off on a rocketship at a constant speed for a while, then turn around and come back.

We know that moving things experience time more slowly, so I'll think that when you get

back, you should be younger than me.

But from your perspective, the earth (with me on it) is doing the moving, receding and

then returning, so you think that I should be younger than you.

Who's right?

We'll use the fact that time rotates to sort this all out.

Ok, so from my perspective, every second that passes I stay in the same place, while every

second that passes, you get farther away, and then closer.

Simple enough.

From my perspective, you'll take ten seconds to get back.

And since you're moving, I'll think that time is passing more slowly for you, so I'll calculate

that your journey, for you, takes eight seconds.

Now – and this is the important bit – since you're moving, what you think of as the forward

direction of time will be rotated relative to _my_ perception of time.

So on your outward journey, the seconds will tick away like this.

And on your return journey, the seconds will look like this.

From your perspective, your journey does indeed take eight seconds!

But almost immediately, we also see the solution to the twins paradox: right here.

This bit of my time is unaccounted for by you.

During your entire journey, you'll think that time is passing more slowly for me than for

you (and indeed it is – here, and here, add up to only 6.4 seconds), but because of

your change in velocity when you turn around to come home, your notion of time rotates

and skips right over a large swath of my time.

Which amounts to precisely – you guessed it – the missing 3.6 seconds.

And this is the resolution to the twins paradox: because you changed velocity, your notion

of simultaneous times rotates, so your accounting of how time passes in parts of the universe

far away from you will have gaps in it.

Well, in reality it wouldn't have gaps, because you couldn't instantaneously change direction

– you'd have to fire your rockets to start heading home, and during that acceleration

your notion of time would have very very quickly rotated through the missing gap in my journey,

allowing you to properly account for the missing time.

In summary: during your outward and return journeys, ten seconds would pass for me, and

I'd calculate eight seconds as passing for you.

Eight seconds _would_ pass for you, and you'd calculate 6.4 seconds as passing for me during

your outward and return journeys, and 3.6 seconds as passing for me during your acceleration

(even if it was basically instantaneous).

So we both agree that when you come home, you'll be younger!

And indeed, this is what happens when you send an atomic clock flying around in in an

airplane: it records less time as having passed than a twin atomic clock that stays on the

ground.

PS the time rotations I've been talking about are actually called "Lorentz Transformations",

and they're the way that most working physicists think about special relativity and things

like time dilation, relativistic doppler shifts, and so on.

Trying to understand relativity just by using basic equations for time dilation and length

contraction (like is often done in beginning physics classes) will often lead to confusing

apparent contradictions, because they don't take into account the full changing of simultaneity

of events, and so on.