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If there's a way to artificially accelerate light, or when it's naturally faster such as around a blackhole, then in that case, would it be possible to go past the 299792458 m / s limit?

Qmechanic
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No Name
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    Light itself is a red herring. You can't increase $c$. – J.G. Aug 01 '23 at 20:57
  • The local speed of light is always c, but see https://physics.stackexchange.com/q/59502/123208 – PM 2Ring Aug 01 '23 at 21:13
  • But why do you want to accelerate light? Sending spacecraft at a significant fraction of c is likely to be beyond our capabilities for a long time. OTOH, we could possibly do interesting things with time dilation if c were a lot lower. ;) – PM 2Ring Aug 01 '23 at 21:17
  • Not exactly what you're asking, but you might be interested in the theoretical possibility of the Alcubierre drive. However, before you get too excited, note that an Alcubierre drive requires exotic matter that might not actually exist in Nature. – Andrew Aug 02 '23 at 01:08
  • @Andrew I really wish more people would point out that the author of that proposal does not take it seriously himself - it is just an interesting paper exercise using GR. – m4r35n357 Aug 02 '23 at 09:13

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First of all, this question is missing something very important about the constant $c$. This only matches the speed of light (really an electromagnetic wave that satisfies Maxwell's equations) in a vacuum. Thus, when light propagates in a medium other than the vacuum, it will almost certainly not travel at $c$. The following Wikipedia article gives a fun extreme example of this phenomenon called "slow light:" https://en.wikipedia.org/wiki/Slow_light

The reason that the speed of light is a speed limit for any massive object is that the energy it takes to accelerate an object asymptotically approaches infinity as the speed of the object approaches $c$. This is a natural consequence of Einstein's special theory of relativity, whose fundamental postulate is that the constant $c$ is an invariant in every reference frame. While this might be extremely weird, it happens to produce results in remarkable agreement with our experimental observations. This idea originally came from a consideration of Maxwell's equations, which seem to produce frame invariant wave equations that propagate at the speed $c$ in every reference frame. It is almost a miracle that Maxwell was able to formulate his equations given that they pre-dated Einstein's theory!

Matt Hanson
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The special role of the speed of light in relativity is about the speed, not about light. That is, there is one particular special speed in the universe which all observers agree on. This speed acts as a scaling factor between space and time. It so happens that light travels at that speed, and so it's called "the speed of light" for historical reasons. But it might better be called "the speed of gravity" or "the speed of causality".

Eric Smith
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If we assume that the speed of light is fixed, that assumption leads to models that match reality extremely well. If we throw out that assumption, we do not know how to preserve that match to reality. Since that match to reality is fundamental to reasoning about physics, we have no way to reason about a variable speed of light.

I should add that in the current state of metrology, the speed of light is a fixed quantity by definition: we calibrate distance measuring instruments using clocks and an adopted value for the speed of light. If you seek a varying speed of light, you must redefine either time or distance relative to our present understanding.

John Doty
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  • Keep in mind that you have to be very careful in saying that the speed of light is fixed. It would be better to say that the speed of time is fixed for a local observer. For example, is you were to drill a tunnel through a large planet, then shine a light through that tunnel, then that light would actually be slower than without the planet because time in and around the planet has dilated. – foolishmuse Aug 03 '23 at 16:46
  • @foolishmuse We don't normally account for effects like that with a varying speed of light, but rather with a non-Euclidean metric. – John Doty Aug 03 '23 at 20:26
  • It can be described mathematically to suit the model of a constant c, but it still comes down to a varying c in the real world. Looking back at the OP, we come to realize that c is not as "fixed" as we might think. I find it very helpful now that I can visualize time and spacetime. – foolishmuse Aug 04 '23 at 15:13
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    @foolishmuse When we compare optical rulers to rigid material rulers, they agree within the limitations of material rulers. It's only when we assign coordinates to places and compute distances using Euclidean rules that the $c$ appears to vary. So, the phenomena prefer models in which $c$ is fixed and spacetime geometry isn't Euclidean. – John Doty Aug 05 '23 at 13:00
  • I guess it all depends on whether the Observer is riding on the photon, or is watching from afar. – foolishmuse Aug 05 '23 at 13:38
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    @foolishmuse Observers don't ride photons, nor do they watch them from afar. – John Doty Aug 05 '23 at 13:49
  • I was just out for a hike and realized that my leg is 1 m long. According to the standard you've put forth, next to a black hole my little toe would be 1 m long. I just can't work with that concept – foolishmuse Aug 05 '23 at 16:32
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    @foolishmuse No, assuming your little toe is rigid, it would theoretically be the same length it always is. Carry a ruler with you and measure it and you get the same answer. The ruler could be optical. But how you get "next to" a black hole is tricky conceptually. Remember, the radial parameter in the Schwarzschild metric isn't distance as measured by anything: it's the circumference of a sphere centered on the hole divided by $2\pi$. Not a Euclidean radius at all. – John Doty Aug 05 '23 at 16:43
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There is an alternative to string theory known as Loop Quantum Gravity which basically holds that space is particulate, with each particle being about 10^-30 meters in size.

While I was reading an early paper on LQG, it struck me that in a different universe, if the size of these particles was smaller or larger, then the speed of light would be faster or slower. So I developed the speculative theory of Dilating Loop Relativity which says that energy from a body leads to a dilation of these particles and thus to a slowing of time, and hence gravity.

So although LQG is still theoretical, and DLR is certainly speculative, it gives rise to thought about what is it that determines the speed of light. I can't help but wonder if c is only this speed in our universe.

foolishmuse
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    How do you define distance and time to allow the speed of light to vary? – John Doty Aug 01 '23 at 21:55
  • @JohnDoty The concept is that light must pass from particle to particle. If the particles are larger, then it takes light longer to pass from one to the next. Eventually, at a black hole the particles are so large that light can't make the jump at all. So you can think of how many particles there are in 1 metre of length of space. But remember that light is only a proxy for the speed of time. It is these particles that define the rate of time. – foolishmuse Aug 01 '23 at 22:00
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    What do you mean by "larger"? How do you measure? – John Doty Aug 01 '23 at 22:52
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    How do you determine what "1 metre of length of space" is? – John Doty Aug 01 '23 at 22:53
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    https://www.bipm.org/en/si-base-units/metre – John Doty Aug 01 '23 at 22:56
  • @JohnDoty obviously you cannot use what you are measuring to measure what you are measuring. But you could take a stick that is one meter long and use that. This is very similar to the idea that time dilates and therefore you cannot measure one second of time and on Earth against one second of time in distance space because they both measure one second, but we know that they are different – foolishmuse Aug 02 '23 at 01:32
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    How do you know that the stick is 1 meter long? – John Doty Aug 02 '23 at 11:56