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It's well known that electromagnetic fields contains energy but do they gravitate?

When we talk about the composition of the universe it's now accepted that the 74% is dark energy, the 22% is dark matter and then the remaining 4% is the rest of ordinary matter/energy that we can see or measure. Are the electromagnetic fields considered in this 4%?

Níckolas Alves
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Andrea Scaglioni
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  • Are the electromagnetic fields considered in this 4%? Yes. And what do you mean by gravitate?. – jinawee Dec 17 '13 at 14:00
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    Every contribution to the stress-energy tensor has a gravitational effects. Also, the radiation-dominated era, when radiation made the greatest contribution to stress-energy, was before the universe ~50k years old. Today, it's not very important, though still there. – Stan Liou Dec 17 '13 at 14:04
  • @jinawee i mean if it exerts a gravitational pull like ordinary matter...For example : An electron generates a gravitational field and an electric field right? This electric field contains energy so,it will generate another a gravitational field ? – Andrea Scaglioni Dec 17 '13 at 14:05
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    Gravitation is described in General Relativity as curvature. Every type of mass/energy/stress/pressure affects curvature, so yes. – jinawee Dec 17 '13 at 14:07
  • @jinawee Thank you..it wasn't so obvius for me..but now it's natural for me to ask from where the energy to provide the electric field come from.. – Andrea Scaglioni Dec 17 '13 at 14:13
  • Possible duplicates: http://physics.stackexchange.com/q/22876/2451 , http://physics.stackexchange.com/q/72819/2451 and links therein. – Qmechanic Dec 17 '13 at 14:20
  • @Qmechanic sorry but electromagnetic fields and electromagnetic radiation are the same thing ? it's sound confusing to me – Andrea Scaglioni Dec 17 '13 at 14:27
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    @user2794: in general electromagnetic fields are not only radiation, but the moral is the same as in those answers. If it has energy, momentum, or stress, it's going to have gravitational effects. The energy and momentum density of an EM field are described by the Poynting vector, and the stress by the Maxwell stress tensor. – Stan Liou Dec 17 '13 at 14:56
  • @StanLiou Ok,so..just for clarify the whole question : Electromagnetic fields and electromagnetic radiations exert a gravitational pull like they are the same thing ,togheter with everything that have a mass ,aganist the anti-gravitational pull of dark energy in the universe ..right ? – Andrea Scaglioni Dec 17 '13 at 15:09

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Partly this answer is just gathering together the comments above, though there are a couple of points that haven't been mentioned.

Firstly, as mentioned in the comments electromagnetic waves do gravitate and the links in the comments cover this well. In the early universe (for the first 47,000 years after the Big Bang) EM radiation was the dominant contribution to gravity, and as a result this era is known as the radiation-dominated era. However radiation dilutes more rapidly than matter as the universe expands, and for the last 13.7 billion - 74,000 years matter has outweighed (no pun intended) EM radiation. I can't find a figure for the current fraction of the critical density made up from radiation, but it's very small.

You asked about the effect of charge. In practise all large objects are approximately neutral so charge contributes little to their gravitational fields. However in principle charge has an effect but it probably isn't what you think.

For example the spacetime curvature around a stationary black hole is described by the Schwarzschild metric. If you now add charge to the black hole the curvature is described by the Reissner-Nordström metric. The charge decreases the curvature and indeed if you add enough charge you can make the event horizon disappear to leave a naked singularity.

John Rennie
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    Something around $8\times 10^{-5}$ - http://physics.stackexchange.com/questions/31018/omega-r-from-wmap-results (There are only WMAP results used. I don't know the figure after the "Planck" results.) Ah yes, that's total for neutrinos and electromagnetic field. – firtree Dec 17 '13 at 18:10
  • Photons are about half of that, not less than 10%. – firtree Dec 17 '13 at 18:16
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    "the links in the comments cover this well" Please include all the links and content from the comments and then flag them for deletion so we don't need to waste time reading through and trying to figure out what's what – endolith Jan 14 '16 at 19:30
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It is tempting to say that radiation gravitates since we know that E=m*c^2 and and mass gravitates. Further, gravity starting with its inverse square law is a consequence of conservation of momentum according to Bertrand theorem(https://en.wikipedia.org/wiki/Bertrand%27s_theorem#:~:text=In%20classical%20mechanics%2C%20Bertrand's%20theorem,orbits%20are%20also%20closed%20orbits. ). We know that radiation has momentum too and does observes momentum conservation. Further, we know that Maxwell equations show that it is possible to regard radiation like a gas that has a pressure(https://en.wikipedia.org/wiki/Photon_gas ). So the conclusion is that radiation does gravitate but it also suffers an expansion pressure force like a gas, and it is the balance between these two forces that decides the final state of radiation in an unbound space.

Riad
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