Two Spaceships question

[losfomot]

OK....

 

Two Spaceships Head out of the solar system... They position themselves exactly 1 million km apart and then accelerate, in the same direction, to a velocity of 1000 km/s. They maintain this velocity until they leave the galaxy (Yes I know this would take a long time). Once out of the galaxy they both coast at this velocity for the duration of my question.

 

So the situation we have is this:

 

2 ships coasting at 1000 km/s (we'll say relative to the last star they passed while leaving the galaxy) parallel to eachother at a distance from eachother of 1 million km. Oh and they are in intergalactic space (as inertial as it gets).

 

They each have bright lights attached so they can see eachother and they are moving directly towards a distant galaxy (which they will probably never reach at their velocity). Also there is no rotation of any kind.

 

So.. on to my question(s).

 

A - In the race toward the distant galaxy, does each ship see themselves as in the lead? (They are a million km apart and they have a velocity (away from our galaxy), the light takes about 6 seconds to reach the other ship. So each ship should see their counterpart as he was 6 seconds ago rather than as he is 'now'... therefore, each ship should see the other ship as lagging behind about 6000km (1000km/s • 6 seconds))

 

However...

 

The ships are moving uniformly. They have exactly the same velocity. They are in an inertial frame of reference.

 

B - Are they not at rest WRT eachother?

 

C - If the answer to A and B are both yes, isn't this a contradiction? (ie. When 2 objects are at rest WRT eachother, one can point a laser at the other... shoot... and hit the object. But if A is true, then if one of the ships pointed a laser at the other ship, they would be pointing a laser at the ship's position 6 seconds ago and miss the ship entirely. Of course, this assumption could be the error of my thinking so maybe I'll make that Question # D)

 

D - When 2 objects are considered at rest WRT eachother, can one point a laser at the other... shoot... and hit it no matter how far away it is? (keeping in mind that there is no gravity or any other force acting on the laser, and the shooter is very very good)

[swansont]

I think your confusion stems from trying to analyze things in two different frames of reference at once. The spaceships are at rest with respect to each other. They will not see any lag, and they can point the laser right at the other ship and it will hit.

 

What someone in another frame will see is different.

[[Tycho?]]

I think your confusion stems from trying to analyze things in two different frames of reference at once. The spaceships are at rest with respect to each other. They will not see any lag' date=' and they can point the laser right at the other ship and it will hit.

 

What someone in another frame will see is different.[/quote']

 

 

I dont think I understand this. The laser will either hit or not hit, all frames of refferences must agree on this, as the light of the laser would be visible on the ships hull, or it wouldn't.

[Jacques]

The spaceships are at rest with respect to each other.

But they will need to point the laser where the other spaceship will be in 6 seconds remembering that they see the spaceship where he was 6 seconds ago.

[J.C.MacSwell]

The spaceships are at rest with respect to each other.

But they will need to point the laser where the other spaceship will be in 6 seconds remembering that they see the spaceship where he was 6 seconds ago.

 

Which in their reference frame is the same position. So they don't think (do the math), they just aim and shoot.

[Jacques]

OK I see they don't have any speed relative to each other. But from our point of view the laser won't hit ???

[swansont]

'']I dont think I understand this. The laser will either hit or not hit, all frames of refferences must agree on this, as the light of the laser would be visible on the ships hull, or it wouldn't.

 

The laser will hit in all frames, but the specifics of who see what and when (i.e. simultaneity issues) will look different to different observers.

[swansont]

The spaceships are at rest with respect to each other.

But they will need to point the laser where the other spaceship will be in 6 seconds remembering that they see the spaceship where he was 6 seconds ago.

 

The other ship will be in exactly the same spot 6 seconds from now, since it is at rest in that frame.

 

An observer in a different frame will conclude that the beam was aimed ahead.

[Jacques]

An observer in a different frame will conclude that the beam was aimed ahead.

 

How does that observer be able to conclude that ?

[swansont]

How does that observer be able to conclude that ?

 

Because the beam hits, and the spaceships are moving in the other frame.

[losfomot]

I think your confusion stems from trying to analyze things in two different frames of reference at once. The spaceships are at rest with respect to each other. They will not see any lag' date=' and they can point the laser right at the other ship and it will hit.

 

What someone in another frame will see is different.[/quote']

 

CLICK!

 

OK, I got it straight... I had it straight before too... it's just every once in a while I'll think about it and my brain goes CLACK!... THIS DOES NOT COMPUTE.

 

Thanks.

[Jacques]

The captain of the spaceship knows he is moving at 1000 km/sec relative to the last star of the galaxy. To aim his laser he have two choices: use the star reference frame or use his ship reference frame. They are both valid reference frame. How will he decide which frame to use ???

[timo]

The captain of the spaceship knows he is moving at 1000 km/sec relative to the last star of the galaxy. To aim his laser he have two choices: use the star reference frame or use his ship reference frame. They are both valid reference frame. How will he decide which frame to use ???

Oh, that´s easy: If he´s a lazy guy he´ll use his own reference frame as the targetting is trivial in it. If, on the other hand, he is a frequent visitor of http://www.scienceforums.net then he will most certainly find a more complicated way of targeting. During his completely incomrehensive calculations he will mixup classical and relativistic view. This will result in a miscalculation. The laser will miss. The captain will post this here as an experiment that clearly disproves relativity.

[losfomot]

The captain of the spaceship knows he is moving at 1000 km/sec relative to the last star of the galaxy. To aim his laser he have two choices: use the star reference frame or use his ship reference frame. They are both valid reference frame. How will he decide which frame to use ???

 

He has no choice but to use his own reference frame, in which the other ship is not moving. So he simply points at the other ship and shoots.

 

If he complicates things by trying to look at it from the reference frame of our galaxy, he may decide to shoot ahead of the other ship, in which case he will miss it entirely.

 

You could look at it this way: Why would the captain choose that last star (from which he is moving at 1000 km/s) as a reference point over any other star? If he chose a star in some other galaxy and computed that the ships are moving at 1,234,654 km/s WRT that star, would that help him compute where to fire the laser in order to hit the other ship? No. It wouldn't help him any more than using the closer star as a reference point.

 

It's simply easier to disregard everything else in the universe and focus on whether or not the other ship is moving relative to himself (which it isn't), and then shoot accordingly on that basis alone.

 

 

 

by the way, my figure of about 6 seconds in my initial question is wrong, it should be about 3.3 seconds

[Jacques]

He has no choice but to use his own reference frame

Why ? Is the spaceship frame more valid than the star frame or any other star frame ?

[timo]

No, it´s just easier to use.

[J.C.MacSwell]

As a third choice he could spin on his own axis and hope his calculations compensate for it.

[losfomot]

He has no choice but to use his own reference frame,

 

Why ? Is the spaceship frame more valid than the star frame or any other star frame ?

 

 

Sorry, poor choice of words. He could use another reference frame... but it complicates things unnecessarily and could lead to mistakes.

[swansont]

The captain of the spaceship knows he is moving at 1000 km/sec relative to the last star of the galaxy. To aim his laser he have two choices: use the star reference frame or use his ship reference frame. They are both valid reference frame. How will he decide which frame to use ???

 

It won't matter, as long as he applies the equations properly. As has already been said, his own frame is easiest. If he uses the star's frame he will transform everything and then undo it all again.

 

For similar reasons, it's often easiest to do physics collision calculations either in the rest frame of one particle, or in the center-of-momentum frame. You can choose any one you want, but why make the problem unnecessarily harder?

[Jacques]

OK So using any frame to calculate the pointing direction of the laser will give the same result ?

[iglak]

if in the frame of the shooter, the shooter has to aim more than 12 seconds ahead of the other ship, right?

 

it takes 6 seconds for light to get to the shooter frame, so the shooter sees the target at it's position 6 seconds ago. they are in the same reference frame, so differences in perception of time are not a factor.

but the shooter can't simply aim 6 seconds ahead, because it takes 6 seconds for the laser to get to the target. but light, when shot from a moving vehicle, does not have a velocity added to the vehicle's velocity. so if shot parallel (6 seconds ahead of the target), the laser will hit where the target was 6 seconds ago, and will not travel in the direction of the ships at all. to compensate, the shooter needs to shoot at where the target will be more than 6 seconds from now. not just 6 seconds; because when shot at an angle, the light will take longer to travel the 1million km.

[swansont]

if in the frame of the shooter' date=' the shooter has to aim more than 12 seconds ahead of the other ship, right?

[/quote']

 

In the frame of the shooter, the target isn't moving. "12 seconds ahead" is not meaningful because v=0.

[Jacques]

What I understand is that velocity have no meaning by itself. Only relative velocity have a meaning. What is the relative velocity betwen the two spaceships ? 0. So no correction needed.

If we sented a bullet instead of a laser pulse, intuitively we know that the bullet already have the same velocity as the spaceship and we wont add an angle to catchup the other spaceship. From the star point of view it is like the light emitted by the spaceship already have this velocity toward the other galaxie or that if there is a medium to transmit the light, the medium goes at the same speed as the spaceship. Just some thaught...

[Daecon]

the shooter needs to shoot at where the target will be more than 6 seconds from now. not just 6 seconds; because when shot at an angle, the light will take longer to travel the 1million km.

 

But surely the light is moving in a plane that is absolute, and not at an angle? As in, the light is moving forward and not moving "sideways" so you can just extrapolate the location and position of the other ship and concentrate, still only needing a 6-second window?

[iglak]

so if shot parallel (6 seconds ahead of the target), the laser will hit where the target was 6 seconds ago, and will not travel in the direction of the ships at all.

that's supposed to be if shot perpendicular... sorry.

 

 

 

 

In the frame of the shooter, the target isn't moving. "12 seconds ahead" is not meaningful because v=0.

oh yeah ^^;

ok, so then from the shooter frame, the distance that the target will travel in the right amount of time from the rest frame.

 

what if he didn't know he was moving though? would he be able to shoot in the right place?

 

 

But surely the light is moving in a plane that is absolute, and not at an angle? As in, the light is moving forward and not moving "sideways" so you can just extrapolate the location and position of the other ship and concentrate, still only needing a 6-second window?

i'm sorry, but i don't know what an absolute plane is. the only plane i'm looking at is the one that contains both parallel lines of the movement of the spaceships.

the light leaves the target in all directions. the light going perpendicular from the target will not reach the shooter, because by the time it gets the the opposite side, the shooter will be 6 seconds ahead. only the light going at an angle will reach the shooter. light can't go parallel at any speed and still go perpendicular at the speed of light, because then the light will be going faster than the speed of light at an angle. so, it will take more than 6 seconds for the light from the target to reach the shooter.

 

then, now that the shooter can see the target, the shooter has to aim to where the target will be to hit the laser. same as before, the shooter has to aim greater than 6 seconds ahead of where the target is now; greater than 12 seconds ahead of where the object is seen.