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Name: chris
Status: other
Age: 30s
Location: N/A
Country: N/A
Date: 1999 


Question:
with regard to space travel and the speed of light,if a gun is fired on an aeroplane the bullet travels at 1000mph plus the speed of the plane 500mph (1500mph relative to an independant observer)therefore would in not be possible to build a series of space rockets with around 30 extra "jockey sections". the first ship would attain a speed of say 18000mph, the next ship would then be released from its possition on the back of the first ship (remember we are travelling in a vacuum with no friction )the speed would then increse to 36000mph,this second ship would have around 30 further ships on its back each one on top of the other to give a cumulative speed increase,by the time we had got to say, the twentieth jockey in a series of thirty ,this ship would be carrying a series of 10 further ships to be released.what are the drawbacks of this suggestion,because the speed in a vacuum would be cumulative and the build up of aggregate velocity must surely eventually take it to the speed of light (theoretically)can you please shoot holes in this assumption and tell me why this idea will not work.am i wrong?????


Replies:
What you're describing is very similar to actual practice of dividing rockets into stages. The objective is to maximize the speed of the final stage, given a fixed amount of fuel. Obviously you are dividing the energy contained in the fuel among all the stages, so in the end you'd like to minimize the energy given to all stages except the payload. One way to do this is to minimize the mass of the throw-away stages. Usually, minimizing the mass also means minimizing the complexity.

No matter what you do, however, this approach will not get the payload stage to the speed of light, because as you approach the speed of light more energy is required to increase the speed.

Tim Mooney


The problem is that the speed of light (in a vacuum) always appears the same to all observers, whether the different observers are moving relative to each other or not. Say you have your series of spaceships nested lke Russian dolls and begin launching them, each traveling at a speed if 18,000 mph relative to the one before it. Even if you launch 30 stages like this, making the sum of their velocities well over the 186,000 mph speed of light, the 30th ship will NOT travel faster then the speed of light, relative to the first ship. In fact, if the first ship then shines a light down the line, each of the succeeding ships will see the light beam rush past it at the standard 186,000 mph.

How can this possibly happen? If each stage travels 18,000 mph faster than the previous stage, shouldn't the nth stage travel (n-m)186,000 mph faster than the mth stage? Well, no. Compared to the first stage, the succeeding stages see time go more quickly and see distance longer. To explain how this works, I'll use an answer to an earlier question.

"If you took a step in a train going the speed of light , would you be going faster than the speed of light ?"

No, the theory of relativity gets around that quite nicely. It is impossible to get an object with mass (such as a train) to travel at the speed of light, because that would require infinite energy.

You might then ask if you could walk 3 miles an hour on a train traveling 1 mile an hour less than the speed of light, thereby going 2 miles an hour faster than the speed of light. The answer here is yes, you could walk 3 miles an hour on a train traveling 1 mile an hour less than the speed of light, but no, that would not make you go faster than the speed of light. The reason for this is an effect of relativity known as time dilation. From the viewpoint of the train, light still travels at 3 x 10^10 cm/s, and you're only moving at 3 mi/h. So you're traveling MUCH slower than the speed of light.

Now when we say the train is traveling at speed of light - 1 mi/h, we actually mean that it is traveling that fast relative to some external observer. From the viewpoint of that external observer, time has slowed down for you. Let's say one of the cars of the train is 30 m (3 x 10^3 cm) long. If you shine a light from the back of a car toward the front, the observer will see that it takes 67 s for it to reach the front of the car. From the viewpoint of the train car, it only takes 10^-7 seconds for the light to travel the length of the train car. So, time has slowed down for you by a factor of 67s / 10^-7 s = 6.7 x 10^8. So, your velocity, which seems to you to be 3 mi/h faster than the train's velocity, only seems to the observer to be (3 mi/h) / (6.7 x 10^8) = 4.5 x 10^-9 mi/h faster than the velocity of the train - still slower than the speed of light, no matter whose point of view you take.

How does this work? The reason is that one of the fundamental laws of nature, according to the theory of relativity, is that light always travels at the same velocity from the viewpoint of ANYTHING, no matter how fast that anything is traveling relative to anything else. This can make it appear to observers traveling at high speeds relative to each other that the OTHER guy is getting shorter, or heavier, or his time is slowing down, but as far as the other guy is concerned, all those things are happening to you.

Confusing, but pretty cool.

Richard Barrans Jr., Ph.D.


Chris -

Einstein shot holes in your theory. Following his theory, as speed increases mass increases until - in theory - at the speed of light your mass is infinite. As mass increases, the amount of energy required to make the same change in speed would increase... until... well, how do you find the energy to accelerate a mass of almost infinite size?

Except for this, your theory works. If Einstein is wrong (could be) your are in good shape.

You might find reading on the subject of relativity to be interesing to you.

Larry Krengel


From Newtonian physics, we learn that F = ma. Force = mass * acceleration. We can rewrite this as a = F/m. Or acceleration is Force versus mass. If there is no force, there is no acceleration. Also, imagine if the mass were really huge--infinite, then there would be no acceleration also.

Now, on to your problem. Einstein basically disproved Newtonian physics with the Law of Relativity. A number of different things are stated, but some of these are:

As you approach the speed of light, time slows down. This has actually been proven with experiments and different things. It does happen. You have probably even heard of this.

Another thing that is less well known, is that, as your velocity increases, so does your mass.

For most of us, this doesn't affect us. The speed of light is so huge that usually Newtons laws are accurate enough to suffice. We don't worry about the change in mass because it is so slight (even for planes traveling faster than sound) that it is almost imperceptible.

In your case, however, as you attemp to approach the speed of light, the mass of each rocket would increase to the point where you simply can't accelerate any part of your ship anymore. With the technologies we use, this inability to overcome would occur quite soon with regard to the speed of light--even at 1/100000 the speed of light the mass increase is so overwhelming we couldn't accelerate something large enough to hold a man.

Also, as a point of interest, it doesn't matter how you build the ship, whether you build it in one huge part or 1000 small ones.

Think about this for a bit: it doesn't matter whether you appy a given force 1 second or half that force for 2 seconds. The final velocity will be the same. Some equations for this:

F = ma (Force = mass * acceleration)
v = at (Velocity = acceleration * time)

Now, thinking about this a bit more, we can also state that: if an object is traveling at a given velocity, we know what the sum of the forces acting on the object were. This is a bit far to go, but looking at what we did before, it doesn't matter how long the force was applied, so lets assume 1 second in our equations.

v = a * 1, or v = a. Now, F = ma. Rewrite as a = F/m. Now v = F/m, or: v * m = F

This is the formula for momentum. This is interesting because it means, no matter how an object got to a given velocity, the force acting on it was the same. This also means, it doesn't really matter whether the force was applied in 30 bursts or 1 short burst. None of that matters.

Also, we have to realize, all these equations ignore friction and the law of relativity. Since you are interested in space flight, friction can safely be ignored. The law of relativity must be delt with. Fortunately, it doesn't affect the conclusion in this case. Einsteins equations are very similar to Newton's. They just adjust the mass so it is relates the object's mass to the speed of light.

So, it takes exactly the same amount of force to accelate a bullet to 1500 mph whether it is shot out of an airplane standing still or an airplane flying at 1000 mph. The difference is simply with an airplane flying at 500 mph, most of the force was spent in getting the airplane with all the bullets and a pilot and all that to 500 mph. But the portion of force acting on the bullet to get it to 500 mph was the same.

In your case most of the force would be spent accelerating the modules that never achieved the final velocity.

The reason scientists build rockets in stages in this manner is not because it is the most efficient way, but because it is the only way we can get the amount of fuel needed aboard the craft. A rocket engine is mostly fuel, and when the fuel is burnt, the engine is dead weight, so space craft are built to dump their dead weight. The less mass, the less force is required.

Anyway, I hope this explains a little why it is so difficult to get to the speed of light and why it doesn't matter so much how you build your ship as long as you are applying force to get it to move, it is always going to be hard to get to the speed of light.

Thanks,
--Eric Tolman



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