I will make this a brief post in the hope that it will generate some interest from people who might know something of the subject (Chronos et al). If it does, I can add the background cites.
Question: is there any research/theory on the speed of light through dark matter? The speed of light varies depending on the medium and is a maximum in a vacuum (cite needed). Dark matter exists (it is believed) and so is there any reason to postulate that the speed of light is different in areas of high concentrations of dark matter (ie galaxies like ours)? The distribution of dark matter throughout the universe is non-uniform, that is how it was detected (cite needed), so the speed of light should be non-uniform.
Also, the speed of light can be calculated from fundamental constants in Maxwell’s equations (cite needed). This agrees quite well astronomical observations. If dark matter effects the speed of light, are the constants wrong?
Thanks
I beleive you are confusing c, the universal constant from Einstein, with the speed of transmission of EM in a particular medium.
Both are commonly called “the speed of light” by non-experts. They are utterly different things.
The speed of light (EM transmission) in a glass fiber is different from the speed of light (EM transmission) in a vacuum. No laws are broken, no paradox results.
Dark matter having a different transmission speed would no more threaten the universal constant c and our current theories than the existence of ordinary glass does.
Not to say that dark matter, whatever it is, might not have other properties which upset some aspects of current theories. But what you’re talking about isn’t it.
I think he’s asking about whether, having formulated our theories about light and measured its speed in a galaxy full of dark matter, there is any speculation concerning whether our c is actually just the speed of light in dark matter, with the real c as yet undiscovered.
I must say this is a new one to me. I think it is highly unlikely that dark matter throws off our theories and measurements that bad. But I’m not a physicist and haven’t heard about this issue being discussed by them.
Virtually all the theoretical components of dark matter are ordinary matter without any exotic properties. They just don’t give off electromagnetic radiation. Even nonbaryonic dark matter candidates are all within the bounds of the Standard Model.
I think the problem is that dark matter has been made into some mysterious other rather than just a collection of dark particles. If you think of it as just a collection of dark particles, as yet unidentified, any exoticness goes away. As a series of particles (although they can be clumped into planetoids or other massed objects) they’re an inconceivably tiny fraction of the volume of space, just as the stars and planets we see in galaxies are inconceivably tiny fractions of the volume of space they occupy. The dark matter halos around galaxies are essentially vacuums as good if not better than the galaxies themselves. So no measurable local variation from C should be expected.
The whole point of dark matter is that it doesn’t interact with light, or at least interacts with it to a minimal degree. So we should certainly expect to slow light much less than, say, hydrogen, and perhaps not at all. Plus, even though it’s a good bit more abundant than normal matter, it’s still pretty thin stuff.
That said, the normal matter between the stars, being not a perfect vacuum, does have a small but nonzero effect on the speed of light through space. It’s not much, but it’s enough that high-energy neutrinos might be able to move faster through space than photons do.
The speed of light and c have both been measured independently, and agree wonderfully. And there’s really no mechanism by which dark matter (or anything else) could affect c.
Thanks!
I didn’t realize that c and speed of light are different constants. However, my main question is limited to the speed of light. Bringing in Maxwell’s equations was not helpful to my question since it has nothing to do with defining the speed of light (besides specifying the speed of an electromagnetic wave). However, given that c is numerically equal to the measured astronomical value of the speed of light implies that dark matter does not effect the speed of light. At least if the effect is there it is probably within the margin of error.
Strictly speaking, if the speed of light differs from c, it’s not a constant. Anything that travels at any speed other than c can, in principle, travel at any speed whatsoever less than c. Neutrinos, for example, generally travel at very close to c (relative to our reference frame), but one could have a neutrino just plodding along at five miles per hour.
The critical error the OP is making is thinking that “the speed light travels at” is a constant. It is not.
OTOH, “the speed of light”, the value c, IS a constant. It just has a really crappy & misleading name. It should have been called the speed of time, or the speed of information, or the speed limit of the Universe. Almost anything would have been better than the name it got: “speed of light”
The constant c is a fundamental feature of Nature as we understand it. It truly is a constant, unchanging in all circumstaces and at all times. The speed of propagation of EM in a vacuum not under the influence of acceleration / gravity is equal to c. They are not the same thing, but they have the same value.
When we add either acceleration, or non-vacuum, or both, to the situation, the speed of EM propagation decreases to a value less than c. The constant c doesn’t change.
We can get further into accelerated & non-accelerated reference frames and all the rest, but I don’t think that’ll improve matters.
This doesn’t sound right to me. Wikipedia says the current estimates to be equivalent to one hydrogen atom per cubic meter. Interstellar matter clouds less dense than that can affect spectra.
That’s not that I disagree with you in the grand scheme of things - dark matter apparently does not affect the speed of light. But I do think you would expect a measurable variation if it affected light.
I prefer to call it atramentous corporeity. Whatever we choose to call it, missing dark matter is a placeholder for some substance or field that creates a gravitational effect without significant interaction with electomagnetic fields. That that is essentially all we can currently say on the topic without advancing into bareass speculation.
I am well aware that the speed light travels at is not a constant.
In fact, that is the crux of my question. The speed light travels at is a function of the medium through which it travels including the effects of gravity.
I cluttered up my question by attempting to say too much and introduced extraneous concepts-though I am glad I did as I learned something important.
Nevertheless, the speed at which light travels is a function of the medium through which it travels. Is there any theory/speculation that considers whether light traveling through dark matter (at least one of dark matter’s well-known effects: gravity certainly does change the speed light travels), effect the speed? If so, given that c and the speed of light agree so well, what does that say about the equivelance?
well, we can go at least one step further. Am I correct in saying that Einstein showed that gravity by itself changes the speed at which light travels.
Also, the purpose of atramentous corporeity is to explain observed gravitational effects. Can we not therefore say that atramentous corporeity does in fact have an effect on the speed at which light travels?
Or, more precisely, it’s one or more (probably more) substances and/or fields. There’s no reason to expect that it’s all the same stuff, whatever it is.
To the extent that it has a gravitational field, it has a gravitational potential, and a gravitational potential will result in a time dilation, so you would get a delay if photons were to travel near a concentration of dark matter. But in current models of gravity, that’s not interpreted as a change in the speed of the photons, but as a change in the distance they have to travel.
The reason that conventional matter slows the propagation of light is that atoms contain positive and negative charges. When the light is propagating through the matter, the atoms become polarized, which means there is some current, called polarization current. Those currents then themselves cause radiation, which is at the same frequency as the original light, but phased so that the combination of the original light and the light radiated by the matter is slowed a little relative to just the original light. So it isn’t really true that the original light slows, although that’s the easiest way to describe it. It’s the combination of original and scattered light that is slower. The important concept is that it’s the fact that normal matter is made from charged particles, not that it has mass, that leads to a slower speed.
The non-baryonic dark matter, which is what is often referred to as just dark matter*, doesn’t interact with light like regular matter, and may not have any charge. If dark matter is made of electrically neutral particles, the above mechanism for “slowing” light wouldn’t apply, and so there wouldn’t be any reason to expect light to be slowed by dark matter.
*Baryonic dark matter is just regular matter like planets. The term came about because it’s possible to see how much “light matter” there is, because you can see the stars. So when you try to estimate how much mass there is in a galaxy, say, you can get a good idea of how much light matter there is, but you can’t see the dark matter, because it’s, well, dark.
