[Bill S (OP)]

Consider Einstein’s thought experiment in which he imagines himself travelling at a speed that is approaching that of light, holding a mirror in front of his face.  If, as was believed at the time, light travelled at about 300,000kps relative to the aether, then, as Einstein and his mirror reached the speed of light, the light trying to leave his face would not do so, and would therefore not reach the mirror, so would not be reflected back.  In this thought experiment, as Einstein reached light speed his reflection vanished from the mirror.  Einstein reasoned that this ran contrary to Galileo’s concept of relativity.  According to which, someone travelling at a constant velocity, without reference to any object outside his or her frame of reference, should not be able to distinguish between being in motion and being stationary.  If one were not able to see one’s reflection in the mirror, in which the reflection was visible when stationary, then one would have an experiment to distinguish between being stationary and being in motion. 

Recast Einstein’s thought experiment in terms of Galileo’s below-deck scenario: a sailor would know if the ship were moving at the speed of light because his reflection would vanish.  In Galileo’s scenario all the insects etc. move within the moving cabin exactly as they did in the stationary cabin because the entire system is moving in unison.  If one shone a light across the cabin its speed would be measured as 300,000 kps, whether the ship was moving or not. 

Prior to Einstein’s discoveries it would have been assumed that this was because the speed of the ship was being added to, or subtracted from the speed of light, as would be the case with the various creatures in Galileo’s scenario.  Until Einstein came to the conclusions that arose out of his thought experiment it would have been reasonable for him to assume that the speed of the light leaving his face would have been measured as 300,000 kps, plus his speed through the aether.  Thus, why would he have concluded that his reflection would vanish from the mirror when he travelled at light speed? 

Until he made his later discoveries, would he have had any reason to assume that the light travelling from his face to the mirror would have been measured as anything other than almost 600,000 kps by an observer outside his RF? Unless, of course, Einstein’s motion through the aether caused an aether wind which effectively reduced the light’s speed between Einstein and the mirror to zero as he reached light speed.  To avoid this, Einstein and his mirror would have to be completely shielded from the aether, apart from a small portion which they carried with them in their RF, in order to equate this thought experiment with that of Galileo and his below-deck goings on. 

It seems as though it would have been necessary to have been in possession of the conclusions drawn from Einstein’s thought experiment in order to have been able to formulate and analyse it.         

[jeffreyH]

You cannot consider relative motion without taking into account time dilation. The history of relativity does not only involve Einstein. He was basing his ideas on those of others.
https://en.wikipedia.org/wiki/History_of_special_relativity#Light_constancy_and_the_principle_of_relative_motion

[Bill S (OP)]

Thanks for the link, Jeffrey, I'd love to have time to read all that.  A bit at a time, perhaps.

[CPT ArkAngel]

In newtonian mechanics, nothing has a constant speed. Speed is relative. Maxwell's equations require that light has a constant speed, as later experiments have demonstrated.

Light has a constant speed of light relative to what? Physicists asked. Relative to some aether? According to the Michelson-Morley experiment, the answer was no. It seemed it was absolutely relative to the measurement and therefore, the concept of local time emerged. Later, Einstein has limited the constancy of the speed of light to a locality in spacetime with his General theory of Relativity.

[Bill S (OP)]

Later, Einstein has limited the constancy of the speed of light to a locality in spacetime with his General theory of Relativity.

 

Explanation, please.

[CPT ArkAngel]

In General Relativity, light has a constant speed locally only, due to gravity (different relative gravitational potential=different relative speed of light).

[opportunity]

In General Relativity, light has a constant speed locally only, due to gravity (different relative gravitational potential=different relative speed of light).

And when we are in the middle of nowhere, where gravity has no actual effect because there is no mass anywhere near for light years, then how does this thought experiment play out? The only gravity would be between the face and the mirror here, right? Accordingly therefore approaching the speed of light using a face and a mirror has new conditions in play making them extra massive. We can't just talk about light without considering the mass of the face and the mirror, right? We could use the analogy of a spaceship approaching near light speed and what happens to the occupants as they are brushing their teeth looking in the mirror, right?

[jeffreyH]

You are then in special relativity territory. The only difference is flat spacetime. Time dilation and length contraction still go hand in hand with relative motion. Only now inertial motion dominates in the absence of other external forces.

[opportunity]

I hate blowing smoke, but relative motion to "what" in the middle of nowhere? Yes, flat space time, yet the same flat space-time that is responsible for FTL spatial expansion?

[Colin2B]

relative motion to "what" in the middle of nowhere?

You can always set up a reference frame for comparison. The subject here is a thought experiment making certain assumptions.

Yes, flat space time, yet the same flat space-time that is responsible for FTL spatial expansion?

Where does it say that? Certainly not a statement we would make.
@jeffreyH is refering to SR.

[jeffreyH]

Hydrogen atom A and hydrogen atom B are moving relative to each other in intergalactic space. From the frame of reference of hydrogen atom A, hydrogen atom B is moving and A is stationary and visa versa. So where is the problem?

[PmbPhy]

Bill, ole buddy ole pal. "How you doin?" (A Joey Tribbiani line)

Einstein reasoned that this ran contrary to Galileo’s concept of relativity.  According to which, someone travelling at a constant velocity, without reference to any object outside his or her frame of reference, should not be able to distinguish between being in motion and being stationary.  If one were not able to see one’s reflection in the mirror, in which the reflection was visible when stationary, then one would have an experiment to distinguish between being stationary and being in motion. 

That is incorrect. All one would know is that they were moving relative to the aether. You just argued that above.

If one shone a light across the cabin its speed would be measured as 300,000 kps, whether the ship was moving or not. 

Incorrect for the same reason I gave above.

Until Einstein came to the conclusions that arose out of his thought experiment it would have been reasonable for him to assume that the speed of the light leaving his face would have been measured as 300,000 kps, plus his speed through the aether.  Thus, why would he have concluded that his reflection would vanish from the mirror when he travelled at light speed? 

In his 1905 SR paper Einstein didn't use that argument. Instead he argued that if he was moving along with an EM wave then the wave would be a static EM field and would be in conflict with Maxwell's equations. He also argued that search for an aether has not found one.

It'd be useful for you to read his paper. Have you read it yet?

[Bill S (OP)]

Toting an O2 cylinder around with me, but still breathing, thanks.  Sorry to hear you are having a lot of pain.

I think I’m going to have to take this in “bite sized” chunks.

Einstein reasoned that this ran contrary to Galileo’s concept of relativity.  According to which, someone travelling at a constant velocity, without reference to any object outside his or her frame of reference, should not be able to distinguish between being in motion and being stationary.  If one were not able to see one’s reflection in the mirror, in which the reflection was visible when stationary, then one would have an experiment to distinguish between being stationary and being in motion.

That is incorrect. All one would know is that they were moving relative to the aether. You just argued that above.

Which bit is incorrect?  All of it?

[Colin2B]

Which bit is incorrect?  All of it?

The bit about how he thought about it is. As Pete says, and I’ll quote Einstien “I should observe such a beam of light as an electromagnetic field at rest.” In other words, the wave would seem stationary, but this was not possible according to Maxwell’s equations, which describe the motion and oscillation of electromagnetic fields.
So he either believed Maxwell was correct or the wave was stationary. He chose Maxwell for the same reason we do, the maths is correct and consistent with experiments of Faraday, Gauss etc.

[Bill S (OP)]

Thanks Colin.  I'll go back to my original notes to see what I need to change.  There will probably be other errors linked to this.

[PmbPhy]

Toting an O2 cylinder around with me, but still breathing, thanks.  Sorry to hear you are having a lot of pain.

Thanks bro. I'm one of the silent victims of the opioid scare. Its a deadly one to say the least.

Which bit is incorrect?  All of it?

When you said that you could use a mirror and a beam of light to determine that you're "really" moving. This contradicts what you started out emphasizing. You noted that in the aether theory light moves relative to the aether. Then you later concluded that in that theory you could use a beam of light to say "I'm really moving. What you can actually conclude is not that but that you're moving relative to the aether rest frame.

Think of the aether like you would a body of water and you can't go wrong. And I mean that as stated, i.e. Bill can't go wrong. Many others here too. But we know how wrong others can be. Lol!

[Bill S (OP)]

Thanks Pete.

Bill can't go wrong.

Wanna bet?

[Janus]

Recast Einstein’s thought experiment in terms of Galileo’s below-deck scenario: a sailor would know if the ship were moving at the speed of light because his reflection would vanish.  In Galileo’s scenario all the insects etc. move within the moving cabin exactly as they did in the stationary cabin because the entire system is moving in unison.  If one shone a light across the cabin its speed would be measured as 300,000 kps, whether the ship was moving or not. 

Prior to Einstein’s discoveries it would have been assumed that this was because the speed of the ship was being added to, or subtracted from the speed of light, as would be the case with the various creatures in Galileo’s scenario.  Until Einstein came to the conclusions that arose out of his thought experiment it would have been reasonable for him to assume that the speed of the light leaving his face would have been measured as 300,000 kps, plus his speed through the aether.  Thus, why would he have concluded that his reflection would vanish from the mirror when he travelled at light speed? 

Until he made his later discoveries, would he have had any reason to assume that the light travelling from his face to the mirror would have been measured as anything other than almost 600,000 kps by an observer outside his RF?       

Maxwell's equations had already arrived at the conclusion that

c= 1/sqrt(eu)

where e is the permittivity and  permeabiltiy of the vacuum.

in order for an observer outside of the ship's reference frame to measure the light as traveling at nearly 600,000 kps relative to himself, One or both of these properties would have to be smaller.   But how could the relative motion of the ship change the properties of the vacuum for him?

[Bill S (OP)]

permittivity and  permeabiltiy come under the heading of things I will look at, one day, time permitting - perhaps; so I don't know what influence they would have on the outside observation.

Acknowledging that no craft could travel at c, but bowing to Einstein's thought experiment:  what would be the external observer's measurement of the speed of light within the craft?  My instinctive thought would be that she would measure both as c. which would mean that the light was perceived as stationary relative to the craft.

[Colin2B]

what would be the external observer's measurement of the speed of light within the craft?  My instinctive thought would be that she would measure both as c. which would mean that the light was perceived as stationary relative to the craft.

No, think about it a little longer and I’m sure you will get it.