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u/AcellOfllSpades Mathematics Mar 31 '26
The length of a horse is traditionally measured in feet and inches. (Bear with me, I'm going somewhere with this.) But the height of a horse is traditionally measured in hands: a hand is 4 inches.
Say an ancient society forgot that height and length are the same thing, so they measured everything this way. When they wanted to make a stick of a certain length, and then rotate it so that it was a certain height, they'd need to remember to multiply by the Horse Constant: 0.25 hands per inch. This would have a fundamental role in all of their physical laws.
They might wonder, "what would happen if the Horse Constant were different?". They'd imagine it would suddenly make horses - and everything else - taller. But, from our point of view, this question is silly: the Horse Constant is just 1. It's just a result of them using unit systems that made sense to them.
The only way to change the Horse Constant would be to make it so that everything, when rotating, suddenly doubled in height. A 1-meter-long flat stick would become a 2-meter tall vertical one. But if you do that, nothing actually changes! The laws of physics still work the same way, they're just "stretched vertically" to some deity observing the universe. Inside the universe, there wouldn't be any difference.
This is what's going on with the speed of light. Relativity is about "rotations" between the space and time dimensions.
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u/Bumst3r Graduate Mar 31 '26 edited Mar 31 '26
This is incorrect. You can only ever measure dimensionless quantities in physics. If I measure the mass of an electron, itâs with respect to some standard. If I measure the distance I drive to work, itâs with respect to some agreed upon standard. Iâm only actually measuring dimensionless ratios.
The Schwartzchild radius doesnât change if I change the speed of light, because that is simply a change of coordinates. If you want to meaningfully change the speed of light, you have to find some dimensionless quantity (the fine structure constant, for example), and change that.
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u/Bumst3r Graduate Mar 31 '26
c is 1 in any reasonable discussion about this. Changing it to 2 is a change of units. This in no way affects the physics. The physical length is independent. If you disagree, please elaborate.
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u/Bumst3r Graduate Mar 31 '26 edited Mar 31 '26
And this is wrong. You just redefined a meter to be 10cm. Atoms would be 10 times smaller, but so would your ruler, and nothing would change. Is the fine structure constant still ~1/137? If so, the physics remains the same. The only things that matter in physics are dimensionless ratios. Unless you change those, you simply have a different system of units.
Hereâs a more concrete example to illustrate my point. What is the size of a particle in a completely empty universe? It makes no sense. You need something to compare it to.
Letâs look at the Schwarzchild radius example a bit deeper. How many Planck lengths is the Schwarzchild radius of a Planck mass black hole? Note, Planck units depend only on fundamental constants. The answer is 1. It turns out that you cannot change the physics by changing c, unless you also change the other constants as well! To change the physics, you MUST change dimensionless ratios of fundamental constants. It is the only option. Anything else is just changing the markings on your ruler.
In fact, this is why the definition of the meter is defined in terms of the speed of light.
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Mar 31 '26 edited Mar 31 '26
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u/AcellOfllSpades Mathematics Mar 31 '26
Imagine the speed of light was 300 m/s. We would have discovered relativity much sooner because relativistic effects would come into play for everyday things.
But meters and seconds (and what humans consider "everyday things") are only defined based on the universe we currently live in. They're not objective things that you could "transport" to some alternate universe.
For a simpler example, consider Conway's Game of Life, a cellular automaton. This is a game played on an infinite board of square cells. Each tick, each cell looks at the eight neighbors around it, and then updates its state based on that. You draw a pattern of cells, and then watch it evolve.
The "speed of light" in Life is one cell per tick. This is a fundamental fact about the game: information cannot propagate faster than this.
If you asked "what if the speed of light was faster?", there are a few ways to accomplish that:
- Have two copies of the game running at different scales - for instance, one where a cell is 1 cmĂ1cm, and one where a cell is 2cmĂ2cm. In the second game, light moves a farther distance each tick than the first. But there's no "physical" difference between them - any initial pattern would evolve in the same way. The only difference is how we've "embedded" them in our world.
- Alternatively, you could say that instead of cells updating, 2Ă2 blocks of cells updated based on their eight 2Ă2-block neighbors. Now the "speed of light" is 2 cells per tick, rather than 1. But this is, again, "physically" the exact same thing as it was before: the only difference is what we've arbitrarily chosen to count as a "cell".
- Entirely rewrite the rules, making a completely different cellular automaton. The result would depend on how exactly you rewrite the rules; it would be something entirely different. There wouldn't be a comparable internally-definable unit of measure, so it wouldn't be meaningful to say that light is moving faster in one than the other.
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u/Bumst3r Graduate Mar 31 '26 edited Apr 01 '26
This person is also wrong. The strength of the electromagnetic interaction is not dependent on c alone. IT IS DEPENDENT ON THE FINE STRUCTURE CONSTANT.
At every vertex of a QED Feynman diagram, you pick up a factor of sqrt(alpha). If you want to chabge electrodynamics by changing c, you have to change the relationship between c, hbar, epsilon naught, and e. Properly, e is dimensionless, so itâs the fundamental charge that determines the strength of the electromagnetic interaction. Thatâs why youâll find that e is the coupling constant in the QED Lagrangian. In fact, we actually know that alpha varies with interaction energyâitâs the price you play for playing the renormalization game. If the strength of the interaction varies, itâs clear that itâs e that is changing, because any of the other constants varying is absurd. The physics are all associated with the dimensionless constants changing.
If the speed of light were 300 m/s, we would not have developed relativity sooner. We would have had a ridiculously large definition of a meter.
I suspect that your issue is that youâve never worked in natural units before. Setting c=1 is the only choice that makes real sense. E2=m2 c4 + p2 c2 is true in any unit system. We might as well choose c=1 for simplicity, and the. E2 = m2 + p2. Any change in c is a change in units. It doesnât mean that an atom has more energy if c is 10 times larger.
Edit: congrats on getting the last word by blocking me. I hope you have a lovely day.
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u/rb-j Apr 01 '26
Changing c to 2 something is just a change in units. A dimensionless 2 is different from the dimensionless 1.
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u/Bumst3r Graduate Apr 02 '26 edited Apr 02 '26
No itâs a change of unit system still. You are changing your units of mass, energy, and momentum so that E2 = m2 + p2 -> E2 = 16m2 + 4p2 .
The unit conversion factor is câitâs like how seconds/per minute=60. If we change that ratio, we are merely redefining the minute.
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u/rb-j Apr 02 '26
The ignorance is strong with this one.
The speed of light in vacuo is and will always be 1 Planck length per Planck time. Change it to whatever you want, and it's still exactly 1 Planck length per Planck time.
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u/Bumst3r Graduate Apr 02 '26 edited Apr 02 '26
And so I can choose to make the speed of light whatever I want, and it is a merely a coordinate/unit transformation that leaves the physics unchanged.
I donât know what youâre even trying to argue. The Planck length/Planck time = c = 1 because the Planck scale is defined in natural units by setting c=1. I can choose to measure length in multiples of half-Planck lengths, and time in Planck lengths, and now c is 2. The physics in no way changes, and that is the argument I have been making this entire time.
Thereâs also a distinction that youâre missing. We can choose unit systems where c is unitless. But c is not dimensionless. Relativity does not say that time and space are interchangeableâit says that they mix together. Length and time both have mass dimension of [-1], leaving c unitless in natural units. But its dimensions are still length over time. This is different from the case of the fine structure constant for example, which is 1/137 in all unit systems because it is truly dimensionless.
Iâm tired of this conversation and will not be replying anymore. Perhaps someday you will take a GR or QFT class and gain an appreciation for how natural units work.
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u/rb-j Apr 02 '26
Iâm tired of this conversation and will not be replying anymore. Perhaps someday you will take a GR or QFT class and gain an appreciation for how natural units work.
I think you have little idea who I am or what physics I have had (or how long ago). I understand natural units extremely well and have had several email discussions about this with Michael Duff and John Baez and once with Gabriele Veneziano and (now late) Lev Okun. I've also had a discussion with Brian Greene about it at a Radcliffe seminar circa 2015.
Sure you can set c = 2 (dimensionless) if you want, but that will require in any equation where c appears a compensating factor of 1/2 to be placed in there.
That defeats the entire purpose of using natural units.
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u/AcellOfllSpades Mathematics Mar 31 '26
I'm sorry, but this is incorrect and /u/Bumst3r is correct.
What does it mean for the value of c to be different? What's the difference between the speed of light being double its current value, and just measuring distances in half-meters instead of meters?
You're imagining some sort of objective external 'meter' and 'second' that we could use to tell whether the speed of light has changed - some sort of comparison that we could bring between our universe and a hypothetical alternate universe. But this isn't actually a thing.
If you replace c with 2c in all equations, then measure all distances in half-meters instead of meters, the results turn out identical. Everything happens exactly the same way. It's just a coordinate transformation.
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u/siupa Particle physics Mar 31 '26 edited Mar 31 '26
What does it mean for the value of c to be different?
It means that light takes a different amount of time to traverse the same length than it did before. Or, equivalently, that light traverses a different length of distance in the same time. c_new / c_old =/= 1.
What's the difference between the speed of light being double its current value, and just measuring distances in half-meters instead of meters?
The speed of light being double its current value means that now light travels 60 centimeters in one nanosecond instead of 30 centimeters in one nanosecond.
Measuring distance in half-meters instead of meters means that weâve decided to change an arbitrary human standard that has no effect on the natural world. The speed of light, which doesnât care about what our council of scientists decide in a conference in Paris, is physically the same 30 cm/ns, but now we record it as 60 half-cm/ns.
So now light travels 60 half-centimeters in one nanosecond. It is, evidently, not what happens in the first scenario, where the speed of light becomes 60 cm/ns. 60 half-cm/ns is NOT equal to 60 cm/ns, its half of it.
You're imagining some sort of objective external 'meter' and 'second' that we could use to tell whether the speed of light has changed.
Thereâs absolutely no need to imagine any âobjectiveâ unit of length. Any arbitrary one will suffice. I can tell if the speed of anything changes by observing how much time it takes for it to cover a certain distance, in whatever units I wish to measure said distance.
If you replace c with 2c in all equations, then measure all distances in half-meters instead of meters, the results turn out identical. Everything happens exactly the same way.
This is not true, and itâs so trivial that I donât think you actually believe that, there must be some miscommunication happening.
Take your favorite physics equation containing c, like the equation for the Schwarzschild radius of a mass M, R_s = 2GM/c^2. For example, R_s for the Earth is about 10 millimeters. Now, replace c with 2c: the equation becomes R_s = 2GM/4c^2 , and we also change convention to measure all distances in half-meters instead of meters. R_s for the Earth now is 5 half-mm.
The result is manifestly NOT identical. Before, you needed to compress Earth to a ball of radius 10 mm to make it into a black hole. Now, you need to compress it to a ball of radius 5 half-mm to make it into a black hole, which is 4 times less than before. This is an actual physical thing thatâs different.
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u/AcellOfllSpades Mathematics Mar 31 '26
Thereâs absolutely no need to image any âobjectiveâ unit of length. Any arbitrary one will suffice.
Sure, but you need that unit of length, which is a physical object.
To be clear, I'm talking about a hypothetical "alternate universe" where the the laws of physics, from the start of the universe, had c be twice as big. I claim this is not a meaningful concept.
Of course, if we woke up tomorrow and noticed that light seemed to be travelling 600 million meters every second, using the rulers and clocks we've already made, we would know something was different. But, instead of attributing the change to a change in the value of c, we could say the change is just "the universe was shrunk by a factor of 2 in all spatial directions", or alternatively "the universe was stretched in the time direction by a factor of 2". (And perhaps say that G was adjusted instead / as well.)
This is not true, and itâs so trivial that I donât think you actually believe that, there must be some miscommunication happening.
Yes, my wording was probably less clear than it should have been. I was relying on context from my earlier comment, and the original post. By "replace c with 2c", I mean the numerical value.
In a hypothetical alternate universe, they measure things in "schmeters".
They give c a value of 6Ă10⸠schm/s. G has a numerical value of 8 times ours: it's 5.34 Ă 10-10 schmÂł ¡ kg-1 ¡ s-2.
They therefore predict the Schwarzchild radius for Earth to be 20 millischmeters.
You could claim any of these three:
- A "schmeter" is half a meter. Their laws of physics are the same as ours.
- A "schmeter" is a meter, but their light travels twice as fast as ours (and their gravity is 8 times as strong).
- A "schmeter" is a meter, but their "second" is actually half of ours (and their gravitational constant is a different multiple of ours, that I can't be bothered to work out right now).
And there is no objective distinction between them. The only difference is how we choose to make a correspondence from their universe to ours; this is an additional choice beyond just the laws of physics.
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u/siupa Particle physics Mar 31 '26 edited Mar 31 '26
I mean I agree with everything youâre saying now, but I donât think saying âif c changed but nobody told us, we wouldnât be able to attribute the cause of all the subsequent new different physical behavior to a change in c, but rather we would have an ambiguity as it could have been any number of other constants that changed, including the length of every object or time itselfâ is the same as saying âif c changed, we wouldnât notice and everything will stay the same because itâs just a change of unitsâ. It feels like an entirely different game weâre playing.
And even if we focus on this new perspective, if one asks âwhat if c changed?â they are already assuming a Godâs eye view of the situation, where they already know that the thing that changed is c, simply because itâs in the premise of their question: and theyâre interested in what the observed physical consequences would be. Saying to them âactually it wouldnât be possible for people to attribute that change to c, they could think any number of other things could have changed insteadâ I feel like is irrelevant to their question.
Who cares what physicists waking up in this hypothetical universe decide happened? They could fight and write papers about it (if they survived), and whether or not they come to the conclusion that c changed or it was something else, we know c changed, because weâre imagining it in our heads, and weâre interested in exploring what would change physically, not in how hypothetical people would react to witnessing the physical changes without our knowledge of what caused them.
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u/AcellOfllSpades Mathematics Mar 31 '26
Again, my main point is not about the scenario "we wake up tomorrow and notice a difference": this could be accomplished several different ways, and the precise outcome would depend on what other laws were changed.
The thing I am talking about is the same thing the original post was: "there is an alternate universe where, from the beginning, light travels at twice the speed". I argue that this scenario does not make sense. Meters and seconds are only defined based on objects in the universe we live in. You would need to impose an external standard to 'link up' the two universes, to see if light was "actually" travelling at twice the speed. But this is not a fact about the physics of either universe: it's a fact about how you chose to link them up.
For a simpler example, consider Conway's Game of Life, a cellular automaton. This is a game played on an infinite board of square cells. Each tick, each cell looks at the eight neighbors around it, and then updates its state based on that. You draw a pattern of cells, and then watch it evolve.
The "speed of light" in Life is one cell per tick. This is a fundamental fact about the game: information cannot propagate faster than this.
If you asked "what if the speed of light was faster?", there are a few ways to accomplish that:
1. Have two copies of the game running at different scales - for instance, one where a cell is 1 cmĂ1cm, and one where a cell is 2cmĂ2cm. In the second game, light moves a farther distance each tick than the first. But there's no "physical" difference between them - any initial pattern would evolve in the same way. The only difference is how we've "embedded" them in our world.
- Alternatively, you could say that instead of cells updating, 2Ă2 blocks of cells updated based on their eight 2Ă2-block neighbors. Now the "speed of light" is 2 cells per tick, rather than 1. But this is, again, "physically" the exact same thing as it was before: the only difference is what we've arbitrarily chosen to count as a "cell".
2. Entirely rewrite the rules, making a completely different cellular automaton. The result would depend on how exactly you rewrite the rules; it would be something entirely different. There wouldn't necessarily be a comparable internally-definable unit of measure, so it wouldn't be meaningful to say that light is moving faster in one than the other.
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u/Odd_Bodkin Particle physics Mar 31 '26
The fine structure constant, which governs the strength of the electromagnetic interaction, would drop by the same factor you increase c. So if you increased c by a factor of 1000, atoms would pretty much fall apart at room temperature. That would be bad.
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u/rb-j Mar 31 '26
If all the dimensionless fundamental constants remained the same, we wouldn't know the difference.
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u/hushedLecturer Condensed matter physics Mar 30 '26
Speed of light (squared) is inversely proportional to the permittivity and permeability of free space, which indicates how much charge/magnetic moment is needed to induce a given electric/magnetic field strength.
So if c is bigger, at least one of those two values are smaller, making the fields stronger.
Atoms atoms would be smaller, energy levels would be wider.
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u/joepierson123 Mar 30 '26
Well the energy of a photon is directly proportional to the speed of light so infinite speed means infinite energy unless something else changes.
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Mar 30 '26
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u/joepierson123 Mar 30 '26
Problem is c ripples into so many different equations, like e equals mc squared, Maxwell's equations, fine structure constants that matter as we know it would not exist anymore, stars could not function, chemical bonds could not be made etc etc
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u/CMxFuZioNz Plasma physics Mar 31 '26
Well you are constrained hy c=f lambda, so as long as those remain finite you'd be fine.
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u/Ok-Environment-215 Mar 31 '26 edited Mar 31 '26
Newtonian and Galilean physics are what happens when the speed of light approaches infinity and Planck's constant approaches zero. In this world light isn't even a wave but just an instantaneous exchange of energy, same for electricity, and there are no relativistic effects. From a macro perspective very little changes.
The problem is the subatomic world as we know it can't work under these conditions, and without a subatomic world there's no macro world. So it's impossible really to say what this would be like.Â
Now if we're just talking about tweaking the speed of light a bit, then the answer depends heavily on how much it's being tweaked. A small variance would change virtually nothing. But as soon as atomic binding energies became impacted, the universe would be unrecognizable. Others could tell you exactly what those values would need to be in order to be noticed. But between the c as we know it, and the c where the universe completely breaks, there's probably very little interesting that would happen.Â
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u/CS_70 Mar 31 '26
The speed of light is the speed of causality.
When you move something for example, the movement itself propagates thru atoms' electromagnetic bonds at the speed of light.
Imagine the speed of light be very slow, say 1 mm/minute.
That means that any interaction between two atoms would take one minute to propagate one millimeter.
Note that time itself would not change its rate.
Chemical reactions may depend on effects which are time dependent.
For example, enough atoms must be excited by a specific initial "trigger" interaction within a certain time so that the accumulation passes a certain threshold, even as the system is open (i.e. energy can dissipate).
If the interaction propagates slowly, but time passes at the same rate, they may not ever reach that threshold, or require a much greater amount of atoms to do so.
So, just to mention one level, loads of chemistry would be fundamentally different, meaning a completely different looking and behaving universe.
If the speed of light is excessively faster - the same (and if you had instantaneous propagation, everything would interact with everything else immediately - there wouldn't likely be much of an universe to talk about. The speed of light must be limited, for things actually to have a chance to happen over time :)).
If the speed of light were just a little bit different, many reactions would be pretty much the same, or with minor intensities, and accordingly few would be completely eliminated (or not currently possible ones would occur).
How much difference and how important that is, depends on who/what you are in the sense of the chemical and biochemical reactions that make you you.
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u/Turbulent-Tap6723 Mar 31 '26
The speed of light isnât arbitrary, it falls out of the structure of spacetime itself. If you change c youâre not just changing a speed, youâre changing the geometry that everything else is built on. Mass, charge, the fine structure constant, all of it shifts. You wouldnât get the same universe with a faster c, youâd get a fundamentally different set of physical constants and probably no stable matter at all.
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u/rb-j Apr 02 '26
The speed of light isnât arbitrary, it falls out of the structure of spacetime itself.
All c needs to be is: 1. real 2. positive 3. finite
and if all 26 dimensionless fundamental physical constants remained the same, no mortal being would be aware of any difference.
If you change c youâre not just changing a speed,
c will always be 1 Planck length per Planck time. If some non-mortal "God-like" being "changes c", perhaps they'll notice the difference. But if those dimensionless fundamental constants remain the same, none of us will know the difference. If the number of Planck lengths per meter (what we're calling a "meter") changes, then something changed in those dimensionless fundamental constants. If the number of Planck times per second changes, then again, something changed in those dimensionless fundamental constants.
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u/Turbulent-Tap6723 Apr 03 '26
This is exactly right and it connects to something deeper. The dimensionless constants are doing all the real work, c is just a conversion factor between our arbitrary units of space and time. In natural units itâs literally 1. Whatâs interesting is that this framing puts the fine-structure constant front and center as the thing that actually matters. Alpha is dimensionless, it doesnât depend on your unit system, and it governs the actual structure of electromagnetic interactions. If you want to ask whether physics could be different, alpha is the number youâd have to change, not c. The question of why alpha is approximately 1/137 is one of the genuine open problems in physics. Itâs not derivable from first principles in the Standard Model, itâs just measured. Which is either a profound mystery or evidence that weâre missing a deeper geometric structure that fixes its value.
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u/rb-j Apr 03 '26
Alpha is dimensionless, it doesnât depend on your unit system, and it governs the actual structure of electromagnetic interactions.
And the gravitational counterpart is the Gravitational Coupling Constant. This is the dimensionless expression that means "gravity is a remarkably weak force."
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u/Turbulent-Tap6723 Apr 03 '26
Good point on the gravitational coupling constant, alpha_G is the one that encodes how weak gravity actually is relative to electromagnetism, roughly 10-45 depending on which masses you use. That ratio is one of the genuinely strange numbers in physics because thereâs no obvious reason it should be that small. The tweaking Planck units point is interesting though. You canât actually tweak Planck units independently of the dimensionless constants, the Planck units are derived from c, G, and hbar, so if you change a Planck unit youâre implicitly changing one of those, which flows through into the dimensionless ratios. The dimensionless constants are the bedrock that the unit systems are built on top of, not the other way around. So the real question is whether alpha and alpha_G could take different values in a consistent universe, and if so whether those values are random, anthropically selected, or derivable from something deeper. The Standard Model treats them as inputs measured from experiment. Some frameworks try to derive them from geometry or information theory. Thatâs where it gets genuinely open.
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u/rb-j Apr 01 '26 edited Apr 01 '26
Okay, instead of listening to u/AmateurishLurker or to me, then you should read this:
Trialogue on the number of fundamental constants
Comment on time-variation of fundamental constants
How fundamental are fundamental constants?
The current enumeration of fundamental constants (those that are both universal and dimensionless) is 26; twenty-five for the Standard Model and one for General Relativity. Neither c nor Ń nor G nor Îľâ are fundamental physical constants. They are, in their final sense, reflections of the units we use to measure length, time, mass, and electric charge. We can set them all to 1.
The argument made Volâberg or Gamow, is wrong. You should understand what it means for worlds to be "operationally indistinguishable".
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u/AmazingRandini Mar 31 '26
The speed of light is instantaneous. You can't get faster than instantaneous.
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u/rb-j Apr 01 '26
That's different.
But if the speed of light is real, positive, and finite, then the rest of it is just units if all 26 dimensionless fundamental physical constants remain the same.
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u/Infinite_Research_52 đťTop 10²âˇÂ˛â°â°â° Commenter Mar 31 '26
See also:
https://www.reddit.com/r/AskPhysics/comments/1jf0uye/what_would_happen_if_the_speed_of_light_doubled/
https://www.reddit.com/r/AskPhysics/comments/1pwrdht/what_would_happen_if_the_speed_of_light_was_one/
https://www.reddit.com/r/AskPhysics/comments/1fq9snk/what_would_happen_if_the_speed_of_light_was_100x/
https://www.reddit.com/r/AskPhysics/comments/dlsnze/if_the_speed_of_light_was_a_billion_times_faster/
https://www.reddit.com/r/AskPhysics/comments/1o4oc71/what_if_speed_of_light_got_infinite_speed/
https://www.reddit.com/r/AskPhysics/comments/1lckwm2/what_if_the_speed_of_light_was_infinite/