I think that's, in a certain sense, not really correct. Without some form of Occam's Razor, you can't do science — or at least, you can't come to any definite conclusions, because there are always infinitely many models that would explain any observation.
Turning that around, it means that for any fixed amount N of data in support of your hypothesis, there's an ugliness/arbitrariness threshold H_0 such that if your hypothesis is more ugly/arbitrary/possessed of entia multiplicata than H_0 you shouldn't accept it yet.
I'm afraid that's a category error. I wasn't talking about science. I was talking about truth.
Many truths are literally incomprehensible. There are many systems out there in the world that may even consist entirely of atomic parts that we fully understand but we still can not understand the totality of all the parts, that simply consist of too many bits (in the information theoretic sense) for us to ever understand. The truths are true nevertheless.
I'm a bit old fashioned that way. The idea that there is no objective reality, or that if we can't understand it it can't exist, may be very exciting philosophies among the word-loving crowd, but I do not find them useful models of reality.
It actually wouldn't surprise me that much to discover that we can't quite nail down the laws of physics the universe operates on even in principle for some reason. (And I don't mean with any sort of copouts like "it might be a simulation" or anything; I mean the laws of physics that appear to govern the universe we appear to live in, regardless of whether that is itself sitting on top of something else.) Based on our progress in understanding the universe, I'd still guess that it's reasonably likely that we can find all the basic, atomic principles. But I wouldn't be stunned to be wrong.
That's honestly not so "weird" at all. There are TONS of examples of objects and substances having that kind of behavior in nature. If I crash my car into something, the metal "remembers" what happened to it. If I run electricity through an inductor, the magnetic field "remembers" that electricity was being run through that space and doesn't just disappear the moment the electricity is turned off. Momentum is the same thing. Humans remember what happened to them because consciousness records everything that happens to us. What prevents gravity from being something similar? Gravity itself may mean exactly the same thing as space having a kind of memory and operating fundamentally by the law of cause and effect. Things may seem weird only in the absence of such a clear principle.
Recall the operant definition of "weird" here is something that fits into the sentence "I find the idea of 23% of the universe being invisible particles that we are completely incapable of detecting is weirder than the idea that we have gravity slightly wrong."
Mathematically, if we add non-linear terms to gravity, it isn't that weird that you might get a memory-like effect, among other things. But would someone who agrees with the previous sentence agree that this is still less "weird" than 23% of the universe being so invisible that we still can't pick it up, even in 2017?
That there's stuff that has a mass but doesn't interact well with light, or that space is able to curve itself without mass, thus also rendering Einstein false.
This immediately raises further questions: How long does space remember? How's the exact mechanism? Where and how does empty space store that information? What happens to E=mc2?
Clearly, Occam's Razor can only be applied correctly if you can distinguish what you really know from what are actually your assumptions. Not only do people hear what they want - but those who don't know don't know they don't know.
However, I get suspicious when someone argues by analogy. Your "my car remembers a crash, furthermore humans do remember, therefore space could also remember gravity" does not seem reasonable to me.
I am also not sure why my post got downvoted.
Edit: As I think about it, "but Einstein showed that you can bend space without mass" again is an argument by analogy. It doesn't matter what analogies exist. The theory should be tested according to empiric evidence. Pulling your analogies together judt for the sake of abandoning a different theory - where's the point in that?
Did you mean the metric tensor g_{\mu\nu} or the Ricci curvature tensor R_{\mu\nu}?
"Gravitational field" is one of those shifty things in GR that makes one sympathetic to MTW's refusal to identify the term with any particular mathematical object.
You're right though; there are plenty of exact solutions to the Einstein Field Equation which are vacuum solutions -- no matter, just gravity.
Which variants are these?
cf. section 8.3, https://arxiv.org/abs/1112.3960
Several of the relativstic MOND theories (see chapter 7 of the same review) are detailed, and I don't see how any of the 8.3 set matches the quote above.
It's already a few years old, though, so maybe you've seen something newer.
> We also have a Standard Model of particle physics that, whatever its known flaws may be, doesn't immediately obviously have a hole for dark matter in it
There's a hole in chirality, which was part of the motivation for WIMPs as CDM.
There's a hole in the Strong-CP problem, which was part of the motivation for axions as CDM.
There's a hole in the hierarchy problem, also part of the motivation for WIMPs but also the motivation for Little-Higgs-like CDM.
Those particle dark matter candidates came from BTSM gauge theorists, not relativists.
> I also am sympathetic to MOND solutions as well.
If you make MOND relativistic you wind up adding a field to G = T; which side it's on doesn't really matter - it'd mostly be either down to an aesthetic choice based on coupling strength, or social considerations (this goes back to at least 1918! [1]).
[1] https://arxiv.org/abs/1211.6338 (and sure enough even in 2017 you will not be arrested by the EFE police if you put the cosmological constant onto the T side as a source, and you might even want to, e.g. in quintessence modelling, or in the cosmological frame; also cf. the choice between the Einstein frame and the Jordan frame for scalar-tensor gravity)
The Bullet Cluster isn’t the incontrovertible
evidence for particle dark matter that you have
been told it is
As an astronomer in the field of clusters, I don't know anyone who takes MOND-like theories seriously. Any non dark matter explanation for the bullet cluster would have to be extremely contrived.
However, there's lots of scope for taking an unphysical and even incomplete theory seriously [this supergravity researcher puts it well: https://arstechnica.com/science/2013/05/earning-a-phd-by-stu... ]
Relativistic approaches to MOND might eventually raise interesting questions about the EFT, and those are what a QG phenomenologist lives for. So it's easy to see why she'd take MOND seriously without taking it too seriously.
(As an example, Verlinde's recent work is partly provoked by MOND, and it's also nice to see a string theorist step outside of the AdS comfort zone and confront what we see in our sky).
the probabilities and number of clusters imply we'd expect
a bullet cluster
Hayashi & White 2006 -- BC is apparently consistent with LCDM.
Farrar & Rosen 2007 -- BC is apparently not consistent with LCDM.
Nusser 2008 -- BC possibly not consistent with LCDM.
Angus & McGaugh 2008 -- BC is more consistent with MOND cosmology than LCDM.
Llinares et al. 2009 -- BC is more consistent with MOND cosmology than LCDM.
Lee & Komatsu 2010 -- BC is apparently not consistent with LCDM.
Forero-Romano et al. 2010 -- BC is apparently consistent with LCDM.
Thompson & Nagamine 2012 -- BC is apparently not consistent with LCDM.
Watson et al. 2014 -- BC is apparently consistent with LCDM.
Krajlic & Sarkar 2015 -- BC is apparently consistent with LCDM.
Bouillot et al. 2015 -- BC is apparently consistent with LCDM.
Thompson et al. 2015 -- BC is apparently consistent with LCDM.
But that's not the case. We have petabytes of data from space (if not more). Your theory needs to match all of them, give or take noise. The common example of the Bullet Cluster [1] comes up a lot, but it's just one particularly exciting example of the problem. I've seen people create variants of MOND that try to address the Bullet Cluster problem, but you start having to have some sort of really weird gravity memory effect in space or something, where space itself "remembers" that it used to have a lot of matter in it and still has a small amount of gravitation in it despite no longer containing the corresponding mass. Which I suppose isn't necessarily out of the question given that we're intrinsically discussing a non-linearity in gravity, but it's still a bit strange.
By the time you work out what MOND is going to need to look like in order to explain everything we see in space, it ends up losing its "Oh, it's just one more simple term on gravity" nature and starts looking pretty weird itself. It's constrained by the real data to be at least as weird as dark matter itself, because the data itself is weird. Given the history physics has had with "calling its shots" and identifying the necessity of particles years or decades before they are detected, dark matter is something that I consider at least not out of the question.
I also am sympathetic to MOND solutions as well. After all, "the math looked awfully weird by the time we finished working out the theory" also has a long and illustrious history in physics too! What's true is true, regardless of how we feel about how beautiful it is. We also have a Standard Model of particle physics that, whatever its known flaws may be, doesn't immediately obviously have a hole for dark matter in it, so perhaps the "we've called our shots before" argument doesn't apply so well here. There is something to be said for not trying to push a new particle in there. But if MOND is the answer, you're likely to find the final result to have non-trivial weirdness in it, too.
[1]: https://en.wikipedia.org/wiki/Bullet_Cluster#Significance_to...