Dear Pragnan,

   Photons are very special particles as the gravitons or gluons because they are bosons allowing the interaction. In particular, photons are particles with rest mass zero which have always a constant velocity for every observer (independently it his/her state of motion. Thus you cannot accelerate them or to change their state of motion.

  This point (beside others, which distinguish quantum behaviour from the classic) , makes it very different to what happens with Newtonian particles, where the second law makes proportional the force with the acceleration.

Dear Prof. Baldomir,

Thank you very much for the answer. This answer is clear and interesting. Therefore, precisely, photons don't follow newton's second law (F=ma). But this behavior is not just restricted to photons. Conduction current in conductors also take to a constant velocity and don't accelerate despite application of a force. Similarly, small ball bearings dropped in a barrel of viscous liquid, in viscosity experiment, takes a constant velocity (can't accelerate) despite application of the gravitational force.

On the other hand, fundamental particles (which also exhibit quantum mechanical behavior) can be accelerated in different particle accelerators.

     Dear Pragnan,

     The photons can never be accelerated and this is a key point to understand how they are radiated or absorved by an electric charge. This property hasn't anything to do with quantum behaviour, it depends much more of the special relativity.

     Nowadays, devices as quantum dots (excitons associated to electron-hole pairs, Wannier excitons) can modify their frequency ( thus their energy) taking into account confinement for a given potential . There are also others of low energy working as excitions of the molecular electronic structure (Frenkel excitons) and which are very important in nanoptics (mainly in luminiscence) due to only allow certain transfer of photons between molecules. This technique is used in tv colors, etc... The same could be said for  the optic conductivity in quantum wells, quantum wires, etc...The collective photon transport can be quite different of the photons in vacuum, which are the ones that I have mentioned and even in vacuum we need distinguish between virtual and real photons. This tell us that we need to take care with the answer to your general question.

Dear Pragnan,

The difference between a quantum particle and a classical (what you name Newtonian) particle, is that while the former behaves as dictated by the Schrodinger equation, the latter behaves according to Neuton's laws of mechanics.

The fact that say specify "not wave" behavior, but "particle-like" behavior, doesn't help.

Let me no go to details.

A photon is not a tennis ball, it is a wave-packet. A tennis ball is a rigid object moving with a well defined linear momentum. A wave-packet doesn't have a well-defined linear momentum, because it is a superposition of frequencies. As you know, there is a relation p = (h/c)\nu, therefore the wave-packet is a superposition of linear momenta. Therefore, while the wave-packet travels with some group-linear momentum, the wave-packets also widens because of that spread in linear moments. With the tennis-ball such a widening does not exist.

As to accelerating wave-packets, the photon cannot be accelerated, but the wave-packets of other quantum particles yes can be. I advise you to read about the Ehrenfest theorems. 

A tennis ball is a wave-packet, just as the photon. However there are two remarks to be made.

1.A tennis ball is composed of a lot of elementary particles, which are wave packets as well.

2.The wave lengths of the tennis ball are that small, that we may treat the ball in the classical way.

1. The fact that the tennis ball consists in many elementary particles is not relevant, because the question addresses the movement of the tennis ball as a whole. This movement has a wavelength practically zero. One can check it using λ = h/mv and h=2π×10-27. 

A similar problem was posed by A. Zeilinger, namely, what is the wavelength of a walking man.

2. We treat the tennis ball classically not because the wavelength is small, but because it is small in comparison with the linear dimension of the tennis ball. Thus, we can't perform interference experiments with the ball. For increaseng the wavelength so as to be comparable with the linear dimension of the ball, we have to reduce the ball velocity to the order of magnitude of 1cm in 1020 years.

Please compare with the age of the Universe.

Charles,

The movie on the youtube is very lovely, I liked it, but Pragnan asks something else: by what the particle properties of a photon, or another quantum particle, differ from the "particle" properties of a classical object, e.g. of a tennis ball (which is quite big a "particle").

Best regards (P.S. dispite my criticism the downvote is not mine, and it is quite irritating when people downvote without explaining with what they disagreed.)

Charles,

I already said it. In my last comment, after saying

"we have to reduce the ball velocity to the order of magnitude of 1cm in 1020 years",

I said

"Please compare with the age of the Universe."

This age is of the order of only 1010 years.

Best regards!

Ha, ha! Congratulations Charles!

I subscribe to the complaints.

Now, speaking seriously, the experiments with fluoro-fullerenes show what I said. The fullerenes are of big linear dimensions, and quite massive. To obtain interference with them, Zeilinger's group exploited transversal velocities in a beam of fullerenes. The longitdinal velocities weren't sufficiently small. One can find articles about those experiments in quant-ph. 

I also know that there were talks about doing interference with microbes. But I didn't see any reports.

You see, a major question physicists ask, is where is the limit between the classical and the quantum world. The world should, in principle, behave quantumly. But the classical objects have too big linear dimensions and masses, and that impedes generating visible interference, i.e. fringes of width bigger than the lnear dimension of the object. For overcoming this problem, one should reduce the velocity of the object, but the example with the tennis ball shows that we can't do this sufficiently.

(Of course, the measurement problem in quantum mechanics tells us that there may be additional differences between classical and quantum objects, but that is already out of the scope of Pragnan's question.) 

I said it Charles, in previous answers, and I repeat: the question posed here is what is the difference between the particle properties of a clasical object, and the particle properties of the quantum object.

The quantum object is a wave-packet. The wave-packet is not a rigid body as a tennis ball, it undergoes dispersion. As to acceleration in a field, for this one has to solve the Schrodinger equation in the field.

The measurement problem is more complicated than you say. (First of all we can do measurements if we don't require a higher precision than the wave-packet allows. The dimensions of the wave-packet and the velocity are of importance. For instance at high evergies and well localized wave-packets with low dispersion the situation changes.) This is why I didn't want to get into this.

Dear Charles,

==> the wave function is not even a wave. It is a complex valued helix in configuration space

What you say is interpretation, and interpretations are many. About each interpretation there are arguments and counter-arguments, and it's whole books filled with them. We can't cover this here. As to your interpretation, my counter-argument is that only matter influences on matter. For getting a click in a counter, there has to be some form of matter impinging on the detector. The abstract formulas that we write on the paper stay on the paper, on the detector impinges matter. We don 't know what form of matter is the wave-function. If we knew we would understand how the collapse works.

But, again, this discussion is outside the scope of the question asked by Pragnan. If you like to discuss such issues, it would be better to post a question, and there will be answers and comments.

Best regards!

Charles,

I quite dislike a polemic about an issue alien to the question itself.

Well, when I objected to your statements as

==> the wave packet is not an object at all. . . .  In fact the wave function is not even a wave. It is a complex valued helix . . .

what I thougt is that you attack the idea that the wave-function is a wave, ontologically.  And what I said was that interpretations are many.

Now, I repeat, the answers to a question should first of all be useful to the poster of the question. So, look at the question and look at your explanations about "helix in the complex configuration space".

Of course, I am no one to tell people what to say and what not to say, but a user posts a question for getting help. A helpful answer should be first of all intuitive for the poster of the question. Sometimes, rigorous mathematical explanation may be very nice, but of no use for the question poster.

I, for one, think that Pragnan got a couple of intuitive answers. To other discussion I am glad to participate, but under appropriate questions.

Charles,

If I would say that your claim is unhelpful, incorrect and invalid, I am sure that you would feel offended.

This is not a way.

If you criticise me, criticism is wellcome, but you have to motivate it, you have to explain in what, do you believe, I am wrong. For instance, you criticise me about my saying that the quantum object is a wave-packet. What for do we write the Schrodinger equation? Not for obtaining the wave-packet that desribes the evolution of the quantum object?

Just to throw at people that what they say as unhelpful, etc., can't replace argumentation. They can say the same about you, also without adding argumentation, if somebody wants to be impolite. But this site would become a site of exchanging offenses, instead of a site of physics. I, for one, won't contribute to that.

Best wishes!

Charles,

We will never end this polemic, s.t. it's better to stay each one of us with his/her opinion. There is an ancient saying "de gustibus non discutabimus" (if I made some grammar mistake please correct me).

Can't you see that we argue about interpretation?

You say, "The wave function is a part of the calculation", i.e. a mathematical tool, and you speak of information.

And I believe that the wave-packet is more than that. I believe that what hits the measurement apparatuses is indeed a wave-packet, but in each trial and trial of an experiment we have collapse.

Only an experiment can decide between us, if such an experiment is possible.

You see, you criticize me that say things dogmatically. But you, what you think you do when you claim that the wave-function is no more than a mathematic tool? I repeat, it's interpretation here, epistemiologic (yours) vs. ontologic (mine). So, I stop here, because for the moment, no experiment is known to me to decide between the two.

(By the way, I believe that I have in hand an argument that the collapse hypothesis is not completely correct. But I have to put the things on the paper, in an organized form.)

Best wishes!

A simple answer would be the uncertainty principle!!

 Another way to look at this issue is real experiments, take for example the double slit experiment, the photons behave like waves through interference and diffraction, however in the photo electric effect,  they just behave like particles (quanta), they collide with electrons giving them energy quanta. On the other hand, the electron just do the same, electrons behave like waves in diffraction through crystals and of course they collide and can be accelerated as particles in many other phenomena. 

@Pragnan

The principle difference is that quantum particles also act as a wave- yes even things like an electron. Each wave has a frequency associated with it that allows to behave as a wave and a particle.