The fastest you can go.

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ACU-LP
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The fastest you can go.

Postby ACU-LP » Thu Jan 08, 2009 3:56 am UTC

I was thinking earlier (and even posed this question to the people of general, but to no avail), about relativity.
Now, at our levels of speed, to an external observer, by going faster, you get to your destination quicker. However, if moving close to the speed of light, the faster you go, the longer it takes you to get to your destination (to an external observer).
So I'm wondering what the 'critical speed' is. What speed do you have to be going to get to your destination the fastest?
If that made any sense at all.
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Re: The fastest you can go.

Postby Carnildo » Thu Jan 08, 2009 4:14 am UTC

ACU-LP wrote:However, if moving close to the speed of light, the faster you go, the longer it takes you to get to your destination (to an external observer).


Where do you get this from?

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Re: The fastest you can go.

Postby ACU-LP » Thu Jan 08, 2009 4:18 am UTC

Well; in relativity, if you go exceptionally close to the speed of light, and though you are going very fast, to an external observer, due to length dilation making your length tiny, it takes longer to get to your destination (so in theory, you're actually going slower)
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Re: The fastest you can go.

Postby ++$_ » Thu Jan 08, 2009 4:24 am UTC

ACU-LP wrote:Well; in relativity, if you go exceptionally close to the speed of light, and though you are going very fast, to an external observer, due to length dilation making your length tiny, it takes longer to get to your destination (so in theory, you're actually going slower)
What? If an ant is moving at 500 m/s, it reaches a destination exactly as fast as an airplane moving at 500 m/s. I fail to see what length has to do with this.

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Re: The fastest you can go.

Postby Jack21222 » Thu Jan 08, 2009 4:34 am UTC

ACU-LP wrote:Well; in relativity, if you go exceptionally close to the speed of light, and though you are going very fast, to an external observer, due to length dilation making your length tiny, it takes longer to get to your destination (so in theory, you're actually going slower)


If you're moving close to the speed of light, your personal time slows down. An outside observer would see you going at whatever speed you're going.
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Re: The fastest you can go.

Postby ACU-LP » Thu Jan 08, 2009 4:40 am UTC

So it turns out my physics teacher was wrong.
Nevermind.
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Re: The fastest you can go.

Postby Rentsy » Thu Jan 08, 2009 4:43 am UTC

I think that the best speed for getting somewhere fast is as fast as you can go.

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Re: The fastest you can go.

Postby Swivelguy » Thu Jan 08, 2009 4:48 am UTC

ACU-LP wrote:Well; in relativity, if you go exceptionally close to the speed of light, and though you are going very fast, to an external observer, due to length dilation making your length tiny, it takes longer to get to your destination (so in theory, you're actually going slower)


As you go fast, distance contracts in the direction of your motion. So if you go faster, then distance you have to go seems smaller, and you get there faster. In fact, you can go so fast that you get to your destination, no matter how far, in (what seems to you like) 1 second. Faster always gets you there sooner.
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Re: The fastest you can go.

Postby ACU-LP » Thu Jan 08, 2009 4:50 am UTC

Maybe I got the technicalities mixed up, but when you go close to the speed of light and say, travel to another star, you'll get there in the 'future' of the external observer; to you, the time will have passed as normal, but to them many more years would have passed....Or at least that's how it has been explained to me.
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Re: The fastest you can go.

Postby Charlie! » Thu Jan 08, 2009 5:08 am UTC

ACU-LP wrote:Maybe I got the technicalities mixed up, but when you go close to the speed of light and say, travel to another star, you'll get there in the 'future' of the external observer; to you, the time will have passed as normal, but to them many more years would have passed....Or at least that's how it has been explained to me.

This is correct. The way this happens actually looks differently depending on what frame you're in:

If you're you, time will pass like normal but distance will contract along your direction of motion, so it will looks like a short trip at normal speed.

If you're an observer on the earth, the traveler's time will slow down but distances will stay the same (well he'll look shorter but it won't change his speed), so it will appear to be a long trip with slowed rate of time.
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Re: The fastest you can go.

Postby danpilon54 » Thu Jan 08, 2009 3:45 pm UTC

I think the important distinction is that the spaceship is not making the Earth's time go faster. According to the Earth, the ship will take d/v time to get there, same as always. The faster the ship, the less time it takes to get there.
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Re: The fastest you can go.

Postby gmalivuk » Thu Jan 08, 2009 4:07 pm UTC

ACU-LP wrote:So it turns out my physics teacher was wrong.
Nevermind.

While that's certainly possible, I would like to put forward the countersuggestion, which is that you simply misunderstood your physics teacher...
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Re: The fastest you can go.

Postby sgt york » Thu Jan 08, 2009 7:01 pm UTC

++$_ wrote:What? If an ant is moving at 500 m/s, it reaches a destination exactly as fast as an airplane moving at 500 m/s. I fail to see what length has to do with this.

That's some ant.

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Re: The fastest you can go.

Postby ++$_ » Fri Jan 09, 2009 12:47 am UTC

sgt york wrote:
++$_ wrote:What? If an ant is moving at 500 m/s, it reaches a destination exactly as fast as an airplane moving at 500 m/s. I fail to see what length has to do with this.

That's some ant.
The ant is in the airplane, of course.

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Re: The fastest you can go.

Postby Charlie! » Fri Jan 09, 2009 1:27 am UTC

++$_ wrote:
sgt york wrote:
++$_ wrote:What? If an ant is moving at 500 m/s, it reaches a destination exactly as fast as an airplane moving at 500 m/s. I fail to see what length has to do with this.

That's some ant.
The ant is in the airplane, of course.

Perhaps it is some sort of super-ant. OR, wait!

Ozzymandibles, king of ants! Look on my works, ye termitey, and despair!
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Re: The fastest you can go.

Postby HeroicFail » Fri Jan 09, 2009 2:27 am UTC

Charlie! wrote:
++$_ wrote:
sgt york wrote:
++$_ wrote:What? If an ant is moving at 500 m/s, it reaches a destination exactly as fast as an airplane moving at 500 m/s. I fail to see what length has to do with this.

That's some ant.
The ant is in the airplane, of course.

Perhaps it is some sort of super-ant. OR, wait!

Ozzymandibles, king of ants! Look on my works, ye termitey, and despair!

I, for one, welcome our insect overlords.

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Re: The fastest you can go.

Postby Herman » Fri Jan 09, 2009 2:30 am UTC

gmalivuk wrote:
ACU-LP wrote:So it turns out my physics teacher was wrong.
Nevermind.

While that's certainly possible, I would like to put forward the countersuggestion, which is that you simply misunderstood your physics teacher...


For the sake of the rest of the class, I hope so...

Interesting piece of trivia from relativity: At what speed will a journey of distance x, in a "resting" frame, last time x/c in your frame? (In other words, how fast must your spaceship be moving for a journey to Alpha Centauri to be 4.3 years by the time-dilated ship's clock, assuming acceleration and deceleration happen very quickly?)

Spoiler:
c*sqrt(2)/2, or around 70% of lightspeed

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Re: The fastest you can go.

Postby ACU-LP » Fri Jan 09, 2009 11:38 am UTC

gmalivuk wrote:
ACU-LP wrote:So it turns out my physics teacher was wrong.
Nevermind.

While that's certainly possible, I would like to put forward the countersuggestion, which is that you simply misunderstood your physics teacher...
I can see where you're coming from, but he spent more time attempting to be the anti-christ and being generally evil than he did teaching. And I had a full grasp of relativity before those classes (nerdy childhood)....but not after (when another person's way is forced onto you, it kind of sticks, right or wrong).
But then again, I have been known to be wrong before.
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Re: The fastest you can go.

Postby Klotz » Sat Jan 10, 2009 12:45 pm UTC

If a glass spaceship is going fast, to an outside observer the people inside the ship are moving in slow motion.

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Re: The fastest you can go.

Postby Tass » Sat Jan 10, 2009 9:16 pm UTC

Klotz wrote:If a glass spaceship is going fast, to an outside observer the people inside the ship are moving in slow motion.


Although the difference of what one observes is dwarfed by the effects to what one sees such as redshifts, dobbler effect to their movement rates and so on...

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Re: The fastest you can go.

Postby Herman » Sun Jan 11, 2009 12:35 am UTC

Klotz wrote:If a glass spaceship is going fast, to an outside observer the people inside the ship are moving in slow motion.


Interestingly, vice versa. To people on the spaceship, the observers will also appear to be moving in slow motion.

Relativity is weird.

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Re: The fastest you can go.

Postby Klotz » Sun Jan 11, 2009 3:12 am UTC

The barn door paradox is weirder.

Although the difference of what one observes is dwarfed by the effects to what one sees such as redshifts, dobbler effect to their movement rates and so on...


Let's say they're far away and travelling perpendicular.

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Re: The fastest you can go.

Postby Solt » Sun Jan 11, 2009 11:31 pm UTC

ACU-LP wrote:Maybe I got the technicalities mixed up, but when you go close to the speed of light and say, travel to another star, you'll get there in the 'future' of the external observer; to you, the time will have passed as normal, but to them many more years would have passed....Or at least that's how it has been explained to me.



I thought it was the other way around. Time will pass normally to the external observer- meaning it will take distance/velocity seconds for you to get there. To the person on the ship, time will pass slower meaning they will have traveled faster than the speed of light when their time frame is resynchronized with the observer's.
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Re: The fastest you can go.

Postby gmalivuk » Sun Jan 11, 2009 11:40 pm UTC

Well, yeah. But to get to another star, this means that many years will have passed for the observer back home, while less time has passed for the fast-moving person in the ship.
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Re: The fastest you can go.

Postby Cpt. Red » Mon Jan 12, 2009 1:05 am UTC

Are you asking in what speed will the length dilation (of the craft) affect the time (for an external observer) so that it will appear to take a longer time to travel from A to B?
If that is indeed your question the the answer is never. The length of the craft (in the direction of the speed) wont decrease faster then the time it takes for the craft to travel that length.
If this was not the question you were asking then poo on me...


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