6 ms·

Speed of light is not really just speed of light, but universal speed limit of causality. Light (electromagnetic waves just happens to be one of the few things

Speed of light is not really just speed of light, but universal speed limit of causality. Light (electromagnetic waves just happens to be one of the few things capable of hitting that limit. Even the effect of gravity is limited to speed of light. PBS Space time has a good video on this - https://www.youtube.com/watch?v=msVuCEs8Ydo&t=5s https://www.youtube.com/watch?v=msVuCEs8Ydo&t=5s. So to answer your question, no it is not possible for anything to affect us any faster than the universal speed limit of causality (that light happens to be able to hit).

internet_person 7y ago
There is at least one known exception: neutrinos from a supernova. (Don't get too excited, this doesn't contradict Einstein, as we'll see.) In the hot, dense core of a supernova, photons will scatter around for a while before escaping. Neutrinos, which interact much less often than photons, and travel also more or less at the speed of light, get out basically right away. The difference can be as much as a few hours! Of course it's the high-energy photons that are more dangerous, so the neutrinos are really more like an early-warning system. See e.g.: https://en.wikipedia.org/wiki/SN_1987A https://en.wikipedia.org/wiki/SN_1987A
- oAlbe 7y ago
  That's still not an exception though. Neutrinos escape first by virtue of the fact that they don't interact with matter and don't bounce around, while light does. They aren't travelling faster than light, just travelling a shorter distance.
  - xmprt 7y ago
    The question wasn't about how fast neutrinos can travel but whether we can detect something before we see it.
    - onetimemanytime 7y ago
      So x event happened 550,000 years ago and the radiation from it might kill us. You say that we may get a warning from neutrinos before the harmful stuff kills us?
    - chrischen 7y ago
      But we detect things with particles other than just light.
- saberdancer 7y ago
  This is very misleading. In fact, neutrinos are not going faster than speed of light (c), they are going faster than photons that are emitted from the supernova. Not just that, but lights when it passes through a medium slows down depending on it's refractive index (speed of light in water is about 225 000km/h while in vacuum it's 300 000km/h). This is important distinction as recently we had an uproar when it looked like neutrinos are slightly faster than c (in the end, it was a measurement error). You can observe effects of particles going faster than speed of light in a medium if you look at photographs of Cherenkov radiation.
  - Erlich_Bachman 7y ago
    That comment never wrote that neutrinos were faster than the speed of light?
    - pavs 7y ago
      No, but he did write "There is at least one known exception", the misleading part.
  - Lukas_Skywalker 7y ago
    I'm sure it's a typo, but for others: speed of light is 300'000 m/s in vacuum, not km/h.
    - Cyberspy 7y ago
      It's not a typo - it's 299,792,458 m/s - approx 300,000,000 m/s or 300,000 km/s
  - lmilcin 7y ago
    The correct way to say it is that the speed of light is the speed of causality only in vacuum. In any medium the speed of light is slower but the causality or other fields (for example gravity field) that are not impeded by matter can and will propagate faster than the speed of light.
- throwaway2048 7y ago
  Its possible to exceed the local speed of light in many classes of material. https://en.wikipedia.org/wiki/Cherenkov_radiation https://en.wikipedia.org/wiki/Cherenkov_radiation
lenkite 7y ago
The speed of shadow can be faster than light though. https://physics.stackexchange.com/questions/335537/can-a-shadow-move-faster-than-the-speed-of-light/335573 https://physics.stackexchange.com/questions/335537/can-a-sha...
- anonytrary 7y ago
  I think those answers are actually wrong. When the object casting the shadow moves, the shadow remains in the same place for an observer in it until the light from the source reaches the observer inside the shadow. I know that's a weird explanation, so consider: t0: S~~~>O U (shadow exists) t1: S~~~~~~> U (shadow exists) t2: S~~~~~~~~~>U (shadow !exist) Where "S" is a light source, "O" is an opaque object, "~" are photons traveling to the right, "U" is the observer, and "t0", "t1", and "t2" are times (increasing). At time "t0", "U" thinks he's in the shadow. At time "t1", "U" thinks he's in the shadow. At time "t0", "U" thinks he isn't in the shadow, since the photons are now hitting him. A similar calculation/thought-experiment can be done for shadows with "angular momentum", in case you think the tangential velocity of the shadow will exceed the speed of light.
  - lenkite 7y ago
    Thanks for that nice diagram! What happens when the light source moves and the shadow is far bigger than the object casting it ? Wouldn't the speed of the shadow on the surface be faster than the speed of light ?
    - anonytrary 7y ago
      For example, when a light source is super close to an object and the shadow gets super big really far away, and the light source moves? t0: SO U1 (U1 in shadow) t1: S~~~~~~~~~~>U1 (shadow leaves top first) O~~~~~~> . ~> . . . ' U2 Dotted line is the path of a fixed point on the shadow during the time S moves. I didn't do the precise math, but I'm pretty sure the tangential velocity of the shadow along the dotted line won't be greater than the speed of light. The curvature of the "wave front" formed by the tips of the arrows ">" above will be lesser than the dotted line curvature, so the photons near the top of the diagram hit the dotted edge before the ones towards the bottom. This is because the source, S, takes time to move away from O. Note that the wavefront formed by the photons moves radially outward from S, but ascii art is limiting.
andrewjrhill 7y ago
TIL. Very interesting.
stronglikedan 7y ago
> can the reverse happen with space events where light reaches you then something else? I read that as: Can something come along and snuff us out after we've observed the light wave component of the event, such as a shock wave of some sort?
- oarabbus_ 7y ago
  >I read that as: Can something come along and snuff us out after we've observed the light wave component of the event, such as a shock wave of some sort? That is the only way it can happen.
abtinf 7y ago
My (limited) understanding is that the limit is only applicable to movement through space. The expansion of the universe is not bound by the limit, as it is expansion of space itself. Thus it seems incorrect to talk about a “universal speed limit of causality”. Am I missing an essential concept?
- dodobirdlord 7y ago
  > Thus it seems incorrect to talk about a “universal speed limit of causality”. Am I missing an essential concept? Probably. Why would points becoming more distant due to expansion of space be a counterexample to the speed of light being a limit on the speed of causal propagation? Spatial expansion doesn't move anything, so it can't propagate information.
  - marcus_holmes 7y ago
    It doesn't move anything, but it does mean that things that are now far away used to be closer, and therefore had the ability to affect us, but now don't. Not that it matters, because the speed of light is still the limit: if they could influence us when they were closer, that would have happened by now.
  - aeorgnoieang 7y ago
    > Spatial expansion doesn't move anything, so it can't propagate information. It certainly moves matter-energy in different regions of space with respect to one another.
LifeLiverTransp 7y ago
Everytime i here this- it sounds like a cheesy speed limit introduced by a post-grad into a late-night hacked together simulation for which s/h/it couldnt aquire enough server power for real parallel execution.. we should make it our job, to label this project as a "F" for all to see.. Ignore me.

internet_person 7y ago

There is at least one known exception: neutrinos from a supernova. (Don't get too excited, this doesn't contradict Einstein, as we'll see.) In the hot, dense core of a supernova, photons will scatter around for a while before escaping. Neutrinos, which interact much less often than photons, and travel also more or less at the speed of light, get out basically right away. The difference can be as much as a few hours! Of course it's the high-energy photons that are more dangerous, so the neutrinos are really more like an early-warning system. See e.g.: https://en.wikipedia.org/wiki/SN_1987A https://en.wikipedia.org/wiki/SN_1987A

oAlbe 7y ago
That's still not an exception though. Neutrinos escape first by virtue of the fact that they don't interact with matter and don't bounce around, while light does. They aren't travelling faster than light, just travelling a shorter distance.
- xmprt 7y ago
  The question wasn't about how fast neutrinos can travel but whether we can detect something before we see it.
  - onetimemanytime 7y ago
    So x event happened 550,000 years ago and the radiation from it might kill us. You say that we may get a warning from neutrinos before the harmful stuff kills us?
  - chrischen 7y ago
    But we detect things with particles other than just light.
saberdancer 7y ago
This is very misleading. In fact, neutrinos are not going faster than speed of light (c), they are going faster than photons that are emitted from the supernova. Not just that, but lights when it passes through a medium slows down depending on it's refractive index (speed of light in water is about 225 000km/h while in vacuum it's 300 000km/h). This is important distinction as recently we had an uproar when it looked like neutrinos are slightly faster than c (in the end, it was a measurement error). You can observe effects of particles going faster than speed of light in a medium if you look at photographs of Cherenkov radiation.
- Erlich_Bachman 7y ago
  That comment never wrote that neutrinos were faster than the speed of light?
  - pavs 7y ago
    No, but he did write "There is at least one known exception", the misleading part.
- Lukas_Skywalker 7y ago
  I'm sure it's a typo, but for others: speed of light is 300'000 m/s in vacuum, not km/h.
  - Cyberspy 7y ago
    It's not a typo - it's 299,792,458 m/s - approx 300,000,000 m/s or 300,000 km/s
- lmilcin 7y ago
  The correct way to say it is that the speed of light is the speed of causality only in vacuum. In any medium the speed of light is slower but the causality or other fields (for example gravity field) that are not impeded by matter can and will propagate faster than the speed of light.
throwaway2048 7y ago
Its possible to exceed the local speed of light in many classes of material. https://en.wikipedia.org/wiki/Cherenkov_radiation https://en.wikipedia.org/wiki/Cherenkov_radiation

lenkite 7y ago

The speed of shadow can be faster than light though. https://physics.stackexchange.com/questions/335537/can-a-shadow-move-faster-than-the-speed-of-light/335573 https://physics.stackexchange.com/questions/335537/can-a-sha...

anonytrary 7y ago
I think those answers are actually wrong. When the object casting the shadow moves, the shadow remains in the same place for an observer in it until the light from the source reaches the observer inside the shadow. I know that's a weird explanation, so consider: t0: S~~~>O U (shadow exists) t1: S~~~~~~> U (shadow exists) t2: S~~~~~~~~~>U (shadow !exist) Where "S" is a light source, "O" is an opaque object, "~" are photons traveling to the right, "U" is the observer, and "t0", "t1", and "t2" are times (increasing). At time "t0", "U" thinks he's in the shadow. At time "t1", "U" thinks he's in the shadow. At time "t0", "U" thinks he isn't in the shadow, since the photons are now hitting him. A similar calculation/thought-experiment can be done for shadows with "angular momentum", in case you think the tangential velocity of the shadow will exceed the speed of light.
- lenkite 7y ago
  Thanks for that nice diagram! What happens when the light source moves and the shadow is far bigger than the object casting it ? Wouldn't the speed of the shadow on the surface be faster than the speed of light ?
  - anonytrary 7y ago
    For example, when a light source is super close to an object and the shadow gets super big really far away, and the light source moves? t0: SO U1 (U1 in shadow) t1: S~~~~~~~~~~>U1 (shadow leaves top first) O~~~~~~> . ~> . . . ' U2 Dotted line is the path of a fixed point on the shadow during the time S moves. I didn't do the precise math, but I'm pretty sure the tangential velocity of the shadow along the dotted line won't be greater than the speed of light. The curvature of the "wave front" formed by the tips of the arrows ">" above will be lesser than the dotted line curvature, so the photons near the top of the diagram hit the dotted edge before the ones towards the bottom. This is because the source, S, takes time to move away from O. Note that the wavefront formed by the photons moves radially outward from S, but ascii art is limiting.

andrewjrhill 7y ago

TIL. Very interesting.

stronglikedan 7y ago

> can the reverse happen with space events where light reaches you then something else? I read that as: Can something come along and snuff us out after we've observed the light wave component of the event, such as a shock wave of some sort?

oarabbus_ 7y ago
>I read that as: Can something come along and snuff us out after we've observed the light wave component of the event, such as a shock wave of some sort? That is the only way it can happen.

abtinf 7y ago

My (limited) understanding is that the limit is only applicable to movement through space. The expansion of the universe is not bound by the limit, as it is expansion of space itself. Thus it seems incorrect to talk about a “universal speed limit of causality”. Am I missing an essential concept?

dodobirdlord 7y ago
> Thus it seems incorrect to talk about a “universal speed limit of causality”. Am I missing an essential concept? Probably. Why would points becoming more distant due to expansion of space be a counterexample to the speed of light being a limit on the speed of causal propagation? Spatial expansion doesn't move anything, so it can't propagate information.
- marcus_holmes 7y ago
  It doesn't move anything, but it does mean that things that are now far away used to be closer, and therefore had the ability to affect us, but now don't. Not that it matters, because the speed of light is still the limit: if they could influence us when they were closer, that would have happened by now.
- aeorgnoieang 7y ago
  > Spatial expansion doesn't move anything, so it can't propagate information. It certainly moves matter-energy in different regions of space with respect to one another.

LifeLiverTransp 7y ago

Everytime i here this- it sounds like a cheesy speed limit introduced by a post-grad into a late-night hacked together simulation for which s/h/it couldnt aquire enough server power for real parallel execution.. we should make it our job, to label this project as a "F" for all to see.. Ignore me.