# B Speed of Light in Dark Matter

1. Jun 5, 2018

### Sanborn Chase

Does anyone know the speed of light in dark matter?

2. Jun 5, 2018

### Ken G

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.

3. Jun 5, 2018

### Sanborn Chase

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?

4. Jun 5, 2018

### Orodruin

Staff Emeritus
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.

5. Jun 5, 2018

### Sanborn Chase

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

6. Jun 5, 2018

7. Jun 5, 2018

### Drakkith

Staff Emeritus
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.

It takes longer to travel, but the path it is traveling is also longer, so the speed still ends up being c.

8. Jun 5, 2018

### Sanborn Chase

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.

9. Jun 5, 2018

### Drakkith

Staff Emeritus
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.

10. Jun 5, 2018

### phinds

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.

11. Jun 6, 2018

### Orodruin

Staff Emeritus
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.

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.

12. Jun 6, 2018

### Sanborn Chase

13. Jun 6, 2018

Staff Emeritus
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"?

14. Jun 6, 2018

### Sanborn Chase

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

15. Jun 13, 2018

### nnunn

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'.

16. Jun 13, 2018

### Sanborn Chase

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

17. Jun 13, 2018

### Drakkith

Staff Emeritus
Would you count astronomical observations as measurements of the permittivity and permeability of deep space?

18. Jun 13, 2018

### Sanborn Chase

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

19. Jun 14, 2018

### nnunn

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

20. Jun 14, 2018

### phinds

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.