Slashdot Mirror
Alien Solar System Much Like Ours
MrGort writes "Wired News reports that British astronomers say they found the first sun-like star with a giant gas planet in an orbit similar to Jupiter's, which leaves plenty of room for worlds like Earth and Mars. This system is a quick 90 light years away. The similar solar system to ours means that this gas giant could attract most of the debris, allowing smaller planets closer to the sun to develop like ours did!"
I agree with you that we shouldn't be too pessimistic, however the Wright Brothers' flight was more of an engineering challenge than a scientific one. They required no novel physics to accomplish their feat, only the application of known physical laws. It will be possible for us to explore our own solar system using known physics by using nuclear propulsion (fission and some day fusion) and even solar sails. However, travel to other stars in less than a human life-time in our frame of reference will require super-luminal speeds. There is no physics known yet that will allow us to achieve this. So, interstellar travel will be a lot harder for us to achieve than the Wright Brothers' first powered flight.
Stick Men
(a) Most physicists think gravity is transmitted at light speed. Very few (and none who believe in General Relativity) think gravity is instantaneous.
(b) (I Am A materials scientist) "Solid" matter is composed of atoms bound together by electromagnetism. When you "push" a solid object, displacement waves (essentially sound waves), travelling from atom to atom inform the material that you are pushing it. For sufficiently fast pushes and short timescales, even a block of carbon steel looks like a wobbly jelly. This is important in impact engineering, for example, and mechanical engineers and materials scientists deal with stress waves in solids all the time (plastic torsion waves are the most "fun"). Nothing is perfectly solid.
Your "stick to europa" would have to have unphysical infinite rigidity for instantaneous transmission. In real life, assuming you could make a stick to europa (not in itself unphysical, just extremely unlikely), a wave train would travel down the stick when you displaced one end, displacing the material of the stick. This would happen at the speed of sound in the stick, which is always significantly lower than light speed (since it is determined by interatomic interactions, themselves subject to light speed) So yes, conceivably, the drum would make a sound, but the sound would come some time after you pushed the other end of the stick, since the stick would be acting like a wobbly jelly on such a scale, as all atomic matter must.
You can even see this in action - surely you've seen the high-speed movies of bullets hitting apples, with deformation waves crisscrossing the surface? All solids behave that way, it's just the waves travel very quickly (but not nearly as fast as light...) in some solids such as hardened metals.
Re. slower than light travel - if you get fast enough (i.e. a sizable fraction of c), then, even if it takes dacedes to get where you're going, time dilation will mean that far less time passes for the crew of a spacecraft - so, if you're going fast enough, a trip of 90 light-years, say, could be accomplished within the natural lifetime of the crew without FTL travel.
There turn out to be practical problems with this. Any craft that carries its own fuel with it - including the more practical breeds of antimatter drive - will be limited to a crusing speed of about 0.1-0.2C by the specific impulse of their fuel. The only thing that could approach speeds at which time dilation would be significant is a beamed core antimatter drive (that uses the charged particle shower from an antiproton annihilation as the reaction mass), but that requires unrealistic amounts of antimatter (positrons are easy to make, but antiproton synthesis is very inefficient, and will remain so unless new physics is discovered).
In principle, some kind of sailcraft driven by a stationary laser or maser array could reach relativistic speeds, but the array would be very expensive to build and very large (we need to focus on a planet-sized sail at a range of many light-years). It would also work wonderfully as a weapon capable of melting cities to slag at a range of hundreds of AU (or even light-years, depending on configuration), so I suspect non-proliferation agreements would prevent it from being built in the first place.
In short, the only hope for relativistic travel at less than colossal cost is new physics.
It's been far too long since I read a non-fiction book with spaceships in it, but can't you (in theory) propel a spaceship by shining a very powerful light out of the back, using the photons themselves as the reaction mass? Then could you get nearer to c?
You can, but the problem is generating the light in the first place, and the fact that light has a lousy ratio of momentum to energy (it has to be very, very bright to generate significant thrust).
Most light sources that are bright enough to move a ship at any reasonable acceleration (e.g. fusion bombs wrapped in other matter or just shining on a shield block that can tolerate gamma rays) waste matter - the energy to mass ratio of a fusion bomb is much worse than that of the photons you're driving the ship with. This means you'd be better off just using a magnetic bottle to deflect the plasma resulting from the fusion explosion, and you'd still end up with specific impulse too low for relativistic flight.
A light source that doesn't ablate or otherwise lose mass has to be relatively dim (either a hot block of solid matter or a confined plasma ball), which means getting anywhere will take an extremely long time.
The forms of light propulsion that I've seen considered involve generating the light somewhere else (e.g. a laser array) and just reflecting it off the craft's sail. You still have a drive that's horribly inefficient energy-wise, but the energy source doesn't have to travel with the craft.
For reference, power to thrust is 3e8 W/N for a photon drive (energy to momentum ratio is C for photons).