[InclinedPlane]

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 different models and then test those predictions. And that is precisely what happened about 20-30 years ago. A lot of work was done to pin down what type of dark matter makes up the majority of it out there.

For example, you can point a telescope at a set of neighboring galaxies and look for brightening effects due to gravitational micro-lensing from a chance alignment of a "macho" (e.g. orphaned gas giant planet) along the line of sight. Surveys were set up and indeed found that there were orphaned "macho" objects in our galaxy, but the statistics showed that they were orders of magnitude too rare to make up the bulk of dark matter we know about from other studies. Another line of evidence involves studying the large-scale structure of the Universe (e.g. the layout of galaxies, galaxy clusters, etc.) and comparing it with various computer simulations of models with different assumptions on the composition of the mass of the Universe (e.g. 100% "ordinary" baryonic matter, various percentages of "special" dark matter such as cold and hot dark matter, WIMPs, etc.)

From this and many other lines of evidence we came up with very strong evidence that the vast majority of the mass budget of the Universe is in the form of so-called "cold dark matter" which is composed of weekly interacting massive particles other than neutrinos (neutrinos are dark matter, but we've been able to place an upper limit on how much they contribute to the dark matter budget of the Universe, because they are detectable to a degree, and it's only a fraction).

So that's it, just a simple matter of comparing the predictions of different theories with observations and eliminating the theories that do not predict what we actually see out there in the Universe.

[mkup]

(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 expansion? It must push galaxies away from each other, right?

[InclinedPlane]

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.

[Tloewald]

Same place sunlight goes.

[jeggers5]

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?

[codeflo]

Well yes, when the light hits a mirror, a certain percentage (say 95% of the energy) bounces back, which can hit another mirror and so forth. There's no limit, but after a few dozen bounces, the remaining light is virtually undetectable.

Really, the exact same thing happens in a standard white painted room, the two differences being that the mirrors reflect more of the light (so that a dimmer light source will suffice to reach the same level of illumination), and that the reflection is directed instead of diffuse (this only changes the shape of the reflected light, not its amount). Maybe you can explain what's confusing you.

[scott_s]

Mirrors are not perfect reflectors; some light will be absorbed.

[Retric]

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 get's get's warmer from the balls energy.

[Tloewald]

Yeah... Warmer is kind of mixing metaphors since warmth is just a statistical aggregation of stuff moving around. The point is that photons result from electrons changing energy levels and disappear when they hit an electron and change its energy level. A big change makes a high energy photon which might bump into an electron, resulting in a higher energy electron and a new, less energetic photon. This keeps on happening.

Seen purely as electrons -- one electron had a lot of energy, now a lot of electrons have a little. Seen purely as photos, one photon had a lot of energy, now there are lots on very low energy photons.

Overall, the collective term for this is entropy -- over time we get fewer opportunities for big photons to get created, until it's all small changes in energy and small photons -- total entropy -- and everything is background radiation.

[lepton]

The reflection is not perfect; the mirrors absorb some light (and warm up...).

[syncerr]

I assume light collides with atmosphere as well. Only in a perfect vacuum, light would travel forever?

[InclinedPlane]

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.

[creamyhorror]

(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)?

[InclinedPlane]

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.

[scott_s]

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