How can light be detectable everywhere?

[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 traveling 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?

 

[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

 

[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.

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

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.

 

[Mario Lanza]

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

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?

 

[Mario Lanza]

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.

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 realized that the Hubble telescope has recorded photons that were generated before our Sun even existed.

 

[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.

 

[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 $50,000\, {\rm photons}/({\rm mm}^2{\rm s})\cdot 30\,{\rm mm}^2\cdot 10^{-0.4/2.5}\approx 1,000,000\, {\rm photons}/{\rm s}$

 

[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.

 

[russ_watters]

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.

 

[Mario Lanza]

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.

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 realize that was a mistake.

I'll come back with further questions.

 

[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.

 

[russ_watters]

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.

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

 

[Mario Lanza]

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.

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.

 

[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...)

 

[Mario Lanza]

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...)

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!

 

[ucsdhopeful]

Can we see light not directed towards us?

 

[russ_watters]

No...did you read the thread?

 

[juanrga]

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 traveling 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?

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.

 

[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!

 

[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.

 

[xts]

Diffraction, interference, etc. are real phenomena. I never denied that!

I am just appealing to common sense: diffraction is not visible in such contextes, as OP asked about: you don't see the Arago's spot while watching solar eclipse...

 

[juanrga]

@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!

As may be explained in any QED treatment photons are not localizable. I.e., there is not position eigenstates |r> for them, therefore there is not classical description of photons as «tiny iron balls, moving in straight lines» because their position r(t) does not exist at any time t.

Most of traditional photonic astronomy is based in classical optics {*}, but the application of quantum optics (aka QED) to astronomy is a modern field of research.

{*} Neither in classical optics, photons are modeled as «tiny iron balls, moving in straight lines»; in rigor, they are not modeled at all.

Precisely the fact that classical optics could not explain modern experiments was the reason for the introduction of the concept of photon and this was the starting point of quantum theories.

 

[sophiecentaur]

Diffraction, interference, etc. are real phenomena. I never denied that!

I am just appealing to common sense: diffraction is not visible in such contextes, as OP asked about: you don't see the Arago's spot while watching solar eclipse...

Diffraction is seen in all contexts aamof. Your single photon scenario is an exceptional condition and needs to be treated with "common sense". This means not making simple classical assumptions about its behaviour.

 

[russ_watters]

xts is right here, guys - you're getting waaay too far down into the weeds for the level of the question in the OP. There is no need to confuse the OP with such details when the OP's question is easily (and, IMO better) explained using a bullet-from-a-gun analogy. All the OP wanted to know is if a single photon can hit more than one detector.

 

[Phrak]

...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.

Yeah. Like you say. However, one quanta of electromagnetic energy will be detected on only one side of the sun. There is only one 'photon', to use your words. So how can the absorbtion of this photon manifest as non-absorbtion 20 billion light years on the other side of the sun? What caused this photon to go one way rather than the other? Or, to be less presumptive, how is it that the energy was detected on one side of the sun rather than the other?

This is the quantum mechanical enigma. Quatum mechanical theory is an acausal theory with no better explanation for this acausality but to resort to comparison to a classical roulette wheel, as the mathematics fails to describe it. The mathematics defers to 'random variables' which are not mathematical abstractions at all, but classical objects dependent on the expectations of the outcome of experiments in classical mechanics due to incomplete information. It's foundations consist of both classical mechanics and subjective ignorance. Go figure. Quanutm mechanical theory is self contradictory nonsense in this regard that works amazingly well--after each adustment to conform to experimental results.

 

[sophiecentaur]

xts is right here, guys - you're getting waaay too far down into the weeds for the level of the question in the OP. There is no need to confuse the OP with such details when the OP's question is easily (and, IMO better) using a bullet-from-a-gun analogy. All the OP wanted to know is if a single photon can hit more than one detector.

But we ARE "down in the weeds" whenever we talk of these things. There is no straightforward answer.
To address your last sentence, one photon can only interact with one 'detector'. But that detector could be absolutely anywhere in space. However, if you shine your beam through a hole, the chances are that the photon will interact with a detector that happens to be lined up with the hole. The 'bullet from a gun' analogy works often enough in practice (because of the statistics involved) but there are many many common instances in which it doesn't so you just can't make assumptions. Bullets from guns doesn't easily explain why the common or garden CD has those iridescent colours or why your TV aerial works the way it does (not exactly rare experiences).
Newton found that the corpuscular theory of light was not good enough and no one, since, (of any Scientific Weight) has actually gone back to it. This is despite the popular vision of photons as little bullets.
To my mind, the corpuscular theory is / was very easy to grasp, with no scientific knowledge at all. The wave theory is much much harder to appreciate and, given half a chance, people have reverted to the pre-Newtonian ideas and not actually progressed to what QM actually has to say about the nature of photons.

Interestingly and slightly apropos: I heard Brian Cox on the Radio this morning. He said that everyone should be taught about Quantum Theory in School and that it was basically easy. I have a feeling that he may have found it all a lot easier than your average School pupil. He might be surprised to find how few people reach the Formal Operational stage of cognitive development. This QM is seriously difficult stuff and can easily be confused with intellectual and academic anarchy.

 

[sophiecentaur]

Yeah. Like you say. However, one quanta of electromagnetic energy will be detected on only one side of the sun. There is only one 'photon', to use your words. So how can the absorbtion of this photon manifest as non-absorbtion 20 billion light years on the other side of the sun? What caused this photon to go one way rather than the other? Or, to be less presumptive, how is it that the energy was detected on one side of the sun rather than the other?

This is the quantum mechanical enigma. Quatum mechanical theory is an acausal theory with no better explanation for this acausality but to resort to comparison to a classical roulette wheel, as the mathematics fails to describe it. The mathematics defers to 'random variables' which are not mathematical abstractions at all, but classical objects dependent on the expectations of the outcome of experiments in classical mechanics due to incomplete information. It's foundations consist of both classical mechanics and subjective ignorance. Go figure. Quanutm mechanical theory is self contradictory nonsense in this regard that works amazingly well--after each adustment to conform to experimental results.

You write as if there is a better alternative available at the moment. In any case, what's wrong with acausality?

 

[Phrak]

You write as if there is a better alternative available at the moment. In any case, what's wrong with acausality?

Describde acausality. This is not an easy task!

 

[sophiecentaur]

Describde acausality.

You introduced the word acausal first! ! I was just re-using it. haha

But 'if you know of a better hole to go to . . . . . . .'

 

[Phrak]

You introduced the word acausal first! ! I was just re-using it. haha

But 'if you know of a better hole to go to . . . . . . .'

A fair question. In the usual sense of the word:--

Pick a small patch of space at time t1 in any inertial frame. There was another patch of space in the past at t0, all within the past light cone of t1. Sorry for the clumsy language. For the state of the system at t1 to be acausal with repect to the region at t0 would mean that it would not be possible to calculate the state of t1 given the state of t0.

 

[sophiecentaur]

The fact that familiar processes appear to exhibit causality could just make it a difficult thing to do without. But causality may just be a 'local' thing. Newton's laws and all the others are very handy for most of our lives but that could be as far as it goes..

 

[juanrga]

All the OP wanted to know is if a single photon can hit more than one detector.

From https://www.physicsforums.com/showpost.php?p=3574503&postcount=18

If that photon is absorbed by a detector at Earth, then it was not absorbed by the a spacecraft detector and vice verse.