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Comment by dd444fgdfg | Hacker News Reader

It's hard for me to have a useful mental model on this subject. As we know from the article, time passes differently for the top than the bottom, and this effect is in everything including you and me (obviously). What I find hard even less intuitive to grasp though, is whether or not this is cumulative. It should be right? 0.00000000000000001% slower on one side, well that adds up quickly. It can't be cumulative or my feet would be a lot older than my head, but I don't get how that's not true.

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Back of the envelope calculation. Say you are really tall, 2 meters. That's "only" 2000 millimeters. So even additively, that's still only 0.00000000000002% difference between your head and feet. Let's say our two meter giant lives 100 years. That's a bit over 3 billion seconds, and for their entire life, their head and feet continue to age differently at this rate. If I haven't dropped a decimal point somewhere, that's still only a life time difference of less than a microsecond (6x10^-7).

So "adds up quickly" is relative (heh) in this case, and inconsequential in absolute terms, at human scale.

You’re feet are older than your head for relativistic reasons, as you say. But not “a lot” older.

yeh it's the "lot" bit I'm confused about. like, aren't they always diverging, therefor the age difference should be a lot, right?

How much is a lot? The parent comment came with 6x10^-7s over 100 years and for 2m tall people. I haven't checked the calculation, but we'll go with that. Is that a lot? If yes, why? Or if not, why not? Let's say it's not 6x10^-7s, but 10s over a lifetime. Is that a lot? 1 minute? 1 hour? So what if it's 1 hour?

This difference causes tidal forces in your body. These tidal forces are minuscule compared to any other everyday interaction that you do. I haven't done the calculation, but I would wager that the tidal effect of even 1 hour over a lifetime is completely dwarfed by everyday interactions and effectively invisible in human everyday life.

Even if it were, there's trillions of cells in your body. The time differences would still have to be incredibly pronounced for it to matter at this scale of entropy.

well, what does pronounced mean? how much time difference is too much?

when I give it more thought, it feels to me that when we say time, we're really say how fast things move relative to other things, and that makes it easier to reason about how different parts of your body are not upset about experiencing time differently

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