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In my story, those who are transported back in time do not age if they have to live through the years in the past. You don't start aging again until you reach the point in time you left.

I was curious though, in this situation, would it still be feasible for one's hair and nails to grow (basically meaning they'd still have to groom themselves)?

What would have to happen for these two facts to coexist? Ir perhaps I'm overthinking this.

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    $\begingroup$ What does "age" actually mean? Biologically, it is the decay of the body, caused by permanent cell death and insufficient recreation. If you stop this, hair wont grow. But also no healing will take place. So you should reconsider why you dont age or what you mean by it. $\endgroup$ – thst Jun 12 '17 at 7:04
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    $\begingroup$ If your hair doesn't grow you go bald. $\endgroup$ – Pieter B Jun 12 '17 at 8:47
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    $\begingroup$ You seem to be looking for something scientifically plausible, but what is the basis for your original premise that people don't age? $\endgroup$ – user16107 Jun 12 '17 at 10:15
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    $\begingroup$ I suppose the notion underlying the premise is that aging is a process which is continuous in time, starting at a person's birth, and that no ageing takes place outside that time interval. The categorical problem with that idea is that actually no time should pass subjectively at all. That would mean that the body "freezes" and does not change at all and only "thaws" once it has reached the point in time from where it traveled. The hybrid of "time passes and physics work as usual for the time traveler, s/he simply doesn't age" is unworkable. $\endgroup$ – Peter A. Schneider Jun 12 '17 at 10:58
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    $\begingroup$ If your time travel mechanism is magical, you are overthinking it. You can just handwave it as a consequence of the magic for time-traveling. If you time-travel mechanism is non-magical (just pseud-scientific), you'll have a really hard time explaining this without blasting away suspension of disbelief. $\endgroup$ – T. Sar Jun 12 '17 at 11:47
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Aging is related to the accumulation of DNA damage in the cells.

Hair growth is simply a physiological process, like also red cells production in the blood stream and skin renewal.

If you stop all the physiological process I think it is going to be pretty hard for your characters to survive in the past more than few weeks: no skin renewal mean no wound can ever heal, just to give one consequence.

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    $\begingroup$ So I think this answers my question. I can have a lack of aging (as it relates to the accumulation of DNA damage in the cells but still have all the rest of the physiological processes. $\endgroup$ – Olandir Jun 12 '17 at 7:22
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    $\begingroup$ @Olandir most aging theories currently involve telomeres which are like "end caps" for DNA chains. They get shorter and shorter as we age, and it's thought to be the reason our DNA starts to have trouble replicating itself as we get older. If telomeres could replicate without eroding, we could theoretically regenerate our cells forever. learn.genetics.utah.edu/content/basics/telomeres $\endgroup$ – wetcircuit Jun 12 '17 at 8:42
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    $\begingroup$ @Olandir, growing hair and nails is more like pooping than aging. So if your character does that, the hair and nails should also grow. $\endgroup$ – Syzygy Jun 12 '17 at 9:03
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    $\begingroup$ +1, and I agree. The body has a lot of elimination points for various toxins; hair and nails serve other purposes but are also used as garbage dumps: We can see toxins like heavy metals being eliminated in them. Same goes for ear wax, tears, sweat, urine, feces. You also have sperm and egg production, and neural production. QUESTION: I understand if the cells don't age: Can they be killed? Can new living individuals be created? If two time travelers mate can they procreate? Does baby age? Healing creates new living cells, also needed for replacing lung and other linings daily. $\endgroup$ – Amadeus Jun 12 '17 at 9:36
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    $\begingroup$ You'll probably die much faster. The reason your stomach does not burst open is because the stomach lining repairs faster than it can be digested. If this stops, you'll digest a hole in your stomach. $\endgroup$ – Nelson Jun 12 '17 at 10:21
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I would suggest that you're overthinking this. Your premise is insufficiently detailed for this question to need answering for your story. The part of your world that you have lots of detail on - go into detail on them. The part of your story that is more hand-wavy: stick with "normal" for details you're not covering, because "normal" will not feel out of place for your readers. So in this case, I would assume your character does grow hair and nails.

Think of it this way. Do you think that T.H. White had an explanation for this, for Merlin? No, not in the slightest. And if it was interesting to write about, he would've written about Merlin combing his hair or trimming his nails just like anyone else.

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    $\begingroup$ I think this is right. Not only is explaining these details not interesting/not needed, but it also draws attention to the implausibility of the story's premise. Sometimes a rather farfetched premise can be the basis of a good story, but in that case you want readers not thinking about it. $\endgroup$ – user16107 Jun 12 '17 at 15:04
  • $\begingroup$ Someone's excessive care in grooming habits can be explained by mages -- if you include belief in magic as part of your societal norms -- using hair and nail clippings as a focus for a spell used to control him or her. Vodoun (voodoo) and Santeria have "voodoo dolls" which need a connection to the victim; often hair and nail clippings (and fresh blood) are used to establish that link. This was used in one story in a negative way. Characters burgled a resident's home to get hair or nail clippings. They found nothing that could be used, suggesting the resident was familiar with magic. $\endgroup$ – Yoshi Bro Jun 17 '17 at 1:14
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Assuming that one's body remains constant throughout time travel, you would not age when you jumped to a different time. At least, that's how it is presented in sci-fi - the person steps into the machine, and pops out exactly the same in a different time.

Warning: reality might be quite different from stories that sell well. In which case, all bets are off. Relativity suggests that as a person approaches the speed of light, time slows down for that person, to where a one month journey for you the light speed traveler might be a 50 year time span for everyone else.

However, that's a one way trip. Relativity suggests you might be able to fast forward, but not rewind.

Note than in a lot of popular sci-fi, people don't travel at light speed, they instead warp space/time, so they avoid that paradox completely by just not physically traveling very far.

Oh no, I've gone crosseyed...

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