The Quintessential Quantum Question

[Jeff Root]

I think this may be the quintessential question about quantum theory.

Why can a photon be detected only after the entire photon has
interacted with the detector?

That appears to be true whether the "detector" is a long antenna
wire or a single electron.

-- Jeff, in Minneapolis

[John Jaksich]

This my attempt--but may sound overly simplistic or naive.

It is a result of Heisenberg's uncertainty principle.

[Grey]

I think this may be the quintessential question about quantum theory.

Why can a photon be detected only after the entire photon has
interacted with the detector?

That appears to be true whether the "detector" is a long antenna
wire or a single electron.

I think this has the same problem as any "why" question in physics. We don't know why the universe is the way it is, we just know that it is that way. Electromagnetic interactions are quantized, that's just the way the world works. So you can't interact with half a photon. Any interaction that takes place is an interaction with a whole photon. And even in the case of an antenna picking up a radio signal, I'm pretty sure that if you look at the interaction on a quantum level it would still be a series of individual radio photons interacting with individual electrons in the wire.

[John Jaksich]

I think this has the same problem as any "why" question in physics. We don't know why the universe is the way it is, we just know that it is that way. Electromagnetic interactions are quantized, that's just the way the world works. So you can't interact with half a photon. Any interaction that takes place is an interaction with a whole photon. And even in the case of an antenna picking up a radio signal, I'm pretty sure that if you look at the interaction on a quantum level it would still be a series of individual radio photons interacting with individual electrons in the wire.

I certainly agree but if my memory serves me correctly---there is an interpretation of QM which speaks of how a photon will be detected in a two slit experiment. Probablity amplitudes (?) of a photon can be detected in one of two slits depending on whether one or the other slit is closed.

[Strange]

Why can a photon be detected only after the entire photon has
interacted with the detector?

Because that is what "quantized" means?

[HenrikOlsen]

I certainly agree but if my memory serves me correctly---there is an interpretation of QM which speaks of how a photon will be detected in a two slit experiment. Probablity amplitudes (?) of a photon can be detected in one of two slits depending on whether one or the other slit is closed.

That's the attempt at figuring out which slit the photon goes through. It fails because those photons going through when one of the slits is closed act according to the probability distribution from the much smaller interference effect from the edges of the slit (nearly the same as if they were particles) and only those going through when both slits are only adds to the interference pattern of the main probability distribution.

The effect of blinking one of the slits is a pattern which is two (or three if both slits blink) different property distributions overlaid, with the relative intensity of the patterns proportional to the relative amount of time of each slit configuration.

[HenrikOlsen]

Why can a photon be detected only after the entire photon has
interacted with the detector?

Because that's what they do.

Sorry, that's as close to an answer you'll get to a "why" question.

This my attempt--but may sound overly simplistic or naive.

It is a result of Heisenberg's uncertainty principle.

Horse before carriage, Heisenberg's uncertainty principle is a consequence of quantized interactions, not the other way around.
First the physical world, then humans models for it. Not the other way around.

[EigenState]

Greetings,

Because that is what "quantized" means?

Better to say that is what it means to be a quantum.

Best regards,
EigenState

[John Jaksich]

Because that's what they do.

Sorry, that's as close to an answer you'll get to a "why" question.


Horse before carriage, Heisenberg's uncertainty principle is a consequence of quantized interactions, not the other way around.
First the physical world, then humans models for it. Not the other way around.

Thanks for the clarificatiions

--john

[Jeff Root]

Why can a photon be detected only after the entire photon
has interacted with the detector?

I think this has the same problem as any "why" question in
physics. We don't know why the universe is the way it is, we
just know that it is that way.

Why does smoke rise?
Why do we need to eat?
Why does rubber stretch?
Why do lightbulbs burn out?
Why are candle flames yellow?
Why does lightning make thunder?
Why is iron stronger than aluminum?
Why is the Sun brighter than the Moon?
Why is ocean water saltier than lake water?
Why does the Sun's spectrum have dark lines in it?
Why does a magnifying glass make things look bigger?

Perhaps my question can only be answered, "That's just
the way it is", but if so, it is absolutely, definitely, certainly
NOT because it contains the word "why"!

And even in the case of an antenna picking up a radio signal,
I'm pretty sure that if you look at the interaction on a quantum
level it would still be a series of individual radio photons
interacting with individual electrons in the wire.

That's a very interesting notion! I'll have to research it!

-- Jeff, in Minneapolis

[Jeff Root]

Why can a photon be detected only after the entire photon
has interacted with the detector?

Because that is what "quantized" means?

Interesting.

A photon can only be detected after the entire photon has
interacted with the detector because "quantized" means
"a photon can only be detected after the entire photon has
interacted with the detector."

That isn't really what you meant to say, is it?

Why can the cart only be behind the horse?

Because that is what "harnessed" means.

Hmmm. Doesn't quite work for me.

-- Jeff, in Minneapolis

[korjik]

Whether it works for you or not is utterly irrelevant.

You cannot detect half a photon. You cannot detect any part of a photon. You can only detect whole photons.

Even if you dont like it.

[Ken G]

A photon can only be detected after the entire photon has
interacted with the detector because "quantized" means
"a photon can only be detected after the entire photon has
interacted with the detector."

Your objection reminds me of people who object to "survival of the fittest" on the grounds that "fittest" is defined as "those that survive." What they miss is that "survival of the fittest" is not a statement about what survives, it is a statement about what matters. Evolution happens because some survive and some don't. Photons interact the way they do because they are quantized. So yes, this is the quintessential question behind quantum mechanics, but it's answer is that this is why we have quantum mechanics. If you ask, why do things fall, and someone says "because of gravity", you can then ask "but isn't gravity essentially the observation that things fall?" Yes, it is, and that's how we do physics. We observe that things fall, so we have gravity, and we observe that the whole photon has to interact, so we have quantum mechanics. So you are not asking a question about quantum mechanics, you are asking what quantum mechanics is.

[Jeff Root]

korjik,

I didn't say or imply that I don't like the fact that we can only
detect whole photons. If you think I did, you misunderstood
what I said.

What I said was that saying photons are quantized becaused
"quantized" means "quantized" doesn't tell me anything I didn't
already know. Just like saying that "harnessed" means the
horse is in front of the cart doesn't explain why the horse needs
to be in front of the cart. Strange's suggestion/comment/question
was pointless. Not wrong, but pointless.

So was your comment. My original post includes the assertion
that we can only detect whole photons. All you did was repeat
what I had already asserted.

-- Jeff, in Minneapolis

[grapes]

I may be just shooting in the dark, here, but I'd like to help Jeff answer his questioning. What about that detector? Even in a one slit experiment, with a detector, how does the detector affect the experiment? How does the detector (which kind of detector?) keep from affecting the experiment? What precautions have been taken, what advances over the years have been made in detection technology?

[EigenState]

Greetings,

I may be just shooting in the dark, here, but I'd like to help Jeff answer his questioning. What about that detector? Even in a one slit experiment, with a detector, how does the detector affect the experiment? How does the detector (which kind of detector?) keep from affecting the experiment? What precautions have been taken, what advances over the years have been made in detection technology?

I believe the insertion of interference into the discussion is a point of confusion. The original question posed was:

Why can a photon be detected only after the entire photon has
interacted with the detector?

That really has nothing to do with interference phenomena--the slits in such experiments are not detectors. It has everything to do with the fact that the photon is a single, indivisible quantum of energy which by definition cannot be partially used. Photon interactions with matter are an all or nothing event. Even in the case of scattering, be it elastic or inelastic, the incident photon is annihilated and the scattered photon is created.

Regarding interference phenomena, the nature of the detector is not relevant. Nor does the relative spatial positions of the detectors matter. All that matters is if one attempts to determine which slit the photon passed through. Do that, by any conceivable approach, and the interference pattern is lost.

Best regards,
EigenState

[Strange]

That isn't really what you meant to say, is it?

Pretty much (although EigenState has said it better). A photon is a quantum; which means it is indivisible; which means it all interracts in one go.

A better horse and cart analogy might be: why do we have to have a whole number of horses pulling the cart, why can't we have 2.3 horses?

reading it again, the way you phrase the question ("after the entire photon has interacted") it sounds a bit like you imagine that the photon starts interacting, more and more of it interacts, eventually it has all interacted and then we see the result. Which isn't how it works (as Grey says).

[profloater]

Why does smoke rise?
Why do we need to eat?
Why does rubber stretch?
Why do lightbulbs burn out?
Why are candle flames yellow?
Why does lightning make thunder?
Why is iron stronger than aluminum?
Why is the Sun brighter than the Moon?
Why is ocean water saltier than lake water?
Why does the Sun's spectrum have dark lines in it?
Why does a magnifying glass make things look bigger?

Perhaps my question can only be answered, "That's just
the way it is", but if so, it is absolutely, definitely, certainly
NOT because it contains the word "why"!


That's a very interesting notion! I'll have to research it!

-- Jeff, in Minneapolis

Aha there is more than one "why", Aristotle was onto that and he had only a smattering of English. Why did the stick hit me over the head? It was his mother's idea.

[Strange]

Perhaps my question can only be answered, "That's just
the way it is", but if so, it is absolutely, definitely, certainly
NOT because it contains the word "why"!

True. But the answers to any of these can be followed by another "why" (or "how") until it bottoms out and all you can say is either "that's the way it is" or "we don't know".

[Jeff Root]

That isn't really what you meant to say, is it?

Pretty much (although EigenState has said it better). A photon
is a quantum; which means it is indivisible; which means it all
interracts in one go.

Which is completely useless as a reply to my question.

Q: Why is a saw-edged blade used to cut bread?
A: Because that is what "bread knife" means.

Q: Why can't we see light with wavelengths longer than red?
A: Because that is what "infrared" means.

Q: Why can a photon be detected only after the entire photon
has interacted with the detector?
A: Because that is what "quantized" means.

A better horse and cart analogy might be: why do we have to
have a whole number of horses pulling the cart, why can't we
have 2.3 horses?

That's an analogy to my question. My horse and cart analogy
was an analogy to your response to my question, like those
above.

Q: Why do we have to have a whole number of horses pulling
the cart, why can't we have 2.3 horses?
Q: Because that is what "horse" means.

No useful information is conveyed by such a reply.

reading it again, the way you phrase the question ("after the
entire photon has interacted") it sounds a bit like you imagine
that the photon starts interacting, more and more of it interacts,
eventually it has all interacted and then we see the result.

I make no assumption about that. My question allows for all
possibilities. If one could not allow the possibility that interaction
might take some time, then the question would not be askable.
I asked the question to learn the answer, not to present a logical
paradox.

Which isn't how it works (as Grey says).

Is there actually evidence that photon-charge interactions are
instantaneous? I don't think there is. On the contrary, I'm pretty
sure it takes time for an electric charge to emit or absorb a photon,
and the time can be measured statistically, like the wavelength of
monochromatic light can be measured statistically.

-- Jeff, in Minneapolis

[Grey]

Jeff, nobody knows the answer. Nobody is sure why certain things in the universe are quantized. Planck introduced the quantum hypothesis pretty much as a mathematical trick to fit the blackbody radiation curve. From what I've read, he was planning on doing the standard calculus-style trick of separating something into discrete segments, and then letting the size parameter go to zero so that it's back to a continuum. It was something of a surprise to find out that it only fit the observations when the size parameter was a specific value. Since then, we've found Planck's constant cropping up in pretty much every small-scale phenomenon that we've investigated. It is a mystery why that's the case, or why that constant should have the specific value it does. When particles interact via the electromagnetic force, those interactions occur in little quantized bits, and nobody really know why.

I'm sorry that you seem really unhappy with that state of affairs, but that's the way of it. We also don't know why electric charge comes in little discrete bits of a certain size, or why the number of baryons is conserved in particle interactions. There's a whole bunch of stuff that we just don't know. Maybe someday we'll replace quantum theory with something else, and we'll be able to "explain" why electromagnetic energy is quantized. But it's a pretty sure bet that even if we do, there will be features of that new theory that aren't explainable, that we just have to accept as part of how the theory works. Until then, all anyone can do is keep repeating, "we don't really know".

[Strange]

Which is completely useless as a reply to my question.

I guess I don't really understand what it is that you are asking then (this happens to me a lot!).

Are you actually asking "why are photons quantized?" ?

[Jeff Root]

Jeff, nobody knows the answer. Nobody is sure why certain things
in the universe are quantized. ...

That reminds me of the joke about the census taker explaining to a
person being canvassed that they are trying to find out how many
people live in the US. The person replies, "Why ask me? I have
no idea!"

I'm sorry that you seem really unhappy with that state of affairs,

Even if you can't explain why photons are quantized, maybe you
can explain to me why I seem unhappy with that state of affairs.
What did I say that gave you that impression? Is it just from
something I said in this thread? Is it based on something I said
elsewhere? Or what? Please be as specific as possible. I have
no idea what gave you the impression that I'm in any way unhappy
about photons or anything else being quantized.

-- Jeff, in Minneapolis

[Grey]

Even if you can't explain why photons are quantized, maybe you
can explain to me why I seem unhappy with that state of affairs.
What did I say that gave you that impression? Is it just from
something I said in this thread? Is it based on something I said
elsewhere? Or what? Please be as specific as possible. I have
no idea what gave you the impression that I'm in any way unhappy
about photons or anything else being quantized.

It's not that you seem unhappy that photons are quantized. It's that you seem unhappy that nobody can give you a good answer for why that's the case. It seems like everyone in this thread has given some variant on the only answer we have: "we don't know, that's just the way things seem to be". And you keep responding with things like "[that] doesn't tell me anything I didn't already know" or that such a response is "completely useless as a reply to my question".

We know that the electromagnetic force is quantized, and we call those quantized bits photons. We have no idea why the world should work that way, and we have no suggestion of an underlying mechanism that should make it that way. So we can't provide any better explanation than that. If you really already know all that, why are you asking the question, and then getting annoyed at people when they give you the only answer we can give? It's admittedly not much of answer, but there's no better one available, and there won't be a better one unless and until we replace quantum theory with something else.

[Jeff Root]

I guess I don't really understand what it is that you are asking then
(this happens to me a lot!).

Are you actually asking "why are photons quantized?" ?

Yes.

I asked it in the same way that I imagine someone who knows
nothing about quantum theory would ask. Say, any physicist in
1905. Asking it that way is more general than asking why photons
are quantized, because the latter assumes that we all agree on
exactly what "quantized" means. I was asking for an explanation
of the phenomenon, not a name for it.

The basic fact, as I understand it, is that an observer doesn't
know that a photon is being or has been emitted or absorbed
until it has been completely emitted or absorbed. I'm looking
for a description / explanation of why it works that way rather
than some other way. Maybe if you consider what I'm asking
for to be a description instead of an explanation, it will seem
more tractable.

-- Jeff, in Minneapolis

[Jeff Root]

Grey,

Thank you.

My complaint about your first reply wasn't that you couldn't
supply an answer-- it was that you pinned the problem on my
question being a "why" question. My question may well be
unanswerable with current knowledge-- and might even be
unanswerable with any knowledge-- but it isn't because the
question is asked with a "why".

My complaint about Strange's reply was that it was completely
circular. It didn't say anything that wasn't in the original post.

Maybe you guys were just trying to say "We don't know" in
a way that sounds more interesting and helpful than that.

-- Jeff, in Minneapolis

[EigenState]

Greetings,

Perhaps this might help some. Again, it is much better to state that a photon is a quantum rather than it is quantized. The terms are really not interchangeable. Being a quantum, it is the smallest known packet of electromagnetic energy. It is the fundamental unit of electromagnetic energy. It is therefore indivisible. That is our definition of a photon. Based upon that definition, one cannot postulate about a photon not interacting as a unit--it is axiomatically all or nothing.

As far as stimulated or spontaneous transitions, the absorption or emission of the photon is instantaneous. The easier case to imagine is spontaneous emission. The lifetime of the excited state is certainly finite. But when it does radiate, the creation of the emitted photon is indeed instantaneous.

I cannot answer the question why those are the observed and theoretical results. But they are.

Best regards,
EigenState

[HenrikOlsen]

Which is completely useless as a reply to my question.
Q: Why can a photon be detected only after the entire photon
has interacted with the detector?
A: Because that is what "quantized" means.

A: It's what they do, we don't know why.

Would you have preferred that?

[Grey]

My complaint about your first reply wasn't that you couldn't
supply an answer-- it was that you pinned the problem on my
question being a "why" question. My question may well be
unanswerable with current knowledge-- and might even be
unanswerable with any knowledge-- but it isn't because the
question is asked with a "why".

Perhaps. But anytime someone answers a "why" question in science, you can ask another "why" about the answer to that one, and eventually you get to a point where you just have to accept that the universe seems to work that way. So there are always a huge number of unanswered "why questions: Why was there a big bang? Why is there gravity? Why are there three kinds of quark color, two kinds of electrical charge, and only one kind of mass? And so forth. I felt pretty sure (from interacting in other threads) that you knew enough about the situation to be aware that your question was more like "why was there a big bang?" than like "why does smoke rise?". I apologize if I made a false assumption.

[korjik]

Grey,

Thank you.

My complaint about your first reply wasn't that you couldn't
supply an answer-- it was that you pinned the problem on my
question being a "why" question. My question may well be
unanswerable with current knowledge-- and might even be
unanswerable with any knowledge-- but it isn't because the
question is asked with a "why".

My complaint about Strange's reply was that it was completely
circular. It didn't say anything that wasn't in the original post.

Maybe you guys were just trying to say "We don't know" in
a way that sounds more interesting and helpful than that.

-- Jeff, in Minneapolis

Actually, it did supply your answer. You cannot detect a photon before it finishes interacting with the detector because before it has finished interacting, nothing has happened. You cannot be half-interacted because you cannot split a photon in half.