Gravity slows down time. Therefore time passes faster for my head compared to my feet, "adding up to approximately 90 billionths of a second over a 79-year lifetime." I understand this isn't much, but it's not nothing.
Now, I'm fine with the typical example of time dilation: a clock on Earth will tick faster than a clock in a fast-moving spaceship. Once the spaceship comes back, the clocks can be compared and the discrepancy observed.
But I have trouble picturing the fact that discrete parts of a single object can experience time differently. How does it impact the communication between, let's say, my brain and the nerve endings in my toes? What are the consequences of my head being older than my feet?
What happens in a tall grandfather clock, where the pendulum affects the hands? How can the lights stay on on the top floor of a skyscraper when the electricity comes in from the basement?
Best Answer
The difficultly you have imaging it could be because you imagine time like a series of panels in a cartoon and wonder if you are missing a foot in some panel.
But better to imagine a 4d space that labels events. An event is a combination of a place and a time. For instance the event when-where a light goes on, or the when-where a section of DNA unzips so that it can replicate.
The universe is made of such events. And they don't particularly care what labels you assign them. They don't care if you call an event origin or whether you labeled it as later or before the origin or if you labeled it as being in the x direction of the origin or in the y direction of the origin. Perhaps there is no natural path between one event and the other.
What you need is that you have events and that they affect each other. Take the DNA unzipping. It does this because molecules around it get close enough to make it do so. How do they get close? Firstly the parts to make it could be nearby or it could be part of something else nearby or it could be moving towards that area and inertia carries it on.
If it is inertia then it arrives when-where it does because of the speed it had. If it is something larger falling apart then it depends on the rate things fall apart (which depends on the things around it) and if it forms then it depends on the rate thibgs form (which depends on the things around it). But no matter what the things start to happen at one event (at one when-where) and finish at another event (another when-where) and while the labels don't matter that curve with a fixed endpoint and a fixed starting point and a whole path on 4d spacetime it has a length. And that length is determined by the geometry. So that is where gravity comes in. Those rates are measured against the 4d length of that curve.
And that curves length isn't the length it looks like in a picture it is the one given by the equations.
Which means there is a nice way to visualize it. Imagine that everywhere along spacetime there are all these curves. And that things won't do their things until they get paid money. And imagine that the money they are paid depends on this length, not the visual length, but the one given by the equations.
So its like you pay people less when deeper in gravity well so they do everything less quickly. Radioactive particles decay less quickly. Cells repair themselves from radioactive damage less quickly. Brains register pain less quickly. Signals travel less quickly. Because every measure of rates is affected by the lengths if these curves.
So your foot feels like signals from your head are arriving more often than they do because the foot is acting slower. And the signals from the foot to the head seem to arrive less often than the do because the head does things faster.
With this idea of everything waiting to get paid you can see that we have no idea who is getting paid the normal rate and who is underpaid. We just know that each one only does things when it has been paid enough and that curves inside you foot and curves inside your head have to measure based on the geometry then.
So you head feels like there are more breaths in your lungs per thing it does and you for feels like there is less. Same with beats of tour heart. They might feels like you are eating less or more often than it does other things.
But that just means the feedback they give about whether they are getting oxygen compared to how much they are using is based on that. Blood can circulate and those cells that need it most can get ready to grab it and if the amount coming back it too depleted that can signal your lungs to breath more often. You lungs don't have to keep track of it was because you head was working harder and so more cells there needed oxygen or whether it was because you stood up and so your feet were now in a deeper well.
All the things that affect your body affect your body not just the rates of time. Just like if you paid different regions different amounts or they had less or more holidays. So your feet take less oxygen because they do everything more slowly. But they also multiply more slowly, repair more slowly, and so on. Since the rest a bit slower your brain when learning to control them it learned about that, just as it learned about the exact amount of time it takes to send a signal through the specific paths ways from your brain to your foot.
Just as you'd learn if another country had way more holidays you'd learn to send packages sooner. But this is a small effect compared to a cell just being farther away and your body is adaptable.
So you deal. You just have to accept that every single thing happens slower the good and the bad. And these effects will be totally swamped by bugger effects like a bacteria getting in the way. So you won't notice.
Mostly I tried to address the cognitive bias that might have been making it difficult. Every last of your body is someplace at every time, but the rate that they react to things depends on the path they take.
There are other similarly small effects. There is a Doppler broadening of binding energies because of the special relativistic time dilation from the motion. Bit that is covered in what I said, the special relativity notion requires that it move through spacetime so the length of the path takes the notion and the gravity well into account.
So increasing the temperature of your body also makes it age slower, but when that happens it can also break things by making them hit harder against each other.