[EvaH (OP)]

Victor wants to know:

I recently had a personal eureka moment when I realised that there can be no such thing as "free space", as in the permittivity, permeability, and impedance of free space and the radar range equation for free space. If there is nothing there how can it have properties? So I suddenly thought these properties must refer to dark matter.

This got me thinking further. Why should electromagnetic waves such as light have a fixed speed? Is it possible the 30,000,000 m/s is only because we are not measuring over a sufficiently large distance? If light has to plough through dark matter could it eventually come to standstill? Could the strings making up the photons simply stop vibrating and become dark matter?

What do you think?

[alancalverd]

You have the argument back to front. EM radiation does not require any medium to propagate. Therefore its speed is constant and independent of direction in vacuo.

If we calculate its speed from Maxwell's equations (which derive from electrostatic and electromagnetic laboratory measurables) we find there are two constants with the dimensions of permittivity and permeability (lab measurables) which determine the propagation speed in any or no medium. We can measure both in any material m, so it is convenient to use the terms "relative permeability μ_m (permittivity ε_m)" which is dimensionless and characteristic of the medium, and "permeability μ₀ (permittivity ε₀) of free space".

My navigation instructor said "start from where you are, then you won't get lost before you take off." It's a good idea to begin with what we know, rather than invoke hypothetical concepts like strings and dark  matter that we don't know. You can bounce light and radar signals off the moon, and receive radio transmissions from deep space probes. As far as we know here's nothing in between.

Gravitational and doppler shift equations seem to give the correct answers when tested experimentally.   
 

[Bored chemist]

If there is nothing there how can it have properties?

We can measure them.

[evan_au]

So I suddenly thought these [electromagnetic] properties must refer to dark matter.

Since Maxwell's time, we have got very good at detecting electromagnetic effects - the MRI machine detects subtle signals from the spin axis of protons. There is an even more subtle magnetic effect from neutral neutrons.
See: https://en.wikipedia.org/wiki/Neutron_magnetic_moment

This leads researchers to think that the mysterious dark matter must not interact at all with the electromagnetic field. Therefore it will not affect the permeability μ₀ and permittivity ε₀ of free space.

Is it possible the 30,000,000 m/s is only because we are not measuring over a sufficiently large distance?

We have very accurate measurements of the speed of light to space probes, a couple of which have passed the edges of the Solar System; these measurements are important for spacecraft navigation. These measurements are accurate enough to detect a tiny asymmetry in the infra-red radiation from one space craft.
See: https://en.wikipedia.org/wiki/Pioneer_anomaly

A supernova 170,000 light-years away showed that light and neutrinos travel at almost the same speed.
See: https://en.wikipedia.org/wiki/SN_1987A#Neutrino_emissions

Over longer distances, detection of a neutron star merger 130 million light-years away shows that light and gravitational waves travel at almost the same speed.
https://en.wikipedia.org/wiki/GW170817#Gamma_ray_detection

A gravitational field permeates the entire universe, forming the substrate for transmitting gravitational waves.
Similarly, an electromagnetic field permeates the entire universe, forming the substrate for transmitting electromagnetic waves.

[Bored chemist]

mysterious dark matter must not interact at all with the electromagnetic field.

It's worth noting that light is electromagnetic in  nature.
They called the mystery stuff "dark matter" precisely because it did not interact with light.
If it interacted it wouldn't be "dark".

[alancalverd]

A gravitational field permeates the entire universe, forming the substrate for transmitting gravitational waves.Similarly, an electromagnetic field permeates the entire universe, forming the substrate for transmitting electromagnetic waves.

Strewth, Ev, that's a dinkum googly

In physics, a field is a physical quantity, represented by a number or tensor, that has a value for each point in space-time

That makes sense with gravitation because all the gravity in the universe is associated with all the mass, and when a mass moves the resulting perturbation can in principle be sensed everywhere. But I can switch on a searchlight and create photons that whizz from A to B without having any effect at C, so no evidence for a substrate field.
Care to elaborate before the bails come off, or have I smashed the universal EM field out of the park?
   

[RobC]

Deriving the speed of light from Maxwell's equations is similar to Bell's inequality in that both are universally accepted but I have never seen an easy to understand explanation for either.

[evan_au]

That makes sense with gravitation because all the gravity in the universe is associated with all the mass

I agree.
The universe is full of mass, and all mass attracts, via gravity. We can see the effects of gravity at large distances, so the idea of a universal gravitational field is not novel.

The universe is also full of electric charges. In contrast, electric charges both attract and repel. As far as we can measure, the amount of positive and negative charges in the universe is identical, and mostly cancels out to 0 even in very small volumes (ok, a thundercloud is not a very small volume...).

This means that electromagnetic fields cannot be felt at large distances, and so forces at the scale of a minor planet and larger is dominated by gravitation.

Perhaps this is why the idea of a universal electromagnetic field sounds novel?

when a mass moves the resulting perturbation can in principle be sensed everywhere. But I can switch on a searchlight...

Clarification: It is accelerating masses that perturbs the gravitational field and produces gravitons. By itself, uniform motion of masses does not.
Similarly, it is accelerating charges that perturbs the electromagnetic field and produces photons. By itself, uniform motion of charges does not.

Clarification: It is the reflective mirror of a searchlight that deflects photons.
In contrast, with our current understanding of gravity, we know of nothing that can deflect gravitons. Effectively, a gravitational source is omnidirectional.
To compare apples with apples, you need to use an omnidirectional photon source (eg a star).

Clarification: Visible photons have an energy around 1eV (as an order of magnitude). Astronomers can detect individual photons with high efficiency.
We understand less about gravitons, but some guesstimates put their energy at around 10^-23 eV. We have real trouble detecting them, even in immense numbers.
This assigns them less energy than AM radio photons.
See: https://en.wikipedia.org/wiki/Graviton#Energy_and_wavelength

If you had a good enough telescope, you could detect the light from a star 10 billion light years away.
- And we can still detect microwaves from 300,000 years after the Big Bang
- Some researchers are trying to detect gravitational waves from the Big Bang (a much more difficult challenge - but if they succeed, they should be able to look back less than 1 second after the Big Bang!)

Conclusion:
So, given a suitable comparison, I would suggest that both photons and gravitons can be detected throughout the universe.
- So I think a substrate electromagnetic field is useful, at least as an analogy.

PS: I have trouble seeing a cricket game, at the moment - but I have it on my bucket list....

[alancalverd]

Deriving the speed of light from Maxwell's equations is similar to Bell's inequality in that both are universally accepted but I have never seen a simply understood explanation for either.

try www.maxwells-equations.com  The video is excellent and links the weird-looking 3-dimensional vector calculus to actual laboratory experiments with electromagnets and capacitors.

[alancalverd]

The universe is also full of electric charges.

This is where our understanding diverges. All the electric charges we know of, are associated with massive particles. And whilst there are plenty of them,  the interstellar density is almost zero, not always neutral over any distance, and always fluctuating at any point. The propagation of photons seems unaffected by such fluctuations, which would surely broaden the spectral lines from distant stars.

The problem with your substrate EM field is that it smells awfully like aether, so c depends on direction, which is doesn't,  whereas we can detect variations in the gravitational field as massive bodies move around.

[syhprum]

I have never quite forgiven Einstein  for abolishing the aether  I still have lingering doubts that some thing like it must exist

[alancalverd]

It wasn't Einstein. Maxwell demonstrated that it was unnecessary, and nobody has found any experimental evidence or reason for it since.

It did however feature in radio communications textbooks at least until the 1950s.

[Bored chemist]

And  Albert A. Michelson and Edward W. Morley nailed the lid on ether's coffin in 1887.

[PmbPhy]

I have never quite forgiven Einstein  for abolishing the aether  I still have lingering doubts that some thing like it must exist

Why?

[evan_au]

The propagation of photons seems unaffected by such fluctuations, which would surely broaden the spectral lines from distant stars.

The thing about photons (and gravitons) is that they can pass through each other without affecting each other.
- At least until you get up to gamma-ray energies, where they can produce electron/positron pairs...

So the sort of spectral broadening you expect to see in a star is:
- Thermal broadening because the emitting atoms are moving very quickly
- Pressure broadening, because the gas has an increasing density as you get closer to the surface
- Rotational broadening, because the star is spinning

None of this has to do with the propagation of light through space as a perturbation of the electromagnetic field, propagating at c...
- As described by Maxwell
- Quantised by Einstein
- Relativised by Einstein
- Energised by Planck (and Einstein, again...)

See: https://en.wikipedia.org/wiki/Spectral_line#Line_broadening_and_shift

[alancalverd]

My point precisely.
If there was a fluctuating interstellar medium responsible for the propagation of light, those fluctuations would broaden spectral lines from distant objects, so you would have to add "distance broadening" to your list.
If the receiver is moving through the field, as we are, the value of c would have a directional dependence, which it doesn't.
If photons propagate by making waves in the field, you could detect a single photon off-axis by local distortion of the field, but you can't see a laser beam in a vacuum.

[evan_au]

you can't see a laser beam in a vacuum.

A laser is a non-isotropic source.
- Even with non-isotropic sources (eg the dual-slit experiment), the photon could appear at extreme angles (just with very low probability)
- The same applies to any practical laser with a finite diameter - there is always some probability of off-axis photon refraction at the source and off-axis detection at a distance

A simplistic analogy: "Light travels like a wave and hits like a particle...".

You can't detect where a photon is when it is transit, because it is a probability wave that spreads out in all directions from the source.
- If you do try to detect it, you collapse the wave function, and it appears somewhere (with some probability distribution)

It is the same with an isotropic source like a star or supernova - any particular photon could appear in your detector billions of light-years away in any direction (with some probability distribution).

[alancalverd]

A laser is a non-isotropic source.

which is why you can't see the beam from a point at 90 degrees to the axis. But if it were propagating through a universal field, you would be able to detect the field distortion and thus extract energy from the beam, thus producing "distance broadening" or simply "distance shift."

[evan_au]

But if it were propagating through a universal field, you would be able to detect the field distortion and thus extract energy from the beam, thus producing "distance broadening" or simply "distance shift."

So you have invented a universal field that produces an impossible result, and then say that can't work.

I put it to you that the electric & magnetic field strength is defined everywhere in the universe (you would need to pick a frame of reference, as the relative strength of electric and magnetic field is frame-dependent).
- We know this is true, because we can see stars billions of light years away, and their electromagnetic radiation has passed through all that distance without coming to the "end" of the electromagnetic field.
- Any photons passing through each position in space will temporarily disturb the electric and magnetic field at that position in space
- And you will detect real photons if you put a light detector there.
- That includes photons from any lasers or masers that happen to be pointing in that direction

Why is this so strange?

[alancalverd]

From my point of view I don't see the need for a universal and allpervasive field. Projectiles and massive particles of all sorts fly through space precisely because there is nothing there to stop them or slow them down, though gravity bends them a bit. Why do photons need a medium? 

And if there is a medium, how come it is as uniform as a theoretical vacuum, but only and always in a vacuum?

If a conductor moves in a magnetic field, a current will flow in it. If an insulator moves in an electric field, you will get charge separation across it. Are these effects noticed in deepish (i.e. explored and measured to date) space? 

Or have I firmly grasped the wrong end of the wombat's tail?