# Do photons of different energy interfere with each other?

1. Jul 27, 2013

### Ahmed Abdullah

I am learning quantum mechanics from Feynman Lectures on Physics. Where he says that amplitude of alternatives that can't be distinguished interfere with each other. I am trying to understand some simple phenomenons using this principle.
My query is, when photons of different energy come from two different source and are detected using a 'single' detector, do their amplitudes interfere. It seems quite straight forward from the aforementioned principle , that the answer is no. Because when the detector receives a photon , its energy can be determined and we'll know from which source it came. So these two alternatives are distinguishable and should not interfere with each other.

But quick google search revealed to me that photons of different energy do interfere with each other for reasons I am incapable of understanding at this moment. Plz explain the matter in basic terms for I am a novice (I can easily follow Feynman, though .. .. pity he haven't explained it there :( ). Thx in advance .

2. Jul 27, 2013

### Staff: Mentor

Depends on the measurement you do. In general, yes, and you can get the same as in acoustics with electromagnetic radiation. If you try this with visible light, it is hard (but possible) to adjust the frequencies with the necessary precision.

That is one of the important points which depends on the measurement. If you try this with radio waves, for example, it is not true - you don't even see single photons, you measure the electric field in the limit of many photons.

3. Jul 28, 2013

### Ahmed Abdullah

Suppose my detector do have one photon sensitivity and the source is dim enough then my argument is correct isn't it.

Last edited: Jul 28, 2013
4. Jul 28, 2013

### Drakkith

Staff Emeritus
Only if you actually have a detector that can determine the energy of a single photon. A photomultiplier, for example, can detect single photons yet it cannot measure their energy with any real precision by itself.

5. Jul 28, 2013

### Naty1

Can you provide the source......lecture number and approximate time?

I'd like to understand the context of Feynman's statement.

Thanks.

6. Jul 28, 2013

### Ahmed Abdullah

Volume III, chapter 3

" If you could, in principle, distinguish the alternative final states (even though you do not bother to do so), the total, final probability is obtained by calculating the probability for each state (not the amplitude) and then adding them together. If you cannot distinguish the final states even in principle, then the probability amplitudes must be summed before taking the absolute square to find the actual probability. "

audio cassette V3 Ch03 - at approximately 42:30

7. Jul 28, 2013

### Naty1

Are these the Sir Douglas Robb lectures, U of Auckland??

If so they are likely in the [free] youtube series, Feynman quantum mechanics....

8. Jul 28, 2013