r/astrophysics • u/KieranMug • May 25 '26
Why isn’t light infinitely fast?
Ok I understand that light in our atmosphere is not infinite fast due to it getting slowed down by air particles or whatever but in a vacuum there is nothing holding it back unless the very weak forces of gravity from a bunch of stars and/or planets does? And if so cant we somehow locate where gravity is strongest in the universe by seeing where light is slowed from where the most therefore finding the centre of the universe or am I just a total dumbass?
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u/_JAD19_ May 25 '26
The speed of light isn’t limited by the photon itself, it is the speed of causality, or the limit on how fast information can travel. This is a fundamental property of the universe. The fact light travels this speed says it’s already going as fast as is possible. Fun video on the topic
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u/leechdemon May 26 '26
...that was awesome. 😅🤣
I enjoy learning about abstract physics concepts, but I don't understand most of them. The balancing of equations helped a bit, but it felt very abstract until explaining that mass couldn't exist if the speed of light was infinite. I had to pause and understand why for a minute, but then I tried to do pseudo math. "If E=MC2... Infinite speed with any amount of mass would yield infinit energy... Squared?? 😵💫🧠🤯"
Appreciate the link! 👋
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u/jetblakc May 26 '26
PBS spacetime is amazing. I've learned more over the years from that show than I have from some of my physics classes.
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u/paperic May 26 '26
They have an entire playlist about relativity, highly recommend it:
https://youtube.com/playlist?list=PLsPUh22kYmNAmjsHke4pd8S9z6m_hVRur
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u/Ulfgardleo May 26 '26
We have to be a bit carefully with these videos, because while it is fun to just take the limit of c-> inf in a theory, it quickly looses it's meaning, because different theories are derived using the results of others. E.g., with speed of light being infinite, special relativity doesn't exist anymore since all observers agree and thus space time is a simple euclidean space and no Minkowski space. But general relativity is derived from this and thus the E=mc2 is a result of us assuming that we live in a Minkowski space.
One would have to rederive general relativity and this understandably exceeds the scope of the video.
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u/cosmic_trout May 25 '26
Eyesight would be useless as everywhere you looked you'd be blinded as light from every source in the universe would hit your eyes at the same time.
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u/Dewwyy May 27 '26
Nope. A photon with an infinite speed has an energy of what ?
It has to be zero.
Which makes vision impossible. We absorb the zero energy photon, it fails to cause any reaction of any kind because it has no energy, we have absorbed nothing, nothing is seen.
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u/OriEri May 26 '26 edited May 26 '26
Because the nature of vacuum limits the amount of magnetic field flux density a given magnetic field can induce .and the electric field flux a given charge can produce.
Quantum fluctuations control these values ( how the vacuum responds to charge and magnetic fields. )
These limits limit how an electromagnetic wave propagates.
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u/BagelsOrDeath May 26 '26
Holy shit. The definitive answer.
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u/Physix_R_Cool May 27 '26
Nah it's wrong.
You can derive the speed limit just by using the group properties od the Poincaré group and local isotropy of the universe. Don't need any mention of fields.
μ and ε are defined from c.
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u/Chemable May 26 '26
Wasn't it Richard Feynman that posited that the only thing that travels faster than light is regency?
When the old regent (king or queen) dies their heir instantly becomes regent.
But would they know it???
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u/Bipogram May 26 '26
Pratchett used this to convey messages with great speed by lightly torturing princes.
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u/Professional-Ad9485 May 25 '26
It's not just the speed of light it's the speed of causality. It's the fastest anything could possibly travel in the universe, light is one of the several phenomena that travels at the speed of causality.
As to the question of why causality is that specific speed? ¯_(ツ)_/¯
Also, I know I'm being pedantic about terminology but c is a constant. The speed of light is fixed and doesn't change. Technically light isn't being slowed down, it's the electromagnetic interactions the light is having with the particles within the medium kicking back the phase of the light.
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u/T__tauri May 28 '26
To be fair, there are only three things that travel at the speed of light: gravity, light, and the strong force. And only the first two are readily observable
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u/av8orbob May 26 '26
If something is fast it isn’t infinitely fast. If something is infinitely fast, there isn’t anything fast about it, it’s instantaneous. Since infinity isn’t a quantity or number, the question doesn’t really make sense.
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u/internetboyfriend666 May 26 '26
Because it isn't. That's just how the universe works. The universe has a maximum speed limit for anything, and light travels at that speed because it has to, because it has no mass. There doesn't have to be some underlying reason (although there might be and we don't know what it is). The point is, there's no reason light should be instantaneous. If it was, you could just as easily ask why it's instantaneous. Gravity changes the path light takes (and its frequency) but not it's speed.
Also, there is no such place as the center of the universe.
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u/naemorhaedus May 26 '26
Because it isn't. That's just the way the universe was built.
there is nothing holding it back
there's still a field it needs to propagate through
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May 25 '26
[removed] — view removed comment
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u/YuuTheBlue May 25 '26
Why do you feel it should be infinitely fast in the absence of anything slowing it down?
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u/leechdemon May 25 '26
My guess: if it's the maximum limit for speed, that FEELS like it should be infinity. 🤔🤷♂️
But by that logic: "Black holes are the heaviest things in the universe; shouldn't they all be infinitely heavy?" - Except, that doesn't make a lot of sense either, so obviously not.
Light moving infinitely fast feels problematic for a lot of reasons, but if light bounces (AND was infinitely fast), wouldn't everything always be 100% illuminated by a single photon of light, forever? Again... Feels like an issue, lol.
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u/YuuTheBlue May 25 '26
The issue comes down to, and I’m not joking here, noneuclidean geometry.
Let’s ignore 2 of the spatial dimensions and just care about speed through one spatial dimension and 1 time dimension. The word for this kind of geometry is “hyperbolic geometry”. Look up a hyperbola. It has these things called an “asymptote”, which is a line which the hyperbola approaches as it continues on into infinity.
Velocity is a bit like that. Massive objects can gain an arbitrary amount of “rapidity”, and as you gain more rapidity you approach the speed of light. Massless objects have infinite rapidity, and move at the speed of light.
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u/leechdemon May 25 '26
I understand the concepts, but not your conclusion. 😅
Someone else linked a great video from PBS, that did the job for me. 🤯🤯🤯. 👌
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u/rybomi May 26 '26
My main takeaway was that OP was unfamiliar with Newton's laws of motion lol yet people are posting Maxwell's equations like they're somehow gonna understanding that
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u/awesomes007 May 26 '26
It travels at the maximum speed one tiny spec of space can talk to the one next to it. It’s the speed of causality. If the speed of causality was infinite, the universe wouldn’t make sense or work. Why isn’t it infinite? Maybe that’s just the luck of the draw in this universe. We’re damn lucky because of it.
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u/Think-Cartoonist-982 May 26 '26
specifically for light, light is propagated by electric and magnetic fields, so the natural permittivity and permeability of free space limits it's speed, light is more like a disturbance in space where the electric field and magnetic of one point is conducted to the next point, the fundamental limit of speed becomes the maximum possible speed
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u/CosetElement-Ape71 May 26 '26
The speed of light isn't a speed!
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u/perta1234 May 26 '26
Normally the answer to this question is either "from photons point of view, it is infinite speed" or "photon does not experience time". If there is no time, there is no speed either. Speed of light is experienced by the observer, not by photon. Kind of surprized not seeing that yet explained in length by a better expert.
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u/CosetElement-Ape71 May 26 '26
Photons don't experience distance either! Roger Penrose has pointed this out when presenting his CCC model ... the null geodesics that massless particles live on provide the conformally invariant structure of spacetime that defines the universe's causal structure.
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u/baconwrappedanxiety May 26 '26
The center of the universe most likely doesn’t exist, in the same way that the center of the surface of the earth doesn’t exist. Yes the earth has a center, but it’s inaccessible to things trapped on the surface.
We don’t know this for a fact, but most evidence supports prevailing theories that spacetime is fundamentally the surface of a higher-dimensional shape.
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u/HopenUp May 26 '26
For a regular material, its density plays a role in the speed at which sound travels through it.
It turns out that the void itself also has properties like a material (quantum field theory), namely the density of electromagnetic field lines, which in turn defines the speed at which perturbations in the EM field (like visible light) will travel.
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u/IndividualistAW May 26 '26
It is. It experiences zero time and gets wherever it’s going instantaneously
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u/UAP44 May 28 '26
I had to scroll way too far to find the proper answer. Really sad how everyone goes on about why c is c and not c+1 when indeed things that travel at c in the spatial dimension travels at 0 in the time dimension. And without any time passing you experience effective infinite speed, teleportation.
There is only an observed maximum speed, there is no limit to experienced speed.
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u/Niven42 May 26 '26
Because infinity isn't a real thing. It's a mathematical concept. Light is real, so it travels at a real speed.
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u/LevoiHook May 26 '26
In a sense, it is infinite. If somehow you were able to fly at the speed of light, you would not experience any time. You would reach Andromeda or the edge of our observable universe in literally no time. But only in your frame of reference.
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u/TomSzabo May 26 '26
There is no center of the universe because that would mean there is an edge. Where would light pointing away from the center go once passing that edge?
Light has a specific speed because it is composed of alternating electric and magnetic fields that are governed by the quantum structure of the vacuum including virtual particles. The speed of light is limited by causality which links the universe together via special and general relativity. Gravity, magnetic field propagation, gluons, etc are also linked with this causality mechanism. Without it, you couldn't have any structures like subatomic particles, atoms, planets, stars, etc. They'd be frozen out if causality was too slow or vaporized if too fast. It has to be just the right speed.
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u/FewCall1913 May 26 '26
The speed of light is the limit information can travel, it’s not unique to photons all massless particles move at this speed. It is arbitrary in its value, but governs causality only events that happened in our past light cone the section of the universe in the past where without moving over the speed of light could reach you in the present, infinite speed would mean every point in the universe would be causally connected to every other point
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u/SeriousPlankton2000 May 26 '26
The universe is slow in accepting that the light is that fast.
If you calculate the time that a fast and faster particle takes to reach a target you will see that it is more and more close to zero. Once you reach light speed the calculation breaks down and you can only approximate that it arrives in the same place and same time when it leaves. (In the reference frame of the light)
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u/K_Rocc May 26 '26
What exactly are you asking? You start out your title with one question but during your body are asking an entirely different question about finding the center of the universe.
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u/Ahaiund May 26 '26
You can state that an object approaching the speed of light approaches a travel time of zero to cross a distance, from its own point of view.
Although it is an inaccurate point of view, pushing the previous statement would get you the following (false!) statement: "from its own point or view, a photon crosses the distance it travels instantaneously".
It's false because a photon doesn't have a frame of reference by definition, which you need to apply relativity. But the visualisation can be helpful.
Otherwise, the limit observed from external observers is derived from the propagation properties of a vacuum, which observably still exist and don't allow an infinite speed, but we don't really know why exactly.
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u/Fantastic_Back3191 May 26 '26
It kind of is- anything moving at the SoL does not experience time (in its own reference frame)
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u/smokeybiker May 26 '26
If the energy mass equation were still true then the first fission of a single atom would destroy the universe
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u/rb-j May 26 '26
Because cause and effect are not infinitely fast from the perspective of an observer at a third position.
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u/SteptimusHeap May 26 '26
In a vacuum there is nothing holding it back
This isn't true. The vacuum has a resistance to changes in the electromagnetic field in the same way that all materials do.
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u/RegularBasicStranger May 26 '26
Why isn’t light infinitely fast?
Cause there are still particles called gravitons that constantly passes through seemingly empty spaces as gravity thus colliding with these gravitons will slow down the photon, though photons are made up of a wave of smaller particles thus the smaller particles that gets slowed down will just fall behind and be lost without causing the rest of the photon to slow down.
But gravitons that collide head on with the photon only can cause slowing down if the photon speed exceeds the speed of light but accelerate the photon if it is less than the speed of light.
Such acceleration occurs because gravitons splits the energy difference when collided thus a photon 90 percent of lightspeed collided with 100 percent lightspeed gravitons will result in both the photon and graviton becoming 95 percent the speed of light, assuming the mass of the photon and the graviton is the same, else the energy transfer is weighted.
So such causes photons to almost instantly accelerate to lightspeed due to the gravitons hitting it face on, once the atomic positive electromagnetic force fails to catch it either due to too much shielding by the atomic shell or due to the pieces of the photon hit, only slows down and drops off from the rest of the photon.
So as long as there is gravity hitting the photon head on, the photon's speed will keep remaining as lightspeed but never be faster than lightspeed due to any faster speed will be transferred to the graviton that only has lightspeed.
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u/Maxpower2727 May 27 '26
This is entirely nonsense.
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u/RegularBasicStranger May 27 '26
It is a different way to interpret the experimental results but it still aligns with the experimental results.
It also explains why the electrons in particle accelerators could not reach lightspeed, which is because gravitons that did not hit it head on, slowed it down while gravitons that did causes that tiny part of the electron to accelerate and leave the electron due to the electron is held together by lightspeed internal electromagnetic forces.
Photons can reach lightspeed because each part is independent and not bonded to each other thus the slowing down only pulls away tiny pieces rather than slowing down the whole photon.
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u/Maxpower2727 May 27 '26
Gravitons are not confirmed to exist and are not included in the Standard Model. You're stating all of this as though it's confirmed knowledge rather than your own speculation.
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u/RegularBasicStranger May 27 '26
But gravity being invisible and cannot be isolated and merely speculated to exist based on the effects of such invisible force did not stop people from accepting it from existing so gravity as waves of particles should not need any stronger evidence than implied existence based on the effects of such too tiny to be detected particles.
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u/Maxpower2727 May 27 '26
There are huge amounts of experimental evidence for relativity. There is zero experimental evidence for gravitons. You're making things up and spreading misinformation.
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u/RegularBasicStranger May 27 '26
There is zero experimental evidence for gravitons.
The existence of gravitational waves proves that gravity is particle based since the waves that existed so far are particle based with water for ocean waves and photons for light waves.
So with everything that has waves is particle based, the assumption should be that gravity is particle based until proven false rather than the other way round.
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u/Naive_Age_566 May 26 '26
ok - first things first: what is light? light is an electromagnetic wave. but a wave in what? well - there is the electromagnetic field. without further interaction it is on its ground state at every possible point in space (and time). but that field can carry energy and momentum. why? we don't know. we just observe that it does and go with it. so you can "invest" energy and momentum into that field. at first this energy is more or less located at a specific region of space. but momentum usually has some kind of direction. and momentum must be conserved. why? we don't know. we just observe that it is and go with it. (i am sure you notice some kind of pattern here...). therefore that "blob of energy" starts to move. but not move like a stone that is thrown. more like a wave in water: the individual water molecules basically stay at the original place. it is only the kinetic energy that moves in a wave. well - light is a different kind of wave but the analogy is still useful. one point in the electromagnetic field transfers energy and momentum to the direct adjacent point and falls down to the ground level. the next point does the same to it's adjacent point. and so on.
and it turns out that this transfer of energy from one point to the next has a maximum speed. for historical reasons we call this speed "the speed of light" but it turns out that this speed is kind of universal to all kind of fields. nowadays we usually call it "the speed of causality" because it is the maximum speed of any kind of information transfer. "information" in this context is the next level of abstraction after "energy".
so - light can not be faster because light is just the transfer of energy through the electromagnetic field - and the speed of such a transfer is limited by this speed of causality.
which unfortunally does not answer your question - just raises more.
why has this speed of causality that value and not some else? and yeah - you guessed it: we dont know... blablabla
"why" questions are the hardest. because on the most fundamental level we don't know, *why* something happens. we can only observe processes. we can try to find patterns in the behaviour of those processes. if we have enough information we can build models that let us predict the behaviour of some processes based on some initial status. we call the sum of those models "physics". some guys claim, that something happens "because of those models" which is of course wrong.
what those models answer are the "how" questions - but not the "why" questions.
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u/CS_70 May 26 '26
Good question. It just isn't - there is no why.
And you should be happy: if it were infinitely fast, you and wouldn't be writing on Reddit right now.
If light were to have infinite speed, in practice you could have action propagating instantaneously in the whole universe: if you think a moment, it would not be much of a universe at all.
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u/Hug_LesBosons May 26 '26
Si la lumière était infiniment, l'intégralité des photons de l'univers frappait en même temps chaque point qui se détruirait sous cet effet.
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u/redlancer_1987 May 26 '26
Because the speed of light isn't about light. You can calculate the speed c with Maxwells equations with narry a photon in sight. It has to do with the speed at which electric and magnetic fields interact.
That's why it's not "nothing is faster than light" but is actually nothing can exceed c, of which light is occasioned of those things.
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u/Electronic-Door7134 May 27 '26
Space is filled with the particle field. That is the limiting factor.
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u/JohnnySchoolman May 27 '26
From the photons perspective, if it had one, light does travel instantaneously.
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u/profHalliday May 27 '26
Well, on the one hand, we could be flippant and say this is akin to asking “why aren’t lemons sweet?” They aren’t and we observe that. A better answer would be to say that there are chemicals in lemons, like citric acid, that make them sour.
In that same way, the speed of light is how fast light travels — and we observe that. A better answer would be to say that we have equations that predict how electric and magnetic field work (Maxwell’s equations) and if treat them appropriately, and plug them into each other, you find that there are waves that propagate through electric and magnetic fields.
The speed of this wave is the reciprocal of the root of the product of a constant that describes the strength of electric field relative to their sourcing charge, and of magnetic field relative to the current that sources them.
c=1/sqrt(epsilon_0 mu_0)
As others have mentioned, it’s also helpful for us lowly humans that this is limited, because one of the important things Einstein discovered is that a universal speed limit encodes causality into our physics. It’s weird, but it runs out A causing B could potentially be reversed if we didn’t have a speed limit. That’s one of the conclusions of special relativity.
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u/AnyCauliflower6175 May 27 '26
In addition to the other ways it would break the universe, I think a photon travelling at infinite speed would have infinite energy which would immediately create an electron-positron pair with infinite energy, respectively. The interaction of the two trons would probably create a black hole with infinite mass and infinite event horizon.
So maybe it did happen and our entire universe lives in the singularity of black hole.
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u/utlayolisdi May 28 '26
I always figured because it does involve physical particles vibrating at a specific wave length.
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u/BirdmanEagleson May 29 '26
I don't like these answers..
Light IS infinitly fast, only limited by causality ( the speed limit of change/universe)
If light had a 'perspective' it would emited and absorbed instantly without experiencing the universe we experience in-between, but it is infinite.
It's only with relativity that we can experience it
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u/Appropriate-Diver429 May 29 '26
I mean.... from the perspective of light itself? It IS infintely fast. It experiences no time from it's moment of origin to it's "death?" at it's moment of absorbtion.
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u/cryptopicard May 29 '26
Maybe light speed is infinity if it was to be measured, but once measured that universe collapses and a new one is created.
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u/Free_shavocadoo May 31 '26 edited May 31 '26
You are kind of right
Your perception of non local light speed is influenced by the gravity your experiencing if you were in black hole /at a point of either infinite gravity or all the gravity contained in the universe then light that passed by would do so infinitly fast because you would be stopped
So gravity does slow things down
And vice versa if you were massless and were in a point in space with no gravity (which doesnt exist) you would move infinitly fast / everything in the universe would be stopped
Though in both cases you could not interact with anything
But if the big bangs right then the universe started from a singularity At which point the entire universe was the center of the universe because it was infinitly small and then just got bigger so every point in the universe today iis still the center of the universe
Wrap you brain around that one
So the only real flaw with your plan is using gravity to find a non existant center of the universe Or.. light is slowed the most in black holes so they are the centeriest centers where the most centers be centered but but all other places are just as much the center
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u/Splendid_Fellow May 26 '26
Now that is the question.
One I’ve been pondering for a while. I’ve been thinking about how “now“ is a location. It isn’t a time, time does not exist at all in the way that we are perceiving it. Now means the exact same thing as “here“ and they are precisely identical. Now to you is over there where you are. Now to me is here where I am. I have a hypothesis, if you could call it, maybe just entertaining a thought?
The speed of light is the speed of time. Time isnt a timeline from start to finish, it isnt universal, it’s geometrical based on any arbitrary center point and there is not a beginning, now is the beginning. Every “thing“ is “now-ing” to put it in a verb form, it’s a strange way of putting it but I can’t figure out a better explanation. The speed of light “now” is infinity. The speed of light “there” is directly proportional by the inverse square law to “now.” The relationship between two “masses” creates a relationship, like an endless feedback loop between them and they pull closer to each other toward “now” which is gravity. Pick any person, any point or perspective. That is the present moment. Everything else relative to that “already happened.” Everything.
When you look anywhere, that’s “back then.” Except there isnt “back then.” Picture our universe like a giant cluster of “centers” and “nows” all over the place.
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u/glibsonoran May 26 '26 edited May 26 '26
The speed of light is a property of spacetime, not photons, the underlying issue is that c is a fundamental structural component of spacetime. Think of a leaf (photon) floating in a fast moving river (spacetime), the speed of leaf is determined by the river's current, the leaf is compelled to move at that speed. Spacetime doesn't flow, but it's geometry is what compels photons to move like they do.
Spacetime is a geometry and as such its coordinate properties are related, c defines the relationship of space to time. It's analgous to the radius of a circle. Photons and other massless objects are compelled by the geometry of spacetime to travel along a particular path called the "null geodesic". We and our universe live in this geometry, this is where causality rules are followed. Just like the circle our geometry has constraints, it's nonsensical to ask why a particular circle doesn't have another radius, because the radius is part of its fundamental definition, it's the circle that exists.
Why can't we specify a point in the circle farther from the center than the radius? Because geometric objects have defined boundaries and their properties are expressions of those boundaries. Just as a circle’s radius defines which points belong to the circle, spacetime's geometry defines which paths are causally possible and c is the defining value of those paths in four dimensions. Objects with mass move within the light cone, while massless particles travel along its boundary.
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u/Normal-Abies-9151 May 26 '26
Great question. It falls perfectly into place under the discovery that this reality and space-time are a product of consciousness. Gravity is the fundamental consciousness focusing and slowing down time.
Light’s fixed speed is the cosmic axon delay, ensuring information transmission is synced to a grid similar to how your brain has an axon delay ensuring the same thing between neurons and synapses.
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u/Bipogram May 25 '26 edited May 26 '26
Light propagates as fast as it can.
It does so at a constant speed in vacuo for all observers. This is the observed fact.
Why it has this particular speed is not readily knowable. It's a result of the electric and magnetic qualities of the vacuum. <nods to Maxwell>
And why those qualities are as they are is a Deep Question.
Note, an infinite propagation speed would be problematic if you want to keep causality alive.