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Comment by ChuckMcM | Hacker News Reader

When I first learned about this in college it really really bothered me. (obviously it still does to some extent)

That time is not constant on a scale that lets us measure it from space vs the earth much less from upstairs to downstairs is somehow deeply unsettling. If you're traveling at 10 m/s as you go out of the gravity well you go faster and faster because time speeds up, but your velocity per unit time stays the same.

Something that is really hard for my brain to wrap around in any meaningful way.

During some radio work this became actually a thing in that if you're doing time-of-flight measurements of originating RF energy and one receiver is in a platform flying above and one is on the ground, at high time resolutions you start having to correct for difference in the rate of time. This is definitely something you have to do if you want to get really accurate time from GPS satellites. So freaky it hurts.

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PBS SpaceTime has a treatise on relativity. There are a dozen videos worth watching on this topic alone. The video about “the speed of causality” is the one that caused me to level up:

https://youtu.be/msVuCEs8Ydo

Here is a 12 video playlist on relativity:

https://www.youtube.com/playlist?list=PLsPUh22kYmNAmjsHke4pd...

Here is a six video series on measuring space and time:

https://www.youtube.com/playlist?list=PLsPUh22kYmNBkabv9M4fX...

It didn’t bother me so much when I finally understood that the duration perceived by an observer is simply equal to the path integral of distance along their worldline.

When you think about it, you will realise there’s almost nothing else it could reasonably be.

I don't get what this means at all. Can you try to Eli5

To figure out how far a distance you traveled to get to my house, I need to know which path you took.

To figure out how long a time you traveled to get to my house, I need to know which path you took.

Our ordinary intuition is instead,

"To figure out how long a time you traveled to get to my house, I need to know the time on my clock when you left and the time on my clock when you arrived."

But that turns out to be wrong if you care about a high degree of precision or if you were traveling very fast.

Also because there is no meaning for the first value (there is no “synchronicity” between separate clocks).

Might be reasonable, but it means that comparing timestamps between two observers requires knowing the observers' entire history back to some reference timestamp no? Still a headache.

Also, if we added the speed of light to an object's speed you'd get really weird results like you'd see a car coming towards you before the car actually arrived.

Trying to stay alive in such a world would be quite a challenge.

All the cars I see coming, I see them before they arrive

Cars are not superluminal.

I meant like this: https://youtu.be/_pEiA0-r5A8

You would see the image of a car before the actual car arrived

How do you feel about it vs leap seconds vs leap smears? I really wish timestamps were tied to TAI instead of UTC time (and therefore subject to leap seconds).

> If you're traveling at 10 m/s as you go out of the gravity well you go faster and faster because time speeds up, but your velocity per unit time stays the same.

You seem to be confusing coordinate time with proper time. The time you experience as you get out of a gravity well is proper time, which, as a sibiling comment said, is something you obtain when you do the path integral of the path you travel. This time is not exactly affected by dilation. Anything traveling with you will not have its time go faster or slower. Anything that is not traveling with you, say stationary on Earth, will indeed have its passage of time shifted with respect to you.

Coordinate time is the time observed by an observer at the center of the gravity well, looking at you (not always, but generally so for most calculations. Also, observer = origin of frame of reference) and that is what is morphed and bent around. Since you're an observer as well, you also see other points experience time differently, since from your point of view, they're moving away from you.

I was pretty bothered just knowing that time is a real thing and not some construct.

That really depends on what you mean with real thing, I would say our understanding of time and space is heading towards them being emergent phenomena and not fundamental ones. As far as we know, there is no thing in the universe called the time that you can just measure, you can only look at the ordering of events. And then we invent a variable called time that lets us express the laws of physics reproducing the observed events in a simple way, but that is much more a convenient mathematical and everyday tool than a real thing than one usually realizes.

Time is absolutely a construct, just like the iPad I’m typing this on is a construct. I really can’t be sure it exists, but the model I have in my head that I’m tapping on a large flat surface with my fingers agrees with my observations. If in my next tap I get a painful burning sensation in my finger and I hear the fire alarm go off I may need to change my model to consider that I was mistaken and I may be touching a what appears to be a flame instead.

Isn't time "merely a man-made method of measuring the motion of the Earth around the Sun?"

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