I always wonder if gravity could be a force that runs across infinite quantum universes, but really, while a fun mental experiment I have no idea how to do the maths for what that would mean...
A Physicist Who Denies That Dark Matter Exists
151–160 of 193 posts
Re: A Physicist Who Denies That Dark Matter Exists
#152Earlier quoted context omitted.
How do we know dark matter isn't just really transparent matter? Like, we can't see it in space but it's there, not reflecting or deflecting enough light for us to detect it.
Because it would collide with itself. Dark matter isn't just "not visible" - it's non-electromagnetic. As observed it didn't aggregate when colliding with itself, so there aren't big dark matter made black holes around, nor gravity anomalies from dark matter nebula or the like. Two dark matter particles headed directly at each other would pass through each other and be only slightly deflected by their gravitational i…
Not necessarily. Dark matter could be made of small dense objects. Because they are small, we wouldn't see absorption of light. And because they are small, they will very seldom collide. These kinds of object are called MACHOs.
Based on observed dimming events while monitoring many stars, we have ruled out brown dwarfs as more than 20% of the total. Other possibilities still remain, but none have shown evidence of being sufficient to explain dark matter. However we have not ruled out the possibility of it being something that we haven't thought of yet.
Re: A Physicist Who Denies That Dark Matter Exists
#153Earlier quoted context omitted.
You can observe it via gravitational lensing. For instance: http://scienceblogs.com/startswithabang/2011/04/20/how-gravi... .
We've seen instances already where gravitational lensing was explained through other means (very dim clusters, dust, and even camera issues), we also don't know if the space time on those scales is "smooth", if it isn't then light will not travel in a straight line which would account for the lensing effects. The spacetime curvature, mass/energy, and gravity have quite a few interesting interactions as in gravity as…
> we also don't know if the space time on those scales is "smooth", if it isn't then light will not travel in a straight line which would account for the lensing effects.
Except space isn't perfectly smooth. It does curve, and that curvature is gravity. That's what general relativity is all about. If you want to postulate some way for the universe to curve absent observable mass, go ahead: you're doing nothing but restating the dark matter problem in a (minimally) different way.
Re: A Physicist Who Denies That Dark Matter Exists
#154Earlier quoted context omitted.
It obviously doesn't interact with ordinary matter very much, or else it would have been observed non-gravitationally by now. It could interact a little bit, though. Consider neutrinos: they almost don't interact with ordinary matter at all, but they can be observed with enough patience and care, and they are definitely real. Neutrinos were actually an early candidate for dark matter. The numbers didn't work out, but…
> And sure, dark matter is falsifiable. If observations are made where gravity appears to deviate from what's visible and can't be explained by adding invisible mass, that would falsify it. Whats to stop us from "adding epicycles" when that happens?
The same thing that stopped the addition of epicycles to the geocentric model of the solar system: a better theory that explains the observations more naturally.
Re: A Physicist Who Denies That Dark Matter Exists
#155Earlier quoted context omitted.
I am not a physicist. > Remember that dark matter isn't just some arbitrary fudge factor that gets inserted into the observations to make the numbers come out right. I look at dark matter/energy as a measurement of what we don't know - it is not necessarily matter or energy (although it could be). It is an effect or the sum of effects that we are not aware of. It's almost like the predictions that Mendeleev made with…
> I look at dark matter/energy as a measurement of what we don't know - it is not necessarily matter or energy (although it could be). Dark matter is called matter because it has mass, i.e. it experiences gravitational attraction to itself and to other forms of matter. Dark energy is called energy because it acts like radiation, i.e. it has negative pressure. > Personally, I don't like dark matter because my "hack" a…
> Dark matter is called matter because it has mass, i.e. it experiences gravitational attraction to itself and to other forms of matter.
> Dark energy is called energy because it acts like radiation, i.e. it has negative pressure.
I was going to post something similar, but tempered a little, since some participants in this discussion are skeptical of the existence of these things. I think the dark energy statement is fine, but would weaken the dark matter one to say, e.g., it's called matter because we can observe effects (lensing, rotation curves, etc.) which can be explained by the presence of matter.
In this way, dark matter and dark energy definitely exist, in the same way that centrifugal and coriolis forces definitely exist: they explain concrete observations, and by squinting at the formulae we see that the terms we've introduced have units of mass (dark matter), energy (dark energy) and force (centrifugal and centripetal).
The effects explained by centrifugal and coriolis forces can also be explained in a different way, by taking into account the rotation of our frame of reference. Articles like this one show that the jury's still out on whether the effects of dark matter and dark energy can be explained in a different way.
AFAIK dark matter is quite widely accepted as a fundamental explanation, rather than a "fictitious" one like centrifugal/coriolis forces, while it's common to treat dark energy as a "placeholder" at the moment: it may be a form of energy which permeates space, or it may be an effect of something else (e.g. some unexpected behaviour of spacetime, like discretisation or some other change of metric).
While we're still some way from understanding dark energy, I have no problem with models of dark matter (although SUSY's looking less likely every day); after all, why should we demand that all matter interact elecromagnetically?
On the other hand, when theorising about what dark energy might be, I think it's fine if that explanation encompasses the effects of dark matter too. Just because we discovered the dark matter effects first, it doesn't mean they must be explained first.
Re: A Physicist Who Denies That Dark Matter Exists
#156Earlier quoted context omitted.
Remember the discovery of Neptune? I mean, not directly, since everyone from that time is dead, but.... That's a nice example of a prediction being wrong, inventing an as-yet-unobserved chunk of matter to fix it, and that invention then being correct. Of course, there's a counter-example in the inner solar system in the form of Vulcan. It was invented to explain Mercury's orbit, but the solution ended up being Relati…
> Of course, there's a counter-example in the inner solar system in the form of Vulcan. It was invented to explain Mercury's orbit, but the solution ended up being Relativity. So it's not always the right answer. If anything, the Vulcan hypothesis was not bold enough. It was a much greater leap of faith to assume that Newton's mechanics was just a low energy approximation, hence the twist in Mercury's orbit. Anyway,…
It's how science works, but it's not how politics and human psychology work. Trying to get a paper published that directly contradicts the established theories is almost impossible because people don't like their life's work being thrown away as incorrect.
This is why science advances one generation at a time.
Re: A Physicist Who Denies That Dark Matter Exists
#157Earlier quoted context omitted.
> would require like extra dimensions with funny topology for gravity to be traveling though. Why? I think I can use simple force formula such as F=k/r without any idea of extra dimensions or topology.
Geometrically if a force radiates uniformly it either A)has infinite density B)drops off as 1/r^2 We know that gravity drops off as 1/r^2 for all cases except galaxy rotation curves so for it become 1/r for just this specific scale requires that from gravity's perspective the surface area of a sphere usually but doesn't always increase as r^2. Which in turn means it doesn't move through the 3 spacial dimensions that…
The electromagnetic force (specifically, the Coulomb interaction) is 1/r^2. The strong force is actually effectively constant at long ranges, since it becomes energetically favorable to create an antiquark/quark pair. The residual strong force is exp(-r)/r. The weak nuclear force is exp(-r)/r^2 (or exp(-r)/r? I can't really tell with the links on hand--I just know for sure there's an exp(-r) term in it). Gravity being 1/r wouldn't be a surprise given the other known forces.
Re: A Physicist Who Denies That Dark Matter Exists
#158Earlier quoted context omitted.
There used to be this theory of Phlogiston, which was a material substance of heat (if you put a lit match under a brick, phlogiston would move from the match to the brick.) Today of course, this sounds ridiculous. But my physics teacher was always keen to point out that the people involved weren't stupid and that it was an important theory because it was a good starting mental model of how things actually worked and…
This also reminds me of the Michelson-Morley experiment[0]. A pretty sofisticated setup (at the time) was preparated to prove the fairly established idea that light propagated through Aether. Well, the failure of the experiment motivated a lot of scientists to improve and test other models. I can't exactly recall right now, but Einstein's work was supported by the work of many other scientists. It's not the case that…
Michelson-Morley experiment failed to substantiate the existence of ether. That was 1887. Much after that, in 1905, Einstein's special theory of relativity stated you don't need ether and any medium for light to travel, as long as one abandons the idea of absolute time.
The comment by Mordehai Milgrom, " the emperor has no clothes, that dark matter is our generation’s ether." rings true to me. So far no dark matter has been found by LHC.
They used to say 95% of the universe was dark matter. Now they say 68% is dark energy and 27% dark matter. Still only 5% normal matter. Could it be that there is 100% normal matter, 95% of which are transient and temporal in vacuum? If we were to follow Milgrom, and not invent substances, it seems plausible that we still have more math work to do.
Re: A Physicist Who Denies That Dark Matter Exists
#159Earlier quoted context omitted.
Okay. Your consciousness doesn't exist. Plz go away you scary philosophical zombie!
Weak attempt. The mind is what the brain does. The very same way your browser appears on your screen. Good philosophers, mostly eastern ones, tried very hard to establish a distinction between products of imagination and adequate representations of what is. The classic example is how the brain conditioned by hundreds of thousands of years by Mother Nature readily mistakes a rope for a snake. Notice, that there is not…
Don't change the subject. Explain how consciousness arises, and describe an experiment that replicates it.
Re: A Physicist Who Denies That Dark Matter Exists
#160Earlier quoted context omitted.
Remember the discovery of Neptune? I mean, not directly, since everyone from that time is dead, but.... That's a nice example of a prediction being wrong, inventing an as-yet-unobserved chunk of matter to fix it, and that invention then being correct. Of course, there's a counter-example in the inner solar system in the form of Vulcan. It was invented to explain Mercury's orbit, but the solution ended up being Relati…
I am not a physicist. > Remember that dark matter isn't just some arbitrary fudge factor that gets inserted into the observations to make the numbers come out right. I look at dark matter/energy as a measurement of what we don't know - it is not necessarily matter or energy (although it could be). It is an effect or the sum of effects that we are not aware of. It's almost like the predictions that Mendeleev made with…
Do you know of a summary of the phenomenons we cannot explain without resorting to "dark matter"?