Science/FTL Technology

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Faster-than-light (or superluminal) technology is crucial to Multiverses. It allows the movement of people, objects, information and resources between star systems, where even reaching nearby stars from one solar system would take years at sublight speeds.

FTL Travel

FTL travel is the movement of objects at faster-than-light speeds. Light speed (or c) is 300,000,000m/s; by means of comparison, sound in the Earth's atmosphere (at STP) travels at 343m/s.

Why can't I travel faster than light?

Light speed is a fundamental barrier, and objects cannot be accelerated to faster-than-light speeds, as per the Principle of Invariant Light Speed (one of Einstein's postulates of the theory of special relativity). Why?

Imagine a car travelling along at 10m/s (22.4mph). If the driver of one then throws a tennis ball from the window at 10m/s, then what is the ball's velocity relative to the ground? 20m/s, right?

At these speeds, conventional wisdom holds. However, it is not the case that velocities add- percentages of velocities add. It is just that, in our world of relatively slow motion, these percentages are as near as dammit 100%.

Now imagine that you are in a spaceship, observing another spaceship heading towards you at 75% of light speed. If it has a headlight, what is the velocity of a photon coming from that light? As you may have guessed, it's not 175% of light speed. The faster you go, the lower the percentage that is added. It reaches zero at light speed, so velocities will never exceed c. Your time and distance coordinates distort to allow lightspeed to be equally far away from people moving with respect to each other.

There are other objections as well. As your speed increases, your mass increases. This is negligible in a car or a plane, but becomes very noticeable at relativistic speeds. And, as you will know, the greater your mass, the more energy you must input to accelerate. When you reach c, your mass is infinite, but you couldn't get this far anyway- as you came close, the energy requirements for accelerating would become greater than the energy content of the universe.

This is all well and good, but has it been proven? Yes. Relativity is backed up by over a hundred years of rigorous testing. here is a Wikipedia article on tests of special relativity. It's part of our daily lives- if you use a GPS, the satellites transmitting to you are affected by their greater distance from the Earth (time moves slower the stronger the gravitational field), and if this was not compensated for, such mechanisms would not work. The orbit of the planet Mercury cannot be explained by the Newtonian system, and only when relativity is factored in.

And what if another theory comes along? This will not make all this information redundant. A new theory would have to explain everything that relativity has done and more, or do it in more detail.

What methods have been suggested to allow FTL?

As you can imagine, the light-speed barrier is a major problem for our interest in leaving Earth. It also doesn't allow fast-paced science fiction outside of single star systems (though Firefly is a good example of science fiction within one star system, using no FTL).

Lots of very intelligent people have considered the matter. All solutions suggested are highly speculative and most, if not all, are almost certainly impossible, or at least impractical. Here are some of the methods given:

The traversable wormhole: Wormholes would be a shortcut through space, provided by a pair of singularities that had somehow 'found' each other and connected. Kip Thorne and Mike Morris demonstrated in a 1988 paper that wormholes which can allow travel between distant parts of the universe are compatible with general relativity. However, they are fundamentally unstable- very large quantities of negative energy would be required to stabilise one, allowing transit. Negative energy densities are possible, as demonstrated in the lab with the Casimir Effect, but this mechanism would not scale well up to the scales required to produce enough of the stuff (billions of miles across). It's hard to find solid figures on the quantity of negative energy required, but Matthew Visser of Victoria University (in New Zealand) says a one-metre wormhole would require a (negative) Jupiter's mass-equivalent in negative energy to remain stable.

But you would then have to escape the event horizon on the other side of the wormhole (impossible without FTL) and, of course, you have no idea where you are going.

Alcubierre drives: In 1994, Miguel Alcubierre suggested that a kind of 'warp drive' would allow FTL travel. In this system, the ship would cause the expansion of space behind the ship and the contraction of that before, allowing the bubble of space in which the ship sits to travel faster than light (relativity does not prohibit this, to the best of human knowledge).

Of course, there are serious problems. First, the only objects capable of producing such warping are black holes, which are not manipulatable (at least at the current time). Second, negative mass is required to sustain the bubble (around -10^64kg, where 10^64 is one followed by sixty-four zeroes, to transport a small ship across the Milky Way. 10^64kg is more than the estimated mass of the universe. A bubble large enough to enclose a starship 200 meters across would require a total amount of negative energy equal to 10 billion times the mass of the observable universe.). The negative mass must also be limited to a wall of thickness 10^-32m, just higher than the Planck length, which is clearly impractical. Other problems include incoming particles heating the interior of the bubble to temperatures not seen since the Big Bang, saturation with gamma rays from Hawking radiation, the ship cannot see where it is going or steer, and when the ship stops it will emit an enormous burst of gamma rays and high-energy particles in the direction of travel, destroying the destination.

The Krasnikov Tube: Proposed by Serguei Krasnikov in 1995. The Krasnikov tube is a hypothetical construct generated (using hypothetical technology) behind a spaceship travelling at high relativistic speeds. It does not increase the velocity of the travelling ship, but enables any ship travelling back along the tunnel to arrive at the starting point just after the ship left.

With one tube, the method does not violate causality, but with two, it does. Causality is not something we're just going to throw away, and imagining that the production of a second tube would be forbidden whereas the first would not is not reasonable. Also, it would require enormous amounts of negative energy to stabilise.

Tachyons: It has been proposed that ships could travel at FTL speeds by converting their mass into tachyonic mass. Tachyons always travel faster than light, and travel faster the less energy they have. There are two main problems with this: tachyons can't exist and even if they did, there is no known way to convert real matter into tachyonic matter.

FTL Communications

This is the transfer of information at speeds exceeding c. FTL communications are often found (and left unexplained) in science fiction, and the most common term for an 'FTL radio' is an ansible.

Suggested methods for FTL communication:

Quantum tunnelling: Quantum tunnelling is a process in which particles cross boundaries by 'borrowing' (for want of a better term) energy temporarily from their surroundings. This occurs a lot, but any given particle only has a very small probability of being able to tunnel. Raymond Chiao measured the speed of spin zero particles during tunnelling at 1.5 to 1.7 times light speed (not good enough, but this would be a start).

However, this is its phase velocity only, and a superluminal phase velocity doesn't allow transmission of FTL information. Also, channels allowing this kind of movement cannot be laid out faster than light.

Quantum entanglement: Learn about the science behind this here. This is thought to be the best method for FTL communication to occur in the real world, and has been demonstrated to allow simultaneous observation of state collapse in two places at once.

However, it might not allow transmission of information, but rather only allowing two observers to observe the same wavefunction collapse at once, without being able to change what happens.

Mass Adjustment

There is FTL travel in the Multiverses Wiki multiverse. It is the One Big Lie of the wiki- it is not based on fact at all. The process revolves around mass-adjustment technology.

Momentum is conserved within all systems, even in non-Newtonian systems. Momentum is the product of mass and velocity, so changing the mass will result in a change in velocity given that momentum is conserved. For example, say an object is moving along with 10Ns of momentum. It has a mass of 10kg and a speed of 1m/s prior to adjustment.

The mass-adjustment field then halves the mass of the object. In response, its speed doubles, to 2m/s.

Scaling things up: Say we have a ten-tonne starship travelling at half the speed of light. We then halve its mass, and the speed will double so that the ship is now travelling at light speed, bypassing most of the tricky problems. There is no violation of causality, thermodynamics, and so on. As the field is projected around the ship, incoming particles are similarly affected and do not cause any exceptional change in conditions.

FTL communication is possible by sending objects with FTL capabilities to the destination, carrying information.

Appearance

Direct detection of an object travelling at FTL speeds is impossible unless the detector is travelling parallel to, and at the same speed as, the object. This is because detectors receive or emit information at the speed of light or lower, and the object in FTL is no longer where the signals come from.

Where the object travels through space, photons going through the mass bubble are refracted, and diverted in all directions. Particles, too, will be dispersed. Photons reflected from the ship will travel outwards and leave the field in which the FTL travel is occurring, and the exhaust particles which have left the mass bubble will emit photons. As such, evidence of the object's passing will arrive, but only after the object has passed. Given how obvious spaceships are against the background of space, these traces can be detected from millions of miles away. The trail is not all as it seems, however, and since the emissions from the object coming from closer to the source will reach the detector first, an object at FTL heading towards a detector will appear to be receding at the speed of light.

Races with FTL technology will be aware of this, and adapt their sensors and computers accordingly.

Here's a test: spot the logical flaws in mass-adjustment technology.

Answers.

We now use Hyperdrive (and so Ultradrive and Uberdrive.)