This maybe naive, but how do we know nothing travels quicker than light?

Question

Given that if anything did we wouldnt be able to see it?

Some good and some interesting answers.

Given you can hear a fart before you smell it would rule out, as light is faster than sound, of a faster than light fart smell. :)

I thought all electromagnetic radiation travelled at the speed of light, not just light?

Answer 1

There is a common misconception that relativity forbids superluminal speeds. It does not. It only postulates the speed of light to be a universal constant, identical to all observers in all inertial frames.
This postulate has been experimentally confirmed hundreds of times by now.
Now, the consequences of this postulate (plus another one, stating that all inertial frames are equivalent) are all proven mathematically. And there is absolutely no doubt whatsoever, that (provided, that the postulate is true) they are true.

This consequences include things like time dilation, energy-mass equivalency (the famous mc^2 formula) etc.
They do NOT include a statement that nothing is faster than light.
They DO state that it is impossible for an object that has a mass to reach the speed of light (it would require infinite energy), but, first, no all objects have mass (photons do not for example), and, more importantly, if something is ALREADY moving faster than light, there is nothing in relativity that would forbid that.
There is aslo a possibility for some exotic things - like tachyons, with imaginary mass, or, particles, moving backwards in time (as it was once believed positrons did) - that would move faster than light too - there is no limit to fantasy.
Also, there really ARE distant galaxies, that are currently moving away from us faster than the light (yet we can still see them, contrary to another common misconception).

One problem with superluminal motion (and a pretty big one if you ask me) is the causality principle. It states that an event can never happen earlier in time than it's cause (makes sense, doesn't it? You first turn the switch on, and then the bulb lights up, not the other way around).
It can be shown in relativity, that if two observers, moving (fast enough, yet slower than light) with respect to each other, could communicate quicker than light, the causality principle could be violated, and cause all kinds of paradoxes - (such as killing your own grandfather before your father is born). Relativity, as a formal theory, does not have any problems with it. But I, as I am sure any other sane person, do.

But there are ways around that too... Like I said, there is no limit to fantasy. After all, we know that some laws of physics get violated every now and then - particles get created "out of nothing" in vacuum all the time for example, but only for a very short period of time - perhaps, causality can be "slightly" violated the same way? Also some laws (notably, thermodynamics) are statistical in nature - they can't predict the behaviour of every particle every time, just the average common trend. The causality could be one of those - perhaps it only holds most of the time, but not always? Etc.

So, to answer your question finally, yes, theoretically it is possible to move faster than light. We even know for sure that some things (distant galaxies) do.

Answer 2

The result that nothing can travel faster than light comes out of Einstein’s special relativity. Einstein postulated that (1) The speed of light is the same to all observers and (2) that the laws of physics are the same in all inertial frames (this means that they are the same to anyone whose viewpoint moving with a constant velocity). Using this you can derive all of the equations governing special relativity. There are many equations that show this result and I will outline a few for you.

Firstly there is the relativistic mass equation which is
m=m0/(square root(1-(v^2/c^2)) where m0 is the rest mass of the object, v is its velocity relative to a stationary observer, c is the speed of light and m is the mass of the object as seen by the observer. Notice 2 things, firstly that when v=0 the denominator is one so the observer and object are in the same reference frame and the observer sees the object having its rest mass. Secondly that for 0c. This will produce an error since for v=c the denominator is 0, a number you cannot divide by and for v>c the denominator is not real. This is because this equation is showing that as v increases, m increases up to the point v=c at which m is infinite, however this cannot be possible since this would mean the energy of the object is infinite and this violates the laws of thermodynamics.

Secondly there is the transformation of velocities. The equation to find the speed of an object as seen by another observer using the speed seen by you is v=(u+v')/(1+v'u/c^2) where v is the speed in the frame travelling at velocity u to the frame you are observing the speed as v in. Taking the limit as u tends to c we see that v=c i.e. the fastest that an observer in any frame can see an object move is c and that is when the reference frame moves at speed c to that observer.

There is some talk in quantum mechanics or general relativity that you can go faster than light although we have not yet found a solid theoretical backing for this or any experimental evidence that suggests this. If you are interested in these there is a wealth of physics general interest books or magazines out there, which often contain chapters or articles on this subject, it is very popular at the moment.

Jez

Answer 3

General Relativity (GR) states that no information may travel faster the c (which is about the speed of light in a vacuum).

Remember that this is just a theory, but all the current evidence points to it being correct. Effectively as you get close to the c you mass increase in such away that at the speed of light you mass will be infinite and so you would need infinite energy to go faster.

Note that it is "information" that can't travel faster the light - subatomic particles do travel faster than light and under quantum mechanics information "appears" to travel faster than light but in both cases no information can be carried and so GR is obeyed.

So to answer your question, we don't know 100% but "current theory" suggest it is impossible - but theories are over turned so so who knows in the long term...

Answer 4

I don't know if that is exactly a fact. I can tell you that tachyons are particles that scientists define as traveling faster than the speed of light. Tachyons are theoretical and have not yet been observed. Probably something or a great many things in our universe travel faster than light but we can not, as yet, observe it. In any case we wouldn't be able to detect it via our eyes so proving this theory will rely on technological developments.

Answer 5

Because in basics terms, nothing can go faster than light because energy=mass remember Einsteins theory? E=MC2 i.e. energy=mass multiplied by the speed of light squared? So whatever energy or shall we say fuel, you use would weigh so heavy and take up so much space (mass) that your (spaceship/vehicle) would be unable to carry it. Think of loading a railway engine with so much coal as fuel that it could not move or is even crushed with the weight of it's (fuel) That's the barrier to obtaining the speed of light.

Answer 6

There actually is a theoretical particle (called a tachyon) that can never travel *slower* than light. It also has some other funny characteristics such as imaginary mass and the ability to go faster by *losing* energy.

But, so far, nobody has ever seen one (and there have been a *lot* of people looking ☺)


Doug

Answer 7

All science is theory, alot of it proven but in some cases unproven. With our current understanding nothing travels faster than night, I could go into the theory but it would bore you. So in the future we may redefine our understanding of the current limitations making travel faster than light possible.

Remember we once thought the world was flat, and the sun revolved around us, so things change our understanding increases

Answer 8

That nothing can travel faster than the speed of light was an assumption made by Einstein in his now famous theories. We do not know if it is true or not, but those theories have stood the test of time and much experimental work.

Answer 9

It's all theory based. May be some time soon, some one with eitehr come up with an alternative theory or will actually find a way of going faster than light ....... time travel.

It's a theory in the same way folk use to think the world was flat ...... until someone proved them wrong!

Answer 10

I think it is theoretically possible to get from A to B faster than light, but still you can't move faster than light.