In this article, I will once again break stereotypes and discuss the speed of light propagation. The renowned Sir Al baptized it c (from the English constant) and stated that it is 299,792,458 m/s in a vacuum as it travels from point A to point B. But have we measured it? If it's not so, what do we do?
Firstly, let me tell you that at this stage, the speed of light propagation doesn't change much for us. However, if in the coming centuries we start colonizing space, it will play a significant role. But possibly, our perception of the world around us might change.
Speed is a vector quantity, meaning it has both magnitude and direction. So does direction play a role in its value? For example, a person descends a slope much faster than they ascend it.
To measure light, we would need a laser and two clocks with photodetectors. We would shine the laser, the beam would pass through each clock, and we would see the chronological moments it passed through. Then we would divide the distance between them by the time measured by the clocks and find the speed. Correct? No. Why not, though?
Because the clocks would need to be perfectly synchronized. This can be achieved, but if one clock starts moving to reach point B, due to its motion, the measured time will change. If we place both clocks at point M and move them to points A and B, since the speed can be affected by direction, they might measure different times. And if we bring them back to M to check, any deviations due to their opposite motion compared to before may have been canceled out. Therefore, we cannot measure the speed in one direction; what we measure, though, is the speed for light to go somewhere and come back.
To avoid clock synchronization, we can measure how much time it will take for the beam from the laser to pass through our clock, reflect on a mirror, and return to the clock. This experiment has been conducted, and we have measured that the average speed resulting from the time it took for light to pass through our clock twice is c. However, this does not rule out the possibility that it may have traveled at a speed of c/2 in one direction and at infinite speed in the other...
Let's imagine another experiment that might make more sense. I've traveled to Mars. You're here on Earth. I arrive on Mars, grow potatoes, but I don't have a clock, and at some point, you send me a message saying, "Thinking of you. Time sent: 12:00." I know the distance between Earth and Mars and calculate that since light travels at a speed of c, it took 10 minutes for the message to arrive, so I adjust my clock and reply, "Thinking of you too. Time sent: 12:10." You receive the message and check your watch, and the time of receipt is 12:20, so everything is fine.
If light didn't propagate at the same speed in all directions, what would change in this experiment? Let's reconsider.
You send me a message saying, "Thinking of you. Time sent: 12:00." If light travels at a speed of c/2, it would take 20 minutes for the message to arrive. Assuming incorrectly that it travels at c speed in all directions, I adjust my clock and reply, "Thinking of you too. Time sent: 12:10." However, the actual time of sending the message is 12:20. You receive the message, check your watch, and the time of receipt is indeed 12:20, as it might move at infinite speed in this direction, so everything seems fine. However, in reality, we would lose 10 minutes in our communications with Mars.
This affects our perception of the universe because if light traveling in a certain direction moves at a much greater speed, it leads us to see the present of the star we are observing rather than its past (which we believe we are seeing). Conversely, if it moves at a slower speed, we see an even more distant past of the star (than what we believe we are seeing).
The only thing we can say with certainty is that light on a round trip seems to move at an average speed of c. However, how fast it travels on the outbound and inbound journeys is still a mystery to us...
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