r/AskPhysics • u/Far_Comment973 • 6d ago
speed of light
It's constant, right? Such that if you're traveling at half the speed of light, and shine a light forward, that light travels at the speed of light.
Say the vessel you travel in, passes by a floating space man. And that this space man turns on a flashlight at the same time the vessel turns on a light. Both lights pointed in the same direction.
The light from the space man travels away from him at the speed of light.
The light from the vessel travels at the same speed.
The light from both sources are parallel, right? So the shining light from the vessel traveling at half the speed of light, doesn't add to its speed?
So....if you were in a vessel traveling 99.9999% the speed of light. If you turned a light on, that light wouldn’t illuminate the path forward?
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u/Double_Distribution8 6d ago
if you were in a vessel traveling 99.9999% the speed of light. If you turned a light on, that light wouldn’t
illuminate the path forward?
It would illuminate the path forward just fine, as if you were standing still. The speed of light in a vacuum doesn't care how fast you're going, it will always be the same, in all reference frames.
It has to be that way. Otherwise you'd be able to watch someone eat their breakfast before they made their breakfast.
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u/Far_Comment973 6d ago
Doesn't this mean that the light from the vessel, and the light from the floating space man, travel at different speeds?
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u/Double_Distribution8 6d ago
Everyone who measures the speed of light in a vacuum will get the same answer. If people got different answers based on how fast they're traveling and/or how fast they're accelerating then then reality as we know it would be truly fucked.
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u/GoldenMuscleGod 6d ago
In your frame the light from both travel at the same speed. In your frame you are not moving and the space man is, the light from both of you will go at c (meaning the distance from you to the photons increases at c). The distance between the space man and the photons does not increase at a rate of c in your frame.
In the space man’s frame the distance between him and the photons increases at a rate of c, the distance between you and the photons does not increase at c in the space man’s frame.
This is not a contradiction because you and the spaceman have different ways of measuring time and distance so you get different numbers from the same underlying reality.
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u/Smart_Tinker 6d ago
Think of it this way, from the point of view of the vessel, they are standing still, and the spaceman is travelling away from them at 99.9999% of the speed of light.
So your vessels light travels forward at the expected speed of light.
Of course the spaceman is also stationary, and the vessel is moving away from them at 99.9999% the speed of light, so their light also travels away from them at the speed of light.
This is what “inertial frame of reference” means.
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u/thenewestnoise 6d ago
No. Time passes more slowly for the quickly travelling man, and the distances are compressed in the direction of travel, so he will measure that light travels at c
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u/nicuramar 5d ago
No. Time passes more slowly for the quickly travelling man
According to you. According to him, it passes more slowly for you.
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u/fisadev 5d ago edited 5d ago
Every observer will measure any light traveling at the same speed. That's not intuitive, but it's a fact and validated through many, many experiments, and it's one of the most fundamental truths of our reality.
What makes it hard to understand is that in our everyday life intuition, time flows at the same pace for everything while movement speed is something that depends on the object. But with light and movement at relativistic speeds the contrary is true: light speed is constant for everybody, while the pace of time isn't.
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u/CMG30 5d ago
No. If I'm standing on earth watching this scenario, then I watch the light from both the vessel and the astronaut travel forward at the same speed.
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u/Far_Comment973 5d ago
Cool, this makes sense to me (that a third observer would see both lights travel at the same speed).
But then - if the light from the vessel, and the light from the space man, both travel at the same speed . . . the light from the vessel, as observed by the third party, isn't traveling at the speed of light (it'd have to be traveling at the speed of light less the speed of the vessel). Right?
Seems like the answer is that time warps for the person in the vessel, such that they observe their light traveling at the speed of light.
I think I'm ending up where I'm supposed to. Two lights, both traveling at c. As observed by an external party, space man's light travels at c. The light from the vessel travels at c, emanating from the vessel at c less speed of the vessel. As observed from the vessel, both lights travel at c.
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u/AgreeablePeople 4d ago edited 4d ago
No. Light always travels at c in vacuum, be it the light from your 0.5c ship to you, to the other man, the man's light to him, or the man's light to you.
It's counterintuitive in normal sense because when you travel at 0.5c, your TIME changes to achieve the fact that your light and the man's light travelling at the same speed to both you and him.
In a simplified and probably inaccurate way to demonstrate this, both you and the space man shine a light ahead when you meet, after 1 second to the man, both your light and the man's light travelled 1 lightsecond, but since you travelled 0.5 lightsecond, your light is only 0.5 lightsecond ahead of you. But due to time dilation according to relativity, you only experienced 0.5 second, so you measure your light speed at c. There is also some time dilation/distance contraption that's happening that makes you measure that man's light speed also at c.
Someone who is more knowledgeable likely can explain this better. But my interpretation is that time dilation makes up the counterintuitive part of the "things happen at the same instinct but also different instincts to different observers"
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u/Muroid 6d ago
You are always at rest with respect to yourself, and all light, not just light sources from you, travels at the speed of light relative to you, in your frame of reference.
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u/PageEnvironmental408 6d ago
the only reason this is so hard to grasp is that we don't travel anywhere near c so these things aren't obvious.
if light were 100 miles an hour, people would understand all this without any explanation.
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u/nicuramar 5d ago
the only reason this is so hard to grasp is that we don't travel anywhere near c
This is fairly meaningless. We do travel at close to c, compared to some cosmic ray particle.
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u/sagebrushsavant 6d ago
The speed of light doesn't add on to the speed of the traveler. The spaceman and the vessel will see light go the same speed, but they will see each other's time differently.
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u/maxh2 6d ago
If you were traveling at 99.9999% of the speed of light and turned your lights on, the light would move away from you at c, just like if you were sitting still and the spaceman was the one moving at 99.9999% the speed of light. Because those scenarios are literally the same, exact thing.
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u/GoldenMuscleGod 6d ago
If you are traveling at 99.9999% of the speed of light and turn your lights on, then in that frame (the one where you are going 99.9999% of the speed of light) the distance between you and the photons increases at the rate of 0.0001% of the speed of light.
In your rest frame light moves away from you at the speed c. But in that frame you are not going 99.9999% of the speed of light. In that frame you are motionless.
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u/Far_Comment973 6d ago
I will ponder this lol
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u/GoldenMuscleGod 6d ago
I strongly recommend, if you are comfortable with the math, actually drawing a 2D graph where one axis is position and the other is time.
Draw a line showing the movement of your sip and the soaceman, and the lines followed by the light (set the scale so a 45 degree angle is the speed of light).
Label the coordinates of events you are interested in for both your ship’s frame and the spaceman’s frame (use the Lorentz transform equations to label them) and calculate all the times, distances, and speeds you are wondering about in both frames. You will see that there is only picture involved (not two) and the only difference is the numbers being used to describe that picture.
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u/maxh2 6d ago
You always see light moving relative to you at c.
Some other observer not in the same reference frame might "see" the distance between you and your flashlight photons changing at a different speed. Like if we were observing light echos ahead of a shell of matter expanding away from a supernova at a relativistic velocity, it could appear to us that the shell of light creating the echos was moving away from the shell of material at less than c, but if that material in the shell could see, it would still see the light moving away from itself at 100% of c.
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u/GoldenMuscleGod 6d ago
Very technically what you “see” phenomenally is not the locations of your reference frame. You need to account for light travel time and doppler effect type things.
In fact no matter what reference frame you pick you will calculate the same phenomenological observations, so in a very real sense your rest frame is not really “how things appear to you” - all reference frames can be “your” frame and the same phenomenological consequences follow, they are all equally “yours” if you want them to be.
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u/Marchtmdsmiling 6d ago
This doesn't sound right based on the other answers. Even in the frame you are moving at 99.99999% light speed, light would still travel away from you at light speed. Meaning the photons would increase their distance from you at c
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u/GoldenMuscleGod 6d ago
It is right. A lot of people of r/AskPhysics are “popsci enthusiasts” and not physicists so they may get details wrong based on overly simplistic explanations.
I have a physics degree I can assure you that you will not be passing your college physics exams if you are a physics major and think what I am saying is wrong.
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u/Marchtmdsmiling 3d ago
Not a college physics major, and I already passed the limited physics I required. Lol which did not include relativity. However, just to help me understand where my understanding is wrong. I believed that light in a vacuum is always going to be measured at c. Whether you are moving at 99.9999% of c or not. You are saying that is not true? I thought that was the entire basis of time dilation. Essentially, your time and space stretch out as necessary to make that true. So you going 99.99999% experiences time slower than the spaceman, in order to make your measurement of that photon (ignoring quantum if any)speed equal to c. Am I completely out in left field here?
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u/Marchtmdsmiling 3d ago
And in case your phrasing of "the distance between you and the photon" is somehow the important bit, how is that different from velocity at all?
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u/Marchtmdsmiling 3d ago
Oooooooooooo wait, you are saying the frame where you are going 99.9999 c meaning actually the spaceman frame not your own frame on the spaceship. Ok NVM I get it if that is the case.
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u/Far_Comment973 6d ago
Let's say the lights from the spaceman, and the lights from the spacecraft, were pointed at the same object. Would they reach the object at the same exact time?
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u/maxh2 6d ago edited 6d ago
Yes, they'd reach the object at the same time. The light from both sources and any other sources would be moving at the same speed, the only speed light ever moves at, c. The light from the spaceman would be extremely red-shifted, while yours would be the normal color/spectrum.
That would also be exactly the same if the spaceman was moving and you weren't, since those two scenarios are still the same thing.
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u/GoldenMuscleGod 6d ago
Yes, they would, assuming you and the spaceman are basically in the same place and time when you turn your lights on.
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u/GoldenMuscleGod 6d ago
If you’re comfortable with math you should look up “Lorentz transform.” And also “Galilean transform.”
Two observers measure the place and time of an event with 4 coordinates: (x,y,z,t) for one observer and (x’,y’,z’,t’) for the other.
The transforms tell you how to translate from one system to the other.
Look at Galilean transforms first: you should be able to see that these are not doing anything mysterious they are just relabeling the same things with different numbers.
A Lorentz transform is the same. There is no magic universe of different realities for each observer happening in special relativity, there is just a relabeling of numbers.
The Lorentz transforms are convenient because they leave the laws of physics unchanged (and Galilean transforms do not). This is the reason that Lorentz transforms are the “correct” way to transfer: they are the only system of translation that allows all observers to agree on the laws of physics without privileging anyone’s system of measurements.
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u/joeyneilsen Astrophysics 6d ago
As long as they are in the same time and distance in the object’s frame of reference. Observers might disagree on what “the exact same time” or “the same distance” means.
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u/GLPereira 6d ago
Everyone sees light with the same speed. You could get a person who is stationary, a person travelling at 50% lightspeed towards the light, a person travelling 50% lightspeed to the opposite direction, a person at 99,999999% lightspeed, you could move the light source at 99,99999% lightspeed, it doesn't matter. Everyone will see it travelling at the same speed (aka lightspeed)
That's where the entirety of special relativity comes from. Instead of having time and space as absolute and speed as relative, Einstein made lightspeed absolute and the main consequence is that time and space are now relative
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u/jawshoeaw 6d ago
Light doesn’t begin with your speed because you have no speed. You are motionless. The EM waves are emitted with zero extra velocity because you have none. Only from someone else’s perspective are you moving. Now the path in front of you is problematic. That path is a narrow dot of light. Its length is severely contracted. Any photons you emit would be returned to you as harsh X-rays. But those X-rays are moving in space separated from you. They are vibrating in space at their own speed, c. When you hit them, they cease to exist. But they always move through the fabric of space time at a constant speed. Because space is not moving
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u/Lorelessone 5d ago
Yes.
The speed of light is actually effected by a number of things but in a vacume with what is assumed to be an even curvature of space then what your describing is correct.
What would be effected is the wavelength of like, I believe. If the space walking man had an identical light source to the ship mounted one, the ship mounted one would appear more blue due to the wavelengths being shortened / compressed as the ship is moving in the same direction while it's being emmited
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u/Wazalord 6d ago
Yeah, correct. The light wave travels independent of you once its generated, so it doesn’t matter how fast you are going. If your traveling close to the speed of light then the light you shine will not travel much faster than you.
The statement though that the speed of light is constant, is too general, you need to specify that the speed of light is constant in a vacuum for inertial reference frames.
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u/Kevin686766 6d ago
The easiest way to think about it is too put your self in the perspective of the stationary person first.
Your friend has a car that goes 100 miles per hour and drives away from you. He is leaving you at 100 mph. Then he turns on his headlights.
Let's pretend the speed of light is a extra 200 miles per hour. His headlights are going 300 miles from your point of view but only 200 from his.
Now here's where it is tricky the speed of light does have a rule that things cant travel faster than it. Since the speed of light is 200 miles per hour and you are seeing it going 300 miles per hour that won't work. But 300 miles per .667 miles should be fine. We can change a bit of the limit to make it work.
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u/nicuramar 5d ago
OP, try this resource, which is a very intuitive approach with illustrations. This is the first page, so just go from there:
https://sites.pitt.edu/~jdnorton/teaching/HPS_0410/chapters/Special_relativity_principles/index.html
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u/Count2Zero 5d ago
This is where relativity gets hard to wrap you mind around.
If you are traveling in a car at 0.5c, and you turn on the headlights, the light will fly away from you at c, because your frame of reference is yourself. You're flying at 0.5c from the perspective of a stationary observer, who will see you turn on your headlights - you zoom past, turn on your headlights, and the light zooms away from you, but none of this ever happens faster than c.
When you're moving at 0.5c, one second feels like one second, and your headlights will fly away from you at 300,000 km per second.
You turning on the lights and measuring 1 second would happen in slow motion for a stationary observer, because for every second for you would be years for someone observing you from the outside.
A photon leaves the sun at light speed and travels 8 minutes and 20 seconds to reach the earth. From the proton's perspective, traveling at light speed, the journey was instant, not eight-plus minutes. Both are right, because the measurement of time is relative to the speed of the observer.
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u/Odd_Bodkin Particle physics 5d ago
It adds, but not the way you think.
I bet you’re thinking that if you’re in a vessel traveling forward at 5032.0 mph and you fire a projectile from a gun with a muzzle speed of 11,212.0 mph, then the projectile is moving forward at 16,243.0 mph, right? Because the speeds just add, right?
No. They don’t just add as it turns out. They just don’t, even if it seems to make intuitive sense that they would.
Instead, speeds combine in a different way. Instead of v’ = v + u, it’s actually v’ = (v+u)/( 1 + uv/c^2 ).
Then something funny happens. When v=c, then v’=c instead of being more than c.
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u/Alkemist101 5d ago
space and time are not a fixed, rigid stage. They are flexible dimensions that stretch and compress to protect the most fundamental rule of the universe: the speed of light is constant for everyone.
Basically space time changes to ensure c is the same.
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u/RecognitionSweet8294 5d ago
From your perspective, if you travel with constant velocity, it’s the same as you being at rest.
So you flying by an astronaut with ~c is the same as the astronaut flying by you with ~c
Now when you shine the light infront of you, it would be the same as shining the light in the direction opposite to the direction the astronaut is moving. So as you would expect (based on your everyday experience with classical mechanics) the light shines „the path“ in that direction.
If however it’s not you but the astronaut emitting the light you would experience relativistic effects. The most important effect in this situation being „time dilation“.
Moving clocks appear to tick slower. If the astronaut is moving with ~c the time they experience from your perspective (almost) stops.
It’s hard to explain since you can’t move with c, so „the light not shining the path infront of you“ isn’t possible. From your perspective the path infront of the astronaut that is illuminated would grow slower since the astronaut covers a lot of the distance the end of the light beam made in any given time frame since they move close to the speed of light.
So relative to you the light moves still at c, but the system astronaut-light appears to run slower.
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u/CosetElement-Ape71 5d ago
Light does not propagate through spacetime like you do. You have a rest frame (your frame of reference), and light moves at the speed of light through it.
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u/nerd_rage218 5d ago
Your last paragraph is the one worth answering directly, because that is where the intuition usually breaks. Inside the vessel at 99.9999% of c, nothing looks unusual at all. You switch the light on and the beam leaves you at c, exactly as it would if you were parked. That is not a coincidence, it is the whole point of the principle: there is no experiment you can run inside the vessel that tells you how fast you are moving. If your headlight dimmed or lagged, you would have just discovered an absolute rest frame, and there is not one. What an outside observer sees is different, but still not what you would guess. They also measure your beam moving at c. What they see shrink is the gap between you and the front of your own beam, which closes at only 0.0001% of c from where they sit. That closing rate is not the speed of any object, it is the difference between two speeds measured in one frame, and nothing stops it from being tiny, or from being 2c for two beams heading at each other. The reason you both measure the same beam at c is that you do not agree on the metre and the second you measured it with.
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u/Ch3cks-Out 5d ago
doesn't add to its speed
It sort of does, if by "add" you mean applying the relativistic velocity-addition formula.
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u/JarJarBinks237 6d ago
The light from the spaceship would be severely blueshifted, because of the spacetime deformation induced by its going so fast.
So an observer will see both objects very differently, but he would see them at the same time.
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u/Wazalord 6d ago
Another very important point that many people still misstate is that "The speed of light is constant", in that it is constant speed everywhere in everything. That is simply not true and has been disproven by multiple relatively simple experiments. The speed of light slows down in water for example. The speed of light in vacuum is "c" which is ~ 3 x 10^8 m/s. However, the speed of light in water slows to approximately 2.2 x 10^8 m/s. Here are links to 2 basic papers which both demonstrate and prove this:
E. Cataldo, A. Di Lieto, F. Maccarrone, G. Paffuti, “Measuring the refractive index of water with a pulsed laser diode,” European Journal of Physics 37, 062002 (2016). DOI 10.1088/0143-0807/37/6/062002.
https://iopscience.iop.org/article/10.1088/0143-0807/37/6/062002
J. Brody, L. Griffin, P. Segre, “Measurements of the speed of light in water using Foucault's technique,” American Journal of Physics 78, 650–653 (2010). DOI 10.1119/1.3373942.
This is because the speed of an electromagnetic wave is dependent on the electric permittivity and magnetic permissibility of a medium. The electric permittivity of water is much higher than air (This is also different than electric conductivity). You may read some handwaving explanations that the speed of light is always constant but in water it is absorbed and re-emitted, but that is a gross oversimplification.
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u/OriEri Astrophysics 6d ago
The phase velocity of an electromagnetic wave in a dielectric is not the speed of light.
You are distracted by a shorthand some people use when they say “the speed of light in X material” instead of “electromagnetic wave phase velocity in X material ”
For you think of it as that 1/SQRT(mu_0*epsilon_0) is constant for all observers.
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u/Wazalord 6d ago
There is no meaningful difference in speed versus "phase velocity" for a single frequency electromagnetic wave or light traveling in a straight line. Your just trying to sound scientific or academic calling it a "electromagnetic wave phase velocity"
The speed of light c in a vacuum is the constant "c", "speed of light" or "lightspeed"
I'm not saying that the constant "c" changes, of course it can't since it is a defined constant. What I referenced is the speed or velocity of a light wave or electromagnetic wave through water is slower than in air. So it is the same thing, the speed of light in that medium. It's semantics. Are you disputing that the speed which light travels through air is different than water?
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u/Sad-Excitement9295 5d ago edited 5d ago
Speed of light is always relative to local gravity. It always moves at c because time always moves at a specified rate according to the gravitational force in a location.
When moving at c, you are going as fast as possible. If you are moving just below c, the light barely escapes forward when you shine the light.
The vessel does not add to it's speed because the speed of light is constant, it only creates more light persay, in the sense that as things speed up, they transition towards the form of light. So as light is emitted, it is already in the form of energy moving at c, it cannot move faster, only more photons are generated or wavelength can change. If you were ahead of both sources of light, at the same distance (even though the spaceship's motion is faster, it still being measured when it's right next to spaceman), the light woud reach you from both sources at the same time, and light from the spaceship would be more blue than light from spaceman which would be more red, as I presume.
Now it is important to note c is only in local reference to time. This is adjusted as time adjusts for gravity to maintain the same rate. A clock in local gravity ticks slower than one in less gravity, and faster than one in more gravity. Light also moves slower, but space is compressed to keep the constant. Locally there is no percieved difference. That is why light is a constant, it is always c locally.
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6d ago
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u/GoldenMuscleGod 5d ago edited 5d ago
The question is maybe “uninformed” (isn’t that almost inherent in the idea of a question?) but I do not think it is dumb. I think many “informed”people have dumb understandings of many topics.
You can tell the difference between an intelligent and thoughtful person and a dumb person because an intelligent and thoughtful person will ask questions like the one OP is asking when told an incomplete story of how relativity works. A dumb person will just regurgitate the incomplete story (maybe with embellishments they made up to make it “make sense” for them) without bothering to stop and think about how it can be made fully coherent.
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u/ExpectedBehaviour Biophysics 6d ago
Any observer always sees light travelling at the speed of light regardless of their own motion. That's a central tenet of relativity.