Q&A: A Question About Understanding Special Relativity
A Question About Understanding Special Relativity
Question
Hello Rabbi Michael,
I’m reading your book Freedom Arrives, and on page 145 you talk about Einstein’s special relativity, and I just can’t manage to understand it.
The example is of a carriage traveling at the speed of light that sends a beam of light upward at the speed of light from point A, and from the perspective of the person sitting in the carriage it reaches point B at the same time, if there is a mirror, etc. A few questions:
1. You wrote that according to Einstein and his colleagues, the speed of light remains constant—so what causes it to move diagonally then? Shouldn’t there have to be some force acting on the light to make it move diagonally?
2. Even if a force is acting, and for some reason the light still keeps the speed of light even though apparently there should have been acceleration here, how is it logically possible that they arrive at the same time? After all, the light traveled a greater distance?
3. And if it’s a fact that this is how it works, how did Einstein arrive at it just by thinking? What logical move is there here? This seems like thinking against common sense, so what exactly is the thought experiment here? If he had empirical information that this is how it happens, then I don’t understand what is so great about the thought experiment.
Thank you,
Answer
Hello.
1. The magnitude is constant, but not the direction (speed, not velocity). If light is given an initial velocity with two components, it moves diagonally. No force is needed—just an initial velocity. Light has no mass.
2. Because time flows faster (that itself is what I showed there). If time flows faster, then it is possible to cover a greater distance “in the same time.”
3. He assumed the principle of the constancy of the speed of light (I don’t remember at the moment whether this was based on reasoning or whether he had some empirical datum). From there one can continue by thought alone. The greatness is that from the constancy of the speed of light he derived the change in time and lengths between systems moving at different speeds.
Discussion on Answer
Indeed. Time flows faster in the frame of the beam and more slowly in the frame of the carriage.
Is it possible to ask a question about the thought experiment involving the moving train and the two lightning bolts striking the ends of the train?
You can try.
Sharp.
Thank you, Rabbi.
Did you mean that for the traveler in the carriage, time passes more slowly (not more quickly as you wrote)? That is, that he has “more time” in one second to cover a greater distance?