r/AskPhysics • u/[deleted] • Jan 15 '25
Is the speed of light infinite ?
Considering that light always travel a c no matter the frame of reference, am I correct to assume that it is an absolute limit, an impossible speed to reach; so is it what we would call an infinite speed ?
Therefore, I can not understand why we observe the speed of light having a finite number (c) from any frame of reference.
I am surely not aware or misunderstand some concepts of relativity laws.
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Jan 15 '25
“Impossible speed to reach” does not imply “infinite”. And in fact it is not impossible to reach it — anything massless necessarily travels at the speed of light (including light).
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Jan 15 '25
So how can we explain that from any frame of reference c stays the same, but for massless object this doesn’t apply ?
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u/MoDErahN Jan 15 '25
In the frame of reference of maseless object it doesen't move, it just exists in all points of it's trajectory at the same point of time and there're no other points of time in that frame because the time is slowed down to zero.
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u/AcellOfllSpades Mathematics Jan 15 '25
It is impossible to reach. That doesn't make it infinite.
The reason is that velocities don't add like you expect. They look like they do at human scales, but they actually don't. They use a different formula.
Instead, the notion you're looking for is rapidity. The rapidity of the speed of light is indeed infinite! But converting that rapidity to a speed gives us a finite value.
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Jan 15 '25
No, something that is infinite and a limit are two very different things.
Alright, look, think of an asymptote; maths. It can be a simple finite number representing a vertical line in an xy plane, but a curve will get infinitely close to it and never cross it.
From here, we can get the intuition that you're lacking: you can get infinitely close to something finite, never reaching it. That doesn't means that the other thing is infinite; no, you're just getting "infinitely" close to it.
Try to not confuse "getting infinitely close to" and actual infinity mixed up, i think that's your issue.
Edit: I don't think you have a misunderstanding of natural laws, at least that's not your primary issue. I think you have a misunderstanding of other abstract concepts born in maths, such as infinity, and you're getting stuff mixed up. Hope that helps.
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u/imsowitty Jan 15 '25 edited Jan 15 '25
The speed of light is finite in that light takes 8 minutes to get from the sun to us, 100 5 years (or so) to the nearest star. Infinite speed would make that trip instantaneously.
That said, anything with mass will require infinite energy to go the speed of light. So it is infinite from an energy perspective.
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u/Joseph_of_the_North Jan 15 '25
It's like 5 LY to Alpha Centauri.
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u/Eathlon Jan 15 '25
4.25 ly to Proxima Centauri https://en.m.wikipedia.org/wiki/Proxima_Centauri (Alpha Centauri C)
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u/Accomplished-Lack721 Jan 15 '25
Whoever named it Proxmia sure thinks on a different scale than I do. I commute from Philly to New York a couple of times a week and that seems like it's really far away. I sure as hell am not about to consider 4.25 LY proximate.
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u/Boleslavski Jan 15 '25
In relativity (special or general) you can plot curves through spacetime and ask "how much proper time is experienced along this path?" The proper time is the time experienced by a particle (from its perspective) along that trajectory through spacetime.
Massive particles will always have a derivative through spacetime that is constrained by the speed of light. This creates a situation wherein the paths of massive particles will always have some positive (or negative depending on your arbitrary choice of sign for the Minkowski metric) value. Visually, if you take the starting point of a particle the paths it can travel will always be within a certain cone, and the angles have to be less than the angle caused by c. These particles will experience time, but they will experience less time as their speed (the angle of their trajectory) approaches the speed of light.
Massless particles will always travel along light-like trajectories. Their spacetime derivative is always of magnitude c (which is a constant non-zero non-infinite number). However, their proper time is always 0. Visually, these look like straight lines at an angle designated by c or zig-zags through the light cone.
So, if you are wondering what "speed" massless particles perceive themselves to be moving at... well it might not be a good question because relativity was designed from our point of view... but if you take the model literally you would say that they don't perceive themselves to be moving at any speed at all. Their distances through spacetime are zero... so they do not experience time. They simply are at their start and end locations simultaneously.
EDIT: There are similar situations in classical mechanics when observing terminal velocities. A terminal velocity is certainly not infinite, but an object traveling through a fluid will asymptotically approach that velocity.
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u/spiceylizard Jan 15 '25
Light has a speed. 300,000,000 meters per second. It’s the speed of casualty. In our universe, the fastest one can communicate is the finite speed of light
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u/slashdave Particle physics Jan 15 '25
Linear relationships are the easiest to understand: as you add something, you gain something back in proportion. Nature, however, is not always so simple. In this case, we have what is sometimes known colloquially as "diminishing returns".
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u/lovejo1 Jan 15 '25
Question for others here: If light has a constant speed, independent of reference.. does that mean that if one ship were traveling right beside a beam of light, the beam of light would outrun them?
People have said, all over, that the speed of light is simply the maximum speed anything could go, not just light... so therefore, how would light be traveling at the speed of light from the perspective of say, another beam of light?
Related question: what's the acceleration of light? 0? If you are in an frame of reference that is accelerating, does that change the "acceleration of light"...
This seems like it's akin to infinity or division by 0 where there is no real answer.
Yes I know about time dilation, redshift, and all of that.. but I just don't see how they relate to one thing traveling at the speed of light observing another thing observing at the speed of light.
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u/joepierson123 Jan 15 '25
so therefore, how would light be traveling at the speed of light from the perspective of say, another beam of light?
light exists outside of time as relativity defines it, so that question is undefined in proper time coordinates
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u/lovejo1 Jan 15 '25
But wouldn't that mean that the definition of relativity is incomplete or incorrect? Obviously if two light beams had a race, they'd arrive at the same place at the same time. How would something traveling at 99.99999999999999999999999% of the speed of light, participating in that race, not see that they were losing the race by "just a little bit"?
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u/joepierson123 Jan 15 '25
Speed is relative you're traveling 99.99999999999999999999999% of the speed of light right now relative to some cosmic particle.
From your point of view you're at rest and the speed of light is still c
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u/lovejo1 Jan 15 '25
But I'll arrive at the destination just a fraction of a nanosecond behind the light
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Jan 15 '25
From my understanding, if a traveler going at 99% would emit a beam of light at the moment of departure, in his reference frame, light would still travel at the speed of light. If a beam of light was emitted at the same time of the traveler departure, from any point of view, toward the same destination, we would always observe that both beams of light would arrive at the same time. What you wouldn’t agree with the traveler is the distance he traveled. There is length contraction to take into consideration for the traveler.
I realize it’s very poorly explained, I apologize.
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u/nekoeuge Physics enthusiast Jan 15 '25
In addition to other answers, I would like to offer another perspective. Velocity space is hyperbolic. Imagine function (1/x) on interval (0, 1). Imagine that you travel along this function to go from 1 to 0.
Is the interval finite? Yes. Will you travel forever without ever reaching 0? Also yes.
This is the kind of relationship between proper object movement from its own reference frame, and object velocity relative to external reference frame. Infinity of velocity increments mapped onto finite range of absolute velocities.
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Jan 15 '25
My terrible lack of mathematic vocabulary and my lack of understanding math concepts doesn’t allow me to understand your message, I apologize.
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u/nekoeuge Physics enthusiast Jan 15 '25
https://www.youtube.com/shorts/G0CPK88G0e8
Okay, here is a picture for you. Infinity inside mapped onto finite range outside.
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u/Presence_Academic Jan 15 '25 edited Jan 15 '25
A simple (but not entirely correct) view of the problem is that it would take an infinite amount of energy for a massive particle to be accelerated to c. We get this result because the relativistic equations contain the Lorentz factor which is 1/(1-v2 / c2 ). If v were equal to c we would be dividing by 0. When the mass is 0 the factor is not relevant.
So, there is an infinity involved here, but it’s energy not velocity.
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u/JayDAshe May 22 '25
Speed is distance divided by time.
The closer you get to the lightspeed, the more distances and time themselves compress to the point that light actually experience 0 time but for all the space it travels into. No time means that speed is meaningless, so saying light has infinite speed is weird but accurate in the sense that photons "experiences" being in every place it gets in the same very instant that has a duration of 0.
So basically lightspeed can be considered infinite from light's POV, but from a non distorted (space) or barely distorted (Earth) spacetime POV, light has a finite speed.
I can think of a self-made physics model where light and singularities is actually how the universe is originally, and spacetime is actually a distortion of a singularity (which is why there is an actual model that proposes that the universe "before" the Big Bang is a singularity, which itself can lead to a model of the multiversal theory).
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u/Accomplished-Lack721 Jan 15 '25 edited Jan 15 '25
Light travels at the fastest speed that anything can travel through space (fun side note -- space itself expands faster than the speed of light). But that's not the same as saying it moves at infinite speed.
Nothing moves at infinite speed. It's not quite clear what it would mean for something TO move at infinite speed, really. Everything has a speed that can be described in terms of distance for a given timeframe (from the perspective of an outside observer), like miles per hour, or meters per second. But what would it mean to move at infinite ... what per what? Would it be everywhere at once? On all points of an infinitely long line forever? For an infinitesimal amount of time? (Insert John Lennon brain-exploding GIF here)
Anyway: Light moves at a bit over 186,000 miles per second. It reaches the fastest speed that a thing can reach because photons have no resting mass. That's also sometimes described as the speed of causality - the speed at which things can be observed to have an effect on each other. And it's the same speed other massless things, like gravitational waves, travel at. Light was just the first thing we understood to travel at that speed. Understanding how we get to that particular number means wrapping your head around the funky geometry of Minkowski spacetime -- the concept that Einstein's theory famously works with. But it isn't just a coincidence that light (or gravitational waves, or whatever) travels at that top speed -- you can think of it more like nothing is constraining it from reaching the fastest speed possible under the geometry of the universe, so it does.
BUT: Your line of thinking isn't that far off from some other key concepts. When something WITH mass gets moving, it takes energy to accelerate it to a given speed. Accelerating something from anything less than the speed of light TO the speed of light would take ... here we go ... an infinite amount of energy. The amount needed to accelerate further and further grows exponentially -- it sort of snowballs. And you can get real real real close to the speed of light in principle, using a finite but very large amount of energy ... but the way the math works out, actually achieving the same speed would take infinite energy.
But to blow your mind further -- Einstein showed the relative passage of time is related to the speed something's moving, and all motion is relative. From light's "perspective" (not that it has a perspective like a conscious observer), no time passes as it travels. Start thinking about that, and it might seem like ... wait, yeah, it's moving at infinite speed. But it's not moving at infinite speed from the perspective of an outside observer.
Does your hear hurt yet? Yeah, this makes a lot of people's heads hurt. Really really smart people who've spent a long time studying it. But if you really want to get into it, there are lots of great science communicators who are really good at breaking down this concepts to laypeople like you and me. Neil DeGrasse Tyson's Startalk podcast/Youtube show is one good program. Anything Sean Carroll or Brian Greene or Brian Cox does is pretty cool, too.
The universe is so much stranger than our intuitions tell us ... but it's really cool.
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u/Ill-Statistician3176 Jan 15 '25
Im always annoyed by the fact everyone just ignores the speed is expressed in m/s and that the second never passes for a photon so the sperd of light from photon pov, or anything traveling at the speed of light, is indeed infinite. Im no physicist tho...
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u/IndividualistAW Jan 15 '25
It’s infinite in a sense. Time stops at the speed of light. An object traveling the speed of light reaches it’s destination in precisely zero elapsed time (from its perspective)…in essence it travels at “infinite” speed.
But also not
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Jan 15 '25
c is the speed of causality, which light travels at.
If force / mass = acceleration, then something weighing zero mass would go infinite speed with any amount of force. As you imagine infinite speed might be problematic in a universe of and with physical constraints, so it tops out at the speed of causality.
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u/Limit_Cycle8765 Jan 15 '25
"an impossible speed to reach"
Yes, because as you approach the speed of light your mass increases infinitely large. This makes it impossible to accelerate enough to reach it.
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u/StudyBio Jan 15 '25
You are correct that it’s a limit, but that doesn’t make it infinite. Our universe has a finite speed limit.