# Einstein's lightening-hit-train thought experiment

In Einstein's version the onboard observer sees the flash from the forward strike first because while the light from the strike was propagating he and the train moved forward toward the location of the source of the flash. This gives the flash a shorter distance to reach the observer than does the flash from the rear strike.

Brian Greene in his book The Elegant Universe presents a variation on Einstein's version. Two kings sitting at the ends of a long table on a moving train are to sign a peace treaty when a lamp at the midpoint of the table is lit. (Neither king wants to be first to sign.) The lamp is lit and both kings sign simultaneously to the satisfaction of those aboard.

But onlookers from the bank see the forward-facing king sign first because while the light was propagating he and the train moved forward toward the lamp, thus shortening the distance the light had to travel to reach the forward-facing king.

My question: If in the Einstein scenario the onboard observer saw the forward flash first, why didn't the onboard observers in Greene's scenario see the forward-facing king sign first? In other words, if in the first case the forward movement of the train shortened the distance between the source of the light and the onboard observer, why, in the second case, didn't the same thing happen so that the onboard observers saw the signings as sequential, not simultaneous?

The two thought experiments are constructed differently. In one (Einstein's) the "stationary" observer on the platform sees the two events as simultaneous because that's the way Einstein set it up. The thought experiment could have been constructed so that the passenger on the train sees the flashes of light at the same time in which case the platform observer would have observed them happening a different times. Greene's variation is set up so that the observers on the train see the light and sign at the same time. What do they care is some observer on a platform is zipping by outside?

Your final sentence is the key one...

'In other words, if in the first case the forward movement of the train shortened the distance between the source of the light and the onboard observer, why, in the second case, didn't the same thing happen so that the onboard observers saw the signings as sequential, not simultaneous?'

You have assumed that the forward movement of the train shorted the distance that light had to travel. That is only true from the perspective of the onlookers on the bank. For the observer on the train, the light has travelled half the length of the train.

The observer on the train sees the two flashes at different times because they were not simultaneous in his frame of reference, not because the light from the two events has travelled different distances in his frame.

The forward movement of the train does not change distances on the train from the perspective of the observer on the train.

"But onlookers from the bank see the forward-facing king sign first because while the light was propagating he and the train moved forward toward the lamp, thus shortening the distance the light had to travel to reach the forward-facing king."

Only if onlookers from the bank see the light traveling at constant speed (independent of the speed of the light source). If, as shown by the Michelson-Morley experiment in 1887, the speed of light does depend on the speed of the light source, onlookers from the bank will see the kings signing simultaneously:

https://en.wikipedia.org/wiki/Emission_theory "Emission theory, also called emitter theory or ballistic theory of light, was a competing theory for the special theory of relativity, explaining the results of the Michelson–Morley experiment of 1887. [...] The name most often associated with emission theory is Isaac Newton. In his corpuscular theory Newton visualized light "corpuscles" being thrown off from hot bodies at a nominal speed of c with respect to the emitting object, and obeying the usual laws of Newtonian mechanics, and we then expect light to be moving towards us with a speed that is offset by the speed of the distant emitter (c ± v)."

http://philsci-archive.pitt.edu/1743/2/Norton.pdf "The Michelson-Morley experiment is fully compatible with an emission theory of light that CONTRADICTS THE LIGHT POSTULATE."

http://books.google.com/books?id=JokgnS1JtmMC Banesh Hoffmann, Relativity and Its Roots, p.92: "There are various remarks to be made about this second principle. For instance, if it is so obvious, how could it turn out to be part of a revolution - especially when the first principle is also a natural one? Moreover, if light consists of particles, as Einstein had suggested in his paper submitted just thirteen weeks before this one, the second principle seems absurd: A stone thrown from a speeding train can do far more damage than one thrown from a train at rest; the speed of the particle is not independent of the motion of the object emitting it. And if we take light to consist of particles and assume that these particles obey Newton's laws, they will conform to Newtonian relativity and thus automatically account for the null result of the Michelson-Morley experiment without recourse to contracting lengths, local time, or Lorentz transformations. Yet, as we have seen, Einstein resisted the temptation to account for the null result in terms of particles of light and simple, familiar Newtonian ideas, and introduced as his second postulate something that was more or less obvious when thought of in terms of waves in an ether. If it was so obvious, though, why did he need to state it as a principle? Because, having taken from the idea of light waves in the ether the one aspect that he needed, he declared early in his paper, to quote his own words, that "the introduction of a 'luminiferous ether' will prove to be superfluous."

• You say: " If, as shown by the Michelson-Morley experiment in 1887, the speed of light does depend on the speed of the light source, " That's not true. The MM experiment showed exactly the opposite. That's the whole basis of special relativity: The measured speed of light is independent of any motion relative to the observer or light source. Feb 2, 2017 at 15:48
• From the first reference you cite. "Although there are still proponents of this theory outside the scientific mainstream, this theory is considered to be conclusively discredited by most scientists." Feb 2, 2017 at 16:14
• "this theory is considered to be conclusively discredited by most scientists" Still, the fact remains: In 1887 (prior to FitzGerald and Lorentz advancing the ad hoc length contraction hypothesis) the Michelson-Morley experiment UNEQUIVOCALLY confirmed the variable speed of light predicted by Newton's emission theory of light and refuted the constant (independent of the speed of the light source) speed of light predicted by the ether theory and later adopted by Einstein as his special relativity's second postulate. Feb 2, 2017 at 16:56
• Pentcho: Many thanks for your references to the emission theory. This is new to me. Naturally I thought the MM experiments showed lightspeed independent of any motion of the source. However, my question is why the onboard observers didn't see the forward-facing king sign first. This would be consistent with Einstein's explanation that the onboard observer saw the forward flash first because it had a shorter distance to travel. It's the apparent inconsistency that bothers me. Feb 3, 2017 at 16:31