# Yeah, but WHY is c the speed limit of the universe? **URL:** \ **Category:** Factual Questions\ **Tags:** science-math\ **Created:** [October 15, 2024, 5:03pm UTC](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837 "2024-10-15T17:03:26Z")\ **Posts on this page:** 20\ **Page:** 3
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [October 15, 2024, 11:25pm UTC](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837/41 "2024-10-15T23:25:13Z")
Perhaps the question can be recast in the reverse. It isn’t that c is a funny number, and why is it so, but rather, whence the slew of other constants that define the more mundane aspects of the nature of the universe. The fine structure constant was mentioned upthread. We know it is really really important and fundamental to the way a lot of matter works. But have no clue as to why it has the value it does. Us, as observers of the universe, are built out of such constants, and the final way we see the universe is defined by them. Asking why we see c as having a certain value is really asking why all the other defining constants lead us to measure c the way we do, and why they have the values they do. And for the most part we have no clue. It isn’t c’s fault that we are created the way we are. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [October 15, 2024, 11:31pm UTC](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837/42 "2024-10-15T23:31:18Z")
> [@Stranger\_On\_A\_Train](#): > > Fundamentally, the speed of light, _c_ is related to two other natural constants, the electric permittivity of vacuum, ε0, and the magnetic permeability of vacuum, _μ_ 0 [in a specific ratio](https://scienceworld.wolfram.com/physics/PermittivityofFreeSpace.html). And if either of these values were different by even a very tiny proportion, we would have a very different universe from the cosmological scale right down to the subatomic level even if the relative behavior of spacetime was the same. If \epsilon\_0 times \frac{1}{\epsilon\_0} were anything other than 1, then yes, it would be a very different Universe indeed. \mu\_0 is by definition 1/\epsilon\_0. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [October 15, 2024, 11:48pm UTC](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837/43 "2024-10-15T23:48:18Z")
> [@Saint\_Cad](#): > > I’ll try again. What is it about the structure of the universe makes the velocity of light what it is instead of something else like 15 mph? In the details my answer is “I have no idea”. Conceptually… The universe where you measure the speed of light at 15MPH with the miles and hours that we know of doesn’t work, doesn’t even make sense. I know you are asking why couldn’t in a different universe c could appear to be slower than it is in ours but in any universe at all like ours at the speed of light the same things will happen as described by @UncleMoose and whatever that speed is measured as will be the seeming limit there whatever units are used. It’s the point where we can’t go any faster in any universe that we can make sense of. > [@UncleMoose](#): > > The shortest explanation I’ve seen for the numbers (that fall out of Einstein’s equations) is that for a photon, the speed of light is where the time dilation is infinite (time slows down to zero) and the length contraction in the direction of motion shrinks to a zero size. So the actual speed isn’t random, it falls out of the general relativity equations. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [October 16, 2024, 12:02am UTC](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837/44 "2024-10-16T00:02:03Z")
> [@Stranger\_On\_A\_Train](#): > > This just isn’t true; while massless particles can only move at _c_, and massy particles at some relative speed less than this, This is what I intended to convey. ‘defined’ was not the right word to use there, but things don’t move faster than the speed of light as you say. Obviously any number of miles per hour can always have something added to it to define a speed. ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [October 16, 2024, 12:09am UTC](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837/45 "2024-10-16T00:09:24Z")
The problem is that the very definition of speed is distance times time, and what Einstein discovered is that neither space nor time have the simple additive qualities that had been presumed since Newton’s day. In Einsteinian physics the speed of light is a constant, and space and time are defined in reference to that rather than the other way around. > [@Francis\_Vaughan](#): > > Perhaps the question can be recast in the reverse. It isn’t that c is a funny number, and why is it so, but rather, whence the slew of other constants that define the more mundane aspects of the nature of the universe. And for those who take the stance that the speed of light is the speed of light is the speed of light, @Francis_Vaughan’s answer is probably the closest we’re going to get. Namely that if _C_ simply is what it is, where do all those other constants get the precise ratios to _C_ that they have? ---
**Author:** ![wolfpup](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/wolfpup/32/10618_2.png) [@wolfpup](https://boards.straightdope.com/u/wolfpup)\ **Post date:** [October 16, 2024, 1:29am UTC](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837/46 "2024-10-16T01:29:45Z")
> [@Chronos](#): > > > [@FlikTheBlue](#): > > > > Sure. But it being meaningless and random isn’t really an answer. It’s just an explanation as to why the particular value that c happens to be can’t be determined through scientific inquiry. What that number is? Sure, we can answer that. Why that particular value? As you say, it’s most likely random, and so trying to answer why that particular number is unlikely to give a useful answer. > > Really, that’s not even a question about c. It’s a question about the units we use. c is 1, and 1 is not a value that inspires us to ask “why”. This is the position I tend to favour. The “speed of light” is just a euphemism for a fundamental property of spacetime. It’s the speed of causality in the universe. It’s usually expressed as a peculiar large number just because of the units we happen to choose to express it. My other little nitpick is with the idea that photons are “massless”. They are, and they aren’t. The correct statement is that they have zero **rest mass** , but since photons are never at rest, this is pretty meaningless. But in reality photons have momentum, or else how could solar sails work? The formula for momentum in classical physics is p = mv, so zero rest mass equals zero momentum, but not when a particle is moving extremely fast, close to c. Here the momentum is p = \frac{h}{𝜆} where h is the Planck constant and 𝜆 is the wavelength of the photon. Another way to look at it: a photon has energy, inversely proportional to its wavelength, and energy is mass. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [October 16, 2024, 1:37am UTC](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837/47 "2024-10-16T01:37:04Z")
> [@wolfpup](#): > > Another way to look at it: a photon has energy, inversely proportional to its wavelength, and energy is mass. Ordinary classical objects like a brick have momentum due to (rest) mass and (negligible) additional momentum due to relativistic mass effects. Photons have no (rest) mass and no rest-mass based momentum but have non-negligible momentum due to relativistic mass effects. Both have both; it’s just a matter of which flavor predominates. ---
**Author:** ![md-2000](https://avatars.discourse-cdn.com/v4/letter/m/9d8465/32.png) [@md-2000](https://boards.straightdope.com/u/md-2000)\ **Post date:** [October 16, 2024, 1:50am UTC](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837/48 "2024-10-16T01:50:28Z")
The speed of light is the _speed_ of _light_. Light is an electromagnetic wave (or a particle - take your pick), so the speed at which a photon travels is how fast the elctromagnetic wave propagates. Due to the nature of the universe, the constants mentioned above, elctromagnetic waves in a vaccuum propagate at a certain speed. The weird thing that’s hard to wrap you head around, is this is the same speed for every observer, no matter how fast those observers are moving WRT each other. If Bob is zippng past you toward a light source at 1/2C and he and and you measure the same speed of light C from that source, it’s because time is slowed and distance in direction of travel shrunk in his world compared to yours. (From your point of view. From his, it all looks normal) The paradoxes of Einstein space that we try to map into our Newtonian brain are hard to figure out. ---
**Author:** ![Pasta](https://avatars.discourse-cdn.com/v4/letter/p/ecccb3/32.png) [@Pasta](https://boards.straightdope.com/u/Pasta)\ **Post date:** [October 16, 2024, 3:38am UTC](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837/49 "2024-10-16T03:38:24Z")
@Saint_Cad and others: Let’s start with much simpler standards for distance and time units. Let’s base our distance units on the radius (suitably defined) of a ground-state hydrogen atom; and let’s base our time units on the oscillation period of radiation from the lowest level atomic transition of a hydrogen atom. We can return to messier standards in a bit. But for these standards, we have our basic distance unit being 1 “H radius” (symbol R, say) and our basic time unit being 1 “H period” (P). In such a unit system, a person might be 3.4x1010 R tall and might live for 6.2x1024 P. If we measure the speed of light, we’ll find it to be around 2300 R/P. You could then ask: why is it 2300 R/P and not 8 R/P or 99 million R/P? Could it have been something very different? The reason I chose such basic unit standards here is that it showcases immediately how we are not, in fact, free to think about all three items independently (distance standard, time standard, and speed of light). To wit, we can “easily” calculate how the radius of the hydrogen atom depends on c, how the transition radiation’s period depends on c, and thus how their ratio depends on c. These dependences aren’t free to be whatever. The very structure of spacetime is part of the physics that underlies both standards. Upon doing these calculations, one finds that the radius of the atom scales as 1/c and the radiation period scales as 1/c^2. Thus, it “turns out” (but it had to be) that the ratio of the radius to the period scales as c. If the speed of light were to suddenly halve, the size of a hydrogen atom would double and the period of the radiation would quadruple. Then, when we establish our distance standards and time standards in this modified universe, we would amazingly still find that the speed of light in this modified universe is 2300 R/P. It apparently didn’t change at all! Saying the same thing in equations: the laws of physics relate these size and time standards according to 1~\mathrm{R/P} = \frac{3\alpha c}{16\pi}. Here, \alpha is the fine structure constant. No matter how you try to scale c, you will always find that the measurement of R/P scales by the same amount. That is, your distance and time standards will always scale suitably so that any re-scaling of c is unobservable. And, importantly, **the numerical value 2300 reflects nothing about the speed of light** and everything about the nuances of our chosen standards. In this case, the numerical value derives from a few mathematical constants and the fine structure constant. That is, 2300 is simply \frac{16\pi}{3\alpha}; it is unrelated to c entirely. If you ask why is c equal to 3x108 m/s and not 87 m/s, what you are actually asking is “If I look at what I base my ‘meter’ on and what I base my ‘second’ on, what fundamental physics and corresponding emergent phenomena lead to those things having the ratio that they do?” That’s answerable, but it’s not the same as “Why does c have the numerical size that it does?” In fact, the “numerical size” question isn’t about c at all but about the physics that underlies (and relates!) the distance and time standards chosen. For the hydrogen standards, the answer to “Why is c equal to 2300 R/P and not 87 R/P?” is directly visible in physics underlying the standards. If we try to do this instead for “rotation period of the earth” and “size of earth”, then there is a lot of smoke and mirrors in our path, but at the bedrock bottom of it all is the same story: the distance and time standards _must_ be fundamentally related by the very spacetime they reside in. The analogy of “horizontal” vs. “vertical” directions (say) being on equal footing can enter here. In the case of spacetime, the rule is just that “movement in a spatial direction can never be more than movement in the time direction”. When worded that way, **there isn’t even a place to _put_** a numerical value for c. It just doesn’t have one, fundamentally. Not any more or less than “up” versus “sideways” distances have numerical constants relating them. Any concept of c having a numerical value is, well, just down to units. Just like “2300” is unrelated to c but fully related to our unit standards and their physics. A separate question might be “Why is c so big?” or more to the point: “Why is c so much bigger than speeds we experience in everyday life?” or more to the point still: “Why do we typically see things that move in the time direction much more than in spatial directions?” For everyday objects on earth, we need to have velocities commensurate with typical chemical binding energies and atomic masses so that it’s not all a dissociated mess every time two things touch. Similar lines of discussion, with different physics involved, could connect to gravitationally bound systems, say. The point that I hope comes through the clearest in all this is that there isn’t even a place to put a numerical “input” value for c in physics. If you want to “change” it numerically in a given unit system, you need to change something else that changes how your distance and time standards relate, like \alpha (say) in the hydrogen example. “Changing” c does nothing, because there’s nothing to change. The law that “You can never move more in space than in time” doesn’t have a number associated with it. And this restriction is all that the speed of light amounts to. (If I haven’t moved the needle here, consider this a starting point for further conversation.) ---
**Author:** ![Monty](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/monty/32/47_2.png) [@Monty](https://boards.straightdope.com/u/Monty)\ **Post date:** [October 16, 2024, 3:41am UTC](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837/50 "2024-10-16T03:41:24Z")
> [@Lumpy](#): > > the very definition of speed is distance times time I thought it was _distance divided by time_. > [@Lumpy](#): > > And for those who take the stance that the speed of light is the speed of light is the speed of light, @Francis_Vaughan’s answer is probably the closest we’re going to get. Namely that if _C_ simply is what it is, where do all those other constants get the precise ratios to _C_ that they have? Yeah. I get the feeling the OP is essentially asking _Why is there a speed of causation?_ > [@md-2000](#): > > Light is an electromagnetic wave (or a particle - take your pick) Don’t physicists usually choose both at the same time? Or am I confusing that concept? ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [October 16, 2024, 10:29am UTC](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837/51 "2024-10-16T10:29:03Z")
> [@Pasta](#): > > (If I haven’t moved the needle here, consider this a starting point for further conversation.) That was beautiful. Thank you. It’ll be eye-opening for the people stuck on “speed of photons”. If they can un-stick themselves. ---
**Author:** ![RitterSport](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rittersport/32/6326_2.png) [@RitterSport](https://boards.straightdope.com/u/RitterSport)\ **Post date:** [October 16, 2024, 12:47pm UTC](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837/52 "2024-10-16T12:47:26Z")
> [@Pasta](#): > > “You can never move more in space than in time” I love this!! It’s is a much better explanation than what I tried to hack together. ---
**Author:** ![mozchron](https://avatars.discourse-cdn.com/v4/letter/m/c2a13f/32.png) [@mozchron](https://boards.straightdope.com/u/mozchron)\ **Post date:** [October 16, 2024, 1:02pm UTC](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837/53 "2024-10-16T13:02:59Z")
My chemistry professor in college had a good answer for these types of questions. “God only knows and she ain’t about to tell” ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [October 16, 2024, 1:03pm UTC](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837/54 "2024-10-16T13:03:00Z")
c isn’t just a speed limit. It’s also the square root of the proportionality constant between energy and mass, which are different sides of the same coin. E = m c^2. In a physics class I took, we worked through the derivation of that equation, using high school math. ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [October 16, 2024, 2:04pm UTC](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837/55 "2024-10-16T14:04:00Z")
Look at it this way – either the speed of light (or of causality, or what have you – the Speed Limit of the Universe) has a finite value, or it is infinite. If it is finite, it will have some particular value, which is the case we have. Exactly why that particular value isn’t evident. It ties in to a lot of thinhgs, but that doesn’t say why the value it is. If the value was different, the universe would functio a little differently. But that’s OK – we live in the universe as natural laws define it. If the value were slightly different, we’d still be asking the same question – why _that_ particular value. And we’d be shrugging slightly different shoulders and sayin g we don’t know. (Or else we wouldn’t be here at all, shrugging shoulders, or doing anything, because the laws of the universe in that case didn’t allow for our kind of sentient life. But then, we wouldn’t ever know that, would we?) The other possibility is that this “limiting speed” is infinite, and I’m not sure what sort of universe that would result in. I’m reminded of the saying that “we have time so that everything doesn’t happen at once.” Maybe a lot of things that in our universe take time because of the finite speed of light would all be happening at once. I’m sure that would be a bureaucratic nightmare for the God of the Physicists, if He exists. ---
**Author:** ![filmore](https://avatars.discourse-cdn.com/v4/letter/f/7993a0/32.png) [@filmore](https://boards.straightdope.com/u/filmore)\ **Post date:** [October 16, 2024, 3:45pm UTC](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837/56 "2024-10-16T15:45:20Z")
> [@Pasta](#): > > If the speed of light were to suddenly halve, the size of a hydrogen atom would double and the period of the radiation would quadruple. Does this have anything to do with why the universe is expanding? In your example with the hydrogen atom, would the universe itself also double in size if the speed of light were to suddenly halve? Is the speed of light slowing down, and thus, the universe is proportionally expanding so that _c_ is always _c_? ---
**Author:** ![Pasta](https://avatars.discourse-cdn.com/v4/letter/p/ecccb3/32.png) [@Pasta](https://boards.straightdope.com/u/Pasta)\ **Post date:** [October 16, 2024, 4:47pm UTC](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837/57 "2024-10-16T16:47:53Z")
> [@filmore](#): > > Does this have anything to do with why the universe is expanding? It’s all much more… uninteresting than that. There is a subtle shell game in that part of the discussion. In particular, when I say, “If the speed of light were to suddenly halve, the size of a hydrogen atom would double and the period of the radiation would quadruple,” that is actually incompatible with what I conclude later, which is that it makes no sense to talk about doubling or halving the speed of light because there is no numerical quantity to double or halve. However, in this early part of the post, I’m trying to roll with the conceptual idea of (say) halving the speed of light in order to point out that it necessarily does nothing observable (and thus is meaningless) because anything you try to use as your distance or time standards are based within a coherent spacetime and will “change” accordingly. A cleaner way to say this is just that a numerical value of c never even enters the problem in the first place. It shows up only due to arbitrary historical choices. ---
**Author:** ![Buck\_Godot](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/buck_godot/32/6573_2.png) [@Buck\_Godot](https://boards.straightdope.com/u/Buck_Godot)\ **Post date:** [October 16, 2024, 4:49pm UTC](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837/58 "2024-10-16T16:49:42Z")
> [@Saint\_Cad](#): > > This is exactly what I am talking about. That is not an answer, you are arguing the number comes from the units. Why isn’t _c_ 10% faster than it is? Why isn’t it 5x10^100 plank units per second? Your answer is like if I asked why Bugatti has a top speed of 305 mph and instead of the engineering, limiting factors of friction on asphault, &c. you say its because in 1 hour it goes 305 mile but if you measured it in meters per minute you would get a different number (although not a different speed). OK so lets rephrase your question to ask why does light move 2.2x10^6 times faster than a Bugatti. The speed of light is the conversion factor between time and distance in 4th dimensional space time. Effectively what we are saying is that due to the laws of physics and asphalt etc. the Bugatti can only move 1 meter in space of every 2.2x10^6 meters it moves in time. So your question isn’t so much a question about relativity than it is about material science. If we “changed” the speed of time to 12 mph, the laws of physics that governs friction internal combustion and wind resistance would also change such that the speed of the Bugatti would still be 2.2x10^6 lower. ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [October 16, 2024, 6:56pm UTC](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837/59 "2024-10-16T18:56:18Z")
The “speed” of light is a question relating more to language than to physics. Unfortunately, physicists are limited to the common language when they try to explain what the math is telling them. Consequently, the language of explained physics is often analogy, with all the limitations that produces. I remember when “what is the speed of dark?” was commonly asked, not always humorously. We see it again today with “dark energy” and “dark matter.” “Dark” comes from either “unseeable” or “unknown” but has a multitude of other connotations that obscure the underlying meaning of “our observations and measurements are telling us something’s odd out there and we don’t know what it is, although that won’t stop us from writing about it 24/7.” It gets especially bad when whimsy is involved in naming. Strange and charm are types of quarks, which have three colors of subparticles that are in no way colored. (Truth and beauty were tried out for a while, before the prosaic top and bottom won out.) Quarks are spin 1/2 particles, but don’t spin in any way the common language would give a clue to. I think it would help if the physicists started out with this disclaimer before trying to explain what’s really going on. It might prevent the insistence on getting an answer that satisfies the common language when one doesn’t exist. ---
**Author:** ![Saint\_Cad](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/saint_cad/32/18907_2.png) [@Saint\_Cad](https://boards.straightdope.com/u/Saint_Cad)\ **Post date:** [October 16, 2024, 6:57pm UTC](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837/60 "2024-10-16T18:57:26Z")
I suppose another way to put it is: what other constants could I change that would a result in a change in c? What if I changed permittivity by 1/2? Would the speed of light even change? If so would it double? Become 1/8 the current amount? What about the fine structure constant? Would changing that change the speed of light? I suppose someone would say they are all interrelated but I’m Thanos here with OCD so I snap my fingers to make the reduced Planck constant to exactly 1.05 x 10^-34. What would happen to the speed of light if anything? Maybe that doesn’t answer the question of what causes c to be what it is but at least it would show the interdependencies of the variables. [Previous page](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837.md?page=2) [Next page](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837.md?page=4)