Speed of light maybe not constant after all!:

So far, I have found a number of sources that state that two separate light sources in the two slit experiment does not produce an interference pattern.

Originally posted by Kepler
1. Yes, of course. Radio waves interfere and that's just light of a different wavelength.

Any references / descriptions to confirm this? How would we even know if radio waves interfere? Are you possibly confusing interference between the electric signals produced by radio waves in an aerial with the actual photons interfering?
I can find no reference confirming interference in light from two separate sources, and even the two slit experiment apparently requires light of only one frequency. So in your radio wave interference, do different frequencies interfere, or only within a given frequency?

In principle light (or any wave phenomenon) from different sources can interfere, although in many cases you won't notice a significant effect since most of the time the waves won't be coherent. It should also be noted that you can't really have light of a fixed frequency (although the frequency spectrum can be sharply peaked about a certain frequency).

I was told that gravity alters the spead of time itself? - that if you take 2 watches and place one on the moon and one on earth the one on the moon runs slightly slower. (it might of been faster, but there is a difference) The extreem being a black hole where time becomes a tricky concept as even light can't escape?

Originally posted by e4chris
I was told that gravity alters the spead of time itself? - that if you take 2 watches and place one on the moon and one on earth the one on the moon runs slightly slower. (it might of been faster, but there is a difference) The extreem being a black hole where time becomes a tricky concept as even light can't escape?

I think you are confusing a few things here.

Here is an accessible article for the layman on relativity.

http://en.wikipedia.org/wiki/Introduction_to_special_relativity

Originally posted by KazetNagorra
In principle light (or any wave phenomenon) from different sources can interfere, although in many cases you won't notice a significant effect since most of the time the waves won't be coherent. It should also be noted that you can't really have light of a fixed frequency (although the frequency spectrum can be sharply peaked about a certain frequency).

I know that any wave phenomenon can interfere - if in the same medium, but I am yet to be convinced that light is a true wave phenomenon.
Can you explain what you mean by 'coherent'.
Why does a single laser result in an interference pattern which coincides over time and over many photons, but two separate lasers do not give an interference pattern?
From what I can tell so far, the single laser interference pattern is caused by each individual photon interfering with itself, and only with itself. If photons interfered with each other, then surely the coherence problem would cause even single laser systems to result in no discernible interference pattern.

Most importantly though, I cant think of a quantum mechanical explanation for two photons interfering with each other.

Originally posted by KazetNagorra
I think you are confusing a few things here.

Here is an accessible article for the layman on relativity.

http://en.wikipedia.org/wiki/Introduction_to_special_relativity

oh yes i don't know if it makes the clock go fast or slow - but it changes it - I have read a brief history of time! did not understand a word! other then the bit about the turtles

do you really understand this stuff?

Originally posted by twhitehead
I know that any wave phenomenon can interfere - if in the same medium, but I am yet to be convinced that light is a true wave phenomenon.
Can you explain what you mean by 'coherent'.
Why does a single laser result in an interference pattern which coincides over time and over many photons, but two separate lasers do not give an interference pattern?
Fr ...[text shortened]... I cant think of a quantum mechanical explanation for two photons interfering with each other.

http://en.wikipedia.org/wiki/Coherence_(physics)

Two lasers can in fact interfere.

The quantum mechanical explanation for two photons interfering with each other is straightforward; both have a wavefunction which opens up the possibility of interference.

Originally posted by e4chris
oh yes i don't know if it makes the clock go fast or slow - but it changes it - I have read a brief history of time! did not understand a word! other then the bit about the turtles

do you really understand this stuff?

The more you know, the less you understand. 😉

Originally posted by twhitehead
Any references / descriptions to confirm this? How would we even know if radio waves interfere? Are you possibly confusing interference between the electric signals produced by radio waves in an aerial with the actual photons interfering?
I can find no reference confirming interference in light from two separate sources, and even the two slit experiment ...[text shortened]... r radio wave interference, do different frequencies interfere, or only within a given frequency?

From Wikipedia but it sums up other works on the subject:

In physics, interference is a phenomenon in which two waves superimpose to form a resultant wave of greater or lower amplitude. Interference usually refers to the interaction of waves that are correlated or coherent with each other, either because they come from the same source or because they have the same or nearly the same frequency. Interference effects can be observed with all types of waves, for example, light, radio, acoustic, and surface water waves.

Note the bit in bold. Waves with the same source will interfere but so will waves of differing sources provided the frequencies are the same. I'd suggest that the same source bit is redundant because the waves will have the same frequency if they are from a single source.

You can detect radio wave interference using a radio wave detector of some sort. It is the basis of radio interferometry used in radio astronomy.

Originally posted by Kepler
You can detect radio wave interference using a radio wave detector of some sort. It is the basis of radio interferometry used in radio astronomy.

Unless I am mistaken this is a case of a single source?

Originally posted by twhitehead
Unless I am mistaken this is a case of a single source?

Not necessarily. You can have multiple radio sources within a single galaxy. In any case, whether or not radio interferometers are looking at single or multiple sources does not change the fact that radio waves, or any waves for that matter, from separate sources can interfere provided they have closely similar frequencies. I mentioned radio interferometry simply because you doubted that radio wave interference can be detected.

Two applets that let you play with two sources or multiple sources:

http://www.phy.hk/wiki/englishhtm/Interference2.htm

http://www.phy.hk/wiki/englishhtm/multipleSources.htm#anchor

Originally posted by Kepler
Not necessarily. You can have multiple radio sources within a single galaxy.

But each of those sources goes through all 'slits' when doing interferometry, correct?

I mentioned radio interferometry simply because you doubted that radio wave interference can be detected.
I am not disputing it can be detected, I just wanted to make sure that we were talking about the type of interfering we are discussing and not some sort of interference going on in a radio aerial.

Two applets that let you play with two sources or multiple sources:
Once again, I have no doubt that for waves on water, there will be interference from multiple sources. I am trying to establish whether or not light waves are truly equivalent to waves on water ie that they behave the same under all circumstances.

Having read up on radio interferometry, I see that the interference is not necessarily done physically at all, but can be done entirely in data.
I see I need to give this some serious thought.

If we take the two slit experiment, and separate the slits so that the resulting images on the screen are separate, we see two bands with no interference patterns. Radio interferometry suggests that with a highly sensitive screen, we could record the two bands and discover the interference pattern by combining them ins software.
I find this hard to reconcile with quantum mechanics. Very interesting.

Originally posted by twhitehead
But each of those sources goes through all 'slits' when doing interferometry, correct?

[b]I mentioned radio interferometry simply because you doubted that radio wave interference can be detected.
I am not disputing it can be detected, I just wanted to make sure that we were talking about the type of interfering we are discussing and not some sor ...[text shortened]... ng them ins software.
I find this hard to reconcile with quantum mechanics. Very interesting.[/b]

Radio waves are light waves, but of a different frequency than visible light. Interference of radio waves works exactly the same as interference of visible light.

Why is it hard to reconcile this with quantum mechanics?

Originally posted by KazetNagorra
Radio waves are light waves, but of a different frequency than visible light. Interference of radio waves works exactly the same as interference of visible light.

Yes, I do realize that. It just that I have not heard of the two slit experiment with radio waves so I am having trouble reconciling the two.

Why is it hard to reconcile this with quantum mechanics?
In quantum mechanics a particle is a probability wave of where it will be. But it will always be somewhere.
When you do the two slit experiment, a particle goes in all possible paths and the probabilities interfere with each other.
If we are talking about two separate particles, then we are saying that they are somehow interacting with each other when they create the interference pattern even though when they hit the screen they hit in two separate places. So when a photon leaves a slit and hits the screen, it not only knows all the possible paths it may take, but also all the possible paths that every other photon that could possibly hit the screen may take. This makes quantum mechanics even weirder.
It also suggests that the screen cannot distinguish in any way between the two photons. It cannot possibly know which source a given photon came from.
I haven't fully understood the page on coherence, but I am guessing from what I said here, that if the two beams are polarized in different directions then there will be no interference pattern.