Discussion in 'Science & Technology' started by Hrothbern, Jun 7, 2018.
Who knows? The experiments haven't been tested thoroughly yet.
There are some very highly-regarded mathematicians/physicists who think there
is no such thing as dark matter and that the six-sigma experiments are tainted
and/or that there are mathematical difficulties that lead to results that seem
to imply the existence of dark matter.
Eric Verlinde is one of the mathematicians/theoretical physicists I was
referring to. I'm not saying he is right or wrong, just that there are
credible alternatives to the dark matter/ dark energy theories.
A recent lecture of his:
Be careful of going too far down some of the rabbit holes of the alternative
theories or you will end up with the Bogdanov Brothers. Then, you might
decide to have a look at recent photos of them and you will have nightmares
But you're going to do it, despite my warning. Shrug.
First, there are some doubts how solid this evidence for a sterile neutrino is:
Second, if these neutrinos are real, they still cannot explain dark matter. The problem with neutrino dark matter models is that neutrinos don't have enough mass to explain dark matter. Typically these models require neutrinos to have a mass of at least a few keV. Nobody knows the mass of neutrinos exactly, but there is very solid evidence that the mass must be < 1eV. If the data here is taken at face value, the new neutrinos would have a mass difference of 0.2 eV, which is far less than would be required to explain dark matter.
Sterile neutrinos would be an amazing discovery for themselves, but these here will not explain dark matter. Of course, they could be just a glimpse to a much broader theory that could, but for that to be possibility, you would need much more amazing discoveries than this one.
I assume that by "dark matter" you mean discrepancies between predictions made
by the standard (or some other) model and the best observations we have at
Or have you already accepted that it's a missing particle?
I'm not trying to be nasty or disingenuous about that, it's just that I'm never
sure what some people actually mean by the term when they use it.
"Accepted" is too strong a word. In my semi-informed opinion (this is way to far from my specialty that I can claim any expertise here), there is strong, but not ultimately conclusive evidence that there is non-baryonic dark matter and that hasn't been explained by modified theories of gravity, yet. So I consider it highly likely that it is indeed a missing particle (or multiple). I cannot rule out that you can construct a theory of gravity that explains all observations without resorting to non-baryonic dark matter, but so far I am not convinced by anything that is proposed. I use "dark matter" as a shorthand for all of this.
In this case, it doesn't matter: To answer the question "Does this explain dark matter?", you need to assume that there are missing particles, build a model which describes what would be required of such particles and conclude that whatever they may have found here does not meet those requirements and thus cannot explain dark matter -- no matter how you define it. To validate or invalidate a hypothesis you need to first assume that it is true, work out the consequences and then validate or invalidate those.
Thanks, Uppi, that's what I thought you might mean.
I agree with most of what you said, except...
That word likely bothers me a bit.
Can't agree with that at all. It's one way, but not necessarily the only way.
It is the only way to answer this particular question. If you want to test a hypothesis, you need to assume it is true for a moment. That doesn't mean you need to commit to that assumption in any way. It is like a proof by contradiction in math, where you assume the opposite of what you want to prove and then show that it leads to absurd consequences. That doesn't mean that you believe your initial assumption in any way (In fact you want to prove the opposite). In this case the hypothesis that a particular particle explains dark matter requires the hypothesis that dark matter is missing particles. So you calculate the models if that were true and by doing that you can show, that no, this cannot explain dark matter. But even if it could explain dark matter, you could still be of the opinion that there are other models that explain the dark matter question better (although the definition of "better" is always a combination of metaphysics and personal opinion).
Absolutely correct. But you are welded onto the one idea of a missing
particle and disallow any hypotheses that don't assume a missing particle.
No. Assuming that for 5 minutes until the hypothesis is falsified can not be described as "welded" in any way or form.
I still don't get why it is the only way, but I'm more than happy to
bow my head to your superior knowledge of this field.
And please, Uppi, don't think I'm trying to put you down in any way
for your opinion. I'm very glad you spent the time to answer. Thanks.
It's articles like this one from earlier today that confuses me...
My excerpt from the article...
However, despite numerous experimental efforts, there is no direct proof that dark matter exists. This led astronomers to the hypothesis that the gravitational force itself might behave differently than previously thought. According to the theory called MOND (MOdified Newtonian Dynamics), the attraction between two masses obeys Newton's laws only up to a certain point. At very small accelerations, such as those prevailing in galaxies, gravity becomes considerably stronger. Therefore, galaxies do not tear apart due to their rotational speed and the MOND theory can dispense with the mysterious star putty.
The new study opens up the possibility for astronomers to test these two hypotheses in an unprecedented regime.
What exactly confuses you? I mean there is a lot of potential for confusion in that article, but if you are more specific, maybe I can try to resolve some of that confusion.
Thank you for the kind offer.
Yes, the article is very superficial, but I am still struggling to see where
there is a need to assume particles in a test of the hypotheses that could be
made about the observations in it.
Can't this all be done with field equations or some other approach? Or am I not
understanding some form of "duality" that makes it irrelevant whether one uses
"fields" or particles?
It sort of is all done with field equations, the particles are just implied. But let me start from the beginning:
The most well tested field equation we have for gravity are the Einstein field equation. Those relate mass and energy to time and space. For laboratory tests these work very well: There are experiments with very high precision that test this equation and so far, all these tests were positive. They also work very well for describing "small" scale astronomical systems, like our solar system.
The problem occurs when you look at large scale astronomical systems like galaxies or the entire visible universe. If you put in all mass and energy that you know about from observations, you will notice some discrepancies of how the universe should behave to how it does behave.
There are two possible solutions to this:
The first one is to modify the field equations in a way that all works out with just visible mass and energy. This is what these alternative theories of gravity try to do. This isn't easy, because the new equations need to fit all observations that we have made. It is easy to introduce a term that works for a specific galaxy, but it has to work for all galaxies without becoming overly complicated. The problem is, you will never run out of possible ways to modify the equations, so "We need to modify the equation" is not a proper scientific hypothesis. To transform the idea into a specific hypothesis, you need to make assumptions how the equations should look like and then you can derive predictions from that which you can compare to observations.
The second possibility is to keep the equation as it is and fill in mass and energy that we cannot see (yet). The "missing" mass and energy are called dark matter and dark energy. Now, the field equation doesn't care much what you put in there - it doesn't have to be particles. For example, dark matter could consist of ordinary matter clumped together in many small black holes, which we would have no way of detecting. Again, you will never run out of possibilities of invisible stuff to put in, so "There is dark matter" isn't much of a scientific hypothesis, either. Again you need to make assumptions about the properties of the dark matter, so that you can derive predictions about the behavior of the universe.
Theoretical physicists concerned with gravity need to publish something, so they will keep up making new hypotheses about dark matter or the lack thereof. Over time, certain theories and classes of theories will become favorites, because they explain observations best, are not overly complex, or have the best justifications for the assumption they make. Of course, these criteria are quite soft, so there will be at least some gut feeling involved in this. To progress beyond these gut feelings, you need to find differences between predictions that these theories make and then compare these to observation to see which theory matches the observations best. To do that you need to commit to the assumptions of these theories long enough to calculate the consequences. So you go: If the equations would look like this, this galaxy should behave like that. Or if the dark matter distribution would look like this, the galaxy should behave in that different way. That is what they are trying to do in the paper this story is about.
The most successful model of the universe and dark matter is ΛCDM. To avoid the "it could be anything up to invisible pink unicorns"-problem, it makes certain assumptions of how dark matter behaves. It doesn't explicitly require dark matter to be some kind of particle, but it is difficult to see how these properties could be realized without some kind of particle or particle-like object. So it is implied that dark matter is composed of some kind of new particles. However, if you could propose some kind of dark matter which doesn't require new particles but still fulfills the assumptions of ΛCDM, this would not invalidate the theory. At them moment, I am not sure, how much this has been tried and how successful such approaches could be.
Since ΛCDM is so popular, the best way to make an alternative theory more popular is to show that it is better than ΛCDM. If it's worse, why should anyone go for it? This is why these studies usually compare with ΛCDM. If you could find an observation that ΛCDM could not explain, this will not prove that dark matter doesn't exist or isn't some kind of particle, but the next best theory might be quite a bit worse at explaining everything so that more people switch to alternative theories instead.
The field-particle duality isn't really relevant here, since the Einstein field equation is not a quantum field theory. So you cannot really work with the duality here. However, since QFT is the best theory we have to describe particles, any new particle is assumed to have a corresponding quantum field. This doesn't affect the gravity calculations, but if you search for a specific particle with an associated field, you usually need to involve the field-particle duality to calculate how that hypothetical particle would interact with whatever experiment you want to perform.
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