[Xcelerate]

This is really cool. If you look at this graph of the Standard Model interactions http://upload.wikimedia.org/wikipedia/commons/4/4c/Elementar... you will see the ways in which all of the particles we currently know about interact.

If you'll notice though, there's one interaction between ALL of the particles that is missing: gravity. Gravity affects anything with energy. Photons, leptons, quarks -- they are all attracted to each other because they possess energy (negligible, unmeasurable attractions, but still extant).

Wouldn't it be interesting if the only way that dark matter interacted with the other particles was through the gravitational force? Maybe from some alien's perspective it would constitute the matter of everyday life, but because it didn't interact with any of our particles except through gravity we would be missing out on a large aspect of our universe!

Furthermore, is it that far-fetched to think there might exist particles that do not interact at all with the ones we have discovered? Gluons, for example, only interact with themselves and with quarks. Some other particle may interact with nothing we are familiar with -- and thus we could never study it. Is it even "real" then?

(Any particle physicists on here, please feel free to educate me further!)

[mattheww]

There are a lot of people thinking about what kind of structure the model of dark matter particles could have. And there are a lot of different ways for there to be more than one dark matter particle, and even if the only way those other particles interact with normal matter is through gravity, there are still ways for us to understand things about them.

At the moment, the most popular dark matter theory comes from supersymmetry. In this case, there's only one dark matter particle, and all of the rest of the particles interact with normal matter, in pretty much the normal way, since all of the underlying structure of the model is almost identical.

[jared314]

I keep hearing about the death of supersymmetry. Are there new developments keeping it "alive"?

[mattheww]

Unfortunately, the idea that supersymmetry is "dead" has been propagated by journalists covering science who don't know any better and people with an agenda. Generally, it's people who just don't know any better. Unfortunately, you see comments supporting the idea even on HN.

The important thing to understand about supersymmetry (SUSY) is that in the most general case, there are approximately 105 new parameters. That's far too many to probe in a meaningful way, so most models choose between 2 and 5 to vary, and fix the rest. Then a bunch of models are chosen that hopefully cover a wide spread of different behaviors.

The true part is that several models have been excluded, basically as well as the LHC is going to be able to exclude/discover anything in the current energy regime. However, some of these models were just not chosen very well to begin with (but have historical importance) and others were chosen to have maximal signal strength.

So as time goes on, the search for SUSY turns away from "easy" models and looks at more complicated ones. With 105 parameters, there's a lot of parameter space unexplored.

[tb]

> Some other particle may interact with nothing we are familiar with -- and thus we could never study it. Is it even "real" then?

Philosophically, this is equivalent to the question of whether other universes, which do not interact with ours and therefore we cannot study, exist or are "real". It is not a question that Science can answer.

[Xcelerate]

I think it's very interesting that there's reasonable questions about things that science can't answer. If you can't study it, can't predict it, or can't reproduce it, then it isn't in the domain of science.

[Jach]

> can't predict it

This is the one a lot of people miss out on. Standard theories predict several types of "other universes"--thus that question is answered from science's view, it's just not an answer people like. See http://arxiv.org/pdf/astro-ph/0302131v1 for a general overview. "Containing unobservable entities does clearly not per se make a theory non-testable."

[kamaal]

Oh well,

That question really has far reaching implications. Because if we say Science is what we observe and describe as per our interpretations of logic(And the language of logic - 'Math') then our science is really broken. Because what we can observe doesn't often turn out to be true and what is true is not often observed.

Because look at it this way. We are now saying Dark Matter doesn't interact anyway with light nor something else. Hence observing, detecting or modeling them out through conjectures manufactured through thin air is nothing more than what religion was some centuries ago.

Anything unexplainable was attributed to some form of divinity in times before.

We know it exists, but we can't show you, can't explain you what it is, how it looks is the text book definition of god throughout centuries.

[drostie]

It is only "broken" if you assume incorrectly that the goal of science is to Discover Truth. Moreover it is broken in a more direct way: there exist certain models which are mathematically equivalent but which describe contradictory states of being. A reasonably good example of this is heliocentrism vs. geocentrism: classical mechanics allows you to say "the Earth is at the center of the Solar System, there are gravitational, Coriolis and centrifugal forces around it affecting all of the stuff in space", but it also allows you to say "The Sun-Jupiter barycentre is at the center of the Solar System, and the only force we need is gravity." There is no experiment which can distinguish between those two; they are mathematically equivalent.

(A slightly better example comes from quantum mechanics. In the "Schrodinger picture" there is a "wavefunction of the universe" which changes from moment to moment, while the definitions of space and momentum stay the same. In the "Heisenberg picture" the wavefunction stays the same while the definitions of space and momentum change. You would think there would be an ontological difference to the question, "is the state of the universe different from the state of the big bang?" but, in fact, on this description there is no observable difference, and science could never settle the question.)

This does not reduce science to a religion; science simply studies the observable differences and must be content with not knowing everything -- which most scientists are already content with, since they have to deal with matters of uncertainty and the distinctions between correlations and causations.

Dark Matter does interact with other things, but it does so indirectly, because it has mass and therefore warps spacetime. This is not actually the first use of gravitational lensing to observe dark matter; in fact, earlier it had been used to settle the question of whether dark matter felt any electromagnetic force at all, by looking at galaxy collisions. The prediction would be that the dark matter clouds of two galaxies would more or less "go through each other" in a collision while the luminous stuff would "bump into each other". This was observed as early as 6 years ago, see http://chandra.harvard.edu/photo/2006/1e0657/ .

We certainly can show you, and we can explain to you what it is. The only problem is the same problem that neutrinos have: it's just very hard to detect these particles because they don't have an electric charge and therefore don't care about the electrons which make all the rest of chemistry happen. Our best tool for understanding dark matter is still gravity; its a force which we know the dark matter feels.

[lutusp]

> ... there exist certain models which are mathematically equivalent but which describe contradictory states of being. A reasonably good example of this is heliocentrism vs. geocentrism: classical mechanics allows you to say "the Earth is at the center of the Solar System, there are gravitational, Coriolis and centrifugal forces around it affecting all of the stuff in space", but it also allows you to say "The Sun-Jupiter barycentre is at the center of the Solar System, and the only force we need is gravity." There is no experiment which can distinguish between those two; they are mathematically equivalent.

How do heliocentrism and geocentrism represent "contradictory states of being"? They are trivially related to one another, and are mathematically equivalent as you point out. They represent a simple example of geometric relativity.

Consider a gravitational slingshot maneuver, a way to harvest some of a planet's orbital momentum to accelerate a passing spacecraft. If you make the planet the frame of reference, or the sun, or the spacecraft, the math and physics come out the same. No "contradictory states of being".

> A slightly better example comes from quantum mechanics. In the "Schrodinger picture" there is a "wavefunction of the universe" which changes from moment to moment, while the definitions of space and momentum stay the same. In the "Heisenberg picture" the wavefunction stays the same while the definitions of space and momentum change.

No,. this isn't really a "better example" -- Dirac demonstrated the mathematical equivalence of Schrodinger's wave mechanics and Heisenberg's matrix mechanics. Again, the difference is only apparent and superficial.

[drostie]

What's different in both cases is ontology, which is a fancy way of saying how things actually are. It may help to think of how you would program a computer to simulate either universe, or for that matter it may help to reduce the computation to something much simpler, like the following two functions:

    def odd_sum(n):
        return sum(range(1, 2*int(n), 2))

    def square(n):
        m = int(n)
        return 0 if m <= 0 else m ** 2

Those two functions are mathematically indistinguishable if all you are doing is putting in various values. There happens to be a vast mathematical identity that the sum of the first n odd numbers is the n'th square number, allowing for that indistinguishability. But to claim that they're exactly the same realization of this function is obviously mistaken.

Similarly, there is a qualitative difference between a wavefunction of the universe which changes and a wavefunction of the universe which does not change. It does not matter that the two positions can be made mathematically equivalent; one is X and one is not-X, and science is permanently incapable of figuring out which it really is.

[scott_s]

We are now saying Dark Matter doesn't interact anyway with light nor something else. Hence observing, detecting or modeling them out through conjectures manufactured through thin air is nothing more than what religion was some centuries ago.

It interacts gravitationally with other matter.

[derleth]

> It is not a question that Science can answer.

It is, though: Science suggests that they don't exist because science favors simpler models to more complex ones, as long as the simpler model still accounts for all the evidence and makes correct predictions.

[pdonis]

Gluons also interact gravitationally, like anything with energy. So would any other hypothetical particle, since it would have to have energy to exist at all.

[Xcelerate]

I said earlier in the post "there's one interaction between ALL of the particles that is missing: gravity", so it was implied. Perhaps I should have made that a little clearer.

[rwmj]

I thought the interaction with the Higgs is supposed to create mass?

[aroberge]

Two things, greatly oversimplified: 1) all forms of energy (including mass) are sources for gravity. 2) Interaction with the Higgs gives rise to what we observe as mass of particles; without this interaction, we'd have massless particle, with essentially the same energy, but moving at the speed of light. The net gravitational effect would be very different from what we observe though...