# Would motion explain the missing 'dark matter'?

## Main Question or Discussion Point

From previous answers - according to the general theory of relativity, kinetic energy contributes to gravitational mass. A body in motion posseses kinetic energy plus mass. Energy and mass are equivalent under GR. Particle colliders routinely confirm this prediction. Crash two particles together at relativistic velocities and you create particles with more mass than the sum of the two crashed particles.

If we look at the motion of the objects in the universe, would this not explain the missing 'dark matter'? If not, perhaps it means the whole universe is moving quite fast...(we would never know) ?

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phinds
Gold Member
2019 Award
From previous answers - according to the general theory of relativity, kinetic energy contributes to gravitational mass. A body in motion posseses kinetic energy plus mass. Energy and mass are equivalent under GR. Particle colliders routinely confirm this prediction. Crash two particles together at relativistic velocities and you create particles with more mass than the sum of the two crashed particles.

If we look at the motion of the objects in the universe, would this not explain the missing 'dark matter'? If not, perhaps it means the whole universe is moving quite fast...(we would never know) ?
No. I do not wish to be rude, but the whole idea is just silly.

You say "perhaps it means the whole universe is moving quite fast." RELATIVE TO WHAT ???

You, personally, right now, as you read this sentence, are moving at 99.999% of c from some frame of reference. Does this make you any heavier?

Chronos
Gold Member
Relativistic mass does not contribute to the gravitational field of a body. Otherwise, you could create a black hole merely by sufficiently accelerating an object. This kind of confusion is the reason physicists dislike the term relativistic mass.

First of all, the concept of relativistic mass doesn't contribute to gravity.

Second, one thing that we know about dark matter is that it's "cold" (i.e. it's not moving near the speed of light). One of the big questions in the late 1980's was whether dark matter was "cold" (i.e. moving very slowly) or "hot" (i.e. moving very quickly).

We know it's "cold" because "hot" matter would smear out large scale structures. We've seen large scale structures since the early-1990's, so we know that dark matter isn't "hot." The prevailing idea on how galaxies formed was that the superclusters formed first and then galaxies formed later. The "hot dark matter" scenario was that galaxies formed first and then grouped into clusters, and that idea is dead.

(Also, I find it generally a good idea to be nice to newbies. One thing that newbies with questions need to know is that after spending lots of times arguing with people that have very strong crankish ideas on how the universe works, so often people that ask a simple question get a ton of bricks dropped on them.)

phinds
Gold Member
2019 Award
(Also, I find it generally a good idea to be nice to newbies. One thing that newbies with questions need to know is that after spending lots of times arguing with people that have very strong crankish ideas on how the universe works, so often people that ask a simple question get a ton of bricks dropped on them.)
Yeah, I often tend to get too snippy. Sorry about that, Oracle. This is a great forum, so don't let SOB's like me drive you off.

Drakkith
Staff Emeritus
Yeah, I often tend to get too snippy. Sorry about that, Oracle. This is a great forum, so don't let SOB's like me drive you off.
Someone forget to walk you and throw you your nightly treat Phinds?
(Should I feel bad for poking fun at an old man??)

phinds
Gold Member
2019 Award
Someone forget to walk you and throw you your nightly treat Phinds?
It was the treat. They were completely out. Really pissed me off. Had to go to bed hungry.

(Should I feel bad for poking fun at an old man??)
Dog. That's an old DOG (junkyard variety). And yes, you should.

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mfb
Mentor
Well, kinetic energy does contribute to the total energy density of the universe - the kinetic energy of neutrinos is relevant, for example.
For ordinary (baryonic) matter, it is negligible. Typical relative velocities are something like 200km/s, this gives a factor of ~1.0000002.