281–290 of 302 posts
> The next-generation radio telescope - the Square Kilometre Array @SKA_telescope will be able to test this theory, and directly confirm or invalidate its predictions. 13/17 It is rather surprising that the model predicts the properties of a LambdaCDM universe.
As a casual observer this is is what gets me excited! We'll get our answers one way or another.
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Yes, good, you've saved me some typing along these lines. (Well, in retrospect, maybe not that much :D ) In typical vacuum solutions of the Einstein Field Equations the metric is what determines the available geodesics[0]. Putting an isolated object into a vacuum solution causes it to "select" an appropriate geodesic from the ones available, and if left alone, that object will at every time be somewhere on that geode…
A couple little things I won't turn into literal ETA3, just in case you've already been reading the long post above. :) (2) below is also mostly for me, I think. 1. Thanks, Michael Weiss. This is essentially the same argument I'm making, only terser. http://math.ucr.edu/home/baez/physics/Relativity/GR/expandin... 2. One important thing about the expansion of the universe is that people hear "expansion of space" and r…
In the EFE, if we move the cosmological constant factor to the right handside, where the stress–energy tensor is, then stress–energy tensor and the cosmological constant factor have opposite signs and stress–energy tensor can overcome the cosmological constant factor.
When we say EM-forces/Strong/Weak forces "cancel out" the expansion of the universe, it isn't because of those forces are holding matters together. But they contribute to the stress–energy tensor and "offset-ed" the effect of the cosmological constant factor.
Even without those forces, as long as the stress–energy tensor is greater than the cosmological constant factor, the solution of EFE will be non expanding.
Does this sound right to you? Thanks.
> Negative masses are a hypothetical form of matter that would have a type of negative gravity – repelling all other material around them. Unlike familiar positive mass matter, if a negative mass was pushed, it would accelerate towards you rather than away from you.
> The gravity from the positive mass galaxy attracts negative masses from all directions
...why aren't these contradictory?
intuitively... wouldn't a negative mass be repelled by positive gravity?
(I am not a physicist of any kind, obviously)
In the author's Twitter thread https://threadreaderapp.com/thread/1070302359325151233.html
there is this image: https://pbs.twimg.com/media/Dtp7YjWW4AEIvSp.jpg
..which seems to answer my question
If I understood the picture:
- positive mass gravities attract each other and result in acceleration towards each other
- negative mass gravities attract each other, but the negative mass means this attractive force results in an opposite acceleration (they behave as if repelled)
- a positive mass and a negative mass have gravities which repel each other so the positive mass accelerates away from the negative, but the negative mass inverts its gravitation repulsion into an acceleration towards the positive (I find it hard to imagine how this interaction actually plays out, it's a bit counter-intuitive.. I guess they must cancel out rather than chasing each other faster and faster?)
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A different hand-wavy explanation. The places that an electron can be found are described by a wave pattern. If the electron is staying in place around a nucleus, that wave pattern has to be a standing wave that reinforces itself. To reinforce itself it has to wrap around the nucleus an integer number of times. This explanation doesn't just explain why it doesn't fall in, it also explains why there are discrete shell…
I'm not really interested (here) in the reality of the interaction between the electron and the nucleus. I don't think the existence of an electromagnetic field is a good argument against its own existence as argued by FabHK further up. I also don't think FabHK's argument works as an analogy to my problem with negative mass. The electromagnetic field is not self-reinforcing in the same way. Imagine that you're holdin…
(Though it escapes me why you would assume that I believe in theories about negative mass, which actually seem to me to be very speculative and not very likely.)
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To approximate with Newtonian laws: F=ma Meaning that positive and negative masses will accelerate in opposite directions in response to the same applied force. The effect is different, but it's a bit like how when you are in a car and hit the gas pedal, and your kid is in the back seat holding a helium balloon, the kid feels pushed backward into the seat but the balloon flies forward toward the windshield. F = GMm/r…
We have an intuitive ~~example~~ approximation of this in real-life: balloons in a car[1]. [1]: https://www.youtube.com/watch?v=uAJlg8MDAlU
Paper: https://arxiv.org/pdf/1712.07962.pdf Not a physicist, but my summary of the author's point would be: If you allow for matter with a negative mass and plug it into a n-body simulation. You seem to get what looks like dark matter from first principles. Namely, a halo of non interacting matter around galaxies in just the right "non-cuspy" shape. Their conclusion is that this is worth considering, as almost all ma…
Here's an argument: If you think about it, "mass" actually means two different things. One one hand, there is mass as the "charge" of gravitational interaction, that is, the gravitational field generated by a body is proportional to its mass. Let's call this concept "gravitational mass". On the other hand, you have mass in the sense of "intertial mass", a body's resistance to change of velocity. Although we kinda tak…
E.g. assume two bodies, one with mass M, the other with mass -M. The standard gravitational model describes a "repulsive" force on each of these objects, in the direction opposite each other. However, since the object of mass -M has negative inertial mass, this force would cause it to accelerate toward the object of mass M.
This results in these objects not accelerating relative to each other, and perpetually accelerating relative to an independent observer. One might attempt to explain this by saying that the negative mass has a growing negative kinetic energy level, but (it seems as a layman) this would violate the second law of thermodynamics.
Edit: Actually just negative inertial mass does this all by itself; it is just in the opposite direction if the gravitational mass is positive.
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Because physicists would find it totally unsuitable for their requirements?
Why match is better for that? To compute prediction, you will need to use computer and compiler, so it better to talk directly in these terms because of (1) (see above).
Here comes the Alcubierre Warp Drive!
To quote the author https://twitter.com/Astro_Jamie/status/1070343971338153984 > It is quite likely that the negative masses in this study are a mathematical tool, rather than real physical matter. This rules out a lot of sci-fi, which is disappointing of course!
Why crap on the fact that this may lead to a warp drive.
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Exactly where my mind went. Does the perspective described in the paper make the Alcubierre drive seem more plausible? Or is there something about the nature of the dark fluid hypothesized that would make it useless for something like that?
>Or is there something about the nature of the dark fluid hypothesized that would make it useless for something like that? Assuming it was, literally, some kind of magical FTL juice... it all seems to be outside the galaxy, which means tens of thousands of years' travel just to get to it. If we're willing and able to do that, why bother with FTL at all?
