Have scientist determined light moved the fastet? Or are they searching for something else? What’s so special about light that it would moved the fastest?
Einstein figured out the fastest speed that ANYTHING could move.
This speed happens to also be the speed that light moves.
There is a certain speed in the universe by which all other speeds are measured in reference to, c, 186,000 miles per second. This speed defines one’s “four-velocity” through space-time, so that one is always going at that speed through the four dimensions (3 space, 1 time). While standing still, you’re going at c through time. When you have some speed through space, you take away some of your speed through time (hence time dilation), always keeping the combination at a constant c.
Light happens to go at that speed, as do a few other things. As far as current physics is concerned, anything that is currently going slower than c can never ever reach it or exceed it. Strangely enough, it also means that anything going faster than c can never ever slow down to slower than c, and things that are going at c must always go at c forever and ever.
Said another way … The problem is with the phrase “speed of light”, not with light itself. All the stuff in the universe (even you) “moves” at *c *for a particular technical definition of “moves”.
*c *is the fundamental speed of life, the universe and everything. Which includes light, radio waves, xrays, magnetism, and as far as we know, gravity. And people, dogs, planets, spaceships and worms.
They should never have called it “the speed of light”. That just confuses laymen into thinking light (i.e. the part of the electromagnetic spectrumv visible to humans) is somehow special. it isn’t.
What are some examples of this?
That’s correct, though it’s worth pointing out that light does not have four-veloccity, i.e. it’s four-velocity is undefined.
Nobody knows. Faster than c ‘stuff’ is entirely theoretical.
Only hypothetically, but tachyons.
Hypothetical particles called ‘tachyons’. They’re generally thought to be only a mathematical artefact, and their existence causes certain problems – they lead to unstable vacua in quantum field theories, for example. The original bosonic version of string theory lead to the emergence of a tachyon state, which is one of the reasons it’s generally thought to be unphysical; the tachyon state can be gotten rid of by introducing a supersymmetry (that relates bosons to fermions, and thus enables the theory to contain the whole observed particle spectrum), hence, modern string theory is more aptly known as superstring theory.
Also, if tachyons exist, they should emit copious amounts of so-called Cherenkov radiation, which is radiation emitted by particles going faster than light in some optical medium – if they’re electrically charged, this radiation will be in the form of light (it’s the reason why the interior of nuclear reactors has this healthy glow), however, even neutral tachyons should emit gravitational Cherenkov radiation. Since the tachyon loses energy through this radiation, and has the paradoxical property of accelerating with the loss of energy, it keeps emitting more and more radiation – the faster it goes, the more radiation it produces; the more energy it loses through this radiation, the faster it goes, such that the amount of energy given of by a single tachyon quickly diverges to infinity. That’s something somebody ought to have noticed by now…
As for the reason of why light is special, it’s thanks to its masslessness – turns out (from special relativity) that anything with positive rest mass is constrained to move slower than c, anything with zero rest mass moves exactly at c, and anything with imaginary rest mass (i.e. tachyons) – or negative mass squared – is constrained to always move at speeds faster than c.
Thanks for that explanation. I can’t say that I understand the nuances but I think I got most of it.
I agree. A good layman’s term would be “the speed limit of the universe”. I think we should all start using that instead of the speed of light around here and maybe it will catch on. If the term were ever adopted, it would at least stop kids from trying to outsmart their flashlights based on a misperception.
The speed of the universe. It’s not just a good idea. It’s** the law**.
A good explanation and leads naturally into an explanation of why the Lorentz transformation looks like the Pythagorean Theorem.
As I understand it, c refers to light passing through a vacuum. Light can be slowed down when passed through other media, or so I’ve heard claimed
Is this correct? Does c itself change, depending upon the media? If not, your last sentence would seem impossible.
Ditto Half Man Half Wit. I think that is all correct.
But I would like to elaborate on the phrase “particles going faster than light in some optical medium”.
The speed c is the speed that light (and some other things) travel when there is nothing else around. However, when light travels through glass or water or some other clear substance, it goes slower. Maybe it is more accurate to say that it isn’t light anymore, but something more like sound traveling exclusively through the electrons. Whatever we call this, its velocity is lower than c, and it is also possible to shoot some kinds of particles into that substance at a speed faster than this. We aren’t talking, though, about particles moving faster than c. We are talking about particles moving at a speed lower than c and higher than the propagation velocity for this electron sound or embedded light.
When …whatever you want to call it … leaves that media and is back in vacuum, is it light again? Does it speed up to c?
Yes, you can think of it that way. But perhaps a better way to think about the whole issue is to think of the whole path of the light beam as a series of individual legs. In vacuum the photons are traveling unimpededly. When they hit a piece of glass the beam hits an atom, gets absorbed and then retransmitted and then hits another atom and gets absorbed and then retransmitted and so on and on and on until it emerges out the other side. Then the photons move without interruption.
Any individual photon always travels at C. That’s a function of its being a massless particle. All massless particles will do this. But the measurement of a light beam over a distance is the sum of the times it takes to travel all the legs of the journey. The absorption and retransmission time is longer for certain substances than in others and adds to the total length of the journey. We say that the speed of light through a piece of glass (or water or diamond or any transparent object) is lower than through an equivalent length of vacuum because of this. But if we could do the measurements, every photon measured would always travel at exactly C for every leg of the path in between atoms.
It’s really two different definitions of the speed of light. In a perfect world you’d see capital C defined as Einstein’s Constant, the theoretical speed that a massless particle must always travel, and lower case c for the total speed of a beam of light through a physical substance. Mostly people ignore that distinction and just talk about the speed of light. That’s what causes so much confusion. They’re really two different things.
An intuitive – if not entirely correct, since things at that level just aren’t very intuitive any more – picture of light travelling through a medium would be in steps of absorption and re-emission; each step takes some time, so in total, it’s gonna take longer for light to cross some distance than it would if it travelled unimpeded. (EDIT: I didn’t see Exapno’s post before I posted mine.)
How this relates to what Napier said is roughly that ordinarily, we think of light as a propagating disturbance in the electromagnetic field. However, in matter, it isn’t (just) that any more, but rather also a disturbance in position and velocity of charged particles, i.e. electrons, because of course the electromagnetic field influences these quantities. So the electromagnetic wave propagating through the medium moves the charges around – more precisely, it makes them oscillate, the light wave being periodic and all – and of course, oscillating charges create electromagnetic radiation, of the same frequency as the incoming radiation (the absorption and emission stuff I mentioned above), but somewhat delayed, out of phase. If we sum up all this oscillating and radiating, we then get a wave of the same frequency, but a shorter wavelength – and since wavelength times frequency is the wave’s propagation speed, that means it’ll be somewhat slower.
As for the Cherenkov effect, one could now ask why physics should care if something in a medium moves faster than the local light – basically, there’s nothing wrong with doing that, so why would it lead to any observable physical effects? The answer is that it’s a bit like a sonic boom: any charged particle entering a medium leads to disturbances in the electromagnetic field, due to exerting Lorentz forces on the local electrons. That of course means electromagnetic waves; however, normally, those cancel out, they interfere destructively, so we don’t see light all the time when charges move. If now a particle moves through the medium from x to x’ faster than light can travel the same distance, electromagnetic radiation created at x can’t annihilate that created at x’, simply because it doesn’t have time to get there in time. In fact, if the particle moves exactly at the local lightspeed, wavefront hits wavefront, leading to constructive interference – a sort of ‘attached’ shock front of light, analogous to that of a sonic boom.
I should note, for completeness, that Cherenkov radiation only occurs in media that are electric insulators – the reason for that being that in a conductor, charge disturbances created by passing particles can restore themselves without emitting light.
Look, my kid is almost 18. Did you think about the fact that he’s IN the universe before you posted that? To any teenager, that “speed limit” stuff is like a red cape.
Thanks a LOT. He and his high-risk friends are down in the Home Depot parking lot zipping around at 29 centimeters every millionth of a second. According to them, they’re not going to bed until they “break that ƒ%&#n speed limit”.
If they do, it’s your fault.* I* am not going to rewrite all those physics books, bub. I’ve got laundry to do tomorrow.
Soooo…did we discover the Universal Speed Limit (USL) was c, and then discovered light in a vacuum travelled c. Or did we discover that any massless object travelled c and said well photons are massless! Or did we discover that there is a speed limit, and any massless object would travel this limit, and light is massless, therefore, any massless object and c must be the numerical value of the speed limit, and light travels at c and therefore the speed limit must also be c…
And… is the value of c only observationally determined, or is there any theoretical reason at all for c?