Indeed it does, cato. Of course, take one look at the gravitation thread to see that some people deny this Please Register or Log in to view the hidden image! . Anyways, the electromagnetic radiation (light) that fills the universe (think Cosmic Microwave Background for instance) has an energy density and a pressure. It affects, among other things, the expansion of the universe (the expansion of the universe is a gravitational effect). If the energy density of electromagnetic radiation was not included in the total gravitating energy density of the universe, we would get the Hubble 'constant' wrong. This is only one of the simplest reasons in a long list of experimental reasons why we believe that light can generate gravity.
Oooohhh! THAT'S why.... Please Register or Log in to view the hidden image! I got lost on the "most people say light behaves" part.... Please Register or Log in to view the hidden image! Yeah man! I want some of what your' havin'!!! Yaba Daba :m:
The experiment that shows light has particle like qualities is the fact that you can charge up a zinc plate with UV light.
Wrong, it does not generate gravity. It does however follow geodesics (i.e. curvature created by matter) just like all other matter and waves. No. Some particle-like characteristics - maybe.
Don't listen to Aer. Physicists responsible for carrying out and interpreting modern experiments in the gravitational field say light does. Check my postings in the gravitation thread for specific direct quotes and references where you can examine the evidence that physicists base these conclusions one yourself. Aer is right only if basically the entire body of physicists and all their experiments are wrong.
Nothing you've posted shows why light would create gravity. In fact, the only statements I've found by physicists say exactly the opposite, and that is curvature of spacetime must be created by matter. Photons are not matter.
Aer, what about the posts I made in the gravitation thread where I directly quoted gravitational physicists as saying, and I quote again, "The density rho is made of material density ... and also of radiation energy density ... " Gravitation p. 713
I do not have that book in front of me, perhaps you cannot scan the pages and send them to me? I say cannot as that probably wouldn't be legal..
It would indeed be illegal I think. I wish that I could show you the book, but all the same I am not lying to you. If you want to look at something on the web check out the online lecture notes I posted where Sean Carroll derives the stress-energy tensor for the electromagnetic field. Light contributes to gravity because it has a stress energy tensor.
can you give me some links physics monkey? I would like to know more about this idea of gravity producing photons.
cato, The following set of online lectures notes http://pancake.uchicago.edu/~carroll/notes/ is a good source. These notes have actually been published in book form and the author is a reputable physicist. The stress-energy tensor for electromagnetism is first discussed on pp. 20-22. An axample of emag contributing to gravity can be found in the Reissner-Nordstrom black hole. This kind of black hole is charged and produces an electric field, the electric field contributes to the energy density of the hole thus increasing the curvature. You can read about that on pp. 202-208. Also, the case I mentioned of cosmological expansion is discussed on pp. 220-222. The author explicitly demonstrates how the energy density of radiation contributes to the expansion rate and also discusses the radiation energy density in terms of photons.
Here is a link that isn't as technical but still says the same thing and gives some other examples: http://www.madsci.org/posts/archives/jul2001/994971027.As.r.html Also, starting from John Baez's website you can get a lot of info on the subject written for non-physicists: http://math.ucr.edu/home/baez/
OK Physics Monkey, after doing some reading through various sources, I think I can now provide a summary of the situation here. First of all, there appears to be a distinction between the terms 'gravitation' and 'gravity' in modern physics that I was not aware of until now. Gravitation is the tendency of two bodies to move toward each other. Gravity, essentially, is the curvature created by matter. Now, when you consider 'the tendency of two bodies to move toward each other' you must account for the kinetic energy and direction of travel relative to the body creating the curvature because, depending on this kinetic energy, the path through spacetime will change and thus alter the 'tendency of two bodies to move toward each other' (think of orbital velocity, escape velocity and the such..). Now, what this means is that kinetic energy does not create curvature of spacetime as I've been saying all along and this includes photons, they will not create curvature either.
I agree, assuming that the Hubble constant is really constant. Of course, even if it isn't the actual exact age, it is a good estimate.
It is not constant. You can calculate H for every moment of time, where T is the age of the universe at that moment. It is getting smaller over time. The current age of the universe is estimated to be about 13,7 billion years. That number comes from the studies of the cosmic microwave background radiation. This gives the Hubbles constants value of H = 1/432043200000000000 = 2,31e-18 1/s = 2,3e-18 m/s/m No need for the energy density of the universe. You would need to know the energy density to see how is the universe curved, though. I think.
fo3, Thanks, but I did already know that the Hubble constant isn't really constant. It is however changing very slowly which is why people often refer to it a constant. I'm sure you know all this. Also, I agree that one way to determine the Hubble constant experimentally is by observing the microwave background to determine the rough age of the universe. My point was that it is possible to calculate the Hubble 'constant' using GR from a knowledge of the energy density of the universe. In fact this is one of the ways we know that dark matter and dark energy probably exist since the theoretical calculation comes up short if you don't have extra 'dark' matter that isn't seen.