How Old is Light? - Exploring Einstein's Theory

[Drakkith]

But you have not committed anything on my assertion that time dilation and length contraction are dependent on each other.

They aren't dependent on each other, they both are the result of the fact that light travels at the same speed for all inertial observers. I wish I could go into detail about this, but I confess I do not know the details!

But am I right if I think that all this confusion is there because the marriage between theory of relativity, which is basically classical and quantum mechanics, (which takes birth from the womb of classical mechanics, has some extraordinary non-classical postulates;) is not yet complete

Nope. Relativity is a classical (non-quantum) theory. Nothing in this thread has anything to do with quantum physics.

 

[Let'sthink]

How do you say that , Drakkith, photon is name given to the particle associated with electromagnetic waves/field hence quantum physics has to be very much in as was also hinted in Nugatory's response?

Also I do not buy that argument of introducing a parameter c (length/time) and assume that that makes the two effects independent. Rather Relativity broke the myth that time is absolute!

 

[Nugatory]

But you have not committed anything on my assertion that time dilation and length contraction are dependent on each other.

It's not so much that they are dependent on one another as that they're both manifestations of the same underlying fact, namely that the speed of light is the same for all inertial observers. But yes, you need both (along with the relativity of simultaneity, which in some ways is more fundamental than the other two) to construct an internally consistent theory with a constant speed of light.

But am I right if I think that all this confusion is there because the marriage between theory of relativity, which is basically classical and quantum mechanics, (which takes birth from the womb of classical mechanics, has some extraordinary non-classical postulates;) is not yet complete

That marriage was successfully consummated about 75 years ago, with the discovery of quantum field theory. Unfortunately, QFT charges a much higher mathematical price of admission than either special relativity or non-relativistic QM; it's not generally considered an undergraduate-level topic.

 

[Drakkith]

How do you say that , Drakkith, photon is name given to the particle associated with electromagnetic waves/field hence quantum physics has to be very much in as was also hinted in Nugatory's response?

While its true that photons are the quanta of the EM field, relativity treats light as a classical wave, not as photons. As I said before, quantum physics has nothing to do with anything in this thread. (In other words, the theory of relativity works just fine whether you treat light as a classical wave or as a quantum object)

 

[Let'sthink]

Thank you very much Nugatory, I cannot ask a question or comment anything in a field where I am not admitted. But I am sure the total story of space time mass and energy is not yet completely written or written off!

 

[Nugatory]

I am (almost) completely serious when I say that the best thing you can do is to try to forget that you ever heard the word "photon" until after you have learned special relativity and are starting in on quantum mechanics. Einstein figured out special relativity without photons, and you can too.

But if you aren't going to take this advice... Take a look at the papers referenced in the first two posts of this thread: https://www.physicsforums.com/threads/what-is-a-photon.879128/

 

[Let'sthink]

I welcome your zeal of replying, Nugatory! When I accept your first advice, the first thread of this discussion loses its place.
I shall follow your second advice and then come back to you with my confusions!

 

[Let'sthink]

I have read one of the articles you, Nugatory, referred to and and am able to appreciate that photon is quite different from any other particle. I am in the superposition state of clarified and confused ones and will take quite some time for wording a clear question. Thank you for all your answers! But I do not understand what is the great idea in letting the ug student treat photon as he wants and also answer his questions as per his understanding in a round about manner and ultimately let him know at pg level that photon is no particle at all. But that is just observation and not a question!

 

[Drakkith]

But I do not understand what is the great idea in letting the ug student treat photon as he wants and also answer his questions as per his understanding in a round about manner and ultimately let him know at pg level that photon is no particle at all. But that is just observation and not a question!

In a perfect world every student would be taught things in the clearest manner, the best order, by the best teachers, and with the best source material. Unfortunately perfection is in a superposition of "doesn't compute" and "pipe dream".

 

[Let'sthink]

Thank you Drakkith for your reply and for using similar diction!

 

[Mister T]

But I do not understand what is the great idea in letting the ug student treat photon as he wants and also answer his questions as per his understanding in a round about manner and ultimately let him know at pg level that photon is no particle at all. But that is just observation and not a question!

The relevant issue here is the meaning of the word "particle". In the classical particle model, it's simply something so small that its size can be ignored when compared to all other relevant distances involved. So a traffic engineer can model a car as a particle, but an auto mechanic cannot. When calculating a home run distance a baseball can be modeled as a particle, but a major league pitcher cannot use that model when executing his pitches.

In quantum mechanics the word "particle" means an entirely different thing. A photon is just such a particle, as is an electron. If you've ever taken a chemistry class you run into the seemingly senseless warnings about not thinking of an atomic electron as you would think of a planet orbiting its sun. The planet can in that case be modeled as a classical particle, but the electron cannot.

If you read an article written by a physicist like N. David Mermin you see that he speaks of photons bouncing off mirrors and traveling the length of a train car as he derives a formula for the relativistic addition of velocities. But he is a physicist, so he can get away with it! He knows the difference between a classical particle and a quantum particle. For someone who doesn't that practice can lead to all sorts of misconceptions.

 

[Let'sthink]

When I was a ug student I used to think em waves or Planck's photon as pure energy. Classically whenever we talk of energy it is of the particles only whether with classical particles or classical waves. For waves also because in the classical domain energy of the wave is ultimately the energy of the particles of the medium. In case of light and after advent of relativity we find there is no necessity of a medium. But em waves have energy. So these waves must be particles in the guise of waves. Also these so called particles are strange they cannot be retarded and hence can never come to rest and we have to presume their rest mass as zero. For other particles the contribution to energy comes both from mass and motion but in case of photons it is all motion. The discussion on this thread has broadened my idea of photon and now I think I can think it as a purely QFT object whose energy, direction and plorization can only be conceived and position is just indeterminate or un-conceivable naturally it follows it cannot have a conceivable velocity at a given time.

 

[GeorgeDishman]

we have to presume their rest mass as zero.

"Presume"? Try following the references here.

 

[Let'sthink]

Thank you George Dishman, I went through some references suggested by you some par in detail and others not in so much detail. It was a very fruitful and useful exercise for me.

 

[.Scott]

Light, we are told, cannot be influenced by the speed of it's source (we cannot add to the speed of light by putting the torch on the front of the train). Hence it should not be influenced either by the sideways motion of the train.

To be clear, the light will be affected - but not it's speed. Suppose we have a high speed train passing a stationary train on a side track. And we have two stationary photon detectors positioned in front of the trains - each measuring photons from one train. We look at the light from each just as it passes. Photons from each train arrive at the detectors at the same time, but the detector watching the moving train will see heavier, bluer photons.