[jeffreyH]

Gravity has nothing to do with rotation. It is a function of mass only.

Now I've only been skimming this thread but Alan's post should be read and understood. It is a very important point and says a lot about translation in space over rotation. If a gravitational field actually rotated then the speed of information transfer becomes Galilean. Just think about that for a moment.

[jeffreyH]

Now if electromagnetic energy has a gravitational field, photons travelling through otherwise unoccupied space will tend to clump together (since gravity always sucks) , so the intensity of light received from distant stars will increase with distance, by self-focussing. And there will be stars out there that we can't see because their self-focussed beam isn't pointing in our direction.

Really?

Now that is a very good point. I can't see how anyone could refute it. We see stars, therefore light does not beget gravity.

[Kryptid]

Now if electromagnetic energy has a gravitational field, photons travelling through otherwise unoccupied space will tend to clump together (since gravity always sucks) , so the intensity of light received from distant stars will increase with distance, by self-focussing. And there will be stars out there that we can't see because their self-focussed beam isn't pointing in our direction.

Really?

Let's consider the case of a single star. How can the photons from that star focus each other into a beam if they are radiated in basically a spherically-symmetrical fashion? The pull in all directions would be about equal, so what direction do you propose the beam should point? Let's also not forget that the gravitational pull between something like rays of light would be incredibly weak, so distortions in the paths of electromagnetic rays would be overwhelmingly dominated by the gravity of planets and stars and even small asteroids.

A pair of X-ray photons with an energy of 100 keV (1.6022 x 10^-14 J) would have a mass-energy equivalence of about 1.783 x 10^-34 kilograms per photon. If the two start off 1 nanometer apart, they will experience a gravitational force between them of: F = (GMm)/r², F = ((6.674×10⁻¹¹)*(1.783 x 10^-34)*(1.783 x 10^-34))/(10^-9)², F = 2.1209 x 10^-60 newtons.

Let's compare that with the gravitational force that each photon would experience from Alpha Centauri A if they started out 4.366 light-years away from it, F = (GMm)/r², F = ((6.674×10⁻¹¹)*(1.783 x 10^-34)*(2.18735 x 10³⁰))/(4.130236 x 10¹⁶)², F = 1.5258 x 10^-47 newtons. So even a star over 4 light-years away is pulling on each photon with a force more than 7 trillion times higher than they are pulling on each other.

The existence of gravitational lensing indicates that massive objects can pull on beams of light. So the beams of light must also be able to pull on those massive objects, otherwise we have a violation of Newton's third law.

How can the idea of a kugelblitz work if energy has no mass and therefore no gravity? https://en.wikipedia.org/wiki/Kugelblitz_(astrophysics)

[Halc]

I think perhaps this thread is full of misinformation, and we need to consider what we are saying.
I too think I am making gross generalizations by my statements.

I've been in the energy-has-mass camp, but I'd like to question some of the arguments for that side:

I'm pretty sure energy creates gravitational fields as well, does it not? Otherwise, it seems like you could violate the first law of thermodynamics. In principle, you should be able to convert the entire Earth's mass into energy. If that got rid of its gravitational field, then you could move a weight away from the Earth at no energy cost. Then turn the Earth's energy back into mass and let the weight fall under the influence of the newly-created gravitational field. Rinse and repeat as often as you'd like and you'd have a source of infinite energy.

My bold.  How exactly might one propose to get this energy away from Earth 'at no cost'?  Light doesn't work since it loses energy as it climbs out of a gravity well.  What sort of energy doesn't do that?  If we can't identify it, then this argument falls apart even if the conclusion was correct.

I can produce reactionless thrust if energy has no mass.   Consider two stationary weights.  I let them fall towards each other, but before they hit, I change one of them to energy, which I hold on to for a while.  The second mass now has momentum with no reaction mass.  That's reactionless thrust.

The arguments for the other side seem to have faults as well.

so you are telling me that electromagnetic radiation has a gravitational field, and therefore F≠GMm/r²?

It seems that all radiation has mass and is therefore subject to gravity.  EM radiation cannot escape a black hole, nor can gravitational radiation.  But the EM field and the gravitational field are still felt, implying that neither is something that travels, light speed or otherwise.  If gravity was something that traveled at light speed, gravity wouldn't be able to escape a black hole.

Kryptid's response to the 'focused light' from stars is spot on.  For all I know, a laser beam of light might be capable of holding itself together in that manner, but light from a star has equal 'density' in all directions, so the light has no imbalance to divert itself.

If a gravitational field actually rotated then the speed of information transfer becomes Galilean.

Nobody suggested a rotating field.  That's not the same as a field from a rotating object as compared to the field from the same object not rotating.  The former is presumably stronger since there is more mass/energy to the object.

[Kryptid]

How exactly might one propose to get this energy away from Earth 'at no cost'?  Light doesn't work since it loses energy as it climbs out of a gravity well.

My point was to illustrate that if energy does not have gravity, then turning the mass of the Earth into energy gets rid of the gravity well and thus you can move a weight away from the Earth at no energy cost. Since that violates conservation of energy, my conclusion was that turning the Earth's mass into energy mustn't actually remove the gravity well and therefore energy has gravity.

[Yahya A.Sharif (OP)]

Gravity doesn't suddenly spring into existence out of nowhere. It has energy and energy is conserved.

The law of conservation of energy applies to our universe, but no-one knows if it applies before the big bang or not.
Mass popping from nowhere " before the big bang" will have gravity propagate with c and finite range, for mass to have infinite gravity range it should exist infinitely in time in the past so that its influence reaches infinite , but infinite time in the past is not logical.

[Yahya A.Sharif (OP)]

Energy causes gravity the same as mass , this is a well known fact according to Einstein's  theory of general relativity.

[alancalverd]

In principle, you should be able to convert the entire Earth's mass into energy.

That raises an interesting scenario, with cosmological significance. Suppose we spontaneously convert a spherical mass into photons. The resultant electromagnetic pulse will be a spherical shell, expanding at c.  From outside, the gravitational field will be unchanged, appearing to be generated at the original center of gravity, but inside the shell, the field will be zero.

We don't need to evaporate an entire planet to do this. Any mass-energy converter (like the sun) will be kicking off spherical EMP, with a decreasing local gravitational field but no change noticeable outside the "shell".

How does this affect our view of the mechanics of an expanding universe?
 

[jeffreyH]

When the sphere of photons reach objects in the gravitational field of the converted object gravity will stop having an effect. Anything orbiting the original mass will now move away with a possible momentum boost from interactions with the expanding photon sphere. Everything just expands away from the source.

[alancalverd]

Mass popping from nowhere " before the big bang" will have gravity propagate with c and finite range, for mass to have infinite gravity range it should exist infinitely in time in the past so that its influence reaches infinite , but infinite time in the past is not logical.

There's the source of misunderstanding! "Range" means the absolute limit of a quantity: the plane will fall out of the sky after 1500 miles, however long we wait,  because its fuel range is limited, but there is no theoretical limit to gravitational influence - if we wait long enough, with a sufficiently sensitive instrument, we will detect the creation of a distant mass. "Extent" means its actual position radius at any given time, which for gravity appears to be ct, and for our aeroplane, can be any value less than 1500 miles. 

Colloquially, we sometimes use range to mean present radius where the context is unambiguous, e.g. "Target radar range is 10 miles, my gun range is 5 miles, so I can't hit the target, Captain."  "Wait."

[Yahya A.Sharif (OP)]

if we wait long enough, with a sufficiently sensitive instrument, we will detect the creation of a distant mass.

"if we wait enough " this is the problem , we can't wait infinitely so that gravity be everywhere to infinity.
However gravity behavior is exactly a definition of a finite range:

The gravity of the energy of a rotating object doesn't reach some places  in the universe " equals zero , is not available there , etc" and that is exactly the definition of a limited range.
Suppose there is a planet 3E^8 meters away from us , this planet rotates to create new gravity,  after 0.5 seconds its gravity range will be 1.5E^8 that means every object inside its range will be affected by its gravity and that according to the inverse squared law, and every object outside its range won't be affected by its gravity including us, we are out of range.
The limited range is not fixed its extendable and increases with time by the speed of light c.
If gravity propagate with c , and at some moment there are some places which are covered with gravity influence and some parts in the universe which are not then there is a limited range.
It is simple and obvious as this: gravity didn't reach us and available elsewhere then we are out of range and there is a limited range, gravity propagate with c then this range extends with c .
What equation is used in such case ? the equation calculate g for any given r but, if gravity didn't reach us that means g=0 for distance equals 3E^8 , making the equation invalid for this calculation.

If gravity doesn't exert its influence infinitely then it has a finite range that is exactly what I claim

[jeffreyH]

Where is the boundary for gravity? If its range is finite then there must be a boundary. Is it the edge of the observable universe? Well, no, it can't be, since more of the universe will become observable over time.

The concept of infinity being a limit is not the same as infinity being a point in space. Infinity is not anywhere. Not physically. Gravity will never not have an effect, however small that effect might be. That is all it means.

This shouldn't be so hard to grasp.

[Yahya A.Sharif (OP)]

Where is the boundary for gravity? Is it the edge of the observable universe?

Newton has the idea of infinity in his work including infinite speed of objects, infinite gravity speed and gravity existing infinitely, all these concepts of infinity are wrong,  according to his idea if the sun disappeared suddenly the earth will lose its orbit at the same time of the sun's disappearance , gravity propagates with c means no infinity for its influence, if its influence reaches us with speed c then we have at a time three significant points , a point at which gravity exists , gravity propagates and cover such point, a point at which gravity doesn't exist this point is not covered by gravity propagation yet, and a point at the edge  in which gravity suddenly drops to zero this point is the BOUNDARY of gravity. Unfortunately we live within the gravity coverage of masses in the universe and the gravitational equation satisfies within this coverage , but the equation won't satisfy beyond its coverage.And for a rotating object experiment we would see how gravity has finite influence and range.

[Yahya A.Sharif (OP)]

Cut to the chase.
"How does gravity exert its influence infinitely?"
How could it not do so?

Because it has finite propagation speed.

[Colin2B]

Cut to the chase.
"How does gravity exert its influence infinitely?"
How could it not do so?

Because it has finite propagation speed.

Your answer is totally illogical as has been pointed out to you.
You are playing silly word games with infinite and finite, neither of which you fully understand.
End of

[evan_au]

Where is the boundary for gravity?

It would be anything outside the light cone of our masse.

According to the Big Bang theory, all the mass of the universe started off inside the light cone of the rest of the universe.

However, the addition of "Cosmic Inflation" to this theory means that there is some (now distant) mass that might be outside our light cone.

See: https://en.wikipedia.org/wiki/Inflation_(cosmology)