Quantum Physics Discussion Forum


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Comment by Michael Ivsin
I like your assertion that light has no momentum. Is anybody taking that seriously (besides you and I), since it has many implications?

Please comment on the sentence: "Light is virtual and, while light changes its path instantly through optical interaction, photon can also transform to real energy with the operation of absorption and end its existence."

Given that Compton scatter calls for very high energy photons, do you think this effect is core-based rather than electron based. Perhaps something exotic such as neutron breakup?

I assume you are referring to my page photons.htm with your question regarding the photon momentum. Actually, nobody has so far challenged my treatment of the photoeffect given there, so I take it that it is a valid consideration. As a matter of fact, that page is, apart from the main page, by far the most requested page of my site, i.e. it seems that on the contrary the established view of light as particles is not being taken seriously by many people.
Similarly to the derivation of the Compton effect, I applied energy and momentum conservation for a two-body collision here to show that the energy transfer from a photon to an electron is much too small to account for the photoeffect. Some people might object that the presence of the nucleus changes the situation, but the point is that energy and momentum conservation are direct consequences of Newton's laws and are therefore in principle unjustified if non-force 'interactions' are involved (see my page regarding Energy and Momentum Conservation).
. As far as I am aware, there is no known interaction force between photons (perceived as particles) and electrons or protons. This in itself renders therefore the concept of a photon mass and momentum inconsistent and meaningless.
I have treated the Compton effect separately on my main page. Actually, corresponding experiments seem to indicate that this is electron based. However, as far as I am aware, no experiments have in fact been performed with free electrons but only with solid targets, and by coincidence (or not) the wavelength(energy) of the photons has always been about the same as the size of the lattice structure of the material. The latter circumstance is well known from optical physics to cause effects which are not being taken into account at all in the Compton theory.
Your 'Light is virtual....' sentence is apparently your (or somebody else's) philosophical reflection on the nature of light, and as such is OK in my opinion (although one should bear in mind that light can in principle also effectively end its existence by means of destructive interference).

Michael Ivsin (2)
In the dual-slit electron interference experiment I hear that the interference effect goes away if the electron is illuminated by light prior to entering the slits. If this is the case then there would be experimental confirmation of light and free electron interaction.

For the Compton effect, photon's interaction with structures that are in the neighborhood of light's wavelength makes sense as it confirms selective absorption of light in gasses. The question is, what happens to the photon once it has been absorbed?

But of course light cannot end its existence at destructive interference because it continues propagating forward (each photon is on its own).

Finally, since light has no momentum, laser has no recoil. That is what I mean by implications.

I am not sure if the double slit experiment has anything to do with the photon momentum even if you assume the latter exists. Anyway, as you may be aware, I do not accept the concept of matter waves, because it is in my view inconsistent. You could for instance in principle let two matter waves interfere to yield zero amplitude everywhere, which however would violate mass conservation. On the other hand, electromagnetic waves are massless and one can therefore superpose two plane waves to zero (furthermore, energy conservation is a concept from classical mechanics and does not apply here). It would be a rather academic question whether the two waves are still there but just not visible or if they have destroyed each other, as the electric and magnetic fields are the only physical realities of the light wave, quite in contrast to the suggested matter waves (an electron has for instance both a charge and a mass as well as other properties).

The scattering of light waves, as applied in the Compton effect, has in principle nothing to do with absorption. In the former case, the light is simply reflected according to the rules of classical optics, in the latter it is destroyed and results in the release of a photoelectron which however could recombine again to re-emit the radiation (although this would generally have a different frequency). Both processes can occur parallel to each other, which in my opinion could explain at least some features of the 'Compton Effect'.

Although the 'radiation pressure' effect seems to indicate that photons have momentum, this is in my opinion more likely due to electrons that have been released from the material by the light, although in principle there could be a contribution due to the classical Lorentz- force (see the main page ).

Michael Ivsin (3)
Just thought you may have heard about the electron interference pattern disappearing when the setup is illuminated by light -- for whatever reason.

Agree that radiation pressure should be discussed in an intrinsic form as 'pressure on mirror surface' -- to eliminate other mechanisms. Here we have NASA building solar sailing craft (with reflecting mirror sail) thinking it will work. I am surprised nobody directed a laser on a mirror to get some measurement -- seems like an easy thing to do.

I have read about the effect that the interference pattern disappears if the electron beam is disturbed by light or other sources (actually this should only happen if the wavelength of the light is smaller than the distance between the slits). One would expect this to happen if the coherency of a wave is destroyed by some interaction, but in my opinion this would indicate anyway that one is not dealing with electrons here but with light waves (x-rays) (see also the Dualism entry on the main page). Experiments regarding the interference of 'electron' beams do in fact not only completely neglect the possible presence of x-rays, but yield results that are quantitatively inconsistent or at least inconclusive (see http://www.ati.ac.at/~summweb/ifm/main.html, where known and unknown experimental problems prevented for instance a systematic study of the wavelength (energy) dependence of the interference pattern, whereas the presented results disagree with theory by up to a factor 2 and more (unfortunately, the full publication (Dissertation) is only available in German, but you may be able to compare the experimental data anyway)).
Note added later: by coincidence, the experiment discussed under the above link uses electrons deflected by a wire charged at a given electric potential. Because of the interaction of the particles with the obstacle, one can in this case apply the Schrödinger equation for the resultant potential. In principle this could therefore result in a deflection exhibiting interference like effects (see also my Reply(2) to Fotonios Malakas below).

As I mentioned earlier, the recoil effect produced by light could have a number of reasons which have nothing to do with a photon momentum.

Comment by J.L.
Interesting discussion point Michael. I was once asked if light has mass, and was forced to mumble that no, but, well, it has energy so... The reply talking of 'particle collisions' appears to maintain that a photon does not have mass.
I am a beginner, so bear with me, but cannot mass energy be turned into photon energy? Is this not one of the fundamentals of Einstein's mass energy equivalence? Perhaps the reverse process has not been observed to take place because a photon is such a small packet of energy that many such packets are required to impart sufficient energy.
One thing in favour of light having mass is the way it is influenced by proximity to other masses (eg the way you can see stuff behind the sun). Can this space-time distortion be said to move a massless electromagnetic wave?
One thing I am 'relatively' (sic) confident of is that energy cannot be destroyed. Energy tends to dissipate towards chaos, sure, but never disappears.

It may be so that mass can be turned into light, but if you turn something into something else, this logically means that the former has to vanish to create the latter, i.e. if a mass is turned into energy, the mass disappears.
Besides, as mentioned on my page regarding Energy and Momentum Conservation, the concept of 'energy' can strictly speaking only be applied to classical mechanics and does therefore not make any sense for light. Light is an electromagnetic wave with a certain frequency and amplitude that acts on matter in a way which technically allows you to roughly describe it in terms of 'photons', but this depends in reality on many factors and can not simply be summed up by the 'energy' and and number of 'photons'. In fact, some phenomena like the photoeffect clearly invalidate the assumption that light consists of particles with mass, momentum and kinetic energy (see my Photon page).
I would not completely exclude the possibility though that light is affected by masses (although this would be without any theoretical foundation), but considering the electromagnetic nature of light, it makes common sense to assume that the deflection of light is in these cases caused by electric or magnetic fields. The sun (or any other astronomical object) is surrounded by a plasma halo of charged particles which, due to the gravitational field, induces a radial plasma polarization field and the latter is likely to have a refraction effect on light (this is different from the usual optical refraction which is already taken into account in these cases).

Question by Simon Pennington
I wonder if anybody can answer me this question as it realy relates to a lot of strange experiments regarding non-interactive measurements... such as the Zeno effect. Most of these systems involve mirrors ...
Is the reflection off mirrored surfaces in fact a re-emitted photon as in changed energy states of the "stuff" in the mirror at a quantum level or genuinely the same photon bouncing off the surface?

First of all, one should if possible avoid the notion of a 'photon' as it is usually wrongly associated with discrete, particle-like objects (see my page photons.htm). Light is an electromagnetic wave field which has no discrete structure but is continuous. It is the absorption of this wave field by the atoms in the detector material which gives the impression of a discrete nature (e.g. the photoeffect). However, in the case of reflection, the wave is not absorbed but just scattered into a different direction. This is a completely coherent process where the phase and frequency of the wave is exactly maintained. The energy states of the 'stuff' in the mirror are responsible for this reflection, but only passively; there is no change of the energy states associated with reflection as no light is absorbed; effectively, the light bounces therefore just off the surface.

Simon Pennington (2)
This being the case, there is a degree of exactitude of the energy state of the "stuff" in the mirror as it produces a coherent reflection.. this also occurs in Lasers, so my second question concerns this phenomenon which to my mind i find confusing: given notions of uncertainty, how can all those coherent waves be created?

Reply (2)
The coherent waves are already created from the outset when the atoms in the laser emit the radiation: the atomic transitions used in a laser are typically very slow, i.e. associated with very long waves (coherency). The mirrors do nothing but exactly reflect the waves that fall on them (this is because the 'stuff' in the mirror acts like a forced oscillator: it oscillates at whatever frequency you expose it to and this causes the radiation to be reflected at exactly the same frequency). You could even argue that the mirrors in the laser make the radiation less coherent as they fold the wave (which can be many kilometers long) into a space less than a meter long. The point is that due to this 'backfolding', the amplitude increases by a corresponding amount for those waves where the distance between the mirrors is a multiple of half the wavelength, whereas for other wavelengths the increase is much less. Certain 'wave modes' will therefore appear as pronounced, which gives the impression of an increased coherence. However, this is only a consequence of the way the mirrors causes the individual waves to interfere with themselves, not with the coherency of the waves as such (one could say that the mirrors just reveal the coherency that is already there in the first place) (see also my entry regards Lasers on the main page).
Anyway, this is my view regards this problem and other physicists might disagree (not that somebody has expressed his opinion to me as yet).

Simon Pennington (3)
.. so in "laymans" terms the coherency is a "lucky catch" that excludes the incoherent waves by virtue of harmonics (if you will) created by the length of the mirrored chamber?

Reply (3)
You could say this, although as mentioned above, the excluded waves are in principle not less coherent (the length of all wavetrains is essentially the same). They just have the 'wrong' wavelength to set up a standing wave in the laser cavity and can therefore not build up to the same amplitude. As a result, an interference pattern develops in 'frequency space' in a similar way as an interference pattern develops in real space if waves are spatially constricted by obstacles (where in some places the intensity is enhanced and in others it is reduced). This is merely a re-organization of the individual waves rather than a change of the coherency as such. Only by isolating one of the harmonics can one effectively make the light overall more coherent, but this is in any case limited by the coherency of the wave emitted originally by the atoms (it is only at this point that the coherency is determined by quantum effects).
Lasers therefore merely concentrate the light both spatially and spectrally but do not increase the overall power of the emission. In fact, lasers usually have a continuous output power about the same as the average pocket torch (1 Watt or so). It is only the extreme concentration of the radiation which makes laser light so powerful.

Comment by Lior
I think your description of the absorption of a photon by an electron is wrong. It is a known fact that a FREE electron can NOT absorb a photon. The energy is conserved and the momentum is conserved only if the there is a nucleus close which takes the missing momentum.
In the photoelectric effect you missed the point again. The crack of the matter is that there is a cutoff wavelength which above it no matter how much power is supplied (of radiation) no absorption is seen. But as long as your radiation is below that cutoff the power of the radiation is irrelevant.
And finally the photon has momentum which is (h- the plank constant)/(lambda - the wavelength of the photon). The photon has NO MASS only virtual photon but that is another story.
By the way there is interaction between electron and photon. This interaction you know when you look at your TV. The electron trajectory is bent because of electric and that is a photon. It is the classical limit of Quantum field theory where many photon are treated classically.
I did not realy understand the problem but the physics part is not treated correct. When talking about the atom there is only quantum mechanics. The description of the acceleration of the electron in the atom by radiation is totally incorrect. There is no gradual acceleration by radiation there is only emission or absorption of a photon. The electron is jumping from one energy to the other and not gradually changing its orbit. The electron is not circling the nucleus it lives around it with a probability to at a specific poing in space. The orbit description is classically and it is not relevant here.
For a more detailed calculation you'll have to look for quantum field theories and interaction of radiation and matter books.

I am sorry to say that it is you who is missing the point here:
the assumption of energy- and momentum conservation would necessarily imply that a photon is a particle with a mass (energy- and momentum conservation follow directly from Newton's laws). The mass μ of a photon would be related to the momentum p by μ=p/c , but as shown on my page photons.htm, it would be much too small to transfer enough energy to an electron in an elastic collision such as to enable photoionization (you are probably referring to the 'rest mass' of a photon above which is indeed zero in the theory of relativity, but since light moves with speed c, the mass would be non-zero (=p/c)).
Furthermore, no dynamical physical process can occur instantaneously, so each change of the energy state of an atomic electron takes a certain time. The harmonic oscillator is the standard model in QM for describing the absorption or emission of light by atomic systems (see for instance Cowan (The Theory of Atomic Structure and Spectra) or Heitler (The Quantum Theory of Radiation)). I have used a semi-classical argument for this on my page photons.htm which should at least give a good order of magnitude estimate of the time required for photoionization. The point is that the time resulting from this interaction model time is very short (about 10-8 sec for sunlight) and consistent with experiments, in contrast to the usual (invalid) estimate based on the energy density of an electromagnetic wave. Of course, this picture only applies if the frequency of light exceeds the ionization frequency (which is a separate criterion). (please note that quoting the before mentioned publications does not mean that I generally agree with all of the views presented there (or in other publications for that matter); many concepts of quantum mechanics are in my opinion inconsistent as indicated elsewhere on this site).
I don't quite understand your remark regards the deflection of electrons in a TV: this is caused by a static electric field and not by electromagnetic waves. I assume you refer here to the assumption of Quantum Field Theory that static fields consist of virtual photons, but this view is inconsistent as an interaction between the virtual photon and the electron would again require a static interaction force between the two.

Comment by Fotonios Malakas
Quantum physics says that matter has wave-like characteristics. The probability of finding a particle in a given space is associated with the square of the amplitude of its wave at the specific spatial coordinates through the known energy equations. This approach I find strange!
From the experiments that the wave nature characteristics of matter were proposed (youngs double slit and single slit experiments...), the interference at the screen proposed that matter has wave like characteristics....isn't it possible that the screen too may be consider to be of wave nature? If this is taken into consideration, then can't we say that matter is in fact a form of wave interference itself ?

Your suggestion seems like the next logical step forwards, but it merely pushes the inconsistency of the wave-particle dualism to its limit:
first of all, even if you accept the wave nature of particles, the wavelength of a resting object is infinite as it is inversely proportional to the momentum. This means there would not be any additional interference effects introduced by the screen. However, this is not the crucial point here. More important is the circumstance that 'matter waves' are indeed only mathematical constructs for describing the probability of finding the particle at a certain point, but as such are no objects that would enable a physical interaction. There is no force field associated with 'matter waves' (as it is for instance the case for electromagnetic waves) and as such these waves could not possibly physically interact with anything. It is only the individual particles that can cause a reaction on the screen if they impact on atoms in the screen material.
However, the physical interpretation of 'matter waves' through 'probability amplitudes' is in my opinion anyway flawed as the amplitude of any wave has to be positive as well as negative, yet probabilities can only be positive. In other words, adding up the unsquared amplitudes of a matter wave (interference) is a physically undefined operation, whereas adding up the squared amplitudes would obviously not give any interference effects. For electromagnetic waves for instance, adding the amplitudes of two waves means adding the electric and magnetic field vectors, but for 'matter waves' no such interpretation is possible, which questions therefore the concept of 'matter waves' in the first place.

Fotonios Malakas (2)
Ok...so if there is not a wave nature to matter......then how do you explain that the particles changed the direction of motion while passing through the slit (slits)...at least some force must have acted on them in order to change their directions and produce a diffraction pattern on the screen, and not just straight hits on their original course of motion (all at same spot on original straight line of target) ?
A mathematical construct cannot act as a force....its only imaginary, right ? and why was a wave diffraction pattern produced in time and not just random hits?....if u can find a force to explain the first question that is, the change of direction of the particles.

To clarify my above reply: I do not claim that the state of motion of particles can only be affected by the classical force laws. The recombination of an electron and ion (or other atomic processes) can for instance not be adequately described classically, and quantum mechanical principles (i.e. the Schrödinger Equation) have to be applied. The point is that the Schrödinger Equation is still based on the electrostatic interaction force between the electron and ion. Without this interaction the Schrodinger Equation would be meaningless. Applied to diffraction this means, that the slit could in principle exert a (non-classical) 'force' on the particles, but only if there is an electrostatic interaction between the two. This would for instance be the case if you pass electrons through a slit which is electrically charged, but there is no way that an electron (let alone a neutral particle) could 'feel' the presence of a neutral slit, i.e. the direction of motion can not possibly change (whether classically or quantum mechanically).
The logical conclusion is that (for the diffraction of charged particles) either is the slit charged or the diffraction pattern is actually not caused by the particles but by light (x-rays) emitted together with the particles. For neutral particles obviously only the latter option exists unless there is an as yet unknown interaction force (for which also a kind of Schrodinger Equation applies) (see also my Reply(3) to Michael Ivsin above).

Question by Julian S.
I have always been intrigued by the concept of teleportation and with the recent advances in quantum teleportation at the University of Innsbruck, the results are encouraging (http://www.quantum.univie.ac.at/research/photonentangle/teleport/). However, it should be noted that this form of teleportation doesn't allow physicists to teleport the photon itself--only its properties to another, remote photon.
I'm an aspiring science fiction writer and what I want to do in my story is achieve teleportation by actually teleporting the people themselves, not their properties. So, I began thinking about how I was going to do that. I thought about creating a teleporter that works by converting matter into energy and reconverting the energy back into matter and being able to pass through walls and ceilings, sort of like radio signals.
1) Since we are talking pure theory here, is the matter/energy conversion process teleportation in the literal sense, at least theoretically?
2) Does conversion of matter into energy and vice-versa in this case imply the destruction of the person undergoing the process, only to be replaced by a replica in theory?
3) What else could be the best method for my story?

As you (and the report you quote) indicate, the photon teleportation experiments are at best a case of tele-copying rather than the teleportation you have in mind. I would even question the former interpretation as the concept of 'entangled photons' is based on a logical flaw: as an illustrative example, consider two identical cars that set off from a given point into different directions with identical speeds and with the same amount of fuel; under these conditions, both cars will break down at the same moment due to lack of fuel, but it is of course wrong to conclude that this circumstance proves some mysterious connection (i.e. an 'entanglement') between the cars. The latter simply break down at the same time as a consequence of the initial conditions (i.e. the same amount of fuel). In the same sense is any alleged photon 'entanglement' merely a subjective coincidence due to the initial conditions of the experiment (see also my entry regards Hidden Variables on the Home Page in this context).

These aspects besides, your idea of a teleportation through matter-energy-matter conversion faces several fundamental problems:
the energy released even by a small amount of matter would be equivalent to igniting a nuclear bomb in your laboratory. You would therefore have to convert and teleport each atom one by one in order to be able to control the release of energy. This would take such a long time that, even if it could be completed during the lifetime of the person being teleported (it might more likely take millions of years), during the conversion you would have part of the person already converted and the other part not. The remaining part could therefore hardly stay alive and you would just end up with a dead torso in the end. Of course, for a lifeless object this problem does not exist, but even here one faces another fundamental problem, that is the transport of the energy (a teleportation on the same spot obviously makes little sense): in order to be able to penetrate walls for instance, the interaction of the energy with matter must be vanishingly small, but so would be then the interaction with the receiving station, i.e. you would only be able to re-convert a small fraction of the energy (neutrinos for instance can penetrate the earth with a very high probability but they penetrate also any measuring device and only a very small fraction of neutrinos are therefore detected). You will consequently lose a lot of the initial information and end up with a rather imperfect copy of the original. These are fundamental problems which I can not see to be bypassed. I don't think that there is therefore any chance that teleportation will ever become reality in the sense you have in mind.
Of course, if Star Trek (and others) can do it, so can you, but I would consider this more a case of Phantasy than Science Fiction.

Julian S. (2)
In your professional opinion, do you think a person could theoretically survive the conversion of matter into energy, or would the individual be replaced with a replica who was literally born into existence once the energy was reconverted back into matter?
Reply (2)
This is really more a question of medicine as well as philosophy, maybe even religion. I personally would relate the definition of life to the physical integrity of the body (at least to a certain extent and as far as the central nervous system is concerned). The physical integrity as such is obviously destroyed if the body is converted into energy, but one could argue that as long as the body can be re-constituted exactly, the physical (maybe even the personal) integrity is identical to the original one. The point is that this is unlikely (maybe even impossible) to happen. Nature is not just like a game of chess where you can reverse a move and continue from exactly the previous position. Even for lifeless matter, a given object is more then just a number of atoms put into certain positions, but it is the result of certain chemical (i.e. quantum mechanical) reactions that a) take a certain amount of time and b) are not exactly predictable nor exactly reversible. As the saying goes 'The Whole is more than the Sum of its Parts' and 'The Whole' seems to be beyond the reach of science here.
As mentioned above already, there would be the additional problem of information loss when the radiation is transmitted to its destination, which would lead to your copy relating to the original merely like a TV picture with all its disturbances.
However, as indicated in my previous reply, these aspects should be rather academical as the vast amount of energy produced in the conversion of the matter (hypothetically assuming this works) would result in everything within a radius of miles to be pulverized, which not only means that the teleporting person but also everybody else in the surrounding area would die. The only safe way to preserve both the physical as well as spiritual integrity seems to me therefore to use more conventional ways than teleportation to get to your destination. This is almost guaranteed to be faster, less costly and does not involve blowing up a whole city in the process. But then again, does Science Fiction have to stick to the laws of logic and science if science itself doesn't in many cases (see my HomePage)?

Comment by Alim Naji
Regarding Richard Feynman's double slit experiment: light is shone onto a surface with a small hole or narrow slit. The light passes through this hole, and shines on a second surface that has two holes cut in. The light spreading out from these two holes finally lands on a third surface making an interference pattern. As soon as a detector is placed, the pattern created is just two blobs of light, no interference pattern as if they know when they're being watched. Imagine that you could do this experiment on a cosmic scale by using the light from distant objects outside of our galaxy, which has reached you by two different routes. This would occur as a result of gravitational lensing. This is when light is bent around a massive galaxy that it passes to reach you. Numerous different paths of light are available and you can choose two to create an interference pattern. This pattern would show that the light from both paths reached you as a wave. If you monitor each photon, it would then give you blobs of light and no interference pattern. These distant objects are at least 10 billion light years away. So according to the double slit experiment, your choice wether to observe the particles or not will effect how the photons appear on the screen before they get there, which means that they would have to have known wether you would detect them 5 billion years before our solar system was formed. If we could perform this delayed-choice experiment it would show that the past of the quantum world is influenced by factors associated with how we choose to conduct our observation today.

First of all, I assume you are addressing the same issue here as in Wheeler's 'Delayed Choice Experiment'.
The paradoxical (and indeed logically impossible) conclusion of being able to change the past in the quantum world is simply a consequence of the erroneous particle picture for light. As shown on my page /photons.htm, light has, contrary to common opinion, to be considered as an electromagnetic wave field in order to account for the photoeffect. Since an interference of two coherent wave fields (as in the double slit experiment) is only possible if they are allowed to superpose, the interference pattern will of course disappear if one focuses on one of the light sources. The optical construction of the telescopes in the thought experiment illustrated under the above mentioned link does in this case prohibit a free superposition of the two light waves. The only choice that has been made here is to allow or not allow the two waves to superpose at the detector. The wave field of both light sources in front of the telescopes is completely unaffected by the latter (which could easily be proved by putting another (semi-transparent) screen in front of the telescopes). Of course, as light is an electromagnetic wave and not a particle, it is anyway obvious that the concept of a 'path' of the light signal is not only irrelevant here but conceptually wrong, i.e. the 'delayed choice' experiment is as much nonsense as the idea of light consisting of particles and being able to go through two slits simultaneously.

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