As Charles Francis said, the proper time of the photon is zero. With the photon you cannot associate an observer, because real observer cannot travel with the speed of light.

I am not sure what you mean by physical explanation, but let me try. If you have any particle with positive mass, it cannot move with the speed of light, but it can approach this speed. There is a special relativistic effect called dilation of time, which you know I guess. The more fast the object is, the slower its clock ticks. The formula describing this dilation has the property that as the speed approaches the speed of light, the time dilation becomes infinite. Thus, if you regard the photon as the limiting case, its proper time will shrink to zero.

Another question is why the dilation happens. But I believe nobody can answer it in terms of anything more familiar. We just know from the experiment that Einstein's postulates of special relativity hold and they inevitably imply the time dilation. We don't know what's the process behind the time dilation, it is simply a fact. Notice that in classical physics, the time is the same for all observers. But this fact also needs, in principle, the explanation. Why is time the same for all observers? But since the notion of time is so familiar to us, we don't ask for any other reason. However, when we learn from special relativity, that time IS NOT the same for all observers, we are looking for the reason. But it cannot be explained in terms of anything more familiar.

Dear Charles

How about two entangled photons how can they feel each other almost instantly. This means that photons can observe and their observations are almost instant and any observer must have time.

Dear Martin

There is interesting question that you asked which is " why the dilation happens?'

Actually the physics of time dilation are all covered behind its answer. Special relativity is only a special case of time dilation. Its not just related to relative velocity, its related to something more general and more comprehensive.

What do you mean on "photon's clocks"?

I think his question has a point, let's push it a bit, a mass-less-observer let's say sitting on the photon (MLO let's say) should be able to measure time normally, by normally, I mean, for that observer the time should run in it's own frame (activities of mass less things ? ).. on the other hand, for all the other observers (massive observers = MO) moving with a speed less than the speed of light must see the the MLO's clock frozen ! But according to MLO, it's clock should run, if not, then why not ? I don't know ! 

A photon does not experience "time passing". Thus, however long its journey, even billions of light years, from the photon's perspective, it is instantaneous. So, I do not believe photon be a good entity for clock.

Dear all;

Thank you for your interest and participation.

Unfortunately we are stick with the equations of special relativity and I think many of us knows well that these equations don't apply on objects moving at speed of light. The equations of special relativity is just an approximation for such limit. Time dilation is not just due to the relative movement of two observers relative to each other. This explanation lead to many paradoxes one of them the twin paradox. First we need to return back why photons are moving at speed of light and in all directions. Why if there is a variable displacement current an EM wave formed and instantly propagated at speed of light. This means the space around us is actually moving at such speed in all directions and it takes every zero mass particle with it at such speed. 

What made me ask such a question is if the equations of Lorentz transformations are derived assuming the velocity of light real while the velocity of classical observer relative to a third common reference frame as imajinary then the  relative velocity will lead to non zero factor at the denominator of Lorentz factor. This means non infinite value for the time dilation.  The complex relative velocity is becasue photons are quantum objects that their position and time are uncertain relative to classical objects, so if added to classical velocity the relation suppose to be complex relation and here the difference appears between traditional Lorentz transformations and the modefied one. The traditional Loretnz tranformations are only applied to transformations between classical objects not for trnasformations between classical and quatnum particles, otherwise quantum mechanics will not need its complex form.

Dear all, 

In any area of the space there are a medium or EM-fields, which hinder the motion of photons with the maximum possible speed "C".

The hypothesis that the slowing of the speed of a photon is due to their absorption and re-radiation, and between these interactions the speed of the photon is strictly equal to "C", does not hold water.

Therefore, there are no photons, which travel at the speed "C", they all travel at a slower speed than the constant "C" that used in the equations of SRT ( depending on their frequency).

Thus, the clock in the rest frame of the photon will show a slowing of time in accordance with frequency of the photon and optical density of the medium.

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В любом районе пространства имеются среда или ЭМ-поле, которые препятствуют движению фотонов с максимальной возможной скоростью "с".

Гипотеза о том, что замедление скорости фотона вызвано их поглощением и переизлучением, а между этими взаимодействиями скорость фотона строго равна "с", не выдерживает никакой критики.

Поэтому не существуют фотоны, которые перемещаются со скоростью "с", Все они перемещаются с меньшей скоростью, чем константа "c" в уравнениях SRT, в зависимости от их частоты.

Таким образом, часы в системе отсчета фотона покажут замедление времени в соответствии с их частотой и оптической плотностью среды.

Sadeem,

Here is the catch-22 that I think you are overlooking.  Try to construct a clock in the photon's reference frame.  Anything you do to create a differentiated state, necessary for the ticking of a clock, causes the "apparatus" to move slower than c.

For example, you could have the photon bounce back and force between one side of a fiber and the other (or between two mirrors), but it's path would be slightly non-straight and therefore slightly slower.

In fact, each bounce would be what Charles describes as a destruction and creation.  whenever a photon interacts such that its energy or other property changes, we may effectively assume the old photon is destroyed and a new one created.  So a photon cannot change state, and cannot experience time.

Interestingly enough, the rest of the universe can change state during the life of a photon.  But not while the photon is present.  To keep it present, its direction would have to be changed, destroying and recreating it.