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Comment by TrainedMonkey | Hacker News Reader

I was wrong, see edit below.

No, there is still an lower limit of 4.37 years to get to alpha-centauri within any frame of reference no matter how much you accelerate within conventional space.

What relativity does, is that if you travel close to speed of light time taken for the trip as measured on ship versus on earth could be different by many orders of magnitude.

edit: Was wrong, as per [0] "This is something many readers don't understand well, so it bears repeating: The journey times as experienced by those on the ship are not limited by the speed of light. Instead what they experience is the planetary reference frame getting relativistic."

[0] http://en.wikipedia.org/wiki/Space_travel_using_constant_acc...

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That's not true. From the perspective of the travelers space will appear to contract dramatically - though the speed of light remains constant, the distance traveled is reduced.

lfuller is right.

For example, if you have a ship capable of continuous acceleration at 1g, you can reach any point in the universe on the order of ~50 years.

Can you continuously accelerate at g for 50 years without going over the speed of light? According to wolfram alpha http://www.wolframalpha.com/input/?i=speed+of+light%2Flittle...

it would take a year to get to the speed of light at 1g (if my equation was right.) Then you can't speed up anymore.

> it would take a year to get to the speed of light at 1g (if my equation was right.)

It isn't. Wolfram Alpha apparently doesn't know about the relativistic rocket equation. Try here:

http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.h...

Your equation is correct in a Newtonian universe, but does not consider relativistic effects.

Generally when we talk about an object's acceleration, we are talking about its acceleration in its own reference frame. By definition, an object's velocity in its own reference frame is always 0, so acceleration does not become more difficult (ignoring the fact that the surrounding matter will be impacting you at higher speeds). From the perspective of an outside observer who is not accelerating, time would appear to slow down on the accelerating object, and the outside observer would record a decreasing acceleration.

Fun fact, from the perspective of the observer on the ship, time outside the ship would also appear to slow down.

So why is it difficult to maintain a constant acceleration? Is 1g really that impractical right now?

I'm being accelerated at 1g right now and I'm about 40...so in another decade or so I should be able to get to anywhere in the universe!

Jokes aside, there's no fuel source we know of that would allow a ship to accelerate continuously for 50 years at 1g.

1) Maintaining an acceleration requires fuel.

2) Even though you don't observe yourself moving at increasing speeds, you do observe the rest of the universe moving at increasing speeds. This means that small particles impacting your ship will hit it with increasing energy.

After a year of acceleration at 1g, a stationary observer would observe a speed of 215184620 m/s or 71.7% the speed of light.

After two years: 89.9%

After four years: 97.2%

After eight years: 99.3%

Wait why can't you get under 4 years if you accelerate really fast?

Because it's 4.367 light years away [1].

[1] https://www.google.com/search?q=distance+to+alpha+centauri

But from the point of view of the traveler, it an take far less time.

But from the point of view of both the Origin and the Destination(assuming they maintain similar frames of reference) it takes... 4 years.

That may be true but I was questioning the parent comment which said ANY reference frame

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