What are tachyons and why do we think they exist?
Tachyons are hypothetical particles with negative mass squared. That mean the instead of moving on time-like curves (bounded by speed of light), they travel on space-like curves (faster than the speed of light).When just consider special relativity, Tachyons can either exist or not. If they exists they basically break causality, ie, they can deliver information between to events faster then speed of light. And if you can “turn” them, then tachyon can start from a point ‘A’ travel to another point ‘B’, turn and come back to ‘A’ before it ever lift point ‘A’. (which make them a sci-fi fantasy)If we consider not only special relativity but also Quantum mechanics, the existence of Tahcyons is must more dramatic. Causality is way too important to just to do without in QM, and Tachyon are “Theory breaker”. In QM (more correctly yin Quantum field theory) Tachyon are signature of instability. This is closely related to super-cooled water (google it) where a small fluctuations can make them freeze very quickly.Since we haven’t seen the universe destroyed lately, no one believe we are in unstable state. Hence there is no Tachyons in nature.
I'm a physics layman here. So, if tachyons exist, surely they must make time travel backwards. Could this mean that if tachyons were to be observed, we could know the future of the universe?
Classically, this is correct. It would be possible to know what the universe will be like by reading signals from tachyons. (Un)fortunately, our world doesn't behave classically. It behaves as a quantum system. So, what does relativistic quantum theory have to say about tachyons?Well, in RQM systems follow what is called a (hyperbolic) wave equation, such as the Dirac equation or the Klein-Gordon equation, and this should include any tachyons. Indeed, a tachyon can be modeled easily in quantum theory by making the mass of the particle purely imaginary. Wave equations have the property that one of the following is always true:a) Any disturbance can only spread no faster than c, where c is a constant appearing in the wave equation (in relativistic wave equations, c is of course the speed of light).b) It is not possible to solve the equation for any initial data which is zero outside some interval. In other words, it is impossible to localize our tachyon signal to some finite region of space. This makes the concept of propagation velocity meaningless, but it also makes it impossible to encode a signal in the tachyon field - since to do so you would need to first create the signal in some interval and let it propagate.In the case of quantum wave equations, which horn of the dilemma we take depends on whether we allow some of the solutions. Either way, the conclusion that we cannot learn anything about the future from observing tachyons is inescapable in quantum theory.EDIT: This does not mean that tachyons are no different than other particles in quantum theory. If you put a tachyon field in your quantum theory, the theory becomes unstable - because [math]m^{2} < 0[/math] - and if the field is "pushed" enough, it will decay until you reach a theory with no tachyon. In string theory, a tachyon appears naturally and it represents the instability of the D25-brane (or D10-brane in superstring theory). Some physicists have speculated that a tachyon, called the inflaton, could have existed in the very early universe, which caused the universe to decay to a new state; this process of decay could be responsible for inflation.
If tachyons exist, how can we prove it? If it is impossible to prove that they exist and they violate fundamental laws of physics, why are they even discussed in the scientific literature?
They violated all laws of Physics. If they were to exist they would have imaginary mass and thus God only knows if they would sense Gravitation..:)If they have any charge, we would be bombarded by this extremely powerful particles.. which we are not…So, it does not take too many neurons to conclude that they don’t exist.The only reason they exist in literature is because theorists abhor critical sense… It is like artists being subject to norms, colors schemes… they don’t like and they will allow each other to concoct whatever scenario (even the ones obviously incorrect) for sake of freedom of exploration.Marco Pereira's answer to What is the world like in the eyes of a theoretical physicist?Theorists live in unlimited freedom of thought. That is the reason they don’t like my theory. My theory uses evidence and uses simplicity (Occam’s Razor) to get attention and due respect.What theorist would even consider simplicity as a constraint for their flights of fancy..:)For instance, when I say that the Universe has one extra spatial dimension, I provide evidence:Proof of an Extra Spatial Dimension by Marco Pereira on Hypergeometrical UniverseWhen I say that all of their work is incorrect (because it is based upon General Relativity and GR is FUNDAMENTALLY WRONG), I provide an argument showing that ALL ASTRONOMICAL OBSERVATIONS REFUTE GR.Marco Pereira's answer to What scientific ideas or theories are blocking progress?So, I like to speak about evidence, observation and I wouldn’t ever try to publish something that has no support (like Tachyons).
What are muons and tachyons?
Muons are short-lifetime charged particles that are created in some nuclear reactions. They occur naturally when cosmic rays strike the upper atmosphere. http://cosmic.lbl.gov/SKliewer/Cosmic_Ra... Tachyons are hypothetical particles that can go faster than light. At some point some physicist noted that the equations of relativity prohibit a mass from going at the speed of light, but they don't prohibit a particle from existing which only goes FASTER than light (never equal or slower). They have the same "light barrier" we do, but from the other side. Physicists called these particles "tachyons" then worked out what the theoretical properties would be. I don't know if anyone seriously believes they exist, it's just that such a thing is consistent with the laws of physics.
Why do physicists deny the existence of tachyons?
Tachyons are faster-than-light particles which means we can't just say they exist because they violate many physical barriers that we already know exist. For them to be at least possible, you need to stretch a lot of laws to account for them. For instance, Einstein's relativity theories stated that the speed of light was a physical barrier of speed. One, this means that you can never observe one directly, or it's after-effects, as it's travelling too fast to affect anything. Two, relativity suggests that these particles may sort-of time travel, which naturally makes it difficult to assume behaviour. Also, while string and m-theory offer up the possibility of extra spatial dimensions. 10 and 11 respectively. If tachyons did exist, it would break the mould and add at least 20 new dimensions into the fold. The reason why physicists don't accept tachyons so readily is because like most people, they like things to be simple. They don't see why the universe should account for these cheeky little particles.