[Bill S]

No. I am saying the opposite. You cannot match or exceed c.

That's what I would have expected you to say.  I must have misinterpreted #14 as saying something else.  I think that arose because you said: "the object will experience no time or motion". 

I'm still not clear as to whether you would propose this as  a reason for saying that a photon doesn't "experience" time.

[jeffreyH]

Time is irrelevant for a photon. Time is relevant for particles with mass. They interact and form atoms and molecules and ultimately life.

[Bill S]

Time is irrelevant for a photon

I've followed several lines of reasoning on this subject, and I have to say that's the best "get out" I've met.  I'll try to remember to reference you if I ever use it.

This just leaves the question as to whether, or not, the equations of SR can justifiably be extrapolated to "c".

[Colin2B]

This just leaves the question as to whether, or not, the equations of SR can justifiably be extrapolated to "c".

It would be reasonable to say that certain values tend towards certain limits as speed tends towards c.

[Bill S]

It would be reasonable to say that certain values tend towards certain limits as speed tends towards c.

Are you trying to trump Jeffrey's get-out clause? 

[Colin2B]

It would be reasonable to say that certain values tend towards certain limits as speed tends towards c.

Are you trying to trump Jeffrey's get-out clause? 

It’s not really a get out clause, it’s just the way things behave.   

[jeffreyH]

Just wait until I get deeper into quantum mechanics.

[Bill S]

It’s not really a get out clause, it’s just the way things behave.

I accept that; but the question was whether one could justify extrapolating SR to "c", rather than just an "infinitesimally" small distance away.

Just wait until I get deeper into quantum mechanics.

I look forward to hearing about your discoveries.

[Janus]

It’s not really a get out clause, it’s just the way things behave.

I accept that; but the question was whether one could justify extrapolating SR to "c", rather than just an "infinitesimally" small distance away.
 

One cannot. For example the time dilation equation is 
t = t`/sqrt(1-v²/c²)


if you try to make v=c, you end up with a division by zero which is undefined.
It is an asymptotic function with no solution for v=c

[Bill S]

Absolutely!

So does that mean that we cannot know if something travelling at "c" "experiences time?

[Colin2B]

So does that mean that we cannot know if something travelling at "c" "experiences time?

Absolutely 

To have the capacity to experience implies (as currently understood) that the experiencer has mass.

[Bill S]

To have the capacity to experience implies (as currently understood) that the experiencer has mass.

Thanks, a straight answer is always welcome.

Opinions are, as you are aware, quite sharply divided on the (speculative?) question of photons and time.  I'd be interested to know where you stand on that; and why.

[Colin2B]

Opinions are, as you are aware, quite sharply divided on the (speculative?) question of photons and time.  I'd be interested to know where you stand on that; and why.

Where I stand is as I stated:

It would be reasonable to say that certain values tend towards certain limits as speed tends towards c.

Or as @Janus put it in more detail:

It is an asymptotic function with no solution for v=c

The important fact, which has a solution, is that photons travel at c relative to us. This is measurable.

[geordief]

c can (must?) be viewed as  a limit when applied to massive particles .

Can it also be viewed (in a different sense) as a limit when applied to massless particles?

I may well be wrong ,but it is the quantity from which all other measurements depend (are a function of).

If we we to take some other quantity as the benchmark  would we find that the value we found for c  would have to  be expressed as a limit?

I also understand that c is a function of the permittivity and the permeability of a vacuum .Are there any conceivable circumstances where those quantities might vary?

[Bill S]

Thanks, again, Colin.

To me, your statement: "To have the capacity to experience implies (as currently understood) that the experiencer has mass."  says something like: 

Because nothing with mass can be accelerated to "c", and since we cannot reasonably attribute "experience" to a massless object, we have no way of knowing what the relationship between a massless object and time, as we measure it, would be. 

Bear with an ill-informed, pedant, and tell me if I've got that right.

[Colin2B]

Bear with an ill-informed, pedant, and tell me if I've got that right.

That’s pretty well it.
However, if we consider the photon from our reference frame we can say that the time dilation we measure would be extreme, so decay time (drawing a parallel with Muons) would be close to infinite - which, I assume, is why we don’t see photon energy as being distance dependent.

[jeffreyH]

Bear with an ill-informed, pedant, and tell me if I've got that right.

That’s pretty well it.
However, if we consider the photon from our reference frame we can say that the time dilation we measure would be extreme, so decay time (drawing a parallel with Muons) would be close to infinite - which, I assume, is why we don’t see photon energy as being distance dependent.

All those who are actually here to learn read Colin's answer carefully. You will learn something very important about relativity and time.

[Bill S]

All those who are actually here to learn read Colin's answer carefully. You will learn something very important about relativity and time.

No doubt, everyone expects me to ask how you can be close to infinity; but that’s not for this thread.

There’s the bit about considering the photon from our reference frame.  To me, that says that any practical measurement can only be made from our RF.  We might theorise about how, it might look from some other RF.  We might even construct a mathematical argument to back up our theory, but we can never actually place ourselves in a RF that would not be ours, once we were in it.

 “…time dilation we measure would be extreme…”  We can measure time dilation to a very high accuracy, but whatever the extremity we reach, there will always be a limit that stops short of “c”.

“…so decay time (drawing a parallel with Muons) would be close to infinite - which, I assume, is why we don’t see photon energy as being distance dependent.”

If by “infinite” we mean that it is so extreme that we cannot reasonably, distinguish between its value and infinity, then we would be faced with the possibility that at “c”, muons might not decay.

“…which, I assume, is why we don’t see photon energy as being distance dependent.” 

If we apply the same principle to the photon, its energy would appear never to reach zero, in our RF, no matter how far, and for how long, it had travelled. 

OK, Colin, what have I misinterpreted; Jeffrey, what have I missed?

[jeffreyH]

All those who are actually here to learn read Colin's answer carefully. You will learn something very important about relativity and time.

No doubt, everyone expects me to ask how you can be close to infinity; but that’s not for this thread.

There’s the bit about considering the photon from our reference frame.  To me, that says that any practical measurement can only be made from our RF.  We might theorise about how, it might look from some other RF.  We might even construct a mathematical argument to back up our theory, but we can never actually place ourselves in a RF that would not be ours, once we were in it.

 “…time dilation we measure would be extreme…”  We can measure time dilation to a very high accuracy, but whatever the extremity we reach, there will always be a limit that stops short of “c”.

“…so decay time (drawing a parallel with Muons) would be close to infinite - which, I assume, is why we don’t see photon energy as being distance dependent.”

If by “infinite” we mean that it is so extreme that we cannot reasonably, distinguish between its value and infinity, then we would be faced with the possibility that at “c”, muons might not decay.

“…which, I assume, is why we don’t see photon energy as being distance dependent.” 

If we apply the same principle to the photon, its energy would appear never to reach zero, in our RF, no matter how far, and for how long, it had travelled. 

OK, Colin, what have I misinterpreted; Jeffrey, what have I missed?

You have an excellent grasp of the idea.

[Bill S]

Thanks, Jeffrey. Perhaps I'm getting somewhere, slowly; but, at the moment, I'm mot sure where.