A Good Idea On How To Reach The Speed Of light, And Possibly More...

[adam1]

Hi All,

So I'm gonna start off by reiterating something I read earlier that really got me thinking, someone asked the question 'What would happen if you fired a gun whilst you were on a train moving at the same speed as a bullet' and the answer was as follows, "Every body persists in its state of rest or of uniform motion in a straight line unless it is compelled to change that state by forces impressed on it." (Newtons first law) what this means for the bullet is that, much like if u through a ball up in the air it would come straight back down not fly to the back of the Train at 1000 Mph acting exactly the same as it would if you were not moving only being effected by you and gravity, the bullet would act as it would if you were not moving too it would still leave the gun traveling at 1000 mph. Now here is the interesting part, if you were moving already at 1000 mph and you fired the gun and the bullet began to move at an additional 1000 mph relative to you and the train, then relative to the ground the bullet would be moving at 2000 mph. Then I got to thinking about scaling it up to something bigger, for example, if I were to have a rocket that could house two other rockets, like Russian dolls, that could propel themselves progressively faster as their size and weight decreases, now lets work this out from things we know, rockets that leave our atmosphere reach up to 25,000 Mph so lets say once this speed is achieved we initiate the launch of the next rocket from within, lets say to the reduced size and weight, this rocket can reach 30,000 Mph on its own, launching within the other rocket would give the second rocket a starting speed of 25,000 Mph before it has even took off, so it fires out and reaches a speed of 55,000 Mph then the next rocket, that's max speed would be further still, lets say 40,000 Mph due to its smaller size and weight, through this method it seems to me it would be completely possible to reach nearly 100,000 Mph. Now if you scaled this up further still then you could plausibly reach any speed you wanted to by just working out how many ships you would need at at what speed each of them would need to project themselves, and because this would all be happening in space these speeds could be easily sustained for long periods of time, making it a plausible and effective means of space travel.

Well guys there is my idea, poke some holes in it if you can because I really hope I haven't just figured out a real way of reaching these speeds, I'm far to dumb for that haha

Thanks for reading

Edited January 3, 2014 by adam1

[dharak1]

The size of the rockets would be massive. If you are talking about having a Saturn V inside a rocket and having one blast off inside the other the one housing the Saturn V would be HUUUGEE. If you mean one coming out the front of the other then that is just staging.

[Spartwo]

Sadly that won't work for getting past the light limit.It would make more sense to do it in one stage.

[adam1]

you are right, the size of the rockets would be huge, but if it would work then it would be entirely worth doing maybe a ship built in parts then put together in space would work?

[K^2]

This is basically how rocket stages work. And it runs into the same sort of problem. If each earlier stage has to be 20 times or even more larger than than the next one, you plausibly can only ever have a few stages. Eventually, you still run into the same problem as with a single stage. The faster you want your final speed to be, the exponentially larger the rocket and amount of fuel needed has to be. Staging simply lets you drop heavy engines you no longer need to carry a smaller rocket.

And, of course, as you near the speed of light, speeds no longer add the same way they normally do, so you'll never get past speed of light. But even getting to any noticeable fraction of speed of light is practically impossible with chemical rockets, and insanely difficult even with nuclear propulsion.

[stupid_chris]

You just described edxactly the point of general relativity.

This is exactly the purpose of the whole theory. So that, if you are in a rocket moving 299 792 400m/s (the speed of light being 299 792 458m/s) and then fire a gun pointing forward, the bullet doesn't break the speed of light.

The whole thing rotates around the fact that time is relative to the frame of reference you observe it in. So, in a rocket moving at that speed, time would go at a slower rate inside it, so that objects within it never end up going faster than the light in the static frame of reference. When adding velocities, the usual formula is s = v + u, where s is the final velocity, u the original velocity, and v the added velocity. But that is only true when the speeds are not relativistic ones. Actually s is the speed of the object relatively to a static point of reference, v the speed of the frame of reference the speed is measured according to, and u the speed of the moving object measured in the frame of reference u. So in a rocket going at 1000m/s, a bullet fired at a speed of 1000m/s in the ship would be going at 2000m/s on a static point of reference. But this is false at relativistic speed (near the speed of light)

In reality, the real equation is this:

Where s is your final speed, u the speed of the moving point of reference, v is the mesured speed, and c is the speed of light.

That means the faster you would end up going, the slower you will actually end up going, because of relativity.

So no, won't work because of relativity.

EDIT

As an example: if you are originally going 1000m/s and want to go 1000m/s faster, your final speed would be (1000 + 1000) / (1 + 1000*1000/299792458²) = 1999.99999997774699887917m/s (only showing the first 20 decimals here, it's so close to 2000 I had to use an online precision calculator)

However, if you are originally going 299 792 000m/s and fire a bullet at 1000m/s, you would end up with this:

(299792000 + 1000) / (1 + 299792000*1000/299792458²) = 299792000.00305543458625864747m/s

This means, when looked at from a static point of reference, the rocket and the bullet inside it would appear to be moving in slow motion. This is actual time compression for the people aboard. So a ship moving at 99% of the speed of light could take 100 000 years to cross the milky way, while the people aboard it would only age 80 years aboard it because time is slowed within it (gross approximation fo the time they would age, not accurate).

Edited January 3, 2014 by stupid_chris
messed up my number >.<

[Karriz]

you are right, the size of the rockets would be huge, but if it would work then it would be entirely worth doing maybe a ship built in parts then put together in space would work?

Well, I'm not sure how many launches you'd need to build such a massive ship using chemical rockets. Thousands, or even more?

Getting anywhere close to the speed of light (0.1c perhaps?) with a reasonably sized ship would require some kind of nuclear propulsion. Or whatever plasma/antimatter stuff scientists might come up with eventually.

[tavert]

You just described edxactly the point of general relativity.

This is exactly the purpose of the whole theory. So that, if you are in a rocket moving 299 900m/s and then fire a gun pointing forward, the bullet doesn't break the speed of light.

That's special relativity. General rel is gravity curving spacetime.

[K^2]

Special Relativity is a sub-field of General Relativity. So technically, saying that this is so according to GR isn't false.

[stupid_chris]

Special Relativity is a sub-field of General Relativity. So technically, saying that this is so according to GR isn't false.

Yeah, all of that sutff is included within the general relative theory.

[K^2]

You should be checking your numbers to see if they make sense, though.

However, if you are originally going 299 792 000m/s and fire a bullet at 1000m/s, you would end up with this:

(299792000 + 1000) / (1 + 299792000*1000/299792458²) = 149896228.99975502763573572439m/s

[stupid_chris]

You should be checking your numbers to see if they make sense, though.

No, that's the right number. Calculate it yourself. Remember that this is relative velocity. The ship itself would appear to be moving at nearly this speed. Relativistic velocities are wonky things.

Ignore me, I am saying silly things.

Edited January 3, 2014 by stupid_chris

[koshelenkovv]

You mistyped number in a calculator.

Bullet, which was fired forward, cannot end up moving slower than train.

Right answer is 299792000,00306 m/s.

[ZetaX]

No, that's the right number. Calculate it yourself. Remember that this is relative velocity. The ship itself would appear to be moving at nearly this speed. Relativistic velocities are wonky things.

It is obviously not the right number as

a) plugging it into a calculator

a trivial plausibility check (it is slower than the initial speed)

shows.

[stupid_chris]

Yeah, my username is showing off. Replugged my things, ended up with a normal number (which I just fixed in my post). I think I switched u*v to u+v on the bottom part of the equation.

Pretty sure I have not had the required amount of coffee before trying to math.

[K^2]

Here is the check. If the bullet is traveling at half the speed of the train, something went wrong.

[stupid_chris]

Here is the check. If the bullet is traveling at half the speed of the train, something went wrong.

Weeeeeeelll, I logic'd myself by only plugging the speed of the train in and getting a similar number, and only realized after how dumb I was. Herp derp. I've had my coffee now at least.

[Dispatcher]

Approaching the speed of light via chemical rockets would require a fuel/ propellant mass greater than the mass of the entire universe.

[Aghanim]

Why sometime physics is a ..... ?! Like the universe hates everything that tries to travel at speed of light except light itself

[Skyler4856]

Your mass would increase as you traverse farther through the stages and closer towards the speed of light.

[Suko]

You just described edxactly the point of general relativity.

This is exactly the purpose of the whole theory. So that, if you are in a rocket moving 299 792 400m/s (the speed of light being 299 792 458m/s) and then fire a gun pointing forward, the bullet doesn't break the speed of light.

The whole thing rotates around the fact that time is relative to the frame of reference you observe it in. So, in a rocket moving at that speed, time would go at a slower rate inside it, so that objects within it never end up going faster than the light in the static frame of reference. When adding velocities, the usual formula is s = v + u, where s is the final velocity, u the original velocity, and v the added velocity. But that is only true when the speeds are not relativistic ones. Actually s is the speed of the object relatively to a static point of reference, v the speed of the frame of reference the speed is measured according to, and u the speed of the moving object measured in the frame of reference u. So in a rocket going at 1000m/s, a bullet fired at a speed of 1000m/s in the ship would be going at 2000m/s on a static point of reference. But this is false at relativistic speed (near the speed of light)

In reality, the real equation is this:

http://upload.wikimedia.org/math/2/0/3/2035aab1ba5af2e1ff296512b6a57779.png

Where s is your final speed, u the speed of the moving point of reference, v is the mesured speed, and c is the speed of light.

That means the faster you would end up going, the slower you will actually end up going, because of relativity.

So no, won't work because of relativity.

EDIT

As an example: if you are originally going 1000m/s and want to go 1000m/s faster, your final speed would be (1000 + 1000) / (1 + 1000*1000/299792458²) = 1999.99999997774699887917m/s (only showing the first 20 decimals here, it's so close to 2000 I had to use an online precision calculator)

However, if you are originally going 299 792 000m/s and fire a bullet at 1000m/s, you would end up with this:

(299792000 + 1000) / (1 + 299792000*1000/299792458²) = 299792000.00305543458625864747m/s

This means, when looked at from a static point of reference, the rocket and the bullet inside it would appear to be moving in slow motion. This is actual time compression for the people aboard. So a ship moving at 99% of the speed of light could take 100 000 years to cross the milky way, while the people aboard it would only age 80 years aboard it because time is slowed within it (gross approximation fo the time they would age, not accurate).

Yep, I was going to say basically that but much more simplified. It is NOT possible, by any means, to travel at the speed of light. Furthermore, "c" is a constant, that means the speed of light is always the same, regardless of the relative speed of the object you are moving on with anything, 'cause for example the Earth itself is rotating around the Sun at incredibly high speeds, but that does not make us any closer to the speed of light.

[brooklyn666]

Special relativity tells us that locally, nothing can travel faster than C. To build off of your metaphor, if a person on a train traveling at .5c fires a gun with a bullet traveling in the forward velocity vector at .5c, that bullet will NOT be traveling at c.

Your method "could" be used to propel a vessel arbitrarily close to c, but not with chemical rockets.

[K^2]

Special relativity tells us that locally, nothing can travel faster than C.

Very important distinction, by the way. Though, it only becomes meaningful in context of General Relativity.