7 ms·

The speed of light, c, is the speed of light in a vacuum. The speed of light is variable given the medium by which that light traverses. Also, when we speak o

The speed of light, c, is the speed of light in a vacuum. The speed of light is variable given the medium by which that light traverses. Also, when we speak of traversal the implication is closing distance across a space. Space is also variable. According to the Special Theory of Relativity space is curved and that curvature varies according to the forces acting upon it. The general assumption is that such a curvature is uniform according to the force imposed upon at a given point relative to that force, but there is no reason to make such an assumption. More simply space is curved and that curvature may vary in unexpected ways regardless of whether an observer is outside or effected by the force imposing the curvature. That said the safest prediction is to say we all travel at various speeds of light given enough space and duration compared to a competing observation.

pdonis 6y ago
> According to the Special Theory of Relativity space is curved No, it's the General Theory of Relativity that says that spacetime is curved. Spacetime is flat in Special Relativity. > curvature varies according to the forces acting upon it No, in GR spacetime curvature varies according to the matter and energy that are present. Gravity is not a force at all in GR.
- cygx 6y ago
  Gravity is not a force at all in GR. Rather, the gravitational force is a pseudo-force like the Coriolis force.
  - pdonis 6y ago
    That's one way of viewing it, yes.
irjustin 6y ago
downvoted because time traveling through a medium vs traveling through curved space are not anywhere near the same underlying principles[0]. The implication being suggested is - if I was trapped in a block of plastic or... underwater, I would experience time different compared to someone standing on land just because "c" is slower. Clearly not the case. To be specific, when "the speed of light" is discussed like this, it usually means the "speed of causality"[1] which is the underlying meaning of "c" in relativity. The speed of causality through a medium (i.e. water) is the same as a vacuum if spacetime curvature is the same. [0] https://www.youtube.com/watch?v=CiHN0ZWE5bk https://www.youtube.com/watch?v=CiHN0ZWE5bk [1] https://www.youtube.com/watch?v=msVuCEs8Ydo https://www.youtube.com/watch?v=msVuCEs8Ydo
- scarygliders 6y ago
  Upvoted you and thanks. I was hoping someone would mention "the speed of causality", because to my layman's mind, I've always found thinking about "the speed of causality" makes vastly more sense of the universe to me and makes spacetime/relativity/and-all-that-jazz far easier concepts to comprehend. For example, if a powerful laser beam was shot off in a vacuum towards an observer who is far away, it's not the "speed of light in a vacuum" which dictates the time at which the observer detects that laser light, it's the speed of causality in that volume of spacetime which determines when that light beam reaches them/is detected.
- austincheney 6y ago
  Space is a medium of variable density that impacts the speed of light and everything else. That is clear with regard to solar winds and termination shock. The curvature of space is directly related to time travel. Strange how you completely ignored that much larger portion of my comment to punch a straw man. > is the same as a vacuum if spacetime curvature is the same. If that were true particle physics wouldn’t need dark matter/energy to balance conditions that are irregular upon observable matter alone. https://en.m.wikipedia.org/wiki/Causality_(physics) https://en.m.wikipedia.org/wiki/Causality_(physics) Causality, at least in physics, is an abstract notion and not an empirical notion. In causality effects occur in the same order as their respective causes which is substantive for logical consider, but is not necessarily measurably accurate or expected.
  - irjustin 6y ago
    While there are a lot of big words being used, there appears to be a gap in understanding the core principles related to the article. > If that were true particle physics wouldn’t need dark matter/energy to balance conditions that are irregular upon observable matter alone. You responded to my vacuum statement as if we agree the vacuum is perfectly empty. In relativity, the vacuum is not considered empty[0]. Maybe I should have used "patch of space", but more importantly I said "if the spacetime curvature is the same". That literally defines how causality moves through it so that would be inclusive whatever was there including dark matter/energy, which brings us to... > Causality, at least in physics, is an abstract notion and not an empirical notion... but is not necessarily measurably accurate or expected. No it's not... From the wiki you quoted: "In Einstein's theory of special relativity, causality means that an effect can not occur from a cause that is not in the back (past) light cone of that event. Similarly, a cause cannot have an effect outside its front (future) light cone." We're talking about relativity here, not philosophy or another topic where causality isn't well defined. It is not abstract. It is explicitly defined as "c" in Einstein's equations. I'm not sure what you're trying to prove/disprove here. You can say I attacked a straw man, but the original comment was edited. So now, I don't know. I recommend PBS SpaceTime[1] - I still don't understand a lot of things in this space even after watching many videos multiple times, but it really helped put pieces together. [0] https://en.wikipedia.org/wiki/Vacuum_state#:~:text=According%20to%20present%2Dday%20understanding,into%20and%20out%20of%20existence https://en.wikipedia.org/wiki/Vacuum_state#:~:text=According.... [1] https://www.youtube.com/channel/UC7_gcs09iThXybpVgjHZ_7g https://www.youtube.com/channel/UC7_gcs09iThXybpVgjHZ_7g

pdonis 6y ago

> According to the Special Theory of Relativity space is curved No, it's the General Theory of Relativity that says that spacetime is curved. Spacetime is flat in Special Relativity. > curvature varies according to the forces acting upon it No, in GR spacetime curvature varies according to the matter and energy that are present. Gravity is not a force at all in GR.

cygx 6y ago
Gravity is not a force at all in GR. Rather, the gravitational force is a pseudo-force like the Coriolis force.
- pdonis 6y ago
  That's one way of viewing it, yes.

irjustin 6y ago

downvoted because time traveling through a medium vs traveling through curved space are not anywhere near the same underlying principles[0]. The implication being suggested is - if I was trapped in a block of plastic or... underwater, I would experience time different compared to someone standing on land just because "c" is slower. Clearly not the case. To be specific, when "the speed of light" is discussed like this, it usually means the "speed of causality"[1] which is the underlying meaning of "c" in relativity. The speed of causality through a medium (i.e. water) is the same as a vacuum if spacetime curvature is the same. [0] https://www.youtube.com/watch?v=CiHN0ZWE5bk https://www.youtube.com/watch?v=CiHN0ZWE5bk [1] https://www.youtube.com/watch?v=msVuCEs8Ydo https://www.youtube.com/watch?v=msVuCEs8Ydo

scarygliders 6y ago
Upvoted you and thanks. I was hoping someone would mention "the speed of causality", because to my layman's mind, I've always found thinking about "the speed of causality" makes vastly more sense of the universe to me and makes spacetime/relativity/and-all-that-jazz far easier concepts to comprehend. For example, if a powerful laser beam was shot off in a vacuum towards an observer who is far away, it's not the "speed of light in a vacuum" which dictates the time at which the observer detects that laser light, it's the speed of causality in that volume of spacetime which determines when that light beam reaches them/is detected.
austincheney 6y ago
Space is a medium of variable density that impacts the speed of light and everything else. That is clear with regard to solar winds and termination shock. The curvature of space is directly related to time travel. Strange how you completely ignored that much larger portion of my comment to punch a straw man. > is the same as a vacuum if spacetime curvature is the same. If that were true particle physics wouldn’t need dark matter/energy to balance conditions that are irregular upon observable matter alone. https://en.m.wikipedia.org/wiki/Causality_(physics) https://en.m.wikipedia.org/wiki/Causality_(physics) Causality, at least in physics, is an abstract notion and not an empirical notion. In causality effects occur in the same order as their respective causes which is substantive for logical consider, but is not necessarily measurably accurate or expected.
- irjustin 6y ago
  While there are a lot of big words being used, there appears to be a gap in understanding the core principles related to the article. > If that were true particle physics wouldn’t need dark matter/energy to balance conditions that are irregular upon observable matter alone. You responded to my vacuum statement as if we agree the vacuum is perfectly empty. In relativity, the vacuum is not considered empty[0]. Maybe I should have used "patch of space", but more importantly I said "if the spacetime curvature is the same". That literally defines how causality moves through it so that would be inclusive whatever was there including dark matter/energy, which brings us to... > Causality, at least in physics, is an abstract notion and not an empirical notion... but is not necessarily measurably accurate or expected. No it's not... From the wiki you quoted: "In Einstein's theory of special relativity, causality means that an effect can not occur from a cause that is not in the back (past) light cone of that event. Similarly, a cause cannot have an effect outside its front (future) light cone." We're talking about relativity here, not philosophy or another topic where causality isn't well defined. It is not abstract. It is explicitly defined as "c" in Einstein's equations. I'm not sure what you're trying to prove/disprove here. You can say I attacked a straw man, but the original comment was edited. So now, I don't know. I recommend PBS SpaceTime[1] - I still don't understand a lot of things in this space even after watching many videos multiple times, but it really helped put pieces together. [0] https://en.wikipedia.org/wiki/Vacuum_state#:~:text=According%20to%20present%2Dday%20understanding,into%20and%20out%20of%20existence https://en.wikipedia.org/wiki/Vacuum_state#:~:text=According.... [1] https://www.youtube.com/channel/UC7_gcs09iThXybpVgjHZ_7g https://www.youtube.com/channel/UC7_gcs09iThXybpVgjHZ_7g