# Light As A Wave

by fresh
Tags: light, wave
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P: 40,236
 Originally posted by Adrian Baker I'm not sure that this is correct Doc Al. If c is the same for all observers, then the energy of a photon must be fixed for all observers surely? I really don't understand this energy loss with redshift, perhaps you could expand your reply to show me how this could be so?
You must understand that the energy of a photon is given by its frequency (E=hf) not by its speed. I presume you know that, else your overall question would not make sense. Due to the Doppler effect, the frequency/energy of a photon is observer dependent. To understand where the energy "went", look up the relativistic doppler effect. Bottom line: the energy doesn't "go" anywhere, it's just measured to be different in different reference frames.
 Perhaps it should be on another thread though?
To get into the details of Doppler and cosmological redshift, I would suggest the Relativity & Cosmology forum.
P: n/a
 Originally posted by Adrian Baker Thanks for the ideas here, and I understand the stretching idea of a photon as the universe expands and this gives a good mental picture of what is happening. But I still have a problem with this. For an observer, the energy of a photon is given by E=hf Where is the time factor in this then? Mathematically, Energy is lost. Is the Maths wrong, or is there a correction factor to E=hf that I am unaware of? I can't see how Relativty helps either, although that maybe because my Maths isn't good enough.

The math is not necessarily “wrong”, but is “different” from the classical math way of explaining the situation. If you use the modern math way, then the phenomenon is hard to visualize. If you use the classical math way, using the traditional “distance” and “time” factors, then the phenomenon is easy to visualize.

I am a “classical relativist”. This means that I prefer to describe the functions of nature in terms of classical mechanics and relativity, while using modern experiment and observation as a guide to the latest laws of nature that have been observed and discovered.

Since we don’t say that a sound wave “loses energy” when it is redshifted. I see no reason to say that a light wave/photon “loses energy” when it is redshifted.

However, modern relativists prefer to say that a photon “loses energy” when it is redshifted and “gains energy” when it is blueshifted.

But, all we basically have in a redshifted beam of light is a lower frequency per second, just as we have in redshifted sound. In other words, the train whistle moves away from you, and so you receive fewer sound waves per second, meaning you hear a lower pitch of sound than what is being emitted. So, the sound wave energy is not “lost”, it just takes longer to get to you, because the whistle is moving away from you.

If 1,000 cycles are emitted in 1 second, it might take 1.2 seconds for you to receive all of them.

If a distant galaxy is indeed moving relative to the earth at 3 c, it will take at least three times longer for you to see a 1 second burst of light from that galaxy. In other words, 3 seconds (or more), rather than 1 second. All the waves/photons are accounted for. You will receive them all, but over 3 seconds rather than over 1 second. Now, whether each individual photon of light is “stretched out” in space, or whether there is just more distance between any 2 of them, I don’t know.

In my opinion, the spectroscopes that measure the light redshift, act as if they are receiving a stretched out series of light “waves” rather than a stretched out series of point-particles. So, in this respect, light acts like a wave, with the waves being elongated by the relative motion of the distant galaxies.

Another point, I do not like the expression that “space is expanding”. There is no observational evidence that “space is expanding”. The observation information tells us that the galaxies look as if they are “moving through space” away from us, just as everything else moves through space, either toward or away from us, or with us.

Basically, the “space is expanding” idea comes from Einstein’s 1905 Special Relativity theory that put a “speed limit” on matter so that supposedly, it can’t exceed the speed of “c”. But since the redshifts indicate that some galaxies are “moving faster than c”, relative to the earth, the physicists had to come up with some way to keep the galaxies from “violating” this 1905 Einstein proclamation of the “speed limit”. Thus, they came up with this “space is expanding, so the galaxies really aren’t moving” idea, which I think is silly.

Another problem I see with modern physics is that the physicists deny an “ether” or a “light propagating medium”. So, because of this, they tend to think of “photon energy being reduced” when they see redshifted light, since they don’t want to say that “photons” are moving slow relative to the earth when they are first emitted by a distant galaxy, and they gradually speed up relative to the earth as they travel toward the earth. When they deny the “ether”, this irritates the heck out of me. Based on Einstein’s own theories, especially his important 1911 gravitational redshift theory, the gravitational fields of the universe act just like an “ether”, and they regulate the speed of light locally, near the astronomical bodies that generate the local gravitational fields, and I personally think they also regulate the speed of light in deep space.

So, look at this modern description of the situation:

“If one divides a comoving distance by the present cosmological time (the age of the universe) and calls this a "velocity", then the resulting "velocities" of "galaxies" near the particle horizon or further than the horizon can be above the speed of light.

This is the paradox of the ambiguous phrase space expanding faster than the speed of light. An unambiguous rewording of the phrase can now be made:

for a "galaxy" towards or beyond the horizon, its "velocity", defined as comoving distance from the observer divided by the present cosmological time, can be greater than the speed of light”

And look at this:

“In this concept, light leaving a distant galaxy is traveling at velocity c toward us in that local space but since space itself is expanding and that local space is moving away from us, the velocity of a light packet toward us (defined in a certain way) is less than c until the packet gets to our local space and the velocity can even be negative for some early portion of the time light travels to us.”

What this second quote means when it says, “the velocity of a light packet toward us... can even be negative for some early portion of the time light travels to us”, literally means a photon or light wave is “aimed” in our direction, but it is moving away from us. It’s moving “backwards”, just as it is being emitted by the distant galaxy. The latest theory is the light photons/waves literally “speed up” as they travel toward us, and by the time they reach us, they are traveling at “c” relative to the earth, but much faster than “c” relative to the galaxy that emitted them, but the physicists don’t come right out and say this in classical terms. They say, “the velocity of a light packet toward us is negative as the light is first being emitted by a superluminal galaxy.” But this is exactly the same thing that I’ve just said. A “negative velocity” means the light is “aimed toward us”, but is “moving away from us” as it is being emitted.

So how can this be? How can the light ever get to us if it is moving away from us?

If you consider that the gravitational fields that surround all the astronomical bodies in the distant galaxy act as a “light propagating medium” or at least as a “local light speed regulator”, then they control the speed of light to “c” within that galaxy, but as the light gradually leaves that galaxy, on it way toward the earth, it passes through other areas of space where the local fields of other astronomical bodies and galaxies regulate its speed, and since they are not moving away from us as rapidly as the galaxy that emitted the light, the net effect on the photons is that they “speed up” relative to the earth.

So, after billions of years, as the waves or photons gradually work their way toward the earth, they move through more areas of space where the galaxies closer to us are not moving away from us very fast at all, and so, relative to us the light is traveling much faster as it gets closer to us. By the time it enters our own galaxy and our solar system, then its speed is being regulated locally by our own local fields generated by our own local astronomical bodies, and we receive it here at earth traveling at “c” relative to the earth.

This speeding up process gradually “stretches out” the space between the photons or the “wavelength” of the light, and that is why we receive the light as being redshifted.

This is basically regular old Doppler relativity, but with the “local” gravitational fields of space regulating the “local” speed of the light, and all through the space between us and the distant galaxies, the local fields are moving apart with the expansion of the universe. If the universe is “expanding”, then those local fields on large-distance levels, are moving apart, since the galaxies that generate the local fields are moving apart. But the galaxies near us aren’t moving away from us as rapidly as the 3-c galaxy that emitted the light, so the light gradually speeds up relative to us.

A pure “Einstein” relativist will howl at what I’ve just said, and will call me a “heretic” and a “crackpot”. However, they describe exactly the same thing, but in more vague and mysterious modern terms that most people can’t understand.

So, why is there a “c” speed limit for matter on the local level, but not on the universal level? Again, based on Einstein’s own theories, locally a mass has to be forced through a local gravitational field, and apparently the field puts up a resistance to that motion. That would explain why particles can not be accelerated to light speed at the surface of the earth. But, the distant galaxies are not moving “through” the earth’s local field, or through any “field”, if they are moving through “empty” space, so on a universal level, there are no fields through which the galaxies are moving that would put up a resistance to their motion, so they are free to move as fast as they want to.

What I’ve just given you is actually modern cosmological theory, explained in a classical relativity manner.
P: 261
 Originally posted by Adrian Baker Light diminishes inversely proportional to the square of its distance from the source.
Is there any mathematical illustration for this?
 P: 31 http://www.physicsforums.com/latex_images/119455-0.png The inverse square law (ISL) equation is presented in the .png. The divisor is the same as the equation for the surface of a sphere with radius r, and P being the original emitted power. The law assumes the energy is being detected somewhere on an expanding sphere. You can find references stating that a laser does not obey the ISL. Additional material on the ISL equation is at, http://prancer.physics.louisville.ed...ual/node7.html Still, I have yet to find an authoritative reference that unequivocably states that a single photon is not subject to the ISL.
 P: 261 Thank you so much Frank! That was really interesting! Thanks for the link!
 P: 419 David, thank you for taking the time to write such a long, detailed and interesting reply to my query. You certainly take an unusual line! Thanks too Doc Al. I'm going to go away and ponder for a bit now......[:)]
 P: 4 That would explain why particles can not be accelerated to light speed at the surface of the earth. Say you want to fire a bullet at the speed of light, you first need to find a gun that can generate a muzzle velocity of c. If you have a gun that generates only .9c muzzle velocity then your bullet will travel at .9c too. A particle is a bullet and a gun is the particle accelerator in my analogy.

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