Time Travel and the Anthropic Principle

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tomandlu
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Time Travel and the Anthropic Principle

Postby tomandlu » Tue May 05, 2015 3:55 pm UTC

In my book(s) there is FTL and, consequently, time-travel. However, paradoxes are not allowed due to the Novikov principle (which is a lot of fun to play with when there are concious entities involved).

At the end of the book (err, spoilers?), the main character meets an alternative version of himself who has lived for several billion years, who says to him something like:

"Do you know why intelligent life exists?"

"Luck, I suppose," said Louie. "Although there seems to be quite a lot of it around."

"It exists because, without it, the universe couldn't. For instance, do you know how difficult it is to grow grapes if pi is a different number? Now, who do you think sorted that out?" And the old man, looking very pleased with himself, ate another grape.

"So you're god?" said Louie, unable to keep the mockery from his voice. "You travelled back in time and created the universe?"

"I've lived a very long life, and I've only learnt two things with true certainty - firstly, that however long you live, you will remain a fool and, secondly, if gods exist, they are conspicuous only by their absence." The old man paused to spit some seeds into the gorse bush. "Personally, I think it's the stupidest idea I've ever heard of, but everyone does keep putting it in the suggestion-box, so I suppose there might be something in it. But no, I am not a god - just someone with a lot of time on their hands. And I didn't create the universe - I just did a bit of the wiring."


So, my question is, does anything about this seem insurmountably absurd and/or illogical as a solution to the anthropic principle? Also, is pi a silly thing to pick? Most of the values associated with the anthropic principle are a bit obscure for the general reader, and I suppose it could be taken as a joke...
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Re: Time Travel and the Anthropic Principle

Postby speising » Tue May 05, 2015 4:33 pm UTC

The value of pi isn't a physical constant. It's mathematical and could be calculated in any universe (with the concept of flat space?).

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Re: Time Travel and the Anthropic Principle

Postby tomandlu » Tue May 05, 2015 4:38 pm UTC

speising wrote:The value of pi isn't a physical constant. It's mathematical and could be calculated in any universe (with the concept of flat space?).


Well, I was thinking more that if pi was, e.g., 3, then the universe couldn't form in the first place (or would form in such a mad fashion that life couldn't exist, or much else either).
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Re: Time Travel and the Anthropic Principle

Postby Sizik » Tue May 05, 2015 4:41 pm UTC

I don't see how you could have a (consistent) universe where 3.14159... = 3.
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Re: Time Travel and the Anthropic Principle

Postby tomandlu » Tue May 05, 2015 6:28 pm UTC

Sizik wrote:I don't see how you could have a (consistent) universe where 3.14159... = 3.


Me neither - that's rather the point... Still, as much as the notion tickles me for some reason, I think I'll have to make it clearer it's a joke/metaphor, and have him make some throwaway remark about epsilon or something. That is unless anyone can give me a plausible pi justification - I'm thinking of interfering at the moment of creation, rather than fiddling the numbers after the event.

Or, could you have a different value for pi if the topology was such that all straight lines went down-hill, but curved lines didn't - as though the universe was just an unending series of conic dips at all scales and locations.
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Re: Time Travel and the Anthropic Principle

Postby PM 2Ring » Fri May 08, 2015 5:36 am UTC

Please don't meddle with pi. FWIW, I've read a couple of stories that did that and it annoyed the hell out of me.

As others have said, pi is defined in terms of an ideal Euclidean circle, so it's the same in any consistent conceivable universe, unless you can conceive of a consistent universe where the laws of logic and mathematics are not isomorphic to those in this universe...

Sure, in a non-Euclidean space the circles will behave differently to Euclidean circles, but it's usual to describe circles in such spaces in terms of how they deviate from the Euclidean circle. A key difference is that only in Euclidean space is there a constant ratio between a circle's circumference & its diameter, or between its area & the square of its radius. Even in ideal spaces of constant non-zero curvature those ratios vary with the size of the circle. In more complicated spaces where the local curvature varies from place to place things get even messier, although it's (usually) possible to tackle geometry in such spaces by subdividing them into small regions that are approximately flat.

Note that I've only been talking about ideal geometrical circles here - I haven't said anything about physical approximations to such circles. As mentioned in the thread Pi and Approximations mixing up geometrical circles with physical circular things can be a source of confusion.

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Re: Time Travel and the Anthropic Principle

Postby Neil_Boekend » Fri May 08, 2015 6:10 am UTC

You could take a different constant instead of π. If you take α (α a little different and you have different chemical reactions. α much different and you have no stable atoms or no unstable ones in addition to completely different chemical reactions.) you'd use a physical constant instead of a mathematical one.
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Re: Time Travel and the Anthropic Principle

Postby tomandlu » Fri May 08, 2015 8:17 am UTC

Okay...

"It exists because, without it, the universe couldn't. For instance, do you know how difficult it is to grow grapes if pi is a different number?"

"Pi? How can pi..."

"Or was it epsilon? Anyway, whatever it was, it made soft-fruit a bloody nightmare. Now, who do you think had to sort that out?" And the old man, looking very pleased with himself, ate another grape.


Thanks all.
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Re: Time Travel and the Anthropic Principle

Postby sevenperforce » Sun May 10, 2015 8:07 pm UTC

I like it.

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Re: Time Travel and the Anthropic Principle

Postby jewish_scientist » Thu May 14, 2015 1:37 pm UTC

You may want to replace 'epsilon' with 'light's speed' because the general public is familiar that c is special in some way, just like they know that pi is special. Even though they do not know why its special, what changing it would do or even what it means, they understand that it is different from other values.

Also, you may run into a weird paradox. What if the old version of the character is eating grapes because he remembers that when he was the young version, he saw the old version eating grapes. I realize that that sentence makes no sense, but it is a paradox. This is a list of what happens chronologically, in reference to the main character:

1: Main character born or comes into being (time travel stories usually like to mess with a character(s) origin).
2: Stuff
3: Meets an old man, who is himself, eating grapes.
4: More stuff
5: Remembers that the old man he talked to, who was him, was eating grapes during Step 3.
6: Get some grapes.
7: Meets young man, who is himself.
8: Eats grapes and talks to young man, who is himself, creating the memory of this conversation in the young man's mind.

Looking over that, it still does not seem clear. You could either crack a joke about it (maybe there is a running gag that the main character hates grapes) or just say that the main character forgot about the conversation as he aged.

There is a quote from Albert Einstein that the old man reminds me of. To paraphrase it, 'There are only two things that I believe to be infinite; the universe and man's stupidity. I am not sure about the Universe.'

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Re: Time Travel and the Anthropic Principle

Postby tomandlu » Thu May 14, 2015 4:25 pm UTC

jewish_scientist wrote: (maybe there is a running gag that the main character hates grapes)


Good guess :) (although it's one reference rather than a running gag).

As for the rest, well the old man is several-hundred millenia old amongst other things...

Just thought I'd add... some of this came from a dream I had. I was walking along a country road in India when I came across an old man on the roof of a small shack selling hardware. He came down, gave me a stick, and told me to go and kill the snake that lived in some nearby trees. I went off with the stick and got to an avenue of trees, and this huge snake came down and lifted me up into them. I dropped the stick deliberately, and the snake turned into a lamia and put me back on the ground. I went back to the shack, climbed onto the roof, and waited for a young man to walk down the road.
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Re: Time Travel and the Anthropic Principle

Postby Neil_Boekend » Thu May 14, 2015 8:37 pm UTC

It is a risky event wrt paradoxes but that is a direct result of meeting himself. However, I see no paradox there. Perhaps the old version of the man is eating grapes because he didn't want to cause a paradox.
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Re: Time Travel and the Anthropic Principle

Postby krogoth » Fri May 15, 2015 4:09 am UTC

The bit that'll really burn your noodle is when he does later create it, and leaves the flaws there because they were, but then were nolonger required, but he knew he already fixed those.
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Re: Time Travel and the Anthropic Principle

Postby tomandlu » Fri May 15, 2015 12:02 pm UTC

The fun bit to write is when people realise that they can't change the past, but they can re-interpret it. So, if an unknown stranger snatches your kid, just go back in time and be the one to snatch him...
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