Why does light not reach any distance instantly?

[ghwellsjr]

OK -

Light always moves at c = 300,000 km/sec in a total vacuum devoid of mass or other energy in the same space. It always moves at c no matter what the IFR is. Light doesn't have "it's own IFR."

This is good, and correct so far.

You agree that light moves.

So, light does move instantaneously with respect to itself from any point A to a second point B. It is in any other IFR that time goes by depending on that IFR in relation to an IFR containing point A to point B.

This doesn't seem hard to understand.

If you are trying to say that light moves along a null worldline, so that the Lorentz interval between any two events on the worldline of a light beam is zero, you'd be correct. You might even add that the Lorentz interval is zero in any reference frame (though this would be redundant, because the Lorentz interval is independent of reference frame, so if it's zero in one, it's zero in all).

Again, you agree that light moves.

What you actually said may seem clear to yourself, but it's not going to be generally understood :-(. I was going to suggest the above as being what you were trying to say, but I'm not really sure it is!

The problem is that "moves with respect to itself" is problematic. In order for something to "move", it must experience time. But the whole point is that light can't be said to "experience time".

Now it seems to me, following your line of reasoning, you are saying that light can't move. But since you twice agreed it does move, I must be misunderstanding what you are saying. Can you please clarify?

Your phrasing implicitly assumes it can, as nearly as I can tell. It also seems to assume that there is such a thing as a "reference frame" for light (if I'm understanding it correctly). This is widely known to be false, if the usual definition of reference frame is used. There's a FAQ article on this that shouldn't be too hard to find.

As a practical matter, people have different understandings of words and what they mean. The problem is particularly acute in technical fields, technical "jargon" has a very precise meaning. A general procedure for making sure communication is happening is to try to find a phrasing that's acceptable to everyone, so that one is sure that one communicates what was intended.

If I had a penny for every time I completely misunderstood something written in popular language, and answered a completely different question than what they were intended to ask, I'd be rich.

It would appear that somebody deserves another penny here, just not sure who.

 

[ghwellsjr]

Thus, by what pervect states, is that light is independent of IFRs. Thus, from any point A in the universe to a second point B, no matter what IFR is chosen, there will be a time difference between emission and reception.

It would also seem that if light speed, c, is independent of IFRs, then the time difference will always be (distance between A and B))/c no matter what IFR is chosen.

Am I on the correct path here?

No, you've got the cart before the horse. An IRF is defined or created or fabricated or established by using the definition or postulate or stipulation that light propagates at c in all of them. This then establishes what time and distance mean. This is why time and distance are relative to the IRF chosen. It's not the other way around.

You have so clearly illustrated the concept that c is always c (in a total vacuum) no matter what IFR is chosen and there is no standard IFR for light, just as the universe has no built in clock and no center.

Pervect, or whoever, please address the issue of SR when light travels through a medium, energy, or subject to gravity which "slows light speed down". Where do SR or even GR rules fit?

That's a different subject outside the scope of this thread and should be brought up in another thread.

 

[stevmg]

You say "tomato", I say "tomahto", you say "potato", I say "potahto", "potato, potahto, tomato, tomahto, let's call the whole thing off."

Not necessary to call the whole thing off but agree on zero time for light internally. Also agree on no matter what IFR chosen, time from A to B is (distance)/c. But distance AB will vary by what IFR chosen. That's the old flashing light in the front and back of a moving train paradigm.

I will take that penny, thank you.

 

[ghwellsjr]

You say "tomato", I say "tomahto", you say "potato", I say "potahto", "potato, potahto, tomato, tomahto, let's call the whole thing off."

Not necessary to call the whole thing off but agree on zero time for light internally. Also agree on no matter what IFR chosen, time from A to B is (distance)/c.

I will take that penny, thank you.

Again, it's not zero time for light internally. The concept of time doesn't apply to light internally. It's not like the number of pennies in your piggy bank when it is empty, it's like there's no piggy bank. So please don't continue to say that time for light is zero or instantaneous or anything equivalent.

Of course, after you define an IRF by using the definition that light propagates at c in all of them, then you can also say that the distance that light travels between A and B is time multiplied by c.

Are you aware that we define distance by how far light travels in a given time?

 

[stevmg]

-ghwellsjr

Thanks for the clarification. I am learning the jargon word-by-word. Yes, I am now quite aware that in space-time, distance is measured by c*t (such as a light-year). Hence, because there is no time in a light beam, no distance is appreciable. I appreciate that in a light beam, one cannot talk about zero time. I got it.

I LOVE learning this stuff but am at a very primitive point.

Still have to find out the meaning of the gamma and beta functions, though they are not germane to this forum.

 

[pervect]

Thus, by what pervect states, is that light is independent of IFRs. Thus, from any point A in the universe to a second point B, no matter what IFR is chosen, there will be a time difference between emission and reception.

It would also seem that if light speed, c, is independent of IFRs, then the time difference will always be (distance between A and B))/c no matter what IFR is chosen.

Yes. Note that the distance between A and B will depend on which IFR you measure it in.

Pervect, or whoever, please address the issue of SR when light travels through a medium, energy, or subject to gravity which "slows light speed down". Where do SR or even GR rules fit?

As far as SR goes (this means no gravity):

The "c" in the SR formula s basically means the speed of light in a vacuum, though sometimes people skip over this part for whatever reason (my opinion is that it's just too long to keep repeating).

The speed of electromagnetic radiation, including light, will depend on the medium it travels through, and the motion of the medium. See for instance the Fizeau experiment, http://en.wikipedia.org/w/index.php?title=Fizeau_experiment&oldid=578154337 for experiments of the speed of light in moving water.

When gravity enters the picture one has to deal with the effects that are called "gravitational time dilation" due to general relativity.

In that case, it becomes necessary to decide which clock to use to measure the speed of light, since all clocks do not run at the same rate due to the effects of gravitational time dilation.

If you set up a local frame of reference by using a local clock and a local meter stick, in the small region of space where the speed of the light is to be measured, you will find that the speed of light in a vacuum does not change, and is always "c".

If you set up a global frame of reference, usually using some centralized "master clock", you will find that the speed of light in a vacuum, measured in this manner, is not always equal to "c".

For instance on the Earth, the usual global frame of reference is TAI time, (international atomic time), a coordinate time standard.

http://en.wikipedia.org/w/index.php?title=International_Atomic_Time&oldid=581789205

Actual physical clocks need to be adjusted to keep TAI time.

In the 1970s, it became clear that the clocks participating in TAI were ticking at different rates due to gravitational time dilation, and the combined TAI scale therefore corresponded to an average of the altitudes of the various clocks. Starting from Julian Date 2443144.5 (1 January 1977 00:00:00), corrections were applied to the output of all participating clocks, so that TAI would correspond to proper time at mean sea level (the geoid).

Thus, if you measure the speed of light using the time scale defined by the TAI standard (which requires adjusting the readings of clocks depending on their altitude), you'll find that the speed of light in a vacuum is not c. If you don't adjust the clocks for TAI time, and use a clock at the same height above sea level as your light beam is to measure the speed of light in a vacuum, you'll find that the speed of light in a vacuum is always "c".

 

[stevmg]

DQ = Dumb Question:

As light travels through total vacuum space, does the presence of any other electromagnetic energy have a GR time dilating effect? (Assume that the mass equivalent of the "other electromagnetic energy" would be E/c^2)

Dumb Question, no?