7 ms·

The difference is that light travels through space, but not time (similar to how a vertical line does not travel the x axis, only the y axis). The ball travels

by umvi 6y ago

The difference is that light travels through space, but not time (similar to how a vertical line does not travel the x axis, only the y axis). The ball travels through both space and time. The faster you go, the less you travel through time. Thus, if the ball were travelling at the speed of light, it would not travel through time either and would follow the same path as light.

pdonis 6y ago
> The difference is that light travels through space, but not time This is a common pop science statement, but it's not correct. A correct statement is that the concept of "speed through spacetime", which is what has to be split into "speed through space" and "speed through time" in the pop science statement, does not apply to a light ray. In more technical language, the tangent vector to the light ray's worldline is not a unit vector, it's a null vector, and the concept of "speed through spacetime" only makes sense for a worldline whose tangent vector is a unit vector.
- CamperBob2 6y ago
  I wouldn't say it's incorrect at all. From the point of view of a photon, no time elapses between its the origin and destination endpoints.
  - pdonis 6y ago
    > From the point of view of a photon, no time elapses between its the origin and destination endpoints. No, this is not correct. The correct statement is that the concept of "elapsed time" does not apply to a photon; it only applies to timelike worldlines, not null worldlines. To put it another way, if your statement were true, it would mean that the origin and destination events were the same point in spacetime. But they're not; they're distinct points in spacetime. Which means that, since the spacetime interval along the worldline is zero, you can't use the interval to distinguish points on the photon's worldline. And the concept of "elapsed time" requires that you be able to do that. So the concept of "elapsed time" can't be used for a photon.
    - umvi 6y ago
      It's impossible for an object with mass to go the speed of light. But you can observe what happens as you approach it: Let's say I put you in a spaceship and accelerate you to 50% the speed of light toward the sun. From an inertial viewer's perspective you are travelling toward the sun at half the speed of light and it takes you ~16 minutes to crash into the sun. But from your perspective it only took ~14 minutes to crash into the sun[0]. Repeat the experiment except I accelerate you to .99c. From an inertial viewer's perspective you are travelling toward the sun at nearly speed of light and it takes you ~8 minutes to crash into the sun. But from your perspective it only took ~1 minute to crash into the sun. Repeat the experiment except I accelerate you to .999c. From an inertial viewer's perspective you are travelling toward the sun at nearly speed of light and it takes you ~8 minutes to crash into the sun. But from your perspective it only took 20 seconds to crash into the sun. Repeat the experiment except I accelerate you to .9999c. From an inertial viewer's perspective you are travelling toward the sun at nearly speed of light and it takes you ~8 minutes to crash into the sun. But from your perspective it only took 6 seconds to crash into the sun. Repeat the experiment except I accelerate you to .99999c. From an inertial viewer's perspective you are travelling toward the sun at nearly speed of light and it takes you ~8 minutes to crash into the sun. But from your perspective it only took 2 seconds to crash into the sun. See what's happening? As you approach the speed of light, the amount of time that elapses until you reach your destination approaches zero. So from an inertial observer's point of view, time has completely frozen for travelers approaching light speed. [0] Using time dilation formula from this page: https://www.phy.olemiss.edu/HEP/QuarkNet/time.html https://www.phy.olemiss.edu/HEP/QuarkNet/time.html
      - pdonis 6y ago
        > you can observe what happens as you approach it Yes, but you cannot extrapolate from this to say that the time lapse for a photon would be zero. A photon is not the limit of objects with mass going closer and closer to the speed of light, because "closer and closer to the speed of light" is frame-dependent, but a photon's speed being c is not. I can find an inertial frame in which each of your objects is at rest, and in that frame, you are the one who is "close to c" (in the opposite direction). But that doesn't mean your elapsed time approaches zero. By contrast, it is impossible to find any frame in which a photon is at rest. The two types of objects are fundamentally different. In more technical language, the action of Lorentz transformations on photons is fundamentally different from their action on timelike objects. So it is simply not valid to view photons as some sort of limit "as speed approaches c" of timelike objects.
  - phizy 6y ago
    There is no "point of view of a photon." Photons do not have frames of reference. So you are completely incorrect insofar as what you are saying is physically nonsense.
    - CamperBob2 6y ago
      Again, take it up with Tyson and others with doctoral-level credentials who've made a career out of explaining these subjects to the unwashed laity. I'm not one of those people. Nobody posting on Hacker News is, as far as can be discerned. If you and others in the thread feel that these popular authorities are spreading misinformation or using inappropriate analogies, doesn't it behoove you to raise an objection with them directly? Or perhaps with the appropriate faculty committees at their institutions?
      - pdonis 6y ago
        > If you and others in the thread feel that these popular authorities are spreading misinformation or using inappropriate analogies, doesn't it behoove you to raise an objection with them directly? If they want to make money by getting "the unwashed laity" to buy their books, why should I stop them? I simply don't buy them myself. If other people want to get told comforting nonsense, that's their problem. Caveat lector. > Or perhaps with the appropriate faculty committees at their institutions? Which would be pointless and absurd, since, as I have already said, the claims in question are not being made in textbooks and peer-reviewed papers. I suspect you are trolling.
      - phizy 6y ago
        So you take no responsibility for spreading misinformation if the prior source is not corrected? Sounds like you're acting in bad faith then. You can win an internet argument anywhere, but if that's all you want to achieve here, then maybe stop trying to debate physics.
shahar2k 6y ago
does that mean that velocity is constant "speed of light" and only direction in spacetime changes?
edem 6y ago
Light also travels through time it just never makes progress. It still has a 4 vector (x, y, z, t) is is just that its `t` component is zero.

by umvi 6y ago

pdonis 6y ago

> The difference is that light travels through space, but not time This is a common pop science statement, but it's not correct. A correct statement is that the concept of "speed through spacetime", which is what has to be split into "speed through space" and "speed through time" in the pop science statement, does not apply to a light ray. In more technical language, the tangent vector to the light ray's worldline is not a unit vector, it's a null vector, and the concept of "speed through spacetime" only makes sense for a worldline whose tangent vector is a unit vector.

CamperBob2 6y ago
I wouldn't say it's incorrect at all. From the point of view of a photon, no time elapses between its the origin and destination endpoints.
- pdonis 6y ago
  > From the point of view of a photon, no time elapses between its the origin and destination endpoints. No, this is not correct. The correct statement is that the concept of "elapsed time" does not apply to a photon; it only applies to timelike worldlines, not null worldlines. To put it another way, if your statement were true, it would mean that the origin and destination events were the same point in spacetime. But they're not; they're distinct points in spacetime. Which means that, since the spacetime interval along the worldline is zero, you can't use the interval to distinguish points on the photon's worldline. And the concept of "elapsed time" requires that you be able to do that. So the concept of "elapsed time" can't be used for a photon.
  - umvi 6y ago
    It's impossible for an object with mass to go the speed of light. But you can observe what happens as you approach it: Let's say I put you in a spaceship and accelerate you to 50% the speed of light toward the sun. From an inertial viewer's perspective you are travelling toward the sun at half the speed of light and it takes you ~16 minutes to crash into the sun. But from your perspective it only took ~14 minutes to crash into the sun[0]. Repeat the experiment except I accelerate you to .99c. From an inertial viewer's perspective you are travelling toward the sun at nearly speed of light and it takes you ~8 minutes to crash into the sun. But from your perspective it only took ~1 minute to crash into the sun. Repeat the experiment except I accelerate you to .999c. From an inertial viewer's perspective you are travelling toward the sun at nearly speed of light and it takes you ~8 minutes to crash into the sun. But from your perspective it only took 20 seconds to crash into the sun. Repeat the experiment except I accelerate you to .9999c. From an inertial viewer's perspective you are travelling toward the sun at nearly speed of light and it takes you ~8 minutes to crash into the sun. But from your perspective it only took 6 seconds to crash into the sun. Repeat the experiment except I accelerate you to .99999c. From an inertial viewer's perspective you are travelling toward the sun at nearly speed of light and it takes you ~8 minutes to crash into the sun. But from your perspective it only took 2 seconds to crash into the sun. See what's happening? As you approach the speed of light, the amount of time that elapses until you reach your destination approaches zero. So from an inertial observer's point of view, time has completely frozen for travelers approaching light speed. [0] Using time dilation formula from this page: https://www.phy.olemiss.edu/HEP/QuarkNet/time.html https://www.phy.olemiss.edu/HEP/QuarkNet/time.html
    - pdonis 6y ago
      > you can observe what happens as you approach it Yes, but you cannot extrapolate from this to say that the time lapse for a photon would be zero. A photon is not the limit of objects with mass going closer and closer to the speed of light, because "closer and closer to the speed of light" is frame-dependent, but a photon's speed being c is not. I can find an inertial frame in which each of your objects is at rest, and in that frame, you are the one who is "close to c" (in the opposite direction). But that doesn't mean your elapsed time approaches zero. By contrast, it is impossible to find any frame in which a photon is at rest. The two types of objects are fundamentally different. In more technical language, the action of Lorentz transformations on photons is fundamentally different from their action on timelike objects. So it is simply not valid to view photons as some sort of limit "as speed approaches c" of timelike objects.
- phizy 6y ago
  There is no "point of view of a photon." Photons do not have frames of reference. So you are completely incorrect insofar as what you are saying is physically nonsense.
  - CamperBob2 6y ago
    Again, take it up with Tyson and others with doctoral-level credentials who've made a career out of explaining these subjects to the unwashed laity. I'm not one of those people. Nobody posting on Hacker News is, as far as can be discerned. If you and others in the thread feel that these popular authorities are spreading misinformation or using inappropriate analogies, doesn't it behoove you to raise an objection with them directly? Or perhaps with the appropriate faculty committees at their institutions?
    - pdonis 6y ago
      > If you and others in the thread feel that these popular authorities are spreading misinformation or using inappropriate analogies, doesn't it behoove you to raise an objection with them directly? If they want to make money by getting "the unwashed laity" to buy their books, why should I stop them? I simply don't buy them myself. If other people want to get told comforting nonsense, that's their problem. Caveat lector. > Or perhaps with the appropriate faculty committees at their institutions? Which would be pointless and absurd, since, as I have already said, the claims in question are not being made in textbooks and peer-reviewed papers. I suspect you are trolling.
    - phizy 6y ago
      So you take no responsibility for spreading misinformation if the prior source is not corrected? Sounds like you're acting in bad faith then. You can win an internet argument anywhere, but if that's all you want to achieve here, then maybe stop trying to debate physics.

shahar2k 6y ago

does that mean that velocity is constant "speed of light" and only direction in spacetime changes?

edem 6y ago

Light also travels through time it just never makes progress. It still has a 4 vector (x, y, z, t) is is just that its `t` component is zero.