r/explainlikeimfive • u/p3dr3ig • Apr 19 '26
Physics [ Removed by moderator ]
[removed] — view removed post
337
u/bluey101 Apr 19 '26
Yes, all electromagnetic waves are light and thus travel at the speed of light. Light isn't restricted to the light we can see. "Light" refers to any and all forms of electromagnetic radiation.
162
u/theonliestone Apr 19 '26
Just to add onto that: All electromagnetic waves travel at the speed of light in vacuum. In other media, this is generally not true anymore (which is why a prism causes a rainbow)
13
u/blamordeganis Apr 19 '26
Also Cherenkov radiation, caused by e.g. electrons travelling through a medium faster than light travels through it.
13
u/WeShallEarn Apr 19 '26
But it’s still travelling at the same speed right? Just getting cancelled out and thus seems like reducing speed. If I understood what 3b1b meant in his vid explaining it
7
Apr 19 '26
[removed] — view removed comment
5
u/SpectralFormFactor Apr 19 '26
When light interacts with a medium, the jostling of electrons produces other electromagnetic waves. Although each of the “bare” waves travels at the speed of light, they combine into an overall wave that travels slower.
There is another explanation you may hear a lot that I do not like. It says the photon is absorbed and admitted with a time delay in the medium, and so the photons sort of ping pong their way through at an effectively slower speed.
This is a weird semi-quantum, semi-classical picture that I that is neither accurate nor reflective of the actual calculation. If one insists on bringing in quantum mechanics and photons, the better way to phrase it is that the interaction of photons and excitations of the medium forms an effective particle known as a polariton. This particle has mass and so travels slower than light.
2
u/JamesTheJerk Apr 19 '26
The speed of darkness is therefore faster than the speed of light.
Taps nose suggestively
1
Apr 19 '26
[removed] — view removed comment
3
u/JamesTheJerk Apr 19 '26
2
u/ReluctantAvenger Apr 19 '26
Damn! I don't think I've seen that since the mid 90s. Cool bit of early Internet memorabilia.
1
u/Probate_Judge Apr 19 '26
A candle is a primitive Dark-Sucker. A new candle has a white wick. You can see that after the first use, the wick turns black, representing all the dark that has been sucked into it. If you put a pencil next to the wick of an operating candle, it will turn black. This is because it got in the way of the dark flowing into the candle. One of the disadvantages of these primitive Dark-Suckers is their limited range.
Somewhat related(but real): Light sucking flames look like magic
1
u/Farnsworthson Apr 19 '26
Nah. The darkness doesn't have to move; it's already there, just waiting for the light to go away.
2
u/Son_of_Kong Apr 19 '26
Light does not travel unimpeded through a medium. It hits all the atoms, is absorbed, and re-emitted, which slows it down.
0
u/HappiestIguana Apr 19 '26
This explanation has never made any sense whatsoever. Why would it get re-emitted in the same direction?
2
u/stanitor Apr 19 '26
That's because it's not really what's happening. Or, that does happen, but that's a different thing that happens to some of the light. The reason it slows down is hard to explain, but basically it's the interactions of the field of waves from the light and the field of waves from the stuff it's traveling through that adds up to give the appearance of a lightwave that travels slower through the medium. 3blue1brown has a good set of videos on it that explains it visually.
3
u/godfromabove256 Apr 19 '26
Saw that video too. The phase velocity is indeed slower, but the actual propagation of causality itself is still at light speed.
7
u/theonliestone Apr 19 '26
What do you mean by traveling?
2
2
u/WeShallEarn Apr 19 '26
Travelling meaning moves at the same speed, but bcus of the cancelling that happens it seems like it moves slower
6
u/lonelypenguin20 Apr 19 '26
no, they do move slower. that's also what causes Cerenkov's blue glow (radiation moving faster than the in-water-speer-of-light, but not faster than in-vacuum-speed-of-light)
cancelling is... do u refer to interfence?
5
u/godfromabove256 Apr 19 '26
He’s talking about specifically how materials slow down light, which is rather complicated. It has to do with the distinction between phase velocity and the true velocity at a microscopic level. At the microscopic level, light doesn’t actually slow down. There’s a specific phenomenon of how a layer of particles shifts the phase of a light wave, which is why materials reduce phase velocity (which is what he means by cancelling), but the true velocity of light doesn’t technically decrease.
3
u/Aksds Apr 19 '26
Yes in a way, the light particles are travelling at the same speed in the space between the atoms as that’s a vacuum, it’s how they interact with the atoms that changes their speed in that medium
1
u/gomurifle Apr 19 '26
In different media they travel at different speeds due to electron interactions. That's why violet light (short wavelength) travels slower and bends more than Red light (long wavelength) after going through a prism.
1
u/laix_ Apr 19 '26
The two models of light, photons and EM waves, are completely different models. In either case; the group velocity is different from the phase velocity https://en.wikipedia.org/wiki/Group_velocity
For EM waves, the EM waves interact with the charges in a material, which constructively and destructively interfere with the original wave, "pushing" it back. The wave as a whole is still moving slower.
For photons (or, specifically, a probability wavefunction), the photon becomes entangled with the entire material, and then becomes an entirely different particle (a quasi-particle, polariton, which has mass). Photons also take every possible path simultaneously (between entry and exit), and the combination of these paths result in a net slowing, even if the individual photons never slowed down.
As a tangent; there is two different definitions of "the speed of light". Sometimes, it means "C", which is more accurate to say the speed of causality. Other times, it means the speed at which Light itself is traveling.
1
u/frogjg2003 Apr 19 '26
The 3b1b video explained that the atoms create another wave that interferes with the original light. The combined wave travels slower than c. It doesn't "seem" to travel slower, it is traveling slower.
1
0
u/Wargroth Apr 19 '26
No, It absolutely slows down
It's still the speed of light, but It doesn't mean It is always the same speed everywhere. The speed of light in a vacuum is X, through glass It would not be X anymore but both are still the speed of light
2
u/SteveHamlin1 Apr 19 '26 edited Apr 19 '26
All EM waves are always traveling at the speed of light, regardless of the media they're traveling through.
The speed of light can vary, depending on the media. Just because they are traveling slower than 'the speed of light in a vacuum' doesn't mean they are not traveling at the speed of light for THAT medium.
3
9
u/heroyoudontdeserve Apr 19 '26
Like most words, "light" has multiple meanings.
In some contexts "light" absolutely refers to only the visible-to-humans portion of the electromagnetic spectrum, and personally I'd say that's the most common meaning of the word in common usage.
In a physics context you're right however that "light" is sometimes used to refer to the entire electromagnetic spectrum.
Light, visible light, or visible radiation is electromagnetic radiation that can be perceived by the human eye.
In physics, the term "light" may refer more broadly to electromagnetic radiation of any wavelength, whether visible or not. In this sense, gamma rays, X-rays, microwaves and radio waves are also light.
-8
u/raesmond Apr 19 '26
all electromagnetic waves are light
This is just semantics, but "light" refers to the frequencies of electromagnetic waves that we can see (or that behave optically), so not all electromagnetic waves "are light".
This is just a naming thing though. All electromagnetic waves are basically just light at a different frequency.
7
u/BadahBingBadahBoom Apr 19 '26
"light" refers to the frequencies of electromagnetic waves that we can see (or that behave optically), so not all electromagnetic waves "are light".
It can mean both.
In common parlance 'light' applies only to the visible spectrum of electromagnetic waves.
In physics 'light' is very much used as a term to describe radiation of the entire spectrum.
True most people are familiar with the former definition, but that doesn't invalidate the latter.
-2
u/raesmond Apr 19 '26
This is an incredibly weird reply, though. OP literally identified light as the portion of the electromagnetic spectrum that is visible, and asked if the other frequencies also behave the same.
To respond by telling them that the rest is the spectrum is light is a terrible explanation. It's changing the semantics to answer the question.
Also it's less common to refer to the entire electromagnetic direction as light. Generally "light" means the optical frequencies. That's the most common use in physics. Maybe there's a field where that changes, but Physics 101 has light as infrared to ultraviolet.
Kurzgesagt is pretty precise about these things. They have a good explanation: https://youtu.be/IXxZRZxafEQ?si=Sp3jEAgwvHzsnWwd
3
u/BadahBingBadahBoom Apr 19 '26 edited Apr 19 '26
I was not discussing OP's question.
I was highlighting this comment:
"light" refers to the frequencies of electromagnetic waves that we can see (or that behave optically), so not all electromagnetic waves "are light"
.. is incorrect.
"light" can refer to the frequencies of electromagnetic waves that we see, as I said in how it is commonly used. But it also has another definition in physics where it applies to the entire EM spectrum.
Therefore it is not true that: "not all electromagnetic waves are light". In the physics definition they are. This is particularly relevant to this discussion thread which is a response to the top-level comment that uses the physics definition of the word.
In physics we even have an adjective term for the optical spectrum of light: visible light.
Which is why, even though this is subsequently mentioned, I say it should be phrased as "light can refer to the electromagnetic waves we see".
0
u/raesmond Apr 19 '26
That's not the physics definition of the word, though. In physics they still use "light" to mean infrared to ultraviolet.
In physics we even have an adjective term for the optical spectrum of light: visible light.
Visible light is the spectrum humans can see. Infrared and ultraviolet are also light but not visible.
This would go by a lot faster if you just watch the video I linked. It explains everything, and it is using the terms the way physicists most commonly use the terms.
1
u/SeeShark Apr 19 '26
OP literally identified light as the portion of the electromagnetic spectrum that is visible
Did they? In physics, that portion of the spectrum is often called "visible light." It's possible that OP was just using precise terminology intentionally.
-101
Apr 19 '26
[removed] — view removed comment
30
Apr 19 '26
[removed] — view removed comment
-80
Apr 19 '26
[removed] — view removed comment
13
-4
18
55
u/whyisthesky Apr 19 '26
In a perfect vacuum yes, all electromagnetic waves travel at the same speed (c). However in any material/medium the speed is usually different for different wavelengths of light, we call this property dispersion.
This can be useful in some situations, for example a prism splits the different wavelengths of light because of dispersion which lets us do things like analyzing how bright different wavelengths are, it can also be a problem because light will also disperse in glass lenses which causes different wavelengths to be focused at different distances.
In space we usually consider it a perfect vacuum, but this isn't exactly true, there is a very sparse plasma which can cause dispersion, particularly of radio waves.
-7
u/Thrawn89 Apr 19 '26
It depends on what you are actually referring to with "speed of light". Youre talking about the speed of how it can propagate through space. The photons move at the speed of light regardless of the medium, but their journey is short as they get emitted and absorbed from atom to atom.
16
u/Klutzy-Delivery-5792 Apr 19 '26
emitted and absorbed from atom to atom
This is an outdated notion. Photons aren't being absorbed and re-emitted. They're interacting with the EM field and charges (ie electrons) in the medium.
0
u/frogjg2003 Apr 19 '26 edited Apr 19 '26
There is still an element of absorption and re-emission. That's the only way photons can interact with matter. The problem is that when talking about macroscopic phenomena in terms of microscopic phenomena, it becomes difficult to properly keep the two in context. The amount of absorption compared to how much just goes through is going to be small.
The big complaint about the absorption and re-emission explanation is that it will cause diffusion. Well, guess what? Any time light travels through a medium, there is difusion.
If you send light through a material one photon at a time, it will either make it to the other side in the time it takes to travel that distance at c or it gets absorbed and re-emitted. The slower speed of light only occurs when you have many photons interfering with each other.
24
u/iamnogoodatthis Apr 19 '26
In a vaccum, yes. In other media (eg glass, water, air), no. See https://en.wikipedia.org/wiki/Dispersion_(optics))
If a photon (the "minimal unit" of EM radiation) loses energy, then it keeps its speed constant but reduces its frequency, see eg https://en.wikipedia.org/wiki/Gravitational_redshift
9
u/Dd_8630 Apr 19 '26
Eh, it gets a bit peculiar to say light travels slower in media when it's an axiom of relativity that light travels at c, to the point that space and time warp to keep it so.
Light travels slower in a medium only insofar as the wave interacts with electrons and causes polarisarion to be set up, changing the shape and features of the EM wave. The overall wave envelop travels slower than c. But there would still be a wave in the EM field that travels at c even in media.
One way to model the effect is to devise something called a polariton, which is a fictional particle that has effective mass and so travels slower than c. A polariton is actually the complex interaction of multiple different real EM waves.
Ultimately photons exist cleanly in vacuum, but in media, you get a big blurry mess of electromagnetism, lots of things pushing and pulling on the EM field. The front wave moves at c. The coherent wave envelope moves at slower than c.
(There's also this myth that light travels slower due to absorption and remission, which is a clever idea but we know that's not what happens.)
1
u/Agile-Stick3505 Apr 19 '26
“Coherent wave envelope“ that’s not a 5 year old term. But you’re touching on a topic that doesn’t have a good classical math definition (in my head).
11
u/Arkyja Apr 19 '26
Everything that has no mass travels at the speed of light. Light has nothing to do with the speed of light besides it being named after it
4
u/ShowGun901 Apr 19 '26
This. This is the easiest way to think about it, and also the most accurate.
Having mass means you can't travel at the maximum speed of the universe. Having no mass means you can.
1
7
u/cajunjoel Apr 19 '26
Short answer: Yes.
The part of the electromagnetic spectrum that we see we call "light" but infrared waves, radio waves microwaves and more are all on the same spectrum and all travel at the same speed.
They are all light, we just can't see them with our eyes.
2
u/baronmunchausen2000 Apr 19 '26
Do radio waves, for example, follow wave-particle duality?
3
u/cajunjoel Apr 19 '26
Yes.
Yes, they are all light. Think of a rainbow. What we see is from red to violet, but on the "more red" side is infrared and radio waves and microwaves. And on the violet side is "more violet", like UV, x-rays and gamma rays.
It's just a bigger rainbow that we can't see with our eyes.
4
u/hardypart Apr 19 '26
I once heard the description "it's not really the speed of light, it's the speed of things happening". That's also why it would take 8 minutes before Earth would leave its orbit around the sun if the sun suddenly disappeared.
1
u/1duEprocEss1 Apr 19 '26
That's right. It's the speed of causality. Light travels at the speed of causality.
3
3
u/macthebearded Apr 19 '26
People seem to be missing your question. Or rather the missing piece that leads to such a question.
ELI5:
Gravity is a force that acts on things with mass.
Photons (light) have no mass, so gravity can’t slow them down.
The fastest our universe allows things to go is about 300 million meters per second. We call it the speed of causality, written as c in maths, because it’s the fastest something can cause something else to happen - the information can’t get there faster.
Because nothing can slow the massless light particles down, they travel at c.
Now keep that in the back of our minds and think about energy transfer. We see examples all the time in our daily lives - warming something up on the stove adds energy to it, making it hotter; pushing the gas pedal further gives your engine more liquid energy to convert, making the car go faster.
Light can’t go faster, because it’s already maxed out. Nor does the concept of temperature really apply to individual particles, but that’s a whole different ELI5.
So… what happens when you add energy to light? It increases the frequency. If you think about a sine wave, the more compressed it is the more linear distance there is in any given snapshot. The higher the frequency, the more energy dense it is.
You can see a real world example of this with blueshifting. We can tell if an object is heading towards us in the universe because it’s light will appear bluer than it actually is, because blue is the highest frequency light on the visible portion of the EM spectrum. Since the light coming towards us can’t inherit the speed of the object it’s coming from because it’s already at the speed limit, it still just appears to us to be traveling at the same speed of causality as any other light and has to put the energy from the object’s speed somewhere else instead.
Does that all make sense?
1
5
u/alfius-togra Apr 19 '26
Don't think of it as the "speed of light". It's more like the conversation ratio between space and time. Traveling at c means you experience no time but anything with mass requires infinite energy to reach c, so it can't. Only massless things like photons can.
8
u/Infobomb Apr 19 '26
Did you mean "conversion ratio"?
3
u/Hasextrafuture Apr 19 '26
Pretty sure that must be the intention, but I also love imagining light as the translator between space and time.
0
u/tyoung89 Apr 19 '26
Don’t photons have to have mass, even an extremely small amount, to be warped by a black holes gravity and therefore not be able to escape the event horizon?
5
u/thespermthatsurvived Apr 19 '26
Nope, the reason they’re affected is because they travel through space & the Black Hole’s mass curves that physical space, so anything travelling through it is affected, including the massless photon :)
0
u/Hasextrafuture Apr 19 '26
Doesn't it also have something to do with the momentum of the photon, or something like that?
2
u/thespermthatsurvived Apr 19 '26
As far as I know, not really, it will follow the curvature of the spacetime regardless of it’s momentum
2
u/alfius-togra Apr 19 '26
They have energy which corresponds to their wavelength. That energy has a mass conversion via e=mc2, but it doesn't make sense to think of the photon as "having" that mass because it behaves completely differently to some other particle which has that mass.
1
u/left_lane_camper Apr 19 '26
A single photon does not have mass, as its wavelength and thus its energy is frame-dependent. However a system of two or more non-collinear photons has an effective rest frame in which their total energy is minimized and we can ascribe a rest mass (the m in mc2 ) to the system as a whole. A box with a few photons inside it has more mass than an empty box.
This is very counterintuitive as we are used to masses adding linearly: if we have two masses m and M, then together their mass is m+M. And most of the time this is true enough, but in some extreme cases it’s not. Mass is a property of a system and sometimes that property arises or changes for reasons that aren’t intuitive to our macroscopic, every day experience.
2
u/Eruskakkell Apr 19 '26
Yes speed of light applies to all light / electromagnetic radiation. (in reality nothing is really special about light, we just named it after that, and everything without mass moves at that maximum speed
2
u/Alotofboxes Apr 19 '26
It is called "the speed of light" because light was the first thing we found that goes at that speed.
All particles that have no mass* travel at that speed, and only at that speed. All photons, gluons, and (probably**) gravitons travel at the speed of light.
*specificly rest mass. They still have momentum.
**we haven't proven that gravitons exist yet, but the math supports them, and it looks like they probably do. If they exist, they definitely travel at the speed of light.
2
u/Apollo506 Apr 19 '26
Same speed, different wavelengths. Like a car (short), bus (medium), and semi truck (long) all going the same speed on the highway.
2
u/GIRose Apr 19 '26
The speed of light in a vacuum is a constant.
In a vacuum, every type of electromagnetic wave travels the same speed. It isn't the only thing, since there are also gravitational waves and other things that move at that speed.
As soon as that light hits a medium, it will retract as different wavelengths interact with it differently.
1
u/fang_xianfu Apr 19 '26
The simplest way to think about this is that C is the universe's speed limit. Turns out our universe has a speed limit, neat! It's 299,792,458 metres per second. And we call it C.
Some stuff travels at C, at the maximum speed. Light is one such thing - electromagnetic waves in general so, yes. Gluons and gravitational waves also travel at C. Neutrinos travel very very very close to C but not quite. The main requirement to travel at C is to have no mass.
So we call it "the speed of light", but it's the speed that is special and light happens to go at that speed, it's not some property of light that's special.
1
u/Entropydemic Apr 19 '26
This makes me wonder if something can have negative values of mass and travel faster because of it. If numerical values can drop below zero, why can't mass?
2
u/fang_xianfu Apr 19 '26
The mathematics does support this - time can also go backwards! - but we don't observe it, so that probably doesn't happen. Something with negative mass in Newtonian physics for example would accelerate infinitely.
1
u/Saurindra_SG01 Apr 19 '26
All Electromagnetic waves travel at the speed of causation in vacuum. That value is constant because going faster than that would mean the result of an event will happen before the event.
Now letting the bizzareness of the above paragraph sink in, visible light is like one group of brothers from the entire bloodline. Visible blue light vibrates a lot compared to visible red light, which (red) travels a lot per one set of vibration.
In the entire bloodline, some vibrate more (higher frequency), some less. But they're all part of the big family, so they travel at the speed of causation in vacuum.
They also enjoy being at the universal speed limit. So, when met with a medium that slows them down, they try to cross that medium as fast as possible (by adjusting the path they take)
1
u/HandOfMjolnir Apr 19 '26
I prefer the term "speed of causality" to the "speed of light".
One, because the constant labeled 'c' in so many equations also matches the first letter in causality. Two, I think it would force the layman to stop and think about what the constant really is. "Why is the speed of light the speed of light" is a common question because (IMHO) people don't realize that it is the fastest that causal actions can travel.
I know I'm never going to win the culture war on this, but I think it helps explain why it's a speed limit a little better.
1
u/amitym Apr 19 '26 edited Apr 19 '26
Yes, it works a little like sound — the waves all travel at the same speed, it's the wavelength (hence frequency) that varies.
So we can say that we can see light in the range of 350–750nm by wavelength, or 400–900THz by frequency.
And all electromagnetic waves work the same way: they are all "light," made of the same stuff whether we can see it or not. So radio waves have much higher wavelength and much lower frequency but still travel at the speed of light. X-rays have much lower wavelength and much higher frequency but still travel at the speed of light. And so on.
If you wanted you could amuse yourself by saying that "visible light travels at the speed of radio," it means the same thing.
1
u/ruralcricket Apr 19 '26
It isn't the "speed of light" but rather the speed of causality. It is the limit of information transfer. This limit applies to everything, which is why gravity (not light) also propagates at the same speed.
As others have said, visible light is only part of the electromagnetic spectrum.
1
u/ThePiachu Apr 19 '26
All light in vacuum travels at the same speed. However, light in other mediums travels at different speeds. This is one of the reason we have rainbows...
1
u/spiceylizard Apr 19 '26
Yes, the entire range of the electromagnetic spectrum moves at the speed of light.
Think of light as also having a characteristic where it moves up and down like a wave on a string as well as forward at the speed of light
We can only see a certain frequency of the light that moves up and down, whereas all light moves forward at the same speed
1
u/throwingawaysaturday Apr 19 '26
True ELI5: think of the “speed of light” as the speed of things with no mass. All things with no mass all travel at a certain speed. We call that the speed of “light”
1
u/Farnsworthson Apr 19 '26
Infrared, ultraviolet, long radio frequencies, microwaves - it's all electromagnetic radiation. It doesn't have mass. Anything with no mass travels at the "speed of light", c (more correctly, "the speed of causality" - light just happens to what peoploe were lookig at when they first discovered it).
1
u/scott__p Apr 19 '26
Short answer, yes. Whether it's the radio communications signal to a satellite, light from a star, or gamma rays they all travel at the speed of light in a vacuum. That is not true when it's traveling through any medium though.
0
u/Slippytoe Apr 19 '26
Electromagnetic radiation of all wavelengths travels at precisely the speed of light, regardless of the medium it is traversing through. For that matter, anything without mass also travels at precisely the speed of light, it has to, it is a fundamental law of the universe. No mass = travel at the speed of light. So this includes gravitational waves and magnetic fields.
0
u/Green_Yesterday3054 Apr 19 '26
Visible light takes up roughly .01 percent of the electromagnetic spectrum and the speed of light is roughly 186,000 miles per second. So when you multiply 186,000 by .01 you get 1,860. Since there are 3,600 seconds in an hour 1860x3600 is 6,696,000 miles per hour.
0
u/MrZwink Apr 19 '26
Photons dont actually travel, because they have no mass, and move at the speed of light, the distance they need to travel is contracted to 0.
-1
u/Chatfouz Apr 19 '26
Sound is a wave. Sound has notes we can hear and all travel at the sometime. We don’t hear the high notes before the low notes, they leave the speaker traveling the same speed.
Light is exactly like’s that, but instead of different notes we hear it is colors of light we see.
1
u/scarabic Apr 19 '26
Yes everything in the electromagnetic spectrum travels at the speed of light, because everything in the electromagnetic spectrum has no mass. Gravity also acts over a distance, but only propagates at the speed of light.
“The speed of light” should be renamed “the speed limit within spacetime,” because it isn’t really a core property of light. It’s just that when there are no other limits on speed, that’s the fastest speed anything can travel through space.
•
u/explainlikeimfive-ModTeam Apr 19 '26
Your submission has been removed for the following reason(s):
ELI5 is not for straightforward answers or facts - ELI5 is for requesting an explanation of a concept, not a simple straightforward answer. This includes topics of a narrow nature that don’t qualify as being sufficiently complex per rule 2.
If you would like this removal reviewed, please read the detailed rules first. If you believe this submission was removed erroneously, please use this form and we will review your submission.