DavidMcC wrote:

CdesignProponentsist wrote:

DavidMcC wrote:

campermon wrote:

Well, although I am aware of no experiment that has measured the gravitation of photons (the effect would be very, very small), GR does state that the energy contribution of photons adds to the gravitational field. Seeing as GR has been verified by a number of tests, it's pretty likely that the photon does add to the gravity field. Unless of course GR can be formulated to ignore photon energy.

What makes you so sure GR is the correct theory of everything, so that all associated maths applies, even without evidence.

Do have have something than GR that you aren't telling us about or going on a gut feeling David?

No, nothing I haven't already mentioned, but that is enough, IMO, to invalidate claims that GR tensor maths proves that photons must be exerting gravity, regardless of electro-weak interactions.

DavidMcC wrote:Not helping!

DavidMcC wrote:

campermon wrote:

DavidMcC wrote:... As I tried to explain elsewhere, mass-energy equivalence is not an absolute, for two reasons: photons do not necessarily exert a gravitational field, and space itself probably has energy, independent of the particles in it. Most quantum gravity hypotheses involve such "zero-point" energy. Thus, although all mass is energy, not all energy is necessarily mass. Capiche?

The bolded underlined bit.

When do photons not exert a graviational field?

How about you giving an example of when photons DO exert a gravitational field. I have aready shown that there is no evidence that they do.

Radiation-dominated era

In physical cosmology, the radiation-dominated era was the first of the three phases of the known universe, the other two being the matter-dominated era and the dark-energy-dominated era. During this era, the dynamics of the universe were set by radiation, which refers generally to the constituents of the universe which moved relativistically, principally photons and neutrinos.

Pulsar wrote:

DavidMcC wrote:

campermon wrote:

DavidMcC wrote:... As I tried to explain elsewhere, mass-energy equivalence is not an absolute, for two reasons: photons do not necessarily exert a gravitational field, and space itself probably has energy, independent of the particles in it. Most quantum gravity hypotheses involve such "zero-point" energy. Thus, although all mass is energy, not all energy is necessarily mass. Capiche?

The bolded underlined bit.

When do photons not exert a graviational field?

How about you giving an example of when photons DO exert a gravitational field. I have aready shown that there is no evidence that they do.

Radiation-dominated era

In physical cosmology, the radiation-dominated era was the first of the three phases of the known universe, the other two being the matter-dominated era and the dark-energy-dominated era. During this era, the dynamics of the universe were set by radiation, which refers generally to the constituents of the universe which moved relativistically, principally photons and neutrinos.

DavidMcC wrote:Yet another use of theory as "evidence". But why am I not surprised?

Pulsar wrote:

DavidMcC wrote:Yet another use of theory as "evidence". But why am I not surprised?

Really? Are you calling the Standard Big Bang Theory 'just a theory' ? I guess I shouldn't be surprised, after you referred to the Einstein Field Equations as 'mathsy stuff'. I'd like to see you try and reproduce the CMB fluctuations without taking into account the photon density. But actual calculations are not exactly your forte, are they?



I don't know why you keep posting in these threads, because you are most definitely not a physicist.

lucek wrote:Let me ask you this Dave. Do you know why there is a speed of light(a maximum velocity not a speed at which light travels at)?

DavidMcC wrote:

lucek wrote:Let me ask you this Dave. Do you know why there is a speed of light(a maximum velocity not a speed at which light travels at)?

At what level is the answer expected? What kijnd of "why" is it? I could say that it is an observed fact that light in vacuo, in the absence of strong gravitational fields, travels at a particular speed, regardless of the observer's rest frame. Special relativity has well explained "why" any massive particle necessarily travels at less than this speed.

DavidMcC wrote:Further to my response to lucek, I am well aware of Cerenkov radiation, generated by particles travelling faster than light can in the medium. I am guessing that he thinks this is an argument for the possibility of travelling faster than light in vacuo. This would be wrong, because, in vacuo, there are no atoms to slow light down, leaving the possibility of fast particles travelling faster than wave-packets of light.
_________
I am guessing at what is in both lucek's and Pulsar's minds, because they do not say.

DavidMcC wrote:I'm still trying to imagine what Pulsar's beef is. Does he think I am claiming that maths is useless to physics? Nothing could be further from the truth. The problem I see is that it is very powerful, BUT can be misused (accidentally or otherwise), for example, by leaving out relevant boundary conditions in determining which siolutions to equations are physically meaningful. I suspect that string theorists such as the late "Sonny" White may have committed that error when claiming to have "proved" that it should be possible to reach the Proxima Centauri system in about 2 weeks, earth time (which implies an average speed of nearly 2 orders of magnitude greater than c).

DavidMcC wrote:Thommo, particles with a finite rest mass have a gravitational field of their own. That rest mass is a function of their internal energy. Photons, however, do not have such internal energy. The post that I made, that you think is wrong, is correct. It is only wrong in your mind. I have tried already to explain all this, but you just ignore it.

Veida wrote:

DavidMcC wrote:Thommo, particles with a finite rest mass have a gravitational field of their own. That rest mass is a function of their internal energy. Photons, however, do not have such internal energy. The post that I made, that you think is wrong, is correct. It is only wrong in your mind. I have tried already to explain all this, but you just ignore it.

Photons have zero rest mass, and zero is a finite amount. So aren't you saying here that photons have a gravitational field of their own?