# Speed of Light in Dark Matter

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## Main Question or Discussion Point

Does anyone know the speed of light in dark matter?

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Ken G
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Since no one knows what dark matter is, it's hard to say theoretically what that speed would be, except to say that there is not any interaction with light anticipated. If there is no interaction with light, the speed is c.

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Doesn't DM interact with gravity, ie. "bends" light? If we change any of the variables in d=rt doesn't it affect all others?

Orodruin
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Don’t mix the speed of light with light being affected by gravity. The speed of light is a local concept and it is, as already stated, expected to be c in the presence of dark matter by virtue of dark matter being dark.

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But it's not a vacuum, right? And a 'bent" ray takes longer to travel, so we've varied t and r.

Drakkith
Staff Emeritus
But it's not a vacuum, right?
Doesn't matter. If dark matter doesn't interact with the EM field, then it can't affect the light like a normal medium (like glass or water) would and the light simply passes through without being slowed. The only reason light slows down in a normal medium is because regular matter does interact with the EM field, enabling it to respond to the oscillating EM fields of the light and act to slow it down.

And a 'bent" ray takes longer to travel, so we've varied t and r.
It takes longer to travel, but the path it is traveling is also longer, so the speed still ends up being c.

ohwilleke
Gold Member
Thanks very much, CWatters. I enjoyed the previous thread you mentioned.
Personally, I think the evidence for a DM halo surrounding our galaxy and others is quite convincing. If that be so it is doubtful mankind has ever clocked the speed of light in anything but dark matter:: the speed of light in dark matter is the conventional measure currently used.

Drakkith
Staff Emeritus
Personally, I think the evidence for a DM halo surrounding our galaxy and others is quite convincing. If that be so it is doubtful mankind has ever clocked the speed of light in anything but dark matter:: the speed of light in dark matter is the conventional measure currently used.
The density of dark matter around Earth is quite low and we haven't observed any change in the speed of light as it moves through different densities of dark matter, so there's little reason to believe that it has any effect on the speed of light at all.

phinds
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2019 Award
the speed of light in dark matter is the conventional measure currently used.
I think you seriously misunderstand the density of dark matter. Inside galaxies where it is prevalent, the density is something less than 10E-20 grams per cubit centimeter which is pretty much a hard vacuum, and remember, that's the AVERAGE. The actual density of matter (regular and dark) outside of solid bodies is WAY less than that. It's not even remotely reasonable to call that a "Medium".

c is the speed of light in a vacuum. There is no medium involved.

russ_watters
Orodruin
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Homework Helper
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Inside galaxies where it is prevalent, the density is something less than 10E-20 grams per cubit centimeter which is pretty much a hard vacuum, and remember, that's the AVERAGE.
The local dark matter density is around $10^{-25}$ g/cm3. It goes up as you approach the galactic center.

Either way this does not address the problem of the OP, which is confusing speed and velocity.

But it's not a vacuum, right? And a 'bent" ray takes longer to travel, so we've varied t and r.
This is not how you compute speed. Speed is the magnitude of the instantaneous velocity, not that of the average velocity over a longer time.

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Staff Emeritus
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Getting back to the original question, it's too ill-defined to answer. What is the speed of light in nitrogen? That depends on the pressure and temperature, and whether you have solid, liquid or vapor nitrogen. There's not one number.

That said, in most of the universe, the answer is "very close to c", because we see no obvious refraction looking at distant sources. But if you want a quantitative answer, you're asking "how long is a piece of string"?

ohwilleke
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Thanks, Vanadium. As you've already discerned I have no qualms about showing my profound ignorance to the public.

BvU
Thanks, Vanadium. As you've already discerned I have no qualms about showing my profound ignorance to the public.
On the other hand, if you are wondering if the permittivity and permeability of our 4-manifold may be affected by some property (that seems to define the curvature of that manifold), then your question seems like a good one. Both Paul Dirac and Rob Laughlin (and many others) have wondered about this.

From page 121 of Laughlin's 2005 book, "A Different Universe: Reinventing Physics from the Bottom Down" (Chapter 10, "The Fabric of Space-Time"),

"About the time relativity was becoming accepted, studies of radioactivity began showing that the empty vacuum of space had spectroscopic structure similar to that of ordinary quantum solids and fluids. Subsequent studies with large particle accelerators have now led us to understand that space is more like a piece of window glass than ideal Newtonian emptiness. It is filled with 'stuff' that is normally transparent but can be made visible by hitting it sufficiently hard to knock out a part. The modern concept of the vacuum of space, confirmed every day by experiment, is a relativistic ether."​

As pointed out by the OP, if the distribution of this substrate is not homogeneous, we've only really measured permittivity and permeability from deep within a well of that 'stuff'.

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Thanks, NNUNN. I'll certainly purchase Mr. Laughlin's book; I appreciate the mention.

Drakkith
Staff Emeritus
As pointed out by the OP, if the distribution of this substrate is not homogeneous, we've only really measured permittivity and permeability from deep within a well of that 'stuff'.
Would you count astronomical observations as measurements of the permittivity and permeability of deep space?

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I assume that's a rather rhetorical question, and it may not be aimed at me. But here goes: yes.

Would you count astronomical observations as measurements of the permittivity and permeability of deep space?
Hi Drakkith - given the need to interpret those astronomical observations, if permittivity and permeability were not constant along the entire path followed by those observed photons, our interpretation of those observations may be skewed. For example, consider a high precision observation of a quasar, in which the observed photons have followed some path though both cosmic voids and superclusters.

I got to wondering about (a) those voids between superclusters, and (b) the spaces within which over-densities of dark mass have fostered the formation of those superclusters. If (ε00) turn out to be functions of some (currently unaccounted-for) property of space, then any time-dependent features of observed spectra (e.g. wavelength) would need more sophisticated analysis.

For example, if the distribution of the condensate of weak hypercharge (which we now call a Higgs-type field) were non-homogeneous, then given the electroweak assumptions underlying electromagnetic phenomena, we'd have a built-in mechanism for adjusting permittivity and permeability

On a related issue, since standard models already accommodate two simple motions of space itself (inflation, expansion), if we relax some assumptions about those models, then we can start to wonder about possible effects of relativistic transverse velocities on our interpretation of high red-shifts.

great time to be an undergrad in astronomy!
Nigel

phinds
Gold Member
2019 Award
... since standard models already accommodate two simple motions of space itself (inflation, expansion)...
This is not correct. There is no expansion/inflation of space, things just get farther apart during the expansion/inflation of space-time. Space is just geometry, not a thing that can be bent or stretched, etc.

Drakkith
Staff Emeritus
Hi Drakkith - given the need to interpret those astronomical observations, if permittivity and permeability were not constant along the entire path followed by those observed photons, our interpretation of those observations may be skewed.
Yes, but skewed how? What would the observable effects be? It can't be anything too drastic, as we don't observe sudden, extreme shifts in the properties of incoming light. The issue is that our current model is remarkably well supported with very little evidence to point to where it may be inaccurate. For example, the redshift and the angular size of galaxies follow each other fairly closely. We don't find galaxies with high redshift that are also very large (angular size). The highest redshift comes from galaxies which are barely larger than a handful of pixels. Nor do we find sudden shifts in the light when we move our observations between galaxies or clusters into the voids between them. In fact, the CMB can be seen when looking through these galaxies, and there is no known association between the placement of these galaxies, clusters, and voids and a change in the CMB as far as I am aware.

If the permittivity and permeability of space changes over time or distance, it changes in such a way as to make it look extraordinarily similar to the expanding universe that the standard cosmological model represents.

nnunn
Gold Member
What if all observations have been skewed, and skewed how is unknown? A link in the inductive chain may be in question. Perhaps the question should be the speed of light without dark matter present. And, again, I labelled this original post as "B" high school level because they didn't offer elementary school level.

nnunn
Drakkith
Staff Emeritus
What if all observations have been skewed, and skewed how is unknown?
What if they haven't been skewed? Just asking what if they've been skewed isn't a very useful question without something more specific to talk about.

A link in the inductive chain may be in question. Perhaps the question should be the speed of light without dark matter present.
Or there may be nothing wrong with our understanding of the speed of light. That's why you need evidence to point to that can't be explained by the standard model but can be explained with another.

I don't mean to shut down your question, but you could ask "What if X is wrong" about anything at all and in any variation. What if our understanding of the speed of light through dark matter is incorrect, but in a way that we don't know and that looks like something else that we can explain through the standard cosmological model? Well then we're wrong and we don't know it!

BvU
Gold Member
Dear Mr. Drakkith, I can't tell you how much I appreciate your very kind responses to my questions. You've been patient and informative with me, and I know you sense my ignorance and lack of understanding. Perhaps it would help if I explained where my attention is centered.

When I was about ten years old my father and I ground an eight inch mirror and built a reflecting telescope. Certainly I didn't understand what exactly we were doing until one night after its completion he aimed it at the night sky, and I stood on my little stool and looked in the eyepiece. There was Saturn about the size of a dime with all of the colors and rings! My heart just melted; not just about seeing another world, which was so amazing to me, but because my father cared enough about me to wish to share this with his son. I wanted to be an astronomer and learn all I could about that glorious mysterious world out there. My future was settled.

Shortly after this that poor man died of a heart attach, and my life changed completely. I could no longer pursue my dream as necessities eclipsed desires. But I've never lost my sense of awe, and for the next sixty years I've read and studied as much as time allowed in pursuit of those secrets.

When I found these forums I was thrilled. After thoroughly reviewing the protocols and rules protecting the use of them, I decided to join and have never regretted it. I'm completely fascinated by the subjects and responses to these threads. Occasionally, I screw up my courage and ask dumb questions. I realize to some of you with a great deal of quite remarkable knowledge you perceive how sophomoric they seem, but I can't help myself. For I I know I'm standing with giants.

If you have the power to "shut down" my questions, and they seem to have irritated you in some manner, go ahead. But as far as I'm concerned I've paid my money, and I'll take my choice. I hope you won't; there seem to be others interested, too.

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BvU, epenguin and berkeman
Drakkith
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