First test of rival to Einstein’s gravity kills off dark matter • Hacker News Board

Re: First test of rival to Einstein’s gravity kills off dark matter

#81

Post by cgvgffyv » Sat, Dec 17, 2016, 12:38 PM UTC

lisper wrote: post #79

cgvgffyv wrote: post #77

Earlier quoted context omitted.

I encourage you to not stop reading after he called you a loon. He dedicates a few paragraphs to explain what he feels is wrong with your talk. Also read the comments, where he says: I am watching the talk (final minutes now) and except for the history, meaning of interpretations etc., he actually understands QM more correctly than many people employed as physicists... (The audacity! Someone that actually watches the…

> I encourage you to not stop reading after he called you a loon. He dedicates a few paragraphs to explain what he feels is wrong with your talk. What makes you think I stopped reading? How do you think I knew that he was attacking a straw man (and hence making a fool of himself)? > read the comments Well, that's nice. I feel vindicated. But that doesn't change the fact that he published a totally unwarranted ad homi…

> What makes you think I stopped reading?

Well his complaints about the talk are after the "loon" bit, and you didn't say anything to the effect of, "well he called me a loon but did explain his position", so I figured.

Re: First test of rival to Einstein’s gravity kills off dark matter

#82

Post by raattgift » Sat, Dec 17, 2016, 1:32 PM UTC

acqq wrote: post #80

raattgift wrote: post #69

Earlier quoted context omitted.

It's not complete for other reasons, several of which are detailed in Verlinde's paper itself; however, it wholly reproduces GR in the EFT limit by design. Indeed, Verlinde starts with standard dS as his unremovable background on which he puts strings. What in GR we would consider perturbations of dS and IR corrections of GR (at ~ galactic scales) emerge from the behaviour of the strings and how they form long-distan…

Thanks for the correction and clarification. For others who, like me, aren't in the field, the "EFT" mentioned is explained, for example, here: https://arxiv.org/abs/gr-qc/0311082 "The problems of quantizing gravity within the experimentally accessible situations are similar to those which arise in a host of other non-gravitational applications throughout physics. As such, the size of quantum corrections can be safel…

> I'd also appreciate a link to the explanation of "What in GR we would consider perturbations of dS and IR corrections."

Here's a quick overview of perturbation theory then an example of how one might apply it in GR.

https://www.wikiwand.com/en/Perturbation_theory

http://theory.physics.helsinki.fi/~inf/Lectures/Lecture2.pdf

IR is "infrared", low-energy physics, as opposed to UV, "ultraviolet", high-energy physics. In this context, UV means strong gravity where quantum effects are expected to be important. Galaxy-galaxy gravitational lensing (which is the subject of the Brouwer et al. paper that's the subject of the newscientist article linked at the top) is purely a weak gravity problem[1], so is in the IR limit of General Relativity. If you want different results from standard General Relativity you can add corrections by hand in a variety of ways; in the galaxy-galaxy lensing case they would be in the infrared.

[1] strong gravity is hidden behind the event horizons of the black holes of the galaxies and, arguendo, if we could see the strong gravity near the black hole centres, it would not be relevant by virtue of being drowned out by all the stars, gas, dust and other matter.

ETA: This is nice and terse, too: https://www.wikiwand.com/en/Non-exact_solutions_in_general_r...

Re: First test of rival to Einstein’s gravity kills off dark matter

#83

Post by lisper » Sat, Dec 17, 2016, 4:06 PM UTC

cgvgffyv wrote: post #81

lisper wrote: post #79

Earlier quoted context omitted.

> I encourage you to not stop reading after he called you a loon. He dedicates a few paragraphs to explain what he feels is wrong with your talk. What makes you think I stopped reading? How do you think I knew that he was attacking a straw man (and hence making a fool of himself)? > read the comments Well, that's nice. I feel vindicated. But that doesn't change the fact that he published a totally unwarranted ad homi…

> What makes you think I stopped reading? Well his complaints about the talk are after the "loon" bit, and you didn't say anything to the effect of, "well he called me a loon but did explain his position", so I figured.

I explained it as clearly as I could: he called me a loon because I said something that was wrong. And he was correct: the thing he claims is wrong is indeed wrong.

What he failed to notice (or at least failed to mention) was that the wrong comment was preceded by me saying, "I am about to tell you something that is wrong because it makes an interesting and illuminating puzzle to figure out why it is wrong." And it was followed by me saying, "Obviously what I just told you can't be right (because if it were I would at the very least have won the Nobel prize in physics), so there must be a flaw in the reasoning. But where is it? Well, it's here..."

Re: First test of rival to Einstein’s gravity kills off dark matter

#84

Post by imh » Sat, Dec 17, 2016, 5:30 PM UTC

andrewflnr wrote: post #11

“But then if you mathematically factor in the fact that Verlinde’s prediction doesn’t have any free parameters, whereas the dark matter prediction does, then you find Verlinde’s model is actually performing slightly better.” What? How does this work?

When you have free parameters, you are actually opening the door to a whole family of predictions. You're saying that of this entire family of models, you'll choose the one that best fits the data. Without free parameters, you have to come in with a fully specified model to begin with.

Loosely speaking, with an incorrect family of models, there's a lot of wiggle room to find one that still looks quite a lot like your data, while that freedom is not there when making a single specific prediction. There are semi-rigorous ways to take that into account while comparing models https://methodology.psu.edu/node/504

Re: First test of rival to Einstein’s gravity kills off dark matter

#85

Post by pavel_lishin » Sun, Dec 18, 2016, 4:55 PM UTC

yongjik wrote: post #49

pavel_lishin wrote: post #5

I'd be kind of bummed to lose dark matter as a concept. I want to hold onto the idea that it's just as rich as our matter, with its own quantum chromodynamics and an equivalent to the electromagnetic force, leading to a "shadow" universe with their own planets, life, etc. that exists side by side us, trying to figure out why their gravity measurements are just slightly off...

IANAP, but I think the most widely accepted view on dark matter is that it is made of "dark" particles that cannot lose energy by radiation. In other words, it cannot condense and form dark stars or planets. When two lumps of dark matter collide they just pass through each other. Sorry for ruining your dream. :/

Yeah, I googled around after my comment, and that explained it pretty well - it apparently also explains why most of the dark matter is in halos around galaxies, since it can't slow down enough to collapse into galaxies themselves, and is on mostly highly elliptical orbits, spending most of the time away from whatever it's orbiting.

I can still dream, though.

Re: First test of rival to Einstein’s gravity kills off dark matter

#86

Post by pavel_lishin » Sun, Dec 18, 2016, 4:56 PM UTC

logfromblammo wrote: post #40

pavel_lishin wrote: post #5

I'd be kind of bummed to lose dark matter as a concept. I want to hold onto the idea that it's just as rich as our matter, with its own quantum chromodynamics and an equivalent to the electromagnetic force, leading to a "shadow" universe with their own planets, life, etc. that exists side by side us, trying to figure out why their gravity measurements are just slightly off...

You can also have the idea that there are one-way interactions between the normal-matter universe and the dark-matter universe. It isn't too difficult to formulate a mathematical system where two subsets are completely identical when considered in isolation, but when interacting, one subset operates as if the other doesn't exist, while the other is easily perturbed by any values in the mirror subset. I'm not sure if…

Wouldn't there be some sort of energy exchange for this to happen? Wouldn't we notice that there's an unexplained loss of energy whenever we do anything?

Or did I just invent a crappy layman-author's explanation of where inertia comes from?

Re: First test of rival to Einstein’s gravity kills off dark matter

#87

Post by logfromblammo » Mon, Dec 19, 2016, 3:50 PM UTC

pavel_lishin wrote: post #86

logfromblammo wrote: post #40

Earlier quoted context omitted.

You can also have the idea that there are one-way interactions between the normal-matter universe and the dark-matter universe. It isn't too difficult to formulate a mathematical system where two subsets are completely identical when considered in isolation, but when interacting, one subset operates as if the other doesn't exist, while the other is easily perturbed by any values in the mirror subset. I'm not sure if…

Wouldn't there be some sort of energy exchange for this to happen? Wouldn't we notice that there's an unexplained loss of energy whenever we do anything? Or did I just invent a crappy layman-author's explanation of where inertia comes from?

Conservation of energy/mass is a rule for the known universe. If we knew all the rules for the dark part of the universe, we wouldn't even be having this discussion.

Anyway, the way the math works out, it is impossible for a known-universe person to even measure the effects the known-universe has on the dark-universe. The numbers can't come back to the known-universe without an additional input from it, and they would be explainable with another hypothesis that did not include a dark-universe. It way be that all effects we observe to be random are actually perfectly predictable to a dark-universe observer.