Is superluminal speed possible?
"Not only is the universe stranger than we imagine, it is stranger
than we can imagine." Sir Arthur Eddington (1882 - 1944)
Close neighbour, only 2.2 million light years away: Andromeda Great Spiral Galaxy.
In order to journey to the stars we will need to travel much faster than is currently possible with existing technology. Even at the amazing speed of light, at 186,000 miles per second, (670 million mph) the journey to our nearest star, Alpha Centauri, would take 4 years. To be accurate, it would take even longer than that because of the time it would take to reach light speed - accelerating at a rate that would produce 2G's force it would take six months, and of course another six months to slow down. To reach our nearest neighbourhood spiral galaxy, the Andromeda Spiral Galaxy, would take over 2 million years. We need to go even faster than light, but is it possible?.
Relativity tells us in no uncertain terms that faster than light speed is impossible, period. This is not because of any technical restrictions (as if that wasn't enough!) but simply because it is a fundamental part of the way spacetime is constructed. Let's look at the reasons why.
Mass and Energy
Imagine I challenge you to a rock throwing contest to see who could throw a rock the furthest. There are two rocks to chose from, a big heavy rock and a smaller lighter rock. It is obvious we would both choose the smaller rock to throw, because being lighter it will go further. To put this on a more scientific footing, we can say that the lighter rock has less mass, and that with a given amount of energy an object will travel faster - and therefore further - the less mass it has.
However, I was unhappy with the result of the rock throwing contest as you are much bigger and stronger than I am, which made it an unfair contest. In effect, you were using a bigger 'engine' then me to accelerate the rock. In order to overcome the problem of our different 'engines', I challenge you to a car race. The car that achieves the fastest time around the race circuit wins the contest. We both agree that this is a much fairer contest as we are now evenly matched - providing our cars are identical.
We are both presented with cars having the same size engine so the winner of the contest should be the driver with the better driving skills. We are both given identical big gleaming powerful cars to drive at high speed (I wish). What you don't know is that I was so upset at loosing the rock throwing contest (being a poor looser) I have cheated this time in order to ensure my victory and have bribed one of the mechanics to secretly place 1000 pounds of lead into the boot of your car. (cackle. cackle). The race duly takes place, and surprise, surprise, I am victorious! My celebrations are short lived however as an official enquiry soon reveals my dastardly scheme. How did you know that I had cheated? It is because although both cars had the same size engine and should have been identical in every respect, your car ran out of petrol, while mine didn't. Furthermore, when driving flat-out down the straight, my car went faster then yours, which it should not have been able to do. Why? Simple isn't it, your car was a lot heavier than mine, and given the same size engine was not only unable to travel as fast, but used more petrol in the attempt. As in the rock throwing contest, we can explain this in more scientific terms. We can say that the more mass an object has the more energy it requires to reach a given speed, and to maintain that speed.
We now know that in order to achieve the highest possible speed we need an object with the lowest possible mass, with the most powerful possible engine to accelerate it. There is something else we need to consider though, and it concerns relativity.
The Special Theory of Relativity
Relativity tells us that as an object increases in speed, so it increases in mass. This sounds impossible, how can an object increase in mass? The answer goes to the very heart of relativity, to Einstein's most famous equation: E = MC2. In plain talk this tells us that energy and mass are interchangeable, and we have witnessed this effect in the detonation of atomic bombs. We do not though, need to understand the workings of an atomic bomb in order to understand the principles involved, but it is a little complicated. The special theory of relativity tells us that the mass of an object increases with high speed. This mass increase is not detectable to anyone travelling with the object, only to observers in the frame of reference where they see the object to be moving. For them, the measured mass of the moving object increases in accordance with the equations of relativistic mechanics until, if the object could reach light speed it would have infinite mass.
In case you are wondering - the 'E' is for energy, 'M' for Mass and 'C' for light speed. In other words, energy equals mass times the speed of light squared.
The Lorentz contraction.
There is another effect of relativity for objects travelling at relativistic speeds. Apart from gaining mass, they also shorten in length in the direction in which they are moving - they get smaller! Let's look at another everyday example to make sense of this.
We shall imagine that we have just bought a brand new sports car. So excited are we with our new purchase that we immediately drive to the nearest race track to put it through its paces. On arrival we cautiously decide to let the track expert test the car first. He sets off round the race track and very soon is travelling at 120 mph. We are delighted with the car's performance but start to worry that it may be too large to fit into our garage, and decide to calculate the cars length as it passes us by. We know the speed is 120 mph, so we set up a very accurate device that times precisely how long it takes the length of the car to pass a fixed point. This imaginary measuring device is so accurate it can measure to within an accuracy of 13 decimal places. It is then a simple matter to multiply the speed of 120 mph by the time taken by the car to pass our measuring device to arrive at its length. The answer comes out at 15.9999999999974 feet. When the car eventually comes to a stop we decide to check our measurement and find that the car is precisely 16 feet long. There is no fault either with our measurement of the car at speed or at rest, the difference is real. In practice we would not be able to detect this tiny difference at such a low speed relative to the speed of light, but at far greater speeds the difference becomes very noticeable. If a spaceship were to travel at 580 million mph (about 87% of the speed of light) the length of the spaceship would be half that of when it was at rest. Why is this?
The effect is known as the 'Lorentz contraction'. You will recall that in the section 'What is Time? we examined the effect that speed has on time, and how the faster we move in space the slower we move in time. This effect is known as 'time dilation'. If we now apply this knowledge to the measurement of our speeding car, we can see why it gets shorter as it goes faster. It is necessary to take into account at this point that all movement is relative. We cannot say for example that a star is moving at 1,000,000 mph, we have to say what it is moving at this speed in relation to. It may be moving away from us at that speed, but may be moving at 5,000,000 mph in relation to another galaxy. In our example with the moving car, the speed of the car is in relation to us and our measuring device. Relativity tells us that it makes no difference which of us is actually moving (we both are when you take into account the rotation of the Earth).
Let's take the example of a high speed space ship whizzing past the Earth. From the perspective of the astronaut onboard he is stationary while the Earth rushes by, and hence sees our clock as running slow. As a result he realises that our indirect measurement of his spaceship will yield a shorter result than when it was measured at rest, since in our calculation (length equals speed multiplied by elapsed time) we measure the elapsed time on a clock that is running slow. If it runs slow, the elapsed time we find will be less and the result of our calculation will be a shorter length. This is an example of the general phenomenon that observers perceive a moving object as being shortened along the direction of its motion.
We can now pull all this information together into one neat package, containing The Special Theory of Relativity, the Lorentz Contraction and Time Dilation. Sounds impressive doesn't it! We can describe our neat little package with the following statements:
1) Speed results in an object gaining mass
2) Speed results in an object shortening along the direction of its motion
3) Speed results in time slowing down
It is worth noting at this point, that the fastest moving particle in the universe is the photon, the particle of light, as it has zero mass. Although Einstein's equations tells us that nothing can be accelerated to, or beyond, the speed of light, the photon exists at light speed, it does not accelerate to it. As a further point of interest, Einstein's equations do not rule out the possibility of particles that exist at faster than light speed, which would result in them being unable to travel at less than light speed.
Armed with all this useful information let's set of on an exciting high speed journey through space in an attempt to travel at faster than light speed (as we have finished sorting our CD's into alphabetical order). We set off, and as we journey through space we gradually increase our speed until we have reached 98% of light speed. At this point a stationary observer would view our spaceship as being 80% shorter than if it were at rest. Our density has increased and time is running slow, not that any of these effects would be apparent to us. We continue to accelerate (this is the Grand Prix GTi version complete with go-faster stripes and a catchy bumper sticker) and reach 99.5% of light speed. At this speed time aboard our spaceship is running at only one tenth that of a stationary observer. We continue to increase speed until we are at 99.9% of light speed. Time is running extremely slowly, our length reduced drastically and our mass has increased 22 times. Our increase in mass results in us having to boost our engines to compensate. We still push on towards light speed and reach 99.999% of light speed. Things are really hotting up now. Our mass has increased by a factor of 224, time has almost stopped, and our size reduced to a mere dot! We continue to accelerate and reach 99.99999999% of light speed - almost there, but things are getting really difficult now. Our mass is now increased by a factor of 70,000, our clock is moving so slowly that it appears to have stopped, and our size is reduced to almost nothing!
At this point we have to give up trying to go any faster. As our speed continues to increase so does our mass, and therefore the amount of energy required to accelerate. Our mass will continue to increase, and as we approach light speed it would begin to approach infinity and the energy required would also approach infinity. At light speed, if it were possible, our mass would be infinite, we would require infinite energy, our size would be infinitely small, and time would stop.
We cannot travel at light speed. The speed of light appears to be an integral part of the nature of space-time, and as discussed in 'What is Time? it would appear that there is only one speed that we can travel at, and that is the speed of light as a combination of our speed through space and through time.
Now here is the 'problem' part that you knew was coming, and it comes once again from the strange world of Quantum Mechanics. We just don't seem to be able to escape from it, do we?
The EPR experiment.
This experiment was devised by Albert Einstein, Boris Podolsky and Nathan Rosen (hence the 'EPR') as a thought experiment to 'prove' that quantum theory was incorrect. The technology did not exist then to actually carry out the experiment but Einstein believed, that in principle, it proved the 'foolishness' of quantum theory. The experiment was designed such that it would result in communication at faster than light speed, which Einstein's theory of relativity showed to be impossible. Neils Bohr's theory of quantum mechanics was at odds with Einstein's relativity, because it allowed instant communication between paired particles.
We have seen from our previous examination of this experiment (What is Quantum Mechanics?) that it has actually been carried out over a distance of 10 kilometres and confirmed as correct, instant communication did take place. Once again, we have a situation where, in the quantum world, the 'impossible', can take place, this time in the form of instant remote communication, which does of course mean that communication is taking place at faster than light speed.
Can light travelling at faster than light speed?
In a paper dated 19 July 2000 A team of scientists announced that they had succeeded in sending a pulse of light through a special chamber at a velocity faster than the speed of light. Scientists from the NEC Research Institute in Princeton, New Jersey, explain how they sent a pulse of light through a six centimetre chamber containing an unnatural form of cesium at the even more unnatural temperature of nearly absolute zero. The pulse of light travelled so fast that its peak actually exited the cesium chamber slightly before it entered. "No intuitive way to explain this observed effect precisely can be found because the 'specially prepared' atomic cell (cesium chamber) is in a state that does not exist naturally," write researchers Lijun Wang, Alexander Kuzmich and Arthur Dogariu in a statement. The team is quick to point out that their work does not violate Einstein's Theory of Special Relativity, which states that nothing can travel faster than the speed of light, because this would entail going backward in time. "More or less you can't go faster than the speed of light," said Wang. But the restriction that applies to things made of matter does not apply to light waves.
In fact, it was by using the waves of different colours of light to amplify each other and create the pulse that the researchers were able to get the light to warp through the cesium cell and reconstruct itself on the other side before it had entered. According to the researchers, the experiment also does not violate the principle of causality, which requires the cause of any effect to precede it in time. The fact that the peak of the pulse of light exits the chamber before it enters is the result of the light waves building a pulse on the other side of the cesium cell that is identical to the one entering it. So it is not exactly the same pulse. "This means that even if the 'effect' appears to precede the 'cause,' you still can't send information - such as news of an impending accident - faster than (the speed of light)," writes Jon Marango of the Imperial College in London, in a commentary on the work also appearing in Nature.'
What do I think?
It would appear that, perhaps, faster than light speed is possible, sort of, for light. It does not however allow for any form of communication, apart from one particle telling its other paired half what state to collapse into, and certainly does not allow it for solid matter; or so it seems so far, but things have a habit of changing.
As for space flight at super luminal speed, alas, I think not. Sorry Trekies. I reckon Einstein got this one right, but there is no need to give up yet.
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