Can we say a photon doesn't exist until the wavefunction collapses?

[Kavi]

I am sure this sounds silly but..

Between being emitted and interacting, a photon gives us no information as to where it is in space. If we know both where a photon was emitted from and where it then interacted, we can say that it travelled in a straight line. But we actually dont know.

So in a way, it seems that the photon doesnt affect the physical world in any way, until it interacts in a specific way, and that results in the wavefunction collapse. For all we know, a photon could travel across the whole universe in going from a point A to point B, which are just a few metres from each other. But this cannot happen because light has a speed, so we must conclude it travelled in a straight line. So we say it went from A to B, and surely it did because we can measure it took distance/c to get there.

Further, the collapse of the wavefunction seems related to information. In a deterministic universe based on classical physics, all wavefunction collapse would cause some change in the physical universe giving us information about the position (and time) of the photon collapse, if we have the technical abiltiy to discover that information, but the information is there. If a photon collapses a certain point it affects the universe in a unique way, and if it collapses in a different way it affects teh universe in a other way. If it is still a wave then we have no idea where it will collapse so we can say it has not affected the universe yet.

The delayed choice experiment seems to show that if we retroactively remove the information, or physical affect in the universe of the wavefunction collapse, then the photon shows an interference pattern, retroactivly ofcourse. Correct me if I am wrong about the delayed erasure experiment below..

1. Run a double slit experiment with detectors but dont look at the screen or detector data.
2. Look at the detector data and the screen data. Data shows photons passing through one screen at a time
3. Run the experiment again.
4. Delete the detector data after running the experiment, but before looking at tehe screen data. Now the screen shows an interference pattern when looked at.

So at point 4 we have removed from our universe the information about which path the photon took. This seems to retrospectively change the data on the screen, but before we looked at it.

But in both cases there is consistency. Lets say at step 4 after erasing the data the screen showed no interference pattern, that would be inconsistent as we there was nothing happening at either detection point, as the universe now has no record of any itneraction at those places.

It also seems to me that this may be similar to entaglement. Entaglement may exist between the screen and the detector data. When the detector data is erased, the screen data also stays consistent. This can possibly also happen faster than light, like entaglement, instantanously.

Also, a photon having left a distant star, could have gone in either direction, towards, or away from us. Its wave could theoretically be distributed over a vast space, but when it is absorbed into our eyes its collapses instantanously, including any part of the wave that at the moment prior to it hitting out eyes, could have been millions of light years away. We simply cannot know where the photon is until the moment it, in its entirety, is absorbed into our eyes, we only know where it can be, which is determined by c, how long ago it was emitted, and the possible paths away from the source it could have taken.

 

[PeterDonis]

@Kavi, there are a number of things that you appear to be misunderstanding about QM.

First, collapse of the wave function is not necessarily an actual physical process; that depends on which interpretation of QM you adopt. As far as the basic math of QM is concerned (I suggest reading the "7 Basic Rules" Insights article that you can reach from a sticky thread at the top of this forum), "collapse" is just an update we make in our math when we know the result of a measurement. So you have to be very careful in thinking about collapse.

Second, nothing in what you are saying is specific to photons. All quantum systems "give us no information" between being emitted (or more generally prepared) and interacting with something that allows us to measure or detect something about them. Whether and how such systems "affect the physical world" in between is another thing that depends on which interpretation of QM you adopt.

Third, experiments like delayed choice do not "retroactively remove information" from anywhere. But I would not recommend even trying to tackle experiments like that one until you have a solid grasp of more basic aspects of QM.

Fourth, entanglement cannot be used to transmit information faster than light. But this is another aspect that I would not recommend trying to tackle until you have a good grasp of the basics.

And finally, while quantum systems do not, in general, have well-defined paths through space when they are not being detected, that does not mean anything goes. Light arriving from stars billions of light years away cannot have waited until the last instant and then gotten to us instantaneously, and it cannot have zigzagged all around the universe prior to reaching us. There are many scenarios, of which this is one, where the behavior of quantum systems is actually quite close to ordinary classical behavior, even if it does not match it exactly. The fact that quantum systems behave highly non-classically in some cases does not mean they do so in all cases.

Note also that discussion of QM interpretations belongs in the interpretations subforum, not this one.

 

[PeroK]

@Kavi perhaps the first thing about QM you need to understand is that the wavefunction does not give the probability distribution of where a particle is at all times. It gives you the probability of where a particle will be detected, if you measure its position. That may sound like a subtle distinction, but it's of fundamental importance.

An electron in an atom, for example, does not have a well-defined classical trajectory around the nucleus. Instead, the atom is effectively defined by an energy state. This is a fundamentally different way of describing nature from classical physics.

 

[Kavi]

OK guys thanks for hearing me out. For the sake of discussion I will respond to the above posts.

@PeterDonis

"First, collapse of the wave function is not necessarily an actual physical process"

Lets take the converse process. An atom absorbs a photon and then it elevates an electron to a higher energy state. This is the electrons point of view of the photons wavefunction collapse. When the photon becomes a particle it is abosrbed by an atom (in general, though there can be also be other mechanisms) and then that energy which was the photon wave is now located within the physical confines of the absorbing atom.

The energy went from being outside the atom to inside the atom, so the speak. So surely, there is some 'mechanical' or systemic process by how this happens. An atom can be modelled as a oscillater, light as frequency, so we may imagine that light as a wave may be causing the atom to vibrate in some way, which then causes a transfer of energy from the external vibration representing the photon wave to the internal state of the atom. This is how the photon collapses from a wave to a particle. Hence i think we simply lack information about how this process happens. But this shouldnt be a purely mathematical construct, but some physical reality, whether we can observe it or not. This is related to the discrete nature of photon-matter interactions, as only certain frequency (energy) photons can be absorbed by a given atom.

So it seems there is a lot more we dont know. We dont know why a photon is only ever absorbed in its entirety, not say, half a photon absorbed by and atom and another half absorbed elsewhere. We also dont know how exactly energy in the form of a photon wave is converted into electron orbital energy.

These are still open questions. The interpretations and math of QM is glossing over these important questinos imo. Currently in QM, its like we are modelling a car, we know what energy goes in and that it can cause motion of the car, and exhaust, but we dont know about how the engine works, what are the mechanisms by which the fuel is converted into motion and exhaust, we just know the mathematics that tells us how fuel going in and motion and exhaust are related.

The second point I agree with.

The third point, yes maybe the wording is not exactly ideal.

"Light arriving from stars billions of light years away cannot have waited until the last instant and then gotten to us instantaneously"

But how do we know this? The speed of light is the time it takes for a photon energy to show some effect via interaction at some position B when emitted from position A. But the process of conversion from a wave to a particle seems to happen instantaneously, it never happens in part but only in whole. We never observe an atom in a state between energy states having only internalised part of the photon energy. So we must say at some point in time the photon is outside the atom, and then at the next point in time it is inside. This looks like an instaneous process. In the same way, we do not know (and it seems cannot know) or observe the photon travelling toward a specific point in space, we only know it went there when the wavefunction collapses to that point.

Again, looking at the example of two possible interaction points. A photon is emitted from point O, it can be absorbed at point A or point B, both equidistant from O, but a million light years apart. Lets say the photon is absorbed at A. The absorption will happen at time T1 which is distance/c. But at the moment (plank time) prior to T1, the photon cannot be said to be just infront of A. We dont know where it is, but perhaps we can say it is distributed over the region of radius c*(T1-1plank time). If the photon energy exists in space-time prior when its a wave, then some of that energy will also be close to B. But in the next time plank time interval that takes us to T1, which equals distance/c, it becomes localised into A. This means in the plank time interval prior to T1, any of the photon energy that was close to B, which is light years away from A, got localised into A by T1, so in a plank time unit, energy light years away from A became localised into A. But this doesnt mean information travelled from B to A, it was just information potential, but as you say no actual information travelled from B to A, but if we say the photon is physically distributed in space, then some of that photon certainly must have 'travelled' from near to B to A in the single plank time prior to T1.

 

[PeroK]

Lets take the converse process. An atom absorbs a photon and then it elevates an electron to a higher energy state. This is the electrons point of view of the photons wavefunction collapse. When the photon becomes a particle

A photon is a particle, by definition. It's never not a particle.

So surely, there is some 'mechanical' or systemic process by how this happens.

Not necessarily. In theoretical physics you are always going to hit the bedrock at the lowest level. The interaction between matter and the (quantized) electromagnetic field is modelled by QFT (Quantum Field Theory). That is the mechanism. There is no additional theory underpinning this.

An atom can be modelled as a oscillater, light as frequency, so we may imagine that light as a wave may be causing the atom to vibrate in some way, which then causes a transfer of energy from the external vibration representing the photon wave to the internal state of the atom.

QFT describes the process and broadly fits this description. Although, the mathematics is somewhat beyond the theory of a classical harmonic oscillator.

This is how the photon collapses from a wave to a particle.

The photon never collapses from a wave to a particle. It's always a particle.

Hence i think we simply lack information about how this process happens.

QM is what it is. It stands up as an accurate and comprehensive model of elementary interactions. You can learn QM if you want to, but the theory itself doesn't depend on your accepting it!

But this shouldnt be a purely mathematical construct, but some physical reality, whether we can observe it or not.

This was the source of many debates in the 20th century. The experimental evidence goes against this "realist" view. In particular, the experiments to test Bell's Theorem showed that nature is definitely not locally realistic in the way you hope.

So it seems there is a lot more we dont know.

You shouldn't project your basic understanding of QM onto the physics community at large.

We dont know why a photon is only ever absorbed in its entirety, not say, half a photon absorbed by and atom and another half absorbed elsewhere. We also dont know how exactly energy in the form of a photon wave is converted into electron orbital energy.

Again, QFT explains this perhaps as far as it can ever be explained. Your requirement for local realism is already 50 years or so out of date. Eventually, in any theoretical physics, the hows and whys stop at the lowest level of the theory. Adding an additional layer of theory under QM would only move the hows and whys another step down.

These are still open questions. The interpretations and math of QM is glossing over these important questinos imo.

You are entitled to your opinion, but it is not mainstream scientific opinion. And, your opinions are based on having only a basic and probably erroneous understanding of QM in the first place.

"Light arriving from stars billions of light years away cannot have waited until the last instant and then gotten to us instantaneously"

But how do we know this?

That's part of QFT.

The speed of light is the time it takes for a photon energy to show some effect via interaction at some position B when emitted from position A. But the process of conversion from a wave to a particle seems to happen instantaneously, it never happens in part but only in whole.

There's no conversion from wave to particle. QFT in the main describes how an intial state results probabilistically in one of the possoble final states. There is a long thread about the mathematics of modelling the interaction between these initial and final states:

https://www.physicsforums.com/threads/confusion-about-scattering-in-quantum-electrodynamics.1053436/

We never observe an atom in a state between energy states having only internalised part of the photon energy.

That's because it's quantum mechanics, not classical mechanics!

So we must say at some point in time the photon is outside the atom, and then at the next point in time it is inside.

No. That assumes a local realism that nature does not have.

This looks like an instaneous process. In the same way, we do not know (and it seems cannot know) or observe the photon travelling toward a specific point in space, we only know it went there when the wavefunction collapses to that point.

Again, looking at the example of two possible interaction points. A photon is emitted from point A, it can be absorbed at point A or point B, both equidistant from A, but a million light years apart. Lets say the photon is absorbed at A. The absorption will happen at time T1 which is distance/c. But at the moment (plank time) prior to T1, the photon cannot be said to be just infront of A. We dont know where it is, but perhaps we can say it is distributed over the region of radius c*(T1-1plank time). If the photon energy exists in space-time prior when its a wave, then some of that energy but also be close to B. But in the next time plank time interval that takes us to T1, which equals distance/c, it becomes localised into A. This means in the plank time interval prior to T1, any of the photon energy that was close to B, which is light years away from A, got localised into A by T1, so in a plank time unit, energy light years away became localised into A. But this doesnt mean information travelled from B to A, it was just information potential, but as you say no actual information travelled from B to A, but if we say the photon is physically distributed in space, then some of that photon certainly must have 'travelled' from near to B to A in the single plank time prior to T1.

This again is trying to force quantum mechanics into a classical mechanics framework. QM is an entirely different description of nature. Nature does not conform to your classical expectations. This has been known for almost 100 years.

If nature was the way you expect it to be, then QM would never have been developed. It was developed precisely because classical ideas could not explain the experimental results. All these classical idea you are holding on to had to be abandoned 100 years ago, in order to develop the theory of QM in the first place.

 

[Kavi]

@PeroK

Just to clarify, when i said photon above, i meant a discrete quanta of energy whether it is in the field, or wherever it is, or whether it is confined to an atom in its localised state. I just dont know what to call a discrete quanta of light in between emission and interaction, when it is in its wave form. Afaik A photon is the discrete quanta of energy that is light whether in wave or particle form. But I know my use in confusing but I am not sure it is incorrect.

I am still interested in the below analogy which has not been responded to.

To recap it. In the past we didnt know about biology and how food is converted into energy into the body, or someone first seeing a car may not know that there is a whole physical mechanism by how fuel is converted into motion and exhaust.

But sticking to the biology analogy. In the past we had less information, we were less evolved, for us in our 'theories' or experiments or observations, all that existed was food in and energy out. There was no concept or understanding of mechanisms by how food is converted into energy inside the body.

In those days we could make some 'theory' that xkg of carbs can give a person some ykg of energy. That theory could be perfectcly consistent and complete, and it was!

This is where we are in QM, as far as I can tell. We know that some discrete quanta of energy is absorbed by an atom, causing an internal change of an atomic orbital configuration, and then outputted via a photon or radiationless de-excitation.

But it is a fact that we do not know how ie by which mechanism a quanta of energy as a wave, external to the atom, is converted into energy inside the atom. This is black box to us, just like in older knowledge of the biology and chemistry, we didnt know how food was converted into energy.

Scientists will need to answer these questions because it is simply a logical requisite of science to understand mechanisms relating to the physical world. To say it matches predictions and hence is complete is the same as saying biology is unimportant as we know that carbs and proteins provide 5 cals per gram and fats provide 9. It matches with observations. If we ask more questions like why does refined sugar spike energy faster than fats then we start to delve deeper into chemistry and biology. In the same way, we can ask why or how light is quantised, or how an atom converts external wave energy of light into internal electron orbital configurations, and these are valid questions, we can vaildly answer that we may never know or cannot know, but to say that those mechanisms certainly dont exist is not a valid answer.

 

[phinds]

I just dont know what to call a discrete quanta of light in between emission and interaction, when it is in its wave form.

You call it a wave, and it is NOT quantized. Light travels as a wave and EM waves are not quantized. Between emission and interaction, there IS no photon and no quantization. The relative velocity of emitter and receiver can cause a wave that starts off at one frequency to be a different frequency as seen by the receiver. That could not happen if EM waves were quantized.

Afaik A photon is the discrete quanta of energy that is light whether in wave or particle form

A photon is the result of, for example, an interaction between a wave and an atom. THAT is quantized but if the wave doesn't have a frequency that works for exciting the atom, then there IS no photon. The best example of this is spectral lines resulting from shining a white light through a gas.

In the same way, we can ask why or how light is quantised

Again, light is not quantized, only the interaction between a light wave and an atom is.

 

[PeterDonis]

Lets take the converse process.

That doesn't change my comment.

surely, there is some 'mechanical' or systemic process by how this happens

In the literature on QM, this kind of hypothesis is known as "hidden variables". Bell's Theorem and other theorems derived from it show that, if there is such a process, it cannot be a local realistic process of the kind you are envisioning.

Again, this is the sort of thing I would not advise getting into at all until you have a sound grasp of the basics.

it seems there is a lot more we dont know.

Many interpretations of QM agree with you that QM as we have it now is not a complete theory, yes. But not all of them do.

how do we know this?

Because physicists have spent almost a century now developing QM as a theory and testing its predictions, in some cases to many decimal places, and that is what the theory says. You need to learn what the theory says.

the process of conversion from a wave to a particle

Is a myth. There is no such thing in actual QM.

Again, you need to learn the basics of QM. You appear to be misinformed, and you can't reason correctly from a wrong starting point.

 

[PeterDonis]

Scientists will need to answer these questions because it is simply a logical requisite of science to understand mechanisms relating to the physical world.

Tell that to all the scientists who, as I noted in my previous post, have used QM to make predictions that are accurate to many decimal places. We have no problem using QM in its current state. Some interpretations of QM, as I noted, say that QM in its current state is incomplete, but others do not. Perhaps in the future we will discover new phenomena that QM in its current state cannot predict, and then we will need a more comprehensive theory to make such predictions. But that hasn't happened yet, and any claim that it must happen is simply an opinion at this point.

 

[PeterDonis]

Light travels as a wave and EM waves are not quantized. Between emission and interaction, there IS no photon and no quantization.

This is too extreme. There are states of the quantum EM field that, even in the absence of interaction with matter, can reasonably be called "photon" states (these are the Fock states). A better statement would be that such states are not encountered naturally but take a lot of expensive lab equipment to prepare, so the light, and more generally EM radiation, that we see just traveling around the universe from natural processes is not in such states, and there is no useful "photon" description of such light in the absence of interaction with matter.

 

[PeterDonis]

The relative velocity of emitter and receiver can cause a wave that starts off at one frequency to be a different frequency as seen by the receiver. That could not happen if EM waves were quantized.

This is too extreme as well. Fock states will Doppler shift. In "photon" terms, this is because the energy of the photon is frame-dependent, just like the energy of any particle. So you cannot rule out quantization of EM waves on these grounds.

 

[PeroK]

But sticking to the biology analogy.

If you learned quantum theory you wouldn't need an analogy with biology. Moreover, biology is an extremely high-level science compared to QM. Mechanisms in biology only take you so far, at which point cell-biology, chemistry and ultimately atomic physics takes over. Biology is even less of a complete theory than QM. Nothing is ultimately explained by biology - as it rests on chemistry and physics. QM is different from biology, not because it is less complete, but that it is the lowest level of the natural sciences. Even if you demand a theory below QM, that theory would have the same characteristic that its fundamental postulates had no underlying mechanism. And, if you want an infinite sequence of theories stacked on top of each other, then that has its own problems.

You could argue, in fact, that the quark model is such an underlying theory - even though it is still considered part of quantum theory. Quarks cannot be isolated and fulfil the role of explaining the particles we can detect - protons, neutrons and mesons etc. But, you can then ask what is the mechanism that give quarks their properties and you are back at square one. And even if quarks were made of mini-quarks, then why would mini-quarks have the properties they have?

Incidentally, even mathematics eventually hits the bedrock of logic and you end up with the axioms of set theory that have no underlying axioms. Ultimately, you end up with a set of axioms (or laws of physics) that you have to accept as the basis of your theory.

 

[PeroK]

https://www.physicsforums.com/threads/confusion-about-scattering-in-quantum-electrodynamics.1053436

Much obliged. The link is fixed now.

 

[Nugatory]

Just to clarify, when i said photon above, i meant a discrete quanta of energy whether it is in the field, or wherever it is, or whether it is confined to an atom in its localised state. I just dont know what to call a discrete quanta of light in between emission and interaction

You call it a photon. However, that's not especially helpful if you do not have a clear definition of "discrete quanta of light", and for that you start with something like http://www.physics.usu.edu/torre/3700_Spring_2015/What_is_a_photon.pdf which is a good starting point if you're not up for taking on a textbook on quantum field theory.

when it is in its wave form.

That entire wave or particle thing is no part of the modern (here "modern" means "newer than when the great-grandparents of today's physics undergrads were in college") understanding of quantum mechanics. It's something that you want to unlearn.

 

[Kavi]

Information may exist in discrete chunks. A sentence can either be a full sentence which makes sense or a partial one which doesn't. In logic a piece of information exists in discrete chunks called well formes formulas, anything not satisfying those laws is not valid information.

In a similar way, a photon carries information to the absorbing atom. For certain it carries atleast information regarding what direction it came from. This how light tells us about physical space, how plants can grow towards light sources.

This information must be maintained from emission to absorption even while in wave form. Hence we can think of photons as discrete blocks of information the same as we think of it as energy.

Directional information of the emitter to the absorbing atom must be a property of the light, the photon is what maintains the discreteness and wholeness of this information.

If photons werent discrete even while in wave form, different elements from different photons would mix up and be absorbed by the atom, the atom would not know where it came from, a single photon would appear to come from different places. The information system wouldnt work.

In the double slit experiment, the constructive interferenxe areas are those that maintain directional information and destructive regions do not.

 

[PeterDonis]

Information may exist in discrete chunks. A sentence can either be a full sentence which makes sense or a partial one which doesn't. In logic a piece of information exists in discrete chunks called well formes formulas, anything not satisfying those laws is not valid information.

In a similar way, a photon carries information to the absorbing atom. For certain it carries atleast information regarding what direction it came from. This how light tells us about physical space, how plants can grow towards light sources.

This information must be maintained from emission to absorption even while in wave form. Hence we can think of photons as discrete blocks of information the same as we think of it as energy.

Directional information of the emitter to the absorbing atom must be a property of the light, the photon is what maintains the discreteness and wholeness of this information.

If photons werent discrete even while in wave form, different elements from different photons would mix up and be absorbed by the atom, the atom would not know where it came from, a single photon would appear to come from different places. The information system wouldnt work.

In the double slit experiment, the constructive interferenxe areas are those that maintain directional information and destructive regions do not.

@Kavi, you should not be making assertions like this. You don't understand QM well enough for that. Your assertions in the post above are mostly, in Pauli's famous phrase, not even wrong.

Instead of making assertions, you should be asking questions. Since you are not doing so, this thread is closed.

 

[bhobba]

If you learned quantum theory

That seems to be the key here.

You need to see a self-contained treatment of exactly what a photon is. I have posted it several times before, but here it is again:

https://www.physics.usu.edu/torre/3700_Spring_2015/What_is_a_photon.pdf

As accessible background see Lenny Susskind's book on QM:
https://www.amazon.com/dp/024100344X/?tag=pfamazon01-20

I know for many people, math is hard. But unfortunately, the laws of physics are written in the language of math.

As an aside, it introduces Quantum Field Theory (QFT), which QM is a limiting case of. In many ways, thinking of particles and their QM behaviour in terms of QFT makes things like interaction and other words often used in QM that are imprecise, precise, ie it resolves an issue raised by the famous John Bell:

https://www.informationphilosopher.com/solutions/scientists/bell/Against_Measurement.pdf

With thanks to @A. Neumaier

Thanks
Bill