# I am the Sub Galactic Lighthouse Keeper

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I live in a dark corner of the Solar System and am equipped with a  button that I can push which releases an expanding sphere of light in the shape of an individual photon

Around me are sited an array of reflecting objects spaced regularly and all at a  distance from the point of emission  of 1 light.second.

What should I expect to see after the first photon has been created ?

Can I expect to see the photon reflected back after 2 light seconds from an entirely random region  with a probability of this occurring dependent on how much of the 1 light second radius sphere around the point of emission is actually covered by my reflecting objects?

I hope I have done right to post in the Quantum section of the Physics subforum....

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10 hours ago, geordief said:

Can I expect to see the photon reflected back after 2 light seconds from an entirely random region  with a probability of this occurring dependent on how much of the 1 light second radius sphere around the point of emission is actually covered by my reflecting objects?

10 hours ago, geordief said:

an expanding sphere of light in the shape of an individual photon

I'm not sure you can really describe a single photon as an expanding sphere of light. That implies that light could be detected/measured everywhere on that sphere.

It may be best to think of the sphere as defining the maximum probability of finding a photon.

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11 hours ago, geordief said:

an expanding sphere of light in the shape of an individual photon

As Strange points out:

1 hour ago, Strange said:

I'm not sure you can really describe a single photon as an expanding sphere of light.

His intuition that you can't represent the individual photon is totally right. You can't.

It's not that the individual photon is well represented by an s-wave or spherical front wave of light. It's more that the probability amplitude has a spherical shape, so that what you say,

11 hours ago, geordief said:

Can I expect to see the photon reflected back after 2 light seconds from an entirely random region  with a probability of this occurring dependent on how much of the 1 light second radius sphere around the point of emission is actually covered by my reflecting objects?

Is a correct intuition too, AFAICT. Every time you send a photon outside, you get a photon back from an entirely random (equally likely) direction after 2 light seconds, in such a way that every direction for the photon getting back to you is equally likely.

The first part of this problem (the photons going out but ending their trip being absorbed in a spherical photographic film) got Einstein seriously worried. It's one of the lesser-known arguments from A. Einstein, so I'm having some difficulty finding it on the web. Maybe it didn't involve photons, but electrons instead. I'll keep looking for it.

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If the photon isn’t observed and the emission is in a random direction, there is a probability sphere that’s isotropic. It’s not going to be detectable everywhere, but is detectable anywhere

Intuition seems right

editxpost with joigus

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@geordief Have you seen the videos of the lectures that Feynman did on Quantum Electrodynamics for a lay audience. (There is also a book.) They are detailed enough to give a good insight into what is going on, even if you don't have the theoretical/mathematical background.

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Suppose we take one of the elements of the reflecting array and (like the squaddies in M*ash,but inversely**)  we displace it radially ,so that now it is either further or nearer to to the point of emission than all the other elements.

How is the probability of the Sub Galactic Lighthouse Keeper (that's a Bonzo Dog reference btw?* )seeing it is the reflector over the others?

**isn't there a scene when they ask for a "volunteer" and all the others step back?

30 minutes ago, Strange said:

@geordief Have you seen the videos of the lectures that Feynman did on Quantum Electrodynamics for a lay audience. (There is also a book.) They are detailed enough to give a good insight into what is going on, even if you don't have the theoretical/mathematical background.

No, but I could  have a look ,but the  picture and sound quality of those videos is annoying for me

Edited by geordief

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As long as the solid angle is the same, I expect no change

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10 minutes ago, geordief said:

that,s a Bonzo Dog reference btw?

I'm annoyed I missed that.

7 minutes ago, swansont said:

As long as the solid angle is the same, I expect no change

It wouldn't be though, would it? If a mirror stays the same size but moves closer/further away, then the chance of it being hit changes, proportionally.

Actually, on further thought. If one reflector moves closer, then there is a probability that you will detect the photons sooner (probability proportional to the relative area (solid angle) of that reflector. If it moves further away then, similarly, there is a probability of detecting the photon later. Or not detecting it at all, because there will now be a gap where it can escape completely.

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34 minutes ago, swansont said:

As long as the solid angle is the same, I expect no change

So even if the element  is displaced extremely close to the point of emission or recedes by,say a distance of 10 light seconds ,the observer notes the same probability  of observation  but with a different time?

Going backwards over my train of thought(before I posted) can the  (single) photon in this scenario have  a different energy level depending on the experimental setup?

Can it have a frequency ,even though there is just one of them -or does the notion of a  frequency just come into play when there is a stream of photons?

Edited by geordief

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1 hour ago, Strange said:

It wouldn't be though, would it? If a mirror stays the same size but moves closer/further away, then the chance of it being hit changes, proportionally.

If the mirror is the same size and you move it radially, the solid angle changes.

58 minutes ago, geordief said:

So even if the element  is displaced extremely close to the point of emission or recedes by,say a distance of 10 light seconds ,the observer notes the same probability  of observation  but with a different time?

If nothing else is affected, yes

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8 hours ago, geordief said:

Going backwards over my train of thought(before I posted) can the  (single) photon in this scenario have  a different energy level depending on the experimental setup?

Can it have a frequency ,even though there is just one of them -or does the notion of a  frequency just come into play when there is a stream of photons?

An extremely small amount of  internet research has given me my answer .

The photon can have any frequency and corresponding wavelength

I think the frequency and wavelength are inversely related  via the value of c.

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10 hours ago, geordief said:

Going backwards over my train of thought(before I posted) can the  (single) photon in this scenario have  a different energy level depending on the experimental setup?

Can it have a frequency ,even though there is just one of them -or does the notion of a  frequency just come into play when there is a stream of photons?

It's a good question.

I think (but I'm not sure) that the frequency or wavelength for a single photon is not something you can measure directly. You can measure the energy and hence deduce the equivalent frequency (or wavelength). If that is correct, then you could say that a single photon doesn't really have a frequency.

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A single photon won’t have a single frequency; that requires a plane wave. You will have some distribution that can be narrow, depending on the source.

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