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A Physicist Who Denies Dark Matter

nautil.us

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Re: A Physicist Who Denies Dark Matter

#171

Post by badosu »

tynpeddler wrote: post #163
badosu wrote: post #44

Earlier quoted context omitted.

Dark Matter is not just 'something we don't understand yet'. It has predictable effects on observations and adheres to some properties, see: https://en.wikipedia.org/wiki/Bullet_Cluster .

That's evidence, not proof. We still don't have enough evidence to say conclusively that Dark matter is as we currently imagine it. And of course the same can be said about MOND. Here's Milgrom's interpretation of the bullet cluster: http://www.astro.umd.edu/~ssm/mond/moti_bullet.html

I am not saying Dark Matter is something that actually exists that cannot be ruled out by other explanations (a revision of GR would be ideal IMO), just that this phenomenon makes the approach much trickier.

Re: A Physicist Who Denies Dark Matter

#172

Post by olavk »

JadeNB wrote: post #144
yorwba wrote: post #52

Earlier quoted context omitted.

Using epicycles just means to decompose the motion into its Fourier series. It may not be the most elegant description, but its not wrong per se , either.

> Using epicycles just means to decompose the motion into its Fourier series. It may not be the most elegant description, but its not wrong per se , either. One issue here is that (as I understand the history) epicycles were not originally proposed in the context of "just add them until you've got something that looks right"; it was proposed that, occasionally, one needed to add one epicycle. After a while it was fou…

Epicycles were added to make the model better fit the observations. There is nothing wrong or unscientific about this. It is just like new planets are added to the model of the solar system when new planets are observed.

Re: A Physicist Who Denies Dark Matter

#173

Post by JadeNB »

olavk wrote: post #172
JadeNB wrote: post #144

Earlier quoted context omitted.

> Using epicycles just means to decompose the motion into its Fourier series. It may not be the most elegant description, but its not wrong per se , either. One issue here is that (as I understand the history) epicycles were not originally proposed in the context of "just add them until you've got something that looks right"; it was proposed that, occasionally, one needed to add one epicycle. After a while it was fou…

Epicycles were added to make the model better fit the observations. There is nothing wrong or unscientific about this. It is just like new planets are added to the model of the solar system when new planets are observed.

> There is nothing wrong or unscientific about this.

To be sure, there is nothing wrong with changing your model in response to data—what would be wrong would be the opposite. What I was calling 'wrong' was two things: (1) a model that can only be tuned post hoc to observations (that is, it's always being changed to fit them, and never predicting new ones); and (2) a model that is so flexible that it can be tuned to fit any observations (again, because it will never have the ability to predict future observations).

Re: A Physicist Who Denies Dark Matter

#174

Post by olavk »

JadeNB wrote: post #173
olavk wrote: post #172

Earlier quoted context omitted.

Epicycles were added to make the model better fit the observations. There is nothing wrong or unscientific about this. It is just like new planets are added to the model of the solar system when new planets are observed.

> There is nothing wrong or unscientific about this. To be sure, there is nothing wrong with changing your model in response to data—what would be wrong would be the opposite. What I was calling 'wrong' was two things: (1) a model that can only be tuned post hoc to observations (that is, it's always being changed to fit them, and never predicting new ones); and (2) a model that is so flexible that it can be tuned to…

The predictions by the system is the movements of the planets in the sky. The Ptolemaic system with epicycles actually did this pretty well, considering the available data. It is just that as better observations and better calculations became available, the model have to be more precise which required more epicycles.

I don't really get the thing about "too flexible". According to Wikipedia there is apparently some myths floating around regarding the science history of epicycles, but I assure there is nothing magical about them - at least they are not any more magical than Newtonian mechanics.

Re: A Physicist Who Denies Dark Matter

#175

Post by JadeNB »

olavk wrote: post #174
JadeNB wrote: post #173

Earlier quoted context omitted.

> There is nothing wrong or unscientific about this. To be sure, there is nothing wrong with changing your model in response to data—what would be wrong would be the opposite. What I was calling 'wrong' was two things: (1) a model that can only be tuned post hoc to observations (that is, it's always being changed to fit them, and never predicting new ones); and (2) a model that is so flexible that it can be tuned to…

The predictions by the system is the movements of the planets in the sky. The Ptolemaic system with epicycles actually did this pretty well, considering the available data. It is just that as better observations and better calculations became available, the model have to be more precise which required more epicycles. I don't really get the thing about "too flexible". According to Wikipedia there is apparently some my…

> I don't really get the thing about "too flexible". According to Wikipedia there is apparently some myths floating around regarding the science history of epicycles, but I assure there is nothing magical about them - at least they are not any more magical than Newtonian mechanics.

I think we may be talking at cross purposes here, but I wanted to give it another shot. I do not think that there is anything magical about epicycles, and my objection to them is scientific; it has nothing to do with their history. My claim is that a scientific tool is valid science only to the extent that it has predictive power. If your model has so many parameters that, by setting them to appropriate values, you can fit any outcome, then (absent some other data telling you which values of the parameters to use) you'll never actually predict anything, only scramble setting them to 'post-dict' existing observations—which is, effectively, useless.

(For example, any finite collection of observations can be 'post-dicted' by a high-degree polynomial, and we can accommodate more observations simply by increasing the degree of the interpolating polynomial. This is not magical, and mathematically totally sound; but its very flexibility means that it is scientifically useless, because there's no reason to believe that our perfect fit to the `n` values we've seen so far says anything about the (`n + 1`)st value we'll see next.)

Re: A Physicist Who Denies Dark Matter

#176

Post by amai »

knodi123 wrote: post #122
amai wrote: post #118

MOND might explain rotation curves of galaxies. But it doesn't explain all the other places in astrophysics, where one needs dark matter. Especially MOND (and any modification to gravity) cannot explain the Bullet Cluster: https://en.wikipedia.org/wiki/Bullet_Cluster#Significance_to... The Bullet Cluster is especially significant, because it shows gravitational lensing by dark matter spatially separated from visible…

> Especially MOND (and any modification to gravity) cannot explain the Bullet Cluster The creator of MOND addressed that: http://www.astro.umd.edu/~ssm/mond/moti_bullet.html

1) "But MOND (or other modified gravity theories) predict that the "DM" should be found around the visible matter not elsewhere"; so they say. However, this last statement is incorrect. For example, in galaxies the baryons are concentrated at small radii but the putative DM according to MOND is far beyond that. "

It is important to stress the word "around". In Milgroms example the DM is still around the baryons, just far out at a different radius. This is the modification you can get, as long as gravity is still a central force in your theory. However in case of the bullet cluster the DM simply doesn't follow the center of the baryons. It is completely separated at a different place. I think he doesn't get this important point.

2) "As I said we do know that there is some (still) dark matter in clusters and that's what they found."

So Milgrom is admitting that even with MOND one needs some dark matter to explain the results. But with only dark matter we don't need to use MOND. So according to Occams razor the Dark Matter theory wins, because it explains more with less.

Re: A Physicist Who Denies Dark Matter

#177

Post by lightlyused »

hwillis wrote: post #34
douche wrote: post #19

Earlier quoted context omitted.

I'm not seeing anything there that isn't a handwavey explanation for "reality doesn't match the models, so DARK MATTER!"

>One of the recent most convincing things was the bullet cluster as described here.[1] We saw two galaxies collide where the "observed" matter actually underwent a collision but the gravitational lensing kept moving un-impeded which matches the belief that the majority of mass in a galaxy is collisionless dark matter that felt no colliding interaction and passed right on through bringing the bulk of the gravitational…

Parallel universes?

Re: A Physicist Who Denies Dark Matter

#178

Post by olavk »

JadeNB wrote: post #175
olavk wrote: post #174

Earlier quoted context omitted.

The predictions by the system is the movements of the planets in the sky. The Ptolemaic system with epicycles actually did this pretty well, considering the available data. It is just that as better observations and better calculations became available, the model have to be more precise which required more epicycles. I don't really get the thing about "too flexible". According to Wikipedia there is apparently some my…

> I don't really get the thing about "too flexible". According to Wikipedia there is apparently some myths floating around regarding the science history of epicycles, but I assure there is nothing magical about them - at least they are not any more magical than Newtonian mechanics. I think we may be talking at cross purposes here, but I wanted to give it another shot. I do not think that there is anything magical abo…

OK how is it you think epicycles differ in this respect from say the keplerian model of the solar system? I struggle to understand what you think is "special" about epicycles. No model of the solar system will allow you to predict the movement of the planets if you don't enter the correct parameters, which you have to find through observation. Given the correct initial parameters, the ptolmaic system with epicycles will yield predictions about the movements of the planet, predictions you can verify or falsify through observation.

Re: A Physicist Who Denies Dark Matter

#179

Post by JadeNB »

olavk wrote: post #178
JadeNB wrote: post #175

Earlier quoted context omitted.

> I don't really get the thing about "too flexible". According to Wikipedia there is apparently some myths floating around regarding the science history of epicycles, but I assure there is nothing magical about them - at least they are not any more magical than Newtonian mechanics. I think we may be talking at cross purposes here, but I wanted to give it another shot. I do not think that there is anything magical abo…

OK how is it you think epicycles differ in this respect from say the keplerian model of the solar system? I struggle to understand what you think is "special" about epicycles. No model of the solar system will allow you to predict the movement of the planets if you don't enter the correct parameters, which you have to find through observation. Given the correct initial parameters, the ptolmaic system with epicycles w…

> OK how is it you think epicycles differ in this respect from say the keplerian model of the solar system?

Just to be clear, I'm not in any way insulting the intelligence of those who proposed them. It was a clever mechanism for dealing with what seemed at the time to be a blemish on an otherwise beautiful theory, and the fact that it turned out to be (factually) wrong does not mean that it was wrong to propose and test it.

With that said, here's my stab at what I see as the difference. The Keplerian model predicts elliptical orbits. To be sure, one must do some tuning of parameters to find the semimajor and semiminor axes of these ellipses; there is no way for someone sitting in a windowless room, with access only to Kepler's laws, to predict exactly, say, the length of a Jovian year, or even the existence of Jupiter.

However, what is crucial is that that person sitting in a windowless room can say, definitively, that, if Jupiter has been observed in these five positions at these five times, then it will be in this position at this sixth time. If it's not there, then the Keplerian model is wrong; it has been falsified.

Now suppose instead that another person in another windowless room knows about, not just Kepler's laws, but also epicycles. Given the same five measurements, he or she can make a prediction (which won't require epicycles). This prediction might or might not agree with the sixth measurement, but, if it doesn't, then that's no problem; just add on an epicycle to account for that measurement. If the result of that correction doesn't agree with the seventh measurement, then just add another epicycle.

To me, these two systems are meaningfully different. It is not the reliance on parameters to which I object, but the extreme flexibility; a model that can literally never be falsified, because it can always be adjusted to match new observations, is worthless as a scientific model.

Re: A Physicist Who Denies Dark Matter

#180

Post by olavk »

JadeNB wrote: post #179
olavk wrote: post #178

Earlier quoted context omitted.

OK how is it you think epicycles differ in this respect from say the keplerian model of the solar system? I struggle to understand what you think is "special" about epicycles. No model of the solar system will allow you to predict the movement of the planets if you don't enter the correct parameters, which you have to find through observation. Given the correct initial parameters, the ptolmaic system with epicycles w…

> OK how is it you think epicycles differ in this respect from say the keplerian model of the solar system? Just to be clear, I'm not in any way insulting the intelligence of those who proposed them. It was a clever mechanism for dealing with what seemed at the time to be a blemish on an otherwise beautiful theory, and the fact that it turned out to be (factually) wrong does not mean that it was wrong to propose and…

That would be a valid criticism if epicycles were added to the model post-hoc to explain previous observations, and adding those epicycles to the model did not improve future predictions.

Is that really how you think epicycles were used?

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