Lets say we want to send a message to Bob, a friend who lives near Alpha Centauri. Lets keep it simple and assume the message is a basic signal, something like a single flash of light. On receiving the message, Bob will ... I don't know ... feed my goldfish, which are staying with him while my cockroach-infested house is being sprayed. Now, Bob is about 4.3 light years away, so if we point a big laser at him and send a quick flash of light, the transmission time will be 4.3 years.
By then, all the fish will be dead. Clearly we need a faster way of sending Bob a signal.
What if we had a solid rod, 4.3 light years long? We pull our end, Bob feels the tug immediately, and feeds the fish. They survive. Remarkable. Nothing is moving very fast at all, but a message is being delivered to Bob far faster then if it were made of light. This idea overcomes all problems of communicating over long distances. I know, however, that there are a few problems with this idea. Here are a couple:
This is how real-time intergalactic communication might become realistically achievable.
If you're living in Orson Scott Card's (or Ursula K. Le Guin's) universe, you could simply use an ansible relay.
For those not familiar with the Ender novels, the ansible is a clever little device to allow instantaneous communication between any two points in the universe. When it was first introduced in Ender's Game, there was no explanation as to how it worked; we were only told that humans learned of the existence of such technology from an alien life form that used it, and once they knew it was possible reverse engineering it was just a matter of time. By the time the next novel, Speaker for the Dead, takes place, ansibles are widespread and essential for communication among the interplanetary human colonies.
Mind you, sending spaceships faster than light is still impossible; the ansible only allows transmission of data. The idea is based on the idea of quantum entanglement, where two subatomic particles are "entangled" in such a way that each one is aware of the other's state, no matter how far they're removed from each other. Alter the state of one particle, and the other responds in the same manner, even if they're light years apart. Sending information encoded in binary through such a device is just a matter of switching states back and forth over and over again.
Unfortunately, there are real and quantifyable reasons why an ansible can't work in the real world, most of them involving a working knowledge of quantum mechanics. The proof of this is left to the reader as an exercise.
Any time I refer to c let us take it that it is in vacuo.
Ok the whole rod thing, what actually happens when you move a rod is that you send a compression wave down the rod. We are used to having rods which seem to be rigid but that is because in the domain of use the speed at which it is pushed is usually a lot less than c. If you have a very long rod moving one end is sending the information along the rod as a compression wave. This wave can't move faster than c. A wave in a rod of 4.3 light years in length will probably dissipate into heating parts of the rod and the distant end will probably not move at all. You can't create a perfectly rigid rod, if from nothing else the uncertainty principle so enough with the rods already.
Quantum entanglement does indeed exist. It is known as the Einstein Podolsky Rosen paradox (or EPR for short). It is not a paradox as no information can be sent using this method. This was explained by Bell with Bell's theorm Allthough you may measure an entangled particle and instantaneously know what state it's partner is in this is not a transmission of information. Let us say that I want you to feed my goldfish if you measure spin up and I spin down. The information is really the code of action to take as a function of the measurement. That code has to be prearranged. It is the passing of the code which is the passing of the information and that cannot be done using the entangled particles.
About the Cesium gas experiment, Tiefling's comment says it all.
My point is that the particle that mediates the graviational field is proabably massless and applying notions of momentum to it is not a valid thing to do. The particle is a manifestation of a field and the test particles dynmics are determined by the shape of the field they find themselves in. (often physics seems like a magical place).
I know that this has not been a very clear explanation, I apologise. I will endeavour to do better in the future. The most elegant description of gravity is through geometry (Einsteins general theory). All of the decriptions, geometric, field, particle, wave, codify the same physcis and are physically equivalent. When thinking about it, it's nice to have many approaches to the problem.
You are right, things might change. The most promising idea on the scence is M-theory. An implication of M-theory is that the global topology of the universe might have extra-dimensions which are foliated onto one another. In this scenario gravity propgates through this higher-dimensional bulk,, while the other forces are constrained the the lower dimensional sub-manifold, (called a brane after membrane). Gravity can effect objects that are not within each others causal light cones. This theory remains just that. Currently experiments are beginning to test it. I may work on some aspect of this in the current year, If I do I will let you know.
Now, what is obvious is that dryers already harness trans-dimensional folding. That's right, even the ones at YOUR laundromat down the street. Of course, the ideal use, at least while we're speaking of dryers, would be to fold in heated gas from around the sun to dry your clothes with, but we haven't become that advanced. No, instead, some alien civilization has spent hundreds of years developing the technology to kidnap all our socks. It's not a conspiracy, really, they just have 12 feet or something and no soft warm cotton to make coverings for their toes with.
Regardless of the final purpose of the missing socks, the socks themselves are proof that trans-dimensional folding technology already exists. All we need to do now is figure out a way to pass a message from one dryer to another just like that hot pink sock that ended up in your dryer last Saturday came from an 85 year old woman living in Cleveland. Once the means to harnessing this awesome power has been discovered, it is simply a matter of placing the desired piece of information in a sock, dialing up the destination on the dryer, putting in 4 quarters, and looking the other way while the interdimensional technology goes to work.
Dryers. Obviously the key to Faster-Than-Light travel.
We need a power source that generates infinite power.
Therefore, I propose, the world's first light speed engine: Buttered toast strapped to the back of a cat inside a dryer in motion!
The cat and buttered toast's infinite energy will rotate in a direction opposite to that which the dryer is spinning at, therefore combined with the power of dryers to tesseract (fold the distance between two points in space/time, as seen in the children's book A Wrinkle in Time), the dryer/cat and toast combo will be used to propel space ships across vast distances in space! No longer are we limited to just socks!
And to think, all this time, we were thinking that matter/antimatter reactions would do it. Who would have imagined that we had all the elements readily available? Time makes fools of us all.
For example, hit a nail with a hammer; it will still take a fraction of a second before the tip "knows" that it should move.
Lots of things aparently move faster than light, but when you get down to examining it, nothing really does.
First, stand in the center of a sphere with a 3x10^8 meter circumfrence, and shine a laser pointer at the wall. Now spin the pointer in a circle in less than 1 second. The spot on the wall will move faster than light. However, the light itself doesn't. Lots of things that don't really exist (a spot of light on the wall isn't a "real" thing) can move faster than light.
Second, a photon is a wave packet. A wave packet has both a phase velocity and a group velocity. The phase velocity is the speed that the peaks inside the packet oscillate, and the group velocity is the speed the wave packet actually propagates. For light, the group velocity is always less than c, but the phase velocity may, in fact, be much greater than c. The experements using cesium gas measured a phase velocity of 300c -- an interesting achievement for the intellectually curious, but in the words of the experementor, nothing earth shattering.
Third, in quantum mechanics, two particles may be entangled. Measurement of an entangled particle will cause its companion to collapse into a definite state. However, there is no way to tell that this has happened until you compare notes afterwards -- via slower than light mechanisms.
Theoretically, a wormhole could be turned into a time machine by taking it on a rocket trip, explained in _Black Holes and Time Warps_ by Kip Thorne. However, last I heard there was debate as to whether gravitational fluctuations would annihalate the wormhole the moment it could be used as a time machine. This is of course assuming you ever could construct a macroscopic worm hole and make it stable.
An interesting result of relativity is that any FTL signal can be used to violate causality.
My opinion is that FTL transport/communication is impossible, mostly because the universe seems pretty good at plugging loopholes.
A practical objection which has not been raised is the relative velocities of the respective solar systems and planets. A number of solutions present themselves:
First, Is it possible to transmit information faster than light? I don't mean with current technology, of course, but does the Theory of Relativity specifically forbid this? It's often said that transmitting information FTL would cause or allow time travel-like paradoxes (i.e.- If you transmit a signal which arrives at its destination before it is sent, you could decide not to send it after it arrives, creating a paradox). But (in my understanding) when people say this, they seem to be confusing light speed with infinite speed. (correct me if I'm wrong, but...) Something travelling at infinite velocity leaves its origin and reaches its destination at precisely the same time (zero travel time). To arrive before its departure (negative travel time), it would have to travel at greater than infinite velocity. This brings me to my second point.
Would an object travelling faster than light travel back in time? Light speed, c, is finite. Twice the speed of light, or 2c, is also finite. However, according to Relativity, as one approaches light speed, subjective time slows, and ultimately stops. Here's how I understand it--A spaceship travelling 2 lightyears at the speed of light (ignoring acceleration and deceleration) would take 2 years to complete the trip, but no time would pass for the passenger(s), because of the time dilation effects. Based on this, a ship travelling at twice the speed of light would travel the same distance in half the time, but would still arrive one year after it left. So, (according to my possibly flawed logic) an object does not travel back in time by travelling FTL. I mentioned subjective time because that was the idea on which, I think, many people based the entire FTL time travel concept. To speculate along those lines, time (from my perspective) would slow down as I accelerate, until it finally stops when I reach c. To an outside observer (assuming they could somehow observe me going that fast), my watch would stop, and I would stop aging. If I were to exceed light speed, would subjective time reverse? This would not seem to have any effect on the outside universe, but would seem rather confusing a hypothetical traveller who suddenly realizes he's already on the vacation to Alpha Centauri he had been planning to go on several months (or years) later. He would have to reset his watch and calendar, and be reminded of all the things he forgot, since during his flight, he would have gotten younger, and lost memories of things that happened before his departure. That gets rather mind-boggling...and I've gotten off my original topic. Hopefully the actual experts can clear some of this up a little. ;)
Even Albert Einstein had no idea what would happen, if someone would travel faster than light. So he called it a paradoxon. Theory of Relativity explicitly forbids traveling faster than light. Mathematically seen, some things were really strange. For example the following equation: l = l' * sqrt(1-v²/c²) (called "Längenkontraktion" in german)
Lets say we travel at 2*c: v = 2*c v² = 4*c² l = l' * sqrt(1-(4*c²)/c²) l = l' * sqrt(1-4) l = l' * sqrt(-3)
The same would happen with time... t = t' / sqrt(1-v²/c²) becomes... t = t' / sqrt(-3) and mass... m = m0 / sqrt(1-v²/c²) becomes... m = m0 / sqrt(-3)
can anyone image what this means in practice ?
An important fact in Albert Einsteins Theory of Relativity is the frame of reference. The man who is travelling near the speed of light does not feel, that he does not get older, nor that he shrinks.
Only the people who see him travelling (or would, if they could) recognize this behaviour.
So, if it would be possible to travel faster than light, then the traveller would remember everything !
I think he would not travel in time.
When we take the definition for velocity as some distance in some time, than we can state, that the time AND the distance must have a kind of "raster", a very small quantity, which cannot be broken into smaller pieces.
Sounds familiar ?
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