6 ms·

I found that the author gave a great setup for an explanation, and then balked at giving the actual answer. > For right now, if you just believe that four-velo

I found that the author gave a great setup for an explanation, and then balked at giving the actual answer. > For right now, if you just believe that four-velocities can never stretch or shrink because that's just the way it is In other words, nothing can go faster than the speed of light because that's the way it is? The author needs to explain why the magnitude of this four-velocity vector is the speed of light! I was hooked after the first few paragraphs, but then felt like it dead-ended in a circular argument.

kalvin 16y ago
She wrote several followup answers in the comments thread, one of which addresses your question (though it might not be satisfying!): http://www.reddit.com/r/askscience/comments/fjwkh/why_exactly_can_nothing_go_faster_than_the_speed/c1gi47a http://www.reddit.com/r/askscience/comments/fjwkh/why_exactl... Someone else wrote my favorite answer: "Basically, the way to think of it is not that light is the fastest thing, but rather that there is a speed, c, which the geometry of space and time demands is the fastest possible speed. One can also work out that anything without mass must travel at this fastest possible speed c. Light is one of those things, therefore light travels at c. It's only an accident of history that we call c "the speed of light": that's the context we discovered c's existence in. As for why it's the speed it is, well, it's the speed in our universe. It's actually much more natural to say c=1 and all speeds are then unitless numbers between 0 and 1. From this point of view c is 300 Mm/s because of how we chose to define the meter and the second." http://www.reddit.com/r/askscience/comments/fjwkh/why_exactly_can_nothing_go_faster_than_the_speed/c1gi276 http://www.reddit.com/r/askscience/comments/fjwkh/why_exactl...
- scorpion032 16y ago
  As of today, meter is defined in terms of the speed of light. We have come a full circle! > The meter is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second. via: http://physics.nist.gov/cuu/Units/meter.html http://physics.nist.gov/cuu/Units/meter.html
  - beza1e1 16y ago
    So how long is a second?
    - a1k0n 16y ago
      via Wikipedia: The duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom. So I guess that means a meter is 30.66331898849836976219 cesium-133 transition periods.
- oewolf 16y ago
  This clarified a few questions I had. Hope you can help me with a couple: why is there even a limit? how do we know there is an insuperable barrier nothing can cross for sure?
  - perlgeek 16y ago
    A physicist can never certainly say that something doesn't happen. We have models that describe the world very well, but those models are never the whole truth (and even it were, we couldn't be sure of it). Special and General Relativity are experimentally very well tested, and these theories predict that a particle that has a speed < c can never acquire a speed > c. There are solutions for speeds larger than c, but those solutions are quite weird, and can't interact with "normal" particles. But the mere scientific approach of observing facts, modeling and verifying means that one can never be sure that something can't be - only in the limits of the models can we be sure that a thing can't be.
siglesias 16y ago
I'm not sure if that's entirely fair. She did shed light on the nature of the limit (the how): it's not a quantum speed limit, where suddenly more energy input to a body moving at c has no effect (as would probably be a classical interpretation), but rather that the very geometry of our universe constantly revises our notion of time as we move about space, such that the idea of a body moving "faster than c" is a violation of that geometry. I call that pretty insightful, for all that the idea of a universal speed limit could possibly imply.
brazzy 16y ago
The question why a certain velocity, c, cannot be exceeded, is what was actually asked (and answered very well). The question why it's that particular magnitude is a different question.
- paulgerhardt 16y ago
  See also Feynman on "Why?" questions: http://www.youtube.com/watch?v=wMFPe-DwULM&t=28s http://www.youtube.com/watch?v=wMFPe-DwULM&t=28s
Confusion 16y ago
Nobody can explain why that is the case, as little as anyone can explain why the universe exists. It exists and has certain properties that define its existence. These are just irreducible facts of nature.
- chroma 16y ago
  Not very long ago, people couldn't explain why things fell to the ground at 9.8m/s^2 instead of some other acceleration factor. The answer now seems straightforward to us. You can't tell the difference between an irreducible fact of nature and a reducible one until you've reduced it.
  - Confusion 16y ago
    That they couldn't explain it did not mean they thought or claimed it was irreducible. The reason for claiming the exact numeric value of the speed of light is irreducible is not because we can't currently explain it: it is because it is part of the definition of the universe. The universe consists of space and time components and they are related by a ratio of c. Would the ratio have been 1, then nobody would have posed the question in the first place. You don't go around asking why pi has the value it has either. It may turn out it is possible to relate these values to other properties and values, but they can never be reduced to them. That would just be choosing a different basis for your description, while that basis is arbitrary.
    - lurker19 16y ago
      I agree with your main point, I think, but c and pi are different. c has a unit and so can take any numerical value under a compensating choice of units. Pi is unitless, however, and cannot be defined as any other value. C and pi are similar also: c would be different, if the universe's geometry where scaled differently, even if meters and seconds were unchanged in transition to the alternate universe (assuming one can formalize what I am saying in a sensible way). Likewise, pi (circumference divided by diameter) would take a different value (or not even be a constant) in an alternate (non-Euclidean) geometry (assuming that a circle could still be defined there, which is more obviously possible than the c/spacetime hypothetical)
moultano 16y ago
The actual answer is just that the speed of light is the conversion factor between meters and seconds. You can measure time in meters and distance in seconds, and convert between them using C. Light goes at that speed because that makes the little packet of electromagnetic energy reinforce itself properly.
crocowhile 16y ago
Yes, I haven't read the other comments but that is a terrible terrible explanation. OP had a genuine question about the nature of things and the person answered bringing up paper and arrows. This is the stuff that made Feynmann mad when they asked him about magnets. The only reasonable answer to the original question is: we don't know. The whole schizofrenic theory of something that can be waves and particles doesn't make sense right now but that's the best we can come up with. Nothing can go faster than light because our equations tell us so but we don't know the mechanisms, exactly as we don't know what is creating gravity, for instance. Physics is full of deep mysteries still.
- panic 16y ago
  Yes, I haven't read the other comments but that is a terrible terrible explanation. OP had a genuine question about the nature of things and the person answered bringing up paper and arrows. This is the stuff that made Feynmann mad when they asked him about magnets. Have you ever read the book QED? It's Feynman's best effort to explain Quantum Electrodynamics to a layman. In the book, he takes a genuine question about the nature of things (how does light work?) and answers it by "bringing up paper and arrows!" (I encourage you to read the book to see what I mean). Feynman disliked "explaining" nature with inaccurate and irresponsible metaphors. But arrows are the way it really is!
  - crocowhile 16y ago
    I haven't read that particular book. I pointed out Feynman's magnets because they were a bit of a recurring joke on reddit (I refuse to use the word meme) and found ironic redditors fell in the same trap. On a margin note, the best book I've read about explaining modern physics to the layman is this: http://www.amazon.com/Evolution-Physics-Albert-Einstein/dp/0671201565 http://www.amazon.com/Evolution-Physics-Albert-Einstein/dp/0...
    - jarin 16y ago
      What's wrong with the word meme? It wasn't invented by reddit, you know (or 4chan either).
crux_ 16y ago
The main takeaway for me is that of a deeper "that's the way it is": * My understanding before: light has a constant speed and nothing can exceed that speed, but up until that limit everything can travel at a variety of speeds. * My understanding after: everything travels through space-time at a constant speed. Light is "special" only in that it travels exclusively through space and not at all through time.
ryanelkins 16y ago
The problem is that even answering that question just leads to another question. It ends up the way conversations go with inquisitive children - an endless stream of "why"s with no real end because we don't know everything. At some point you just have to reach a satisfactory answer and realize that the more you learn the more you come to understand how little you know.

kalvin 16y ago

She wrote several followup answers in the comments thread, one of which addresses your question (though it might not be satisfying!): http://www.reddit.com/r/askscience/comments/fjwkh/why_exactly_can_nothing_go_faster_than_the_speed/c1gi47a http://www.reddit.com/r/askscience/comments/fjwkh/why_exactl... Someone else wrote my favorite answer: "Basically, the way to think of it is not that light is the fastest thing, but rather that there is a speed, c, which the geometry of space and time demands is the fastest possible speed. One can also work out that anything without mass must travel at this fastest possible speed c. Light is one of those things, therefore light travels at c. It's only an accident of history that we call c "the speed of light": that's the context we discovered c's existence in. As for why it's the speed it is, well, it's the speed in our universe. It's actually much more natural to say c=1 and all speeds are then unitless numbers between 0 and 1. From this point of view c is 300 Mm/s because of how we chose to define the meter and the second." http://www.reddit.com/r/askscience/comments/fjwkh/why_exactly_can_nothing_go_faster_than_the_speed/c1gi276 http://www.reddit.com/r/askscience/comments/fjwkh/why_exactl...

scorpion032 16y ago
As of today, meter is defined in terms of the speed of light. We have come a full circle! > The meter is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second. via: http://physics.nist.gov/cuu/Units/meter.html http://physics.nist.gov/cuu/Units/meter.html
- beza1e1 16y ago
  So how long is a second?
  - a1k0n 16y ago
    via Wikipedia: The duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom. So I guess that means a meter is 30.66331898849836976219 cesium-133 transition periods.
oewolf 16y ago
This clarified a few questions I had. Hope you can help me with a couple: why is there even a limit? how do we know there is an insuperable barrier nothing can cross for sure?
- perlgeek 16y ago
  A physicist can never certainly say that something doesn't happen. We have models that describe the world very well, but those models are never the whole truth (and even it were, we couldn't be sure of it). Special and General Relativity are experimentally very well tested, and these theories predict that a particle that has a speed < c can never acquire a speed > c. There are solutions for speeds larger than c, but those solutions are quite weird, and can't interact with "normal" particles. But the mere scientific approach of observing facts, modeling and verifying means that one can never be sure that something can't be - only in the limits of the models can we be sure that a thing can't be.

siglesias 16y ago

I'm not sure if that's entirely fair. She did shed light on the nature of the limit (the how): it's not a quantum speed limit, where suddenly more energy input to a body moving at c has no effect (as would probably be a classical interpretation), but rather that the very geometry of our universe constantly revises our notion of time as we move about space, such that the idea of a body moving "faster than c" is a violation of that geometry. I call that pretty insightful, for all that the idea of a universal speed limit could possibly imply.

brazzy 16y ago

The question why a certain velocity, c, cannot be exceeded, is what was actually asked (and answered very well). The question why it's that particular magnitude is a different question.

paulgerhardt 16y ago
See also Feynman on "Why?" questions: http://www.youtube.com/watch?v=wMFPe-DwULM&t=28s http://www.youtube.com/watch?v=wMFPe-DwULM&t=28s

Confusion 16y ago

Nobody can explain why that is the case, as little as anyone can explain why the universe exists. It exists and has certain properties that define its existence. These are just irreducible facts of nature.

chroma 16y ago
Not very long ago, people couldn't explain why things fell to the ground at 9.8m/s^2 instead of some other acceleration factor. The answer now seems straightforward to us. You can't tell the difference between an irreducible fact of nature and a reducible one until you've reduced it.
- Confusion 16y ago
  That they couldn't explain it did not mean they thought or claimed it was irreducible. The reason for claiming the exact numeric value of the speed of light is irreducible is not because we can't currently explain it: it is because it is part of the definition of the universe. The universe consists of space and time components and they are related by a ratio of c. Would the ratio have been 1, then nobody would have posed the question in the first place. You don't go around asking why pi has the value it has either. It may turn out it is possible to relate these values to other properties and values, but they can never be reduced to them. That would just be choosing a different basis for your description, while that basis is arbitrary.
  - lurker19 16y ago
    I agree with your main point, I think, but c and pi are different. c has a unit and so can take any numerical value under a compensating choice of units. Pi is unitless, however, and cannot be defined as any other value. C and pi are similar also: c would be different, if the universe's geometry where scaled differently, even if meters and seconds were unchanged in transition to the alternate universe (assuming one can formalize what I am saying in a sensible way). Likewise, pi (circumference divided by diameter) would take a different value (or not even be a constant) in an alternate (non-Euclidean) geometry (assuming that a circle could still be defined there, which is more obviously possible than the c/spacetime hypothetical)

moultano 16y ago

The actual answer is just that the speed of light is the conversion factor between meters and seconds. You can measure time in meters and distance in seconds, and convert between them using C. Light goes at that speed because that makes the little packet of electromagnetic energy reinforce itself properly.

crocowhile 16y ago

Yes, I haven't read the other comments but that is a terrible terrible explanation. OP had a genuine question about the nature of things and the person answered bringing up paper and arrows. This is the stuff that made Feynmann mad when they asked him about magnets. The only reasonable answer to the original question is: we don't know. The whole schizofrenic theory of something that can be waves and particles doesn't make sense right now but that's the best we can come up with. Nothing can go faster than light because our equations tell us so but we don't know the mechanisms, exactly as we don't know what is creating gravity, for instance. Physics is full of deep mysteries still.

panic 16y ago
Yes, I haven't read the other comments but that is a terrible terrible explanation. OP had a genuine question about the nature of things and the person answered bringing up paper and arrows. This is the stuff that made Feynmann mad when they asked him about magnets. Have you ever read the book QED? It's Feynman's best effort to explain Quantum Electrodynamics to a layman. In the book, he takes a genuine question about the nature of things (how does light work?) and answers it by "bringing up paper and arrows!" (I encourage you to read the book to see what I mean). Feynman disliked "explaining" nature with inaccurate and irresponsible metaphors. But arrows are the way it really is!
- crocowhile 16y ago
  I haven't read that particular book. I pointed out Feynman's magnets because they were a bit of a recurring joke on reddit (I refuse to use the word meme) and found ironic redditors fell in the same trap. On a margin note, the best book I've read about explaining modern physics to the layman is this: http://www.amazon.com/Evolution-Physics-Albert-Einstein/dp/0671201565 http://www.amazon.com/Evolution-Physics-Albert-Einstein/dp/0...
  - jarin 16y ago
    What's wrong with the word meme? It wasn't invented by reddit, you know (or 4chan either).

crux_ 16y ago

The main takeaway for me is that of a deeper "that's the way it is": * My understanding before: light has a constant speed and nothing can exceed that speed, but up until that limit everything can travel at a variety of speeds. * My understanding after: everything travels through space-time at a constant speed. Light is "special" only in that it travels exclusively through space and not at all through time.

ryanelkins 16y ago

The problem is that even answering that question just leads to another question. It ends up the way conversations go with inquisitive children - an endless stream of "why"s with no real end because we don't know everything. At some point you just have to reach a satisfactory answer and realize that the more you learn the more you come to understand how little you know.