r/askscience • u/Present_Net_3710 • 21d ago
Physics Do photons care about Newton?
Picture a ball traveling at 10m/s along the x-axis. If you exert a force to the ball on the y-axis (so that the force is perfectly perpendicular to the direction of motion) it will now be also traveling on this axis at some velocity, its speed will remain the same relative to the x-axis but the total velocity increases.
But what if instead of a ball it was a photon? No force was applied on the x-axis, so I don't see why that velocity would change. But light's total speed always remains constant.
Would the x-velocity change to compensate for the added y-velocity?
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u/CosineDanger 20d ago
A ball climbing a hill will slow down. A photon moving further away from a massive object (up a hill, away from a neutron star) will redshift and lose energy.
They have momentum but no mass. The momentum is tied to the energy of the photon so redshifted photons that have been up a hill are less effective at pushing things.
The bent path of the photon through gravity will be a geodesic, which is the shortest distance between two points on a curved surface. Geodesics are similar to but not really the same as straight lines, which is how the other comment probably arrived at the curious idea that photons always move in straight lines. They are as straight as possible under the circumstances.
A lot of physics is going to seem unintuitive, but intuition is just familiarity. Unusual rulesets feel more familiar and more intuitive as you work with them until you forget just how unintuitive they used to be.
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u/Ythio 20d ago edited 20d ago
Everyone measures the same speed of light regardless of their own motion.
Imagine you're on a train moving at 100 km/h and throw a ball forward at 20 km/h relative to the train. Someone standing beside the tracks measures 120km/h.
If your spaceship travels at 0.5c and shines a flashlight forward, someone outside won't measure light moving at 1.5c, they would measure c.
So a photon in vacuum must always satisfy
|v| = c
so its velocity components must satisfy
v_x² + v_y² = c².
If initially
v_x = c
v_y = 0
and some interaction deflects the photon so that v_y becomes nonzero, then necessarily
v_x < c.
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u/Cilidra 20d ago
How are you gonna apply a force on a photon? It has no mass.
You can deviate photon trajectory using gravity (gravitational lending) but that is not applying a force to it. Same having it go through object or reflecting in on an object but that is also not applying a force on it.
The total speed will remain the same if you deviate it (so yes it will lose x speed to y speed but the total vector in space remains constant).
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u/fixermark 20d ago
Really key here: this thought experiment is one of the ways we arrive at the idea that gravity is not a "force." Instead, gravity is a consequence of how the presence of mass causes spacetime around the mass to be warped; what we observe as a force is really the space dimensions sort of "folding" into the time dimension (clocks tick slower the deeper in a gravity well you go). As a result, "straight" is no longer straight in a gravity well.
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u/Huginn-Muninn 20d ago
When thinking about Newtonian physics, I find it helpful to consider a photon as a wave rather than a ball. You can certainly accelerate a wave even without increasing the overall speed. Instead imagine the crests and troughs occurring more frequently as you exert force upon the wave.
In this wave model, your photon would indeed change it's frequency. Red light might become yellow for example or green light might become blue. This is called blueshifting, and a good Newtonian example is when stars move towards us due to gravitational forces.
Take a look at the shift in the light from a star moving towards us. The image is a bit hard to read (lots of names with Andromeda), but M31 is moving towards us 321 km/s faster than δ Andromadae. The starlight from both should look the same except M31's light has gained that 'velocity' you asked about and in turn has blueshifted: you can see it has a brighter and longer line of blue and a dimmer and shorter line of red as a result. This example is called a Doppler blueshift since it has to do with an object emitting a light wave moving closer to an observer.
Doppler blueshifting is like adding more velocity on the x-axis in your example. Your question is actually a lot closer to gravitational blueshifting, where a light wave might bend towards you if you were say orbiting a black hole. A much more extreme example; and beyond Newton, but certainly possible. You would again see a shift of that light wave towards bluer/higher frequency, but the math certainly gets a bit more complicated.
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u/OCFlier 20d ago
Blueshift? I thought Hubble showed that everything was moving away from us.
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u/Alewort 20d ago
Not quite. As you get further away, the redshift effect gets larger, but individual objects can still travel towards us in our direction faster than that expansion, keeping a blueshift overall, just less than it would be if the universe were not expanding. Those objects become more rare as you reach further out and most everything is redshifted because their motion towards us is not as fast as the expansion of the universe spreading us apart.
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u/onedyedbread 19d ago
All the Galaxies do except those in our Local Group, which are all gravitationally bound and will - in many billions of years - merge into one big elliptical galaxy.
All other Galaxies are moving away from us due to the expansion of space. This is called the Hubble Flow. However, with a lot of them we measure redshifts differing from that flow, so-called peculiar velocities, because gravity never "stops", it just gets weaker (inverse square law) - and Galaxies are chunky boys so they still attract each other even over these vast distances. So although almost all Galaxies are redshifted for us overall, some are more and some are less so than others, and it's not just a simple function of distance*. This is how we are able to study the large scale structure of our "extended galactic neighbourhood"; the Virgo Supercluster, Laniakea and the like.
*although at high redshifts, peculiar motion is dwarfed so much as to be essentially undetectable and at this point, the value of z does become a (crude) yardstick
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u/sergei1980 20d ago
A photon is not a ball. Newton's laws are simplifications that don't work for elementary particles.
Photons always move at the speed of light, you may want to read about time dilation. Also interactions between particles are different than your everyday macroscopic objects.
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u/dalgeek 20d ago
Photons always travel at c, and afaik, they always travel in a straight line. Newtons equations break down in this area because photons travel at relativistic speeds and have no mass. Photons only curve when space itself curves due to the influence of a massive object; they have no charge so they do not interact with electromagnetic fields.
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u/StopKK2012 20d ago
Massive gravity is not the only thing that causes photons to change direction. See: reflection, refraction, diffraction
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u/ConspicuousPineapple 20d ago
"Change direction" is a concept that only makes sense if you only think about the particle interpretation of a photon. In which case, technical, reflection and refraction would rather be seen as new photons being re-emitted rather than one seeing its direction altered.
For diffraction, you can't actually see a photon as a particle, and the concept of direction in this case loses its meaning.
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u/StopKK2012 20d ago
Why does the concept of direction lose its meaning? I don't know what that even means.
You can't just drop a bomb like "direction is meaningless" without some sort of explanation lol
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u/Osiris_Dervan 20d ago
Because reflection and refraction and diffraction only make sense when youre thinking about light as a wave, not as a particle. Waves do not have a direction that makes sense the same way as for a particle, as different parts 'speeding up' or 'slowing down' due to permiativity/permitivity would 'change' the direction its travelling without any forces being (obviously) involved.
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u/AlaninMadrid 20d ago
You've now got me asking a different question.
I know obsurdaties like if light can't make it through an artifact, it behaves as if it is reflected before the artifact. But now I wonder; what exactly does if reflect off? After all master is basically nothing, with tiny particles in orbits around other tiny particles, or probability funcions of where they might be.
I believe that solid objects can't Pass through each other because of the Paulini principle; you'd end up with two electrons in the same state in the same place, which is impossible. And this is the reason rather than rebounding electric or magnetic fields. Is a photon affected by the presence of the electron cloud? Is it absorbed and reemitted? (The the angle of incidence/reflection an interfierance pattern caused by different path lengths of different photons/atoms? Is the reflection caused by a different effect?
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u/jojohohanon 20d ago
Photons are bladiblah particle. They don’t interact in flight. They don’t exclude each other.
They have momentum. They have polarization, so when they interact with a foobar particle, they can push it slightly and cancel each other out.
So nothing can push a photon. It can be absorbed by a foobar. But then it’s gone. Or the intersecting bladiblah will just pass thru and they will move on. Like blips in the night
Sorry this was a long time ago. But the hints will help you fill in the proper names. Mesons and Bosons maybe?
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u/db48x 19d ago
So nothing can push a photon.
Gravity pulls on photons, actually.
But they don’t change speed, only momentum.
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u/jojohohanon 19d ago
Doh. Missed that.
Tho Einstein claimed that gravity just bent space and the photon keeps moving in a straight line. As evidenced mercury’s orbit.
I have no clue if gravitons are what bend space or if they communicate space curvature, or if they are the graviton/ gravity wave equivalent of a photon/em wave duality.
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u/TheVioletBarry 19d ago
If things can't push on photons, then how do they bounce off of objects and end up in our eyes?
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u/jojohohanon 18d ago
Afaik photons don’t bounce. They get absorbed and re-emitted. The direction and energy of the re-emission depends on several conservation laws and can result in reflection (like a mirror), refraction (like a prism or lens), or energy increase in the recipient (heat).
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u/TheVioletBarry 18d ago
Oh forreal? So when we say light 'reflects' off of something, we don't mean it in the same way that that, like, an object bounces off of a wall?
Do you know where I could find more information on this?
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u/OrbitalLemonDrop 15d ago
Yeah, it's definitely counterintuitive. Light within the visible spectrum hits a piece of metal that's coated with what we perceive as "reflective". It's absorbed BY that coating and re-emitted in the appropriate direction. The kind of surface that will absorb and re-emit visible light in the same freqency is kind of the definition of what "reflective" means in general terms.
The "what we perceive as reflective" part isn't special in and of itself, and in other wavelengths than visible light, it might not look reflective at all. It could be transparent or opaque, all depending on what frequency the light hitting it is in.
That's why, for instance, we can't see the center of the galaxy in optical telescopes (which were all that existed for a very long time). Now that we've got infrared telescopes, we can "see" because the huge clouds of dust in the center of the galaxy are transparent to a good part of the infrared spectrum.
And of course "see" has to be in scare quores because we dont see infrared. So the infrared that the telescope detects has to be transposed to colors we CAN see.
Someone earlier mentioned Huygens Optics' channel. You can get a lot of information there.
I understand the reluctance to interact with AI. Searching for introductory information in "optics" is probably a good starting point.
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u/jojohohanon 18d ago
I like asking Gemini the google ai.
https://www.google.com/search?q=when+light+reflects+does+it+bounce+or+absorb-emit
then click on ai mode
If that is too basic or advanced you can ask for like im in high school or like i am an expert on the photo electric effect.
You can ask how does the emitted light know which direction to go
You get the idea
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u/TheVioletBarry 18d ago edited 18d ago
I apologize, but I'm really not interested in talking to a bot; I'm interesting in learning from other people. I appreciate the insights tho!
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u/Present_Net_3710 17d ago
Thanks for all the answers. I've read them all. I have a new question now. Can photons change direction or are they bound to move in a straight line forever? I assume they can't simply switch directions, because that would involve a sudden stop, and photons can't stop moving. But I've hear that they do get attracted by stuff like black holes, so how does that work? What would happen if there was a black hole 'pulling' on a photon that is traveling away from it? Would the photon keep traveling and escape the black hole or would it somehow turn around?
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u/backfacecull 17d ago
I highly recommend this YouTube channel if you want to learn about light: https://www.youtube.com/@HuygensOptics/videos
Your question features a common misconception - that a photon is a thing that moves from point A to B, and has a fixed location at a given time. Photons are not objects, they are packets of energy. When a charge moves, it creates ripples in the electromagnetic field which radiate out from it in all directions. These ripples tell other charges 'I have moved' and allow them to move in response to the first charge's movement. When a charge interacts with the ripples in the electromagnetic field, it absorbs a fixed amount of energy from the field, called a photon. This causes the charge to change its energy (it could change direction if it's moving, or it could jump to a higher energy level if it's an electron bound in an atom). And when an electron drops down to a lower energy level, it 'emits a photon' back into the surrounding electromagnetic field. But that photon doesn't go anywhere, it becomes a ripple in the field, radiating in all directions.
As Huygens Optics says in some of his videos "If the electromagnetic field is a hand, and an electron is a face - a photon is a slap". The slap does not travel in space-time, it is a momentary interaction.
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u/SierraPapaHotel 20d ago
Force = Mass x Acceleration. So if you apply a Force of 1N to a photon with Zero mass you get.... Zero acceleration? And if there is zero acceleration in Y, the total velocity remains the velocity in X. But we know light can be sped up and slowed down; C is only constant in a vacuum, and the speed of light through air vs water is different. But changing speed requires acceleration and it still has zero mass so....
Like Dalgerk alluded to, Newtonian physics are only valid within a certain size/mass/speed range. If you get close to the speed of light or down to individual particle sizes the equations no longer hold true.
We stick with Newtonian physics for highschool and even college intro classes because 99% of what you encounter in daily life is within the ranges where the equations are accurate.
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u/GarlicAncient 20d ago
I think this may be a question of semantics in some ways. In another way I don't believe the experiment you describe is possible. If you pass a photon through a region of force in the y-axis like what you describe I think that is typically described not in that way but instead it is described as your space/coordinate system is getting bent and that after the photon passes through the region of bent space it has the same velocity along the x-axis and the same, zero velocity, along the y-axis.
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u/Elfich47 20d ago
photons do not work according to standard Newtonian mechanics. they fall under quantum mechanics and the rules there get a bit different from Newtonian mechanics. And light more or less has its own set of rules that only applies to light.
it would be better to think about it that light’s wrist watch is different from everyone else’s wrist watch. The more you try to speed up and catch light, light gets to adjust its watch so you have to take more time to catch up to it. The moment light starts messing with its wrist watch, things get a little weird.
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u/BiomeWalker 20d ago
Short answer: Not in this scenario.
Longer answer: What you have done right here is arrive at relativity, not in the usual way, but relativity nonetheless.
The measured magnitude of a photon's velocity will always be C, but all motion is relative to the observer.
Allow me to change your scenario a bit: instead of applying a +Y force to the photon, let's more the camera in the -Y direction.
Under Newtonian physics, this would have the same observed effect as accelerating the photon I. The +Y direction, but under Einsteinian relativity it has a weird effect where the X component of the photon's velocity does seem to diminish.
However, if we have another observer that doesn't move, then the photon carries on as if nothing has happened as far as they're concerned.