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It is believed that the mass of galaxies is largely made up of "dark matter" hence we are immersed in the stuff are we right in assuming that the speed of light outside this sea of "dark matter" is the same as that which we measure locally

by definition dark matter does not interact with light

However, for any location, we can always produce a system of coordinates for which the speed of light is the determined constant, c. Whether the speed of light (in a vacuum) at a location is faster than c, equal to c, or slower than c is dependent on the choice of the system of coordinates that one uses.

A photon approaching a black hole event horizon? Does the gravity accelerate it to what velocity?

The coordinate system of the distant observer. He says that the "coordinate" speed of light at the event horizon is zero, that a light-clock at that location doesn't tick, and that gravitational time dilation goes infinite. Note that this isn't in any metric per se, the metric is "an artefact of measurement" as it were, and when light stops you can't measure anything.

Quote from: JohnDuffield on 20/03/2015 10:52:25The coordinate system of the distant observer. He says that the "coordinate" speed of light at the event horizon is zero, that a light-clock at that location doesn't tick, and that gravitational time dilation goes infinite. Note that this isn't in any metric per se, the metric is "an artefact of measurement" as it were, and when light stops you can't measure anything.Just say you don't know if that is the case. Stop dancing around your lack of knowledge.

Just say you don't know if that is the case. Stop dancing around your lack of knowledge.

Quote from: jeffreyH on 20/03/2015 11:09:12Quote from: JohnDuffield on 20/03/2015 10:52:25The coordinate system of the distant observer. He says that the "coordinate" speed of light at the event horizon is zero, that a light-clock at that location doesn't tick, and that gravitational time dilation goes infinite. Note that this isn't in any metric per se, the metric is "an artefact of measurement" as it were, and when light stops you can't measure anything.Just say you don't know if that is the case. Stop dancing around your lack of knowledge.That's not exactly fair. Mr. Duffield seems quite certain in his answers and thus probably cannot tell that they are not quite true.Note that his answer uses the language of special relativity, "light clocks", something that is not used as a standard in General Relativity, which allows us to use any physical system as a clock of sorts. According to general relativity, the speed of light at a location where that location is given as the origin of the system of coordinates is always going to be the standard c. One could, of course, prefer certain systems of coordinates on metaphysical grounds, perhaps based on aesthetic considerations, but this is not part of the physics of General Relativity.Note also that coordinate systems usually applied to black holes are only approximations to the physical systems that they are supposed to represent. Mr. Duffield seemingly prefers an approximation with a well known mathematical flaw at the even horizon of the black hole; perhaps his aesthetic or methodological dislike of mathematics drives him to accept those models with known mathematical flaws.

...According to general relativity, the speed of light at a location where that location is given as the origin of the system of coordinates is always going to be the standard c. One could, of course, prefer certain systems of coordinates on metaphysical grounds, perhaps based on aesthetic considerations, but this is not part of the physics of General Relativity.

Jeffrey it's nothing to do with mathematics.

See what Pete said. The speed of light varies in a gravitational field.

Quote from: PhysBang on 20/03/2015 14:36:45...According to general relativity, the speed of light at a location where that location is given as the origin of the system of coordinates is always going to be the standard c. One could, of course, prefer certain systems of coordinates on metaphysical grounds, perhaps based on aesthetic considerations, but this is not part of the physics of General Relativity.This is not true. See for example Einstein talking about the subject ...sorry, you cannot view external links. To see them, please REGISTER or LOGIN:

However, for any location, we can always produce a system of coordinates for which the speed of light is the determined constant, c.