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View Full Version : Can Dark Matter go Faster than Light?



squidworth
2009-Nov-13, 01:31 AM
Since it doesn't interact with em radiation would c still pose a boundary?

JimmyRoberts
2009-Nov-13, 02:29 AM
If it has mass, it cannot move through space at speeds equal to or greater than C. Neutrinos do not interact electromagnetically, but they do have mass and are constrained to move at speeds less than C.

DrRocket
2009-Nov-13, 02:40 AM
Since it doesn't interact with em radiation would c still pose a boundary?

So far as we know, nothing, including information, can be transmitted faster than light. This has nothing to do with interaction with photons. It is a result of the understanding of space and time that comes from special relativity.

If dark matter travels in excess of c, then that will be a big deal and probably require a lot of re-thinking of relativity.

At this point in time we would have hard time determining if dark matter could travel faster than c, since no one knows what it is, and it is only detected by its gravitational influence.

WayneFrancis
2009-Nov-13, 02:49 AM
True we don't know what dark matter is but we can say what some of its properties are. It's these properties that we've actually given the label of "dark matter"

The main properties of dark matter is its mass and its inability to interact with the electromagnetic spectrum.

Because it "hangs around" in galaxies one can surmise that it can't travel that fast because it keeps "hanging around"

Also don't equate it "being dark" to anything to do with "light speed" the scientific term is c. An particle that has zero rest mass, which dark matter does not have, has to travel at c. So saying "light travels at c" is a better description then "light travels at light speed"

squidworth
2009-Nov-13, 03:33 AM
so c is a property of space, not EM? neat, thnx

dhd40
2009-Nov-13, 03:44 PM
True we don't know what dark matter is but we can say what some of its properties are. It's these properties that we've actually given the label of "dark matter"

The main properties of dark matter is its mass and its inability to interact with the electromagnetic spectrum.
(my bold)

My understanding is that the only property we know of is its gravitational effect on "normal" matter. Whether DM has mass or not seems to be unknown, inspite of the word "matter". Am I mistaken?

Grey
2009-Nov-13, 03:51 PM
(my bold)

My understanding is that the only property we know of is its gravitational effect on "normal" matter. Whether DM has mass or not seems to be unknown, inspite of the word "matter". Am I mistaken?Sort of. For dark matter to account for the cluster and galaxy formation that we see, it has to be "cold" dark matter. In this context, "cold" means that it has to be relatively slow moving, so that it can clump gravitationally. On the other hand, particles with a very small rest mass, like neutrinos, end up with a high velocity even if they have only a relatively small amount of energy. So neutrinos, or other particles with a small rest mass, behave like "hot" dark matter, and cannot account for the structure we see. That's why cosmologists are pretty sure the dark matter isn't just a lot of neutrinos. So we do expect that if dark matter turns out to be some new exotic particle, it will be one with a relatively large rest mass.

Cougar
2009-Nov-13, 04:21 PM
My understanding is that the only property we know of is its gravitational effect on "normal" matter. Whether DM has mass or not seems to be unknown, inspite of the word "matter". Am I mistaken?

Yes, I'd say you're mistaken. How do you get such a large gravitational effect without mass? Yes, energy, momentum, even gravitational energy contribute to the gravitational effect, but in this case, what is the source of these contributions? Even if there is such a source, I expect these contributions would be orders of magnitude smaller than the effect needed to match observation. The gravitational effect we see is 5 or 10 times the expected effect from all the matter that we can detect (stars, gas, dust, etc.).

Another property we're fairly certain* about is that dark matter does not interact with the electromagnetic or strong nuclear forces, since we detect its gravitational effect, but there has been no detection via EM or strong force (not for lack of trying).

Of course, this has driven some to try various modifications to our understanding and formulas involving gravity itself. Making these proposed solutions fit all the observations hasn't worked either.
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* Of course, certainty is not really a characteristic of any scientific theory, or even a 'law'.

dhd40
2009-Nov-13, 08:19 PM
Sort of. For dark matter to account for the cluster and galaxy formation that we see, it has to be "cold" dark matter. .... So we do expect that if dark matter turns out to be some new exotic particle, it will be one with a relatively large rest mass.

I understand your arguments. Still, if something else than matter warps space-time ... :shhh:(forget it, its ATM)

cosmocrazy
2009-Nov-13, 08:27 PM
so c is a property of space, not EM? neat, thnx

Yes, constant "C" is the speed at which massless energy/particles and gravity propagate, light (photons) just happens to be used as the most popular example, hence "light speed".

dhd40
2009-Nov-13, 08:29 PM
Yes, I'd say you're mistaken. How do you get such a large gravitational effect without mass?
Yes, similar argumentation as Greys


Of course, this has driven some to try various modifications to our understanding and formulas involving gravity itself.
Im not aware of this and would appreciate just one good link to start from, if you have one at hand
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* Of course, certainty is not really a characteristic of any scientific theory, or even a 'law'.[/QUOTE]

chornedsnorkack
2009-Nov-13, 08:43 PM
Yes, I'd say you're mistaken. How do you get such a large gravitational effect without mass? Yes, energy, momentum, even gravitational energy contribute to the gravitational effect, but in this case, what is the source of these contributions?
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Does the gravitational mass of dark matter equal its inertial mass?

That equality is well checked to hold for various types of light matter. Indeed, a violation of this equality means that gravity and acceleration are not equivalent.

But is dark matter directly observed to accelerate and decelerate as it should given the mass that gravitates and forces it should be receiving?

Also, can mechanical forces in dark matter be ruled out by direct observation?