As they’re looking out there, they just can’t seem to find any kind of evidence that it’s some normal type of matter,” Yeager [Rubin's collaborator] says. It wasn’t black holes; it wasn’t dead stars. It was something else generating the gravity needed to both hold the galaxy together and propel those outer stars to such fast speeds.
Not only can we not see it, we couldn’t touch it if we tried: If some sentient alien tossed a piece of dark matter at you, it would pass right through you. If it were going fast enough, it would pass right through the entire Earth. Dark matter is like a ghost.
Here’s one reason physicists are confident in that weird fact. Astronomers have made observations of galaxy clusters that have slammed into one another like a head-on collision between two cars on the highway.
Astronomers deduced that in the collision, much of the normal matter in the galaxy clusters slowed down and mixed together (like two cars in a head-on collision would stop one another and crumple together). But the dark matter in the cluster didn’t slow down in the collision. It kept going, as if the collision didn’t even happen.
The event is recreated in this animation. The red represents normal matter in the galaxy clusters, and the blue represents dark matter. During the collision, the blue dark matter acts like a ghost, just passing through the normal colliding matter as if it weren’t there.
So the question it prompts, for me, is how can physicalism, as a philosophical principle, be credibly maintained in light of these conjectures? Given that science now acknowledges that it can account for only a small percentage of what is figured to exist, and that the remainder exists in a form that science can't even comprehend, how can such philosophical principles as 'causal closure' be said to hold? — Wayfarer
So the question it prompts, for me, is how can physicalism, as a philosophical principle, be credibly maintained in light of these conjectures? — Wayfarer
Since dark matter has physical effects, as you’ve explained, that classifies it as a physical thing. — Pfhorrest
Physicalism, in at least one rough sense, is the claim that the entire world may be described and explained using the laws of nature, in other words, that all phenomena are natural phenomena. This leaves open the question of what is 'natural', but one common understanding of the claim is that everything in the world is ultimately explicable in the terms of physics. This is known as reductive physicalism. However, this type of physicalism in its turn leaves open the question of what we are to consider as the proper terms of physics. There seem to be two options here, and these options form the horns of Hempel's dilemma, because neither seems satisfactory.
On the one hand, we may define the physical as whatever is currently explained by our best physical theories, e.g., quantum mechanics, general relativity. Though many would find this definition unsatisfactory, some would accept that we have at least a general understanding of the physical based on these theories, and can use them to assess what is physical and what is not. And therein lies the rub, as a worked-out explanation of mentality currently lies outside the scope of such theories.
On the other hand, if we say that some future, "ideal" physics is what is meant, then the claim is rather empty, for we have no idea of what this means. The "ideal" physics may even come to define what we think of as mental as part of the physical world. In effect, physicalism by this second account becomes the circular claim that all phenomena are explicable in terms of physics because physics properly defined is whatever explains all phenomena. — Wikipedia
. So “physical” vs “non-physical” is really a meaninglessness distinction in the end. But that suits physicalism just fine: it’s really just saying to treat all stuff the same way we treat ordinary stuff we’re familiar with, consider it with the same scientific method, etc. — Pfhorrest
But how is this different in any respect from how we derive our knowledge of "bright" matter? — apokrisis
So - science doesn't know what dark matter is, what its components are, or even really that it exists, except inferentially. — Wayfarer
So if we have to redefine matter so it 'passes right through the world' and exists in parallel baryonic matter, then it completely upends every prior idea of 'matter'. — Wayfarer
I think it's noteworthy how sanguine you are about it. I guess it's because you've got a slot for it in your mental model of the world, so it's not a problem. — Wayfarer
As if often observed, it might turn out that dark matter will in the end be like the epicycles of Ptolmaic cosmology - devices introduced to save the appearances, but, in the end, abandoned on account of the reigning paradigm itself being undone. — Wayfarer
So - science doesn't know what dark matter is, what its components are, or even really that it exists, except inferentially.
— Wayfarer
Nonsense. Science doesn't even claim to "know", only to constrain uncertainty through an epistemology of theory and measurement. — apokrisis
it is believed that the visible universe, comprising baryonic matter - stuff made from atoms - comprises only about 4% of the totality, the remainder comprising dark matter and dark energy — Wayfarer
The only thing known, as the galactic mass behaves as if it is subject to the gravity from an unknown source, which is presumed to be a form of matter. — Wayfarer
And yet folk really seem to go for this dark matter mystery. Curious. — apokrisis
Dark matter on the other hand, sounds like a straightforward theory. — Mijin
people think their own wild guesses can be on an equal footing. — Mijin
What about the idea that the mass/energy of 96% of the universe is of an unknown type if ‘straightforward’? Ought not that be considered mystifying or surprising? — Wayfarer
What I’ve said is that ‘dark matter’ undermines the philosophical idea of the ‘causal closure of the physical’. It does this by showing that our ideas of ‘the physical’ must be radically deficient in some way. — Wayfarer
So any version of materialism that assumed absolute knowledge was flawed from the very start. — Mijin
This is just a measure of how much of the universe's total mass/energy is accounted for by each phenomenon. — Mijin
Don’t you think that covers a lot of what goes by the name ‘philosophical materialism’? — Wayfarer
Yet another iteration of "science doesn't know everything there is to know, therefore physicalism is false." — SophistiCat
Don’t you think that covers a lot of what goes by the name ‘philosophical materialism’?
— Wayfarer
No, I don't think that. Do you?
And if so, on what basis? — Mijin
science doesn't know everything there is to know — SophistiCat
So the question it prompts, for me, is how can physicalism, as a philosophical principle, be credibly maintained in light of these conjectures? — Wayfarer
I mean, presently, it is presumed that some unknown substance, provisionally titled 'dark matter', has observable effects on the cosmological observations, but it's nature is unknown. So how can it be known that it is physical? — Wayfarer
You've now agreed with me that physicalism does *not* make the claim that we know everything about the physical universe.
So what it actually is, is just the proposition that physical causes and effects are all that exists, even while we don't yet know of, or fully understand, all physical events and mechanisms. — Mijin
Well you can define physical things so as to include it. That's usually how physicalism continues. At first "physical things" were rocks and such, then they became the less intuitive waves, then the non-inuitive "Probability functions" and now "physical things" pass through each other apparently. — khaled
Eventually we're going to say that consciousness is a "Physical thing". — khaled
I never got the split between physicalism and idealism for this reason, it seems physicalists are playing dirty by changing what counts as "physical" every few decades, leaving no room for something to be "non-physical" — khaled
Nice article in Vox though, and the thrust of it is, if anything, the opposite of Wayfarer's perennial pitch. — SophistiCat
Inherent to the nature of science is the fact that whatever we know is provisional,” Natarajan says. “It is apt to change. So I think what motivates people like me to continue doing science is the fact that it keeps opening up more and more questions. Nothing is ultimately resolved.
Alternatives to physicalism generally suggest that there are mental or spiritual aspects of the universe itself.
Do you think probability waves count as evidence for *that*? — Mijin
I don't want to see any handwaves of it being purely physical until we have a model with explanatory power of subjective states. — Mijin
But it's a big problem for alternative hypotheses, like that our understanding of gravity is flawed. — Mijin
Care to explain in what sense it's 'opposite'? — Wayfarer
Well you can define physical things so as to include it. That's usually how physicalism continues. At first "physical things" were rocks and such, then they became the less intuitive waves, then the non-inuitive "Probability functions" and now "physical things" pass through each other apparently.
I never got the split between physicalism and idealism for this reason, it seems physicalists are playing dirty by changing what counts as "physical" every few decades, leaving no room for something to be "non-physical". Eventually we're going to say that consciousness is a "Physical thing". But at that point the word "Physical" becomes meaningless and redundant, as it should, and so will "Idealism". We'll just have "thingism" — khaled
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