DanMP

It is possible, after all, to misunderstood you, so I apologize. Not necessarily, here are my comments: gravitational lensing: is explained by GR Bullet cluster: that could be very well be DM. BTW, DM of the Bullet cluster is hypothesised on the basis of gravitational lensing. gravitational time dilation: is explained by GR gravitational waves: explained by GR I don't understand, MOND is not competing GR? They don't exclude each other? Ok 😀 Do you have a link on this?

Yes, I agree. The thing is that we may never directly detect dark matter particles, because of their low energy and/or other reasons, so the only thing we can do is to define DM models in such a way that they are in agreement with all the observations/aspects that are (or can be) related to DM. Even better would be to make/define a model that not only agrees with all the observations, but also makes predictions that can be tested by doing new observations or experiments. If I understood correctly, the same approach is used in order to model the structure of protons and neutrons. I never did. I only said ... This is what you said (the bold is mine): And a theory of gravity, like MOND, should cover all the aspects, not only the rotation curves, that’s why I asked you about gravitational lensing, Bullet cluster, gravitational time dilation and now gravitational waves. How is MOND explaining them? If I understand correctly you are saying that the rotation of the stars around the galactic center is influenced by other galaxies in the Universe in the same way as the DM, if exists, would do? I really doubt that, but you mentioned calculation. Can you elaborate or point to a credible source?

I found an article ('Final parsec problem' that makes supermassive black holes impossible to explain could finally have a solution) suggesting that dark matter not only exists, but is also self-interacting. I learned about this few days/weeks ago, from a Sabine Hossenfelder video, but this article is better. I wrote "detect or explain": Anyone that invested time, effort and/or money to detect or figure out what DM is or does, is taking DM as real, so I did the same. You can say that what I did is just garbage, but I did something. You, instead, just claimed that MOND is the correct explanation and, when asked about your favorite theory, you were unable and/or unwilling to respond. You consider this honorable? I think that it's quite the opposite. This is a science forum, we are here to discuss science, not to claim that something is correct and, when asked for arguments, to become aggressive and offend the one who asked. Shame on you!

The scientists trying to detect or explain DM have also taken DM for granted. Are they wrong?!? I am open for other explanations, that's why I asked you about MOND, but your response was disappointing. The scientists trying to detect or explain DM have also taken DM for granted. Are they wrong?!? I am open for other explanations, that's why I asked you about MOND, but your response was disappointing.

I agree, thank you. Now you are being aggressive. I simply didn't address the idea that dark matter may not be the common cause of all phenomena that we attribute to it. Now, if you insist, I do answer: yes, you may be right. On the other hand, what is your point? All scientists who are trying to find and explain DM should stop, because "it is not established that dark matter is the common cause of all phenomena that we attribute to it"? You are serious? Yep. Why?

I searched and found what I learned from Swansont. It was: the cosmic neutrino background. And the relic neutrino energy became smaller, not their speed, as I wrote/implied. I assumed that lower energy means lower kinetic energy, but maybe I was wrong. Anyway, as I said, neutrinos are my second option, and I wrote about relic neutrinos mainly because Mordred option for DM was sterile neutrinos and he said that they are not enough (if I remember correctly). No, not necessarily. Really? You said: Gravitational lensing and gravitational time dilation are not "established science" (more accurate, real facts/observations, because a new theory must be in agreement with the facts, all of them, not necessarily with the previous/existing theories)? You really think that MOND may ignore gravitational lensing and time dilation?

You don't get it, to me is cheating if you postulate the presence of DM just to "exactly match" the observations. The same is with the "peculiar velocities", used in order to justify/cover a difference in redshift.

I learned about relic neutrinos from Swansont in another discussion/thread and, if I remember correctly, he said that they are slowed down due to expansion. We should ask him. (I don't know how). Then it (MOND) sounds like a modified GR rather than a modified Newtonian theory ...

In the same manner as GR though ... I didn't understand your explanation and didn't find any. Sorry for the clipped quote, I'm having trouble to select the text from my tablet. Thank you. This is the first time I receive a positive reply in relation with (a part of) my "theory" on DM (posted 6 years ago). I understood what you said. Sorry if I sound aggressive, but an alternative gravity theory, like MOND, should cover all the aspects, not only the rotation curves, in order to be "the correct one". And I really don't understand how a Newtonian theory can explain gravitational time dilation. Thank you. By the way, your observation (One particle may lose some kinetic energy to another one, but the other one gains it), reminds me 2 things. First, neutrino collisions with electrons (the way we detect them) may slow neutrinos down. Second, if the collision is with another, static, neutrino, the emerging neutrino would be the static one, possibly different, explaining the mystery of neutrino in-flight transformation ...

Neutrinos, of any/all kind, are my second choice for DM particles. I wrote "or something else" ... There is something more: they are slowed down due to expansion.

Maybe not only sterile neutrinos but also relic neutrinos. Or something else. How is MOND explaining gravitational time dilation? The gas we know is made by atoms and molecules. Molecules can transform part of kinetic energy in vibration and rotation, slowing down. Also they may react/transform and ultimately clump. In extreme concentrations/pressure, hydrogen and helium can undergo nuclear fusion. On the other hand, DM particles may be incapable to bond in any way, and just collide with each other, without losing speed. Would such a "model" lead to the halo we know/inferred?

It's not that GR is inaccurate I wrote: "we are incapable to accurately use GR " ... In the real world n is extremely large. And you said : DM is huge. If this is a "mathematical artefact" how can we trust the validity of GR on large scales?!? How/why we can say that GR is in agreement with all observations? It sounds like cheating. Or you are wrong.

I can't believe that you wrote/admit such a thing, namely that we are incapable to accurately use GR. In this case, how/why we can say that GR is correct (in agreement with all observations)?

Weak interaction is an interaction. So, if DM is made of WIMPS, then we should be able to detect them (if their mass/energy is big/massive enough) and we didn't. Also they may collide with each other. If the collision is mostly between each other and elastic there would be no reasons for slowing down and clumping. DM halo may be a sphere because DM particles are interacting/colliding like the molecules in a gas? Reaĺy, MOND turns out to be correct? How MOND explains gravitational lensing and Bullet cluster?

Yes, if we want to use thrusters directly on the asteroid surface in order to push the asteroid off the collision course, a rotation of the asteroid around an axis parallel to its path would complicate things, because the thrusters would rotate and need to be switched on and off. Therefore, in such cases, the gravity tractor technique may be a better choice. But gravity is a weak "force". If I electrically charge a piece of paper, the electrostatic force overcomes the whole Earth gravitational pull, lifting the piece of paper off the table. So I wonder: if there is, or we create, a net electrical charge on the asteroid and an opposite charge on the ship/ships, it would increase significantly the pull, making easier the effort to change the asteroid trajectory? The energy may be provided by the space solar power plants I proposed earlier/above and the charging through ionization and ejection of ions or electrons. If the electrostatic force is bigger than the gravitational pull, and also bigger than the force provided by the ship's thrusters, we can charge the asteroid and the ship with the same charge (negative, by ejecting ions) and push, instead of pull. It would be better for the ship's thrusters to be oriented directly opposite from the asteroid. When the asteroid is not very big and can be split, using explosives or nukes, the rotation and the axis orientation is actually helpful, as I wrote earlier/above, because the parts would be driven away from the original, dangerous, path, by the centrifugal forces.

Larger the mass, longer the time from detection to impact, hence more time for the intervention ... And the spin may be pre-existing, needing just some increase. The risk is that the splitting explosion may not work as expected ... I also proposed something like that: The chemical reactions I mentioned should be violent/powerful enough in order to eject gases and some materials, providing thrust. Heating may also do the job in some instances. Space solar power plants would be spread around the Earth, some of them far enough, so we may have at least one close enough to the incoming asteroid. Also lasers would be used to convey the energy where is needed ... Another idea would be to deflect small asteroids from the belt in order to smash them into the incoming asteroid and deflect it.

I think that we can use rotation/spin in order to deflect the debris/parts away from a collision course. If the axis of rotation of the asteroid is more or less parallel with its path, an explosion should split the asteroid in parts going away from the axis/collision path. If the rotation is too weak, we can increase it, with well placed thrusters, prior to the carefully planned explosion. If there is no spin, or the axis is perpendicular to the asteroid path, we may use thrusters to deflect the entire asteroid. We can use as thrusters craters/wells on a side (at a pole, if it spins), where we initiate some chemical reactions or we simply heat them using large arrays of space mirrors (ultrathin reflecting sheets). The arrays of mirrors should be deployed in advance, in all directions, and used as solar power plants, before being converted into solar guns.

OK, I got it. Now I get your point about the importance of (centripetal) acceleration. Clever. Correct. Still, if frame-dragging is present (and significant), the clocks at rest in the dragged frame would be the fastest ... while rotating (with respect to distant stars) and having(?) centripetal acceleration ... You understood exactly what I intended to convey. Thank you!

No, I can't find a link to the explanation you mentioned: the one you brought up when I said: This confirms what I said: the speed is the relevant cause for the differential aging in the twins' paradox, not the acceleration, nor the frame change, as the classical, linear scenario, suggests. In my modified Hafele-Keating experiment, it makes no difference if the plane continues forward or is turning back. Only the speed/velocity matters. And, by using the mirrors, the twins observe each other as in the classical, linear, scenario, with a relay (the relay scenario is the one where a third twin/clock is introduced, in order to skip the turnaround maneuvers). What inertial frames? Only the tower twin is in an inertial frame. And in the classical Hafele-Keating experiment the tower twin is also in an accelerated frame ... By the way, as far as I know, Lorenz transformations are between inertial frames ... but it turns out that they can be applied successfully between the non-rotating Earth frame and twins accelerated frames. This reminds me of the clock postulate.

I don't quite understand. Can you provide a link to the explanation of Hafele-Keating experiment you are mentioning?

Yes, but I never heard that centripetal acceleration is playing a significant role in explaining the time differences in Hafele-Keating experiment. If you did, please elaborate.

Ok, let's consider a modified Hafele-Keating experiment: The stay-at-home twin would be in a tower, at the equator, and the traveling-twin would fly around the Earth, over the equator, at the same altitude as the stay-at-home twin (in order to have the same gravitational time dilation). The Earth is not spinning and flat, with mirrors placed all along the equator. The twins would see each other in mirrors, exactly as described in the classical, linear, experiment. Now, at the opposite side of the Earth (from the tower), the travelling twin turns the telescope from backwards to forwards, and by this action he is changing his perception from moving away to moving towards the tower twin. In this case there is no acceleration (change in velocity), no actual turning back, but the difference in ageing would be the same as he turns back with instant change in velocity and direction, like in a relay version. To me, this is yet another indication that: for the differential aging in the twins' paradox, not the acceleration, nor the frame change.

Yes, that's why I asked Markus Hanke:

Ok, but the context was the twin paradox and the blueshift observed at the turnaround. The blueshifted signal originated from Earth, clearly not local. Ok, so why did you offer the equivalence principle as an explanation for the blueshift there, in the twin scenario discussion? Anyway, the mentioned/linked discussion was closed and I don't intend to continue/reopen it here. All I wanted here was to make this observation:

Ok, thank you.