Wtf is dark matter?

[Xamonas Chegwé]

but the last decade or two seem to deepen the mysteries rather than shed light on them to my way of thinking.

Try the last century or more!

The "standard model" (as usually defined) refers to particle physics and has been undergoing frenetic revision ever since Rutherford showed there was something smaller than an atom! Protons and Neutrons were thought to be fundamental particles not so very long ago. More and more quarks and fermions have been added since its first formation. The Higgs finally took its seat only last year and there are more and more theoretical maybe-particles queueing up to get a moment in the spotlight!

Likewise, the standard model of cosmology has been similarly revised.

Oh, and the standard model of heredity - remember "junk DNA"?
And the standard model of Biology which once had fungi as a sub-phylum of plants and used to have a group called Coelacanths.
And the standard model of economics that had this wonderful panacea called the "trickle-down effect"!

To some, this constant revision is a flaw in science. A proof that it was never right in the first place. To others, myself included, it is the core strength of science and a huge part of its beauty.

[Xamonas Chegwé]

I just realised I included economics with real science. I feel dirty and ashamed. I... just want to be alone now...

[hackenslash]

I just realised I included economics with real science. I feel dirty and ashamed. I... just want to be alone now...

[surreptitious57]

the proposer of a verified solution that overturns GR will go down as the supreme physicist of the century

How realistic is this though ? Is it more probable based upon existing evidence that General Relativity needs some moderation to accommodate Quantum Gravity
rather than a complete eradication ? Now given that it is a consequence of gravity and that gravity is observable can you explain how it could be eradicated any
way ? And are there any physicists thinking the unthinkable and suggesting that it is Quantum Mechanics that needs modifying and not General Relativity ? Or is
that a question so nonsensical than no serious physicist would even contemplate asking it ? I have to ask it though for it is the only way I shall actually find out

[Brian Peacock]

42.

[JimC]

Whatever happens, it makes no sense to say that General Relativity or Quantum Mechanics will be "overturned". Newtonian mechanics still remains an excellent predictive theory within a clearly defined range of initial conditions; so will GR and QM...

[hackenslash]

Whatever happens, it makes no sense to say that General Relativity or Quantum Mechanics will be "overturned". Newtonian mechanics still remains an excellent predictive theory within a clearly defined range of initial conditions; so will GR and QM...

That depends entirely on tractability. If whatever replaces one or both of them is mathematically more less difficult than what it's replacing, it will be overturned. The only reason we still use Newtonian mechanics is that, within its domain of applicability, it's considerably simpler than GR. If it had been simpler to carry out the calculations in GR than in Newtonian mechanics, overturned is what would have happened to it, and we'd now call Newtonian mechanics exactly what it is, namely 'wrong'.

We know for certain that GR and QM are at best incomplete. Until we know what replaces it, it makes no sense to say that it makes no sense to talk about them being overturned.

Edit: Typo

[hackenslash]

How realistic is this though ? Is it more probable based upon existing evidence that General Relativity needs some moderation to accommodate Quantum Gravity rather than a complete eradication ?

Dunno, and nor does anybody else.

Now given that it is a consequence of gravity and that gravity is observable can you explain how it could be eradicated any way ?

We're not talking about eradicating gravity, we're talking about superseding our model of what it is and how it works.

And are there any physicists thinking the unthinkable and suggesting that it is Quantum Mechanics that needs modifying and not General Relativity ? Or is that a question so nonsensical than no serious physicist would even contemplate asking it ? I have to ask it though for it is the only way I shall actually find out

Roger Penrose thinks it will be QM that capitulates rather than GR, and he's not entirely alone.

[Animavore]

To some, this constant revision is a flaw in science. A proof that it was never right in the first place. To others, myself included, it is the core strength of science and a huge part of its beauty.

You have to invert reasoning to believe it's a flaw. Some people think science has steadily being shifting away from some ancient Truths, whereas it has actually been gradually correcting ancient misconceptions and assumptions. It wasn't that science was never right in the first place, it's that before the scientific method much of our thinking was hopelessly wrong.

[surreptitious57]

Can you give some detail about what Penrose thinks or is it just an untestable hypothesis he has ? I do think it would be
unwise for every physicist to just assume it has to be General Relativity which has to go for intuition is no guarantee of
truth. And we know this because we have of course been here before. Before Einstein for example it was assumed that
all of space and time were absolute and that they were separate dimensions so making assumptions based on intuition
alone is not to be recommeded. Testing all hypotheses and keeping an open mind are the order of the day as per usual
And a lot of patience also for the Universe is not in the habit of giving up its secrets in order to satisfy human curiosity

[hackenslash]

Can you give some detail about what Penrose thinks or is it just an untestable hypothesis he has ?

What detail do you want? Most people think that GR will have to be amended to incorporate quantum principles, Penrose thinks it will be QM that has to be altered.

I do think it would be unwise for every physicist to just assume it has to be General Relativity which has to go for intuition is no guarantee of truth.

It's the more reasonable proposition that it will be GR that will be superseded for the very simple reason that we have a circumstance in which gravity is hugely important but in which our theory of gravity completely fails. We can't claim to have a complete understanding of gravity if our theory breaks in any circumstances, especially when no set of circumstances can be found in which QM fails.

[mistermack]

Re: Wtf is dark matter?

It's obviously an interesting question.

But it might be wise to wait until we work out wtf MATTER is.
That might give a clue as to what dark matter is.

[jamest]

Cynics of the world unite.

[jamest]

One question which arose in my mind, was why dark matter doesn't affect the slower orbital speeds of planets on the outer edges of the solar system in the same manner in which it supposedly affects the orbital speeds of stars in our galaxy?

Classical orbital mechanics tells us that, the further you get from the barycentre (point about which all bodies orbit; in the case of our solar system, this is inside the sun for every body in orbit) of any given system, the slower the orbital velocity must be for any body to remain gravitationally bound (i.e., if faster, it will escape). In the case of stars in the outer edges of galaxies, especially smaller galaxies, we observe that their orbital velocities are similar to those of stars closer to the barycentre of the galaxy.

As a result, most prospective solutions are rooted in the proposal that dark matter dominates in the halos of galaxies. Some have dark matter suffusing all of space, but still dominating in the halo (WIMPs), and some have very small but massive bodies, also largely dominating in the halo (MACHOs).

I overlooked this post, sorry.

Does the dark matter not similarly affect the orbital velocities of the inner stars?

[hackenslash]

No. That's what it means for dark matter to dominate in the halos (i.e., the outer edges of galaxies), while ordinary matter dominates closer in. It permeates all of space, but is dominant where ordinary matter begins to thin out, and is completely dominant in the large-scale structure of the cosmos.