Grandfather paradox

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nezumi
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Grandfather paradox

Post by nezumi » Fri Oct 02, 2020 3:48 pm

So fate is real?

This has blown my mind and I am not a maths person at all, it implies to me that events must happen how they happened and even if you time travel, nothing will change. So, fate then. But you can still try, so free will also. And contrary to all science fiction ever, it is actually quite hard to unravel the fabric of space time, thankfully.

Or I could be completely wrong or exaggerating. Please help!

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shpalman
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Re: Grandfather paradox

Post by shpalman » Fri Oct 02, 2020 3:58 pm

Just because some maths predicts something could exist doesn't mean that it actually exists.

The paper is at https://doi.org/10.1088/1361-6382/aba4bc and appears to be open access.
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nezumi
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Re: Grandfather paradox

Post by nezumi » Fri Oct 02, 2020 4:01 pm

shpalman wrote:
Fri Oct 02, 2020 3:58 pm
Just because some maths predicts something could exist doesn't mean that it actually exists.

The paper is at https://doi.org/10.1088/1361-6382/aba4bc and appears to be open access.
Well, I shall see if I can understand it, and attempt to read. But thankyou for putting it in perspective. Too much madness going on this year, I haven't been able to think straight since mid Feb.

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shpalman
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Re: Grandfather paradox

Post by shpalman » Fri Oct 02, 2020 4:10 pm

To be honest, I wouldn't really understand the maths in the paper either.

Someone who was at Warwick at the same time I was did his PhD on the idea that closed timelike curves from general relativity could explain quantum mechanical entanglement, or something.

It's not obvious that we live in a universe in which CTCs would be possible.
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monkey
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Re: Grandfather paradox

Post by monkey » Fri Oct 02, 2020 4:28 pm

nezumi wrote:
Fri Oct 02, 2020 3:48 pm
So fate is real?

This has blown my mind and I am not a maths person at all, it implies to me that events must happen how they happened and even if you time travel, nothing will change. So, fate then. But you can still try, so free will also. And contrary to all science fiction ever, it is actually quite hard to unravel the fabric of space time, thankfully.

Or I could be completely wrong or exaggerating. Please help!
It's not all sci-fi. There's plenty that show you can't change the past, because you're reacting to things caused by you, 12 monkeys is the first example that popped into my head (as far as I remember it, it's been a while). But this does rule out stepping on a butterfly and ruining things for everyone.

I'm not going to bother trying to get into the detail of the paper, but shalpman's right, just 'cos a theorist has a theory, it doesn't make it so. Still, it's fun to think about.

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basementer
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Re: Grandfather paradox

Post by basementer » Fri Oct 02, 2020 4:39 pm

Kurt Godel showed ages ago (don't know when, but presumably before his death in 1978) that closed timelike trajectories could exist in general relativity. This paper might not be as newsworthy as an average journalist or PR department think.
Money is just a substitute for luck anyway. - Tom Siddell

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shpalman
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Re: Grandfather paradox

Post by shpalman » Fri Oct 02, 2020 4:58 pm

basementer wrote:
Fri Oct 02, 2020 4:39 pm
Kurt Godel showed ages ago (don't know when, but presumably before his death in 1978) that closed timelike trajectories could exist in general relativity. This paper might not be as newsworthy as an average journalist or PR department think.
According to wiki he showed that CTCs could exist in a particular metric; "Its definition is somewhat artificial in that the value of the cosmological constant must be carefully chosen to match the density of the dust grains"

https://en.wikipedia.org/wiki/G%C3%B6de ... rpretation
Following Gödel, we can interpret the dust particles as galaxies, so that the Gödel solution becomes a cosmological model of a rotating universe. Besides rotating, this model exhibits no Hubble expansion, so it is not a realistic model of the universe in which we live, but can be taken as illustrating an alternative universe, which would in principle be allowed by general relativity (if one admits the legitimacy of a nonzero cosmological constant).
Just because it is allowed in general relativity doesn't mean it's allowed in our universe.
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basementer
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Re: Grandfather paradox

Post by basementer » Fri Oct 02, 2020 5:08 pm

shpalman wrote:
Fri Oct 02, 2020 4:58 pm
Just because it is allowed in general relativity doesn't mean it's allowed in our universe.
Agreed.
Money is just a substitute for luck anyway. - Tom Siddell

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JQH
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Re: Grandfather paradox

Post by JQH » Fri Oct 02, 2020 9:40 pm

basementer wrote:
Fri Oct 02, 2020 5:08 pm
shpalman wrote:
Fri Oct 02, 2020 4:58 pm
Just because it is allowed in general relativity doesn't mean it's allowed in our universe.
Agreed.
I'm reminded of the Larry Niven short story in which the universe did indeed not allow* the construction of devices** that created closed time-like loops.

*Your sun went nova if you tried it

**large rapidly rotating cylinders the density of neutron stars
And remember that if you botch the exit, the carnival of reaction may be coming to a town near you.

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sTeamTraen
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Re: Grandfather paradox

Post by sTeamTraen » Fri Oct 02, 2020 9:49 pm

tl;dr: Mathematicians write a paper and their uni has a creative PR department.
Something something hammer something something nail

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basementer
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Re: Grandfather paradox

Post by basementer » Fri Oct 02, 2020 9:56 pm

JQH wrote:
Fri Oct 02, 2020 9:40 pm
basementer wrote:
Fri Oct 02, 2020 5:08 pm
shpalman wrote:
Fri Oct 02, 2020 4:58 pm
Just because it is allowed in general relativity doesn't mean it's allowed in our universe.
Agreed.
I'm reminded of the Larry Niven short story in which the universe did indeed not allow* the construction of devices** that created closed time-like loops.

*Your sun went nova if you tried it

**large rapidly rotating cylinders the density of neutron stars
That came to my mind too, although I couldn't remember the author.
Money is just a substitute for luck anyway. - Tom Siddell

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Martin_B
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Re: Grandfather paradox

Post by Martin_B » Sat Oct 03, 2020 1:06 am

monkey wrote:
Fri Oct 02, 2020 4:28 pm
I'm not going to bother trying to get into the detail of the paper, but shalpman's right, just 'cos a theorist has a theory, it doesn't make it so. Still, it's fun to think about.
Ahem. Just because a hypothesist has a hypothesis, it doesn't make it so. If it's a theory then it's a hypothesis which has been proven (or at least hasn't been disproven!) ;)
"My interest is in the future, because I'm going to spend the rest of my life there"

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