# Dark Matter : Is it science?

Discussion in 'Astronomy, Exobiology, & Cosmology' started by The God, Mar 8, 2017.

1. ### exchemistValued Senior Member

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If you can manage to keep things as they were yesterday it will a pleasure to debate with you. But if you can't manage that, I would advise you to leave, for your own good and ours too.

3. ### Xelasnave.1947Valued Senior Member

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We all just need to be a little more tolerant of each other, avoid being disrespectful when pointing out anothers error and above all be forgiving.
I dont mean to preach but what I point out I suspect was taught to us by our parents irrespective of what religion or non religion they followed.
You have the power to turn the other cheek and not respond if you feel attack and your mood may have a great effect on how the forum moves ahead.
Everyone is different and anyone can make a mistake or fail to live up to expectations, is it not better to encourage better behaviour than jump on bad bahaviour.

Alex

5. ### Xelasnave.1947Valued Senior Member

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I also wish to appologise for my very early posts re pressure as I believe I should not have posted them in this section.
I thought this thread was in the alternative section otherwise I would not have mentioned pressure etc.
Alex

7. ### The GodValued Senior Member

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Yes, you are right because it is backed up by maths and hence not in fringe. But the nature is not obliged to honor all the mathematical solutions.

Take for example singularities (both BB and BH), they are backed up by maths, so they have mainstream honor, they are in text, in review journals and in pop physics journals. But it does not stop non-nutters also not to challenge them. Unfortunate part for these challengers is that they cannot back up the alternative with maths. There is small desperation here, it is the failure of these challengers to provide a sound alternative, not the absolute beauty of the singularity solution.

Another concept... Inflation..one of the founder himself is questioning it now...but the cat is out of bag....none are listening to him. This is also not fringe, it has become the core of the Big Bang Cosmology notwithstanding the cold feet developed by its proposer.

8. ### SchmelzerValued Senior Member

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This is not a question of the possibility to challenge something. To challenge math makes no sense. So, GR has solutions which become singular. This is simply a fact, live with it.

Of course, this is a serious argument for the claim that GR is not true, but can be only an approximation of a better theory. And you will not find many who doubt that - instead, the mainstream believes some theory different from GR is necessary, and this theory has even a name, quantum gravity.

The question if we have observed things which in GR can be described only as BH solutions is an interesting question about the real world, here doubt is allowed. Say, gravastars.
There are strong observational arguments for some part of what inflation theory is. In particular, that in the early universe $a''(\tau)>0$. There are other parts, namely the particle theory why this happens, which are much more open for discussion. The popular presentations of inflation theory are usually wrong. See http://ilja-schmelzer.de/relativity/inflation.php

9. ### rpennerFully WiredStaff Member

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Nature isn't obliged to do anything, including being consistent from day to day. But so far it has been and physics is predicated on making communicable common frameworks to precisely describe the behavior of large classes of phenomena, and has been. Things like Neptune, Cosmic Microwave Background radiation, neutron stars, light amplification by stimulated emission, neutrinos, anti-matter and the Higgs boson were seen first in the theories before begin found in nature, so our physical theories have a pretty good record of describing the observable behavior of nature, even in places we haven't yet looked.

Singularities represent failures of the model. Our physics theory breaks down and cannot describe the ultimate fate of things that fall into black holes because at a certain point our ability to get answers out of GR stops and we need a new theory. Likewise, we can't talk about "before" the big bang because of the singularity in the way. You can't "challenge" the singularities in GR (what would that even mean??), so what it needed to progress in understanding of the behavior of Nature is to replace GR with a better comprehensive theory, for which we have none. It needs to be a comprehensive theory for two reasons, 1) Nature has never been observed switching to different laws of physics as the situation changes, 2) if it is not comprehensive, the only way to check it is to travel to the center of a black hole.

But the black hole event horizon is not a singularity of GR but of a coordinate system that says it is as easy to go one spatial direction as the opposite. When we use a better coordinate system, that problem goes away. And that is where Hawking Radiation comes from, the region of GR far from the singularity where we know it has problems.

Since math is required in all physical theories, use of math cannot be disparaged in physics discussions. Beauty is not a deciding factor in physics, it is a motivational factor used by some to look for new physics.

Yes, and Einstein denied black holes and changed his mind about including a cosmological constant. Physics is about comparing observation to theory, not going back to human authorities and asking their opinion.

10. ### SchmelzerValued Senior Member

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But Hawking radiation has a serious problem too, known as the trans-Planckian problem. Its derivation depends on existing semiclassical theory remaining valid for arbitrary short distances. Distances where we know that GR fails because quantum gravity effects become relevant.

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11. ### The GodValued Senior Member

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....Did I say that HR is not mainstream?

I said many mainstream guys also question it....

Secondly one can always have reservation about any mainstream theory, your point is that it should not be here in the science section. But that does not change the opinion, it just restricts the free expression on this sub section.

12. ### rpennerFully WiredStaff Member

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I would say you did.
That is not precisely my position.
That's my job. To restrict conduct on the Forum in accordance with the forum rules.
Freedom of speech is the right to say stuff without getting in trouble with the Government. The site owners are not the government and they wish to promote particular types of speech. The main science subforums are more restricted than the Fringe subforums.
Isn't that an argument with the formal means Hawking used to derive his thermal spectrum, not the spectrum or the phenomenon itself?
Alternate arguments for existence of Hawking radiation exist:
Alternate derivations which retain the thermal spectrum exist:
https://arxiv.org/abs/hep-th/9506121 ( Phys. Rev. D 52, 4559 )

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13. ### SchmelzerValued Senior Member

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They exist, but are invalid.

And this is quite easy to prove: Compare whatever the derivatio n of HR for a star collapsing into a BH with a star collapsing into a stable star of size $r_S + \varepsilon$. Such a star radiates during the collapse, but does not radiate once the final radius is reached. This result is quite stable and certain, stable states do not radiate.

So, any derivation which does not depend on the difference between the two collapse scenarios is invalid. So, in particular the analogy argument with Unruh radiation fails. Because if valid it would work for a stable star too. The difference is what happens between $r_S$ and $r_S + \varepsilon$, which is many orders below Planck size a second after the collapse. So, the trans-Planckian problem is unavoidable if you don't leave semiclassical theory.
I know. If you cut trans-Planckian frequencies at near-Planck frequencies, you create another mechanism for HR, namely a trans-Planckian structure, which may change in time, thus, create also radiation. This radiation will be caused by that structure, and depends on that structure also if you consider their location. So, in the whole dumb hole business by Unruh you have a flow. A discrete flow of atoms of the fluid. So, the configuration changes in time, and this causes radiation. Once the flow is stationary, you don't see it in the large distance approximation of the equation. So, this is a similar but different effect, which depends on the details of the trans-Planckian regularization. If you regularize it in such a way that the black hole becomes stable in the regularized picture, you have no HR too.

14. ### The GodValued Senior Member

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Thread is getting nice but I must say some cobwebs must go.

Science does not follow democracy, but over the period of time establishment pride and funding has started playing major role. Defining what is mainstream and who all are part of it, may also be dicey. But if few oppose to a concept with valid ground, then it can be safely said that a 'part of mainstream' is not comfortable with the concept.

You while responding to Schmelzer acknowledged that HR derivation by Hawking may have some issues, so to retain it you highlighted existence of some alternative keeping the conclusion intact. If you refer to the problem highlighted by him, then he is not alone, he is a scientist, many are concerned about this problem with HR,....so is it not safe to say that 'a part of mainstream' is uncomfortable with HR?

15. ### SchmelzerValued Senior Member

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Just to clarify: It is not an issue with the prediction that a change of the gravitational field, which leads to a change of the vacuum state, creates some radiation. So, in particular, a collapsing star will also, via this mechanism, create some radiation. It is an issue with the claim that this effect will lead to a permanent radiation for BHs long time after the collapse. It is not an issue with a particular derivation. It is not at all an issue with the temperature - if there is some radiation, independent of how much, this is the only temperature-like natural value available, so it plausibly will have this temperature.

The derivation itself is not the problem. It tells us that the change of the vacuum, which is the result of the collapse from radius $r_S + 10^{-10^{5}}l_{Pl}$ down to $r_S + 10^{-10^{6}}l_{Pl}$, will give some Hawking radiation we observe some seconds after the collapse. We will see it at Hawking temperature, but at the moment when that collapse happened, and the radiation has been created, it was correspondingly blueshifted, by the factor $10^{+10^{5}}$ or so. No problem with this computation itself, if one assumes that semiclassical theory is fine, it is fine. The only problem is that to assume semiclassical theory can tell us something about this is nonsense. So, any other derivations based on the same semiclassical theory will not change the situation even a little bit.

So, you have to go beyond semiclassical theory to improve the situation. That means, you have to speculate about quantum gravity. That means, you cannot derive, but only guess. (Here I have an issue with some computations. You can modify the theory by changing the dispersion relations. But as long as you have a dispersion relation which allows arbitrary large momentum values, the problem is only modified, and the main objection remains - to get HR after some time, you need momentum at the place of creation which goes beyond every limit. To avoid the problem, you have to cut the momentum completely. And then you obtain HR only if you introduce some element of change into the regularization. Like the flow of the liquid in Unruh's dumb hole.)

Last edited: Mar 11, 2017
16. ### exchemistValued Senior Member

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Excuse the interjection, but I would just observe that I really don't think all this agonising about whether some theory can be labelled "mainstream" or not is helpful. In my experience, the term "mainstream" is generally the preserve of cranks, who for personal reasons are obsessed with notions of quasi-religious orthodoxy in science and the supposed suppression of unorthodox views (i.e. theirs).

Whether a theory is "mainstream" or not seems to me to be a false dichotomy. The way I see it, the status of scientific theories is a continuum, with what one might call "settled science" and "discarded theories" at one end, through to novel but unsubstantiated hypotheses at the other. In between are all sorts of rival theories, which have their adherents and their detractors, while the debate on their merits, and further observations to put them to the test, continues.

I have no dog in the fight about Hawking radiation (I brought it up in another thread quite by chance), but it rather looks to me like a hypothesis with a good deal of (though not universal) support, which is awaiting observational confirmation.

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17. ### SchmelzerValued Senior Member

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I disagree. In popular scientific forums, it is often enough used to distinguish between theories which are allowed and what are forbidden. For example:
I would agree. But this is not how science (fundamental physics) actually works. What matters in actual science are publications in reputable journals. A journal is reputable if the papers published there are often quoted in publications in reputable journals. So, string theory is mainstream, and reputable, my ether theory not, even if string theory has reached much less, essentially nothing except good mathematics, but my ether theory predicts the SM gauge group and fermions from a simple model. Such is life. Mainstream is what matters. Because the mainstream distributed the grants.
Not really. There is no chance at all to see HR in reality. Simply because it is far too small.

There is something completely different, an effect of quantum condensed matter theory, which is, in some weak sense, analogical to HR, which may become visible. But this would not be any observational confirmation of HR. Even if popularizations would sell it with such phrases.

18. ### exchemistValued Senior Member

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I take your point about the use of the term on this other site, evidently done to dissuade cranks. I still dislike it and in all honesty had never come across the term at all until I started contributing to these science fora.

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19. ### SchmelzerValued Senior Member

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Nothing unexpected, simply because inside the scientific community non-mainstream theories simply are not discussed at all. They simply do not exist. It is hard enough for theories which are, if one takes into account only scientific criteria, completely inside the domain of modern physics, but, for one reason or another, outside the mainstream. Like the de Broglie-Bohm interpretation of quantum theory, or for some time non-Copenhagen interpretations in general. Or my own ether theory, which is published in a reputable mainstream journal. You will not hear (have not heard in the case of non-mainstream interpretations) about them in any scientific discussion, not because they were rejected as non-mainstream, or based on some scientific arguments. No, they were simply not discussed. Ignorance is an argument in modern science.

In a forum the situation is different. One cannot force the people not to discuss ignored theories without forbidding this explicitly. So, you need a word to distinguish those theories discussed in the scientific community from those ignored, which have to be illegal in an appropriate, reliable forum.

20. ### The GodValued Senior Member

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Exchemist,

Thats precisely why I stated that defining mainstream and who all are part of it, is dicey. But Schmelzer has attempted to clarify and I think that removes the vagueness. Try pondering over: who levels whom the cranks.

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ditto!

22. ### exchemistValued Senior Member

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I assume you mean: "Who defines what a crank is?"

That's a fair question. To me, it is something like the following: a person who obsessively peddles an eccentric idea beyond the degree that normal people would feel is justified by the evidence, if any.

But we are now a long way from the thread title, so perhaps we should either stop or move this to another thread.

23. ### The GodValued Senior Member

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Just waiting for Rpenner to respond to Schmelzer objection on HR. Schmelzer has objected to Rpenner view that HR is well established with an example highlighting the problem with HR, so I think Rpenner must rebut or accept the objection.