# Shortest distance light wave can travel?

1. Aug 13, 2014

### devanshshah

Hey i wanted to know if this answer is already present.If yes then what is the shortest amount of distance a beam of light can travel?

2. Aug 13, 2014

### sophiecentaur

Hi and welcome to PF
Imo, there is no a definite answer to this one and it would depend upon how you actually define a 'beam' (as light is basically a wave phenomenon).
To talk about an identifiable 'beam of light' you would need to have several wavelengths of the light involved or you would really be talking about 'local electromagnetic fields'. Is that a satisfactory answer?

3. Aug 13, 2014

### .Scott

In this case, I take "light" to mean a photon in the visible part of the spectrum.

The problem is with how things develop at the quantum level. You would need to create a situation where you could measure a quanta of energy (the photon) moving from one particle to another. And to know that only a photon could have been responsible.

Many atoms are 1 to 3 angstroms in width. With nanotechnology, I'm sure something could be set up to demonstrate a jump of no more than 4 angstroms from one atom to another.

4. Aug 13, 2014

### johnbbahm

I think if you looked at say a photon from a HeNe laser 632.8 nm.
I do not think the photon could exists in a cavity shorter than the wavelength.

5. Aug 14, 2014

### sophiecentaur

A photon cannot, in any way, be described as a beam of light. You are confusing the two models of EM radiation. Both models work in their own context but when you try to use them at the same time, you tend to fall over.

A "beam of light" would have to constitute a massive number of photons - sufficient for the statistics to kick in and allow you to treat light as a continuum - or, as we say, a wave. A situation in which you can use the wave description and a definite 'beam' must involve a region which is many wavelengths in size.

Otoh, if you are talking about the effect of one atom on another, due to a photon, then that can involve distances much less than a wavelength. (But that's not really a beam)

6. Aug 14, 2014

### .Scott

How many photons per second do I need to cross from one atom to another before I can call it a beam? Once I have established that the mechanism for energy transfer is with photons, all I have to do is measure how much energy is transferred as this process is rapidly repeated.

At what level of energy transfer (Watts) would you be willing to call it a "beam".
Or is there some additional characteristic of a beam that you haven't mentioned?

7. Aug 14, 2014

### rcgldr

Would it be possible for a photon to be released from an electron in a molecule only to be caputured by another electron in the same molecule?

8. Aug 14, 2014

### .Scott

Given the enormous size of some molecules, yes.

9. Aug 14, 2014

### Staff: Mentor

I would probable equate the classical concept of a beam with the quantum concept of a coherent state.

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

However, I suspect that the OP doesn't actually care about it being a beam and just wants to know if the EM interaction has a minimum range. Which it does not as far as we know.

10. Aug 14, 2014

### mishima

Is there no reason to think that the Planck length is this minimum?

11. Aug 14, 2014

### Staff: Mentor

There is no evidence to think that at this time. I am sure that you could come up with a reason, but without evidence it is just speculation.

12. Aug 14, 2014

### Staff: Mentor

I think you need to explain your question further. What made you think of this question, or what sort of experiment or situation are you thinking about?

13. Aug 15, 2014

### sophiecentaur

A beam of light is fairly loosely defined but I would say it's a situation where the majority of the energy is travelling within a limited volume and mostly in a particular direction. This can only happen when the source is many wavelengths wide and when there are enough photons involved (i.e. the energy flux) is high enough for the diffraction pattern to be 'recognisable'.
Recently, people have been leaping on the bandwagon of 'single photon sources' but they are only used in very limited context and I really don't think that understanding of light propagation is best addressed in terms of photons. I haven't read a half decent explanation of simple optical phenomena, using waves, that has been bettered by a photon-based explanation. This thread is a bit of a mixture of the two approaches and is suffering, as a result.

14. Aug 18, 2014

### .Scott

If I catch your drift, rays of light are only "beams" if they are surrounded by the lack of rays of light. So it would be improper to call all the light emitted by the sun "sun beams". Instead, only those rays which pass through holes in the clouds - leaving behind adjacent rays - can be called "beams".

So long as "beam" is well defined, I think limiting the question to "beam", actually makes it easier to come up with experiments where the distance is tiny or the answer is more clearly calculated. It would seem that knowing the upper extent of visible violet light and a clear enough definition of "beam", it should be possible to determine at what point you have a "beam". One could then place a black target an angstrom (or smaller distance) beyond the starting point of the "beam" to create a very short beam. Perhaps the minimum length of the beam is determined by the distance that a few photons will tunnel through the target.

Also, I noticed that the OP asked about "beams" in the post but "waves" in the title.

15. Aug 18, 2014

### sophiecentaur

I don't like to get involved in this sort of 'definitions' discussion as it seldom leads anywhere useful.
The main point about the minimum distance for a photon to go from one atom to another is, perhaps, interesting and it must correspond to interatomic distances in molecules - or less. If you wanted to include the virtual photon interactions between charged particles then you'd have to include sub nuclear distances. (Photons can be gamma rays too)

16. Aug 21, 2014

### Claude Bile

I think the simple answer is - there is no minimum distance.

If I have a beam of light at some point z = 0, then I can propagate it forward an arbitrarily small distance. Of course, there are details regarding classical vs quantum fields, but the OPs use of the term "beam" would suggest the above answer is sufficient.

Claude.