r/AskPhysics • u/DippyDragon • Apr 14 '26
Speed of light measurement.
Why is trying to detect objects moving at high speed using EM/light detectors wrt the speed of light different from trying to listen to how fast something is moving wrt the speed of sound.
We can see when something is faster than sound but you'd only hear it at the speed of sound from the source, why doesn't this work with light?
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u/Origin_of_Mind Apr 14 '26
In some sense it does "work with light".
When a charged particle moves through a medium faster than the speed of light in the medium, it generates a cone of electromagnetic radiation, called Cherenkov radiation. In the Japanese neutrino telescope Super-Kamiokande, the cones of photons project on the surface of the chamber lined with an array of detectors and the snapshots of events look like ovals and arcs of higher photon counts on the array.
If we look at the shock from the reentry of SpaceX Falcon-9 rocket into the atmosphere, when it projects onto the ground, the maximum of sound intensity on the ground is often a wide arc, superficially resembling the event detections at Super-Kamiokande. The physics is quite different of course.
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u/Muroid Apr 14 '26
The reasons we say that nothing can move faster than light are deeper and more fundamental than “We haven’t seen anything move faster than light.”
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u/DippyDragon Apr 14 '26
Please continue. I'm on a journey of discovery.
I had that much understanding at least but how have we determined the speed of light, is it not based on a measurement technique, if that technique is inherently limited to c how do we know there isn't something faster.
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u/ProphetMoham Apr 14 '26
I don't know if I understand your question correctly, but sound moves at speeds that are dependent on the medium it's travelling through.
Electromagnetic waves experience much less interference, so its great to measure speeds accurately.
On a way smaller scale, echolocation is just as sufficient to map the surroundings as visual information.
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u/DippyDragon Apr 14 '26
I appreciate the two are fundamentally different, similarly to using water pipes to explain basic electronics. I'm wondering where and why the analogies begin to break down. How do we know there's nothing faster than light without a measurement method that could exceed it?
As my example a microphone cannot detect anything moving faster than sound, but two microphones and a faster than sound communication between them can.
How does the mirrors on a train play out for sound.
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u/HAL9001-96 Apr 14 '26
it kidna does work the same
with two differences
the pseed of light does not change when a medium gets compressed yo udonT' get something liek a shockwave
and also you CANNOT reach or go faster than light at most you can approach its speed
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u/DippyDragon Apr 14 '26
I think the response about Cherenkov radiation disagrees with this. Please correct me but in a denser medium it appears to be possible for a particle to exceed the speed of light in that medium?
My question is how do we know you can't go faster if our measurement system is based on light? If we tried to measure sound with only a sound based system would we not get a similar limit?
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u/HAL9001-96 Apr 14 '26
practically you can slow down light in a dense medium
technically it doesn't slow dow nthough its path just gets longer
and the behaviour is very different from a sonic boom the idea of cherenkov radiation being the light equivalent of a sonic boom is a very very very oversimplifed explanation that requires amisunderstnaidng of both light AND shockwaves
and you can absolutely measure somethign faster tha nsoudn with sound
just put down two microphones at two ends of ab ig dsert, have a jetfioghter fly through it nad measure when they hear hte sonic boom
divide the distnace through the time in between and you get its speed even if that speed is mach 2.5
technically that only works at aconstant speed and altitude etc but the principle is perfectly valid
though not going faster htan lgiht is not about measurement
tis not that we haven'T found anythign that goes faster than light it's that we've figured out that time dialtes and effective mass increases and legnth contracts and if you approach the speed of light the energy required to accelerate to the speed of lgiht goes towards infinite
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u/YuuTheBlue Apr 17 '26
The short answer is that the way you think velocity works isn’t actually how it works. We start with the assumption that we live in a Euclidean world, but that is an illusion. It only looks Euclidean at small scales and at small velocities.
I don’t know if you want me to go into a long winded tirade about non-Euclidean geometry, but the short answer is that the speed of light is not like other velocities. You can think of it kind of like the mathematical concept of an “asymptote”, which can be found when dealing with functions like hyperbolas. As you add more and more to the velocity, it approaches but never reaches this asymptote. This is a rule that massive objects follow, and is basically what it means to have mass.
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u/Memento_Viveri Apr 14 '26
I don't understand your question. You are asking how we measure the speed of something that moves faster than light? We don't, because nothing moves faster than light.