What if a faster-than-light particle is found? What will be the consequence (severe ones) on laws of physics if a particle that travels faster than light is discovered?
I am looking for a more general answer so that a high school student would be able to understand. Or is it not possible to explain a high school student without bringing special relativity into context?
 A: According to Günter Nimtz, we've already measured superluminal velocities.
Many (most?) physicists disagree with Nimtz' interpretation of his experimental results, probably mostly due to the fact that tachyons potentially violate causality (remember that according to special relativity, faster-than-light means backwards in time when looked at from a different frame of reference!).
However, according to Recami, special relativity can be extended to include tachyons without violating causality (see this PDF if you can stomach the physics).
I haven't really looked into the problem in detail (I've been planning to read through this issue of Annalen der Physik for some time, but never got around to it), but my gut feeling is that tachyonic interactions might indeed be real (I hesitate to call them tachyonic particles because they'll necessarily have some pretty strange features to be consistent with special relativity).
If Recami's analysis is correct, tachyons cannot be used to do anything useful like committing the perfect murder (Bell paradox) or answering questions before they are asked when using an anti-telephone (Tolman-Regge paradox). So even if they do exist, the implications might be far from earth-shattering.
The situation might turn out similarly to the 'spooky action at a distance' due to entanglement: When first discovered, there was a lot of controversy, but in the end the physics community just shrugged, accepted that the world is a bit stranger than we formerly assumed and moved on.
A: Though, we haven't observed any particle traveling faster than light, we've already hypothesized a particle. Through this assumption, we do know several properties that such a particle should have like, it must have an imaginary mass, should never slow down below $c$, etc.
Neither Einstein's nor Newton's theories will get affected by such a particle. Well, SR helped us in calculating those properties. But, there will be serious implications of such particles. Because, they have an imaginary mass and from the energy-momentum relation, we can see that their velocity increases with decrease in energy. They can violate causality, etc. , etc...
There will be a lot of advantages like time-travel, faster than light communication, etc. I'm quite sure that such particles have no place in classical mechanics. Who knows? We may introduce a new theory for such particles or revise SR (which is quite strange)
I think this is enough. Maybe, you can read the Wiki article on FTL and Tachyons.
You can ignore strange terms if you want, but you can still read it...
A: The finding of such a particle would overturn Einstein's theories of relativity, which state that light-speed is a cosmic constant and cannot be passed by anything in the universe. Such superluminal particles would also have the ability to time travel (hypothetically). This would be caused since as a particle approaches the speed of light, time slows down. As the speed of light is overtaken, time supposedly begins to flow backward (sort of like Merlin living backwards in time, if that helps). Another thing is the relationship between energy and speed for such particles. According to Einstein's equations, energy, speed, momentum, and mass are directly related. Superluminal particles therefore have zero mass, infinite speed, and infinite momentum; lastly, as their energy decreases, their speed increases.
