I dont understand enough about light

[Yahweh]

A quick reference shows the speed of light (through a vacuum) is 299 792 458 meters per second. Why cant it go faster?

I've read a few articles on scientists breaking the speed of light barrier, did they really do it?

I've heard that approaching the speed of light causes time to slow and eventually stop, what is the relationship between the speed of light and time?

What does it mean to "quantize a photon"?

I realize the "decaying speed of light" (see http://www.icr.org/pubs/imp/imp-179.htm for reference) is mangled science (pseudoscience) twisted in a way to support to some of the creationist theories on Intelligent Design, I dont need any help in that area.

 

[davefoc]

Yahweh, I'll take a shot at this and then stand back to be corrected by those who know more about what they are talking about.

Two of the things that people might be talking about when they talk about going faster than the speed of light:

1. There is some heavy duty mathematical modeling of the universe that suggests for a brief instant following the big bang the speed of light was faster than it is now. The theory is called inflation.
2. Sometimes particles can go faster than the speed of light in a medium. When they do something called Cherenkov radiation is produced. In no case, with respect to this phenomena, is the speed of light in a vacuum ever exceeded.

As to your your question about why the speed of light is a constant: I don't believe that anybody has an answer to this, but it is at the heart of why the universe is the way it is.

Einstein, didn't explain why the speed of light in a vacuum was constant. He used the experimentally determined fact that the speed of light in a vacuum was constant to make some startling predictions about the nature of the universe. These predictions are unsettling and difficult to understand with our fixation on our standard three dimensional view of the world.

Basically what's going on here with respect to the special theory of relativity is that you have to come up with a theory as to how light can appear to have the same velocity to two different observers that are moving with respect to each other.

The rather strange solution to this is that things in the frame of reference moving with respect to an observer are shortened and clocks in the frame of reference moving with respect to an observer are slowed. Bizarrely both observers see the other's clocks slowing and the others stuff getting shorter.

Interestingly, at least to me, is that the formulas for determing the relationship between the lengths in frames of reference moving with respect to each other were discovered by Lorenz before Einstein had developed his special theory of relativity. Einstein became famous by realizing the full implications of these transformations and that they lead to his famous equation E=MC^2.

 

[Stimpson J. Cat]

Yahweh,

A quick reference shows the speed of light (through a vacuum) is 299 792 458 meters per second. Why cant it go faster?

Light in a vacuum can neither speed up, nor slow down. Our current theories allow for the possibility of particles which move faster than light, but no such particles have ever been detected.

The real question is "why is the speed of light in a vacuum constant in all inertial reference frames?" Nobody knows. We simply observe that it is.

I've read a few articles on scientists breaking the speed of light barrier, did they really do it?

Not exactly. In optics, and particularly in signal processing, there is a concept called the 'group velocity". When a discrete pulse travels as a wave, the speed of that pulse can be smaller, or greater than the actual speed of the wave. No information can be transmitted faster than c, though. What those scientists did was somewhat analogous to sweeping a laser across the surface of the moon fast enough that the dot of light moves across the surface faster than c.

I've heard that approaching the speed of light causes time to slow and eventually stop, what is the relationship between the speed of light and time?

t' = t * sqrt(1 - (v/c)^2)

where t' is the elapsed time for the moving person corresponding to the time t for a stationary person.

As davefoc mentioned, this is a Lorenz transform. From a theoretical point of view, the fact that the speed of light is constant in all inertial frames is a direct consequence of Maxwell's equations, which were already around for a long time.

Making the speed of light constant in all inertial frames, which leads to the Lorenz transformations, is the only way to make Maxwell's equations invariant under changes of velocity. Before Einstein, the assumption was that Maxwell's equations were not invariant under changes of velocity, and that they would be different in different inertial frames of reference. The problem with this view is that it leaves the existence of magnetism as a complete mystery. When you adopt special relativity, it becomes immediately apparent that the magnetic field is simply an electric field viewed from a moving reference frame. In effect, electromagnetism becomes truly unified as a single force.

What does it mean to "quantize a photon"?

It means that electromagnetic waves can only be absorbed and emitted in discrete quantities. A web search for the photoelectric effect should give some insight into quantization, and the origin of quantum mechanics.

Dr. Stupid

 

[Ziggurat]

When talking about Lorentz transformations (going from one reference frame to another), most people tend to focus on length contraction and time dilation as the "weird" effects of relativity. I think this is actually somewhat misplaced, since something more fundamental (and actually easier to understand) is going on. Imagine you have a space-time plot, with one axis being yous spatial position and one your time axis. Classically, if you change to a reference frame moving with respect to your original frame, all that happens is your vertical axis (which marks the same point in space as you move through time) gets tilted to the side. But in relativity, not only does your time axis tilt when you change reference frames, your space axis (which marks every place at the same instant in time) also tilts - they start to come together at the diagnonal line given by the speed of light. This leads to stuff like length contraction and time dilation, but it also means that observers in different reference frames won't agree about whether two spatially separated events happen at the same time. This is part of why faster than light (FTL) travel necessarily means time travel - you can find two different reference frames where the direction of motion for an FTL object is actually going opposite directions - or even not moving at all but just existing everywhere along its trajectory for one instant.

 

[Rat]

So for a physics ignoramus, what's with light bending as it enters a prism or whatever? I was told at school that it slows down. Is this true because of the medium? So the speed of light is not constant? Only in a vacuum or given medium?

Cheers,
Rat.

 

[Ziggurat]

Here's one nice little way of viewing how simultaneity changes with reference frame:

http://casa.colorado.edu/~ajsh/sr/simultaneous.html

I've been looking for some good online diagrams showing the hyperbolic equidistance curves on a space-time diagram, but haven't turned anything up yet.

But here's something I wish they'd use more often in beginning relativity classes, the relativistic space-time metric. In Euclidean space, the distance between two points is r^2 = x^2+y^2+z^2. This quantity is invariant (doesn't change) even if you rotate your coordinate system or shift it to the side. In special relativity, the space-time metric is x^2 + y^2 + z^2 - c^2*t^2. If this quantity is positive, the two points in question have a space-like separation (there's some reference frame in which both evens are simultaneous but at different places). If it's negative, it's a time-like separation (there's some reference frame in which both events occur at the same place but at different times). All special relativity consists of is finding out how to go from one reference fram to another so that this relativistic distance doesn't change. Length contraction results from the fact that when you measure a moving object's length, you measure where the front and the back are the same time in YOUR reference frame. But those two points are not at the same time in the object's own reference frame - if an observer in that object's frame measures the two points you did, he has to use a non-zero time separation, which will decrease the relativistic length, but you'll both agree if you use the relativistic distance. People like to teach relativity talking about light bouncing around on moving trains because the starting point is simple, and although the derivation is straightforward, it's also confusing. But if they'd just START with the relativistic metric (which is very simple) and take that as a given, it's really easy to understand how stuff like length contraction just falls right out.

c is a constant. Light in a vacuum travels at c, but may travel slower in a medium (which is why it bends when it hits glass at an angle). In fact, the speed it goes through a medium can even depend on the frequency (which is why a prism can separate out the different colors of light). The thing to keep in mind is that c is not special or unique to light, it's a property of space and time itself. It's really more the speed at which light (and a few other things) in a vacuum will travel, rather than the speed of light, if you get my drift.

 

[Stimpson J. Cat]

ratcomp,

So for a physics ignoramus, what's with light bending as it enters a prism or whatever? I was told at school that it slows down. Is this true because of the medium? So the speed of light is not constant? Only in a vacuum or given medium?

Remember that light is an electromagnetic wave. When this wave passes through matter, the matter reacts. The electric charges oscillate with the wave. This oscillation produces a new wave. The interference of these two waves results in a new wave with a slower propagation speed, and a deflected angle. This process also accounts for reflection and absorption.

Dr. Stupid

 

[tracer]

Can anyone explain this stuff to someone stupid like me?

Start with maxwell's equations, then the lorentz thingy, then einstien?

James Clark Maxwell for Dummies

Long before James Clark Maxwell walked the hallowed halls of physics, pioneers in electricity and magnetism discovered that it was possible for electric charges to induce a magnetic field, and for a magnetic field to induce an electric current in a wire. Trouble is, in order for this electromagnetic induction to take place, the electric charges or magnetic fields had to be moving. (Actually, it was a little worse than that. Electric charges not only had to be moving, they had to be changing the speed at which they moved, i.e. accelerating or decelerating.) An electric charge moving down a wire is just called an electric current, and was nothing new to these people. A moving magnetic field, or a magnetic field that changed its strength with time, was something kinda new and weird, though.

So, to grapple with these newfound effects, the early physicists formulated four equations that dealt with the interrelationship between electricity and magnetism. These equations described what kind of a magnetic field you'd get in the vicinity of an electric current (i.e. how strong the magnetic field would be, what direction the "field lines" would be pointing, et cetera). These equations also described what kind of an "electrostatic field" (a field that acts on charged particles like the electrons in a copper wire) you would get in the vicinity of a changing magnetic field. These led to all sorts of innovative inventions like the transformer, which turned alternating electric current in one wire into an oscillating magnetic field and back into another electric current in a different wire that wasn't touching the first wire. (Oooo!)

Enter James Clark Maxwell. He saw something even more interesting in the four electromagnetic equations. Namely, he saw that if an electrostatic field existed, and was changing, it would induce a magnetic field even if there weren't any charged particles around! Furthermore, a changing magnetic field would induce an electric field. Then the really neat part: The magnetic field induced by a changing electric field would itself be a changing field, and so would spontaneously induce another changing electric field, which would in turn induce another changing magnetic field, and so on and so forth.

Maxwell took these 4 electromagnetic equations and put them through the wringer. What he discovered was that these induced electric and magnetic fields would act like "waves", with the strength and direction of each field bobbing up and down just like a wave on the water. Furthermore, the waves of the induced intertwined electric and magnetic fields would tend to propagate through space in a given direction.

And when Maxwell used these 4 equations to calculate the speed with which such an electromagnetic wave would propagate through space, he discovered that it would propagate at precisely the measured speed of light in a vacuum.


Maxwell's conclusion:

These hypothetical free-space electromagnetic waves ARE light. Light IS nothing more, and nothing less, than a propagating electromagnetic wave. This is why radio waves (which are a kind of low-frequency light) can induce a weak alternating electric current in a metal radio antenna -- the electrons in the antenna are being pushed around by the electric-field component of the electromagnetic wave.

 

[davefoc]

Just an add on comment to tracer's nice post.

Maxwell's determination of the speed of light from these four equations has always impressed me as one of the most amazing intellectual achievements in history. I have never been quite clear why he is not as famous as Newton and Einstein for it and his other body of work. Any comments?

 

[Zep]

Nicely done, tracer!

So can we book you into some fundie conventions???

 

[Elaborate]

For anyone wanting a better understandign of quantam electrodynamics (basically, how light works) I highly recommend QED by Ricahrd Feynman.

 

[69dodge]

Originally posted by tracer
Long before James Clark Maxwell walked the hallowed halls of physics, pioneers in electricity and magnetism discovered that it was possible for electric charges to induce a magnetic field, and for a magnetic field to induce an electric current in a wire. Trouble is, in order for this electromagnetic induction to take place, the electric charges or magnetic fields had to be moving. (Actually, it was a little worse than that. Electric charges not only had to be moving, they had to be changing the speed at which they moved, i.e. accelerating or decelerating.) An electric charge moving down a wire is just called an electric current, and was nothing new to these people.

A wire carrying a constant current induces a magnetic field around it. (It sounds like you're saying it doesn't.) However, that magnetic field is constant, and so won't induce a current in another wire.

 

[Hexxenhammer]

I'd recommend the Mr. Thompkins books. I can't remember who they're by. They're like relativity for dummies. And funny.

 

[tracer]

A wire carrying a constant current induces a magnetic field around it. (It sounds like you're saying it doesn't.) However, that magnetic field is constant, and so won't induce a current in another wire.

D'OH! You're right. A constant (direct, non-pulsed) current in a wire will induce a constant magnetic field. But, as you say, that resulting constant magnetic field (being constant) will not in turn induce an electric field. It takes an electric field to induce a current in another wire.

 

[davefoc]

Stimpy said something that I thought was worth repeating:

When you adopt special relativity, it becomes immediately apparent that the magnetic field is simply an electric field viewed from a moving reference frame. In effect, electromagnetism becomes truly unified as a single force.

Although, Stimpy probably has a deeper understanding of this then me, I would like to add something here, because it is such an intriguing fact to me.

The fact that two parallel wires with a current flowing through them had been known to attract each other for awhile. I think Ampere discovered this. The reason for the attraction based on special theory of relativity reasoning is that the positive charges in one wire experience a relavistic contraction because of the motion of the minus charges in the other wire. Note that if current was flowing in only one wire, both the minus and positive charges would contract equally with respect to the wire in which current was flowing and therefore no attraction.

Thus, two kinds of forces, magnetic and electrostatic were shown to be only one kind of force that is created either by static charge differentials or the motion of charge carriers that cause apparent charge differentials.