[ARVIND (OP)]

I am not concerned whether light has mass or not. But, physicists say that anything that travels at the speed of light, its time slows down. This is known as time dilation. But, ligt itself travels at the speed of light. So, does this mean that ligt also dilates time.   

[lightarrow]

But, physicists say that anything that travels at the speed of light, its time slows down.

Not at all, maybe popular books, not physicists. Infact it's false.

[Pmb]

... physicists say that anything that travels at the speed of light, its time slows down.

That's not what physicists say. What they say is that time as measured in frame of reference moving at speed v < c runs slower than time as measured in a frame of rest. It can also be shown that nothing can be at rest in a frame which is moving at the speed of light. Or simply put; moving clocks run slow when compared to clocks at rest.

[lightarrow]

If I wanted a debate, I had a lot to wrote about the way you explained it, Pete, but I'm sure you already know it can be put in a better form, so I prefer not to be finicking this time  []

[Pmb]

If I wanted a debate, I had a lot to wrote about the way you explained it, Pete,…

You didn’t like the way I explained it? If so you should have posted your own explanation in hopes of making the answer more clear.

I’m not to happy with my explanation either so here goes again

With regards to “physicists say that anything that travels at the speed of light, its time slows down.”

This is wrong. What physicists really do say is that that nothing can travel at the speed of light. What he must have meant to say (or what he really did hear) was that “physicists say that when an object is moving its time slows down.” Which is correct, but only when it is understood that it only has meaning when its referring to what his time is being compared to and its time is being compared to that of observers who are at rest in the frame in which the other observers are moving.

[lightarrow]

...
What he must have meant to say (or what he really did hear) was that “physicists say that when an object is moving its time slows down.” Which is correct, but only when it is understood that it only has meaning when its referring to what his time is being compared to and its time is being compared to that of observers who are at rest in the frame in which the other observers are moving.

But saying "when an object is moving its time slows down" is however incorrect, for two reasons:
1. It suggests to people that the fact he/she is moving, slows down its clock. False, because if he/she is moving with respect to X, then X is moving with respect to he/she. So, which clock is "slowing down"? And this generates tons of paradox or people attacking the SR claiming it is false....
2. The act of moving doesn't slow down time, it's only that when the twin A and the twin B met again (after B has made a trip with his starship and back), their clocks signs different times. But this only because they have made different path in the spacetime, the twin A made a "longer" path and so his clock obviously signs a longer time interval.

[Pmb]

But saying "when an object is moving its time slows down" is however incorrect, ...

What you did was take that comment out of context. You ignored the part that followed which said but only when it is understood that ... and what followed that. You also made a mistake here in assuming that time dilation (slowing down of clocks) only has meaning when its referring to two twins. I.e. you're confusing the Twin Paradox with time dilation and slowing down of clocks.

[lightarrow]

But saying "when an object is moving its time slows down" is however incorrect, ...

What you did was take that comment out of context.

I know, I've read your post carefully. But, you know, having experience with forum discussions of SR, I can state it's better not to suggest to laymen that "time slows down if you are moving", even if you explain what it means.
As well as the OP, *a lot* of people believes that "the act of moving slows down your clock".

You also made a mistake here in assuming that time dilation (slowing down of clocks) only has meaning when its referring to two twins. I.e. you're confusing the Twin Paradox with time dilation and slowing down of clocks.

I'm not confusing it, I use Lorentz equations too. What I mean is that *physically* you cannot say which clock is actually signing more time, until you compare them to the same two events in spacetime. The "twin paradox" is just the simpler example of what happens.

[Pmb]

]I know, I've read your post carefully.

Then please don’t do it again, okay my friend? Taking a comment out of context means to not understand it since a large portion is missing. The correct statement is

“Consider two observers, one at rest in the inertial system S’ and one at rest in the inertial system S. Let S be moving with speed v relative to S’. Then time has slowed down in system S’ when compared to system S. This means that all clocks in S are running slower that clocks at rest in system S as observed by observers at rest in system S.”

That is what my statement means when its not taken out of context.

Lightarrow – Sometimes I get the feeling that you’re trying to correct me when you already know that what I posted was not incorrect. I hope you’re not doing that on purpose of if so as a joke.

[AndroidNeox]

The only thing about light that would dilate time is its momentum. Just as light is influenced by gravity, light influences gravity. If you put a gram of matter inside of an otherwise evacuated Schrödinger catbox (impervious to anything, including information) it would have the same mass (and gravitational influence) as if the entire gram were converted into light, bouncing around inside the box.

Light, itself, does not experience time.

[Pmb]

The only thing about light that would dilate time is its momentum. Just as light is influenced by gravity, light influences gravity. If you put a gram of matter inside of an otherwise evacuated Schrödinger catbox (impervious to anything, including information) it would have the same mass (and gravitational influence) as if the entire gram were converted into light, bouncing around inside the box.

Light, itself, does not experience time.

I assume that you’re referring to light generating a gravitational field and that field generating time dilation. If the source is radiation then it can be momentum but it need not be. Its perfectly satisfactory to generate a gravitational field with a photon gas which has zero momentum density. All such a gas has to generate a gravitational field is energy and pressure. Those are the only components of the stress-energy-momentum tensor which don’t vanish for such a distribution of radiation..

[lightarrow]

]I know, I've read your post carefully.

Then please don’t do it again, okay my friend?

Ok, didn't intend to annoy you.

Lightarrow – Sometimes I get the feeling that you’re trying to correct me when you already know that what I posted was not incorrect. I hope you’re not doing that on purpose of if so as a joke.

No, the reason is that these concepts are always misinterpreted by laymen, so I believe it's important to clarify them, before legions of people grows believing relativity must be false.

[lightarrow]

Light, itself, does not experience time.

That statement doesn't mean anything.
1. It's impossible to create a frame of reference comoving with light.
2. If you travel at speeds very near to c, you will still experience time, no matter how close to c is your speed.

And this could be one of the many examples of people that misunderstand relativity  []

[Pmb]

]I know, I've read your post carefully.

Then please don’t do it again, okay my friend?

Ok, didn't intend to annoy you.

Lightarrow – Sometimes I get the feeling that you’re trying to correct me when you already know that what I posted was not incorrect. I hope you’re not doing that on purpose of if so as a joke.

No, the reason is that these concepts are always misinterpreted by laymen, so I believe it's important to clarify them, before legions of people grows believing relativity must be false.

Oh! Okay then. Good enough!

Please don't mind me. I get annoyed easily latel. It's this damn flu.

[flr]

2. If you travel at speeds very near to c, you will still experience time, no matter how close to c is your speed.

Your speed never can get 'close to c', because you will always see the photon moving away from you at exactly 'c' no matter what the speed of other non-zero rest mass objects relative to you is.

[Pmb]

Your speed never can get 'close to c', …

When light arrow said “If you travel at speeds very near to c,” he was referring to the speed as measured by an observer who wasn’t co-moving with the object whose speed we are speaking of. He’s speaking about the speed as measured by an observer who is at rest in the frame in which the moving body has speed v. As measured by such an observer any object can travel arbitrarily close to c without violating any law of physics.

[AndroidNeox]

Light, itself, does not experience time.

That statement doesn't mean anything.
1. It's impossible to create a frame of reference comoving with light.
2. If you travel at speeds very near to c, you will still experience time, no matter how close to c is your speed.

And this could be one of the many examples of people that misunderstand relativity  []

Einstein had no problem using the perspective of light in his thought experiments. The statement is correct. Nothing that travels through space at c experiences time.

[JP]

Einstein had no problem using the perspective of light in his thought experiments.

I'd be very interested to see an example where he did that.  Can you provide one?

[lightarrow]

Light, itself, does not experience time.

That statement doesn't mean anything.
1. It's impossible to create a frame of reference comoving with light.
2. If you travel at speeds very near to c, you will still experience time, no matter how close to c is your speed.

And this could be one of the many examples of people that misunderstand relativity  []

Einstein had no problem using the perspective of light in his thought experiments. The statement is correct. Nothing that travels through space at c experiences time.

Maybe you have read some popular book or seen a movie on TV, but I'm talking of physics, instead.
Study physics, not popular books, then we can go on with the discussion...
Regards.

--
lightarrow

[Pmb]

Light, itself, does not experience time.

That statement doesn't mean anything.

O course it does. That's a very common expression in special rleativity.

E.g. -  http://www.askamathematician.com/2011/07/q-does-light-experience-time/

It's not as though light thinks but then again electrons don't think either and wonder "Gee. Time sure seems to be slowly down now that I' all set for my vacation this  comming up friday." lol!