Ask Me Help Desk - How wave propagate?

I have a very basic question about wave propagates.I would like to know how eelectro magnetic wave propagate through vacuum.why a wave moves forward?how massless photons are attraacted bby blackhole

You really have two questions here - one about electromagnetic waves and the other about how light (photons) behave near a black hole.

1. The electromagnetic theory of light was established on firm ground by James Maxwell back in the late 1800's. He showed that light is actually a combination of changing electric and magnetic fields (hence the term "electromagnetic radiation"). With a series of 4 inter-related equations he showed that a changing electric field causes a magnetic field to be created. And that the changing magnetic field causes an electric field to be created. Each of these fields are sine waves at 90 degrees to each other and are self-propogating. The speed of light, c, comes directly from Maxwell's equations. If you'd like to learn more and delve into the mathematics see: Maxwell's equations - Wikipedia, the free encyclopedia

2. Maxwell's equations do a brilliant job of showing how electromagnetic waves propagate in a vacuum, but this was before experiments later showed the dual nature of light - that light can be considered as either a wave (like Maxwell decribed) or as particles called photons. Einstein showed that when considered as particles light photons appear to bend in a gravitational field. It's not because the massless particles are somehow attracted by the gravitational force, but rather that the presence of a large mass causes a curvature in space itself, and from a photon's perspective the shortest distance between two points actually inviolves curving through normal 3D space. One pretty intuitive explanation involves the equivalence between gravity and acceleration. Einstein postulated that it's impossible for an observer in a closed area (such as an elevator) to distinguish between the effects of acceleration and the effects of being in a gravitational field. Consider the following thought experiment: imagine you are in an elevator car that is stationary on earth - you would feel 1 G of force, due to earth's normal gravity. Now consider what it would be like to be in space far from earth and the elevator has a rocket attached that accelerates it at 1g - you will again feel 1 G of force. So a 1G gravitational field feels exactly like 1g of acceleration, and it's actually impossible to tell the difference without looking outside to see whether you are accelerating or not. Now consider what happens if the elevator is accelerating upward and someone shoots a ray gun in through the side. As a passenger in the elevator you would see the stream of photons entering from the side and appearing to bend slightly downward, since the elevator car has moved upward some distance in the time it takes for each photon to traverse the width of the car. Same effect as if you were driving your car down the street and someone shot a stream of water through your open front car window as you drove past - the stream of water would enter through the open front window and may actually hit someone sitting in the back seat, since the car moves forward some distance in the time it takes for the water droplets to moves across the width of the car. If you're traveling at a constant speed the water appears to enter the window at an angle and travels across the car at that constant angle (in a straight line). But if the car is accelerating, the stream of water would appear to people in the car to curve towards the back. Hopefully this isn't too hard to visualize.

OK, so if an observer on an accelerating elevator would see light bending, and if you accept Einstein's equivalence principal, then you can see that light must also appear to bend in the presence of gravity. Of course this doesn't explain why the equivalence principal is true - hence Einstein postulated the concept of gravity causing a bending of space itself, which in turn causes us to perceive that the light rays are bent by gravity.

Hope this helps.

Excellent answer, EB!

Thank you very much for your answer.. If you can please answer to some more doubts..

In order to perceive the bending of light we must travel faster than light as we are traveling in a car?

Can you explain why the wave traverse in a direction?
From where it is getting a force to move forward?

So lights are not attracted by gravity?

Quote:

Originally Posted by iamsv

Thank you very much for your answer..If you can please answer to some more doubts..

in order to percieve the bending of light we must travel faster than light as we are traveling in a car?

No, not at all. In my example of the car and stream of water I did not mean to imply that the speed of the car is greater than the speed of the water coming in from the side. For example: if the car is moving at 10 m/s and a stream of water comes in from the side at 50 m/s, to the observers in the car the stream will appear to come at an angle of arctan(10/50) = 6 degrees.

Actually the observer doesn't need to be moving at all - all that's needed is a large gravitational field somewhere along the path of travel of the light beam. In extreme cases it's possible for a large mass to act as a "gravity lens." Astronomers actually see example where a single point of light from a very distant object such as a quasar can be bent by a massive object between the source and us in such a way that the point appears to be distorted or stretched out. For examples see: Gravitational lens - Wikipedia, the free encyclopedia - I especially like the image of "Einstein's Cross," where we see 4 points of light that all actually originate from a single source.

Quote:

Originally Posted by iamsv

can you explain why the wave traverse in a direction? from where it is getting a force to move forward?

For any wave phenomenon - be it light, or sound, or vibrations on a string - you have a situation where a bit of energy causes a brief local instability that cannot be sustained in place, and so the instability moves along in a way that relieves the momentary local distress. The particular dynamics of this - such as how fast the wave moves - depends on the particulars of the type of wave and the material it is moving through. There's really no external "force" that pushes it along, but rather the energy inherent in the wave itself is able to sustain the movement. For mechanical waves friction will inevitably dampen out the wave. For electromagnetic waves in a vacuum there is virtually no friction that causes the wave to dissipate.

Quote:

Originally Posted by iamsv

so lights are not attracted by gravity?

Light is not attracted by gravity. But the travel of the light is affected by the curvature of space, which is caused by gravity. I hope that isn't too confusing.

Can you please tell me why this instability moves on a particular direction why not it spreads out.

Yu're goint to have to dig into Maxwells' equations to truly understand this. But in essence - the direction of the light wave is determined by the initial directions of the electric field and magnetic field "waves." The electric and magnetic fields are orthogonal to each other, and light propagates in the direction that is orthogonal to both of them. In the attachd figure the elecric field is represented in red as it undulates in the z direction, the magnetic field in blue in the x direction, and the direction of light is along the y axis.