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Assuming that energetic gamma photons travelling in free space at the speed of light have no influence on time and gravity (to my best knowledge no one ever measured a time dilation or gravitational effect caused by a photon or ray of photonic light in free space). If the photon generates an electron and a positron pair, since the matter particle (electron) applies gravitational force and time dilation, from conservation of time and gravity consideration, can we assume that the anti-matter particle (positron) will apply anti-time-dilation (i.e. time runs faster) and apply anti-gravity (i.e. gravity repels)? please read the following paper : http://vixra.org/abs/1609.0047

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    $\begingroup$ No, we can't assume that. Electromagnetic fields do, of course, contribute to the energy-momentum tensor. $\endgroup$ – tfb Aug 12 '16 at 13:53
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    $\begingroup$ No, just because an anti particle has some "reversed" physical properties, we can't assume it will not follow special relativity in exactly the same way as particles do, although you should Google positrons being viewed as electrons going backwards in time. $\endgroup$ – user108787 Aug 12 '16 at 13:53
  • $\begingroup$ Possible duplicate of physics.stackexchange.com/q/391 $\endgroup$ – user108787 Aug 12 '16 at 13:54
  • $\begingroup$ tfb, do you know any measured evidence that massless photons apply time dilation or apply gravitational force? I assume that as long as not proven else, massless photons ,travelling at the speed of light in all the inertial reference frames don't apply time dilation or any gravitational force in the way that matter does. count_to_10,I didn't mean back wards in time, I meant making time run faster to conserve the time dilation caused by the matter particle. $\endgroup$ – Eran Sinbar Aug 12 '16 at 14:06
  • $\begingroup$ No, I appreciate that you didn't mean backwards in time in this post , I just wanted to give you an idea of how particles and antiparticles can be viewed in QFT. Sorry for the confusion. $\endgroup$ – user108787 Aug 12 '16 at 14:14
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The gravitational properties of light are not in question. No direct measurements but enough indirect ones.

  1. Light has inertial mass, from the $E=mc^2$ equivalence. One of the first two tests of General Relativity (I think 1918) was that light is bent as it travels near the Sun, due to its gravitational attraction. This bending of light near heavy bodies has been observed and confirmed lots of times. From conservation of momentum and the equivalence of inertial and gravitational mass the light also attracts the Sun or anything else it comes close to. It's just too small to measure

  2. It is known that for the early time after the Big Bang and inflation (if there was one, as evidence indicates but not 100% agreed) the universe was too hot and was radiation dominated, with light and highly relativistic particles prevalent. The expansion of the universe was consistent with the radiation (estimated as massless particles like the photon) causing the gravitational field then present. Early on the expansion decelerates because of it. After some time the universe cooled enough that matter became more and more prevalent. And as it expanded more also the dark energy.

  3. There are other good reasons to think so. If it is not true energy and matter would also not convert into each other.

  4. The gravity of antimatter is hard to measure, but there are some plans to do so. But we do 'see' the gravity of the energy difference between hydrogen and its fusion products, and the rest is photons and neutrinos. I am not sure if the mass loss of the radiated photons from the sun has been measured.

There's probably other good reasons but those are pretty easy to see.

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  • $\begingroup$ Bob, if light bents it means that mass bents spacetime and light travels through the shortest distance which is a curved line. it doesent mean that light applies gravitational force by itself. regarding the inflation phase, it is a singular anti gravitational phase and its hard to conclude from it that photons apply gravitational force and apply time dilation. photons always produces pair of matter and anti matter and I think that the anti matter applies anti gravity and anti time dilation in order to conserve the zero gravity and zero time dilation of the photonic energy $\endgroup$ – Eran Sinbar Aug 12 '16 at 18:09
  • $\begingroup$ @EranSinbar No, anti-matter has been experimented on quite thoroughly. It does not produce anti-gravity. It is exactly the same as regular matter except with the opposite electric charge. There is no law that conserves gravity and time dilation. $\endgroup$ – Jim Aug 12 '16 at 18:32
  • $\begingroup$ Jim, it is very hard to produce and keep from annihilating a large amount of anti particle neutral anti hydrogen atoms and since the gravitational force is very weak I don't think that there is a consensus regarding the gravitational effect of anti matter. regarding photons, I think that the reason that they have no mass, they are timeless(time stands still at the speed of light ) and they maintain the speed of light in all the inertial reference frames, because they do not interact with spacetime regarding gravity and time dilation. matter does interact with space time . $\endgroup$ – Eran Sinbar Aug 12 '16 at 20:13
  • $\begingroup$ so if we assume that maybe photons do not influence space time regarding gravity and time dilation than when a pair of matter and anti matter are generated from a photon the anti matter applies anti gravity and anti time dilation from gravity and time conservation consideration. $\endgroup$ – Eran Sinbar Aug 12 '16 at 20:16
  • $\begingroup$ that means that the pair of matter and anti matter created from the gamma photon are entangled through time and gravity. $\endgroup$ – Eran Sinbar Aug 12 '16 at 20:17
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Interesting question, though some of the assumptions may be wrong.

Light is energy and it has both the mentioned effects. The reason that there may not be experiments, is that the speed of light, and gravity is same. Due to this, measuring the effects may not be feasible. Because light and its associated gravitational effects both come together, and pass at the same time. Moreover, the effects would be very small to measure. For measurable effects, you have to be very close to a star, but in that case, you can not turn off the star light to measure the difference.

The two particles created out of photon (assuming it did happen) will have same (rest+kinetic) energy as the photon had. Their combined gravitational effect will be same as that of the photon, but they would not move as fast relative to an observer. That is why we can measure gravitational effects of matter.

Net energy of two particles should be same as that of the photon assuming there was no third thing created in the process.

Touching on a topic in comments under Bob Bee's answer -

It is not matter, or anti-matter that creates gravity. It is the interaction of empty space with them that causes curving (gravity). Space curves in a similar manner in presence of both - matter, and anti-matter.

Another way to think is - energy curves space (which is gravity). Anti matter, and matter, both are rest energy equivalent per E = mc$^2$. So, there is no difference between the two as far as gravity is concerned.

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This isn't quite an answer, but instead is a gedankenexperiment which tries to convince you that light does attract things gravitationally. It should be a comment but is far too long.

For this experiment imagine a large container whose inside walls are perfect mirrors (see below for a modification). In this container is a quantity of flammable material and enough oxygen to allow it all to burn, and a timer which will ignite the material. The container is perfectly sealed.

So, initially weigh the container (with its contents). Now, wait until the timer has gone off and long enough after that that all the material has burned. Weigh the container again.

Now note two things:

  1. the container has perfectly reflective walls and is perfectly sealed, so no energy or matter has escaped from it;
  2. the material has dumped energy into the environment inside the container, in the form of light.

Now, we know (and have very good evidence for this) that, in burning, the material has converted some (tiny fraction) of its mass to energy. So the matter in the container now has slightly less mass than it did.

So, after the burning is finished, is the mass of the container and its contents less, more or the same?

  • If it is less, where has the mass gone?
  • (I won't bother with the more case.)
  • If it's the same, then where is the mass that was lost from the burnt material?

Well, of course, it is the same, and the extra mass is in the soup of energy that now fills the container. Light does indeed influence things gravitationally.


Notes.

  1. In real life we can't make the walls perfectly reflective, and some light leaks out: we'd have to account for this loss, which we could do by careful measurement of the emissions from the container.
  2. A better way of doing this experiment involves using a nuclear weapon instead of burning things, as the mass-to-energy conversion is far higher. But nuclear weapons emit things like neutrinos, which leak out and are hard to take account of. Still, this is how these experiments are done in practice, where I come from anyway.
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  • $\begingroup$ tfb,your thought experiment is talking about energy pressure inside a sealed box.in this case I know thatenergy pressure is equivalent to mass. I am talking about photons travelling in free space with no pressure induced that generate suddenly a pair of matter and anti matter particles. I claim that this free travelling photon had no gravity or time dilation effect and since the matter particle has such effect, the anti matter particle needs to have an anti gravity and anti time dilation effect to conserve time and gravity. so its a different case than your example. $\endgroup$ – Eran Sinbar Aug 13 '16 at 7:03
  • $\begingroup$ So, if I take a photon and put it in a box then the mass of the box increases, but if I leave it free (whatever that means) then it has no gravitational effect? Well, I tried to convince you but clearly your level of misunderstanding is too deep for me: sorry. $\endgroup$ – tfb Aug 13 '16 at 10:59
  • $\begingroup$ tfb,I will try to explain myself once more. $\endgroup$ – Eran Sinbar Aug 14 '16 at 5:52
  • $\begingroup$ please be patient,I know its radical. lets diccuss time dilation,its more intuitive. I believe that if if a photon passes near a clock that measure time the photon will not cause time dilation. now this photon pecomes a pair of matter and antimatter particles. now when the matter particle passes near the clock it will slow time so I expect that the antimatter particle will make time run faster in order to keep the conservation of time. $\endgroup$ – Eran Sinbar Aug 14 '16 at 5:57
  • $\begingroup$ by the way I am not trying to convince you since the theory can and will be tested when scientists will succeed measuring the gravity and time dilation of antimatter and I have no doubt that they will measure anti gravity and time anti dilation (time runs faster) .I believe it will take only a few years till they get these results. $\endgroup$ – Eran Sinbar Aug 14 '16 at 5:59

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