# How can light be detectable everywhere?

1. Oct 21, 2011

### Mario Lanza

Hi,

I applogise for the vague wording in the title but this is something that I've been thinking about for a while and I cannot get my head around it. The recent news item about Neutrinos possibly travelling faster than the speed of light got me thinking about this again.

Suppose the Sun is replaced by a single atom and it generates a photon of light for whatever reason.

In 8 minutes or so, a super sensitive detector would be able to detect that single photon on Earth. However, if there was a spacecraft at the other side of "the Sun" it will also detect that single photon. In fact, a detector at that same distance in any direction from "the Sun" would be able to detect that same photon.

In this thought experiment we could extend this time to 10 billion years. In 10 billions years any super super sensitive detector could detect that same photon from any position relative to the original source of the photon. This 10 billion light year radius sphere would be massive indeed. I cannot even imagine how big the surface area would be.

How can a photon be detected in any direction 10 billion years after it was emitted?

What exactly is a photon?

2. Oct 21, 2011

### dacruick

If the premises of your question are correct (which I don't know if they are), then you have an awesome question and I want to see what someone who knows what their talking about says:P

3. Oct 21, 2011

### xts

That is pure statistics.

You may imagine the Sun as a mad cowboy shooting in random directions. If he shot just one bullet - probability the bullet hits Earth is quite small. But if it hit Earth - that means it didn't hit anything else.

And how a Colt bullet can be detected 1/10 s after it had been shot?

For your question - the purely deterministic "Colt bullet" model (which is a simplification - photons exhibit some behaviour, which may not be described in that model!) - is pretty sufficient to answer.

4. Oct 21, 2011

### Mario Lanza

I hope the premise of my question is correct.

Just last night I was out looking at Orion, my favourite constellation.

Whenever I am looking at the stars I wonder how they can be seen by any observer in the Universe, no matter which direction they observe those same stars.

Also, the instrument can be extremely small. In theory at the atomic level.

So, how does the photon do it?

5. Oct 21, 2011

### Mario Lanza

I half get what you are saying and I appreciate that a simplification may mean that I don't fully understand what you are saying.

However, if a star, in a galaxy, is replaced by a single atom which releases a single photon of light then every other object (star, planet, moon, asteroid etc) in that galaxy can detect that photon at differnet times based on distance from the source.

In your analogy, the colt revolver would have to shoot hundreds of billions of bullets just for the local galaxy.

As photons go outside of the local galaxy to distant galaxies it would have to shoot trillions of bullets to potentially hit all possible targets. (Some of which, may not have existed when the photon was created.)

Some of which, may not have existed when the photon was created

In fact, I've just realised that the Hubble telescope has recorded photons that were generated before our Sun even existed.

Last edited: Oct 21, 2011
6. Oct 21, 2011

### dacruick

I think that was xts is saying is that if your "star" emitted only 1 photon, then only sensors along the line of trajectory would see it.

7. Oct 21, 2011

### xts

Stars emit bilions of bilions of bilions of photons... Our Sun emits about 5 1045 photons/s in visual range.

Instruments are not of 'atomic scale'. You watch stars with your eye, its pupil has several milimeters of diameter. You sometimes watch stars with telescopes of 1m of diameter.

The flux from a star of apparent magnitude m=0 is about 50,000 photons/(mm2s) in visual range, while aperture of your eye is above 30mm2. The apparent magintude of Betelgeuse (α-Orionis) is 0.4 - which means that you see $50,000\, {\rm photons}/({\rm mm}^2{\rm s})\cdot 30\,{\rm mm}^2\cdot 10^{-0.4/2.5}\approx 1,000,000\, {\rm photons}/{\rm s}$

Last edited: Oct 21, 2011
8. Oct 21, 2011

### DaveC426913

Yes. What leads you to think that one photon emitted in one direction would be visible everywhere?

If you're thinking of wave particle duality, and thinking of that one photon as equivalent to a radially emanating wave, then the one photon is tantamount to a light source radiating EM waves that are too dim to be detected.

9. Oct 21, 2011

### Staff: Mentor

Even worse: when a photon is detected, it is absorbed and ceases to exist. It cannot be detected again later.

A photon is the smallest possible packet of light.

10. Oct 21, 2011

### Mario Lanza

Thank you. A light bulb has come on!

I have fallen into the trap of using incorrect terminology.

I am not one of those dreaded trolls who just try to provoke an argument for the sake of it.

I really would like a greater understaning of something that has puzzled me for years.

I tried to simplify "the Sun" to a single atomic event. That is why I talked about a single photon. I now realise that was a mistake.

I'll come back with further questions.

11. Oct 21, 2011

### xts

No one accuses you for trolling!

You are just learning - in this case the lesson is: our intuitions fail when we apply them to really large numbers. It seems that the number of photos emitted by the Sun may be big, but if watched from many light-year distance it should be small - more! it should be zero! The intuition is wrong...

Take a calculation. Our Sun (pretty small humble star in a dimmestt corner of Galaxy) emits 3.8 1026W energy. Most of it comes in visual region of light, while typical visual photon has an energy of 1.5eV = 10-19J. It makes that our humble Sun emits about 4 1045 photons per second.

Now compute how many of those photons could hit your (humbliest!) eye pupil if you watch it from 10 light-years distance. The photons are evenly distributed over a 10-ly sphere: $4\pi(10\cdot365\cdot24\cdot3600\cdot300,000,000)^2\approx10^{35}{\rm m}^2$ Your pupil has 30mm2=3 10-5 m2. So if you look at our Sun from 10 light-years distance, each of your eyes would still catch $\frac{4\cdot10^{45} {\rm photon}/{\rm s}}{10^{35}{\rm m}^2}3\cdot10^{-5}\,{\rm m^2} \approx 10^6\,{\rm photon}/{\rm s}$

That is much more than required: eyes of healthy man may see about 50 photons/s as a dimmest spotable light.

Last edited: Oct 21, 2011
12. Oct 21, 2011

### Staff: Mentor

Just to make sure we're clear: only one of those sensors will detect the photon.

13. Oct 21, 2011

### Mario Lanza

Thank you very much indeed for your clear explanation.

That is the sort of information I like rather than Cowboys shooting Colt 45s!

To me, that instantly explains the magnitude of stars, which previously, I knew was obvious, but didn't fully appreciate why.

If I undestand correctly, photons are created but in random directions (hence your mention of statistics earlier). Now I understansd that, many things seem clear.

Last edited: Oct 21, 2011
14. Oct 21, 2011

### xts

No, I won't tell you about duopoly of waves/particles - as it is not related at all to the angular distribution of light emitted by stars and enormous number of photons (which is much bigger than revolver barrels capacity...)

15. Oct 21, 2011

### Mario Lanza

Thank you xts.

EDIT: xts replied to a comment in my original post that I deleted.

I tried to explain that my confusion related to the wave/particle duopoly of light. As even I wasn't convinced I deleted it!

Last edited: Oct 21, 2011
16. Oct 23, 2011

### ucsdhopeful

Can we see light not directed towards us?

17. Oct 23, 2011

### Staff: Mentor

18. Oct 23, 2011

### juanrga

A photon is not a tiny iron ball moving in a straight line in a given direction. It is a quantum particle described rather well by QED. Photons in QED do not have position wavefunctions. They have other properties as momentum and using QED you can compute the probability some emitted photon is absorbed by some matter. If that photon is absorbed by a detector at Earth, then it was not absorbed by the a spacecraft detector and vice verse.

Also I do not understand your query about surface area. In QED particle quantum states are normalized in a box of infinite size, which means that area and volume are infinite as well.

19. Oct 23, 2011

### xts

@juanrga:
in astronomy (as in many other applications, like photography or just human vision) photons are tiny iron balls, moving in straight lines.

Would you propose any experiment showing any difference between QED predictions and 'tiny iron ball' model, in regard to solar photons observed either on Earth or on Moon?
We are not talking about Sir Thomas Young on Earth and his ghost on Moon, performing their double slit experiments at both locations: OP was worried about interference/quantum/duality/however-you-like-to-call-them effects you may observe watching solar light with one detector on Moon and other on Earth. Propose any!

20. Oct 23, 2011

### sophiecentaur

Young's Slits experiment shows that light is not like little bullets. If you give it two slits to go through, your single photon can be detected in all sorts of places and not just in line with one or other of the slits. If you have just one hole, that photon can turn up way off axis when it's 'gone through' that hole. Diffraction is a real effect and it can be seen to effect the statistics of even just a few photons.