Comment by millstone | Hacker News Reader

Comment by millstone | Hacker News Reader

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There's inconsistencies, and then there's real inconsistencies.

Recall some predictions that were later confirmed: the top quark (18 years later) or the Higgs boson (48 years!). These theories were a jigsaw puzzle with a single piece-shaped hole. You could call "inconsistency!" and break apart the whole puzzle, or look behind the couch for the missing piece. (The piece was found!)

We also have real inconsistencies, such as between QM and GR. Not a missing piece but a yawning gap: no single observation can fill in that hole.

Dark matter is a "missing piece" problem. The dark matter hypothesis fills in the cosmological missing mass problem, galactic rotation curves, Bullet-cluster-style gravitational observations, the expectation of non-EM interacting particles, and others. The gap between prediction and observation is the size and shape of a single jigsaw piece; let's just find it behind the couch.

Dark energy is a "real inconsistency" problem. Predictions and observations are orders of magnitude apart. Well hell.

Here's an image explaining the "bullet-cluster" observation. It's a compelling piece of evidence.

Two galaxy clusters have collided. The hypothesis is that normal matter will be slowed down by the collision, while dark matter, not interacting (as strongly?) with itself or normal matter, passes through in the collision. We can determine the locations of the gas by X-ray telescope, and the supposed location of the dark matter (assuming it exists) with gravitational lensing.

https://ircimg.net/slide_33.jpg

(I am not a physicist, just had this explained by a physicist friend of mine).

Indeed, this is the observation most usually cited in support of the matter / particle interpretation of dark matter. It's very non trivial to come up with a decent theoretical extension to GR that explains it. Which is why, I think, that a lot of people feel more confident proposing a new dark matter particle, which does in fact explain it quite nicely.

Perhaps we're wrong in assuming that all of these observations on very different scales (galaxy rotation curves over galaxy cluster collisions to large scale structure) are tied to the same unknown (dark matter), but if you want to explain them all in a simple way, a particle still seems like the way to go.

Remember that elementary particles have been predicted and found many times over the past years, whereas modified theories of gravity haven't seen the same kind of success. Of course this is only anecdotal, and may not mean anything, but I think it's part of the reason for most people's preference for a particle explanation.

The final bullet point in that image contains the answer to the question of dark matter.

The counterargument given by one of the proponents of MOND (a non-dark-matter counterargument) is that the bullet cluster can be explained by a combination of slight modification of GR, and additionally by the presence of a significant (but not crazy) amount of cold regular matter. Cold regular matter can't be easily detected because it isn't emitting any light, but it is just regular matter.

I'm not actually familiar with the specifics so I don't know whether this is actually a good explanation, but it's at least worth discussing.

This is a good explanation, but I'm not so sure the missing piece is so clear in the case of dark matter (and the blog post explained this nicely).

Sure, dark matter can explain those, but what if there's some not yet imagined change to the equations that gives the same results.

Maybe gravitational attraction is not r^2 at some non-obvious scale that we failed to measure (meters? mm? Casimir effect on cosmic dust? Solar system sized gravitational anomalies?)

Why relativity makes Mercury's orbit precess is very not obvious (I mean, the two-body problem is already hard without relativity). The bending of light is a more obvious effect for example.

> Sure, dark matter can explain those, but what if there's some not yet imagined change to the equations that gives the same results.

How do you propose for a modified equation to explain the bullet cluster? That is, we have multiple good observations that show situations where the distribution of mass differs substantially from the distribution of visible matter, and good theoretical explanations why this should happen in the case of dark matter. This is not adequately explained by modified gravitation or other such effects.

Disclaimer: I do think that the Bullet Cluster is good evidence for dark matter, but we can speculate a bit

One is that there's also a possibility that dark matter exists but it's only part of the story.

For the Bullet Cluster I wonder how reliable the estimation of gravitational mass is for cluster of objects, as I understand, it is the images of the galaxies behind it that are lensed by mass in front of it https://en.wikipedia.org/wiki/MACS_J0025.4-1222#/media/File:...

For one heavy object in front of another, it's easy to see how lensing works and estimate masses from there.

But with clusters and "fuzzier" mass distributions I'm not so sure.

Right. A lot of "modified gravity" theories have been proposed in recent decades, investigated, and compared to observations; with work they can be made to match up. Dark-matter theories on the other hand do a somewhat better job of matching observations, which is why they have become the best-liked explanation for the galactic rotation curve puzzle.

Scientists didn't just jump to dark matter at the start, and no cosmologist will tell you that dark matter is definitely the explanation — it's simply the explanation that currently seems most likely.

Thank you for posting this. I often read commenters who dismiss dark matter the way dark energy is dismissed without people addressing the bullet cluster style observations which in my layman view is the most compelling piece of evidence for dark matter

Is there a well defined boundary between QM and GR?

The Planck Scale gives typical dimensions (length, time, energy, mass) for a process where both GR and QM are relevant at the same time. Unfortunately, no experiment will get anywhere close in the foreseeable future.

and yet the universe is full of these objects - black holes, neutron stars, quasars, etc.

No, these objects have nothing to do with the Plank scale. They are described well by the QM and GR we already know and do not show any quantum gravity behavior.

Hawking radiation emitted by a black hole would be a quantum gravity effect. Unfortunately, it is predicted to be many orders of magnitude to weak to be measured.

I thought the Planck scale was way on the QM side of things. My understanding is that the Planck scale is so small we theoretically can never test QM or any other processes below it, making the Planck scale the limits to our understanding of the extremely small.

The plank scale is derived by saying "what length scale appears if we mash together the constants describing gravity and the constants describing QM". One definition of it is "what wavelength photons have enough energy to collapse on themselves and form a black hole". The only thing we know is that when an object is at that or smaller scale we need to consider both QM and GR. It is not "on the QM side of things", because making small wavelengths requires more energy, and more energy is more massive, and mass bends spacetime which requires GR.

In most cases, only one of them maters and the other is negligible: at very small scales QM rules, while at very large scales only GR is important. But in special situations, such as black holes and big bangs, nothing can be neglected and there is no boundary. (AFAIU, IANAP)

What about in between small and large scales? Medium scales? Is there an in-between?

Yes, there is an in-between at which Newton laws hold. Too big for QM and too small for GR to make a difference. Fortunately, this is the scale in which we live.

Small point of clarification, the predictions of newtonian laws are a limit in GR. So, technically speaking, newtonian physics is still following GR for that energy regime.

So, QM at the small scale and GR everywhere else? Where is the line between QM and GR? Atomic level? Quark level?

QM has a classical limit too. At human scales, both QM and GR are at (very close to) their respective classical limits.

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