Photon Interference

[MiloB]

OK, I admit I'm no physicist, but I understand the double slit experiment and why light has interference patterns when shown through the two slits. But, when two beams of light intersect at any angle, each beam passes through the other without any (seemingly) interaction. OK, so the stock answer is that light is acting like a wave and electromatic waves don't interact. If they did, radio communication wouldn't be possible - there would just be static. Yet, in the double slit experiment electromatic waves, i.e. photons, electrons, etc. do interact and interfere with each other. So, how do we have radio or, even how is it possible to see anything at all?

I'm sure someone out there can give a good answer to this.

[sepulchrave]

Electromagnetic waves do not interact with each other, but they do intefere.

Two beams of light will indeed pass through each other, because light cannot be scattered by light.

However at the spot where the two beams of light intersect, the electromagnetic fields from both beams add together.

If the two beams are coherent (that is, the electromagnetic field is smooth all the way across the beam, and the two beams are in phase) and have the same intensity then in some spots the ``high'' parts of the electromagnetic wave from one beam will add to the ``low'' parts of the electromagnetic wave from the other beam and the net result will be a zero electromagnetic field at that spot - the dark fringe in the interference pattern. In other locations the ``high'' and ``high'' parts will add up, or the ``low'' and ``low'' parts will add to make the bright fringe in the interference pattern.

Whenever light from two sources passes through the same spot there is always intereference, but because most lightsources (lightbulbs, the sun, scattered light, etc.) are incoherent there is no interference pattern (neither in space nor in time).

This is why the double slit experiment can only be realized by splitting light from a single source (and typically a coherent source like a laser).

[Frank Merton]

Electromagnetic waves do not interact with each other, but they do intefere.

Two beams of light will indeed pass through each other, because light cannot be scattered by light.

However at the spot where the two beams of light intersect, the electromagnetic fields from both beams add together.

If the two beams are coherent (that is, the electromagnetic field is smooth all the way across the beam, and the two beams are in phase) and have the same intensity then in some spots the ``high'' parts of the electromagnetic wave from one beam will add to the ``low'' parts of the electromagnetic wave from the other beam and the net result will be a zero electromagnetic field at that spot - the dark fringe in the interference pattern. In other locations the ``high'' and ``high'' parts will add up, or the ``low'' and ``low'' parts will add to make the bright fringe in the interference pattern.

Whenever light from two sources passes through the same spot there is always intereference, but because most lightsources (lightbulbs, the sun, scattered light, etc.) are incoherent there is no interference pattern (neither in space nor in time).

This is why the double slit experiment can only be realized by splitting light from a single source (and typically a coherent source like a laser).

Well done.

[MiloB]

Electromagnetic waves do not interact with each other, but they do intefere.

Two beams of light will indeed pass through each other, because light cannot be scattered by light.

However at the spot where the two beams of light intersect, the electromagnetic fields from both beams add together.

If the two beams are coherent (that is, the electromagnetic field is smooth all the way across the beam, and the two beams are in phase) and have the same intensity then in some spots the ``high'' parts of the electromagnetic wave from one beam will add to the ``low'' parts of the electromagnetic wave from the other beam and the net result will be a zero electromagnetic field at that spot - the dark fringe in the interference pattern. In other locations the ``high'' and ``high'' parts will add up, or the ``low'' and ``low'' parts will add to make the bright fringe in the interference pattern.

Whenever light from two sources passes through the same spot there is always intereference, but because most lightsources (lightbulbs, the sun, scattered light, etc.) are incoherent there is no interference pattern (neither in space nor in time).

This is why the double slit experiment can only be realized by splitting light from a single source (and typically a coherent source like a laser).

Thanks. Very good scientific explanation.

However, I'm not sure it gets to the deeper part of the question. As light travels for billions of years across the universe and since the universe must be filled with photons going in all directions it seems quite improbable that a photon would not encounter another photon whose wave pattern was coherent. Same thing with radio waves. Perhaps this could explain the red shift of distant stars? Or, perhaps that is why we can only see 14 billion years away.

I think this gets back to the old question of what exactly is an electromagnetic wave made up of. In other words, what is waving? As far as I have been able to ascertain, no one has been able to answer this question. But, there must be "something" that is waving else there would be no interference when waves are coherent. Granted, photons have energy and so energy is being transmitted somehow and so there is something going on but exactly what, no one knows. Science is comfortable dealing with the material aspects of particles and waves, but delving below that knowledge gets blurred. We've learned that elementary particles are actually standing waves but yet we still don't know what is waving and so it follows we really don't know what matter is at all.

I'm interested in other's thoughts on these subjects. (and to show me the errors of my ways)

[sepulchrave]

As light travels for billions of years across the universe and since the universe must be filled with photons going in all directions it seems quite improbable that a photon would not encounter another photon whose wave pattern was coherent.

Of course it does. But unless you are at the exact place and time, you will not see this interference. If the two photons have different directions, they wil only interfere at a single position and time.

On the other hand, if both coherent photons are traveling in the same direction, in that case there will always be interference, and it will either look like ``no photons'' or a ``double photon''.

Just from the regular lighting in the room you are in right now, there are trillions and trillions of instances of interference happening all the time. But they occur at random places, and only last roughly 10^-12 s (since visible light is in the THz range) - so you can't see it.

In order to see interference, you need to coherent light sources - which is something that is very unlikely to occur by accident.

I think this gets back to the old question of what exactly is an electromagnetic wave made up of. In other words, what is waving? As far as I have been able to ascertain, no one has been able to answer this question. But, there must be "something" that is waving else there would be no interference when waves are coherent. Granted, photons have energy and so energy is being transmitted somehow and so there is something going on but exactly what, no one knows. Science is comfortable dealing with the material aspects of particles and waves, but delving below that knowledge gets blurred. We've learned that elementary particles are actually standing waves but yet we still don't know what is waving and so it follows we really don't know what matter is at all.

We have a pretty good idea about all of those things.

There is (for whatever reason) an electric and a magnetic field that permeates all of space-time. An electromagnetic wave is when these two fields deviates from zero in a periodic manner. These fields can only deviate from zero in certain discrete quantities (again, for whatever reason) - so we have named these deviations ``photons''. The electric + magnetic fields are the thing that is ``waving''.

The deviations in these fields are generated or absorbed by matter that possess an aspect called ``electric charge''.

The deviations in these fields are just one of the many representations of the generalized quantity we call ``energy''. This quantity, ``energy'', can transform from one representation to another by well-defined rules.

[MiloB]

We have a pretty good idea about all of those things.

There is (for whatever reason) an electric and a magnetic field that permeates all of space-time. An electromagnetic wave is when these two fields deviates from zero in a periodic manner. These fields can only deviate from zero in certain discrete quantities (again, for whatever reason) - so we have named these deviations ``photons''. The electric + magnetic fields are the thing that is ``waving''.

The deviations in these fields are generated or absorbed by matter that possess an aspect called ``electric charge''.

The deviations in these fields are just one of the many representations of the generalized quantity we call ``energy''. This quantity, ``energy'', can transform from one representation to another by well-defined rules.

Appreciate the conversation.

If the electric and magnetic fields default is zero and undetectable, how is it that it can be said they exist? Sounds like ether to me.

[sepulchrave]

If the electric and magnetic fields default is zero and undetectable, how is it that it can be said they exist? Sounds like ether to me.

[Emphasis mine]

The emphasized portion is fair, to some extent.

But:

1. Any field used in a field theory has to extend everywhere in space and time.
   
2. Ether was postulated to be a ``thing'' - a real substance with mass, energy, etc. A zero field has neither mass nor energy.
   
3. Quantum vacuum fluctuations are a consequence of field theory - should they be unambiguously detected it would greatly support the theory.
   

So field theory could be thought of as a more modern version of ether theory.

It is also worth remembering that the electric and magnetic fields provide more than just light - they also provide regular electric and magnetic forces. The Sun and the Earth both generate enormous magnetic fields, for example, that stretch out very far in space.

[Rlyeh]

We have a pretty good idea about all of those things.

There is (for whatever reason) an electric and a magnetic field that permeates all of space-time. An electromagnetic wave is when these two fields deviates from zero in a periodic manner. These fields can only deviate from zero in certain discrete quantities (again, for whatever reason) - so we have named these deviations ``photons''. The electric + magnetic fields are the thing that is ``waving''.

The deviations in these fields are generated or absorbed by matter that possess an aspect called ``electric charge''.

The deviations in these fields are just one of the many representations of the generalized quantity we call ``energy''. This quantity, ``energy'', can transform from one representation to another by well-defined rules.

I've got a question, does the electromagnetic wave have any connection to particle-wave duality? Is the probability wave connected? I've read conflicting statements about it. Edited October 25, 2014 by Rlyeh

[sepulchrave]

I've got a question, does the electromagnetic wave have any connection to particle-wave duality? Is the probability wave connected? I've read conflicting statements about it.

Yes, they are connected.

A photon is a discrete energy quanta of an electromagnetic mode. The probability amplitude of a single photon can be mapped to the intensity of the electric and magnetic fields of a circularly polarized light wave.

Most light waves that can be studied experimentally may appear to have smooth variations in the electromagnetic field, but are actually composites of trillions of photons. Remember, any single photon is circularly polarized, but often coherent light is studied in a linear polarization mode. This means that a linearly polarized light ray must be composed of at least two different types of photon wavefunctions.

Wave-particle duality isn't really an issue for photons, because they cannot be localized (or, perhaps more precisely, they can only be localized at the instant of absorption or emission).