# The Naked Scientists Forum

### Author Topic: Is the speed of light constant through out the universe  (Read 6527 times)

#### syhprum

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##### Is the speed of light constant through out the universe
« on: 16/03/2015 09:10:44 »
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

#### PmbPhy

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##### Re: Is the speed of light constant through out the universe
« Reply #1 on: 16/03/2015 15:23:16 »
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
See http://www.abc.net.au/science/articles/2013/03/05/3701140.htm

The speed if light also varies as it moves through a gravitational field.

#### PhysBang

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##### Re: Is the speed of light constant through out the universe
« Reply #2 on: 16/03/2015 18:39:26 »
Strictly speaking, the speed of light, as considered across a finite distance according to a system of coordinates, does vary according to the way that gravity affects those coordinates. 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.

#### syhprum

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##### Re: Is the speed of light constant through out the universe
« Reply #3 on: 17/03/2015 08:04:48 »
I seem to have asked an unanswerable question as by definition dark matter does not interact with light!

#### evan_au

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##### Re: Is the speed of light constant through out the universe
« Reply #4 on: 17/03/2015 10:00:58 »
Quote from: syhprum
by definition dark matter does not interact with light
The somewhat mysterious "Dark Matter" does not seem to interact with the electromagnetic field, so it does not absorb or emit light.

However, Dark Matter does create a gravitational field, and light does interact with a gravitational field (as per PmbPhy & PhysBang).

So, indirectly, Dark Matter does interact with light.

#### Bill S

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##### Re: Is the speed of light constant through out the universe
« Reply #5 on: 17/03/2015 13:40:42 »
Quote from: PhysBang
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.

Please could you say a bit more about that for the benefit of a techno-ignoramous.

#### PhysBang

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##### Re: Is the speed of light constant through out the universe
« Reply #6 on: 17/03/2015 15:17:24 »
According to General Relativity, there is not one true coordinate system. One is effectively free to use any crazy coordinates system that one chooses. Some are better than others for different purposes and some are downright poor for any purpose. One has to state the laws of physics very carefully so that one can translate them from one system of coordinates to another and so as to not mistake a feature of a coordinate system with a feature of physical laws.

In some coordinate systems, simple ones that make it easy to understand what's going on for a lot of cases, one can identify a set of locations--say a fixed distance out from a star--where the effect of gravity appears to make light traveling nearer to the star slow down and light traveling farther away from the star to speed up. When calculating the details of events at different locations, we need to take this into account.

However, we can have a very similar coordinate system that changes that set of locations--say a farther distance out from the star--that acts exactly the same. The difference is that each system of coordinates will claim that the speed of light is c only right around their special set of locations.

There is no right or wrong here; General Relativity doesn't pick out one preferred system of coordinates. It only gives us a way to make sure that the physics that we calculate in one system works out when we translate our description into another system. It also claims that, at any point, the speed of light (in a vacuum) nearby is always c; where one gets into difficulty is trying to determine what a speed, distance, or time is at points that are a finite distance away. There are some restrictions on what we can say about these distant points, and these restrictions form the way that we have to describe cause and effect, but these restrictions do not precede our choice of system of coordinates, they merely shape how we can describe the laws of physics in a coordinate system.

It does get complicated, which is unfortunate, but kinematics in general is not hopelessly complicated, as many pre-Newtonian physicists feared.

#### yor_on

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##### Re: Is the speed of light constant through out the universe
« Reply #7 on: 17/03/2015 18:52:51 »
Now, that depends :)
From a container definition you're perfectly correct, in that ideally there is no gold standard. But locally it exist, and it is locally we define constants, repeatable experiments and physics. And it keeps on working wherever we go, locally. This unspoken presumption of a 'container' is what I think of as the main reason for Einstein trying to define and find a 'fifth dimension' unifying observer dependencies into one ideal container.
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If you consider the idea of a inflation, then define causality as what defines the finished product. Then that will tell you that the constants we find locally must be 'causally linked' throughout our 'infinite universe'. Depending on taste there are several suggestions of how inflation may 'bud off', but as long as we define causality as what will inform us of our place in a universe, it shouldn't matter. We won't leave the constants we have by traveling, no matter how far we go. To assume otherwise is also to assume a region in where (different) constants do battle, defining different types of universes. The way around that is to assume that each universe builds through linking, making an idea of dimensions rather unstable. That as it then becomes a intrinsic property of each universe, its 'laws' and constants defining its topology and other properties. It's no longer different 'surfaces' that connects, it's more like a mathematical expression, in where for example a vacuum as we know it becomes something in each point described by the number three, consisting of its three dimensional property of length, height and width. It's a much clearer concept to me, but it is also one in where we leave the old ideas for something new. And what linked our universe is 'c', locally defined.
« Last Edit: 17/03/2015 20:08:02 by yor_on »

#### Robcat

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##### Re: Is the speed of light constant through out the universe
« Reply #8 on: 17/03/2015 20:45:35 »
A photon approaching a black hole event horizon?  Does the gravity accelerate it to what velocity?

#### JohnDuffield

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##### Re: Is the speed of light constant through out the universe
« Reply #9 on: 19/03/2015 18:08:29 »
A photon approaching a black hole event horizon? Does the gravity accelerate it to what velocity?
Zero!

#### jeffreyH

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##### Re: Is the speed of light constant through out the universe
« Reply #10 on: 20/03/2015 00:19:22 »
Under which coordinate system John? Which metric are you using to determine that? Is it an exact solution to the Einstein field equations? There are 4 exact solutions already. You may have determined a fifth. Can you please elaborate please?

#### JohnDuffield

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##### Re: Is the speed of light constant through out the universe
« Reply #11 on: 20/03/2015 10:52:25 »
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.

#### jeffreyH

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##### Re: Is the speed of light constant through out the universe
« Reply #12 on: 20/03/2015 11:09:12 »
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.

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

#### PhysBang

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##### Re: Is the speed of light constant through out the universe
« Reply #13 on: 20/03/2015 14:36:45 »
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.

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.

#### JohnDuffield

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##### Re: Is the speed of light constant through out the universe
« Reply #14 on: 20/03/2015 17:38:00 »
Just say you don't know if that is the case. Stop dancing around your lack of knowledge.
I do know. If you don't believe me, ask around about it, you'll find that others will vindicate what I say. Gravitational time dilation goes infinite at the event horizon, and the remote observer says the light clock at the event horizon doesn't tick because the coordinate speed of light at that location is zero.

#### jeffreyH

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##### Re: Is the speed of light constant through out the universe
« Reply #15 on: 20/03/2015 17:41:47 »
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.

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.

I am not sure he would even realise a mathematical flaw existed. You have to have a grasp of mathematics in order to appreciate that subtlety.

#### JohnDuffield

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##### Re: Is the speed of light constant through out the universe
« Reply #16 on: 20/03/2015 17:50:56 »
Jeffrey it's nothing to do with mathematics. See what Pete said. The speed of light varies in a gravitational field.

...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 here:

« Last Edit: 20/03/2015 17:52:35 by JohnDuffield »

#### PhysBang

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##### Re: Is the speed of light constant through out the universe
« Reply #17 on: 20/03/2015 18:43:27 »
I do not find it surprising that JohnDuffield spuriously defends his incorrect claim using the same one of two quotations that are apparently all he knows about Einstein. If one observes his behavior, one will observe that the entirety of his claims about relativity theory come down to these two claims. I leave it as an exercise to the reader to determine if this is relevant to an accurate analysis of his claims.

Jeffrey it's nothing to do with mathematics.
I am pleased to see that Mr. Duffield is keeping to his practice of denying the use of mathematics in understanding physics.
Quote
See what Pete said. The speed of light varies in a gravitational field.
Indeed, there is little doubt about this. However, the mathematical details are that one can always create a coordinate system in which, for a given point, the speed of light there is c. This is essential for expanding special relativity to general relativity.
Quote
...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 here:

As always when Mr. Duffield drags up one of the two quotations that he knows, I urge the reader to read the science that Einstein wrote and examine the entirely of the two documents that Mr. Duffield cites. Even a small amount of further reading, even though it might require some mathematical understanding, is enough to reveal a much more interesting physics (and one that one can potentially test) than the one Mr. Duffield would have you believe in and buy his book for.

#### yor_on

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##### Re: Is the speed of light constant through out the universe
« Reply #18 on: 31/03/2015 22:42:24 »
Yes, physics is based on 'c'. Not as a speed of light only, but for a lot of other stuff too.
https://en.wikipedia.org/wiki/Spacetime#Spacetime_intervals for example.
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As long as ones local measurement of 'c' exists, it makes little sense to assume that to be a illusion. You could argue that 'c' only is 'c' under certain circumstances, but you could also argue that what makes 'c' is the origin of SpaceTime, before gravity. That would make it even more of a original constant, as far as I can see. That is presuming that the universe believe in simplicity naturally. But that's the way I presume it to be, and I think most of physics too. There is a complementary description too in where one as well as defining it relative gravity, with what I call a 'container perspective' aka a 'unified universe', instead make it a strict local definition, which it actually is. Then 'time dilations and LorentzFitzgerald contractions' cease to exist for you, locally defined. Because you are the gold standard of any experiment you do, your clock and your ruler defines it. And from such a definition a repeatable experiment consist of us comparing our local definitions (of ones experiment), finding them equivalent according to some strictly local standards. Time dilations etc, are a result of you measuring over frames of reference, relative your local 'gold standard' (as well as me measuring according to my 'gold standard'). And even though we might disagree on such results, we won't disagree on the local definitions we both used reaching them.

(From such a perspective a Lorentz transformation is a proof of causality existing, and in a wider context a 'container' of sorts. Although not necessarily from more than a logic perspective)

If you now after reading this also make your local clock equivalent to 'c',  then 'c' definitely must be a (local) constant everywhere. It's a very good argument to me :) and it simplifies things, which is what I like.
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Some questions draws me to them I'm afraid.
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now consider my statement "Time dilations etc, are a result of you measuring over frames of reference, relative your local 'gold standard' (as well as me measuring according to my 'gold standard'). And even though we might disagree on such results, we won't disagree on the local definitions we both used reaching them."

assume that 'c' is a variable. That means that we can't find an agreement on any of the above, as long as we don't find a 'unified' gold standard that we both can relate our 'local clocks and rulers' to, and so 'fit them' to each other. And a Lorentz transformation won't mean a thing, unless we find that 'hidden container standard' that defines it. In fact, physics disappear for you as there is no way to define it anymore. To disprove this you only need to disprove the statement I made about the equivalence of your local clock with your local measurement of 'c'.  And remember how 'c' is defined, before trying to disprove the equivalence.

(Actually, those sentences should tell you that relativity would have to be wrong, before getting it that way. Relativity doesn't use a 'universal gold standard', it builds on local definitions strictly. Take those away and relativity disappear.)
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To simplify it even more. That I have a computer that works, with (local) standards existing that allow you to have one too communicating, is a proof for the idea of 'c' as a variable to be wrong. And if that is wrong then 'c' is a constant, and so is your local clock (and ruler).
« Last Edit: 01/04/2015 01:05:47 by yor_on »

#### PmbPhy

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##### Re: Is the speed of light constant through out the universe
« Reply #19 on: 01/04/2015 03:23:44 »
Quote from: PhysBang
However, for any location, we can always produce a system of coordinates for which the speed of light is the determined constant, c.
That was a given.

#### The Naked Scientists Forum

##### Re: Is the speed of light constant through out the universe
« Reply #19 on: 01/04/2015 03:23:44 »