Large Hadron Collider refit to solve more questions

[Brian37]

Always happy to hear from a professional. dumb it down a bit for us lay-people tho, wouldja? I can't always tell a subatomic particle from a subway sandwich.

If you spent the night over a toilet bowl, it probably wasn't a particle.

That reminds me of that joke: what does the buddhist mystic say at Subway?

Guy goes into a 7-11 and sees the Buddhist monk behind the counter and asks "Can I have some change?" Buddhist responds "Change must come from within".

[Alex K]

Davka,

I realize I've used more jargon than I was aware of , that's why it's good to tell someone these things informally before you write an article about it. This is written on the phone, so maybe less erudite.

Concerning your various related question:

By matter, people mean stuff which has no sizable pressure and whose mass-energy does not change (much) when space is expanded or contracted. This is opposed to radiation which exerts pressure and is blue/redshifted when space contracts/expands. A further requirement is that it loosely accumulates under gravity as a cool gas would.

In the bullet cluster we are talking gravity, not gravity waves. Gravity waves have not yet been observed, at least not directly. The gravity is indirectly measured because the images of background objects are distorted from the curvature of space which comes with gravity.

Now, you were very astonished by the word frictionless. So, if the dark matter in our galaxy and outside were made of ordinary matter, its atoms would collide and in the course of these collisions would radiate light or radio waves etc, thus losing energy. This would result in a flattening of the dark matter cloud in which the galaxy is embedded, forming a disk, which does not fit the observed velocity profiles of galaxies and galaxy clusters and large scale structures well. After all, in the bullet cluster above, if the dark matter had friction just like gas made of ordinary matter, it would have stopped in a shock wave like the reddish interstellar gas.

Now, the requirement of frictionlessness is not very far fetched: as soon as you have a gas made of particles which do not interact with electromagnetism, i.e. with photons, or only indirectly, the kind of energy loss described above is absent. This is called frictionless because for lack of a way to radiate, the dark matter gas does not dissipate much energy. Again, neutrinos fit the bill, but are simply not plentiful enough nor do they have the correct temperature in the early universe to form the observed filaments in the galaxy survey pic above. They would be what one calls "washed out" if dark matter were hotter. Our ordinary three types of Neutrinos certainly make up a part of dark matter, but, for this reason, not the majority.

LHC and other ways to search for dark matter: first of all it's important to understand that colliders do not aim to "break apart" particles to see what they are made of. Or at least, this is a misleading image. What they do is concentrate as much energy as possible in a collision of two "elementary" particles. The laws of quantum physics then ensure that this energy is randomly funneled into all available channels with different probabilities - among those possibilities is for example producing new heavier particles by converting the available energy partly into mass - nature simply does that for us, all we have to do is provide the energy!

Other detection methods are under way. If you feel like it google CDMS or XENON, or Dama/Libra. Those are what is called "direct detection" experiments. They consist for example of ultra cold blocks of transparentish material which are put into a deep mine and.monitored by photon sensors. The hope is that dark matter particles from our galaxie's dark matter cloud, which would go through the earth like neutrinos do all the time, sometimes get stuck in there - with a mind bogglingly tiny probability - and deposit some energy. These experiments require that there is some sort of indirect interaction between dark and ordinary matter just like with neutrinos. Z or Higgs bosons are often assumed as mediators. That's one possibility.

The third big pillar of DM search is "indirect detection": here one looks at dense regions of galaxies and hopes that dark matter particles can pair annihilate into known particle types, whose effects can get picked up by telescopes such as the LAT of the FERMI satellite.

For other specific dark matter hypotheses, specifically targeted searches exist. A nice candidate hypothesis are hypothetical particles called Axions, which help explain why Neutrons have no electrical field (roughly speaking) and thus have a motivation other than dark matter. Those guys can be changed into photons and vice versa in a strong magnetic field. Similar to the detectors in the mine, a strong magnet is rigged with photon sensors in a light-tight container, waiting for axions from the galactic cloud to produce flashes of light.Keyword: ADMX

Cheers

[Alex K]

P.s. I think your confusion about my talk about friction maybe came from the fact that when you hear matter, you first think of a solid, not a dilute gas. By friction I just meant a shorthand for losing energy during movement because of inelastic processes. Maybe that's not such a great word choice, but I used it for lack.of a better one.

[Brian37]

P.s. I think your confusion about my talk about friction maybe came from the fact that when you hear matter, you first think of a solid, not a dilute gas. By friction I just meant a shorthand for losing energy during movement because of inelastic processes. Maybe that's not such a great word choice, but I used it for lack.of a better one.

KY Jelly takes care of friction.

[Alex K]

P.s. I think your confusion about my talk about friction maybe came from the fact that when you hear matter, you first think of a solid, not a dilute gas. By friction I just meant a shorthand for losing energy during movement because of inelastic processes. Maybe that's not such a great word choice, but I used it for lack.of a better one.

KY Jelly takes care of friction.

Have you been reading Nietzsche again?

[Davka]

P.s. I think your confusion about my talk about friction maybe came from the fact that when you hear matter, you first think of a solid, not a dilute gas. By friction I just meant a shorthand for losing energy during movement because of inelastic processes. Maybe that's not such a great word choice, but I used it for lack.of a better one.

No, I understood what you meant by friction, it's just difficult to wrap my head around. But then that seems to be the case for almost everything about particle physics - classical physics makes perfect sense, we can play with matter and energy and directly observe the relation between energy, mass, friction, inertia, and so on. Particle physics regularly produces results which seem (to me at least) to be counter-intuitive.

So I simply have to accept that whatever it is that is distorting the images of visible bodies in the background must, by dint of its ability to distort space-time, have mass. Yet it does not interact with "normal" matter in any other discernible way, thus being "frictionless."

My brain doesn't like the idea of something that has mass (and therefore gravity) yet doesn't interact with the visible Universe other than via gravity. Question: Dark matter creates a gravity well, but isn't "pulled" by other gravity wells? Do I have that right?

Just for the record, my brain doesn't like quantum entanglement either. I find that the only way I can learn about particle physics is to tell my common sense to shut up and accept that reality is far weirder than I can fully grasp.

As has been said by others, I am extremely grateful that you see fit to spend some of your time answering these (presumably simplistic) laymans' questions. Your above lengthy post in answer to my questions provided a number of "aha!" moments, which are always fun.

Now I need to return to something I actually do understand: fermenting tempeh culture in black beans. Thank the FSM for the macro level of reality!

I look forward to further brain-picking sessions.

[Alex K]

Well I can't tell you what Dark Matter does exactly because noone knows, but all the usual models do not break general relativity, so dark matter bends spacetime and is affected by gravity itself like normal matter. I'm also not saying that it doesn't interact with normal matter at all, if that were the case, all the detection schemes I've outlined above are futile and pointless. Most models include some way to interact with the stuff other than thorugh gravity. *Just not electromagnetism*

You'd be surprised how strange classical physics can be. Strange attractors, the poincare theorem etc.

But even a classical gas of newtonian point masses has this property of zero dissipation if they are not electrically charged, it's not a very quantum or particle physics specific idea.

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I'm making hummus. Close enough I guess

[Roxy904]

I love hummus.
And good explanation too.

[ManMachine]

There's no particle beam in the world strong enough to go through that dense a target...

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At the great CERN roulette wheel my money is on spin-0.

C'mon spin-0.

MM

For what, the new Boson?

Yes, normally I play 00 but a Boson doesn't have a spin of 00.

MM

[Alex K]

Probably a safe bet.