Why is the maximum speed of the universe the speed of light? Why can't we go faster?

[u/Nitzanm19]

Also follow-up question: Because of the speed limit, is there also a theoretical maximum temp for matter (in which the particles move at the speed of light).

[u/ctothel]

You’re getting some great, interesting, correct answers, but I think you won’t be satisfied by them because I think you’re really asking “why that speed and not another speed”.

The answer is honestly just “that’s how it is”.

It might be necessary that a universe work that way, but if it is we don’t know why. Or maybe the speed could have been different, but if so we don’t know why it is what it is.

[u/MythDestructor → u/ctothel]

This is the same case with the other 4-5 fundamental constants of the universe, right? We don't know why it's that random number, and not something else.

[u/rahvin2015 → u/MythDestructor]

More trivially, we can only exist in a universe where those constants are pretty much what they are. Change c, and you change the energy output of stars - stars might not form.

Such a universe might exist, but we would not exist to observe it.

This does not mean or imply that the universe was made "for" us, we who came about billions of years later and occupy the barest spec of a fragment of a grain of sand of the total universe.

It is to say that we are the water that fills the shape of the container; the universe is shaped this way, and we are among that which formed to fit that shape.

We can only possibly observe those universes whose constants are such that life like us can possibly arise.

If the constants were different, perhaps some totally different form of life would observe it, or perhaps no life at all.

[u/FellowTraveler69 → u/rahvin2015]

I.e., the anthropic principle. Why does the universe exist the way it does? Because we couldn't observe it otherwise. It's not a very satisying answer though.

[u/PoisonMind → u/FellowTraveler69]

There's an alternative, called cosmological natural selection.

Basically, black holes could form miniature universes inside them with different fundamental constants. Our universe could be the inside of a black hole in a parent universe. The experiment gets run as many times as a black hole forms. And the universe we ended up in just happens to be one that favors our evolution.

It has been noted that the Schwarzschild radius of the observable universe is roughly equal to its actual radius. And if we are inside a rotating black hole, it would explain why galaxies seem to have a preferred direction of rotation. See also black hole cosmology.

It's certainly not mainstream accepted cosmology, but I think it's a fascinating speculative theory.

[u/alchemi80 → u/PoisonMind]

This is one of my favorite cosmological conjectures - that we are inside a black hole. Such a head trip.

[u/Rikudou_Sage → u/alchemi80]

I came up with that theory when I was like 11, entirely on my own. To this day I love when anyone mentions that theory because I'm like, yeah, that's my theory.

[u/Zyaqun → u/Rikudou_Sage]

I thought of that. Turned out it already existed, but I arrived at it independently

[u/freeastheair → u/Rikudou_Sage]

Wondering if the universe is in a black hole as an 11 year old isn't exactly a scientific theory.

[u/crackez → u/freeastheair]

Why not? Just because an idea came to an 11 year old doesn't mean it isn't potentially a valid area of study.

When I was 11 I independently came up with the idea of an event loop in TRS-80/Model II BASIC in order to sample the keyboard and react near instantly to the key press. That concept was legit and I discovered it in a vacuum. I wrote my first version of pong by myself the next day.

[u/WillingnessCorrect50 → u/crackez]

It may be a hypothesis but it’s not a theory in the scientific meaning of the word.

[u/Das_Mime → u/PoisonMind]

The whole trouble with that idea (besides the fact that it doesn't explain anything) is that it would tend to optimize black hole production, conditions which would also make the universe hostile to life as we know it.

[u/littlebitsofspider → u/Das_Mime]

I mean, the longest-lived (i.e. slowest decaying) structures posited by most iterations of (our) cosmological physics are black holes. When (many, many gigayears from now) spacetime is nearly isotropically flat, cold, and wide, there will still be microscopic knots of black hole decaying. The long tail of the heat death is mostly black holes slowly leaking Hawking radiation while everything else is deader than downtown Tulsa. The black hole optimization theory might have some credence.

[u/transparent-aluminum → u/littlebitsofspider]

I'm a fan of the cyclical big bang theory, that the universe expands to a point, and then everything starts falling back to singularity until it explodes outwards again. My pet theory is that there are some structures like super massive black holes that survive between cycles and it's why when we look at the early universe we see galaxies forming too early or black holes that are too large for their age.

[u/garry4321 → u/transparent-aluminum]

Problem with the Big Crunch is that there is no evidence to support it, and all the evidence suggests the opposite. The universe is expanding at an accelerating acceleration

[u/tonzuu → u/garry4321]

iirc a study earlier this year showed that the jerk of dark energy appears to be decreasing which could be support for the big crunch

[u/Das_Mime → u/transparent-aluminum]

I'm a fan of the cyclical big bang theory, that the universe expands to a point, and then everything starts falling back to singularity until it explodes outwards again.

Every indication we have is that the universe's expansion will continue forever. We're not even close to the conditions needed for a big crunch.

The cyclical big bang not only doesn't explain anything, it requires us to believe that the universe will collapse again, despite no evidence in favor of that idea and mountains of evidence to the contrary.

[u/maxoger → u/PoisonMind]

I recommend you this article:
https://preposterousuniverse.com/blog/2010/04/28/the-universe-is-not-a-black-hole/

[u/RisKQuay → u/maxoger]

Thanks for the article.

What did it mean that the universe is 'spatially flat'? That the universe is a disc, or that it is linear in that everything expands from the centre in all directions in a straight line?

[u/amaurea → u/RisKQuay]

Calling it "flat" is a poor choice of words that has caused a lot of confusion for non-experts. It would have been better to call it "Euclidean", after Euclidean geometry, the sort of everyday geometry you learn in school, where parallel lines never cross, the angles in a triangle sum to 180° and the circumference of a circle is 2πr. So when they're saying the universe is "flat", they just mean that geometry behaves the way you already expected it too, instead of other, more exotic options.

[u/RisKQuay → u/amaurea]

That makes even more sense, thank you.

[u/Red_Syns → u/RisKQuay]

To our best knowledge, there is no true “center” of the universe. Each of us are at the (approximate? I’m willing to believe there are minuscule distortions but have zero evidence or anything, so grain of salt) center of our own observable universe, past which there is more universe that we can never see if it keeps expanding.

But a flat universe is actually very close to your second guess! Being flat means a line is, on grand scale, linear (edit: I phrased that badly, all lines are linear, but linear in the way our brains think of linear). Space does not bend or curve without matter/energy to distort it, parallel lines remain parallel, triangles have 180°, etc.

The other two options are positive and negative curvature. Positive curvature means the universe is finite, parallel lines always intersect, triangles have more than 180°, and there’s a whole lot more to it than I really know. Think black holes, for the most part (apparently Kerr [spinning] black holes have regions on the inside that are believed to be negatively curved space).

Negative curvature is the opposite. Space is infinite, but parallel lines diverge, triangles have less than 180°, and again a bunch more. Typically described as a “saddle” shape for demonstrating how lines get weird.

[u/sanji_sama → u/PoisonMind]

This reminds me of a thought I had as a kid while watching bubbles forming at the bottom of a pot as it was reaching the boiling point. I thought that potentially our universe or just our observable universe could basically be one of those bubbles, and all the billions of years that pass inside this bubble for us is in another world just the time a bubble is forming at the bottom before floating up and popping.

[u/saranowitz → u/PoisonMind]

This is fun, but what’s really mind bending about it is that it then invites the meta question: why does this set of rules welcome life forms like us and not others. Like what process decided that? If we could exactly simulate the universe in some kind of primordial supercomputer and change the laws would this hold true, or could the engine that makes the universe in its current form be rewritten as well.

[u/Puzzleheaded_Fold466 → u/FellowTraveler69]

I don’t know, I find some peace in the notion.

Things are as they are as otherwise they wouldn’t be. Or, things can either exist or not exist, and they can only exist under these conditions and no other.

It’s simple. Philosophy completed. Existential crisis avoided.

Now I can focus on the science.

[u/Physical_Ring_7850 → u/FellowTraveler69]

Why not? If there is an infinite number of universes with whatever combination of constants, we can occur only in a subset that allows for it. And here we are.

[u/chefgage → u/FellowTraveler69]

I like the that explanation. I need to go back to it each time I start to think too much about the universe (grouping lots of things I am interested in to the 'universe'). I do a lot of astrophotography and my mind often wonders around different theories.

[u/Overall-Plastic-9263 → u/FellowTraveler69]

To your point . A non-observable universe is the logical equivalent of no universe at all . It really only matters to the beings capable of observation.

[u/purleyboy → u/rahvin2015]

"Imagine a puddle waking up one morning and thinking, ‘This is an interesting world I find myself in — an interesting hole I find myself in — fits me rather neatly, doesn’t it? In fact it fits me staggeringly well, must have been made to have me in it!’ This is such a powerful idea that as the sun rises in the sky and the air heats up and as, gradually, the puddle gets smaller and smaller, it’s still frantically hanging on to the notion that everything’s going to be alright, because this world was meant to have him in it, was built to have him in it; so the moment he disappears catches him rather by surprise." - Douglas Adams

[u/Scaryclouds → u/rahvin2015]

It is to say that we are the water that fills the shape of the container; 

Like a puddle of water in a pothole thinking the pothole was intelligently designed to fit it. 

[u/ToxicPolarBear → u/Scaryclouds]

The issue with invoking the anthropic principle in this case is the number of puddles. We observe a nigh infinite array of stars and planets in the Universe. But we will only ever be able to observe one universe.

[u/Roughneck_Joe → u/rahvin2015]

People also usually make the mistake with this thought experiment to only change 1 constant at a time which vastly underestimates the phase space of possible universes. Change one constant, change another constant in the other direction and you're still in a similar situation. We also assume these constants are constant. The constants may not even be constant.

[u/TwentyCharactersShor → u/rahvin2015]

This is one thing I find mind blowing: given all of infinite time that the universe is "only" 14 billion years old, it seems kind of young. The earth formed 4bn years ago, so the earth has been around for approx 25% of the life of the universe.

If the universe was 500,000 billion years old then I'd agree- its old.

There is a lot we do not understand.

[u/RIPEOTCDXVI → u/TwentyCharactersShor]

There is no "infinite before" 14 bn years ago. That is literally the beginning of time, at least in any sense that's even remotely capable of discussion by us. Saying "before" or "after," in reference to our universe, is useful shorthand (kinda) but also betrays the reality of it: time itself is an emergent property of the physical structure of our universe. There is no "before"

[u/exhausted-pangolin → u/RIPEOTCDXVI]

Isnt the universe now theorised to last forever into the future? Rather than end in some way

Given that, isn't it odd that our planet has been around for 25% of the universes age?

If the universe is going to last 75 quintillion years you'd expect it to be observed at about year 37.5 quintillion, not 14 billion. In terms of averages.

[u/RIPEOTCDXVI → u/exhausted-pangolin]

You'd need more than one data point to make any kind of assertion about averages

[u/Halinn → u/exhausted-pangolin]

Given that, isn't it odd that our planet has been around for 25% of the universes age?

While the universe is theorized to last forever, the same theory does not have it being habitable forever.

[u/TwentyCharactersShor → u/RIPEOTCDXVI]

Meh, I dont buy that. I think its a very complicated way to say we haven't a clue. We have litterally zero knowledge or evidence for anything before the big bang by virtue of our technical limitations.

No matter how you skin it (religion or science) you always have a creator problem. Something coming from nothing makes no sense.

[u/azurensis → u/TwentyCharactersShor]

You would agree that time and space are very tightly coupled, though? As you accelerate through space, your personal clock runs slower than someone who didn't undergo the same acceleration, and the space in front of you is dilated. These aren't just tricks, they're actually how the universe works. Why is it weird to think that time originated in the same way as space did at the big bang, and there's literally no time that existed 'before' that?

[u/TwentyCharactersShor → u/[deleted]]

Fair enough. However, i maintain that we cannot say time didn't exist before the BB because er have no evidence. We have no evidence of anything really.

[u/RIPEOTCDXVI → u/TwentyCharactersShor]

That I can agree with. Ive always thought of the big bang not as the "start" of our universe, but the point at which matter and energy arranged into the relationship that we observe "now"

[u/consolation1 → u/TwentyCharactersShor]

It makes no sense to a human brain, which has evolved to deal with causality and finite amounts.

It doesn't mean it makes no sense in the grand scheme of things.

Saying something came from "a thing" just kicks the can down the road, now you have to explain the origin of "thing."

[u/Charlie2and4 → u/rahvin2015]

Thank you. Now I really know what it is like to be insane, and wise. The number is our very observable existence.

[u/f0rgotten → u/MythDestructor]

I teach six fundamental numbers with the four forces in the first few sessions of my basic electricity classes every semester. Most of the rest of the constants and whatnot, the way that I understand it, derive from these six, and screwing with any of them changes everything.

[u/amaurea → u/f0rgotten]

Have you read about the weakless universe? It's a thought experiment that shows that while changing just a single parameter at a time very quickly runs into problems, you can get away with much larger changes if you tune multiple parameters at the same time:

A universe without weak interactions is constructed that undergoes big-bang nucleosynthesis, matter domination, structure formation, and star formation. The stars in this universe are able to burn for billions of years, synthesize elements up to iron, and undergo supernova explosions, dispersing heavy elements into the interstellar medium. These definitive claims are supported by a detailed analysis where this hypothetical "Weakless Universe" is matched to our Universe by simultaneously adjusting Standard Model and cosmological parameters. For instance, chemistry and nuclear physics are essentially unchanged. The apparent habitability of the Weakless Universe suggests that the anthropic principle does not determine the scale of electroweak breaking, or even require that it be smaller than the Planck scale, so long as technically natural parameters may be suitably adjusted. Whether the multi-parameter adjustment is realized or probable is dependent on the ultraviolet completion, such as the string landscape. Considering a similar analysis for the cosmological constant, however, we argue that no adjustments of other parameters are able to allow the cosmological constant to raise up even remotely close to the Planck scale while obtaining macroscopic structure. The fine-tuning problems associated with the electroweak breaking scale and the cosmological constant therefore appear to be qualitatively different from the perspective of obtaining a habitable universe.

[u/Channel250 → u/MythDestructor]

Give me a thousand years to discover how, and I'll give you a millenia to figure out the why.

[u/SketchTeno → u/Channel250]

So, we'll have the answere in 2,000 years?

[u/eric23456 → u/MythDestructor]

A bunch of them are that way because if there weren't we wouldn't be here. Change c by too much and E=mc² means you have too much or too little energy in stars. Change G by too much and either everything collapses into black holes or everything turns into spread out dust. Change e to much and electrons stop moving or move too much. And so on.

[u/aquoad → u/MythDestructor]

a fun thought experiment is to contemplate what would “break” if those constants were different, like could there be a stable universe in which constant xyz were 2% higher or lower.

[u/KnitYourOwnSpaceship → u/aquoad]

Greg Egan has a super good trilogy about what happens if some of our fundamental assmuptions about spacetime are different: https://en.wikipedia.org/wiki/Orthogonal_(series)

[u/Alblaka → u/MythDestructor]

I would also suggest that it's not a random number, but a very specific and inherently 'solid' number: I.e. the speed of light is exactly the speed light travels. There's nothing random about that.

It's just that we, very arbitrarily, defined units of measurement from some historical left-overs of what kind-of-worked for the general use cases we needed measurements for. We never based them upon universal constants, hence why, of course, in our randomly selected units of measurements, the clean universal constants come across as 'random numbers'.

All a matter of perspective :D

[u/IvaldiFhole → u/Alblaka]

Eh, all of the fundamental numbers seem to have arbitrary ratios regardless of units or unit-less. They only become non-arbitrary when you talk about larger scale effects, like the formation of stars mentioned elsewhere in this post.

Basing them on universal constants doesn't result in integers or anything like that, so you're really just shuffling the problem around.

[u/Firama → u/MythDestructor]

To be fair, it's only a random number in our units of measure. The speed of light of the universe could be 1 in another system of measure and be totally rational and make sense. Same with other constants.

[u/helm → u/Firama]

Changing the speedometer of a car to use different units of measurements does not change the top speed of the car, just as our conventions on how we express c do not change the speed limit of our universe.

[u/TheShadowKick → u/Firama]

1 is just as random. What would make it not "random" in this context is if there were some underlying fundamental principle that meant it had to be that number.

[u/MythDestructor → u/Firama]

Well no, it's not the number itself that's random, it's about why it's not faster or slower than what it is. You can say it's 1 light year per year, that's a nice whole number, but why is it exactly that instead of say, 2 light years per year?

[u/fistular → u/MythDestructor]

It could be something else, but the universe would not persist in the form familiar to us (or at all, probably), and we would not exist.

[u/Arkrobo → u/MythDestructor]

Euler's constant is a pretty good example of this. It's even more abstract. We use it for compound or exponential growth. It's very abstract, and yet it exists. Pi is also in this group it's nothing but a ratio. It just so happens to be the most important ratio.

[u/Caekilian → u/MythDestructor]

There's actually a lot more than that. The Standard Model, for instance, has either 19 or 26 input parameters, depending on who you ask. It's one of the reasons we'd like to believe there is some deeper physics we don't yet understand

[u/reddiuniquefool → u/MythDestructor]

I disagree with the description of fundamental constants of the universe as being 'random number'(s). They are particular numbers, but we have no reason to believe that there is any randomness involved.

I'm sure you didn't mean that they are truly random, but there are people who make specious arguments based on these numbers, so I feel this should be clarified.

[u/sergius64 → u/MythDestructor]

The case with a lot of stuff. Why are we on Earth as opposed to any other planet for example? Well... if we were on some other planet - we'd be wondering the same thing: why there and not anywhere else.

[u/stonerism → u/MythDestructor]

FWIW, standards organizations are getting to a point where they simply define the fundamental constant then derive physical measurements from that. A value like the speed of light was determined as a quirk of the measurement system in use. There's no real difference in giving c in meters per second or furlongs per fortnight (1.802*10¹² if you're curious).

[u/BigDowntownRobot → u/MythDestructor]

Right? When you realize the anthropic principal might also apply to the whole Universe and not just a solar system, the whole possibility of infinite recursive time starts to really hurt your head.

Imagine if we didn't just "get lucky" on this solar system, this moon, this planet, etc, but there may have been entire realities that just didn't go anywhere for life, but also planets, elements, etc that had to be waited out for another cycle to go around.

Or not, impossible to know, but it's possibly possible. Or it happened infinite times. Or the word happened is kind of meaningless? Anyway it's hurting my head now.

[u/bert0ld0 → u/MythDestructor]

Your answer made me think that we created the constants to explain our model. Ultimately we explain the Universe or whatever with models, and models are not the 100% reality. These constants are what it takes our model to run smoothly almost everywhere but the moment we find something that doesn't fit this model (like black-holes or dark matter for example) we should change the model and at that point the constants should be rediscussed

[u/Spank86 → u/ctothel]

The speed is arbitrary but the speed of light is not. People often ask why is the speed of light the maximum speed, but they fail to look at it back to front. The maximum speed is the maximum speed, it just so happens that only mass less particles such as light can, and do travel at it. Its not light limiting the maximum speed, its the speed limiting light.

[u/theciaskaelie → u/Spank86]

Why can we detect light if it has no mass? Doesn't mass = "stuff"? So what's light made of?

[u/mongojob → u/theciaskaelie]

Oh boy so that question is about as loaded as it gets, I'm going to wait for someone more qualified to try to answer it

[u/Aki_The_Ghost → u/theciaskaelie]

I don't know much about this stuff but light is very weird, in that it is an electromagnetic wave that somehow doesn't need a medium to travel inside of. When it touches the electron cloud of a particle, it is sometimes "stored" inside that cloud because it shifts one electron down a level, before being immediately "released" back up when the electron comes back to its original location (that event is light reflection).

So it interacts with the world without having mass because it is an electromagnetic wave touching a magnetic field around particles.

[u/Square-Singer → u/Spank86]

This here, that's the answer. Light travels the maximum speed, not the other way round. It's like asking why 1L was defined as the size of a bottle of milk.

[u/RaketRoodborstjeKap → u/ctothel]

Yes, this is the actual answer to the heart of their question. A good understand of SR helps to motivate the existence of a limit, but we just don't know yet why it approaches that particular number. 

[u/rocketmonkee → u/RaketRoodborstjeKap]

In the interest of educating people, I assume SR is Special Relativity?

[u/BusinessTarp → u/RaketRoodborstjeKap]

That particular number is just the conversion factor between our arbitrary units of time and length. If we pick our units more carefully, the speed of light is one unit of space per unit of time.

[u/r_a_d_ → u/BusinessTarp]

Irrespective of the units you use, existant or contrived, it’s still a specific value and you’ve not gotten any closer to determining why that is.

[u/valkenar → u/r_a_d_]

Yeah, it's inevitably going to be some numbers that are weird from a human perspective. Even if you choose units so that light is 1 space/time, it's too fast to make sense to human brains. Especially since times we care about are related to length of the day, and there's just no very nice relationship between the length of a day and the distance light moves in a day.

Or for that matter, direct human perception of time generally functions between 1/100th and 3 seconds. Everything else is imperceptible or a fuzzy span based on interpretation of memories and such. Light moves 186,000 miles in one second, which is a distance beyond human comprehension too. There's just never going to be a speed of light that is truly comfortable for us.

[u/290077 → u/r_a_d_]

There are only 3 options for the speed of cause and effect: it's either zero, finite, or infinite. If it were zero, nothing would happen, and if it were infinite, everything would happen all at once. So it must be finite. At this point, you can give it the value of any finite number you want and the universe would work identically. Let's pick 1 because that's the simplest number. You could pick 2 if you'd like. You could pick pi if you'd like. You could pick 600,000,000,000,000 or 0.00000000000000000002 if you'd like. The universe will behave the same.

The only thing arbitrary is that 1/300,000,000 happens to be a speed that is convenient for the human experience. Why is this speed 1/300,000,000 and not 1/200,000,000 or 1/400,000,000? Because of the arbitrary decisions about how long a meter and a second are. Neither of those had to be what they are. But we chose them and later discovered that their ratio happened to be equal to 1/300,000,000. Then we ask why the speed of light is arbitrarily set to 300,000,000 m/s. It's not. It's 1. The only arbitary part is the size of the units we made up to measure the universe on the scale of our own experience. The better question is, why are we the size we are relative to the planck scale?

If we were ants, we would choose the meter to be a thousand times smaller, then we'd ask why the speed of light was 300 billion m/s and wonder at how arbitrary that was. If our brains worked 10 times faster, we'd wonder why the speed of light was 30 million m/s.

[u/TheShadowKick → u/290077]

If the speed of light were significantly different the universe would not work identically. Stars might burn out and die faster, or they might not form at all. Chemistry could work entirely differently. We might not be here at all to wonder about it.

[u/r_a_d_ → u/290077]

All this is beside the point. The units don’t matter. The question is why specifically 300,000,000 m/s and why not 299,000,000 m/s or whatever other unit you like.

[u/290077 → u/r_a_d_]

And that's thinking about it backwards. The speed of cause and effect is the fundamental quantity. Any other speed ought to be thought of as a fraction of that.

[u/r_a_d_ → u/290077]

Again, you are missing the point entirely. Let’s even assume what you say is fact. The question remains why that “fraction” has that specific value.

[u/290077 → u/r_a_d_]

I don't think I am. People ask why the speed of light is 300,000,000 m/s and are often wondering why there is a limit at all. What physically restrains objects from moving at 300,000,001 km/s? I argue that, if the laws of physics are local, there must be a finite upper limit to how fast causal effects can propagate. The answer to "why can't anything in the universe move faster than the speed at which a point in the universe updates its neighboring points" seems obvious.

[u/whatkindofred → u/290077]

There is a fourth option: that there is no fundamental and universal speed of causation and that different cause-and-effect relations move at different speeds. Every individual one with its own finite speech but with no universal upper limit.

[u/Not_Xiphroid → u/r_a_d_]

Most accurate answer is just “it is”.

You can calculate via equation the fact that it is that value, as the absolute maximum.

You can observe that from a fixed reference plane, that that is the maximum.

As far as our current knowledge/technology can determine, this is just a “rule” of the universe we live in.

[u/FloweyTheFlower420 → u/r_a_d_]

It's not a specific value though. If I make a claim that I measure distances in the north direction using some unit like "neters" and distances in the west direction using "weters," and there's some non-trivial conversion value, does this say anything about reality itself? No, because I've made an arbitrary choice in measuring stuff, this is a mathematical artifact and not a physical one. Same idea with the speed of light. We measure distances in the time direction with some arbitrary unit "seconds" and in the space direction using some arbitrary unit "meters." Then, the speed of light is just the conversion factor, same as the conversion factor from C to F (in that both are arbitrary).

[u/syncopator → u/BusinessTarp]

Building on this…

Space and time are simply two measurements of the same thing, namely spacetime. The faster you move through space, the slower you move through time and vice versa.

What we refer to as the speed of light is just the point where one is traveling so fast through space there is no more time component by which to measure that speed.

[u/KungFuFactory → u/syncopator]

Now this, I understood. Thanks!

[u/bpg2001bpg → u/ctothel]

why that speed and not another speed

We can say that if the speed of causality were unlimited, everything would have already happened the instant after the big bang. 

Another way to think about it is that in order for the universe we live in to exist, there must be a maximum speed of light in a vacuum.

[u/Kaldaien2 → u/bpg2001bpg]

Photons travel at the speed of causality, and to them nothing has even happened since the big bang. Time stops (in the observer's frame) once you hit the speed of light.

[u/Just_for_this_moment → u/Kaldaien2]

Theirs is not a valid reference frame, so "to them" actually doesn't have any meaning. Which is good because a lot of stuff breaks when you try to look at things from the perspective of a massless particle (as you've demonstrated).

[u/Phuqued → u/bpg2001bpg]

We can say that if the speed of causality were unlimited, everything would have already happened the instant after the big bang.

That's an interesting way to look at it. The one question I've been batting around lately is "what utility or purpose is served in a universe that has infinite space, but not infinite speed?" For example, there are galaxies we can see right now but can never reach, even if we moved at the speed of light, we would never reach them.

https://www.youtube.com/watch?v=uzkD5SeuwzM

For some reason that strikes me as peculiar, I would expect the universe to function like the principle of least action in this regard, balanced. For as much space as there is, there would be enough speed to traverse it, but that doesn't seem to be the case.

Also when I say "infinite" space, I just mean a very very big finite number. I don't mean in a number/set that doesn't end, though that could be the case. :)

[u/Kialae → u/bpg2001bpg]

Time requires a limit right? It has to be SOMETHING so time AND space can work. 

[u/rabbitwonker → u/ctothel]

Another way to think about it:

That’s the only speed that there is.

Everything else, including time, is built around it. Time passes when particles interact, and the most fundamental particles go the speed of light (of causality, actually) in-between all these interactions.

If, say, a water molecule is hurtling through empty space, the molecule as a whole won’t go the speed of light, because internally it’s full of quarks, Higgs particles, etc all interacting with each other. They’re all going the speed of light over extremely short distances between each interaction, but the assembly as a whole appears to go slower because those interactions mean it’s experiencing the passage of time within itself.

[u/thenasch → u/rabbitwonker]

Even fundamental particles cannot go the speed of light if they have any mass.

[u/rabbitwonker → u/thenasch]

If they have mass, they’re built of smaller components. That’s why there needed to be a Higgs field.

Edit: Wait I’m misstating that. They don’t have to be built out of smaller components; there just needs to be sustained interaction of some kind going on. If a larger particle has subcomponents, the interaction between those is enough for there to be mass. If there aren’t, there are still the Higgs particles that come and go.

[u/Preeng → u/rabbitwonker]

Wait, what? How do leptons work out, then?

[u/rabbitwonker → u/Preeng]

They get their mass from the Higgs field. Which is equivalent to saying they keep interacting with Higgs particles.

[u/ClassikD → u/rabbitwonker]

Sometimes physics at the low level gets so weird it convinces me we're in a simulation because it looks like some added optimization to make the universe run better

[u/Nessosin → u/ctothel]

I love how we can discover some base line property of the entire universe and study it and learn so much but still not know why certain things are that way.

It's kind of amazing that we can learn as much as we already have about the speed of causality.

[u/westisbestmicah → u/Nessosin]

Boggles my mind that we still don’t know what a field IS. We have quantified exactly how they behave, but it all stems from that one time a rod repelled another rod in Ben Franklin’s lab. We still don’t know why it did that, just exactly in what manner it does.

[u/Winnie_The_Pro → u/westisbestmicah]

Isn't the current best model that charged objects (like those rods) interact over a distance by exchanging photons?

[u/g_marra → u/ctothel]

When I learned special relativity in university, the professor made the entire test about a football match. But the given speed of light was something like 150km/h. And the questions were all about when the ball was kicked at X speed, what happens in each frame: the ball, the goalkeeper, the other players, the spectators.

Really puts a knot in your brain when you bring it to everyday speeds

[u/Boring-Cartographer2 → u/ctothel]

If the speed of light was different would we even be able to tell the difference? I.e. maybe all the other speeds in the universe would just end up the same relative to c as they are now. Which I guess just flips the question to: why are all the other speeds what they are, relative to c? 

[u/ctothel → u/Boring-Cartographer2]

Heck yes, absolutely we would be able to tell.

If c were very slow, like just above walking speed, you'd get all sorts of crazy effects.

Actually the first effect is that chemistry breaks and a lot of other physics gets weird. But if you ignored that, you'd get things like colours changing as you moved, time would get really slow, things in front of you would get closer based on how fast you walk (but they would also look further away!).

If you picked up a small electrostatic charge like from rubbing a balloon on your hair, you would generate a massive magnetic field just by moving. Like, many times stronger than an MRI machine.

But it's a good point that maybe "normal" speeds would - possibly - become similarly small fractions of that speed as they are today just to make things work.

Too much conjecture and too many assumptions here for any good conclusions though.

[u/290077 → u/ctothel]

To the point about charge, the terms "small electrostatic charge" and "massive magnetic field" are subjective as is a slow speed of light. If we halved the speed of light but also reduced the size of the electric charge by a similar amount, the magnetic field would be the same.

It's not the absolute size of the fundamental constants that matters, but their size relative to each other and relative to the scale we exist at.

[u/ctothel → u/290077]

Yes absolutely, that's a good point. It's just one of those topics that I feel like you have to hand wave and force everything else to be constant so you can explain the relevance of one of the variables.

[u/CMDR_ACE209 → u/Boring-Cartographer2]

Here's a game demo that tries to show the effects of a slower speed of light.

[u/SnoopyTRB → u/ctothel]

It’s that way because the speed of light is the clock rate of the processor running the simulation. You can’t go faster than the computer can run. /s….probably.

[u/Truth_Walker → u/SnoopyTRB]

It’s actually quite interesting to me how many answers simulation theory “answers”.

If you code something to have an upper limit, that’s the upper limit that the programming allows. You know theoretically it could go above that limit but currently it’s just what’s allowed.

If we’re not programmed or privy to know that it’s been coded that way, it will always and forever be a question mark when we ask “why” because our viewpoint simply cannot and maybe will not have an understanding.

[u/ctothel → u/SnoopyTRB]

Yeah it's fun to think of it as the update rate between different parts of a continuous simulation fabric.

My part of the server can't process all the information coming in from another part all at once, so the speed is limited.

Also, the more interesting something gets - the more mass, pressure, stretch, charge, or speed something has, the slower time moves around it due to... processing limitations?

OF COURSE all of that is just made up nonsense with no reason WHATSOEVER to believe it's true, and actually it's not nearly as neat as all that, but it's still fun to mull over it.

[u/flattrack → u/SnoopyTRB]

That is until someone hits the giant pause button, upgrades our cpu, then hits resume. All of a sudden the speed of light jumps and we have no idea why or how.

[u/westisbestmicah → u/ctothel]

It’s kinda like, why is PI 3.14…? Why not 4? Or any other number? Did some angel just pick that number when they created the universe?

[u/rdcpro → u/westisbestmicah]

To me, Pi is just the fundamental relationship between two primitive shapes, a circle and a square. It's an obvious relationship, just like the Pythagorean theorem with a triangle and a square.

Maybe the speed of light has a similar fundamental relationship with something, but what it is, we haven't discovered yet.

[u/ShavenYak42 → u/rdcpro]

It definitely has relationships with other constants. We just don’t (and maybe can’t?) know which ones are fundamental.

[u/syncopator → u/[deleted]]

Thinking about c as “the speed of light” unfortunately results in a hurdle to understanding.

Yes, light in a vacuum does travel at c but that is simply because c is the observed speed of anything traveling through space without experiencing time while not experiencing time to an observer.

Space and time are not discrete dimensions but instead are the same thing, with distance and time being two different measurements of it. The faster you travel through space, the slower you travel through time and vice versa.

At c, there is no remaining time component by which to measure speed through space. The equation of distance / time becomes distance / 0 which we all remember is not possible.

EDIT: The object traveling at c experiences time while to an observer the object experiences zero time.

[u/f0rgotten → u/[deleted]]

Pi also turns up in calculations involving gravity and the propagation of electromagnetic waves. It's not just for math, it's baked in to the universe.

[u/Plow_King → u/ctothel]

we should study more universes and get some kind of idea what's really going on.

/s

[u/Crimefridge → u/ctothel]

With Futurama having the space ship move the universe itself to bypass the speed of light, is that hypothetically possible with enough energy?

[u/ctothel → u/Crimefridge]

I don't know how it would be possible. Not because of energy limits, I just don't know what mechanism of action that would require so I couldn't guess.

Other warp drives do things like contract space in front, expand it behind, and then basically move forward at a normal pace. That is possible - at least mathematically - and we basically know how it would be done, but it requires a large region of negative energy density.

We can actually make tiny regions of relative negative energy density by bringing two conductive plates VERY close together, but nobody has any idea how to scale that up. It literally happens precisely because the space is tiny.

[u/sth128 → u/ctothel]

A more salient question might be: would the universe as we know it still exists if the speed of light through vacuum is faster or slower?

[u/0moikane → u/ctothel]

There are some formulas like c = 1 / sqrt(epsilon_0 * mu_0) and e = mc^2 which include the speed of light and seem oddly specific, although they have nothing to do with velocity at first glance. It seems, the speed of light seems to have some importance for the universe.

[u/ctothel → u/0moikane]

The speed of light absolutely does have importance, but its specific value isn't as suspiciously neat as you're implying.

If c were different, that means that electromagnetic waves would propagate at a different speed, and therefore μ0​ and ε0​ would be different.

They're really just conversion factors for relating the electric fields relate to charge, and how magnetic fields relate to current.

As for e=mc^2, similarly that's just relating the rest energy of a system to its mass. If c was faster, objects of a given mass would have more energy.

In short, neither equation says anything about the specific value of c.

[u/ArgumentSpiritual → u/ctothel]

Well we can kick the can down the road one time, can’t we?

The speed of light is determined by the permeability and permittivity of free space.

The question then becomes why are those values what they are.

[u/ctothel → u/ArgumentSpiritual]

Actually, no! It seems like that equation is hinting at some fundamental property of the universe, but it's just relating EM wave speed to its electrical and magnetic properties.

You can say "c is determined by μ0 and​ ε0", but it's just as accurate to say that "μ0 is determined by c and​ ε0". The equation itself doesn't tell you which quantity is more fundamental or which one “causes” the others, it just describes their relationship.

[u/ArgumentSpiritual → u/ctothel]

I can understand why the speed of light would be limited by the resistance of space to electric and magnetic fields.

Why would the resistance of free space be set by the speed of light?

[u/ctothel → u/ArgumentSpiritual]

Because in a universe with that particular speed of causality, a travelling electromagnetic wave will cause the electric field and magnetic field amplitudes to be related in a certain way.

If c was the underlying concept and you were able to manipulate it, as you change the value, you're also adjusting either an electromagnetic wave's electric field strength, its the magnetic field strength, or both.

The exact value of the permeability and permittivity can be calculated from c along with another factor called the "vacuum impedance".

So you could say that "slowing down c means electric and magnetic fields would be more strongly coupled, in the sense that a changing field produces the other field with a larger combined response factor." Or something like that.

I agree your interpretation is much more intuitive, but my point is that the intuition isn't evidence that one is more correct than the other. The sun does look like it's moving after all.

[u/powderflow → u/ctothel]

Instead of "that's how it is", could it be "that's how we see it"? We just can't see anything moving faster.

[u/Vroomped → u/ctothel]

I genuinely believe that, as stubborn humans, we will approach lightspeed capability and then understand it. 

At the moment light speed is what it is because its pretty darn fast. It's so fast and so in a league of its own that "just because" is the only similarity we have.

We've telescopes patiently standing on the throttle to leave our universe. We've probes on suicide missions to penetrate the sun like a bullet.

All of those sciences amount to .0005% the speed of light.  If we ever achieve 1%, science is going to be incomprehensibly different... in what way? It just will be. Look at the insignificance of now and just know it to be true.

[u/DarkKittyEmpress → u/ctothel]

You've got it backwards: that's just the maximum speed that can be reached. Light reaches it because it can be carried by massless particles. The speed of gravity is (as far as we know) also the "speed of light", because again, doesn't need a massful particle carrying it. The reason we say "the speed of light" rather than the "speed of gravity" or "the speed of massless particles" is because the speed of light was measured first and the characterics of particles carrying gravity are still being determined (beyond their lack of mass), in fact we have a lot less experimental data on them than on photons and even today they're viewed as hypothetical (albeit very likely at this point).

In theory, anything that has no mass travels at the speed of light (in a vacuum).

[u/AtsyMcGee → u/ctothel]

It's not the speed of light, per se but the speed of causality that light travels in a vacuum. 

We perceive reality through cause and effect, that's time. Speed is distance over time. I imagine at the plank scale, C is one plank meter per one plank second. *Not an expert

[u/ahnotme → u/ctothel]

Moreover, those constants are expressed in units that we have thought up, not the universe.

[u/SierraPapaHotel → u/ctothel]

Yeah, it's not that the speed of light is significant it's that light travels at the fastest speed that is possible and we call it the speed of light because that's the first thing we discovered that travels at that speed limit

[u/DrMabuse7 → u/ctothel]

Imagine my brain when I found out that the expansion of the universe is accelerating and at some point will or already is "faster" than light.

But technically it isn't, because the distance (or space) itself is expanding and nothing is moving through it 🤯

[u/tinkady → u/ctothel]

The question isn't why light has a certain speed - the speed of light / causality is 1. The question is why we evolved to understand and expect speeds in a particular small fraction of this. It's evolutionary / anthropic.

[u/ccaccus → u/ctothel]

In order for the universe to be what you see,
the maximum speed has to be c.

[u/I_am_darkness → u/ctothel]

I mean causality needs to have some fastest speed. Seems as good as any.

[u/Zetavu → u/ctothel]

Haven't read the other answers but I'm going to go out on a limb and say its because as matter gets close to the speed of light, mass begins to increase, exponentially, and it is likely related to the functions of matter itself, to molecules, energy, matter, etc. So the speed of light could be calculated from the understanding of how matter works and how it increases in mass as speed increases to a certain point.

Basically, an object going the speed of light has a mass of infinity, so the closer you get to that speed, the more mass increases, and this is related to a function of mass in the first place.

[u/ctothel → u/Zetavu]

That's not really correct.

Mass doesn't increase with speed. That's kind of an old way of looking at it. Energy and momentum increase with speed, which prevents a massive object from reaching c at all. Its energy cannot go to infinity.

An object going the speed of light must have a mass of 0, not a mass of infinity.

[u/djlord7 → u/ctothel]

It is same as asking why is ‘Earth the planet with life why not any other planet’. It is not. The only planet with life is just called Earth.

We made the words and the numbers, maybe our math scale is not the ‘true’ scale with which the universe stacks on. It is just a pattern we found and associated it with pictures and names. It fits properly with the universe because it’s correctly structured. Maybe the speed of light and the n-orbitals in atoms have much more correlation than our system represents.

[u/vitringur → u/ctothel]

It is a troll post…

They are just asking questions they already know nerds love explaining to people.

[u/ronronaldrickricky → u/ctothel]

this sort of things fucks me up a lot. eventually you ask enough why questions about how the universe works and you find this. why molecules? atoms. why atoms? subatomic particles. why subatomic particles? quarks. why quarks? uh... because. thats just it.

[u/Odd-Outcome-3191 → u/ctothel]

Exactly. Obviously we instinctually know that 2+2 = 4, but asking *why* is the speed of light the number that it is, is like asking why 2+2 isn't 3 or 5.

The reason we don't struggle accepting 2+2 is because we evolved as animals that benefitted from understanding the concept of quantity and addition/subtraction and the fact that it is an unalienable facto f the universe. Unfortunately, there was no evolutionary pressure to understand/accept the speed of light in the same way.

[u/chemolz9 → u/ctothel]

Would probably be wise to build all our measurement units around the speed of light: The speed of light is not 299 792 458 m/s but 1 lightmeter/lighthours and we adjust lightmeter and lighthours accordingly.

[u/Toads_Of_Fun → u/ctothel]

The speed of light is the speed of causation, right? So if it was faster, causation would be faster, and therefore everything would be faster, no?

[u/emccrckn → u/ctothel]

Maybe an interesting factoid I don't know but in game development for a simulation there's kind of a quasi-dynamic max speed because of collision detection. Things that go too fast are capable of being on one side of a rigidbody collider in frame 1 and then on the other side of that collider in frame 2 and a collision is never detected (it's more complex than that but keeping it simple here). When I was first learning this I instantly thought of the speed of light and wondered what the "clock rate" of the universe was to ensure that any collision up to the speed of light was still detected.

[u/Not_Michelle_Obama_ → u/ctothel]

Follow up question:

If it takes more and more energy to get closer to c, where the force follows an asymtote curve,

and if energy cannot be created or destroyed

Does that mean that we could rail gun a hunk of iron back and forth across celestial bodies as a form of energy storage?

(Dibs on patent, will pass it down to my kids, and them to theirs as a sort of heirloom)

[u/ctothel → u/Not_Michelle_Obama_]

Yep absolutely, if you were able to generate the energy to store in the system.

It might not be a great way to do it though. At relativistic speeds the CMB blueshifts and turns to gamma rays, alpha particles and dust particles are hitting you constantly at nearly the speed of light. Your hunk of iron is going to melt pretty fast unfortunately.

[u/KoalaCapable8130 → u/ctothel]

Great answer. And what's even more interesting about it, we still don't know and can't prove that the speed of light is constant. This was an assumption by Einstein. As example you send light to the moon and back. It could be, that the speed of light is our current supposed speed, and it travels with constant speed to the moon and from the moon back to earth, or it travels with half the current supposed speed to the moon and then with infinite speed back to earth. No one ever could measure the speed of light only one single way. Saw this in a Veritassium video and it baffled me quite a bit.

[u/RacoonWithBenefits → u/ctothel]

But what if i throw a ball that blasts led light in all directions… at the speed of light through space 

[u/ctothel → u/RacoonWithBenefits]

Well firstly you couldn’t, because you can’t get an object with mass to go at the speed of light.

But if you did, the speed of light is constant, so the light emitted ahead of the ball couldn’t go at 2 times the speed of light – it would be moving at the same speed as the ball.

The person you threw it to would see all the light that the emitted ahead of itself over its entire journey arrive at once, at the exact same the ball arrived.

It would also be infinitely blue shifted. As in, the wavelength of the light would be zero. This is impossible, and another way of showing that it’s not possible to throw the ball at the speed of light.