fckm

You're thinking of particles again. Stop it
Think about a water wave. You're on a beach, getting a tan, and you notice the waves coming in off the open sea. In this case, the "wave" is a displacement wave traveling through water. The medium is water. So while the water has mass, the actual wave itself doesn't. However, there are fundamental limits to how fast that water wave can travel. The limits are not dependant on the properties of the wave, it's dependant on the properties of the water.
Stop thinking of quantum mechanics. Don't think of particles. Light is not a particle. Think of light as a wave. Light is not a tennis ball flying through the air. A tennis ball flying through the air has mass. Waves don't have mass. Light has no mass. Waves in water have no mass. Sound waves (pressure waves in air) have no mass.
Waves are not "things" which travel through a medium. Waves are chages within that medium.

Have you ever seen one of those stock ticker signs in time square? It's got a bunch of bulbs and you can make words by turning the lights on and off. Imagine letter 'A' scrolling across that board. The letter 'A' is now a wave. The medium is those light bulbs. The wave, the 'A', is made by changes in the light bulbs. But the letter 'A' moving across the sign has no mass. And yet, it can't move infinately fast. It's speed is governed by how fast those bulbs can turn on an off.

Think of Light as being little 'A's being displayed on a giant 3-D sign.

__________________
And if you say to me tomorrow, oh what fun it all would be.
Then what's to stop us, pretty baby. But What Is And What Should Never Be.

GMontag

Both views (light as waves and light as particles) are valid and useful. The wave explanation for light slowing down in a medium is that the speed of light is determined by two constants, the permittivity and the permeabilitity of the medium. Those constants change from medium to medium. The light as particle explanation for the slow down is that the photons are all actually still travelling at c, but whenever they run into an atom, they get absorbed and reemitted, which takes time. On average then, the speed of light slows down.

fckm

Both views are valid. And your right, very illuminating.
For physicists.
For laymen, on the other hand, there's no need to think of light as a particle. It's much too confusing, and people start asking questions like "what makes light move". Unless someone has taken classical physics for a couple of years or a college level course on quantum mechanics, it's probably best to explain things classically.

__________________
And if you say to me tomorrow, oh what fun it all would be.
Then what's to stop us, pretty baby. But What Is And What Should Never Be.

welshbyte

...or taken a _very_ healthy interest in it and borrowed a few books from the library now and then to feed their curiosity

__________________
There are only two industries that refer to their customers as "users". - Edward Tufte

stingc

Just to add to this, the permittivity and permeability are not just empirical constants that happen to be different for every material. It is possible to derive them (at least in principle) using just the permittivity and permeability of a vacuum, along with knowledge of the material's internal structure. Light travels through a medium at the same speed as it does in a vacuum, but its interactions with the atoms inside of it cause them to give off their own fields. This secondary field combines with the main one in such a way that it looks as if light has actually slowed down on macroscopic scales (only the sum of the waves is measurable).

ratiocination

Light travels supposedly in alternating electric and magnetic fields. By Maxwell's theory, this generates a wave that continues to propogate itself indefinetly. The speed of light is the only constant in the universe because of this property, and it is this property that unifies magnetism and electrostatics. Light can move so fast because it has no mass, and because it moves in the form of an electromagnetic wave. As a consequence, all electromagnetic waves move at the same "fast" speed, and this speed cannot be affected by the envornment in most circumstances.

zen_tom

This is a great thread - thanks guys.

I can't get my head quite around the concept of an electric/magnetic field - how does it propagate itself? i.e. Say an electro-magnet is turned on to make a magnetic field. The field may stretch out for say 5", but does it stretch out at the speed of light, or is it instantaneous?

What about gravity (another force-field type effect) how does that propagate? If a planet suddenly appeared next to the moon, would it take time before its gravitational effects were noticable by us, or would they take time to reach us at a distance?

What exactly are these things (electric, magnetic and gravity fields)? I know they exist, but what are they?

stingc

It goes out at the speed of light. It doesn't stop at a few inches though. The range is essentially infinite, although of course the field gets weaker the farther out you go.

The answer to this is the same. Any disturbances travel outward at the speed of light. To be more correct, small disturbances in a very strong gravitational fields will actually travel at speeds less than and equal to the speed of light. The gravitational waves that appear to move more slowly are actually being scattered off of the spacetime curvature.

Are you asking for a definition? They represent how strongly an object placed at a particular point will be affected by electric, magnetic, or gravitational forces. Say you place an uncharged particle of mass m at some point and measure the force on it (F). The (Newtonian) gravitational field is then defined as g=F/m. You make a similar construction for electric and magnetic forces.

The original point of this was that g is independent of which m you choose. Once you know g, you know all there is to know about how all massive bodies will be accelerated. Although this seems like mere mathematical convenience, the field concept eventually took on a reality of its own. Its 'reality' is required for locality, energy and momentum conservation, and so on.

ObieX

Gravity is one of the least undrestood forces, and yes it would take time to reach us. If the sun were to suddenly vanish it would take time for the wave to ripple out to us as well as for the light to reach us to show us that the sun had vanished.

Lets say you hada sheet pulled held flat and pulled tight. If you put a big ball in the midle of it, it would make a curve in the sheet depending ont he weight of the ball. Now we take a marble, we would make that marble spin around this bend in the sheet much like we would have a marble fly around in a roulette wheel. That marble would have to move fast enough so that a0 it does not fall down toward the ball in the center (thus being sucked in by gravity) and b0 slow enough so that it doesnt fly off the blaket (so the forces are balanced and it stays spinning in a circle) Now if you were to pick up that ball in the center it would take time for the sheet to rise and for that ripple to extend out to where the force that keeps the ball spinning is no longer balanced, and the one that wants it to fly away is greater and it would fly away.

Gravity is a tough one. As far as i know, people are still trying to figure out exactly what that force is and how it behaves. Especially when it comes to interacting with other dimensions.

__________________
We Must Dissent.

zen_tom

You've explained very well what they do, and how they act, but not what they are.

stingc

How else do you define an object? I don't know what anything 'is,' nor do I even know what an answer to that sort of question would look like. Give an example of what you mean.

Gravity is understood classically (at least the fundamentals are), but it has been very hard to quantize the theory. Your comment about extra dimensions is referring to string theory, which is one of the postulated extensions of gravity into the quantum world (it actually does quite a few other things as well). Despite being in fashion right now, those ideas are very far from representing anything very coherent so far.

A fundamental problem is that the gravitational field is actually much more complicated than the one I described in my previous post. That field is for Newton's theory. It is a simply a vector (3 numbers) defined at each point in space for all time. Einstein's version has 10 numbers at each point in spacetime, but more importantly, the meanings of those points change depending on the values the field takes on. It is describing geometry. Before general relativity, space and time were considered immutable concepts, but it turns out that they are not. It is much less difficult to come up with a theory describing a field existing on a fixed geometry versus one which changes the very nature of space and time as it 'propagates.' If you want to confuse yourself, try to imagine what it means for something to move when the meanings of distance and time change as it goes along. Now try to find a quantum theory describing that.

fckm

zen_tom:
you're probably more aquainted with a field that you can feel. Gravity. Gravity, like the electric and magnetic fields, is also a field. A field is a mathematical way of describing something that has values throughout all of space.
You are sitting on the face of the earth right now, you feel a certain force pulling you down. Now imagine you are in outerspace, you still feel a force pulling you toward the earth. But now, that force is slightly less. In fact, no matter where you are in space, you will feel a force toward the earth. (if you go out too far, that force becomes really really tiny, practically zero).
Since you feel a force everywhere in space, physicists use a description for gravity called a "field". It simply means that for every point in space, there is an associated force pulling toward the earth.

__________________
And if you say to me tomorrow, oh what fun it all would be.
Then what's to stop us, pretty baby. But What Is And What Should Never Be.

JJRousseau

fckm, how can you say light is not movement? What is a wave if not movement. what is velocity if not movement? Light is not constant, omnipresent or instantaneous. It "travels" from a source at a defined speed. I understand that what we are really talking about is an "effect" on a field. But that "effect" changes its position over a given time.

fckm

When people say movement, a physicist thinks "displacement".
While the light itself is moving, light is not movement.
If you have a wave on a string, that's called a displacement wave. Bits of string Literally MOVE. The Medium is moving. (Of course, the wave itself is also moving.)
If you have a sound wave, the air molecules are moving. They vibrate back and forth.

But with light, the light itself may move, but there is no physical object moving that is associated with the Medium.
Again, refer to my analogy to the stock ticker. You can create little scrolling symbols moving across the stock ticker. The little symbols are like light waves. They move. But the Medium, the stock ticker, is not moving.(It's not like the light bulbs wiggle around or anything).
That's what I mean when I say light is not movement. It's not associate with the dispacement of any medium. Is the light moving? Yes. Is the light movement? No.
At least, that's the way I see it. The term 'movement' is not exactly a precise term in physics lingo, so I suppose the answer is really dependent on how you define 'movement'.

__________________
And if you say to me tomorrow, oh what fun it all would be.
Then what's to stop us, pretty baby. But What Is And What Should Never Be.

JJRousseau

Agreed.

molloby

Light is effectively oscillating electric and magnetic fields.

Most matter is made up of charged particles, which have their own electric fields, which will interfere with how light propagates. In essence matter has a different permeability and permissivity to that in a vacuum.

The speed of light is a function of the permeability and permissivity of the medium it is passing through. 1/root(mu*epsilon) I think.

fckm

permissivity? is that british?
we yanks call it permeativity.

__________________
And if you say to me tomorrow, oh what fun it all would be.
Then what's to stop us, pretty baby. But What Is And What Should Never Be.

mr.montreal

i like that one

molloby

Yeah, that was me getting home from work at 1 am and confusing long words that start with "p". *smiles and shrugs*

Pfhorte

This is easy to explain. Just take a long drive somewhere and back.
Seems to take less time going back, but way more time going anywhere if there are kids in the car.

Suave

ANGELS I TELLS YA!

roderickpsu

Light is generally the result of some event, a burning gas, an excitation of electrons followed by the electrons dropping in energy level, etc....this event provides the initial energy required for photons to travel at the speed of light. Once traveling at this speed...they really don't slow down in a vacuum (space).