[Bill S (OP)]

I may well present three scenarios in this thread; it, sort of, depends on the responses.

In each scenario there is a hypothetical space craft, travelling at c and an astronaut on the craft with a flashlight which he shines from the back of the craft towards the front.  There will also be one or two remote observers, with hypothetical instruments that can be turned on and off very quickly.

(1) The astronaut turns on his flashlight. In his RF the light travels at c to the front of the craft.

Observer A makes her observation as the astronaut turns on his flashlight.  She sees no light because in her RF the craft and the light are travelling at the same speed.

[Colin2B]

Bill, I don't think this is valid.
As soon as the light leaves the flashlight it will be travelling through space at c, that is what the remote observer sees. The craft and the light are travelling independantly.

[evan_au]

there is a hypothetical space craft, travelling at c

If we are going to discuss a fantasy spacecraft, how about we make it travel at 99% of c?

Then we can more sensibly talk about astronauts turning on flashlights, and what happens next...

[PmbPhy]

In each scenario there is a hypothetical space craft, travelling at c and an astronaut on the craft with a flashlight which he shines from the back of the craft towards the front.  There will also be one or two remote observers, with hypothetical instruments that can be turned on and off very quickly.

(1) The astronaut turns on his flashlight. In his RF the light travels at c to the front of the craft.

Observer A makes her observation as the astronaut turns on his flashlight.  She sees no light because in her RF the craft and the light are travelling at the same speed.

One of the was in which Einstein was led to create his special theory of relativity was to imagine himself riding on a bicycle. He wondered what he would observe of he was moving at the speed of light. His resolution of this problem was two postulates, the second of which is that the speed of light is invariant, i.e. has the same speed in all coordinate systems. This led to the conclusion that nothing can travel at the speed of light other than light itself and any particle which has zero proper mass. This means that you're scenario is impossible, i.e. no spacecraft can move at the speed of light.

[Bill S (OP)]

Bill, I don't think this is valid.
As soon as the light leaves the flashlight it will be travelling through space at c, that is what the remote observer sees. The craft and the light are travelling independantly.

I think you lost me there, Colin. You seem to be saying that observer A sees the craft travelling at c, relative to her, and light travelling at c relative to the craft.  This would mean that she measured the speed of light, relative to her, as 2c, so you cannot be saying that.

[Bill S (OP)]

how about we make it travel at 99% of c?

I realise that making the speed anything less than c would bring the discussion into the world of real physics; however, there is a reason for opting for c, which would become obvious in a later scenario.

I wanted to to post the scenarios individually to avoid skipping to the end without considering the steps.

Of course, if step one is untennable there is probably no point in progressing to two.

[Bill S (OP)]

One of the was in which Einstein was led to create his special theory of relativity was to imagine himself riding on a bicycle.

I think I know what you mean, Pete.

He wondered what he would observe of he was moving at the speed of light.

I wondered what observer A would see if the if the craft were travelling at c.  J Richard Gott uses an example in which a space craft travells at c.  It would be tempting to ask if one has to have a PhD to get away with that, if it were not for the fact that Einstein didn't have one when he did it.

This means that you're scenario is impossible

Of course it is.  I was, in fact, working towards establishing that the line of reasoning running through the scenarios resulted in what I saw as an inconsistency. 

I make no apology for wanting to work things out for myself, albeit in an amateurish way, but regret that it sometimes tries the patience of the experts.

[jeffreyH]

Carry on. For the hell of it. I'm listening.

[PmbPhy]

It would be tempting to ask if one has to have a PhD to get away with that, if it were not for the fact that Einstein didn't have one when he did it.

It's refreshing to me to see someone who's familiar with the history of physical theories. Nicely done my friend.   It's unclear to me what you meant by "get away with that." Could you explain that to me please? Thanks.

Of course it is.

I know that you knew that. However it's not clear to me what you think the answer is. After all, you're asking about what happens when the impossible happens. Physics can't answer such questions since the real resolution to the question is that it's not possible.

I was, in fact, working towards establishing that the line of reasoning running through the scenarios resulted in what I saw as an inconsistency. 

What is the "inconsistency" that you have in mind?

I make no apology for wanting to work things out for myself, albeit in an amateurish way, but regret that it sometimes tries the patience of the experts.

I'd like to encourage you to do just that. Such thought processes is a wonderful way of understanding what you're looking for. After all, this is a forum in which we like to encourage our members to do exactly that, i.e. work things out for yourself.

[Bill S (OP)]

Carry on. For the hell of it. I'm listening.

OK, let’s go for the other two scenarios together.

(2)  Astronaut acts as in (1).

Observer A makes her observation when the light is half way from back to front. She would see it “frozen” in that position.
 
(3)  Astronaut acts as in (1).

Observer A makes her observation as in (1).  She leaves her instrument on, so sees no light.

Observer B starts her observation at the same point as A, but has her instrument programmed to turn off and on again such that it comes on again when the light is half way from back to front. She sees, first, no light, then the “frozen” beam.

Question: In the RF of an observer, does the light move only when it is not being observed?

To me, this seems like “fantasy physics”.  The conclusion seems inconsistent with reality as we observe it.  I know that does not necessarily mean it cannot happen, our intuition is often a poor guide.  I wondered what others thought. 

[jeffreyH]

Before proceeding I want to point something out. If we have a rest mass moving at c then we get infinities. What if the velocity is reduced by 1 Planck time interval? The infinities will be gone and finite values for mass, momentum and kinetic energy result.

[Bill S (OP)]

What if the velocity is reduced by 1 Planck time interval? The infinities will be gone and finite values for mass, momentum and kinetic energy result.

That's true, but then neither observer would see the light as stationary, or failing to emerge from the flashlight, if the craft were not travelling at c.

[jeffreyH]

What if the velocity is reduced by 1 Planck time interval? The infinities will be gone and finite values for mass, momentum and kinetic energy result.

That's true, but then neither observer would see the light as stationary, or failing to emerge from the flashlight, if the craft were not travelling at c.

The scenario has a directional bias. If you shine your torch directly away from the direction of motion the photon will leave the torch. However it will from your rf be traveling at 2c. So observers behind you should be able to detect photons except that the energy may not be available to emit them. In theory this one mass, traveling at c, may well contain all the available energy in the universe. Hence the one Planck time comment. Just thinking about conditions at the big bang.

[Bill S (OP)]

The scenario has a directional bias.

 
No problem with that.

If you shine your torch directly away from the direction of motion the photon will leave the torch.

 

Do you mean at 90 deg or 180deg ?

However it will from your rf be traveling at 2c

 Lost!  Can’t find a RF in which light would be measured at 2c.

[jeffreyH]

The scenario has a directional bias.

 
No problem with that.

If you shine your torch directly away from the direction of motion the photon will leave the torch.

 

Do you mean at 90 deg or 180deg ?

However it will from your rf be traveling at 2c

 Lost!  Can’t find a RF in which light would be measured at 2c.

I meant 180 degrees.

If you are moving in 1 direction at c and a photon is moving in the opposite direction at c then from your local frame the photon moves twice as fast as expected. Since we are traveling exactly at c I am entitled to speculate outrageously.

[Bill S (OP)]

Since we are traveling exactly at c I am entitled to speculate outrageously.

Does your outrageous speculation include the equations of relativistic velocities breaking down at c?  This deep into speculative quasi physics, you are entitled to such animadversions, but it would be good to know your reasoning.

[jeffreyH]

Since we are traveling exactly at c I am entitled to speculate outrageously.

Does your outrageous speculation include the equations of relativistic velocities breaking down at c?  This deep into speculative quasi physics, you are entitled to such animadversions, but it would be good to know your reasoning.

Hi Bill

I have had a very tiring week. I will give you an answer to this over the weekend. When I have my thinking head on.

[jeffreyH]

Since we are traveling exactly at c I am entitled to speculate outrageously.

Does your outrageous speculation include the equations of relativistic velocities breaking down at c?  This deep into speculative quasi physics, you are entitled to such animadversions, but it would be good to know your reasoning.

If a mass were to travel at c then length in the direction of travel would be infinitely compressed and time infinitely dilated. So in effect you would go nowhere during an infinite time period. Your universe would then be the photon itself. Since zero volume would contain zero mass you would not even be considered to have converted into photons since you would also have zero energy. Since even the vacuum has energy this is not a tenable situation. This was why I introduced a minimum decrease in speed. It is a quantum decrement. Using the Planck unit of time. That one change removes the infinities and gives a finite energy for the system. This is close enough to c to enable your scenario to have some significance. It would be interesting to calculate the mass at this speed. It might say something interesting about inflation and how time dilation affected the development of the universe once the force of gravity has separated out.

[Bill S (OP)]

All this thinking about who would see what arose from a quote from J Richard Gott who Says:

 “If an astronaut’s rocket were to travel by us at faster than the speed of light, a light beam he sent forward could never catch up with the front of his rocket.  The light beam could never catch up because the front of the rocket would be moving faster and have a head start.  Any athlete should know that catching another runner who is running faster and has a head start is impossible.  The astronaut’s observations would be most peculiar: he would take out a flashlight and shine it towards the front of his rocket, but he would never see the beam of light arrive.  That’s not what an observer at rest would see: rather than perceiving he was at rest, this astronaut would know he was moving, and that’s not allowed by the first postulate.” 

     Surely, if the astronaut is on the rocket he would be travelling at the same speed as the front of the rocket, which would be stationary in his frame of reference, so he would see the light travel at c and would be able to assume he was stationary.

Typically, this sort of thought experiment ignores a range of difficulties when one starts thinking about what an outside observer might see, but, really it is all fantasy physics. []

[Bill S (OP)]

If a mass were to travel at c then length in the direction of travel would be infinitely compressed and time infinitely dilated.

Just a thought, here, Jeffrey; isn't this on a par with arguing that photons don't "experience" time?  An argument to support this would be based on relativity, which doesn't permit massive objects to travel at c; so if it did happen, we would not be able to predict what it would be like.