Note: Astronomer from the Royal Military Academy of Canada
Questions:
I am a college graduate at the University of California, Berkeley, majoring in mathematics and economics. However, I enjoy reading a lot of science-related articles outside of class, so I came here.
In any case, my question here is – about "How did the universe expand faster than the speed of light during its expansion?" The answer to the question – the answer to this question was written by Kristen Speckens. In her answer, she said, information cannot travel faster than the speed of light. I'm not quite sure exactly what information means, but I have a thought experiment that can't be reconciled with the above assertion that information travels faster than the speed of light.
Suppose I live on Earth and I have a friend who lives on Pluto. If I wanted to send him a message, I could send it by radio, but the information would not reach him faster than the speed of light, and it would take a few minutes for the speed of light to reach Pluto from Earth. However, suppose I have a very, very long stick that stretches all the way from Earth to Pluto. If I wanted to send a message to my friend, I could use this stick to type out the Morse code on the surface of Pluto, so that the message could be transmitted almost immediately. In this way, I transmit information much faster than the speed of light.
What I want to ask is is there any problem with this experiment? How did you reconcile this experiment with the claims above?
Reply:
Unfortunately, the idea of you tapping with a stick doesn't allow you to communicate with your friends faster than the speed of light. When you strike a stick, the result of the "knock" is a wave that travels from one end of your hand to the other through the stick. To be able to see this, try the following experiment: Find a friend, stand a few feet away from you, each holding one end of the rope, and close your eyes. Ask your friend to shake one end of the rope and your task is to tell him when you feel the shaking. When you're only a few feet apart, you know what it feels like to be "instantaneous," and that's because the waves your friend sends travel along the rope much faster than a few feet a second.
So let's try the same thing next, but this time on both sides of the football pitch. Now, it takes a few seconds for the wave to reach you, so there's a delay between your friend's shake and your reaction. Then if you make a rope denser and stronger, then you get the stick in the example you gave: the physics is the same between them. So, it will take a certain amount of time to get from Earth to Pluto. So what is the maximum speed of the wave when it passes through the stick? The speed of light, of course!
We can try to experiment with faster communication speeds than the speed of light in a more ingenious way: Suppose your friend on Pluto has a very sensitive probe that can detect tiny gravitational changes caused by Earth on Pluto. To communicate with him, you developed a technique that could change the distance between two planets. At this point your friend, who has been staying on Pluto, detects a change in gravity on the meter, and then you continue to "swing" the Earth to "knock out" the combination of morse codes, is that instantaneous?
Obviously the answer is no: gravity also travels at the speed of light! Although not yet directly detected, waves carrying a gravitational field around an object (gravitational waves) were shown to propagate at the speed of light in an experiment last October that involved the curvature of quasar's light around Jupiter (a result that is still somewhat controversial, but see the detailed answer to this article).
From the material you hit the rod to gravitational waves, and even when light isn't used for communication, for reasons that people haven't figured out yet, the speed of light seems to be a fundamental limit of every branch of physics. That's why I used the word "message" in my previous answer, because there are so many ways to send messages than simply sending optical signals.
And, as long as you don't send the information out together, it doesn't hurt to have objects moving faster than the speed of light. Finally, to give an example, imagine you're in a soccer field packed with people who are doing "wave" sports. This phenomenon arises because you also stand up when you see the person next to you standing up, so that a wave spreads around the stadium. If you do "waves" this way, the fastest it can travel in the stadium is the speed of light, because you have to stand up according to the prompts of the person next to you (or press a button, or shout, or stand for a little longer), but this can only travel at the speed of light. However, suppose the stadium manager wants to make a truly spectacular show and give everyone in the audience a piece of paper with the exact time of standing.
In principle, you can let the waves move as fast as you want, just as you can tell people to get up at any time. So with the precise coordination of time, you can make the wave speed faster than the speed of light. Does this violate the most basic laws of physics? No, because there is no information in the orchestrated wave that results from giving everyone a piece of paper: you give them information on the paper beforehand (when to stand up). In extreme cases, you can tell everyone on paper to stand at the same time because you tell them in advance when to stand, and "infinite" fast waves don't violate physics at all. In "real" waves, when you get a cue from the person next to you, the information about when to stand can only travel at the speed of light.
Suppose the same "stick" (between Earth and Pluto) is pushed and pulled (with Morse code) instead of tapping... It's solid, doesn't the whole "stick" move at the same time? Obviously, the stick does not move at the speed of light, but (in my opinion) the information being sent/received travels at the speed of light.
Unfortunately, your idea also doesn't solve the problem of limited speed of light! The next time you're in the pool (or near the water), stand a foot or two from the edge of the pool and "push" the water with your hands; The water being "pushed" doesn't reach the pool wall right away, but takes some time to get there. Essentially, your "push" action triggers a wave in the water that brings the thrust to the edge. The same thing happens with solid objects, and although you don't notice it, a "thrust" reaches the other side of the stick through a wave that resembles water, and the more "solid" you make the stick, the faster that wave goes. However, as far as we know, the maximum speed of a wave in a solid is the speed of light. So even if you push and pull the stick instead of tapping it, you can still only transmit information at the speed of light.
Trying to communicate faster than the speed of light with a long stick is a fairly common idea, but it doesn't work.
BY: Kristine Spekkens
FY: Landodo
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