Don't think of the photon as a little bullet that is emitted by an electron. The fundamental object is the electromagnetic field. Changes in the electromagnetic field in the vacuum propagate at the speed of light. And because the electromagnetic field is a quantum field, changes (at any given energy level) come in discrete lumps, discrete quanta, which we call photons.

So yes, the photon is born at light speed. That is, the change in the electromagnetic field induced by the electron propagates at the speed of light.

Don't think of the photon as a little bullet that is emitted by an electron. The fundamental object is the electromagnetic field. Changes in the electromagnetic field in the vacuum propagate at the speed of light. And because the electromagnetic field is a quantum field, changes (at any given energy level) come in discrete lumps, discrete quanta, which we call photons.

So yes, the photon is born at light speed. That is, the change in the electromagnetic field induced by the electron propagates at the speed of light.

Dear Remi and Toth

Thank you for attention and answer. Seems there is not any experimental evidence for it, am I right?

Other question: "These efforts show there is an upper bound on the photon mass, although the amount is very small, but not zero."

Heeck, J. (2013). How stable is the photon? Physical review letters, 111(2), 021801

Do you know how they have found that photon has an upper bound of mass?

The question you are addressing is perhaps not properly posed.

When we define a photon we define the electromagnetic field itself, and its interaction, which, in some sense, define the properties of photon itself. Now since photons bring two mutually orthogonal fields is difficult to disentangle them from the Maxwell equations which impose to their propagation the velocity of light.

On the other side if one requires the photon to have a mass the Maxwell equations should be modified (Proca's equations) they acquire a mass term and the Coulomb potential becomes short range, similarly to what happens with Yukawa potentials.

There are articles published on these topics and experiments done to specify the photon mass by looking at deviation from the Coulomb law or by studying the difference in velocities between photon of different frequencies, Upper limits have been set and a research on google may provide a wealth of articles in this direction.

Regarding the light velocity some years ago in alternative to inflation it has been propsed that light travelled faster in the early universe, but it is so hard to put all the pieces of the game together.

I agree with the Toth's comment: light cannot be seen as a bullet, acceleration does not make any sense because we should consider fields. On the other side in general relativity and therefore in cosmology the definition of velocity itself is not that straightforward.

Your question should be posed within a definite physical framework and regarding what we can get on an experiment regarding the exitation and the dexcitation of an atomic system does not make too much sense.

The Best

Pino

In theory - Yes. QED is local field theory.

Your question in actuality hides other important questions:

1. Does have photon an acceleration time?

2. At what velocity does an electron emit a photon?

About the first question you are right: there isn't experimental evidence until now. The further question about photon mass is controversial. My viewpoint is that there are two types of elementary particles: energy particles and massive particles. Photon is an energy particle and therefore it doesn't have a real mass but only an equivalent and virtual mass.

With regard to the second question, in TR I have proved accelerated electron emits the first gamma quantum at the physical speed of light and the second gamma quantum at the critical speed. For this physical event there is experimental evidence and it is represented by the electron annihilation and by the production of  a pair of gamma quanta.

Dear Remi

Maybe this finite time is not detectable.

Dear Giuseppe

Thank you for interesting detail. What is your opinion about:

After 1906 Einstein have derived the second postulate of special relativity the constancy of the speed of light by assuming that the light quanta that he proposed in 1905 were massless particles.

Field, J. Einstein and Planck on mass-energy equivalence in 1905-06: a modern perspective. arXiv preprint arXiv:1407.8507

Einstein theorized that the measured speed of light in a gravitational field is actually not a constant but rather a variable depending upon the reference frame of the observer: Einstein wrote this paper in 1911 in German:

http://www.physik.uni-augsburg.de/annalen/history/einstein-papers/1911_35_898-908.pdf

http://www.speed-light.info/speed_of_light_variable.htm

So, seems concept of photon massless is an assumption in SR, and its speed is changeable in general relativity. Does photon accelerate in gravitational field?

Dear Daniele

Does a single electron annihilate to a pair of gamma quanta?

Victor,

I agree with you but there is a more accurate distinction to do.

Photons is not properly the change of electromagnetic field, it is an offspring of the transitories of  time variable electromagnetic field. The wave is an outcome  of a transitory of a electromagnetic field,  it is not the electromagnetic field itself. The transitories of the EM fields have a side effect which is the EM wave. If the transitories are persistent in time, most of the power is irradiated, or rather it is defined a flow which departs from the sources.

Nonetheless the outcome of the transitory of the field is a perturbation (EM wave) which departs at the speed of light.

Dear Hossein,

you are right. I did the example of electron annihilation in order to confirm that it is possible the production of energy quanta from a massive particle. In actuality we know electron annihilation happens only when electron collides with antimatter (positron). In the Theory of Reference Frames (TR) dynamics of accelerated free electron, without collision, is studied and in that case electron doesn't annihilate but it is subjected to physical changes when its speed increases. In particular at the physical speed of light its electrodynamic mass halves and one gamma quantum (with E=0.255 MeV) is emitted. A second gamma quantum, with the same energy, is emitted at the critical speed.

With regard to Remi Cornwall' s observation I would want to specify that Hossein Havadi asked at which speed of accelerated electron this emits an energy quantum. I suggest to read the preceding my words that gives an answer to the question in the order of the TR. Naturally if one doesn't know or doesn't accept the TR then he must find another answer.

With regard to Stefano Quattrini's idea about the transient phase of the emission of  the gamma quantum by accelerated electron it could be a good idea.

Dear Remi

I meant, I mean interval between separation of photons from electrons to reach the speed of light. If there was an interval.

Dear Daniele

"...electron annihilation in order to confirm that it is possible" In this process, what happens for conservation of electric charge?

Since the photon is a zero restmass "particle" it can only have the speed of c in vacuum. Nevertheless the emission of a photon by an atom is not an instantaneous event as it relates to the lifetime, 𝜏 = ℏ/𝛤 (with 𝛤 being the full width at half maximum) of the excited atom and the size of the emitted photon. Hence the transition- or lifetime of let us say 𝜏= 26,24 ns corresponds to an approximately 8 m long photon travellling at the speed of c. 

I do not think a more detailed question might be realistic since QM is not always commensurate with "common sense".

Dear Hossein,

right question. The answer necessarily involves two cases:

1. electron annihilation with positron in a process of collision. Before annihilation the total charge of electron and positron is Q=-1+1=0. After annihilation the two massive particles don't exist and two neutral gamma quanta are generated. After annihilation the total charge is Q=0 and therefore the Principle of Conservation of electric charge is respected.

2. acceleration of free electron without collision. Before acceleration, resting electron has electric charge q=-1. When electron is accelerated, it  experiences two physical changes, at the physical speed of light and at the critical speed with conversion of electrodynamic mass to energy but electron doesn't disappear and it continues to exist with its electric charge because energy quanta emitted don't have charge. Therefore during the whole process of acceleration electric charge of electron is always q=-1. The Principle of Conservation of electric charge is respected also in this second process.

Dear Erkki

Thank you for your useful detail. I have learned.

My question remains:

Precisely at the moment the electron emits photon, what is the speed of photon?

Dear Daniele

Thank you so much for description.

I have had the same question for many years and finally I think I have worked out an answer for it. It has an accelerating time. Please find the answer in my article ON NEGATIVE AND DARK MATTER IN PHYSICS on ResearchGate.

Dear Hossein: Regarding the upper bound on the photon mass. The theory predicts a photon that is exactly massless. Experimentally, however, we can only establish limits: The upper bound basically says that if the photon has a mass, it must be less than so-and-so. This does not mean that the photon is massive. For that, you would need a lower bound: If an experiment established a lower bound on the photon mass, it would exclude the massless photon, and that would be quite a revolutionary discovery.

As to how that upper bound on the photon mass is established, please see Wikipedia: https://en.wikipedia.org/wiki/Photon#Experimental_checks_on_photon_mass. 

Dear Toth

In my opinion "the upper bound on the photon mass" is important because it might helps us to understand interaction between charged particles and photon better than before. In radiation, does photon accelerates or no? If it has, so photon should gains energy in during its acceleration, and if no, why and how a photon is born at speed of light?

Photons are solutions of the corresponding second order differential equation. This equation holds for the field/manifold/medium in which they travel. The "speed" at which photon travels is a property of this field/manifold/medium. This property is a constant. 

Only objects that have mass or some other kind of charge can be accelerated. This is also a feature of the supporting field/manifold/medium. The property mass or charge is connected to artifacts that cause deformations in the supporting field/manifold/medium. These deformations surround point-like discontinuities in the field/manifold/medium.

Apart from point-like discontinuities also more extensive regions exist in which the field/manifold/medium cannot be represented by a continuous function.

Further, the point-like discontinuities can exist in coherent swarms that act as units, which can be considered as separate objects.

http://vixra.org/abs/1511.0007

http://www.e-physics.eu/TheGeneralizedStokesTheorem.pdf

It is important to understand that a single photon is a conglomerate of fundamental quanta governed by the simple equation E=hn =hf. This means that a gamma ray photon has a frequency (by definition) and contains within it more than 10^20 fundamental quanta.

The fundamental constituents of the Universe at the smallest level are fundamentally tachyonic (speed = to or > than c). Which explains all the quantum equations from first principles. And provided that we understand that the quantum Planck mass is in fact h/c^2, we can explain all the quantum weirdness (see paper below)

Having said this it is important to understand that a single photon can be composed of any number of quanta (up to 10^36). Hence the maximum photon energy is 10^20 eV.

So the starting velocity of these fundamenatl quanta is already c or above, so they either have to maintain c, or slow down in speed to c, to form a photon

Article The formulation of harmonic quintessence and a fundamental e...

The hypothesis  that one photon is made up of a highest number of quanta is a consequence of the mathematical model that has been assumed for photons in Quantum Mechanics and it led to the Indetermination Principle. In the Deterministic Quantum Physics a different mathematical model is used for photon and it is based on the concept of  "rational trigonometry" that leads to different conclusions about the physical nature of photon. The choice of the suitable mathematical models becomes therefore the fundamental question of contemporary physics.

I think that the intoxication of the MM experiment, of SR and of GR on the 19th century's idea of a transmitting medium is at the origin of the debate. In the annexed paper, I discuss how the equation E = mc² is derivated from the idea of a transmitting medium. It explains why photons have a mass but not a rest mass, and it also explains why photons are not accelerated but are created almost instaneously. So, within that concept, the vibration rate of the electron can define the frequency of the photon-wave, but not the wave velocity that is intrinsic to the transmitting medium.

Photons are classed as "luxons" and always travel at the speed of light.  Most particles are "tardyons" meaning slower than light in a vacuum.  Tachyons are a hypothetical particle that always travels faster than the speed of light in a vacuum.  

There is an anonymous idiot who systematically downvotes all my comments without being able to discuss the substance of my comments. Consequently I reaffirm my comment:

The hypothesis  that one photon is made up of a highest number of quanta is a consequence of the mathematical model that has been assumed for photons in Quantum Mechanics and it led to the Indetermination Principle. In the Deterministic Quantum Physics a different mathematical model is used for photon and it is based on the concept of  "rational trigonometry" that leads to different conclusions about the physical nature of photon. The choice of the suitable mathematical models becomes therefore the fundamental question of contemporary physics.

I apologize to serious researchers who follow this question for this due and non-desired specification but that idiot has no effect on me.

Although many problems of modern physics has been resolved, but still many of modern physics problems have remained, so that there is so much effort to solve them. For example quantum gravity and dark energy.

Usually in QM only talk about position and momentum operators. However, there are various theories in physics, but nature is unique. This is not nature's problem that we have various theories; nature obeys simple and unique law. So, we should improve our theories.

Newton and Einstein define the acceleration regardless of the structure of particles (in classical mechanics and relativity). This definition belongs to Newton era or macroscopic level era. In fact the interaction between large objects (e.g. collision of two bodies) under the action of the quantum layer (in fact sub quantum level) has been done.

Although acceleration is used in classical mechanics, but all macroscopic systems are made of quantum particles.

In my view, until we do not solve the problem of acceleration at the quantum scale, we cannot solve the acceleration of the universe (dark energy).

My suggestion is that we need a new definition about particles.

There are two kinds of particles in physics. Some particles like the photon move only with the speed of light, in all inertial reference frames. Let’s call these kinds of particles as Never at Rest condition particles (NR-particles). Other particles like the electron always move with the speed v

Dear Hossein,

I would say that "this precise moment" cannot be defined.

Dear Erkki

Is it relevant to our theories?

Dear Hossein,

I subscribe most of questions that you raise and above all I subcribe your claim: "However, there are various theories in physics, but nature is unique. This is not nature's problem that we have various theories; nature obeys simple and unique law. So, we should improve our theories".  Your claim doesn't mean for me that one only theory must explain all the universe but that for every physical question one only theory is right.

You raise then the right problem of acceleration.  I would want to specify Newton gave a kinematic and dynamic definition of acceleration, based also on the differential calculation, while Einstein preferred to underline the kinematic aspect of acceleration that is mostly in concordance with the concept of spacetime. I think the dynamic aspect is important like the kinematic aspect and I don't see problems, also in particle physics, if we connect the concept of acceleration with the concept of interaction or force. You do then also a distinction between NR-particles (at speed c) and ordinary particles (at  v

Hossein,

“Is photon born at light speed? Or has an acceleration time?”

-  the answer is rather simple – photons are born  “at light speed” practically immediately.

The point is that all /every material object – a particle, galaxy, etc. always move in the [3+2](3+1)D Euclidian spacetime with the “speed of light in vacuum”, c; and having, correspondingly, the  momentum P=mc and the  energy E=mc2 =h*omega, where omega is, in certain sense,  the “de Broglie frequency”. Thus, in certain sense, in Matter there is no acceleration  - as a change of the speed’s absolute value – at all, there are changes of the 4D motion direction and changes of the frequency at interactions.

But the dimensions of the spacetime, i.e. – spatial and temporal – are principally different, since are realizations of principally different Rules/ possibilities “Space” and “Time”, and so there are two different types of particles – “T-particles” that are created after impact with the momentum, which is directed along the t-axis, and “S-particles” that are created after impact with the momentum, which is directed along some spatial direction.

Thus S-particles, e.g., photons, are born and further move with spatial speed that is equal to c. If a temporally directed impact was with the momentum P=m0c [m0= “rest mass”], the appeared T-particle, say, electron, moves with the speed of light along the t-axis. And, since the t-axis is orthogonal to any spatial direction, an additional spatial impact results in a positive spatial acceleration (up to, say the spatial speed V) and in a negative temporal acceleration of the T-particle, , when, since a spatial impact cannot change orthogonal P0, as that Pythagoras said, P becomes be equal to P0/(1-V2/c2)1/2.

At “usual” (or “Newtonian”) interactions in Matter mostly spatial impacts are possible, but if the impact is more then m0c for some particle (for photons it can have practically arbitrary value), the spatial impact can transform in a temporal impact, so, for example, new particles are generated at experiments on accelerators or at interactions of gamma- quanta with electric fields.

More – see https://www.researchgate.net/publication/273777630_The_Informational_Conception_and_Basic_Physics

Cheers

Article The Informational Conception and Basic Physics

Dear Daniele

Theory or law of nature? I think nature obeys of a unique law (for an special case), but a theory comes of human mind. No theory is perfect, and a theory is better than other. For example general relativity is better that Newton's gravitational law, but general relativity also is not able answer all questions.

"I don't see problems, also in particle physics, if we connect the concept of acceleration with the concept of interaction or force. " I agree with you.

"... need a new mathematical model." I agree with you. I am in this way.

"The concept of field then in present physics is very contradictory.." Yes, I think we need a new definition of fields and exchange particles. And physicists are faced on a hard work.

Dear Sergey

I agree with you that photons are born at light speed c in inertia reference frame. But we need to describe its mechanism. Why and how photon is born at light speed?

Dear Tamer

Very good detail, thank you so much.

in a relativistic elastic collision, when energy is transferred from one particle to another, does the mass change?

Dear Hossein,

“…I agree with you that photons are born at light speed c in inertia reference frame. But we need to describe its mechanism. Why and how photon is born at light speed?”

- ? that is explained in the SS post above – not only photons, but all/ every other particles are born always at 4D standard light speed, c;  and further  all/every particle, body, galaxy, etc. move in the (3+1) absolute 4D Euclidian spacetime only with c.

The unique difference [in this context] between photons and T-particles ("having rest mass" see above) is that T-particles are born by 4D momentums having non-zero temporal component and so always move along [c]t-axis (and so, as Pythagoras said – if they have also a spatial speed, V, then V is always lesser then c); when photons are born by purely spatial momentums and so always have 3D speeds in the absolute 3D space be equal to c.

Why that is so? (what is the “mechanism”?) – that in certain sense too fundamental problem that hasn’t answer in physics now. Nonetheless we can suggest  reasonable models for the “mechanisms”, such a [informational] model – see the link to paper in the SS post above – Matter is some huge number of rather simple automata that are united in the system “Matter” by universal interaction – gravity. All automata (particles, bodies, galaxies, etc.) run

[because of the energy conversation law after some initial portion of the energy that resulted in  originating of  Matter]

constantly  with stable identical “operation rate”, what in the kinematics reveals as constant 4D motion of all automata with the identical (by absolute value) speeds; more – see the link.

At that the so called “inertial frame” has no relation to this point, in certain sense. This frame (and any other frame, though) is only a system that contains a subjective “coordinate system” and a set of sticks and specially synchronized clocks, that are intended for some kinematical measurements. These systems are established by some “observers” and so always have some subjective nuances; thus, for example, if some observer synchronizes clocks not with accordance the SR (for example when there are a number of free clocks), the measured speed of light can be non – equal to c. And that would be real light speed, which really isn’t equal to c, an evident example -  the light speed in a moving wagon when light moves orthogonally to the motion direction.

Besides – the physical variable “energy” is only some parameter in physical theory that allows to obtain quantitave estimations of a capability of some body / system of bodies to perform [another physical parameter] a “work”. At that there is no principal differences between particles in Matter – all are made by practically the same scheme.

Thus it cannot be “pure” or “dirty” energy – all interactions result in changing of the frequencies ("m") and/ or directions of particles’/ bodies’ momentums P=mc [4D vector]. For example – a photon having “pure” 1.025 MeV can originate “non-pure” pair “electron+positron”, and reversibly, this non-pure pair can be transformed into “pure” photons…

Cheers

Dear all;

The vibrations of the electrons are happened locally however if we assume the around space is moving by speed of light then these vibrations will turn into propagating electromagnetic waves move at speed of light. We usually assume the electromagnetic fields by default are moving at speed of light but no ask why!

In how many ways can a photon be generated? Atoms emit photons when they change their oscillation mode. Elementary particles emit/create photons when they themselves annihilate. These are quite different processes. Tsjerenkov radiation is another way of photon emission. 

The strange thing with particle annihilation or creation is that in some way this is a process that is controlled in a step-wise way. It is as if the energy is converted in a step-wise way into the mass of the particle or vice versa. If this is true, then the rest mass of the elementary particles is also quantized.

Dear Sergey

I studied your article "The Informational Conception and Basic Physics". In generally, I appreciate your excelent work, especially your FLEs is a considerable concept. I will let you know, in my view graviton moves faster than light speed. I suggest see my work and definition of graviton.

https://www.researchgate.net/publication/260982893_A_new_Definition_of_Graviton?ev=prf_pub

https://www.researchgate.net/publication/279288309_Graviton_and_virtual_photons?ev=prf_pub

Data A new Definition of Graviton

Article Graviton and virtual photons

Dear Sadeem

"...default are moving at speed of light but no ask why!" I agree with you. I think trying to find the answer to this question can lead to the development of our understanding of nature.

Why not c^2 ?

As far as I can see, the framing of the question has a strongly classical flavour, which is why the answer is probably somewhat ill-defined for a quantum entity such as the photon. What, indeed, does ``born at light speed'' mean? It surely implies:

1) that the photon is born at a given instant

2) That ``speed'' is a property you can measure at that very instant

In quantum mechanics, both assumptions are wrong: the emission process always takes significant time, in which one passes from a state in which only an excited atom is present, to another, in which a deexcited atom and a photon are present, passing through intermediate states in which the system is in some coherent superposition.

As for measuring speed, it would involve measuring two positions separated by a given time, and cannot be done instantly.

Dear F. Leyvraz

If a photon is formed of two electric and magnetic fields perpendicular,

and an electron is pressed from one electric field, which is surrounded by a magnetic field, then how the emission of photons by electrons can be described?

https://www.researchgate.net/publication/208380871_Zero_Point_Energy_and_the_Dirac_Equation?ev=prf_pub

Article Zero Point Energy and the Dirac Equation

Speed is a space-time property. Space-time frames are properties of an observer, not the photon (or any other particle). When an observer "sees" a particle, it applies a boost operator in its reference frame to generate its speed in that frame. In the case of the photon, that boost operator is light-speed.

Dear Mike

Is nothing happens outside the frame of reference?

If the laws of physics are the same in all inertial frames, so the physical events happening independently of the observer. Am I wrong?

Hossein,

the way that the receiver of the information can observe depends on the capabilities of this observer and not only on the conditions.

Dear Hans

Without observers, do not happen physical events?

Every particle, is in a sense, an "observer" of every other particle with which it is entangled  or becomes entangled. So it is reasonable to say that every event has an observer in that sense. However, an individual particle has no frame of reference for itself, only for other particles. At the particle level, the variables that relate one particle to another are their entangled quantum numbers. Macroscopic classical observers such as humans re-interpret this in terms of 3+1 space-time. A simple biological organism may have no sense of space-time but "observes" its surroundings by a chemical interaction (its "sense of smell" if you like). 

Dear Mike

Can we describe the physical events from the perspective of a particle such as electron?

Only those events in which that electron is entangled.

Dear Mike

In my view quantum entanglement happens at non-obvious level (space).

https://www.researchgate.net/publication/270339919_Interactions_Between_Real_and_Virtual_Spacetimes

Article Interactions Between Real and Virtual Spacetimes