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Measuring the One-way Speed of Light? (self.askscience)
submitted 13h ago by Yoschi070
So basically I watched the video from Veritasium about the speed of the light or that it was only measured in both directions and not in one direction(https://youtu.be/pTn6Ewhb27k).
In the video, several failed attempts were also mentioned, e.g. a button that activates two clocks, one close to the laser, one distant to it, and stopping the time when the laser hits the clock, the problem would be that the signal from the button may be unequal in one of the two directions and thus falsify the result.
Now my question, if you have a laser that reacts to a button and a clock that also reacts to a button, you align these two devices and then put a rod on these two buttons and then press the rod, then the clock should start when the laser is sent, since the signal no longer depends on the speed of light in different directions?
Since I'm aware that I'm probably not the first with this idea, why doesn't it work?
In the video, several failed attempts were also mentioned, e.g. a button that activates two clocks, one close to the laser, one distant to it, and stopping the time when the laser hits the clock, the problem would be that the signal from the button may be unequal in one of the two directions and thus falsify the result.
Now my question, if you have a laser that reacts to a button and a clock that also reacts to a button, you align these two devices and then put a rod on these two buttons and then press the rod, then the clock should start when the laser is sent, since the signal no longer depends on the speed of light in different directions?
Since I'm aware that I'm probably not the first with this idea, why doesn't it work?
Astrokiwi 1 points 2h ago
This is actually an example of a very classic thought experiment used in courses on relativity.
The core issue in these thought experiments is that they assume that the rod is perfectly rigid - when you push one end of the rod, the other end moves instantly. However, this in itself would count as faster-than-light motion!
What actually happens is that there is no such thing as a perfectly rigid rod - instead, when you push one end of the rod, you just push the molecules you are touching. Those molecules bump into adjacent molecules and push them too, and those push the next molecules, and so on and so on up the rod. You have a pulse or wave of momentum that travels along the rod at a finite speed until the far end of the rod eventually gets pushed. This turns out to be way slower than the speed of light - it's actually the speed of *sound* within that rod.
So you press a button, which fires the laser, and also pushes the rod in order to press the button that starts the clock. The clock has a laser receiver, and stops counting once it receives the laser pulse. The order of events is then:
1. The button is pushed, the laser is launched, the rod is pushed
2. The laser beam is received by the clock's detector, which hasn't started yet. The "pulse" of "push" is still moving along the rod
3. The rod pushes the other button, starting the clock.
This is the common thing to look out for in anything that appears to get around the rules of relativity, travel through time or whatever - usually there's a hidden way that they're breaking the speed of light, such as assuming a perfectly rigid rod or disc.
The core issue in these thought experiments is that they assume that the rod is perfectly rigid - when you push one end of the rod, the other end moves instantly. However, this in itself would count as faster-than-light motion!
What actually happens is that there is no such thing as a perfectly rigid rod - instead, when you push one end of the rod, you just push the molecules you are touching. Those molecules bump into adjacent molecules and push them too, and those push the next molecules, and so on and so on up the rod. You have a pulse or wave of momentum that travels along the rod at a finite speed until the far end of the rod eventually gets pushed. This turns out to be way slower than the speed of light - it's actually the speed of *sound* within that rod.
So you press a button, which fires the laser, and also pushes the rod in order to press the button that starts the clock. The clock has a laser receiver, and stops counting once it receives the laser pulse. The order of events is then:
1. The button is pushed, the laser is launched, the rod is pushed
2. The laser beam is received by the clock's detector, which hasn't started yet. The "pulse" of "push" is still moving along the rod
3. The rod pushes the other button, starting the clock.
This is the common thing to look out for in anything that appears to get around the rules of relativity, travel through time or whatever - usually there's a hidden way that they're breaking the speed of light, such as assuming a perfectly rigid rod or disc.
dryuhyr 1 points 2h ago
Ok, but what if, say, you had an improbably perfect aim, and you pushed the rod through deep space so that both ends hit their respective buttons at the exact same time?
Yoschi070 [OP] 1 points 1h ago
Alternatively, what if you press the rod in the middle between the two buttons so that the delay between pressing the rod and pressing the buttons is the same for both buttons and the different speed of light, depending on the direction, is not relevant here because the speed of sound is not relativistic.
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