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ComputerDude
2006-Oct-13, 07:44 PM
Ok... I think this is probably going to rank as a stupid question but it came up in conversation the other day so I felt compelled to ask the experts :.)

As far as I know (and I'm a "took science in Uni" layman!!), we have invented a thing called Dark Matter to explain the missing mass out there in the universe. I've also been led to believe that as something approaches the speed of light it's mass increases (or seems to increase??). :think:

On the weekend, my friend suggested that Dark Matter (ie the missing matter) could probably all just be explained by the increase in mass of particles being tossed around inside galaxies (at some signifigant % of the speed of light)?? Anyway, I couldn't come up with a good reason for this being false and it has been bugging me ever since.... :doh:

Surely we are misunderstanding the physics and or this has already been thought of and discounted as a possible source of Dark Matter.... Could any portion of this dark matter be "normal" matter travelling at high speed or do we know that dark matter must be something exotic???

Thanks for setting me straight!!

Serenitude
2006-Oct-13, 07:48 PM
If you mean the visible light being infinately heavy due to GR, the thing to remember are that photons are massless for all intents and purposes, so light "rays" don't increase to infinite mass to travel at the speed of light.

Edit: To read about it in much more depth, the Wiki article is a good place to start:

http://en.wikipedia.org/wiki/Photon

Cougar
2006-Oct-13, 08:59 PM
I don't think he's talking about light but rather the massive objects whizzing around in the galaxies becoming more massive due to "relativistic mass increase" and hence accounting for some of the "missing mass."

There are actually quite a few reasons that explain why your argument doesn't fly. (1) Most of the mass in galaxies isn't traveling that fast to make much of a relativistic difference. The tight orbits near the galactic nucleus are moving pretty fast, but that would place "your" missing mass all near galactic nuclei, but observed galactic rotation curves require the "dark matter" to be spread more evenly throughout the galaxies. If it was all in the center, the rotation curves would not be flat, as observed.

I've got to get back to work :) but maybe others can point to the other problems with your idea. Meanwhile, this link (http://en.wikipedia.org/wiki/Dark_matter) will better inform you about the various lines of evidence behind the dark matter idea.

Peter Wilson
2006-Oct-13, 10:48 PM
.... Could any portion of this dark matter be "normal" matter travelling at high speed or do we know that dark matter must be something exotic???
Don't quote me on this, but I believe mainstream position is that all non-exotic explanations have been ruled out.

GeorgeX
2007-Jan-15, 09:22 AM
[QUOTE=Cougar;845130]I don't think he's talking about light but rather the massive objects whizzing around in the galaxies becoming more massive due to "relativistic mass increase" and hence accounting for some of the "missing mass."

There are actually quite a few reasons that explain why your argument doesn't fly. (1) Most of the mass in galaxies isn't traveling that fast to make much of a relativistic difference. The tight orbits near the galactic nucleus are moving pretty fast, but that would place "your" missing mass all near galactic nuclei, but observed galactic rotation curves require the "dark matter" to be spread more evenly throughout the galaxies. If it was all in the center, the rotation curves would not be flat, as observed.

Yea, but all the stars in the outskirts give off massive Particles too, and maybe the great masses themselfes get pretty quick. O.k. the 200km/s of a galactic spiral arm are not really relativistic but add the rotation of a planet inside it and you are at 300 let a nearby Galaxy rotate the other way round 600 plus the larger movement of the galaxys...
and then the outer parts of the universe including the microwavebackground.
They are definitly at a relativistic speed :wall:
The more guys are trying to talk that down- the more I believe noone bothered to think that through.
Whether the mass is dark or relativistic wouldn´t change chandras gravitylensing pics at all, would it? :wall: :wall: :wall:

Ken G
2007-Jan-15, 12:28 PM
There's also a problem with how things could get moving that fast. Even if there were relativistic particles that we had somehow not observed (so it would still be "dark" matter but not as much of it), chances are it would have to be gravity that got them moving. But then there would need to be a large negative gravitational binding energy to explain where they got their energy, and that would actually reduce their gravity relative to their rest masses, not increase it. So it's pretty inescapable that you need large "dark" rest mass.