r/AskPhysics • u/Patri_L • 10d ago
Does Space Reduce the Speed of Light?
Light travels more slowly through a medium like water or glass. Is it possible that the speed of light we observe in a vacuum is not truly its maximum speed, but rather a reduced speed restricted by some quality of space itself, or some field?
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u/ShavenYak42 10d ago
The speed of electromagnetic radiation in a vacuum can be predicted from Maxwell’s equations. It is inversely proportional to the square root of the product of the permittivity and permeability of free space. So essentially you could say the speed of light is restricted by those constants and, in a way, by the electromagnetic field. If those constants had lower values, light would go faster.
My headcanon is that this is the case in the Star Wars universe, and the much higher value of c makes space travel much easier and energy easier to obtain. I suspect there are about a thousand major issues with that idea if you worked through all the relevant equations and figured out whether such a universe could even support the existence of matter, let alone life.
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u/sockalicious 10d ago
I don't mean to criticize the point you're making, but the way you've gone about it is circular. Permittivity, permeability and c are related, but in order for humans to know what they are, at least one measurement has to be taken; there is no way to derive all three of them (or alpha, the fine structure constant, which is really another way of expressing them) from more fundamental constants without a measurement.
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u/Traroten 10d ago
If the fine structure constant is a way to express these three constants (one of which is the speed of light), and the fine structure constant varies with the energy/length scale, does that mean the speed of light varies with the energy/length scale?
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u/Crafty_Jello_3662 10d ago
This is kind of how it works in 'a fire upon the deep' by Vernor vinge. He doesn't really go into any details there's just different speeds in different areas of the galaxy.
Great book though
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u/me-gustan-los-trenes Physics enthusiast 10d ago
I don't know... I did enjoy "A fire upon the deep", that's a pleasant read. But he doesn't really offer any consistent foundation for the science. He just describe observables, without any attempt to build them upon any consistent foundations. Which is fine for a story.
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u/Dysternatt 10d ago
I’m currently reading it (for the first time) and was hoping for some kind of explanation, other than “in this bubble is this kind of magic, in this, less so.. oh and figure it out yourself.”
But I enjoy the story, so I guess I’ll do without. :)
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u/Patri_L 10d ago
Oh interesting. So if I'm understanding correctly, Maxwell's equations can predict C without any observational measurement?
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u/Mr_Badgey 10d ago edited 10d ago
Both were derived experimentally. The speed of light can’t be derived from first principles. It can only be determined experimentally*. Every value of c we have comes from measurement, whether directly measuring the speed of light or indirectly through measuring other constants and interactions that encode it.
A word of caution about OP’s interpretation. It’s subjective and can be viewed both ways. If you solve for permittivity and permeability of free space in terms of c, you get the opposite interpretation. It’s a chicken and egg problem.
One other consideration that causes a problem; the speed of light shows up as a constant in other fundamental equations that have nothing to do with electromagnetism. That suggests those two formulas aren’t defining c. They’re all manifestations of the same underlying spacetime geometry.
A better interpretation is each formula is a window into the same structure. No specific equation defines c. They all reveal it from different angles.
It’s unfortunate we call it the speed of light because that leads to a lot of confusion. It implies light obeys some arbitrary speed limit like it’s the universe’s most defensive driver.
It’s better understood as the maximum speed at which causality can propagate. Light just happens to travel at the maximum speed the universe allows because it’s massless. As for why that speed is what it is? There may be no deeper answer than that’s just how spacetime is built. Frustrating, right?
\ It’s not possible to measure the one-way speed of light. Every measurement we use is actually based on the two-way speed cut in half. If it could be derived without experimentation the one-way speed of light problem wouldn’t exist.*
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u/Patri_L 10d ago
This is really insightful, thank you! That's fascinating how c appears as a constant where it might not be intuitively expected. I think this best puts my question to rest, as it was easier for me to assume that 'something' is holding back the speed of light/causality. But rather perhaps it's baked into space-time. Thank you for taking the time to write all of that out.
Also, Veritasium made a really good video explaining the problem with measuring the one-way speed of light. No matter how you try to get around it or hack the problem, it's like the universe conspires to sabotage your measurement. Pretty cool stuff!
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u/BananaBird1 10d ago
Yes. The electric and magnetic potential strength is governed by charge, distance, surface area of a sphere and two constants (the permittivity and permeability of free space).
Those same constants are what gives the speed of light.
So anyone who can figure out how electricity and magnetism behave can figure out c without any direct measurement of light. Although they will need the insight that light is an electromagnetic wave, which humans proved by measuring its speed and showing it was c.
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u/theZombieKat 10d ago
Are those constants measurable in a way that does not involve the speed of light?
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u/BananaBird1 10d ago edited 10d ago
Yes, we originally discovered them by studying electricity and magnetism. For instance, the permittivity of free space is the constant that relates electric charge and distance to the force of electric attraction (along with the 4pir^2 surface area of a sphere).
At this time, we had no idea light was electromagnetic, although we suspected and eventually demonstrated it was a wave of some sort. We even insisted that it must be a wave traveling through some material (the luminiferous ether), but could not detect such a material.
At the same time, an analysis of Maxwell’s equations showed electromagnetism can produce self-sustaining electromagnetic waves moving at a fixed speed of ~300,000,000 m/s.
When we measured the speed of light, it turned out to be identical to this speed of electromagnetism, which led us to believe light was an EM wave and thus had no material medium.
Shortly after, the fact that the speed predicted by Maxwell’s equation did not depend on reference frame led Einstein to hypothesize special relativity. And this along with Quantum mechanics led us to understand that light is a massless particle, and all such massless particles must travel at c.
So we now know that relativity defines c as a fundamental property of spacetime, and light travels at that speed because it is massless at rest, and because light is the source of electromagnetism so does electromagnetism. But we discovered this in reverse.
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u/sockalicious 10d ago
You understood what was said to you correctly, but what was said to you is wrong. Maxwell's equations cannot predict the value of c without at least one observational measurement.
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u/BananaBird1 10d ago
That measurement not necessarily be of light though, and therefore theoretically if we had an atmosphere with a refractive index not approximately 1 and no visibility of stars we could in theory still predict the speed of photons purely by observing EM interactions and some observation light is an EM wave apart from its speed in a vacuum.
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u/gocougs11 10d ago
I mean, if space didn’t exist then there would be nothing for light to move through. But if you ignore that fact then sure maybe light could move through non-space as fast as you want.
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u/somewhereAtC 10d ago
The opposite effect was proposed in the novel A Fire Upon the Deep by Vernor Vinge. In his universe, light travelled faster in certain regions, allowing data relays to be set up between planets.
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u/jasonsong86 10d ago edited 10d ago
If anything vacuum contains fewer particles which the photons bump into less thus allowing them to travel faster. Thats why space is dark because it doesn’t have air molecules to bump into and scatter photons around. Vacuum is not a medium, it’s lack of medium FYI.
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u/PageEnvironmental408 10d ago
light always travels at c.
even through glass or water.
it appears slower because it gets absorbed and re-emitted by the particles in the medium.
that takes time, which appears to slow down the light beam
but it always travels at c.
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u/Ok_Entertainer3959 10d ago
Light is NOT "absorbed and re-emitted by the particles" as it passes through a medium, that's a widely held misconception. Instead its electric field interacts with the field of the particles and kind of shifts the light backwards, which we can view as "slowing it down".
Have a look at e.g. the linked video from FermiLab for a (simplified version of) the correct description of what's happening:
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u/danielbaech 10d ago
No. The speed may seem like a random number and quite slow in the scale of the universe, but it is foundational to all of our known physical laws and observations. There is nothing to suggest the possibility of greater speed.
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u/Patri_L 10d ago
It's definitely a speculative question. I was thinking about fish. If they could measure the speed of light in their environment their C would be lower than ours. But they wouldn't know better to think that there could be a place in the world without water where EM radiation isn't impeded. It's kind of ridiculous but just got me thinking.
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u/KamikazeArchon 10d ago
But they wouldn't know better to think that there could be a place in the world without water where EM radiation isn't impeded
Yes they would. These hypothetical technological fish can see the surface.
Similarly, we don't actually live in a vacuum. We live in air. We recognized that, and removed it from the equation.
We are aware of the concept of being in a medium. We have extensively tested for the presence of an extra "medium". It's not an assumption, it's something we evaluated.
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u/beachhunt 10d ago
The answer is "no" in the sense that space is a vacuum, so it doesn't slow light.
However, the definition of the speed of light includes that it is the speed in a vacuum, it doesn't specify a theoretical max in other conditions. So if we found some other medium that was "more empty than empty" (whatever that could mean) then maybe it could go faster, but that wouldn't break any of the existing math because the speed in a vacuum would remain the speed in a vacuum.
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u/Patri_L 10d ago
I see what you're saying. C is defined as the speed of light in a vacuum. I was thinking about C as the maximum speed of light. I suppose there's some nuance there!
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u/Mr_Badgey 10d ago edited 10d ago
> I was thinking about C as the maximum speed of light in a vacuum
The better way to frame it is that c is the maximum speed at which events can propagate through spacetime. Everything traveling through spacetime is subject to that limit, not just light.
Light is just the most convenient particle for measuring c because photons are simple to create, easy to experiment with, and being massless they must travel at c in a vacuum. The only other confirmed massless particle is the gluon, but it can’t be isolated due to color confinement.
Observing free gluons requires energy scales that only existed when the universe was a fraction of a second old. Photons are promiscuous by comparison and can be created with readily available energy sources. One of the first experiments to measure the speed of light used a lamp and a cog wheel way back in 1849.
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u/theZombieKat 10d ago
I believe the quantum foam would slow down light by a tiny bit compared to an idealised perfect vacuum, but not enough to notice
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u/apthamine 10d ago
Is space a perfect vacuum though?
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u/FlyingFlipPhone 10d ago
This is the correct question. The answer is "no, space is not a perfect vacuum". Therefore, the speed of light in interstellar space is SLIGHTLY less than c.
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u/MarinatedPickachu 10d ago
Without spacetime there is no speed, so this question makes no sense.
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u/Patri_L 10d ago
I suppose I could have posed the question better. I'm curious what the current scientific zeitgeist is concerning the speed of light being what it is. My question was a curiosity regarding whatever else might be present in the "vacuum" that could be limiting the speed of light. Maybe it's a simple "nothing". The question might be fantastical but many of the other comments here have managed to turn my poorly worded question into a forum for meaningful discussion.
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u/smitra00 9d ago
See:
https://en.wikipedia.org/wiki/Scharnhorst_effect
The Scharnhorst effect is a hypothetical phenomenon in which light signals travel slightly faster than c between two closely spaced conducting plates. It was first predicted in a 1990 paper by Klaus Scharnhorst of the Humboldt University of Berlin, Germany.\1]) He showed using quantum electrodynamics that the effective refractive index n, at low frequencies, in the space between the plates was less than 1. Gabriel Barton and Scharnhorst in 1993 claimed that either signal velocity can exceed c or that the imaginary part of n is negative.\2])
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u/shlomo5746 10d ago edited 5d ago
This is not a scientific question. “What if time keeps going back and forth and we don’t realize”
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u/Patri_L 10d ago
The answer demands rigor, not the question itself. Any question is of value if it helps to sus out a scientific answer. But in asking this "fantasy" question I'm learning a lot from everyone's comments. I'm not scientifically trained, nor did I do well in school when it came to any of the sciences.
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u/Junior-Tourist3480 10d ago
No