Friday, 12 May 2017

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Everything you Need to know about tachyons, theoretical particles that travel faster than light and move backward in time?

What is known about tachyons, theoretical particles that travel faster than light and move backward in time? Is there scientific reason to think they really exist?


What exactly are tachyons? If they can travel faster than light, how much faster can they go?

Answer 1:

You're learning about tachyons in 10th grade? That is quite ambitious. Tachyons are studied in an area called particle physics, and I must say this is a bit out of my league, but I'll give you some general thoughts. Tachyons are hypothetical particles resulting from what physicists call a thought experiment. Back in the 1960s, some physicists wondered what would happen if matter could travel faster than the speed of light, something that is supposed to be impossible according to the Theory of Relativity. So these particles may or may not exist because they have not been proven or disproven by real experiment as of yet. What people have done is apply existing formulas to the unique properties of tachyons (like imaginary mass!). What comes out is a particles that go faster when they lose energy with a MINIMUM velocity of the speed of light and a maximum velocity of infinity! Hope that helps. Theoretical physics is a weird place and is not too far off from philosophy.


Answer 2:

Let's start by saying that no one has ever seen a tachyon -- they don't exist in a universe that makes sense compared to itself. Since this is a question from the 10th grade, I'll go into a bit of detail.
In Einstein's theory of relativity, the "mass" of an object increases as it goes faster, becoming infinite at the speed of light, so it takes an infinite amount of energy (remember E=mc2 means energy and mass are the same) to reach the speed of light. This is why special relativity says we cannot go faster than the speed of light.
So what we talk about in physics is the mass of the object when it is sitting still, the "rest mass." If an object has a positive rest mass, it goes slower than the speed of light; if it is like light with a zero rest mass, it moves at light speed.What we call a tachyon is a particle (a fundamental particle, like an electron) that has an _imaginary_ rest mass. Naively, putting this into the equations of relativity (1) doesn't make a lot of sense and (2) seems like it would allow a particle faster than light.



I say naively because this is not really the full picture. To understand, we need to know what a fundamental particle is. Imagine that the universe is a ball sitting in a valley between two hills. If the ball gets bumped a little up one hill, it rolls back down and wobbles back and forth for a while. What we call a fundamental particle, like an electron, is really that wobble. The mass of the particle is given by how steep the hills are right near the valley. If we have a tachyon, this is really like the universe ball is at the _top_ of a hill between two valleys -- the mass is still given by how steep the hill is, but now the hill is going down. Then the "tachyon wobble" is when the ball moves away from the top of the hill. But then the ball won't go back to the top of the hill; it will go to a valley, somewhere else! So a tachyon isn't a particle in the usual sense because it's not a small wobble. A tachyon is really an instability in the universe, just like a ball at the top of a hill isn't stable. This is why I say a universe with a tachyon doesn't "make sense."
In the standard model of particle physics, this subject actually is very important. There is a particle called the Higgs particle that, in the early universe, was kept at the top of a hill by the high temperatures that were present in the Big Bang. As the temperatures dropped,eventually the Higgs particle could move around and slid down the hill. So as soon as it could tell it was a tachyon, it rolled away from the top of the hill and stopped being a tachyon. When that happened, the whole universe changed, leaving things the way they are today. This Higgs particle is the only particle in the standard model that hasn't been seen in experiments yet, but scientists believe it will be discovered within about 10 years at some new experiments.

"The name 'tachyon' (from the Greek 'tachys,' meaning swift) was coined by the late Gerald Feinberg of Columbia University. Tachyons have never been found in experiments as real particles traveling through the vacuum, but we predict theoretically that tachyon-like objects exist as faster-than-light 'quasiparticles' moving through laser-like media. (That is, they exist as particle-like excitations, similar to other quasiparticles called phonons and polaritons that are found in solids. 'Laser-like media' is a technical term referring to those media that have inverted atomic populations, the conditions prevailing inside a laser.)
"We are beginning an experiment at Berkeley to detect tachyon-like quasiparticles. There are strong scientific reasons to believe that such quasiparticles really exist, because Maxwell's equations, when coupled to inverted atomic media, lead inexorably to tachyon-like solutions.
"Quantum optical effects can produce a different kind of 'faster than light' effect (see "Faster than light?" by R. Y. Chiao, P. G. Kwiat, and A. M. Steinberg in Scientific American, August 1993). There are actually two different kinds of 'faster-than-light' effects that we have found in quantum optics experiments. (The tachyon-like quasiparticle in inverted media described above is yet a third kind of faster-than-light effect.)
"First, we have discovered that photons which tunnel through a quantum barrier can apparently travel faster than light (see "Measurement of the Single-Photon Tunneling Time" by A. M. Steinberg, P. G. Kwiat, and R. Y. Chiao, Physical Review Letters, Vol. 71, page 708; 1993). Because of the uncertainty principle, the photon has a small but very real chance of appearing suddenly on the far side of the barrier, through a quantum effect (the 'tunnel effect') which would seem impossible according to classical physics. The tunnel effect is so fast that it seems to occur faster than light.
"Second, we have found an effect related to the famous Einstein-Podolsky-Rosen phenomenon, in which two distantly separated photons can apparently influence one anothers' behaviors at two distantly separated detectors (see "High-Visibility Interference in a Bell-Inequality Experiment for Energy and Time,"This effect was first predicted theoretically by Prof. J. D. Franson of Johns Hopkins University. We have found experimentally that twin photons emitted from a common source (a down-conversion crystal) behave in a correlated fashion when they arrive at two distant interferometers. This phenomenon can be described as a 'faster-than-light influence' of one photon upon its twin. Because of the intrinsic randomness of quantum phenomena, however, one cannot control whether a given photon tunnels or not, nor can one control whether a given photon is transmitted or not at the final beam splitter. Hence it is impossible to send true signals in faster-than-light communications.



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