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Fubaruk
2009-Feb-13, 09:56 AM
I have a few questions which confuse me a little(not hard) and hopefully you guys could help me understand it a bit better.

Imagine that there are only 2 planets in the entire universe (to keep it simple), Plante A (PA) and Planet B (PB).
PB is moving away from PA at 50%C.
You are on a ship moving 99.999%C relative to PA.
I understand that as you aproach C, your speed through time slows(to a relative observer). So from PA and PB your moving at different speeds through time and space.

1. As you approach C you require more and more energy to accelerate because your mass increases but what is this speed to mass ratio relative to PA or PB.

2. If the ship maintained it's speed but PA increased is speed in relation to PB. Would the ship require less energy to increase it's speed thus acerlerating passed light speed?
3. Back to our universe now, the earth is moving at approx 200miles/sec in relation to the galactic center. If we moved in opposite direction to the oribital path would we require less energy to accelerate that if we went with the orbital path?
4. As I write this I thought that maybe that speed limit is in relation to the amount of mass in the vicinity of the accelerating object if the universe was devoid of all matter would a ship be able to constantly accelerate (I guess you will say in relative to what (grr)).
5. Would being able to negate the effects of gravity on a space ship enable ftl travel?

Thanks for your input and thoughts.

antoniseb
2009-Feb-13, 10:46 AM
You seem like you are asking for an answer that doesn't get into the mathematical details of special relativity. I can tell you that If you are in a spaceship going 99.99999% the speed of light in the two-planet universe you describe, and you put a tennis ball just outside your spaceship as a reference, and burn some fuel to accelerate a little faster, your ship will behave exactly as you'd expect if it were at rest. If you accelerated and added an extra ten meters per second to your velocity, you'd see the tennis ball falling behind you at ten meters per second (as you'd be measuring it). People on Planet A and Planet B would each see your acceleration as very small compared to 10 m/s.

Fubaruk
2009-Feb-13, 10:55 AM
But wouldn't that mean you could accelerate indefinatly by dropping tennis balls outside your window? Thus going much faster than C ( and I don't mean faster than a light beam I mean faster than 300,000,000 m/s)

NEOWatcher
2009-Feb-13, 12:56 PM
But wouldn't that mean you could accelerate indefinatly by dropping tennis balls outside your window? Thus going much faster than C ( and I don't mean faster than a light beam I mean faster than 300,000,000 m/s)
Unfortunately; this is a very common way people tend to visualize this.

The reality is that the faster you go, any changes that you see as being equivalent are seen as forever smaller changes to the planets.

So; they are the same to you, but the planets they seem more and more infinitely smaller as you approach C. So nothing you do can get you to C, because the change gets infinitely small no matter how you see it.

antoniseb
2009-Feb-13, 01:01 PM
Unfortunately; this is a very common way people tend to visualize this.

The reality is that the faster you go, any changes that you see as being equivalent are seen as forever smaller changes to the planets.

So; they are the same to you, but the planets they seem more and more infinitely smaller as you approach C. So nothing you do can get you to C, because the change gets infinitely small no matter how you see it.

Right, but perhaps, satisfying to Fubaruk, is the fact that if you started out knowing how far it was between the planets, that distance would seem to shrink as you got close to c, and so, let's say the two planets were one light year apart when you started out, When you are going 99.9999999% the speed of light, they might seem only miles apart (I haven't done the exact calculation here), so to you it would seem that the journey was *much* less than a year. Accelerating does help the traveler.

Ufonaut99
2009-Feb-13, 01:08 PM
But wouldn't that mean you could accelerate indefinatly by dropping tennis balls outside your window?
Yes

Thus going much faster than C ( and I don't mean faster than a light beam I mean faster than 300,000,000 m/s)
No. (and light beams always travel at (almost) 300,000,000 m/s in a vacuum)

In SR, you can't just "add" velocities; you always go a bit slower, which means you can always get closer and closer to c, but never reach it. But this brings us back to the main point - faster or slower than what? or rather, relative to what? The answer to that has to be "to an observer." That brings us back to your OP :

So from PA and PB your moving at different speeds through time and space.

More accurately, and observer on PA (say "Alice") will observe your ship moving at a different speed than an observer on PB ("Bob") would. In addition, as you walk around your ship, Alice would observe you walking slower than Bob would. This is because you're going faster relative to PA than you are to PB.

1. As you approach C you require more and more energy to accelerate because your mass increases but what is this speed to mass ratio relative to PA or PB.

Firstly, you decide how much energy to put in, and you feel yourself accelerate appropriately. Alice would reckon you've only accelerated a little, and that most of the energy you've poured in went into mass gain. Bob reckons you've accelerated more than Alice thinks (but still not as much as you think).
How come nobody agrees on how much you've accelerated? Because everybody's measuring using their own local clocks and rulers, which are all different from everybody elses since the relative velocities are all different (you think you're stationary, Bob thinks you're at 0.5c, Alice thinks you're at 0.9999c). Relativity provides the tools to transform from one observer's clocks/rulers to another in a consistent fashion, such that everything stays consistent.

2. If the ship maintained it's speed but PA increased is speed in relation to PB. Would the ship require less energy to increase it's speed thus accelerating passed light speed?

No, as discussed above. Your motions are entirely independent of any observers. If you're not accelerating, you can consider yourself at rest and say PA and PB are in motion flying past you. Regardless, that light beam is still going at c ahead of you.

4. As I write this I thought that maybe that speed limit is in relation to the amount of mass in the vicinity of the accelerating object if the universe was devoid of all matter would a ship be able to constantly accelerate (I guess you will say in relative to what (grr)).

You can constantly accelerate anyway. It's just that you'll never accelerate up to light speed. You can never beat the constant, because no matter how fast you go, that beam of light is always going at the constant speed ahead of you.

5. Would being able to negate the effects of gravity on a space ship enable ftl travel?

Nope. Relativity dictates that we can never send a message (and that includes personally delivered ones ;) ) faster than c.

Jeff Root
2009-Feb-13, 03:06 PM
RobA,

Excellent reply to Fubaruk's questions! Maybe perfect!

-- Jeff, in Minneapolis

Fubaruk
2009-Feb-13, 09:38 PM
RobA,

Excellent reply to Fubaruk's questions! Maybe perfect!

-- Jeff, in Minneapolis

yup thanks for that it helps i think although still a little confused

You can constantly accelerate anyway. It's just that you'll never accelerate up to light speed. You can never beat the constant, because no matter how fast you go, that beam of light is always going at the constant speed ahead of you.

I understand that whatever speed you go, light will always be detected at C. but still not totally convinced that you cant go faster than 300,000,000M/s I guess that cant be proven either way atm certainly seems feasable to me still.

loglo
2009-Feb-13, 11:05 PM
We can accelerate sub-atomic particles to relativistic speeds and the equations of SR agree perfectly with how they behave. Same with cosmic rays, with relativistic jets in AGN etc etc. They never go faster than c.

Proof is for maths and court rooms, not for physics where evidence and observations rule. Relativity has a lot of supporting evidence and none of note against it.