# Thoughts/Questions about GR and dark matter

1. Apr 3, 2013

### DLH112

I know light behaves oddly compared to normal matter, I haven't really delved into the math of GR, and that's why I'm here to ask these questions.

1) If all of our observations of the universe are based on light, what determines that the speed of light is the maximum velocity? I'm sure its somewhere in the math I haven't been taught/tried yet.

2) (> i've heard in a another thread that light will just redshift if this happens I think so this is probably wrong)

If something is traveling one way at 1000 m/s and from that something is launched in the opposite direction at 400 m/s the net velocity of the launched object is in the direction of what it was launched from

if dark matter is traveling FTL away from us and behaves similarly then the light would never reach us hence why we cant see it? (but the speed of light is constant, and light behaves weirdly and whatnot so :/ )

sort of unrelated question: since a black hole has mass and we can't see them are they considered dark matter?

i'm prepared for the million reasons why i'm wrong to be yelled at me xD.

2. Apr 3, 2013

### cepheid

Staff Emeritus
The way you phrased the part in bold is strange, because it implies that you think light is a type of matter, but just an "abnormal" kind. Light is NOT matter, whether normal or abnormal. So it shouldn't come as too much of a surprise that it behaves differently from matter.

Your question is, again, a bit strange, because the way you phrased it implies that you think red part has some implication or bearing on the blue part, when in reality the two are totally separate and disjointed thoughts. Let me try to answer the question of why the speed of light is the maximum velocity. What relativity actually requires is just that there is some universal maximum speed. Given the postulates of relativity, this maximum possible speed is required in order to preserve causality, which, loosely speaking, is the requirement that all causes precede their effects, as viewed by all observers. I think you could agree that if causality were violated (i.e. if event A could influence event B, and some observers would see event A as preceding event B, whereas others would see event A as coming after event B), then really the logical foundation for doing physics would break down. So, there is a universal speed limit (i.e. a maximum speed at which matter or information can be propagated).

It just so happens (in nature) that this universal speed limit is also the speed at which the particles that make up light, photons, (or any massless particles) propagate, and there is no reason for this other than, "because that's the way nature is, according to what we've observed."

Please have a look at the threads in our Relativity FAQ section, you may find some of them useful:

https://www.physicsforums.com/forumdisplay.php?f=210 [Broken]

Huh? This wasn't a complete thought, let alone a question.

According to whom? Or, to put it another (more technical) way: in what reference frame? First lesson of relativity: ALL motion is relative. There is no preferred absolute reference frame with respect to which velocities are supposed to be measured. Everyone's frame of reference is equally valid. So it's not meaningful to state that a certain object is moving at a certain speed in a certain direction unless if you also specify which observer that motion is being measured relative to.

Nothing can travel faster than the speed of light. Particles with mass cannot travel at the speed of light, only slower than it.

No, a black hole is not considered to be dark matter. Just because you can't see an object doesn't mean that that object is made out of dark matter (this is not a sufficient criterion). Let me give some background information to set up my explanation. There are four fundamental forces or "interactions" in nature: the strong nuclear force, the weak nuclear force, the electromagnetic force, and gravity. These are the ways in which the elementary particles known to physics can interact with each other. It is pretty clear from observational evidence that dark matter, whatever it is, must be a particle that interacts only by gravity, and maybe by the weak interaction, but nothing else. There is no known elementary particle predicted by the Standard Model of Particle Physics that has the properties that dark matter has been observed to have, so whatever dark matter is, it must be some new type of particle outside of the Standard Model. Black holes, on the other hand, are formed from the gravitational collapse of objects that are made out of ordinary atoms, which in turn are made out of elementary particles from the Standard Model.

Last edited by a moderator: May 6, 2017
3. Apr 3, 2013

### sonic25

If you don't quite understand General Relativity, I'd probably recommend reading a few books explaining fundamental principles behind the theory such as relativity and causation. I know when I was first looking at it this helped me a lot. Then, you can have a closer look at the mathematics later which will hopefully make a bit more sense.

As cepheid said, relatively works around the idea of a cosmic speed limit. This is the speed of massless particles such as photons which make up light. Without such a speed limit, different viewpoints could suggest events happened before or after one another even if they affected one another which would violate causality.

4. Apr 4, 2013

### Chalnoth

In essence, it comes down to all observers measuring the speed of light as being c in their local frames. This was demonstrated by the Michelson-Morley experiments in the late 1800's (which measured the differences in the speed of light between light beams traveling in different directions at different times of the year). The implications of this for fast-moving particles have been confirmed quite dramatically in high-energy experiments performed over the last century.

Right, the light just redshifts.

Dark matter doesn't interact with light. That's why we can't see it.

Yes, in a sense. But there just aren't enough black holes out there with enough mass to matter. More to the point, our most precise measurements of the amount of dark matter come from the very early universe, before anything collapsed into a star, let a lone a black hole.