Lets say for a second gravity in it’s existance is a contant near and around the objects to be mentioned. Let us go a little further and say the sling shot/‘using a planets gravity’ thing (that nasa uses for typically for getting from one body to the next, aka moon and mars travel) also stays in effect near and around the to be mentioned.
Ok enough pre-bullshit. I was in another thread about how space travel requires one day for near or faster than light speed and since this currently seems improbable, then why continue space exploration. Well this is depressing. But that brought me to ask the forum if they had any crack pot ideas on FLT or NLT.
Personally I think If the Straight Dopers had their way, we would already be there but that is just my subtle way of getting the forum ‘fired up’.
Anyways. Back to the thread. Using bodies of similar size, in ascending order ( as in asteriod, moon, planet, sun, system, etc…) classified by A, B, C, … until the mass of the system would equal the mass of the pull of a black hole. Could one use a sling shot orbit from one mass A to mass B etc… and gain enough velocity to sling shot past a blacK hole/‘very massive object’ and gain the obvious FLT. I am just putting this out there because it seems crack pot but I am just curious as to the flaws in how I think.
I mean since the velocity is ever increasing with the pull of the next mass the only fuel needed would be exit velocity fuel but compared to how much it takes without the sling shot, pt a to b travel is far less efficent. As far as I can tell, one could probably harvest fuel as they wait for the ship to come on the next go round. All you need is maybe an alignment of the planets or something to make a more even mass. And you would not be pulled apart by the pull of the black hole because the velocity is already near that speed. I dunno. Help.
I think the major problem with you idea is that the universe is extremely spacious. It will take Pioneer 10 abt 2 million years to go 68 ly. Which means that at that speed it would take nearly 120,000 years just to get to our nearest neighbors 4ly away. WAY too long to wait between slings. Of course it’s not the fastest space ship out there. I believe the ESA holds that record with their ship that visited the comet Halley back in 86.
That being said there are many ways for us to make it to nearby systems in reasonable times. Although everyone of those mean that you will never come back. And even if you did your family, friends would all be long dead. I believe that if we wanted to we could also colonize the entire galaxy in a reasonable amount of time if we tried. Basically this would including to flying to a good system as fast as possible then once there having as fast a turn around as possible, I’d estimate 2,000 years would be a good amount of latency time to get the large population necessary to send out dozens of colonies. I’d say we could do this in about 2 mill years. Maybe even in half that. I do the math but those equations are HARD.
There are ways to reach nearby stars in a reasonable amount of time, but as BR pointed out, you probably ain’t coming back! Orion ships are one good example…
My wild fantasy is to gain total control of gravity: then one could simply place a gravity well of your preferred strength a foot or so in front of your ship (making sure that it stayed a foot in front of your ship!). You could then FALL into near light-speed velocities! Cool!
For convenience, you could make the gravity well at 1 G… how long would it take to reach near light-speed at 1 G? Uh… damn! There’s the phone! Gotta go!
(can someone run the numbers for me? I’m WAY too far from my college physics, plus I’ve had a beer or 3… v=1/2at[sup]2[/sup], right? Oh God!)
I’m not sure you could ever go faster than lightspeed, no matter how many slingshot attempts you did. Isn’t that a clear violation of Einstein? What’s the conceptual difference between firing very big thruster jets and using gravity?
Even if you kept using the slingshot method to gain velocity, you would never go FTL because your mass would increase. The faster you went, the more massive your ship would become. This is the same problem with trying to get a ship past light speed using really huge ass rockets (RHARs). Even with an RHAR, you could not go faster than light because you would get more massive and reuquire even MORE propulsion…
IMHO, FTL travel will never be possible without learning to warp space in some manner – whether by worm holes or warp fields or something not thought of yet…
First of all you cannot ever go faster than the speed of light. Period. If you did your mass would become infinite and you’d be everywhere in the universe at once. There may be shortcuts (ala a trip through hyperspace that shortens the distance you travel) but so far those methods are relegated to sci-fi stories.
That said Bear Nenno is on to the answer to the OP. As you go faster your mass increases. Eventually you (or your spaceship) would have a mass far in excess of the planet you are approaching and you would pull that planet from its orbit.
It is true that even in this case the planet would provide an acceleration to the traveller but that acceleration would be vanishngly small (consider how much you alter the earth’s orbit by jumping).
I suppose that at some point your mass would be so great that the mass (presumably) evenly distributed throughout the universe would tug at you from all directions thus negeating acceleration in any one direction.
And last, but not least, realize that motion is relative as well. I don’t know the math but a spaceprobe slingshotting around Jupiter leaves Jupiter with exactly the same energy in came in at (assume no rocket boost). The probe will speed up as it approaches Jupiter and slow down as it moves away. In the end the two cancel out.
What it does gain is angular momentum relative to the sun. So, from Jupiter’s perspective there is no speed increase but from the suns perspective the probe has gained angular momentum from Jupiter (this is important if you are looking for escape velocity from the solar system).
It’s momentum, not mass that increases at relativistic speeds. The concept of relativistic mass is an ugly hack to make the math work out, but you can just as correctly redefine momentum to account for the same effects. The reason I bring it up is that failure to make this distinction gives rise to mistakes such as:
Which is not at all what current theory would predict.
Now, on to the question, you can gain velocity by successive slingshots, but you run into problems. Firstly, there’s the matter of finding appropriately massive bodies to slingshot from. Secondly is the matter that those bodies are probably not advantageously positioned, and you’ll end up doing a drunkard’s walk, if you can get going at all, wasting gains in speed by increases in distance.
More serious proposals for interplanetary/interstellar drives are the aforementioned orion drive (set off nukes behind your craft), solar sails, and my current favorite plasma solar magnetic sails. According to the page a craft using this method of propulsion could theoretically beat the Voyager probe out of our Solar System.
NASA has done a number of highly experimental probes recently. One of which used an ion propulsion drive. This drive is low impulse, but very very fuel efficient. It has low acceleration, but can get going very fast when you just let it go.
here is a NASA page on possible breakthrough propulsion methods.
here is one on magnetoplasma rockets and reducing travel time to mars.
Ok, so most of the time people talk about mass they are talking about rest mass. What is really happening to an accelerating object is its energy is increasing.
Energy bends spacetime in the same fashion that mass does (they are interchangeable…E=MC[sup]2[/sup]).
So, while saying mass increases may be speaking loosely wouldn’t an object with an insane amount of energy (having been accelerated to near light speed) have an apparent gravitational effect on nearby objects as if its mass had increased?
Think of it this way; if you were standing on the object would you feel an insanely high gravational force? No, you wouldn’t because the rest mass has not changed.
Or - fly along with the object - its mass now isn’t any different than when it was stationary. Gravitation is a global effect not a frame varient artifact.
Ah. I studiously avoided trying to say what current theory would predict, as I do not know. However, we can have a little gedankenexperiment until the math-types show up to provide hard answers.
Certainly we both agree that the energy of the spacecraft increases with increasing velocity. However, I would think that it would have an upper bound, as it has an upper speed limit. But I recognise that you can pump an arbitrary amount of energy into it. Bottom line, I dunno. But the gravitational aspect is what I wanted to discuss.
Imagine we have a spacecraft which we can accelerate arbitrarily. At some point, we will have put enough energy into the spacecraft that if it’s gravitational effect was being effected, it should become a black hole. Assume that from the reference frame where the craft is going .999…9C this is the case. If the craft were to fire a laser (even in a direction orthogonal to its motion) that light shouldn’t escape. However, from the reference frame of the craft, an orthogonally pointing laser should escape with no harm. Granted it will be redshifted due to time dilation, but the fundamental difference is between escaping at all, or not. This is a logical contradiction. From this, I can conclude that our assumptions were somehow flawed. Since the only assumption that I can see we made is about the gravitational effect of relativistic velocities, I conjecture that that is where the mistake lies.
I would welcome a thorough treatment of this subject, with any relevant maths.
This statement sounds misleading. The probe will indeed leave Jupiter going faster than when it came in. The energy to do so was stolen from Jupiter by slowing it down - not much because of the great difference in mass, but it still must slow down Jupiter. This of course is from the point of view of the Sun. Jupiter before the encounter has a kinetic energy of J, and the probe has P. After the encounter, the probe has P+x, and Jupiter has J-x.
If you looked at it from just Jupiter’s point of view, then the probe would leave with the same speed, but that speed would now be directed more in line with your orbit around the Sun.
CurtC:
I’m not positive but I think you’re saying what I was saying. The probe leaves Jupiter with no more speed than it came in with but the probe has gained speed relative to the sun. Let me know if I’m missing something.
Rind and TheNerd:
I’m getting confused on the energy/gravity bit.
Imagine taking the earth and locking it in a big, indestructible, non-deformable box. Floating outside the box you measure a gravitational pull of X.
Now, by the magic of the hypothetical, the entire earth gets zapped and 100% converted into energy. There is no matter whatsoever remaining in the box. Floating outside you still measure the same value of X for gravity.
In one case we have energy. In another case we have mass. Energy is mass, mass is energy. From outside my hypothetical box you have no way of knowing what’s inside merely by measuring the gravitational pull from the box.
Now we’re back to my accelerating you. I am dumping energy into you. Again, energy=mass (E=MC[sup]2[/sup]). Is energy in the form of momentum somehow qualitatively different from energy in the form of photons (I thought way deep down all forms of energy are manifestations of one force)?
As to turning yourself into a blackhole I have no clue. Perhaps energy has a natural repelling force that cancels out the gravitational collapse. This doesn’t happen in a star because the energy from fusion doesn’t rise in lockstep with the addition of mass so eventually the gravitational attraction overcomes the repulsive force of the energy. (Again…this is just speculation on my part.)
E = energy, p = momentum, m = rest mass, c = lightspeed
What this formula says is that radiation that has zero momentum possesses rest mass. Since the Earth prior to being zapped had zero momentum (in some frame) then by conservation of momentum it must have zero momentum after being zapped, and the radiant energy therefore has rest mass.
On the other hand if you are on a rocket the rocket has momentum and therefore it does not have rest mass.
In other words, what matters for gravity is the portion of the energy that you can’t transform away. If I have one billiard ball moving really fast (such that its energy is many times its mass), I can still take a frame of reference where it’s not moving at all, and where the only energy it has is its rest mass. On the other hand, if I have a big box full of a bunch of billiard balls bouncing around every which way at high speeds, then I can’t transform away that kinetic energy: If I put myself into the rest frame of one ball, then the ones going the other direction at that moment are suddenly going much faster, and have enough extra energy to more than make up for the energy “lost” from the ball I’m following.
By the way, there is no limit on the possible kinetic energy of a moving mass. There is a speed limit, but K. E. is no longer equal to .5mv[sup]2[/sup], it’s (gamma - 1)mc[sup]2[/sup], where gamma = (1-(v/c)[sup]2[/sup])[sup]-.5[/sup]. This increases without bound as v approaches c, so you can have, for instance, a proton with the energy of a major-league fastball.
