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- The Minkowski metric is
One aspect of this is that, if you sub `t -> -t'`, that's just as good a solution too. Which would suggest any solution with a positive time direction can have a negative time direction, just as easily. Is this widely assumed to be true, or at least physically meaningful?Δs = sqrt(-Δt^2 + Δx^2 + Δy^2 + Δz^2)There's also Wick rotations, where you can sub `t -> it'`, and then Minkowskian spacetime becomes Euclidean but time becomes complex-valued. Groovy stuff.
I'm not much of a physics buff but I loved reading Julian Barbour's The Janus Point for a great treatment of the possibility of negative time.
The craziest thing I've seen though is the suggestion that an accelerating charge, emitting radiation that interacts with the charge itself and imparts a backreacting force on the charge, supposedly has solutions whose interpretation would suggest that it would be sending signals back in time. [0]
- If you want a good read that (to summarize it quite tersely) takes the idea of 3- or n-body simulations and goes very far indeed with it (i.e. why there is something rather than nothing), I highly recommend checking out Julian Barbour's The Janus Point.
- Is it even meaningful to say that the universe - everything - has an entropy? Isn't it not even meaningful to talk about the entropy of something that is in a non-equilibriated state? People point to experiments about gas particles bouncing off the walls of closed boxes to prove how entropy is always increasing, but it really makes no sense to transfer that to the whole universe.
I have been much more convinced that the universe, starting from a maximally structured configuration, tends to even greater structure, as Julian Barbour wrote about in the Janus Point. Structure emergence, as opposed to catastrophic mixing. My perspective used to be like yours but it has become: the game is only starting.
- I was literally thinking of the same similarities. Barbour's exposition of the principle of least action as being time is interesting. There's a section in The Janus Point where he goes into detail about the fact that there are parts of the cosmos that (due to cosmic inflation) are farther apart in terms of light-years than the universe is old, and growing in separation faster than c, meaning that they are forever causally separated. There will never be future changes in state from one that result in effects in the other. In a way, this also relates to computation, maybe akin to some kind of undecidability.
Another thing that came to mind when reading the part about how "black holes have too high a density of events inside of them to do any more computation" is Chaitin's incompleteness theorem: if I understand it correctly, that basically says that for any formal axiomatic system there is a constant c beyond which it's impossible to prove in the formal system that the Kolmogorov complexity of a string is greater than c. I get the same kind of vibe with that and the thought of the ruliad not being able to progressively simulate further states in a black hole.
- This idea has been explored by Julian Barbour in his book The Janus Point.
http://www.platonia.com/books.html
The related math and modelling goes under the name Shape Dynamics:
https://en.wikipedia.org/wiki/Shape_dynamics
Shape Dynamics - An Introduction
- I'm awaiting Jean Pierre Petit's analysis. He's developed a twin universe theory involving negative mass/energy using bimetric relativity (without the runaway effect). At one point in the development of his theory in the mid 2010s, he stumbled upon: Hossenfelder, S.: A bimetric theory with exchange symmetry.
He contacted her but she refused to collaborate on a common paper, calling him a plagiarist and a crank. She did the same to Tim Palmer (a Royal Society meteorologist) when he approached her with ideas on superdeterminism:
> But Tim Palmer turned out to not only be a climate physicist with an interest in the foundations of quantum mechanics, he also turned out to be remarkably persistent. He wasn’t remotely deterred by my evident lack of interest. Indeed, I later noticed he had sent me an email already two years earlier. Just that I dumped it unceremoniously in my crackpot folder.
http://backreaction.blogspot.com/2019/12/the-path-we-didnt-t...
Back to Petit and plagiarism: compare:
https://link.springer.com/article/10.1007/s10509-014-2106-5
https://januscosmologicalmodel.com/cptsymmetry
with Julian Barbour's Janus Point book blurb:
> In a universe filled by chaos and disorder, one physicist makes the radical argument that the growth of order drives the passage of time -- and shapes the destiny of the universe. Time is among the universe's greatest mysteries. Why, when most laws of physics allow for it to flow forward and backward, does it only go forward? Physicists have long appealed to the second law of thermodynamics, held to predict the increase of disorder in the universe, to explain this. In The Janus Point, physicist Julian Barbour argues that the second law has been misapplied and that the growth of order determines how we experience time. In his view, the big bang becomes the "Janus point," a moment of minimal order from which time could flow, and order increase, in two directions
Source: https://www.amazon.com/Janus-Point-New-Theory-Time/dp/046509...
There are more players in the field with very similar ideas (the refinement of twin universes as a two-sided universe), Petit contacted them all but was never given a reply:
- Neil Turok: https://physicsworld.com/a/our-universe-has-antimatter-partn...
- Our universe may have a twin that runs backward in time: https://www.livescience.com/mirror-universe-explains-dark-ma...
- Neil Turok again a few weeks ago: https://www.newscientist.com/article/mg25734230-100-neil-tur...
And now we have that paper that talks about black holes without a singularity and their coupling with the universe's expansion limited to a maximum k-factor of 3.
This sounds a lot like the conclusion from this paper by Petit:
> Supermassive objects, whose formation will be explained in a future article, are also subcritical objects, Plugstars. The theory gives all plugstars a gravitational redshift of 3. This is exactly what is shown by the measurements of the images of two hypermassive objects located at the center of the galaxies M87 and the Milky Way.
> We predict that this redshift of 3 will accompany future images of hypermassive objects that will appear in the future.
