It really really is.
Funyons, anyone?
Some experiments were done where a jet with an atomic clock aboard was flown around the world, possibly more than once. The clock aboard was slightly out of sync (behind?) a control clock left on the ground. So it’s not just acceleration, but relative motion.
Imagine driving north along a road. You’ll arrive at a certain lattitude north of you in a certain amout of time. Then imagine that you turn off onto a road going northwest. Now you’re travelling in two directions – you’re still going north, but you’re also going west. It will take longer to reach your target lattitude. You can’t add another direction without stealing a little something from another direction. Add a third dimension to this and there’s another direction possible to steal from the other two. Since time is the 4th dimention, you can’t travel in any direction without stealing something from that. So any motion slows down time from the perspective of someone observing the traveller.
Or something like that. I got this from Brian Greene, who I tend to believe.
That is basically correct, tdn, although in the case of the atomic clock flown around the world, there are technically the two competing effects, one is special relativistic due to the plane’s velocity, and the other is general relativistic due to its height above to the ground (a different position in the earth’s gravitational potential). This is all explained here. In any case you are correct that relative velocity plays a role in time dilation, and your heuristic description is basically right. But I do think that therelativity of simultaneity has more relevance to your original argument than time dilation per se.
Since, IIRC, you’re a real live physicist, would Elliot the ET, walking around his moisture farm really add up to hundreds of years because of the vast distance and/or velocities, or does this become moot or indeterminable due to relative simultaneity?
iamnotbatman, thanks for the explanation and the links. I will try to read them without melting my brain.
And they say stoners are unsophisticated and lazy. Pfff!
Neither of the two pictures is exactly right. Relative velocity is all you need to have time dilation, but the magnitude of the inflation does not increase with (or depend in any way on) the distance to Elliot. So, the effects of Elliot walking around his field are no different from the effects of Farmer John up the road walking around his field, if we assume the two home planets themselves don’t have any relative motion between them (which they would in practice, but that’s unrelated relativistically to the absolute distance between them).
(ETA: I was fascinated by how hard a time my brain had calling the alien Elliot in this thought experiment. Elliot is supposed to be the Earth-bound boy, not the extraterrestrial being!)
going back to the OP isn’t there still disagreement about how it is that an airplane can actually fly? Or rather, how a plane’s wing generates lift?
Yep, okay, that’s what I was thinking. As for the Elliot reversal… such are my ways! :evil:
It is perfectly well understood. Some people are just a bit confused and the standard explanation that they give in grade school is wrong.
See this recent SD discussion:
I love that particular comic. And, whooooosh? 
Basically, if Elliot is walking around slowly, and he does so for a short amount of time, then no, there will be no large relativistic time-dilation effects. As Pasta said, the important thing is relative velocity. (Although nit-pick: as the Lorentz Transformations here show, there is a term “vx/c^2”, so the distance between Elliot and me can indeed matter, but in practice this is a very small effect for small velocities unless Elliot is walking in one direction on his farm for thousands of years between the events that are being compared by the two observers)
If relative velocities cause time dilation, then wouldn’t opposite sides of the Earth be different ages?
Well, the earth is not an inertial reference frame, so the equations of special relativity are not applicable to that situation. The two sides of the earth are constantly accelerating towards one-another. Of course we know straight-away by symmetry that clocks on opposite sides of the earth must agree.
Except that they do, and here’s the proof: http://www.acoustics.salford.ac.uk/acoustics_info/duck/
Not time dialation as such but rather, as posted upthread, observers in relative motion can disagree on whether two events happened simultaneously or not.
An extreme example is the thought experiment of fitting a forty-foot pole in a thirty-foot barn: if a forty-foot pole is traveling close to the speed of light, relativistic foreshortening will cause it to appear to be contracted in the direction of travel, meaning that it could (very) briefly fit in a thirty-foot barn. But what about the pole’s frame of reference? In that frame the front and back ends of the pole aren’t in the barn at the same time.
My favorite version of this is the “pole in the universe” paradox (see here) in a universe with periodic boundary conditions. Here the runner is carrying a pole the size of the universe, so that the front of the pole is joined with the back of the pole (the pole is like a giant hoop). What happens when the runner starts moving and sees the universe length-contract so that the pole no longer fits inside the universe?
Sound goes out, no sound comes back in, never a miscommunication. You can’t explain that!
A universe with a periodic boundary condition has a preferred reference frame, and so globally violates the postulates of Special Relativity.