5 ms·

c is technically the speed of causality. The speed at which perturbations in the fields that make up the universe propagate. The speed of light is a consequenc

c is technically the speed of causality. The speed at which perturbations in the fields that make up the universe propagate. The speed of light is a consequence of this, not the cause. Calling c the "speed of light" is putting the cart before the horse. Photons (being massless) have 100% of their velocity in the spatial dimensions and no velocity in the time dimension. They move at the maximum speed that any change in the electrostrongweak force can propagate in our universe because they are not free to do anything else. From a photon's POV a trip across the entire universe happens instantaneously - taking no time whatsoever.

belter 2y ago
> c is technically the speed of causality. How does this notion reconcile with seemingly instantaneous quantum phenomena like spooky action at a distance?
- jlawson 2y ago
  Quantum entanglement ("spooky action at a distance") doesn't allow for communication, so there's no causality between the particles.
  - belter 2y ago
    Thanks for the answer. I had forgotten about the No-communication theorem. This led me into another rabbit hole :-) of why c is 299,792,458 m/s and for example not 499,792,458 m/s or some other value, and the fundamental constants that bring this value. [1] Is there a current theory that tries to justify those constants that bring the current value of c ? Are those values the ones that must be, for the current Universe to be feasible? [1] https://arxiv.org/abs/hep-ph/0205340 https://arxiv.org/abs/hep-ph/0205340
    - pixl97 2y ago
      I'm not sure if there is a good theory on why it ends up being this speed, but if any of the constants were even slightly different we wouldn't be in this universe to talk about it. https://en.wikipedia.org/wiki/Fine-tuned_universe https://en.wikipedia.org/wiki/Fine-tuned_universe
    - dcow 2y ago
      Veritasium has a few recent videos on Action. In one of them I believe he shows speed of light expressed as one unit of action, which implies that it’s the highest “resolution” any propagation can resolve at based on the fundamentally smallest path integral anything can traverse, or something.
    - wisty 2y ago
      The speed of light is 1c and is a fundamental constant. Humans choose to express it in metres and seconds. A meter is huge if you compare it to the Planck length. Humans are pretty big creatures compared to fundamental particles, so we have a big basic unit of length. But seconds are gargantuan, because humans are absolutely glacial if you compare them to the time it takes light to travel a Planck length. It's like a continental plate asking why humans zip around so fast.
      - belter 2y ago
        > The speed of light is 1c and is a fundamental constant. Yes, but why is the value that it is and not higher or lower? From my (basic) research it seems, it could be plus or minus 20% a different value, and the current Universe would still be feasible.
        marcellus23 2y ago
        Well, it has to be _some_ value, right? I mean, if we were in a universe where it was 20% greater, you would be asking the same questions.
        belter 2y ago
        No, the point is that if it was 20% bigger we would not be here to ask those questions :-) Stars fusion or weak atomic interactions would not work. Maybe Hari Seldon decided....
        marcellus23 2y ago
        But you said in your comment that "it could be plus or minus 20% a different value, and the current Universe would still be feasible"
- xenadu02 2y ago
  That touches on a long-running very complicated debate around quantum mechanics, hidden variables, spooky action at a distance, no-communication, etc. For various quantum effects that seem to be paradoxes by classical physics what is happening "under the covers"? Does the delayed choice experiment really send information backwards in time? Even if it appears to do so if we can't at least send information back in time with that mechanism isn't it just sophist philosophy at that point? For my part I'm not smart enough to claim to have answers to anything but my intuition is there are no quantum paradoxes. Delayed choice does not send anything back in time. We don't experience quantum phenomena at the macro scale so our intuition and reasoning are ill-suited to thinking about it. That easily leads us to incorrect conclusions.
chrisweekly 2y ago
That sounds like an interesting theory; is it yours, or do you have any citations / references you could share?
- ameetgaitonde 2y ago
  He's just describing Einstein's Theory of Special Relativity.
  - chrisweekly 2y ago
    Huh. I'm admittedly no astrophysicist, but I don't recall encountering this "photons have no velocity in the time dimension" before. Maybe I'm one of today's lucky 10,000[1]. 1. https://xkcd.com/1053/ https://xkcd.com/1053/
    - bee_rider 2y ago
      But, it is sort of common to observe the somewhat trippy fact that, unlike matter, photons don’t experience time passing, right? (Although, I will be honest, I could not ever really grok relativity like a physicist. However, I can enjoy the Lorentz factor going to infinity as v goes to c).
      - da_chicken 2y ago
        From my understanding, it's more correct to say that time is undefined for massless objects. Through special relativity, our understanding of mass, gravity, and spacetime are linked. If something has no mass, then special relativity can't describe how gravity affects it's spacetime reference. Remember, however, that this explanation is based on the mathematics that explain the observations we've made or theorized. Just as the map is not the territory, the math is not the universe.
        deleted 2y ago
        [deleted]
    - 2y ago
762236 2y ago
[flagged]
stared 2y ago
It sounds like a really nice (and the shortest possible) backronym. https://en.wikipedia.org/wiki/Backronym https://en.wikipedia.org/wiki/Backronym
- aatd86 2y ago
  Isn't it supposed to be c for celerity? Edit.. Oh I see. Nvm
layer8 2y ago
Causation is a bit of a troublesome concept in physics: https://plato.stanford.edu/entries/causation-physics/ https://plato.stanford.edu/entries/causation-physics/ I wouldn’t use causality as a foundational concept ontologically. What’s true is that c doesn’t just apply to the electromagnetic field, but to all fundamental physical fields.
- uecker 2y ago
  There is a more precise concept of causality in physics that is very much fundamental part of all modern theories.
  - layer8 2y ago
    And what concept would that be?
    - uecker 2y ago
      Einstein causality, i.e that no physical effect propagates faster than light. In QFT it it is modeled by the rule that space-like separated measurements commute.
      - layer8 2y ago
        That merely equates the term "causality" to propagating waves. It doesn't introduce any new concept. Ontologically, there are only quantum fields in space-time. The physical laws governing them imply from their mathematics that they cannot contain any propagations faster than c. But attaching the term "causality" to it doesn't really add anything here.
        uecker 2y ago
        This does not equate causality with propagating waves. I also did not claim it is a new concept. Instead I said physics has a precise and fundamental concept of causality.
phkahler 2y ago
Sure, but why did Einstein call the speed of light c?
- layer8 2y ago
  I think the article explains that.
auntienomen 2y ago
"Photons (being massless) have 100% of their velocity in the spatial dimensions and no velocity in the time dimension" I kinda see what you're trying to say, but these words aren't a particularly good match for the math in special relativity. To an observer, any photon's velocity 4-vector looks like V = (c,c,0,0). That's a "c" in the spatial direction the photon's moving, and a "c" in the time direction. So, plenty of velocity in the time direction. What is zero is the _proper time_ along a photon's trajectory through space & time. An observer who's co-moving with the photon (call me if you ever meet one, we can write a paper together) would see a) the photon holding still and b) no time passing.
- uecker 2y ago
  I can't recommend such a co-author. Doesn't get anything done.
phire 2y ago
And when you think about c as the speed of causality, it becomes a lot more obvious why FTL travel/communicate is (almost certainty) impossible without breaking causality.... You are going faster than causality, of course it breaks. The other option is to break relativity, which is what most science fiction media does, often accidentally.
eggn00dles 2y ago
special relativity is undefined at v = c. so no from a photons point of view the trip doesn’t happen in an instant nor does the length of the universe shrink to zero. there is no definition for a photons point of view. time dilation and length contraction only apply when comparing a moving frame of reference to one that is at rest and photons have no rest frame.
xarope 2y ago
is that a highly technical way of saying WE think c is the speed of light, because that's all WE can measure?
rurban 2y ago
No, there is no causality anymore in quantum physics, since the EPR paradoxon was disproved. c stands for the latin for for speed of course. causality is immediate, a higher speed than c. because it's logical, not measurable.

belter 2y ago

> c is technically the speed of causality. How does this notion reconcile with seemingly instantaneous quantum phenomena like spooky action at a distance?

jlawson 2y ago
Quantum entanglement ("spooky action at a distance") doesn't allow for communication, so there's no causality between the particles.
- belter 2y ago
  Thanks for the answer. I had forgotten about the No-communication theorem. This led me into another rabbit hole :-) of why c is 299,792,458 m/s and for example not 499,792,458 m/s or some other value, and the fundamental constants that bring this value. [1] Is there a current theory that tries to justify those constants that bring the current value of c ? Are those values the ones that must be, for the current Universe to be feasible? [1] https://arxiv.org/abs/hep-ph/0205340 https://arxiv.org/abs/hep-ph/0205340
  - pixl97 2y ago
    I'm not sure if there is a good theory on why it ends up being this speed, but if any of the constants were even slightly different we wouldn't be in this universe to talk about it. https://en.wikipedia.org/wiki/Fine-tuned_universe https://en.wikipedia.org/wiki/Fine-tuned_universe
  - dcow 2y ago
    Veritasium has a few recent videos on Action. In one of them I believe he shows speed of light expressed as one unit of action, which implies that it’s the highest “resolution” any propagation can resolve at based on the fundamentally smallest path integral anything can traverse, or something.
  - wisty 2y ago
    The speed of light is 1c and is a fundamental constant. Humans choose to express it in metres and seconds. A meter is huge if you compare it to the Planck length. Humans are pretty big creatures compared to fundamental particles, so we have a big basic unit of length. But seconds are gargantuan, because humans are absolutely glacial if you compare them to the time it takes light to travel a Planck length. It's like a continental plate asking why humans zip around so fast.
    - belter 2y ago
      > The speed of light is 1c and is a fundamental constant. Yes, but why is the value that it is and not higher or lower? From my (basic) research it seems, it could be plus or minus 20% a different value, and the current Universe would still be feasible.
      - marcellus23 2y ago
        Well, it has to be _some_ value, right? I mean, if we were in a universe where it was 20% greater, you would be asking the same questions.
        belter 2y ago
        No, the point is that if it was 20% bigger we would not be here to ask those questions :-) Stars fusion or weak atomic interactions would not work. Maybe Hari Seldon decided....
        marcellus23 2y ago
        But you said in your comment that "it could be plus or minus 20% a different value, and the current Universe would still be feasible"
xenadu02 2y ago
That touches on a long-running very complicated debate around quantum mechanics, hidden variables, spooky action at a distance, no-communication, etc. For various quantum effects that seem to be paradoxes by classical physics what is happening "under the covers"? Does the delayed choice experiment really send information backwards in time? Even if it appears to do so if we can't at least send information back in time with that mechanism isn't it just sophist philosophy at that point? For my part I'm not smart enough to claim to have answers to anything but my intuition is there are no quantum paradoxes. Delayed choice does not send anything back in time. We don't experience quantum phenomena at the macro scale so our intuition and reasoning are ill-suited to thinking about it. That easily leads us to incorrect conclusions.

chrisweekly 2y ago

That sounds like an interesting theory; is it yours, or do you have any citations / references you could share?

ameetgaitonde 2y ago
He's just describing Einstein's Theory of Special Relativity.
- chrisweekly 2y ago
  Huh. I'm admittedly no astrophysicist, but I don't recall encountering this "photons have no velocity in the time dimension" before. Maybe I'm one of today's lucky 10,000[1]. 1. https://xkcd.com/1053/ https://xkcd.com/1053/
  - bee_rider 2y ago
    But, it is sort of common to observe the somewhat trippy fact that, unlike matter, photons don’t experience time passing, right? (Although, I will be honest, I could not ever really grok relativity like a physicist. However, I can enjoy the Lorentz factor going to infinity as v goes to c).
    - da_chicken 2y ago
      From my understanding, it's more correct to say that time is undefined for massless objects. Through special relativity, our understanding of mass, gravity, and spacetime are linked. If something has no mass, then special relativity can't describe how gravity affects it's spacetime reference. Remember, however, that this explanation is based on the mathematics that explain the observations we've made or theorized. Just as the map is not the territory, the math is not the universe.
      - deleted 2y ago
        [deleted]
  - 2y ago

762236 2y ago

[flagged]

stared 2y ago

It sounds like a really nice (and the shortest possible) backronym. https://en.wikipedia.org/wiki/Backronym https://en.wikipedia.org/wiki/Backronym

aatd86 2y ago
Isn't it supposed to be c for celerity? Edit.. Oh I see. Nvm

layer8 2y ago

Causation is a bit of a troublesome concept in physics: https://plato.stanford.edu/entries/causation-physics/ https://plato.stanford.edu/entries/causation-physics/ I wouldn’t use causality as a foundational concept ontologically. What’s true is that c doesn’t just apply to the electromagnetic field, but to all fundamental physical fields.

uecker 2y ago
There is a more precise concept of causality in physics that is very much fundamental part of all modern theories.
- layer8 2y ago
  And what concept would that be?
  - uecker 2y ago
    Einstein causality, i.e that no physical effect propagates faster than light. In QFT it it is modeled by the rule that space-like separated measurements commute.
    - layer8 2y ago
      That merely equates the term "causality" to propagating waves. It doesn't introduce any new concept. Ontologically, there are only quantum fields in space-time. The physical laws governing them imply from their mathematics that they cannot contain any propagations faster than c. But attaching the term "causality" to it doesn't really add anything here.
      - uecker 2y ago
        This does not equate causality with propagating waves. I also did not claim it is a new concept. Instead I said physics has a precise and fundamental concept of causality.

phkahler 2y ago

Sure, but why did Einstein call the speed of light c?

layer8 2y ago
I think the article explains that.

auntienomen 2y ago

"Photons (being massless) have 100% of their velocity in the spatial dimensions and no velocity in the time dimension" I kinda see what you're trying to say, but these words aren't a particularly good match for the math in special relativity. To an observer, any photon's velocity 4-vector looks like V = (c,c,0,0). That's a "c" in the spatial direction the photon's moving, and a "c" in the time direction. So, plenty of velocity in the time direction. What is zero is the _proper time_ along a photon's trajectory through space & time. An observer who's co-moving with the photon (call me if you ever meet one, we can write a paper together) would see a) the photon holding still and b) no time passing.

uecker 2y ago
I can't recommend such a co-author. Doesn't get anything done.

phire 2y ago

And when you think about c as the speed of causality, it becomes a lot more obvious why FTL travel/communicate is (almost certainty) impossible without breaking causality.... You are going faster than causality, of course it breaks. The other option is to break relativity, which is what most science fiction media does, often accidentally.

eggn00dles 2y ago

special relativity is undefined at v = c. so no from a photons point of view the trip doesn’t happen in an instant nor does the length of the universe shrink to zero. there is no definition for a photons point of view. time dilation and length contraction only apply when comparing a moving frame of reference to one that is at rest and photons have no rest frame.

xarope 2y ago

is that a highly technical way of saying WE think c is the speed of light, because that's all WE can measure?

rurban 2y ago

No, there is no causality anymore in quantum physics, since the EPR paradoxon was disproved. c stands for the latin for for speed of course. causality is immediate, a higher speed than c. because it's logical, not measurable.