How Can Light Have Momentum?

A discussion of why light has momentum, how long we've known and what it means for nature and technology.
Link to previous videos mentioned in this video:
• The 2018 Physics Nobel...

[@buckrogers5331]

As for the comet tails, do not confuse light energy with that of charged particles. The tails of comets are pointing away due to solar wind aka charged particles, not light.

[@michaeldamolsen]

Comets have two tails. One is ionized and is produced by the solar wind of charged particles (mostly protons), the other is electrically neutral and is shaped by the photon radiation pressure. According to my google-foo (which may well be flawed), the solar wind pressure at 1 AU distance is around 1-6 nN/m^2 (nano Newton per square meter), whereas the solar radiation pressure is around 9 uN/m^2 (micro Newton per sq. meter). So it seems that the radiation pressure is around 1000 times larger than the solar wind. But that is not the full story. Because the radiation pressure only affects the material it collides with, whereas the solar wind of charged particles can affect dust particles without having to collide. So the effective cross section of the dust is higher for the solar wind when it comes to ionized material. Additionally ionized material is subject to both the solar wind and the radiation pressure, whereas the neutral dust and gas is only affected by the radiation pressure. Adding to this is the fact that the two tails are made from different material. The neutral tail typically contains a lot of ice particles which is highly reflective and thus more visible than the ionized tail. So it is not straight forward to compare the amount of material in each tail, or the effect of the two pressures, just by visual inspection. The result is that the ionized material always points pretty straight away from the Sun, whereas the neutral material retains more of its initial momentum, and thus points a bit towards the direction the comet is moving as the material is released. Apparently, sometimes there is also a third sodium tail, but it is not easily visible and I did not investigate that further. I guess the short answer is "It's complicated." Thanks for inspiring me to look up more details on this topic!

[@core-experience]

You are underrated dude! Simple explanation and visually entertaining as well, great job!

[@atomsandsporks6760]

Thanks! Much appreciated

[@clydekade414]

instaBlaster

[@frankdimeglio8216]

@@atomsandsporks6760 UNDERSTANDING EXACTLY WHAT IS A TWO DIMENSIONAL SURFACE OR SPACE AS invisible AND VISIBLE IN BALANCE, AS E=MC2 IS clearly F=ma (ON BALANCE): The stars AND PLANETS are POINTS in the night sky. Now, very carefully consider what is a galaxy. Get a good look at what is THE EYE, AND notice the black space. E=MC2 AND F=ma IN BALANCE do ALSO FUNDAMENTALLY represent (ON AVERAGE) what is a TWO DIMENSIONAL surface OR SPACE (as what is a BALANCED MIDDLE DISTANCE in/of SPACE), AS ELECTROMAGNETISM/energy is gravity. Gravity IS ELECTROMAGNETISM/energy. Gravity AND ELECTROMAGNETISM/energy are linked AND BALANCED opposites, AS E=MC2 IS F=ma. The stars AND PLANETS are POINTS in the night sky. Now, consider what is the speed of light (c) ON BALANCE. I have explained why THE PLANETS (including WHAT IS THE EARTH) move away very, very, very slightly in relation to what is THE SUN on balance, as ELECTROMAGNETISM/energy is gravity. Gravity/acceleration involves BALANCED inertia/INERTIAL RESISTANCE, AS E=MC2 IS CLEARLY and NECESSARILY F=ma ON BALANCE; AS ELECTROMAGNETISM/energy is gravity !!! Accordingly, the rotation of WHAT IS THE MOON matches it's revolution. It is CLEARLY proven ON BALANCE. E=MC2 IS CLEARLY F=ma ON BALANCE. This NECESSARILY represents, INVOLVES, AND DESCRIBES what is possible/potential AND actual IN BALANCE, AS ELECTROMAGNETISM/energy is gravity !!! TIME dilation ULTIMATELY proves (ON BALANCE) that ELECTROMAGNETISM/energy is gravity, AS E=MC2 IS F=ma ON BALANCE !!! Indeed, TIME is NECESSARILY possible/potential AND actual IN BALANCE; AS E=MC2 IS F=ma IN BALANCE; AS ELECTROMAGNETISM/energy is gravity. Gravity IS ELECTROMAGNETISM/energy. "Mass"/ENERGY IS GRAVITY. ELECTROMAGNETISM/energy is gravity. (BALANCED inertia/INERTIAL RESISTANCE is fundamental.) Objects AND MEN fall at the SAME RATE (neglecting air resistance, of course), AS E=MC2 IS F=ma ON BALANCE; AS ELECTROMAGNETISM/energy is gravity. This NECESSARILY represents, INVOLVES, AND DESCRIBES what is possible/potential AND actual IN BALANCE. Gravity is ELECTROMAGNETISM/energy ON BALANCE. The sky is BLUE, AND what is the Earth is ALSO BLUE. Carefully consider what is a galaxy !!! Energy has/involves GRAVITY, AND ENERGY has/involves inertia/INERTIAL RESISTANCE. “Mass"/ENERGY involves BALANCED inertia/INERTIAL RESISTANCE consistent with/as what is BALANCED electromagnetic/gravitational force/ENERGY, as ELECTROMAGNETISM/energy is gravity (ON BALANCE); as E=MC2 is CLEARLY and NECESSARILY proven to be F=ma ON BALANCE. Inertia/INERTIAL RESISTANCE is proportional to (or BALANCED with/as) GRAVITATIONAL force/ENERGY, as this balances gravity AND inertia; as ELECTROMAGNETISM/energy is gravity; as E=MC2 is CLEARLY and necessarily proven to be F=ma ON BALANCE !!! So, “dark” matter/ENERGY is plainly unnecessary; and it is idiotic. I have mathematically unified physics, as I have explained the term c4 from Einstein's field equations along WITH the fourth dimension, as ELECTROMAGNETISM/energy is CLEARLY AND necessarily proven to be gravity ON BALANCE. E=MC2 is CLEARLY proven to be F=ma IN BALANCE. By Frank DiMeglio

[@ericephemetherson3964]

@@atomsandsporks6760 Where does light get its momentum? In other words: what causes the speed of light? Where does light get its speed?

[@kooky45]

Excellent. Came here because of something missed from a PBS Spacetime video and I'm happy I did cos you fully explained it.

[@masamasri5620]

Absolutely in love with your videos! Incredibly entertaining approach!

[@atomsandsporks6760]

Thanks!

[@farvision]

Thanks for clarifying some things I've been wondering about!!!!

[@alexsandra1009]

Hope you make more of these videos!

[@Vademan]

Andy Weir's book Project Hail Mary brought me here, and I'm glad it did!

[@bobatenin]

Super channel! Not only interesting but also funny!

[@dojinho]

Very lively and fine presentation ! Well done, sir !

[@Maritzaobmpryor]

I love your video, thanks!!❤

[@sarielreigns777]

You deserve Millions of Subscribers.

[@atomsandsporks6760]

Thanks!

[@9nelly5]

Great video. It helped me a lot, thank you!

[@atomsandsporks6760]

Glad it helped!

[@thealtruistmc5020]

Amazing video. A+ content.

[@atomsandsporks6760]

Thanks!

[@gustavomartins6175]

Put the links on the descriptions, A&S! Great video.

[@atomsandsporks6760]

Done. Thanks for the tip.

[@Nahulanham]

Do you have any video's on laser propelled momentum rockets?

[@monu5457]

U r great

[@nonos439]

Thanks man, I got lost. but your video help me a lot and make me realize more buy funny way!

[@joaobaptista4006]

Thankyou for the super video. To visualize the transfer of light momentum see the RADIOMETER -- mechanical gadjet which existed in all secondary schools physics lab.

[@astronics]

Light can push, just point a torch in someone's eye and they will move back!

[@WHYNKO]

Light can even stop something from movie, just point a car headlight at an animal at night... It will stop them☺️

[@sssssnake222]

Or move forward and punch you in the eye

[@psikoexe]

Man believe me, you are one of the best explainers, keep it up.

[@solapowsj25]

Momentum is transferred at the source (atom shell) and continues as free space field modifications along the linear path of light at 'c' with sine transverse waves.

[@phenomenalphysics3548]

12:19 Will you please make a video for those calculations or link me to an already existing video?

[@atomsandsporks6760]

You'll find it covered in any introductory source on special relativity, for example here: hyperphysics.phy-astr.gsu.edu/hbase/Relativ/releng.html it's just a Taylor series of expansion of the relevant relativity equations.

[@The_Green_Man_OAP]

0:08 You can see the laser pulse, so it's reflecting off particles because the experiment is _NOT_ in a vacuum. The laser is _heating the surface_ and _that surface pushes_ these particles away, like a _Crookes Radiometer._ NIII, Action = -Reaction: {←}= - {→}.

[@MrGreen_N]

You are amazing. This is the clearest explanation I've seen so far on youtube. You've just earned a subscriber!

[@petevenuti7355]

What other forms of energy are there becides kinetic?

[@rogerjohnson2562]

Thankyou! So light has momentum, and a vector; is its momentum vector conserved? Does the vector (direction) of lights momentum change when light is reflected? Is some part of 'universal expansion' due to lights momentum?

[@schmetterling4477]

Four-momentum, i.e. a vector made of three-momentum and energy transforms properly under Lorentz-transformations and it is conserved for systems with spatial and temporal translation symmetry.

[@katkatfarkat]

this kinda answers a question i asked in a future video in near past :D Also, i want to point out how light in all its weirdness is actually ideal in mathematical sense: 12:51 "A massless object" I mean this sounds more like math than reality, like an ideal point with no dimensions, unless you would argue tha we have object in reality with no dimension but mass :D

[@reservoirchannel5576]

so, what is mass anyway? if all is energy and waves of energy, what is mass?

[@cosmicinfinity8628]

Make it small compact and crispy. Would be better. This is good though 👍

[@adityapawar8484]

If light does not have rest mass, but has relativistic mass, does that mean with such a high speeds relativistic mass adds up to momentum on a very tiny scale? And can momentum add to mass on a very tiny scale above zero?

[@Ahmed-vs1ui]

Aditya Pawar light doesnt have any kind of mass

[@alexanderm5728]

Relativistic mass is an antiquated concept that even Einstein rejected; it detracts from the otherwise purely-geometric theory of gravity and spacetime. Especially when you're dealing with objects that have 0 rest mass, blindly applying Newtonian intuition to relativistic mass can get you into serious physical trouble. You're much better off just working with rest mass.

[@atomsandsporks6760]

Light doesn't have relativistic mass. Also, physicists generally don't like the idea of relativistic mass the days. It's kinda been phased out as it incorrectly create the notion that the fundamental internal structure of a thing is frame dependent, which it is not. At the end of the day you have some equation with a relativistic correction, initially people conceptually lumped the correction in with the mass, nowadays we conceptually lump it in with the geometry itself. Same equation, different interpretation.

[@philoso377]

What if there is something else that we, including Kepler, are ignorant of, responsible for the orientation of the comet tail ?

[@OFFROADENGR]

I'm sensing that it doesn't actually have momentum, other than it having a force that it is extracted as such. The "push" is really...moving energy. Like a surfer on a wave. Am I wrong?

[@atomsandsporks6760]

Well actually one can take the very *definition* of force as the amount momentum changes in a given time (in "calculus speak" we say the force is defined as the "time derivative of momentum). In a similar vein, the *definition* of energy is in terms of what is called "work" which is the sum of the effect of a force applied over some distance .So force, energy and momentum are actually a directly connected triumverate that are just separated from each other by calculus (specifically energy is the "integral of force with distance" and momentum, as I said, is the "derivative of force with time"). So in reality they all come together, if you say it exerts a force then it has energy and momentum. If you say it has momentum then it has energy and can exert forces and if you say it has energy then it must have momentum and can exert a force. Ocean waves, btw, do indeed have both force and momentum, both in the transverse direction (the direction it's moving up and down and up and down) and in the propagation direction which as you say is what makes surfing possible.

[@Ahmed-vs1ui]

Atoms and Sporks can the momentum of light be calculated by any other equation than E/c=p???

[@michaeldamolsen]

@@Ahmed-vs1ui "Atoms and Sporks can the momentum of light be calculated by any other equation than E/c=p???" - I know this is an old question, but in case someone else is wondering and happen to read this: The energy of a photon is h*f where h is Planck's constant and f is the frequency of light. So the equation could also be written as p = h*f/c. Frequency divided by speed is the wavelength, so that can also be written as p = h/l, where l is the wavelength (usually the Greek letter lambda is used for wavelength, but I could not figure out how to type that here).

[@Ahmed-vs1ui]

@@michaeldamolsen i really appreciate your reply. i did come across this explanation after i posted that comment but thanks anyway

[@StoneShards]

e = p, expands to e = mv, by substitution, so your implication that energy has momentum but no mass is in error.

[@michaelhibbs3683]

I don't like to quibble, but the equation at 1:10 actually gives pressure (=force per unit area), not Pushing Force. Actually, I do like to quibble.

[@hinglish7813]

What a complicated explanation it is..... Simplest explanation is : first what is definition of moment? Ans: Momentum is the ability of anything which can push anything. So light light has ability to push body. Similarly any body with mass has ability to push another body. Pushing ability in mass comes from collision. And this ability in light comes from electric and magnetic field of light.

[@thealtruistmc5020]

I have a question since it is told that Energy = momentum but since electromagnetic waves move at different frequencies and some frequencies have more energy than others so does that mean that some em waves have more momentum?? And since momentum is directly propotional to velocity then that would mean that light travels at different speeds according to its frequency?? Which would then mean that speed of light is not constant someone plz help!!

[@atomsandsporks6760]

Yes, different frequencies have different momentums, a blue photon has more push than a red. But momentum is not related to velocity for light. Remember, in the video we discuss how p=mv isn't the "true" formula and that goes both ways, we talked about how m=0 doesn't actually mean you have no momentum but it's also the case that momentum is not always related to velocity, this is the case not just with light but with any wavey phenomenon. Rather momentum is related to wavelength. So energy, momentum, frequency and wavelength of light waves are all directly connect to one another but all light waves have a speed of c (in a vacuum)

[@thealtruistmc5020]

@@atomsandsporks6760 Thanks very much for replying. 😀😀

[@ericephemetherson3964]

Where does light get its momentum? In other words: what causes the speed of light? Where does light get its speed?

[@LongHairedFreakyDude]

What does it mean for a momentum without mass to still have kg in its si units? Where does this mystery kg come from?

[@Ni999]

I had a complete reply for you and KZbin deleted it - it's their newest thing, arbitrarily destroy equations. I'll try again and you'll have to look stuff up and blame Google. For light - E = pc = (h/λ)c = hf The h is Planck's constant and its units are - m² kg / s m² kg s / s² J s J / Hz The rest is bookkeeping. Mass is equivalent to energy but not exactly the same thing (it's complicated) and Joules are the same regardless.

[@Ni999]

f is frequency in Hz and is speed divided by wavelength - f = c / λ

[@Ni999]

So for light - ρ = h / λ

[@Ni999]

Don't be fooled if you see - E = hν That ν is the Greek letter nu, and still means frequency. It looks like the v for velocity in plain print so we use f instead.

[@Ni999]

The p is really a ρ but no one has a cow if we use p instead. If you noticed, I used both.

[@reluginbuhl]

Isn't E in your presentation actually the total energy and the term "m" is the rest energy?

[@donaldkasper8346]

I doubt outward pressure causes asteroid spin. After all, all planets and their moons also spin. These are electrical effects induced by a field caused by the sun.

[@walnuttv1999]

featured in top 10 videos of the day on walnut.tv

[@neobaud513]

From where does light get its momentum?

[@schmetterling4477]

Special relativity. Almost everything in physics derives from special relativity and local symmetries.

[@typo691]

Really liked this video but you should consider changing the font, it's hard to read.

[@atomsandsporks6760]

Ya, I made the plunge and bought a better font for the newest video. Hopefully it's more readable.

[@typo691]

@@atomsandsporks6760 nice, I'll check it out when I have time.

[@GP-qb9hi]

It's a one line question, what's the one line answer? How Can Light Have Momentum??

[@rafarzempaa5618]

Because light can have orbital angular momentum

[@LiborTinka]

In your other video you said electromagnetic fields don't affect each other: kzbin.info/www/bejne/q4vRiZuInN-ne8k ...and now you say the EM way is able to push atom - now I am confused. I understood it's charges that interact, but EM wave i.e. photon doesn't have a charge.

[@WindsorMason]

YORP!

[@NicolasSchmidMusic]

One equation to explain them all: E^2 = (pc)^2 +(mc^2)^2

[@value8035]

Good Explanation. But Proportional != Equal Notation.

[@atomsandsporks6760]

You have to be a little careful. Energy has units of, well, energy and momentum has units of mass x velocity. They are quantities with different "dimension" and can never be equated just like you can't say 1 second = 3 feet. It doesn't make sense, seconds are units of time and feets are units of length. However, the two are *proportional* with the proportionality constant "c", the speed of light so E =pc. In order to declutter conceptual thinking, what I actually did was chose units so that the numerical value of c was 1 (these are called "natural units"). So the real equality is E in Joules = (1 in velocity) x (p in mass x velocity). So they are proportional and one could never actually equate quantities with different dimensions (i.e. "types of units") but to make things easy to think about one can choose units where the numerical value of that proportionality is 1. Hope that makes sense. Interestingly it's actually fairly common lingo in fields of physics like particle physics to speak, in the lingo, *as if* E=p was actually an equality. This leads to the really weird consequence that times, lengths, rates, areas, etc. all have units of energy. Again it's just a "lingo" , it's not strictly speaking a valid thing to do, but you'll hear bizarre statements like "the duration of the event was 3 per electronvolts"

[@value8035]

@@atomsandsporks6760 Cool.. True that when you don't want to wiggle around many constants in your conceptualization, bundle them and say its 1 (here, its just c is 1). Once the concepts are done, you can always rework the constants.

[@astronics]

Everyone says that he is underated but no one shares...

[@erwinengelsma5498]

Still does not go into the mechanism on why energy has momentum.

[@schmetterling4477]

Relativity.

[@davidhalliday5705]

A typical regurgitation of an all too common misunderstanding of how the E in E = mc^2 is related to the energy portion of the Energy-Momentum 4-vector. What is false is to equate these two “energies”. (The falsehood that E = mc^2 is about something called “rest” mass or “rest” energy, or anything about some special “rest” inertial frame of reference, is an addendum to the first falsehood, above.) Instead, the E in E = mc^2 is the magnitude of the Energy-Momentum 4-vector, while the so-called “energy” component of the Energy-Momentum 4-vector is a very different quantity: let’s call that E_0, for the “zeroth” component of the Energy-Momentum 4-vector. The relationship is E^2 = (E_0)^2 - |pc|^2, where p is the momentum 3-vector; the three “momentum” components of the Energy-Momentum 4-vector. Using E = mc^2, we then see that E^2 = (mc^2)^2 = (E_0)^2 - |pc|^2. Rearranging, this, then, to obtain E_0, yields what the video claimed was the correct equation. Yes. In the special case where momentum is zero, one obtains E_0 = E, as a special “rest” case (because the momentum, p, is zero). So much “hubbub” caused by such a simple misunderstanding!

[@venkata8462]

hey bro are you sure speed of light is 300 million meters per second

[@reservoirchannel5576]

300.000 km/s

[@chriszablocki2460]

It needs momentum? For what? The horse race? 🐎

[@j.maxwell8346]

You live in Austria?? You speak English well.

[@ttschott]

My.... brain

[@erbenton07]

ok, there is a lot wrong in this video. Light is not what causes he cometary tails to point away from the sun, its the solar wind. The solar wind is a vast powerful wind of charged particles. Light itself plays almost no role in cometary tail direction. Light does not cause asteroids to tumble, conservation of momentum does Light causes radiation pressure on solar sails, that's what makes them move. I'm sure the author means well, other than his toilet obsession thing, but he should have researched his topic more. I would suggest PBS Spacetime or Sabine Hossenfelder as more reliable sources of this kind of topic.

[@daemonsoadfan]

your conclusion is that light has momentum but you didn't address the fact that if E = PC then E= MVC then E = 0 since M = 0 for light, so i still don't get it xD

[@charlesstewart4436]

E=mvc ? But v here is c , so E=mc^2. the energy = a tiny mass at absorption.

[@daemonsoadfan]

@@charlesstewart4436 what i was saying is that, when considering the light, then the equation E² = (pc)² + (mc)² becomes E = PC because there is no mass. But my problem is, since P is defined as m*v, which in this case is m*c, then E = PC can be changed to E = m*c*c which is still E = mc² so still 0 if m is 0 xD

[@charlesstewart4436]

In E^2 = (pc)^2 + (mc)^2 the m is rest mass. This is zero for a photon. However the kinetic energy of a photon can be thought of as the energy of a moving mass m' = E/c^2. This is neither zero nor a rest mass. The loophole is that you can regard a photon as 'being' a tiny non-zero non-rest mass! Photons, as you say, definitely have zero rest mass. One can't boost a rest mass to light speed but m' isn't a rest mass. Your not alone in finding Einstein's conception of a photon mind-boggling.@@daemonsoadfan

[@charlesstewart4436]

@@daemonsoadfan I replied days ago yet I see that reply somehow didn't appear. I apologise. Rest mass is a type of matter. Matter at v=0 for observers in a particular reference frame. Inertial mass is a more general idea (its a measure of how much acceleration is produced in a body acted on by a force - the lower the acceleration the greater the absorbing inertial mass or the smaller any impacting mass causing the force). Light can't be accelerated because nothing travels faster than light but it possesses inertial mass because it can be absorbed by matter which accelerates as if it absorbed a particle with a tiny inertial mass, m'=E/(c^2) but NOT a rest mass; NOT matter. Light contains no rest mass which means it doesn't contain a matter particle. Matter is light confined . Radiant energy at c is unconfined light and it has inertial mass. If light had no inertia it would travel infinitely fast. When light bounces off matter it is the repulsion of the magnetic field of the light wave interacting with the induced magnetic field on an electron set in motion by the electric part of the wave.

[@daemonsoadfan]

@@charlesstewart4436 I still see your other comment personnally, and thanks for the answers, but i'm still puzzled, for example i read several times in the past that rest mass = inertial mass in value, i remember they did experiences where they measures that at an incredible precision. So saying the light would have inertial mass but not rest mass is not coherent with that. (Equivalence Principle, on wikipedia)

[@user-pf7dx7cu9d]

The most sad part is that useful informative channel doesn't get as many subscribers as other useless worst channels

[@gigioconio]

What if the light have a mass like 1x10^-60kg..... And why light, its not responsable for the universe expansión .?..??.?.?. I mean beyond the gravity pull of a galaxy, over the Edge of the galaxy .....the light pressure should win......

[@core-experience]

If the light has mass, it would be no more than a particle with extreme speed. But light dont have mass. Think of this as the simplest way of delivering energy besides a physical object like a car hitting you. Can there be a much simpler way? what if something doesn't have mass but can still carry energy? That would be light, the purest form of energy. And without mass, it should not accelerate or decelerate, which means it remains constant relative to anything, traveling at speed of light. As for the last question you asked it would be better said as the total amount of energy of the universe against the gravitational energy(and there are many other factors). The actual working behind this is still not clear tho.

[@RobbieHatley]

Dislike. Clickbait. Third video I've seen in a row that *_fails_* to explain how and why photons have momentum. *Stop dumbing things down.*

[@schmetterling4477]

Photons are irreversible energy exchanges. You have to learn the basics of special relativity and then you will see why energy and momentum always come together as a four-vector. At the end of the day it drops out of the fact that the universe is both geometric and relative. That is all we need to derive these relations from first principles.

[@jensphiliphohmann1876]

I'd refer to a body's rest energy as E₀ rather than just E. The other thing a moving body has is Eₖ which, by the way, also "weighs something". In this sense, a photon is not really _a thing_ which _might_ move but, since it only consists of it's own kinetic energy, it kind of _is it's own_ motion / pure motion. Of course, this holds for any massless particle.