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Comment by jbotz | Hacker News Reader

This a key fact that a lot of people seem to misunderstand, even a lot of popular science journalists trying to explain relativity. But to me it really illuminates the connection between space and time and the inviolability of c. From your own perspective you can travel arbitrarily fast in terms of how much you age inside your spaceship... but as you move faster in space, time in the the Universe outside your ship appears to be moving faster as well such that always more than a year will have passed outside for every lightyear you've traveled, even if it was only days, hours, or minutes for you.

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Vernor Vinge does a great job with this in A Fire Upon the Deep and A Deepness in the Sky. I do wish that he had written more about Pham Numen and the Qeng Ho, back in their Slow Zone heyday.

Alastair Reynolds also does a great job with this in his Revelation Space trilogy, with his lighthuggers.

One of the most interesting parts to me of Pham Nuwen's character was his relationship to his distant descendants.

Humans just aren't wired to consider family members beyond 3 or 4 generations up and down.

Perhaps I'd feel the same general apathy as Pham Nuwen 20 generations down. Then again, he had other familial issues that played a part.

One started with him hiding in a nursing home, after faking his death. I don't recall specifics, but I suppose that it's possible that all of the other characters are his descendants. And they had spent a long time looking for him.

In the other, he'd been resurrected by some godlike AI as a meatspace agent. Millennia after the Qeng Ho era, I think.

I have yet to read Vinge's books, but Joe Haldeman's The Forever War is an excellent book that also makes good use of this effect.

I absolutely recommend all of these. And of course Reynolds's "House of Suns", which sort of takes the "lighthugger" concept up to eleven.

the way i put it is that relativity imposes no limit on how soon you can get from point A to B; it only limits how soon anyone else can do it.

Isn't it the other way around? From the perspective of the spaceship rider, time outside the ship appears to be moving more slowly. That is, as the spaceship rider flies by earth, she will observe earth-bound watches ticking more slowly than her wristwatch.

It's both actually. But that gives an apparent contradiction - how can both observers see that the other party has slower clocks? Who is 'right'?

The answer is that the 'moving' traveler needs to flip the ship around at some point and thrust back towards the starting point. Whoever turns back around will find that it's their clocks that passed more slowly.

And if the universe is spherical?

I think the question you are asking is what if the universe is toroidal. What if you traveled so far you came back around.

You come back around on a sphere too, e.g. traveling by the equator.

In that case you would be traveling a curved path so there would be acceleration even if your speed remained constant.

Isnt a torus curved? What's the argument?

If you use toroidal boundary conditions then you could remain in an inertial reference frame and return back to the same pont.

See https://en.m.wikipedia.org/wiki/Periodic_boundary_conditions

My instinct is that as you fly toward Earth at near the speed of light, earth-bound stopwatches will appear to be going nearly double their normal speed. As you fly away from Earth, the stopwatches will slow to a crawl approaching standstill.

This illustrates a fundamental misunderstanding of time dilation. Direction has no impact, and this example that is often used is terrible for illustrating what is being proposed re. time dilation, because it just has to do with the perception of time as it relates to light reflecting off of a clock within certain frames of reference.

I might be misunderstanding your comment, but you seem to be saying that the change in time is only actually thebvissible perception?

If so, that is not correct. Atomic clocks have measured actual, lasting time differences.

I was saying the opposite. The claim of time dilation has nothing to do with the visual perception of clocks, which is a common (flawed) example that is used. The visual perception of clocks differing by heading towards or away from a clock is just a phenomenon akin to the Doppler effect.

If you correct for this effect caused by changing distance between the parties, you will still observe a difference in passage of time in the two different inertial frames of reference (each observing the other). In both cases the party taking the measurement will observe that time is passing more slowly in the frame of reference that the measurer sees as moving with respect to the measurer's frame, regardless of whether that motion is toward or away from the measurer.

What happens if both parties are moving? They start back to back, fly a distance, both turn around and meet.

Each should see the other as dilated, no?

There is no difference between you moving with velocity X and the other party moving with velocity -X; or you moving with velocity 2X and the other party being stationary; or you being stationary and the other party moving with velocity -2X. There is no experiment you can perform that will tell the difference.

You missed the point that both parties accelerate, we are not talking about constant velocity here.

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