General relativity is not a basic assumption in physics. There is a large feeling that there is something more out there , but that we don't really know what it is . Some theories of dark energy or dark matter work by modifying gravity away from general relativity and in the process create testable predictions.
I believe that modified theories of gravity are stuck on modifying Newtonian dynamics, and their inability to make the jump to modified GR is why they are still disfavored compared to the dark matter hypothesis.
"It might sound like a story from a parallel universe – but it’s true. The Bullet Cluster isn’t the incontrovertible evidence for particle dark matter that you have been told it is. It’s possible to explain the Bullet Cluster with models of modified gravity. And it’s difficult to explain it with particle dark matter"
"Modified gravity cannot successfully predict the large-scale structure of the Universe the way that a Universe full of dark matter can. Period. And until it can, it’s not worth paying any mind to as a serious competitor. You cannot ignore physical cosmology in your attempts to decipher the cosmos, and the predictions of large-scale structure, the microwave background, the light elements, and the bending of starlight are some of the most basic and important predictions that come out of physical cosmology. "
Mr Siegel asserts that dark matter hypothesis is better than any kind of MOND (and, may I add, his opinion is expressed in over-confident and even dogmatic way) because it can "explain" structure of the Universe. However, people in comments doubt that LCDM is particularly good in this area:
"The problem is that dark matter can’t predict the structure of the universe all that well either. Making those dark matter simulations work requires introducing all sorts of epicycles, kludgy mechanisms to make the calculations work out properly. I’m not a big MOND fan, but MOND seems to solve certain problems fairly well, and the galaxy rotation problem is one of them."
I don’t see any valid new argument in the qouted comment, yes, the only strength of MOND are galaxy rotations, as mentioned by Ethan, no, no epycycles are actually present in lambda CDM.
"As soon as you start adding extra fields that couple to the gravitational field, you're no longer modifying gravity. You're doing something that is much closer to what dark matter is doing, with the only difference being that you're invoking an additional particle-free field rather than a field that does have particles. Which is frankly just weird because any field should be quantizable and thus any field should have something that looks like a particle, in principle. Your proposal would basically be to throw quantum mechanics out in order to explain the bullet cluster, which is frankly a lot more problematic than simply adding an extra field to the already existing ones."
If I understand the gist of that, the whole argument for modified gravity being somehow simpler seems to go out the window. The author responded:
"That's how it's called. Don't blame me for the terminology. Sure, the field should have particles if you quantize it, but at low energies the classical mean-field approximation should be good."
It sounds like the Occam's Razor argument about modified gravity being better because it's just a revision of universal laws was conceded somewhere along the line, so why even bother?
Edit:
From my lay perspective, I imagine it like this: we see dinosaur footprints appearing for no apparent reason. Dark matter theorists say it's invisible dinosaurs. Modified gravity theorists say (I thought) that the laws of gravity can be adjusted to explain it as a "natural" phenomenon. But now it sounds like they are saying there is a continuous field that happens to have significant amplitudes in the places that we would otherwise appear to have quantized invisible dinosaurs. But how does that explain at all the reason for the field having an amplitude here and not there? Why is it more pleasing or likely than distinct individual dinosaurs? It all seems like a bait and switch so ridiculous I feel like I must be misunderstanding grossly.
Let’s also not forget Einstein-Cartan theory that takes torsion into consideration and associates it with quantum spin, producing some very satisfying predictions for general cosmology and might even account for the discrepancy in galactic rotations while not causing disruption on the scales we have tested GR so far.
It is better than a feeling. GR cannot be quantitized. If anyone describes the present status of gravity as anything other than a mystery that we are good at predicting the movements of, they are wrong.
This is irrelevant. GR makes accurate predictions. Perhaps we haven’t figured out how to quantise it. Maybe it isn’t a quantum field. Maybe we’re mis-interpreting the mathematics.
Hypotheses are interesting. I find the variable speed of light discipline neat. But they’re just hypotheses. Until they explain experimental evidence better than GR, they will remain hypotheses.
It's not irrelevant if you're looking for new classes of predictions and new kinds of experiments - which is one of the motivations for quantum gravity.
> The concept they tested is similar to Galileo’s apocryphal experiment, in which he dropped two objects of the same mass off the Leaning Tower of Pisa and found that they fell at the same acceleration regardless of their composition.
Quibble: the article refers several times to the theory of general relativity. I always thought it was the general theory of relativity: i.e., it was the theory itself that was general, handling everything the special theory (limited to inertial reference frames) couldn't.
Am I making a distinction without a difference? Are there two forms of the name in free variation?
They are just somewhat different names of the same thing. General relativity includes the special relativity one where special means without accelerated reference frames, i.e. nothing accelerates or is exposed for gravity.
> special means without accelerated reference frames
That's something a lot of people say, but it's not strictly true. Special Relativity is fully capable of describing acceleration in a flat (Minkowski) spacetime[1].
General Relativity is only really needed when there's curvature in the spacetime (gravity).
>“We’ve moved the lab 14 times around the sun,” says Patla. He’s not trying to be cute; it turns out that Earth moves through some distinctive regions in space. Because Earth’s orbit isn’t a perfect circle—its distance from the sun varies—the clock moved through varying gravitational fields.
22 comments
[ 4.6 ms ] story [ 64.5 ms ] thread[0]http://adsabs.harvard.edu/abs/1988PhLB..202..497B [1]http://xxx.lanl.gov/abs/astro-ph/9612099
https://backreaction.blogspot.com/2017/01/the-bullet-cluster...
https://medium.com/starts-with-a-bang/only-dark-matter-and-n...
"Modified gravity cannot successfully predict the large-scale structure of the Universe the way that a Universe full of dark matter can. Period. And until it can, it’s not worth paying any mind to as a serious competitor. You cannot ignore physical cosmology in your attempts to decipher the cosmos, and the predictions of large-scale structure, the microwave background, the light elements, and the bending of starlight are some of the most basic and important predictions that come out of physical cosmology. "
Mr Siegel asserts that dark matter hypothesis is better than any kind of MOND (and, may I add, his opinion is expressed in over-confident and even dogmatic way) because it can "explain" structure of the Universe. However, people in comments doubt that LCDM is particularly good in this area:
"The problem is that dark matter can’t predict the structure of the universe all that well either. Making those dark matter simulations work requires introducing all sorts of epicycles, kludgy mechanisms to make the calculations work out properly. I’m not a big MOND fan, but MOND seems to solve certain problems fairly well, and the galaxy rotation problem is one of them."
https://medium.com/@kaleberg7/the-problem-is-that-dark-matte...
https://en.m.wikipedia.org/wiki/Lambda-CDM_model
"As soon as you start adding extra fields that couple to the gravitational field, you're no longer modifying gravity. You're doing something that is much closer to what dark matter is doing, with the only difference being that you're invoking an additional particle-free field rather than a field that does have particles. Which is frankly just weird because any field should be quantizable and thus any field should have something that looks like a particle, in principle. Your proposal would basically be to throw quantum mechanics out in order to explain the bullet cluster, which is frankly a lot more problematic than simply adding an extra field to the already existing ones."
If I understand the gist of that, the whole argument for modified gravity being somehow simpler seems to go out the window. The author responded:
"That's how it's called. Don't blame me for the terminology. Sure, the field should have particles if you quantize it, but at low energies the classical mean-field approximation should be good."
It sounds like the Occam's Razor argument about modified gravity being better because it's just a revision of universal laws was conceded somewhere along the line, so why even bother?
Edit: From my lay perspective, I imagine it like this: we see dinosaur footprints appearing for no apparent reason. Dark matter theorists say it's invisible dinosaurs. Modified gravity theorists say (I thought) that the laws of gravity can be adjusted to explain it as a "natural" phenomenon. But now it sounds like they are saying there is a continuous field that happens to have significant amplitudes in the places that we would otherwise appear to have quantized invisible dinosaurs. But how does that explain at all the reason for the field having an amplitude here and not there? Why is it more pleasing or likely than distinct individual dinosaurs? It all seems like a bait and switch so ridiculous I feel like I must be misunderstanding grossly.
http://iopscience.iop.org/article/10.3847/0004-637X/832/2/96...
Feelings don’t count in science. Every experiment (I am aware of) to disprove GR has failed. It is the best explanation of spacetime we have.
This is irrelevant. GR makes accurate predictions. Perhaps we haven’t figured out how to quantise it. Maybe it isn’t a quantum field. Maybe we’re mis-interpreting the mathematics.
Hypotheses are interesting. I find the variable speed of light discipline neat. But they’re just hypotheses. Until they explain experimental evidence better than GR, they will remain hypotheses.
Typo. Should say of different masses.
Am I making a distinction without a difference? Are there two forms of the name in free variation?
That's something a lot of people say, but it's not strictly true. Special Relativity is fully capable of describing acceleration in a flat (Minkowski) spacetime[1].
General Relativity is only really needed when there's curvature in the spacetime (gravity).
[1] https://en.wikipedia.org/wiki/Acceleration_(special_relativi...
“General relativity...[is] also known as the general theory of relativity or GTR“ [1]. Einstein himself wrote of der allgemeinen Relativitätstheorie.
[1] https://en.m.wikipedia.org/wiki/General_relativity
Michael from Vsauce has a nice graphic showing that https://youtu.be/IJhgZBn-LHg?t=1016