 2. Gravity is sometimes regarded as a force, and sometimes as just being geometry. The second approach is easier when learning general relativity. As long as the experimental predictions come out right, it doesn't really matter whether one regards it as a force or not.

Question 3. You can't destroy matter, so you can't really answer the question of what would happen if the sun dissappeared. One runs into the "garbage in, garbage out" problem, the mathematical equations become inconsistent if one assumes that the sun dissapears.

The best one can do is ask how fast the gravitational pertubations would travel if one "blew it up". It turns out that one would have to blow it up in a non-spherically-symmetrical fashion to get any effects at all - but in this case, the disturbances would propagate as 'c'.
 
  • #9
bcrowell
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if gravity is a force of attraction ,a natural question arises ,why it can't be repulsive?
It is basically not true that gravity can't be repulsive.

Relativists describe this kind of thing in terms of energy conditions: http://en.wikipedia.org/wiki/Energy_condition

Energy conditions are not directly built into the structure of GR. They are optional add-ons that state generalizations about the behavior of the matter that couples to gravity in GR. This paper http://arxiv.org/abs/gr-qc/0205066 has a good up to date description of what we know about which energy conditions are valid. The basic answer is that none of them are always valid. So gravity can be repulsive in this sense (with "repulsive" interpreted to mean "violating some specific energy condition.")
 
  • #10
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But for commonsake, the rest of 99.9% of the universe, it is positive curviture when the test particle is outside the space of another particle.
I see that negative gravity could exist if the test particle lie within another particle curviture where the energy condition of gravity would be negative. Thanks for the tip.
 

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