Light has Momentum and Kinetic Energy but no Rest Mass. What's up now? | Doc Physics
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- Опубликовано: 8 сен 2024
- Quantum physics keeps getting weirder and weirder. In fact, the explanation comes more from relativity. What's this video doing here? Oh, I guess I'm about to explain the Compton Effect, so I need to know the momentum of a photon.
Times C sqwer.
holllyyyyy shiiitttt I spent so long trying to figure this out, thank you
I read somewhere that this was extremely complicated, but then i watched this amazing video of yours. I knew the formulas but didn't really know how to substitute all of em. thanks a lot for the video. Subscribed
:D
I would say a white sail would work better as the photons would reflect off of it rather than the energy (and therefore mass) being absorbed. Momentum would push it faster because it doesn't have the extra mass. That's just my thought though
+Arciel eleven Or a mirror
White as the surface would be more reflective that way
By black, I mean "absorbing incident radiation" and white means "reflecting incident radiation." So we have to
1) understand why hail is harder to support than same mass rate of water on an umbrella and
2) connect the two situations (umbrella with H2O and sail with light). The analogy is not perfect, as you point out, but it's still useful.
Please consider the conservation of momentum in your analysis. And light has momentum.
You’re already my favorite teacher!!! You are a born natural teacher 👍🏼
Wow better than i expected. Nice explanation
HOLY COW! REDSHIFT?!? I didn't intend to bounce the light so much that the ships gained relativistic velocities!
You've set it up just right. I was hoping that you'd see that the momentum of the system is conserved in each bounce. The photon can be seen as the medium through which the two ships are ACTING ON EACH OTHER. They're just pushing each other apart. Therefore, their center of mass doesn't accelerate.
White because it reflects the light thus doubling the acceleration (if no energy is absorbed).
Yes, as black absorbs
But a black body is an equivalent emitter too.
So why cant it be black?
☯️
I love your understanding, but I will criticize the wording of only your last sentence. In reflection, velocity is changed MORE than in absorption. So, too, with momentum. So the light gives the sail TWICE its original momentum when it reflects. Dang. How can you give more than you have? That's a deep philosophical question.
Great video, Great question⚡️
I keep repeating the first part. Is it planned?
Sorry just curious~ Thanks for the videos! appreciated from far far away from your place^^
I'm reconsidering the answer. The black sail would get hotter, however momentum would be conserved. Assuming both sails are the same size then the sails would work equally. Unless you want to get into thermal expansion of the black sail...
You sure I didn't? I'll try to summarize: relativistic momentum is gamma*m*v. Since gamma is infinity for light, and rest mass is zero, you've got momentum! Do you know what an infinite number of zeroes is? Hint - it's not zero - that would be silly. It's certainly not infinity - that would be RIDICULOUS.
amazing
This really helps me understand the world better
White (well, shiny, I should say) does reflect more. BUT... now tell me whether it's more difficult to hold your umbrella in the rain or the same rate of hail.
I think a white sail, because there will be photons that are absorbed into the colour (we don’t have a perfect white, but bear with me anyway), which will push the sail forward. then when they reflect off the white, it will be like the flashlight example, they’ll push out a bit more energy, Newton’s 2nd law, bobs your uncle and you zoom across the galaxy
Note: all the people who said about the black sail heating up are right too, but this somewhat describes it too
So, sooo much better explained than by my teachers. Insane that I have to look at 7 year old videos to get to someone who knows how to teach
Finally, i understand this, thank you for teaching this 8th grader
Careful! Momentum and kinetic energy are separate concerns, and cannot convert into each other. Also, momentum is transferred to the sail when both reflected and absorbed light hit the sail. But which one transfers MORE momentum? (energy need not be invoked here at all)
you didn't answer the question. what about the mass of photon?
I think he does, he shows in the video that P = mv is not the only equation for momentum.
the way I understand this (and I am total newb to this) is that the speed of light is reality's equivalence of infinity, that means, to accelerate anything with mass to the speed of light you have to have infinite energy, but since photons move at "infinite" speed they can't have mass. Since p=m*v, we have then p=0*infinity, but from math analysis we know that sometimes limit of 0*infinity is a constant greater than 0
btw, does it make any sense?
the black one acts like a black body, so the incident radiation will be absorbed and dissipated throughout the atomic structure causing it to just heat up, this will also radiate heat in all directions thus cancelling the momentum to some degree. The white sail will merely diffuse the radiation via reflection meaning the structure doesn't heat up and so the momentum is purely a result of the incident light.
Reflected light pushes off the sail without heating it. All of the momentum is converted to kinetic energy, whereas with a black sail, the absorbed momentum is converted to heat energy.
to answer the last question it's analogous to a similar momentum problem. Say you want to close a door by throwing a ball. One ball will completely be absorbed into the door (black sail) and another ball will completely bounce off it (white sail). If you need to close the door the fastest, which would it be?
It would be the white sail because only energy is transferred. while the black sail will absorb mass as well as energy. We all know that more mass means more energy needed. Therefore the situation where purely energy is being transferred is the best solution.
(Continued)
... require numbers. As such, without a defined infinity, 1/0 means nothing. You've probably seen all of the little proofs online where people can make 2 = 3. Each proof involves division by 0 at some point. So basically, algebra has a set of rules which allow it to work, and dividing by zero and performing operations on infinity ( + - x / ) are not supported by these rules.
One doesn't need to know all that to avoid mistakes. But, to be absolutely correct, 1/0 does not = infinity.
Another fine question. Draw a careful sketch of the photon hitting multiple sails and think about what is happening to the net momentum of the sails and the photon!!!
The white said reflects incident radiation; therefore incoming light, with it's momentum of plank's constant/wavelength, would impart some of it's momentum into the white sail. I assume all of the light isn't absorbed due to interactions based on wavelength but I don't know for sure. The reflect light loses more energy than was departed onto the sail to change directions and leaves with more energy. The black sail absorbs all incident radiation, receiving all it's momentum.
I believe that what you're thinking of is:
The lim as x goes to infinity of 1/x most certainly is infinity. So 1/x goes to infinity as x gets smaller and smaller, but at x = 0, the lim is undefined.
I can't give you the official definition of infinity, but I do know that it's not a number. i.e infinity is not in the set of all real numbers and all complex numbers.
Infinity is more of a concept than anything, and basic algebraic arithmetic doesn't support it. All of its axioms(...)
White one( assuming it's shiny)
Because if its black the light hits it and gets absorbed. So the P is just half of the shiny. Because if it's a shiny white plate it hits the plate and Ricochet's back applying
2 times the force.
Intuitively I feel that more momentum is transferred when the photon is reflected because the direction of the momentum is altered. Whatever force is required to alter the photons direction is exerted from the sail, and would impart an equal and opposite reaction, pushing the sail a little bit. I cannot explain it. It just seems that changing both the direction and velocity imparts more momentum on the sail then velocity alone.
E=Mc squared, not squor,or however you said it, that was distracting.
If so, which is the better sail?
Hah! I thoroughly enjoyed that video. So educational and yet fun! Thanks for sharing
I think white because if it’s black, it’ll absorb the light which would cause it to have less momentum imparted to it instead of the light reflecting because in reflection, the kinetic energy is conserved while if absorbed you lose some of that to heat. So you ultimately get more from the white one right?
A white sail because the photons of light will be reflected therefore there must be recoil momentum due to the conservation of linear momentum.
battery cell gives energy to light the bulb and something's comes out of that called light so we found a connection between energy and light ,now we know that energy changed and bulb filament defiantly losses something which is photons. we can compare it with nuclear reaction in which one elements diffract into many other particles and loss of weight and mass appeared in energy of heat
so that heat also consider as a diffracted mass colloidal and abraised particulate which are photons in this case
White. The black sail would absorb the incident photons. Whereas they would be reflected of the white one, causing a change of momentum, meaning a force will be exerted on the sail.
1st paragraph, good. In the dark sail after the collision, the motion is random, but with a net p in the direction of the incident photon. So I guess you'd have to have some kinetic energy of the center of mass, too!
I should be more careful. 7/0 is infinity. But what if you multiply 7*(0/0). Surely you wondered what was out there!
I'm glad you liked it! But 1/0? Really? You sure? Would you at least agree that -1/0 = 1/0?
Tell you what. You define infinity, and we'll get this started.
Yes that may be correct, but lim 1/0 doesn't approch infinity. Having a Lorentz factor = infinity seems absurd.
A black sail absorbs the energy and becomes warmer on that side. Therefore the sail moves towards the colder side.
1/0 literally doesn't have a definition. A quick Google search defines infinity as an abstract concept describing something without limit.
I agree, nobody is hurt as long as you're careful not to break algebra. But I think that it's always important to be succinct in maths and the theory behind why 1/0 does not = infinity says a lot about the nature of maths. Limits are an epic area of algebra and are the foundation of calculus and therefore are important in quantum and Newtonian mechanics. :D
The momentum equation is for a single photon of a particular wavelength, correct? How would you go about finding the average number of photons in a beam of light?
Tyrone Slothdrop Ooh. Do you know its intensity? You could get photons per area per second by converting photon energy = hf.
Is the case of black, light will not be reemited, so the ∆p photons will give will be 2 times smaller than the ∆p in case of white, where it will be re-emited
My dude you just saved humanity
I do NOT want to get into thermal expansion of the black sail! But I'm still concerned about your glib declaration of momentum conservation. Are you sure you've thought through its implications?
actually, 1/0 * 0 could be argued to be 1, since the denominators cancel, assuming 0 is a real number.
I wasn't ashamed. Assuming you mean mass flow rate, it would be harder to hold the umbrella against hail because the solid hail wouldn't dissipate energy outward like a water drop would. When asking which sail is the metaphor. I don't even understand the analogy. I thought you were implying the rain being light.
omg the way you rearrange formula's, you do it all before thinking about numbers at all! if my teacher wouldve told me to look at formula's that way i wouldn't be confused with numbers during rearranging formula's and get them right a lot easier! so logical! (i can allmost follow this in one view of the video)
white, because it reflects more light?
It you reflect, the momentum is preserved in the photon reflecting. You want the momentum to be transferred, you can do that by absorbing.
Nope. When light is reflected, it transfers more momentum. Seems counter- intuitive, but think of it like this:
Will I need to supply more momentum(or use more force) to:
1. Catch a tennis ball?
2. Catch a tennis ball and throw it back?
The second represents the reflection of the photon, and the momentum you gain is twice that of the first case.
Let's use classical mechanics to solve this. Momentum is always conserve so v1m1+v2m2=v1final(m1)+v2final(m2)
If photon is absorb, that would mean no recoil which means, assuming v1 and m1 is light, then v1final(m1) is 0. This in turn means all momentum is transfer to v2final(m2), the solar sail.
if change in momentum is -2mv when perfectly reflected, then surely the solar sail would experience a force proportional to 2mv as opposed to the direct transfer from the photon 'hitting' the black solar sail. so my conclusion is you'd get twice the effect from a white solar sail than a black solar sail. Am I missing something??
The sails will be receding from each other at an increasing rate. So, will each bounce, the light would be increasingly redshifted . So, the loss of momentum translates into increasing wavelength.
Aha, but something with infinite density (like the singularity of a black hole) will only have infinite mass if it has finite volume. And a black hole DOESN'T have volume at all...which is how we got to its infinite density in the first place. But really, we should be doing general relativity here.
I like this. I like your definitions. Sadly, though, you won't define 1/0 and you won't define zero using just algebraic operations. Whom does it hurt if you simply set those two concepts equal to one another? We'll be careful when we might accidentally show that 2 = 3.
It is surely 1. But it is also 12. I guess it's pi, too. And two pi. You can't REALLY cancel zero multiplied by infinity.
Can someone tell me what gamma is, not what it's equal to, but what it represents?
Ben Fuller he lost me early on for precisely this reason.
Good vid!!!
I'd have to guess black sail would absorb more energy while the white sail would reflect most of the light hitting it. But I haven't take science in a long time.
I think I could agree to that. Wow!
Neither White or Black? Maybe reflective is the best word? Kinda like a ball bouncing off of a wall.
Black means it will absorb the light more and reemit it in all directions. White means it will do the same but with less absorbing. Reflective means it just reflects stuff without much absorbing.
you're great, thanks alot for your effort
Don't be ashamed! I think it's great that you're still thinking about tricky problems though you don't have to. Question - should rain or the same rate of hail make it harder to hold up your umbrella? Now - is the white sail rain or hail?
What a beautiful colorfullness photons,
Saleh Theory, for the first time, explain the relationship between different colors and different Gyroradius Helical motion of photon. see new difinition of colorfulness of photon video on saleh theory-com chanel
White One .. :D
because the black will absorb the photons . converting them to heat .
so it would not be helpful
They are seriously still spitting out that nonsense today to the kids. But they don't tell them that "undefined" means "a number between zero and infinity." The catch is, you don't a priori know what the number is unless you know HOW the infinite zeroes have been added up. These are advanced math topics. For instance, 1 + 0 - 1 + 0 + 1 + 0 - 1 + 0 +... is not the same as 0 + 0 + 0 + 0 + 0 +... The point is simply that lots of zeroes make 0. But INFINITE zeroes? Not zero. Go get 'em.
A photon decays to electrons and has a rest mass of 2e. But positrons are an issue.
Are you saying light has mass? If light has an undefined mass greater than 0 then then the bending of light by gravity cannot be only explained by Einstein's theory that gravity bends space-time. It gives support to the 'old theory' that gravity is a force between 2 masses.
To expand upon my answer. Only when a photon is emitted from the surface of the sail is momentum transferred to the sail.
Ok. Now the question is how do you account for the loss of momentum that the photon transferred to the sail. It's leaving at the same velocity it arrived. It could hit another sail and transfer more momentum, and keep doing this forever adding more momentum. Are there fewer photons leaving the surface than arrive or does the wavelength get longer?
I'm still not understanding the two types of light you're talking about. Or do you mean the color. I guess the part I don't know enough about is the mechanism of reflected light revealing color vs the energy the incoming light had. So I will go with my original answer of black. Black is the absence of light, ergo; all the light being directly absorbed, nothing bouncing off. If I'm right, would we be able to even measure a difference since visible light is such a small percent of visible light?
This is an awesome video! Although I must point out at 1:07 that 1 divided by 0 is actually not infinity. It doesn't equal anything.
The shiny one, because it takes a greater moment change to reflect something, rather than just absorb it? Newton's third?
Help me with this doc, Suppose a photon of wavenumber x refelects perfectly off our white sail and bounces off with the same wavenumber x in the same direction in came from. Momentum is a vector so delta p is = 2 so the change in momentum of the sail is 2hx?
Black sail absorbs into vibration, rotational and electronic and because of the random orientation of vibration rotation and electronic modes there is no net momentum? Assuming all of the energy is absorbed excluding transnational.
very good video. Wasn't the last equation debroli's law or something?
so accrodin to wht i underestood a photon mass cannot be known r8 .. cause it has a rest mass of zero but gamma has a rest mass of infinity so mass of photon is unkown therefore it can have a momentum r8
i sill dont understand how something without mass can even move.. or transfer energy to anything else.. but in regard to your question, as far as i know a black sail would absorb more light, but if you are ablel to somehow move with the light, then youd only be moving as fast as the connection between you and the light would allow before breaking so it probably makes no difference.
This guy did a poor job of explaining this (and used a poor way to concert the formula). Look up other videos that make more sense.
Well that escalated quickly
white ofcourse
does gravity influence light? or not? or it just follow the path of space being bent by gravity, but wait a minute, space suppose to be empty, well, it's not clear to me but it looks like empty space its not so empty after all. and if light is bent by gravity, then it must have mass, the limit of light must be light itself, like traveling at that high speed and hitting a wall of light, but not sure about that because there should be some friction, but what do you think guys? what other thing would slow down or limit light speed. if light and space is influence by gravity, then mass on (empty space) or light itself it's the wall that sets the light speed limit. just wondering, any ideas?
oosveluzo levso The solution to all your (great) concerns is general relativity, and I have a video on it. Learn on!
if light excerts it's momentum during energy absorption it would be a black sail ofcourse. i'd say light doesn't give momentum when it's being reflected or else it could give the same momentum twice breaking conservation of energy? but what do i know i didnt even go to a university xD (jealous of those who can)
Someone let me know if this is wrong, would the white solar sail be better than black because the photon collides with the sail but is also reflected and not just absorbed so if it is reflected the opposite force of it pushing off the sail gives more force?
I don't understand the part where you go from mc^2 to gamma m0 c^2. Can someone explain what is gamma.
seems like photons do have mass, cuz everything is light e equals mc squared, Matter and light are the same thing
Nope! Definitely didn't think it all the way through. I'll revert to my prior statement. I haven't studied science in a long time. My best subject is nuclear physics/thermodynamics and I'm probably only at a Sophmore/Junior college level. I learned it from my military training.
isnt the m in E=mc^2 just the binding mass to release E amount of energy and have no relation to mass of a photon?
Naa. That equation works on anything. It basically relates energy to mass.
Everything that has mass, therefore, has energy in the form of mass.
It doesn't mean you can turn all of that mass in energy, but it does mean it HAS that energy, just in the form of mass.
I didn't know that E is mc "squrr"...
here you wrote gamma times m knot gives e/c^2. then gamma * m knot * c = momentum.
thus this momentum comes from relativistic mass. so does momentum calculated considering the rest Mass or inertial mass?
I'm a thinking white. If it were black energy would build up until its energy would equal the photons energy and have no effect?
Physics on a Saturday. My work here is done. [Disapparates]
Hey doc! Just a quick question. When you said that gamma is infinity (when applied to light), and since x/0 has no limit, then in theory it could be also -infinity. If so, using -infinity instead of infinity, would it affect the equation E=γmc^2 and the others we got out of it?
One light bounces. One doesn't.
Further, I think you should look at this via momentum, not energy. You're close now.
Then, we could use nuclear fusion reactors to generate such intense light to propel a spacecraft. Would it be efficient?
I prefer the spinning solar sail , black on one side white on the other . What do you mean there is no such thing as a spinning solar sail , its my fantasy !
Nonsense: gamma times zero is zero.
Does the wind affect a black sail differently from a white one? Colour is irrelevant here, in fact the colour will change given the frequency of light - as the human eye only perceives which frequency is being absorbed.
Light is the transitional state of energy, once converted into force-momentum it ceases being light.
Jason Mullings Black and white are not colors. They are indications of reflectivity (emissivity, really).
Doc Schuster For argument sake, White can be defined as all colours and black - the absence of any..
My point being, that the result of light having been converted into kinetic energy would change the original perceived colour of any surface e.g.
upload.wikimedia.org/wikipedia/commons/thumb/2/29/Azobenzene_isomerization.png/300px-Azobenzene_isomerization.png
Is there a way to understand light's momentum from a classical point of view ? (or even QFTesque ?) Or do we just have to accept it ?
cos(arctan(v/c))=1/sqrt(1+v^2/c^2)
Is there a scenario where this is true?
sin(arctan(v/c))=v/(c*sqrt(1+v^2/c^2))
Would it be a black sail because it would lead to a rather inelastic collision with the photons in comparison to a white one where the collision would be more elastic? Black absorbs and white reflects so that's just my hunch.
+Questioning Everyday Oh wait, this would just heat it up rather than push it more... why not a mirror instead of black or white?
+Questioning Everyday Oh wait, this would just heat it up rather than push it more... why not a mirror instead of black or white?
Black. White would transfer half the momentum. I'm reasoning white vs black sails behave similarly to bouncy vs adhesive objects
+Vaysm You should review bouncy vs. adhesive objects!
+Doc Schuster
You end up with the equation p = h/(x) {x=lambda} ...
We know that p=m*v and we know that every wavelength of the visible spectrum gushes with the speed of light i.e 'c' ..
so the equation reduces to m=h / (c*x)..right ?
Now if we know the wavelength of a particular visible beam of light, we can calculate the mass of the photon.. (considering h and c as constant values) The mass of the photon is inversely proportional to its wavelength. This would mean that photons are of variable masses.. Woooh.. Am I right ???
+Doc Schuster Lol, I know how bouncy and adhesive objects work.
Vaysm Do you? Does something that bounces or sticks transfer more momentum to the thing it hits, tough guy?
Doc Schuster No? There's more momentum going in the same direction *total* when something sticks, TOUGH GUY
Lmao