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A Physicist Who Denies That Dark Matter Exists

cosmos.nautil.us

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Re: A Physicist Who Denies That Dark Matter Exists

#101

Post by XorNot »

blueprint wrote: post #83
uf wrote: post #74

Earlier quoted context omitted.

You can apply Occam's Razor here. Which is more likely? That there's stuff that has a mass but doesn't interact well with light, or that space is able to curve itself without mass, thus also rendering Einstein false. This immediately raises further questions: How long does space remember? How's the exact mechanism? Where and how does empty space store that information? What happens to E=mc2?

If you know what Einstein discovered he clearly explained gravity can exist without mass. Look up the stress-energy tensor. Clearly, Occam's Razor can only be applied correctly if you can distinguish what you really know from what are actually your assumptions. Not only do people hear what they want - but those who don't know don't know they don't know.

Where's the energy coming from though? Einstein didn't say "you can curve space for free".

Re: A Physicist Who Denies That Dark Matter Exists

#102

Post by yati »

EGreg wrote: post #88
mikeash wrote: post #28

Earlier quoted context omitted.

It obviously doesn't interact with ordinary matter very much, or else it would have been observed non-gravitationally by now. It could interact a little bit, though. Consider neutrinos: they almost don't interact with ordinary matter at all, but they can be observed with enough patience and care, and they are definitely real. Neutrinos were actually an early candidate for dark matter. The numbers didn't work out, but…

How do we know dark matter isn't just really transparent matter? Like, we can't see it in space but it's there, not reflecting or deflecting enough light for us to detect it.

From what I understand, if dark matter is stuff that only interacts gravitationally, by definition it is transparent to light. Also collisions, being ultimately electromagnetic interactions, aren't observed. So it's not transparent in the sense of glass. But I'm not a physicist, and this thread is interesting :)

Re: A Physicist Who Denies That Dark Matter Exists

#103

Post by XorNot »

fahadkhan wrote: post #46
contravariant wrote: post #9

Earlier quoted context omitted.

It's also worth pointing out that several similar 'holes' in theories have turned out to be correct. Things like missing elements in the periodic table, and fundamental particles like the Ω− baryon or the Higgs boson, where all things that seemed like holes in a theory, but were later discovered to exist.

Black holes were discovered in theory before any there was any evidence too.

Although interestingly we're not necessarily sure they're exactly what we think they are - you can come up with objects which would give the same observations we have but be quite different : https://en.m.wikipedia.org/wiki/Gravastar

Re: A Physicist Who Denies That Dark Matter Exists

#104

Post by zeven7 »

roryisok wrote: post #3

> "If Newtonian physics can’t predict the fixed curves, perhaps we should fix Newton, instead of making up a whole new class of matter just to fit our measurements." I've always felt this way about dark matter, but as a lay person with no background in physics, it's an opinion I'm hesitant to express. if you have a model that predicts something, but the prediction is off by several orders of magnitude, the model is w…

Part of the problem is that fixing Newton is more than just adjusting the equation; it requires making up a whole new constant, which to some is worse than making up a whole new class of matter. Constants don't have explanations; they're just like saying, "I don't know why it works out this way, it just does." It's better to have a reason that explains why something happens than to throw up your hands and say "let's just add a new constant to make things work out."

Re: A Physicist Who Denies That Dark Matter Exists

#105

Post by uf »

blueprint wrote: post #83
uf wrote: post #74

Earlier quoted context omitted.

You can apply Occam's Razor here. Which is more likely? That there's stuff that has a mass but doesn't interact well with light, or that space is able to curve itself without mass, thus also rendering Einstein false. This immediately raises further questions: How long does space remember? How's the exact mechanism? Where and how does empty space store that information? What happens to E=mc2?

If you know what Einstein discovered he clearly explained gravity can exist without mass. Look up the stress-energy tensor. Clearly, Occam's Razor can only be applied correctly if you can distinguish what you really know from what are actually your assumptions. Not only do people hear what they want - but those who don't know don't know they don't know.

I am no physicist and I cannot make heads or tails from stress-energy tensor.

However, I get suspicious when someone argues by analogy. Your "my car remembers a crash, furthermore humans do remember, therefore space could also remember gravity" does not seem reasonable to me.

I am also not sure why my post got downvoted.

Edit: As I think about it, "but Einstein showed that you can bend space without mass" again is an argument by analogy. It doesn't matter what analogies exist. The theory should be tested according to empiric evidence. Pulling your analogies together judt for the sake of abandoning a different theory - where's the point in that?

Re: A Physicist Who Denies That Dark Matter Exists

#106

Post by fader »

pharrington wrote: post #87
roryisok wrote: post #3

> "If Newtonian physics can’t predict the fixed curves, perhaps we should fix Newton, instead of making up a whole new class of matter just to fit our measurements." I've always felt this way about dark matter, but as a lay person with no background in physics, it's an opinion I'm hesitant to express. if you have a model that predicts something, but the prediction is off by several orders of magnitude, the model is w…

This isn't right. Dark matter is a shorthand of saying "our current theory is at least incomplete, there is probably an entirely new class of thing (as in, dark matter is known to be a misnomer), and maybe (with a ridiculously high degree of uncertainty) a separate theory is craftable that precisely explains both what we're currently calling "dark matter" and all other other observed gravitational phenomena. The prob…

The article does mention this:

> In 2004, Bekenstein proposed his TeVeS, or Relativistic Gravitational Theory for MOND. Since then, several different relativistic MOND formulations have been put forth, including one by me, called Bimetric MOND, or BIMOND.

> So, no, incorporating MOND into Einsteinian physics is no longer a challenge. I hear this statement still made, but only from people who parrot others, who themselves are not abreast with the developments of the last 10 years. There are several relativistic versions of MOND. What remains a challenge is demonstrating that MOND can account for the mass anomalies in cosmology.

I'm not a physicist and definitely not qualified to evaluate those claims, but it doesn't seem to be a concern to Milgrom.

Re: A Physicist Who Denies That Dark Matter Exists

#107

Post by pdonis »

roryisok wrote: post #3

> "If Newtonian physics can’t predict the fixed curves, perhaps we should fix Newton, instead of making up a whole new class of matter just to fit our measurements." I've always felt this way about dark matter, but as a lay person with no background in physics, it's an opinion I'm hesitant to express. if you have a model that predicts something, but the prediction is off by several orders of magnitude, the model is w…

> "If Newtonian physics can’t predict the fixed curves, perhaps we should fix Newton, instead of making up a whole new class of matter just to fit our measurements."

The problem with this is that we already know Newtonian physics is wrong. It makes incorrect predictions for planetary orbits in the solar system (the original wrong prediction was the motion of Mercury, but the perihelion shift that is not predicted by Newtonian physics but is predicted by general relativity has now been verified, AFAIK, for all the inner planets). So the way to "fix Newton" is to adopt general relativity instead.

And once you adopt general relativity, you can't just arbitrarily change the equation for "acceleration due to gravity" the way MOND does. That equation is not a free parameter in GR; it's derived from the Einstein Field Equation. So you would have to change the Einstein Field Equation, and that would change all of the predictions of GR, not just the prediction for galaxy rotation curves.

What I've just stated is the standard objection to MOND among physicists. Milgrom mentions relativistic formulations of MOND and says that this issue is "no longer a problem", but I think that's much too optimistic.

Re: A Physicist Who Denies That Dark Matter Exists

#108

Post by blueprint »

XorNot wrote: post #101
blueprint wrote: post #83

Earlier quoted context omitted.

If you know what Einstein discovered he clearly explained gravity can exist without mass. Look up the stress-energy tensor. Clearly, Occam's Razor can only be applied correctly if you can distinguish what you really know from what are actually your assumptions. Not only do people hear what they want - but those who don't know don't know they don't know.

Where's the energy coming from though? Einstein didn't say "you can curve space for free".

Energy is "generated" by activities of 'what exists'.

Re: A Physicist Who Denies That Dark Matter Exists

#109

Post by Confusion »

jerf wrote: post #13
roryisok wrote: post #3

> "If Newtonian physics can’t predict the fixed curves, perhaps we should fix Newton, instead of making up a whole new class of matter just to fit our measurements." I've always felt this way about dark matter, but as a lay person with no background in physics, it's an opinion I'm hesitant to express. if you have a model that predicts something, but the prediction is off by several orders of magnitude, the model is w…

One of the mental problems that I think people get in this area is that you can't help but mentally imagine that there's just these few points of data that we need to hit, and so we have a lot of freedom in how to construct formulae that match the observation, so why are scientists being so obstinate about the possibility that MOND may be true? But that's not the case. We have petabytes of data from space (if not mor…

  The Bullet Cluster isn’t the incontrovertible 
  evidence for particle dark matter that you have 
  been told it is
http://backreaction.blogspot.nl/2017/01/the-bullet-cluster-a...

Re: A Physicist Who Denies That Dark Matter Exists

#110

Post by etrevino »

bartj3 wrote: post #86

> Perhaps somewhere out there, in one of those galaxies I spent my life researching, there already is a known unified theory of physics, with a variation of MOND built into it. But then I think: So what? We still had fun doing the math. His life's work: we had fun doing the math :)

While I was getting my PhD(in History), my advisor told me about meeting Lawrence Stone towards the end of his life (Dr. Stone died in 1999). Lawrence Stone had spent a great deal of his career investigating early modern and late medieval marriages. Grossly summed up, he argued that marriages and child bearing in this era were defined more by convenience than by love and it was only in the eighteenth century that love became the defining prerequisite for marriage (I'm being very broad here). My advisor very gently probed Dr. Stone on how he felt about new research that largely puts the lie to Stone's thesis. It was a big deal, because Dr. Stone's theories had defined how we think about marriage and the family for about a generation.

Dr. Stone said that "no quest for the truth is wasted." (that's a quote from my advisor, not Dr. Stone) And he's right! Even if we've got a far more nuanced view of marriage and the family now, he turned over new stones and raised questions that people had to address. His methodology made further research possible. Even if he was wrong (and he thought that he had gotten it wrong), he still made it possible for later historians to do great things.

All that's to say: I totally respect the attitude of having fun doing the math. (as I think parent does as well) This kind of effort is never wasted.

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