How a clock in outer space moves more quickly than a clock on Earth
In: Physics
In physics 101 you’ll learn about something called “relativity”, or more properly “classical relativity”. The idea in a nutshell is that motion is not absolute, it’s something that only exists as a comparison, *this thing relative to that thing*.
Let me give you an example – you’re floating in black space, you can see nothing just pure black space. Suddenly I appear and slowly float by you, I wave, you wave, and I keep floating on by into the distance.
Who was moving in that example? Were you stationary and I floated by? Was I stationary and you floated by? Were we both moving a little bit? Maybe you and I were both actually fired from cannons moving insanely quickly and you were just a tad bit faster giving the appearance that I was moving past you? *It doesn’t matter*. Neither of us had, by ourselves, a property of ‘motion’ motion only exists as the comparison between us and we could say “I was moving *relative to you* by X feet per second”.
So we had this math for centuries and life was good. But here’s the trick – Newton didn’t have lasers and atomic clocks and computers to measure things insanely accurately so it was all round numbers for him. Come the late 1800’s when were did start having at least fancier equipment we discovered the “classical relativity” equations *didn’t work*. There was “missing energy” and that was a problem.
In a nutshell Newton said if I push on something ‘this’ hard it’ll move ‘this fast’. If you push 2x as hard, it’ll move 2x as fast. What we learned was if you push 2x as fast, it’ll move 1.99999999x times as fast and that doesn’t work for physics, you can’t have missing energy.
What Einstein proposed was that that missing 0.000000000001x of motion basically became motion in *time*. That when you push on things, and they move faster you’re actually pushing them *into a different time*, sorta.
So cutting this short, it’s not that “being in space” makes clocks weird, it’s that we can make things in space move really really really really really fast compared to something on Earth. So if I made 2 clocks and synchronized them perfectly and kept one in New Jersey and the sent the other up into space on a satellite and made it orbit the Earth at a bazillion miles an hour *relative to the New Jersey clock*, the two clocks are now in “in different times” just like they are moving at different speeds.
And yes, we’ve done this exact experiment as I suggested and it does work exactly as I suggested. To make it observable the relative different in motion needs to be HUGE like thousands and thousands of miles an hour different and/or over a long time. I think when we really did the experiment the two clocks were only out of sync by a fraction of second.
Imagine you and a friend stand together in a big open field and one of you starts sprinting as fast as possible due East and the other due north. You are both running as fast as possible and are equally matched so you are both running the same speed too.
As you sprint due East, all of your motion is to the East – you are not going north at all. Your buddy? All of his motion is to the north, he’s not moving to the East at all.
Maybe instead you sprint toward “2 o’clock” and he sprints toward “1 o’clock” and so this time you are both making progress in both the north and east directions. You’re both running the same speed, but you are moving faster to the east than your buddy but slower to the north. Your buddy is moving faster to the north but slower to the east.
Now take this big open field and call it “spacetime” (one word) and instead of “east” the direction is called “time” and instead of “north” the direction is called “space”.
All of us and everything around us – you, me, your cat, your toaster, are all at once moving through “spacetime” at a constant speed – the fastest speed possible (called “the speed of light”) and all of that speed is split between space and time (east and north so to speak).
So the faster something is moving through space (“to the north”) the slower it is making progress through time (“to the east”) and vice versa.
So it’s not that the clock (“time”) moves differently in space simply because it’s in space, rather, because it is orbiting and moving through space much faster than the one stuck on earth, less of its motion is through time and so the clock ticks slower. (Gravity and space curvature play a role too but this is ELI5)
We don’t really have an understanding of how it does this. It’s just a thing we have predicted and then found to be true. So the how or why is unknown, but it moves slower because that is how the universe works. It’s a strange world and it isn’t always intuitive.
Basically, a strong gravity source, like our earth’s core will slow down space-time. They have taken cesium (or atomic) clocks in pairs (to prove the result was not just getting out of sync with each other), and taken one pair really low, like sea level, and another pair really high up into an observatory. When they were both brought back to the same level, the pair from the observatory were shown to be ahead of the pair from the lower height. Admittedly they were fractions of a second off, but the point still stands. Now if we change the high point from an observatory on a mountain to orbit around our planet, that pair would be even more ahead of the sea level pair, even though they were in sync at the beginning of the experiment.
This information we can then extrapolate to a black hole, something that has gravity so intense that not even light can escape it. As long as we do not go past the black hole’s event horizon (the edge of the gravity pull where light no longer can escape) a clock there will move so slow that we might see years pass before they see a second pass.
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