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Comment by abalashov | Hacker News Reader

Wait, I'm confused ...

"For a crew to make the 50,000-light-year journey to the centre of the Milky Way within 10 years, they would have to travel at 99.999998 per cent the speed of light."

If a light-year is the distance light travels in a year, and the center of the galaxy is 50,000 of them away, then how are you supposed to get there in 10 years by traveling "at 99.999998 per cent the speed of light?" 10 years at the speed of light will take you 10 light-years, not 50,000.

Am I missing something?

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http://en.wikipedia.org/wiki/Time_dilation#Time_dilation_and...

Ah, I see, and suspected something like this implicit in the claim. Duh.

I haven't done the math, but is it really possible for time dilation at or around the speed of light to have the effect of compressing 50,000 years into 10? That's a difference of 5,000 times.

By "possible" I mean "as the theory would prescribe," of course...

I don't know, but the Wikipedia article claims that "a constant 1 g acceleration would permit humans to travel as far as light has been able to travel since the big bang (some 13.7 billion light years)".

So I would guess, hell, yes!

(I recommend "The Forever War" by Joe Haldeman for serious time dilation fun.)

Yes, it is. At light speed, time dilation brings the high-speed observer to see the trip as instantaneous : http://en.wikipedia.org/wiki/File:Time_dilation.svg

You can choose ANY time dilation factor and determine a velocity accordingly, because the graph is asymptotic and continuous.

[EDIT: I mean, obviously, the whole thing breaks down with an observer actually AT light speed; assume above that I'm talking about taking limits of the Lorentz factor as velocity approaches 1]

Not only that, but you could go 2M ly in less than 30 years (ship time), at 1g acceleration. http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.h... for more.

Consider this. It is impossible for matter to travel at the speed of light, yes? However, imagine a cannon on a train travelling (speed_of_light - 10 MPH). The cannon shoots a cannonball at 210 MPH. So when the cannon fires, you'd expect the cannonball to go (speed_of_light + 200 MPH), right? Well, no... matter cannot exceed the speed of light. What actually happens (afaik) is time itself is altered as one approaches the speed of light. So the closer you are to the speed of light, the more time is altered.

I don't know any of the details though.

"It is impossible for matter to travel at the speed of light, yes?"

isn't this an oft quoted erroneous conclusion?

I thought the correct statement was that matter could not accellerate to the speed of light. Thus something could instantaneously attain FTL/Light Speed velocity or come into existance travelling FTL/Light Speed?

Sorry, no idea. I don't know any details.

matter could not accelerate to the speed of light

That was Kevin Spacey's claim in the movie K-Pax.

For all I know, we're falling through time at the speed of light.

What still blows my mind is the fact that the Speed of Light is a constant. Meaning: if you're on a train moving a 0.9c, any light emitting from you will STILL travel at c in your reference frame, while at the same time an observer outside of the train will also see the beam of light travelling at c.

As for time dilation, it's a very simple formula called Lorentz Contraction: http://en.wikipedia.org/wiki/Length_contraction. At 0.99996c, you can cross the Milky Way (100,000 ly) in one year.

It depends on which frame of reference you're measuring from.

I assumed the claim was being made from one external to the spacecraft.

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