Is there a limit to the speed a (let’s say spacecraft) can reach in space? If there’s no air friction/resistance, won’t every burst of propulsion cause a craft to increase it’s speed? I’m feeling quite ignorant here, but that’s what you’re all for, right?
Yes, every time you fire the engine, it will increase your speed. When you stop the engine you simply keep coasting at the same speed.
Normally the limit is set by how much fuel you have, and how efficient your engines are. This is not a trivial concern - rockets work by throwing stuff out the back. But let’s say you have a 10-ton spaceship with a 10-ton supply of fuel, and your engines spew out the exhaust gas at 10 miles per second. The end result is that the spaceship and a cloud of hot gas will be moving away from each other at a relative speed of 10 miles per second - or each at 5 miles per second relative to a stationary observer. Which is not much. If you want to go fast, you need engines that has a high exhaust velocity, and use a spaceship which is almost all fuel.
Then of course there’s the speed of light, which no amount of fuel will let you overcome.
So, let’s say that your rocket has such an insanely large amount of fuel that it’s able to get to almost the speed of light. Whenever you toss more stuff out the back, your speed will increase, but it will never reach c. It’s an asymptotic approach, if that means anything to you.
There are other ways to measure speed (such as the “rapidity” or the “proper speed”) which set the speed of light at an infinite value (the scales are not linear). In this case, there’s no upper limit on allowed speeds. Note that both proper speed and rapidity look just like regular linear speed, when you’re much less than the speed of light.
What does speed mean in outer space. If you are chugging along at a few miles per second relative to the earth and a meteor screams past you at a speed of several thousand miles per-second, can’t you just as easily say that the meteor is standing still and you are moving at that “speed”
So long as both of you are moving at a constant velocity (no acceleration and no change of direction), you can’t say which one of you is moving. That was a major insight of special relativity.
On the other hand, speed does have a meaning; if that meteor crashes into you, you’re sure gonna know it was moving fast.
so what you’re saying is:
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a craft in space, going a billion mph, can further increase it’s speed with regualar propulsion?
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(humor this scenario) if I were in a spacesuit, and I were able to “blow” out of the suit [as propulsion], I could constantly increase my speed, and at no point would my propulsion methods become ineffective?
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a craft, upon reaching phenomenal speeds (billion mph), would only feel that sensation if it were to pass by something?
If this is all possible, have any organizations attempted to send unmanned craft into space this way, for exploration of unreachable distances? If not, why not?
Besides colliding with something, what else is known that can cause a craft to slow, stop, or be destroyed (like gravity, magnetism, or whatnot)?
According to relativity, nothing can exceed the speed of light. As one nears the speed of light, the energy necessary to increase one’s speed increases really quickly.
Other than that, your craft might be unstable at those high speeds–relativity postulates some rather strange effects as one travels near the speed of light.
In regards to point #3, if there’s no acceleration, then yes, you wouldn’t feel anything. To understand it, imagine that you’re cruising down the freeway at 65 mph. You don’t feel that you’re moving unless you turn or change your speed.
Yes, an organization has. NASA has done so, four times, in fact. Pioneer 10 and 11, and Voyager 1 and 2 are all out there exploring previously unreachable distances. All four continue to travel away from the earth and sun.
Tris
“To boldly go where no one has gone before.” ~ Gene Roddenberry ~
Yes.
You would soon run out of air to blow out. If you have an infinite supply of air - wait a minute, if you have an infinitely large (and infinitely heavy) fuel tank, you would never be able to move at all.
Well, in perfect vacuum, there will be no sensation at all. In reality, you may have some problems with interstellar gas and dust impacting on the front of your spaceship. I suspect that wouldn’t happen until you get to a significant fraction of the speed of light.
What do you mean? All spaceships and interplanetary probes are propelled by rocket engines of some kind. If you are asking why we can’t go even faster, read my first post. The speed you can reach is limited by how much fuel you start out with, and how efficient your engines are. Engine efficiency is limited by various limitations in our technology. The amount of fuel is limited by the size and cost of our launch vehicles. Best you can do is build a small probe, put it in the Shuttle cargo bay and fill up the rest of the cargo bay with fuel tanks - well, that’s basicall what most probes look like these days. Like the Galileo which managed to get to Jupiter after many years of flight.
Evno wrote:
Technically, yes. Any force applied will result in an increase in speed, but to a smaller and smaller extent as you go faster and faster. This page doesn’t mention it specifically, but the gamma factor can also be applied to mass (at least that’s what I was taught, and the page linked is dealing with particle accelerators, so things are different). As you go faster, your apparent mass (to a stationary observer) increases. Since force equals mass times acceleration, acceleration equals force divided by mass, and you can see how if your engines produce a constant amount of thrust against a larger and larger mass, acceleration will drop.
Years ago, I asked myself, “what if you power the ship using a matter/antimatter engine of some sort?” Given E=mc[sup]2[/sup], as your speed increases, so would the mass of your fuel, thus increasing the energy released (multiply by gamma), as well. I’ve forgotten the precise reason why this still wouldn’t work, theoretically even, but I suspect that it has something to do with the fact that while turning mass into energy is a 100% efficient reaction, the act of trying to turn that energy into useful forward propulsion is not and can not be 100% efficient, and the efficiency drops as you go faster, and it drops more quickly than you ‘discard’ mass by burning fuel (which would increase your acceleration). Someone else, help me out here.
Depends on what you mean by “ineffective.” If an acceleration the equivalent of a gnat pushing on the Earth is “effective,” then yes, it does not become “ineffective.” Of course, as has been already pointed out, you need air tanks the size of an average galaxy or two to reach those speeds, in which case, you’re starting out worse than the gnat to begin with.
Depends on what you mean by “feel.” Looking out a porthole at a billion mph, you’d see the Moon zip by a little less than a second after you pass the Earth, but you’d “feel” nothing other than whatever your acceleration is (Earth’s gravity is a constant acceleration - how often do you “feel” it?).
By the way, a billion mph isn’t all that phenomenal, in relativistic terms - it’s only 0.15% of the speed of light. Gamma is a piddly 1.00000111 (a football field at this speed would only be 4 thousands of an inch shorter than regulation).
I’d say ‘no’, since, as has been pointed out, fuel is limited. The probes mentioned so far in this thread are all coasting - no continued acceleration. IIRC, one (or both) of the Voyagers is doing something like 40,000 miles a second, but that’s just 1/7th or so of your billion mph. Gamma at this speed is a pathetic 1.000000008888889 or so (a football field going this fast would be just 33 millionths of an inch too short). And those are the fastest man-made craft to date.
Colliding with stuff is just about it. That ‘stuff’ can be awfully small, though. If more starlight is hitting the nose of the ship than the back end, you’ll be decelerated by some teensy weensy amount (you’d be ‘colliding’ with more photons than were ‘pushing’ you).
Gravity? Only if you’re going slow will it have a truly noticeable effect. Consider this: the only way for gravity to slow you is to be going away from something (say something with a lot of gravity like a black hole). To be going away from it, you must have come towards it already (which sped you up). The net effect of coming and going is almost, but not quite, no change in speed. If you’re moving slowly, you can use relatively-large sources of gravity (planets compared to the probes we’ve launched) to get a ‘slingshot’ boost in speed (along with a big change in direction). If you’re going really fast, though, the only real effect would be a slight change in course.
Special relativity is a bit hard to wrap your mind around.
Think of it this way. When we look one direction at great distance, we see a galaxy moving away from us at 75% of the speed of light. We look the opposite direction and we can find a galaxy moving away from us at 75% of the speed of light that direction. So it would seem intuitive to assume that the two galaxies would see each other as moving apart at 150% the speed of light. It turns out they see each other moving apart at just under the speed of light.
OTOH, onboard the ship, there’s no apparent limit to your ability to accelerate. It’s not as though the same thrust produces a smaller force of acceleration against your feet. You could accelerate at 1G right up until you ran out of fuel. But as you approached the speed of light relative to nearby stars, you would see your speed change very little. It feels like your speed should be changing by 9.8 m/s^2 but that’s not what you see.
All the previous posters are correct, evno, but I’ll try to make it simpler.
Velocity and relativity in a nutshell:
As the velocity of an object aproaches the speed of light, its mass approaches infinity. An infinitly large mass requires an infinite amount of energy to accelerate. Thus, give enough fuel, an object could achieve 99.99999etc% the speed of light, but it could never achieve the actual speed of light. Think of it as diminishing returns.
As nobody has taken care of this misconception I’ll attack it. Anything to combat ignorance.
The error is in your assumption that your own mass would increase in your own frame of reference. The so-called relativistic mass is a fishy phenomenon, and Einstein himself reportedly never used the word mass for anything but rest mass since 1921 or so. Mixing in E=mc[sup]2[/sup] makes things even worse, as that is only the special case of **E[sup]4[/sup]=m[sup]2[/sup]c[sup]4[/sup] + p[sup]2[/sup]c[sup]2[/sup] **, with p=0.
To a bystander at rest it would appear that your momentum increases, and they would therefore draw the conclusion that you gain (relativistic) mass as well…
Whereas in your own frame you lost weight (as fuel), and accelerated accordingly (but in a Lorentz-contracted universe, with time-dilation etc).
The thing to remember in (special) relativity is that basically nothing strange happens in your own frame of reference. It’s only when viewed from the outside that things start to get complicated.
Oh, well… I see that I have only increased the confusion. I hope Dr Matrix , Chronosor ultrafilter will set things clear. On the other hand, they did a great explanation inanother thread recently, which dealt mainly with Special Relativity, but with some GR as well:
What is the theory of relativity?
Actually, a billion mph is 50% faster than the speed of light.
The speed of light in MPH is:
186,282 [sup]mi[/sup]/[sub]sec[/sub] * 3,600 [sup]sec[/sup]/[sub]hr[/sub] = 670,615,200 [sup]mi[/sup]/[sub]hr[/sub]
It looks like you were off by a factor of 1,000. A million MPH is 0.15% C.
Hey, your explanation seems pretty clear to me.
I agree; explanation clear and happy as can be. Except that tc meant E[sup]2[/sup], not E[sup]4[/sup], in case anyone was confused.
True but a little misleading. No matter what speed you are going, light will always move at the speed of light relative to you. It doesn’t matter if you are emmiting the light or some other object is - it will always move at c for whatever your speed is.
Now if you start a a point and want to get to a point 4 lightyears away you can get there in less then 4 years (actually you can get there a lot faster if you don’t slow down for the 2nd half of the trip and instead shoot right by you point). How do you do this - accelerate at 1g for 1/2 your trip and decelerate at 1 g for the 2nd half. Space and time will actually change to allow this as you approach c from an outside observers point of view.
and no your mass will not start increasing as you approach c (to you it won’t but to a stationary observer it will).
Fuel is a big consideration - I did a rough calculation and came out with something like your ship would have to be 98% matter/antimatter fuel assuming 100 efficency to reach alpha centari in 3 years (to the traveler not the observer) assuming you want to stop when you get there.
tc wrote:
Quite the opposite. You have successfully combatted my ignorance. Bravo!
AWB wrote:
I knew I was going to screw the math up somewhere, which is why, after writing a loooong post with all sorts of examples of ships going various speeds, I deleted 90% of it.
And thinking about what I was calculating last night, I know exactly where my problem was: I divided one billion by 3,600 to get miles/second, but then all the other math I’d done in the deleted stuff “took over” and I started using the result (277,777) as meters/second. This is a great example of why “peer review” is a good thing. Thanks.
Now, if I can only get the pink off my cheeks…