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Could Modified Gravity Kill Planet Nine?

nautil.us

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Re: Could Modified Gravity Kill Planet Nine?

#61

Post by jcranmer »

pfdietz wrote: post #60
adrian_b wrote: post #57

Earlier quoted context omitted.

Such papers only prove that the simplest formula proposed for MOND may be too simple. While it matches well many experimental facts, there also other experimental facts that appear to contradict it. There is an essential difference between a MOND-like theory and a dark matter based theory. For a MOND-like theory one has to choose some mathematical relationship that determines the gravitational forces, given the obser…

But you're just demonstrating the annoying aspect of MOND: it's not one theory, so it fails that first requirement of being science, that it be falsifiable. We see this all the time when MOND is tested. It ends up failing, but then the proponents say "oh, well, it's not that MOND we're talking about". It's like some sort of pseudoscientific cockroach that keeps escaping after you crush it with a shoe.

... so just like supersymmetry and string theory?

Re: Could Modified Gravity Kill Planet Nine?

#62

Post by mr_mitm »

MattPalmer1086 wrote: post #58
mr_mitm wrote: post #55

Earlier quoted context omitted.

LCDM has six parameters, only a few of them are related to the DM distribution. Is that a lot to you or what specifically are you referring to?

The DM distribution itself is effectively a huge number of parameters. You have to have just the right amount of dark matter distributed differently in each case to explain observations and get those best fits. For example, see: https://en.m.wikipedia.org/wiki/Cuspy_halo_problem

Yes, the cuspy halo problem wouldn't be a problem at all if you were to simply adjust the infinite parameters that you suggest that DM has. The fact that there are statistical discrepancies proves my point that no one is adjusting a huge number of parameters.

Re: Could Modified Gravity Kill Planet Nine?

#63

Post by karmakaze »

MattPalmer1086 wrote: post #52
the__alchemist wrote: post #44

Earlier quoted context omitted.

Question, regarding 4:00 in that video. Dr. Becky states that GR, our best theory of gravity, at large scale, predicts dark matter. Is this correct? My understanding is that the models that predict dark matter use Newtonian physics. And more, the problem with GR is that its calculations are complicated, in a partial-differential-equations sense.

No GR does not predict dark matter. But when we look at galaxies and so on, things don't seem to add up if you only look at the visible matter. So, there could be a lot of dark matter we can't see. Or GR might not be correct and MOND (or a relativistic formulation like AQUAL) might be the right answer. It's true I guess that if GR is correct, then we need some kind of dark matter to explain observations. But it might…

It's simpler to say Newtonian gravity and visible matter alone is sufficient to explain the need for dark matter.

Adding GR to the picture doesn't take away the need for dark matter to account for observations.

Re: Could Modified Gravity Kill Planet Nine?

#64

Post by pfdietz »

jcranmer wrote: post #61
pfdietz wrote: post #60

Earlier quoted context omitted.

But you're just demonstrating the annoying aspect of MOND: it's not one theory, so it fails that first requirement of being science, that it be falsifiable. We see this all the time when MOND is tested. It ends up failing, but then the proponents say "oh, well, it's not that MOND we're talking about". It's like some sort of pseudoscientific cockroach that keeps escaping after you crush it with a shoe.

... so just like supersymmetry and string theory?

Yes, just like that.

Re: Could Modified Gravity Kill Planet Nine?

#65

Post by denton-scratch »

I thought MOND only kicked-in in regions of a galaxy where acceleration was much lower than in the Sun's region.

These guys appear to be reasoning that because the Oort Cloud is far from the Sun (i.e. relatively low acceleration) then MOND might apply. But isn't part of the reasoning for MOND that its effects can't be tested anywhere near Earth, because this whole galactic neighbourhood experiences much too much acceleration?

The article was very thin on explaining how MOND could explain anomalous orbits of Oort Cloud objects, or what kinds of anomaly they are trying to address.

Re: Could Modified Gravity Kill Planet Nine?

#66

Post by adrian_b »

the__alchemist wrote: post #44
antonioevans wrote: post #7

Dr Becky explaining the MOND theory and why it's invalid --> https://www.youtube.com/watch?v=HlNSvrYygRc

Question, regarding 4:00 in that video. Dr. Becky states that GR, our best theory of gravity, at large scale, predicts dark matter. Is this correct? My understanding is that the models that predict dark matter use Newtonian physics. And more, the problem with GR is that its calculations are complicated, in a partial-differential-equations sense.

She does not express this fact in the right way.

Whenever the facts predicted by a theory do not match the experimental facts, one could say that the theory has predicted the existence of an unknown factor that has affected the experiments.

Nevertheless, until there is an alternative way to determine the existence of that unknown factor, the right way is to simply say that there is a mismatch between predictions and observations and the reason for this mismatch must be determined in the future. For now, the current theory is not accurate enough.

For instance, when some planetary movements did not coincide with the predictions, it was supposed that perhaps there exists an extra planet which explains the discrepancies between predictions and observations.

This supposition was confirmed only when Neptune was also observed with a telescope. If Neptune had never been observed, perhaps it would have been discovered that the mathematical model of gravitation must be improved.

For now, there are discrepancies between observations and the predictions of the current mathematical model of gravitation. Like in the cases of Neptune and Pluto, there is a supposition that perhaps there exists some kind of dark matter that would be the cause of the discrepancies.

Until the moment when an alternative way to determine the existence of dark matter will be discovered, like the optical observations of Neptune and Pluto, the existence of dark matter remains just a hypothesis that cannot be used for any practical purpose, because it cannot predict anything. Dark matter can be added arbitrarily in any place and this can make any theory of gravity match the observations.

Therefore now we have galaxies that are supposed to be rich in dark matter and galaxies that are supposed to be poor in dark matter, in order to fit the observations, but without any a priori rule that could be used to predict this.

Re: Could Modified Gravity Kill Planet Nine?

#68

Post by adrian_b »

pfdietz wrote: post #60
adrian_b wrote: post #57

Earlier quoted context omitted.

Such papers only prove that the simplest formula proposed for MOND may be too simple. While it matches well many experimental facts, there also other experimental facts that appear to contradict it. There is an essential difference between a MOND-like theory and a dark matter based theory. For a MOND-like theory one has to choose some mathematical relationship that determines the gravitational forces, given the obser…

But you're just demonstrating the annoying aspect of MOND: it's not one theory, so it fails that first requirement of being science, that it be falsifiable. We see this all the time when MOND is tested. It ends up failing, but then the proponents say "oh, well, it's not that MOND we're talking about". It's like some sort of pseudoscientific cockroach that keeps escaping after you crush it with a shoe.

Any specific variant of MOND, with definite formulae for the gravitational quantities, is falsifiable. (Though it may not be very easy to verify or falsify it, because besides the hypothetical "dark matter" there also exists true dark matter, i.e. interstellar gas and dust clouds.)

Any theory based on dark matter is not falsifiable, regardless what model is used for gravitation, because for now there is no constraint on the distribution of the dark matter.

The only way for the theory of dark matter to become a scientific theory is to discover an alternative way to determine where the dark matter is located, besides placing it wherever necessary to remove the discrepancies between the observations of the movements of the celestial bodies and the predictions of the current model of gravitation.

Re: Could Modified Gravity Kill Planet Nine?

#69

Post by nicktelford »

pfdietz wrote: post #60
adrian_b wrote: post #57

Earlier quoted context omitted.

Such papers only prove that the simplest formula proposed for MOND may be too simple. While it matches well many experimental facts, there also other experimental facts that appear to contradict it. There is an essential difference between a MOND-like theory and a dark matter based theory. For a MOND-like theory one has to choose some mathematical relationship that determines the gravitational forces, given the obser…

But you're just demonstrating the annoying aspect of MOND: it's not one theory, so it fails that first requirement of being science, that it be falsifiable. We see this all the time when MOND is tested. It ends up failing, but then the proponents say "oh, well, it's not that MOND we're talking about". It's like some sort of pseudoscientific cockroach that keeps escaping after you crush it with a shoe.

Isn't this essentially the same problem with Dark Matter though? They keep looking for it, not finding it and proclaiming "well, it must be somewhere else!".

I always got the impression that when Dark Matter was initially labelled as such, it was just a name for the discrepancy between theoretical models and observations; and that the name itself seems to have driven this idea that it's the observations that are wrong and not the models.

Personally, when discussing Dark Matter vs. MOND, I think neither should be treated as a concrete "theory", but simply a different perspective on where the problem lies. "Dark Matter" is the idea that our observations are incomplete, and MOND is the idea that our theoretical models are wrong.

Hopefully this conundrum is resolved within my lifetime, because I'd love to know what the answer is. It would be absolutely wild if they're both right i.e. that our observations are incomplete and our models are wrong.

Re: Could Modified Gravity Kill Planet Nine?

#70

Post by mannykannot »

denton-scratch wrote: post #65

I thought MOND only kicked-in in regions of a galaxy where acceleration was much lower than in the Sun's region. These guys appear to be reasoning that because the Oort Cloud is far from the Sun (i.e. relatively low acceleration) then MOND might apply. But isn't part of the reasoning for MOND that its effects can't be tested anywhere near Earth, because this whole galactic neighbourhood experiences much too much acce…

The relevant distance here is not that to the sun, but that to the galactic center, which is within the range over which MOND is proposed to be noticeable. The argument is that, according to MOND, the galactic center would perturb orbits in the Kuiper belt, creating the alignment which has heretofore been taken as evidence for Planet Nine.
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