Nothing can move faster than light moves. But is it coincidence or some physical law that the speed at which light DOES travel is the fastest speed at which light COULD travel? Why doesn’t light travel at 185,000 miles/sec instead of 186,000? Would anything with no mass move that quickly? And does light undergo any acceleration, or is it moving at top speed the instant it’s formed?
All massless particles travel at c. There is no acceleration, since the photon was never standing still, so it doesn’t need to speed up.
Actually, light doesn’t always travel at the speed of light. 186,000 mps is the speed of light in a vacuum (and, I believe there are some other conditions). Add in some air and other particles, and light slows down some. Gravity also affects light and can slow it down (no light can escape from a black hole, for instance).
No cite, but I’ve read articles lately (no, not Weekly World News) where there have been experiments where light was slowed down consideralby (35 mph), I believe.
And why does C-A-T spell cat? Why not dog?
[sub]Sorry, I couldn’t resist![/sub]
Why does anything equal anything? It’s more or less a random phenomenon, IMO.
What about a photon that has come through a medium that has slowed it down such that it is moving at c - x, where x is the amount of retardation throught that medium. When it emerges does it accelerate then to c? If not, does that mean that there is no time between c - x and c?
Is it the exact same photon that emerges from said medium? I always thought perhaps what happened in a medium was that the photon hit something and was absorbed, causing new photon to be emitted and it was the latency in this process which caused the slowing. Just what I assumed so don’t take my word for it.
Fun With Semantics
What Airman Doors, USAF really meant was: Why does a cow travel at the Speed of Cow?
“Light” by any other spelling would still travel as fast.
Light and those pesky little particles with no mass travel at the “Speed of Light” rather that at the “Speed of Those Pesky Little Particles with No Mass” because we can see light. It got our attention, so we measured it first. Then we found those pesky little particles and discovered “Hey! These things go as fast as light.” and were too lazy to change the name for as fast as you can go.
Capt. Kirk: Scotty! We need to get outta here! Now!
Scotty: We no canna git past Those Pesky Little Particles with No Mass Speed! Our warp drive is down an’ alla our Dilithium Crystals are cracked!
I don’t think we’re answering the question here. In my hubris, let me try to restate it…
(1) Why does light travel at the fastest speed allowed by the physics of the universe? How does light do this? Saltire may have answered this one (it just goes unrestrained by the limitations of acceleration and hits the wall at 186,000 miles per second).
(2) Why does spacetime have this limit in the first place? (Anyone care to explain Relativity in 500 words or less?)
Actually, the speed of light in a black hole is still the speed of light found anywhere else in the universe. The black hole doesn’t slow light down. It actually bends space to such a degree that it curves back in on itself (in 4 dimensions). There is no path that light (or anything else for that matter) can follow that will take it out of the black hole’s event horizon.
Another way of looking at it (and I’m not certain which of these examples is considered more appropriate) is to think of climbing a slope. The steeper the slope the harder it is to climb (the more energy it takes). When the slope goes vertical the energy required to climb it is infinite (don’t tell me about mountain climbers…think of this as a mathematical problem on some graph paper). The black hole essentially creates a vertical wall (in 4D) that can’t be climbed by anything. Again, there is no path light can take that will see it over the top and out of the black hole even though it remains travelling at light speed.
So light is still marching along at light speed but it just ends up going in circles (or whatever that equates to in 4D…hypercircles?).
Experiments with lasers have actually managed to stop light…and to make it go about 300 times faster than its theoretical maximum spped.
The 35mph experiment mentioned above relies on the properties of Bose-Einstein condensates, but since then, the same team have slowed light to 1.6kph and then gone on to actually stopp a laser’s light, using a similar technique, but involving a second laser (called a coupling beam) fired into a supercooled gas cloud to make it non-opaque. If this laser is turned off, the other laser pulse stops. Turn the coupling beam back on & the pulse emerges. There are other links from these urls with more information.
Finally, a group have accelerated light up to 300 times its usual speed, without breaking relativity. More details in the link.
The speed at which light propagates in a vacuum is determined by two physical constants, the permitivity of free space, and the permeability of free space.
If you change either value, you change the speed at which light propagates.
For a thorough explanation see: http://doversherborn-regional.k12.ma.us/~zachary_scott/relativity/light.html
The photon always travels at c; it’s the absorption and reemission of the photon by the medium that causes the macroscopic slowing of light.
An interesting way of viewing this is to think of the photon at the Event Horizon as traveling at c and the EH collapsing at c.
Not true. The phase velocity travels at c; it’s only the group velocity that seems to travel greater than c. But nothing is really exceeding light speed.
Quoth Phobos:
It doesn’t. Relativity has no objection to a particle moving at faster than the speed of light, it would just have some interesting consequences. First, either the rest mass or energy of such a particle must be imaginary, since E = 1/(1-(v/c)[sup]2[/sup])[sup]1/2[/sup]*m . If v > c, then that first part has a square root of a negative number. Secondly, any object capable of moving faster than light can also move backwards in time, and transmit information backwards, which opens up the possibility of time travel. Not that there’s anything wrong with imaginary masses or time travel, mind you.
The significance of c isn’t that it’s the fastest possible speed in the Universe. The significance is that it’s equal to one. Einstein told us that space and time are really just two different aspects of the same thing. We should, therefore, be able to measure them using the same units. How many miles long is a day? You need some sort of conversion factor, which turns out to be c. Just like an inch is equal to 2.54 centimeters, a second is equal to 310[sup]8[/sup] meters. Put another way, you can say that 2.54 centimeters / 1 inch is equal to one, and 310[sup]8[/sup] meters / 1 second is equal to one.
Light travels the specific speed it does because of two (or three*) things:
[ul][li]1) The mechanism which releases a photon.[/li]
[li]2) The medium through which the photon travels.[/ul][/li]
*You could split “2)” into permitivity & permeability of free space.
…
The photon always travels at c; it’s the absorption and reemission of the photon by the medium that causes the macroscopic slowing of light.
should read:
The photon always travels at c; it’s the absorption and reemission of the photon by the [virtual particles in the] medium that causes the macroscopic slowing of light.
Chronos:
That is, without a doubt, the most understandable and clear explanation about space and time I’ve ever seen. You should write a book explaining the more esoteric things in science. It would make for a very good read.
Let me be sure whether I have one possible implication of this straight. Is it correct to say that when you receive electromagnetic energy, as through a telescope or radio receiver, from two million light years away, you are receiving energy not only from that far, but that is that old? In other words, is there no way for us to perceive the universe holistically and at present?
There may be a way, but the instrumentation to do so doesn’t exist for us yet.
Libertarian — right. By the theory of relativity, the idea of a global “now” is not a physical reality. Two events, separated by a distance can happen in one order, or another, or simultaneously, depending upon the reference frame of the observer.
To explain more, by “separated by a distance”, called a “space-like curve” in scientific literature, I mean that it would be impossible for an object moving at the speed of light in a vacuum to get from the location and instant of one event to the location and instant that the other happens.
On the other hand, when a time-like curve separates two events, it would be possible for an object moving at or slower than the speed of light to get from one event to the other. In this case, then, every reference frame moving slower than the speed of light will agree on which event happened first, and which event happened second. But they will disagree on how much time elapsed between the two events, and how distant they were from each other.
Ring wrote:
Silo then wrote:
Pardon me??
Photons are absorbed and reemitted by virtual particles???
I’m sorry Silo, but I’m afraid you have no idea of what you speak. On the other hand I could be wrong and you have actually devised a new and wonderful form of quantum physics. Could you perhaps post a cite or further explain your Earth shattering theory?
Could you please explain in layman’s terms why that isn’t simply a matter of scale, or to put it Godelianly, a matter of a set within a container set? It seems to me, by what you’re telling me, that if we can look far enough in distance, we can see the very beginning of time. If that is the case, then it seems reasonable that what we see is, relatively speaking, quite small, and in fact we see the very stuff that we eventually came from. But if what we see is smaller in volume as we look out in distance, then what is the shape of our “viewport”? It seems like we would have to be looking into some sort of cone.
When I look at my coffee table, I can see whatever is happening on it from one end to the other, all at once, because the scale of my observation is larger than the surface of the coffee table. If we were able to increase our scale such that we could see all of the universe at once, would relativity still hold?
The mechanism which releases a photon has nothing to do with the speed of light in a vacuum.
If it did, we’d expect to see slow photons and fast photons flitting about through space. -We don’t.
Maxwell derived the following equation for the speed of light in a vacuum.
v = 1/(eu)^0.5 where v is the velocity, e is the permitivity of free space, and u is the permeability of free space. NO other terms are needed, nor is any appeal to Einstein, or the Gods of Quantum mechanics.
If you are interested in why the permitivity and permeability of space take the values they have, you might need to crack a book on quantum theory of the vacuum, but the OP only wanted to know what governs the speed of light in a vacuum.