# B Are time loops possible?

#### TheQuestionGuy14

I haven't been on here in a while and have forgotten a lot of my physics knowledge, which is annoying.

I just want to know, are time loops possible? Like the ones that appear in movies where a character relives a period of time. Is this allowed by the laws of physics? I know it's not really a physics question but this stuff is important to me, if someone with knowledge of physics is able to explain it. Thanks.

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#### Mister T

Gold Member
Do a google search for closed timelike curves.

Note that the worldline of a person is a timelike curve, so if it were close it's hard for me to make sense of the concept of time.

#### Ibix

No.

Mathematically, we can describe "loops" in spacetime where you can pass the same event twice, but that doesn't operate like Groundhog Day. Every other time you went round the loop would be present every time. And all the potential for paradox that implies lead Novikov to propose his consistency principle, which boils down to "you can't change anything".

Furthermore, although we can describe such structures mathematically, the conditions necessary for them to actually exist are impossible as far as we know (negative energy densities and the like).

#### TheQuestionGuy14

No.

Mathematically, we can describe "loops" in spacetime where you can pass the same event twice, but that doesn't operate like Groundhog Day. Every other time you went round the loop would be present every time. And all the potential for paradox that implies lead Novikov to propose his consistency principle, which boils down to "you can't change anything".

Furthermore, although we can describe such structures mathematically, the conditions necessary for them to actually exist are impossible as far as we know (negative energy densities and the like).
Interesting. So, what's really happening in the movie Groundhog Day? Is it anything of physics or just magic?

#### Nugatory

Mentor
Interesting. So, what's really happening in the movie Groundhog Day? Is it anything of physics or just magic?
Fun movie, zero physics content.

#### Ibix

Interesting. So, what's really happening in the movie Groundhog Day? Is it anything of physics or just magic?
Magic. I have trouble imagining a physical model that could measure Bill Murray's character development and stop the loop when he had done enough to get the girl.

#### TheQuestionGuy14

Fun movie, zero physics content.
Ok, I understand, but I know this sounds crazy, but my friend keeps insisting he lived a day multiple times like the movie Groundhog Day. Why would he say that?

#### Ibix

Why would he say that?
Drunk, stoned, delusional, pulling your leg?

Remember Occam's Razor.

#### Nugatory

Mentor

"Are time loops possible?"

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