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Dark matter scaffolding of universe detected for the first time

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Re: Dark matter scaffolding of universe detected for the first time

#41

Post by InclinedPlane »

mkup wrote: post #26
InclinedPlane wrote: post #24

Earlier quoted context omitted.

Science! We can tell how much dark matter is out there because we can "weigh" it through indirect measures. And then we can take different theories of dark matter (such as, say, the theory that it's all just a bunch of interstellar orphaned planets and "black dwarfs" and what-have-you made up of ordinary matter) and figure out what sorts of implications that would have, make predictions on observable effects of those…

(A bit off-topic question to physics expert on HN) I've read that supermassive black hole accretion is the most energy-effective process of mass to energy conversion in the Universe (50% efficiency or so). I'm just curious: Where does all that energy go? Extremely powerful jets of radiation are emitted into the intergalaxy space and then what? Does it just disappear? Isn't this energy responsible for Universe expansi…

It goes everywhere.

The thing is, the Universe is a big place. So even if you have sagans of tonnes of mass being converted to energy every second in extremely powerful jets around a supermassive black hole (which we do, in the vast majority of galaxies) given enough distance it's still just another tiny point of light in the sky.

However, quasars are just this sort of phenomenon and are so bright that they are visible in telescopes across almost the entire extent of the visible universe.

Re: Dark matter scaffolding of universe detected for the first time

#42

Post by InclinedPlane »

jeggers5 wrote: post #31
Tloewald wrote: post #27

Earlier quoted context omitted.

Same place sunlight goes.

It would be amazing if somebody could answer this too. If I turn a lamp on and off in a sealed room of mirrors, why doesn't the light just keep bouncing off the walls and illuminate the room?

The problem with thought experiments is that sometimes you can create a non-physical situation by accidentally introducing magic which invalidates the whole thing.

For example, in this you have a perfect mirror, which is actually not physically possible and would mean violating several laws of physics such as thermodynamics and electromagnetism.

Another common problem is hypothesizing perfectly rigid materials or perfectly flat surfaces, which can't exist in any matter made out of atoms but which could easily beused to violate the laws of relativity.

Re: Dark matter scaffolding of universe detected for the first time

#43

Post by InclinedPlane »

carterac wrote: post #15

How do these filaments stay stable and not collape under their own gravity due to instabilities? Or, if there is 0 net force causing them to collapse, why doesn't the dark matter drift apart naturally and become less dense and more diffuse over time? Either way, filaments of high density don't seem to be a natural stable state. Can someone explain this?

The dark matter halos don't experience friction, and are incapable of emitting heat radiation to cool off and condense.

If you take a bunch of marbles and put them in a big bowl they will roll around for a while but eventually end up in the bottom of the bowl because they keep running into each other. But if you put in special marbles that just pass through each other and don't experience friction then you'll end up with marbles rolling around the bowl everywhere for ever, which is the way dark matter works.

Re: Dark matter scaffolding of universe detected for the first time

#44

Post by [deleted] »

Retric wrote: post #38
jeggers5 wrote: post #31

Earlier quoted context omitted.

It would be amazing if somebody could answer this too. If I turn a lamp on and off in a sealed room of mirrors, why doesn't the light just keep bouncing off the walls and illuminate the room?

Some of that energy turns into heat every time the photons hit something, but heat is a mix of kinetic energy and light so you do get a fraction of that light bouncing around indefinitely, it's just not in the visible spectrum. Think of it like dropping a ball an a hard vs soft surface. In both cases things bounce. Even on a hard floor the ball stops bouncing after a while, and in both cases the ball / floor / air ge…

[deleted]

Re: Dark matter scaffolding of universe detected for the first time

#45

Post by creamyhorror »

InclinedPlane wrote: post #42
jeggers5 wrote: post #31

Earlier quoted context omitted.

It would be amazing if somebody could answer this too. If I turn a lamp on and off in a sealed room of mirrors, why doesn't the light just keep bouncing off the walls and illuminate the room?

The problem with thought experiments is that sometimes you can create a non-physical situation by accidentally introducing magic which invalidates the whole thing. For example, in this you have a perfect mirror, which is actually not physically possible and would mean violating several laws of physics such as thermodynamics and electromagnetism. Another common problem is hypothesizing perfectly rigid materials or per…

(I posted this in response to another comment but am moving it here since you actually addressed it)

What I'm curious about it, is there a general principle that stops things from possessing a property perfectly? For example, IIRC friction dictates that many energy transformations never convert energy perfectly, leading to far-from-perfect engines and unavoidable power dissipation in electricity transmission. Is there a similar principle that stops collisions/materials from being perfectly elastic, surfaces from being perfectly reflective, etc.? Does it go against entropy never decreasing in a system?

edit: hmm, so are the laws that dictate that perfect objects cannot exist somehow more deeply connected by a general principle (just like Noether's theorem underlies laws in various domains)?

Re: Dark matter scaffolding of universe detected for the first time

#46

Post by jared314 »

mattheww wrote: post #19
Xcelerate wrote: post #2

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 attr…

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 s…

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

Re: Dark matter scaffolding of universe detected for the first time

#47

Post by scott_s »

creamyhorror wrote: post #45
InclinedPlane wrote: post #42

Earlier quoted context omitted.

The problem with thought experiments is that sometimes you can create a non-physical situation by accidentally introducing magic which invalidates the whole thing. For example, in this you have a perfect mirror, which is actually not physically possible and would mean violating several laws of physics such as thermodynamics and electromagnetism. Another common problem is hypothesizing perfectly rigid materials or per…

(I posted this in response to another comment but am moving it here since you actually addressed it) What I'm curious about it, is there a general principle that stops things from possessing a property perfectly? For example, IIRC friction dictates that many energy transformations never convert energy perfectly, leading to far-from-perfect engines and unavoidable power dissipation in electricity transmission. Is ther…

I think you're looking for the second law of thermodynamics.

Re: Dark matter scaffolding of universe detected for the first time

#48

Post by InclinedPlane »

creamyhorror wrote: post #45
InclinedPlane wrote: post #42

Earlier quoted context omitted.

The problem with thought experiments is that sometimes you can create a non-physical situation by accidentally introducing magic which invalidates the whole thing. For example, in this you have a perfect mirror, which is actually not physically possible and would mean violating several laws of physics such as thermodynamics and electromagnetism. Another common problem is hypothesizing perfectly rigid materials or per…

(I posted this in response to another comment but am moving it here since you actually addressed it) What I'm curious about it, is there a general principle that stops things from possessing a property perfectly? For example, IIRC friction dictates that many energy transformations never convert energy perfectly, leading to far-from-perfect engines and unavoidable power dissipation in electricity transmission. Is ther…

Friction is a very complex subject so I won't address it, but other things like the absence of materials that are perfectly flat, perfectly rigid, infinitely strong, etc. are easily explained by the fact that matter is made out of atoms.

For example, there's a recurring thought experiment about the limit of the speed of light that goes something like this: say you have a rigid rod that is one light-year long and you push on one end, won't the other end instantly move, thus proving that you can exceed the speed of light? The problem with that is that it's based on an approximate and intuitive understanding rather than a proper understanding of the physics involved. For a short rod if you push on one end the other end seemingly moves instantaneously, giving the illusion of rigidity, but in actuality what is happening is that you are transmitting forces through the rod at the speed of sound in the material, and if you make movements that are slow compared to the time and space involved then everything will appear instantaneous (since the speed of sound in steel is about six thousand meters per second). However, once you scale things up the intuitive approximation is no longer valid. What happens when you push a long rod from one end is that a displacement wave moves along the rod at the speed of sound until it reaches the opposite end, taking far, far longer to move than a signal travelling at the speed of light. Also, no material can be perfectly flat because at the scale of atoms there are... atoms, which are not flat.

That's generally the biggest reason why we can't have perfect anything, because stuff is made of atoms and atoms are messy. Often times people who let dreams of perfect materials lead them astray fail to take into account the underlying mechanism for the property they are considering (e.g. rigidity is due to forces being transmitted from atom to atom within a material). A good rule of thumb for whether or not an assumption of perfection is going to ruin a thought experiment is whether or not you're ignoring the underlying mechanism for that process. Another is whether or not you're assuming some arbitrarily small amount that you are omitting from the model because it introduces a "tax" that is annoying to account for but that can be easily bounded or instead you are assuming absolute 100% perfection that your whole model is completely reliant upon and even the slightest deviation from perfection would ruin the model.

Interestingly enough, there are a few examples of physically perfect things in the real Universe. For example, superconductors experience 0 electrical resistance to current, and superfluid helium does not experience friction internally, and electrons appear to be perfect point-like charges.

Re: Dark matter scaffolding of universe detected for the first time

#49

Post by mattheww »

jared314 wrote: post #46
mattheww wrote: post #19

Earlier quoted context omitted.

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 s…

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

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.

Re: Dark matter scaffolding of universe detected for the first time

#50

Post by drostie »

lutusp wrote: post #22
drostie wrote: post #21

Earlier quoted context omitted.

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 grav…

> ... 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-Ju…

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
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.

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