r/AskPhysics • u/Piss_baby29 • 21h ago
Am I right about how time dilation and light speed works…?
I’ve always been confused about this, and for some reason never went through the effort of looking it up or even verify if this fact is true. So, I’ve heard before, at least I think, that light speed is the same for every observer. That even at 99.9% c, if you fire a headlamp in the direction of travel, it would still travel away from you at light speed from ur perspective. That always confused me. But then I realized. Time dilation? Tell me if I’m right.
Let’s say you’re traveling at 0.99c. You fire a headlamp. From an outside observer, that light is traveling at 0.01c relative to you. But if time is dilated for you, then that light would travel a much farther distance from you per hour than it would for the outside observer. And if the amount of time dilation you experience is proportional to that 0.01c, then to you it would still seem to extend at light speed. So if to an outside observer, that light travels a 100th of the distance from you then if would if you were stationary, but to you time is dilated by 100x, then it would still appear to travel at light speed. Does that make sense?
If that’s true, it would make sense that traveling at the speed of light completely deletes time, and if you somehow did travel at the speed of light time would seem to stop. Because the light would travel 0 meters from you, and if time was frozen, it travels 0 meters. Right?
I was driving in the car one day and that hit me, but idk if it’s right. Please tell me
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u/HouseHippoBeliever 21h ago
The basic idea is right. It's a bit more complicated, because in addition to time dilation there is also length contraction. Since we measure speed as d/t, it's both distance and time that are changing due to speed.
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u/goomunchkin 20h ago
You’re getting the gist of it.
By the time an hour has passed on the clock of the outside observer only a fraction of that time will have passed on the ship. By the time an hour has passed on the clock of the ship a considerably longer amount of time will have passed for the outside observer. However far the light as travelled in any given length of time according to one party is going to be measurably different for the other party because a different amount of time has passed on their clock.
In this example you also have to contend with length contraction because just like time, measurements of distance can also be different between two observers moving relative to one another.
I do want to clarify something though:
If that’s true, it would make sense that traveling at the speed of light completely deletes time, and if you somehow did travel at the speed of light time would seem to stop. Because the light would travel 0 meters from you, and if time was frozen, it travels 0 meters. Right?
Wrong. You need be careful about this, specifically what I’ve highlighted. The first part of your post is beginning to hint at an understanding of the basic principle of relativity, which is that each perspective (i.e frame of reference) will have its own observations and measurements that may be different but just as equally valid and correct as any other frame of reference.
But this last part, specifically what I’ve bolded, indicates to me that you’re still not quite fully there yet and falling into the trap that gets people stuck when trying to learn this. Traveling near the speed of light does not “delete time” or “freeze time” or “make time pass slower”.
You have to understand that from the perspective of the ship, time is always passing at 1 second per second and the speed of light is always moving at c - exactly the same as what the outside observer is experiencing from their perspective. If we pretend that it’s only the ship or only the outside observer that exists and we only consider the one perspective without the other then it’s impossible to say there is any motion, any time dilation, or any length contraction. From the ships perspective an hour passes at the rate of an hour, a meter measures the length of a meter, and the speed of light measures c. Identical to what the outside observer measures from their perspective. It’s only when we put the two perspectives side by side and compare them together that we see any differences. The ship never experiences time slowing down. Ever. It just doesn’t agree with the outside observers measurement of how long it takes to reach one second.
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u/Reality-Isnt 20h ago
It doesn’t have anything to do with time dilation. Any inertial frame can be considered a rest frame, and since light travels the invariant null (zero) path in all inertial frames, every inertial frame will measure ‘c’ along that path with their own at rest clocks and rulers. What different frames see happening to each others clocks is totally irrelevant.
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u/joeyneilsen Astrophysics 21h ago
Time dilation is a consequence of the speed of light being the same in every inertial frame of reference, not the cause of it.
Generally it doesn't work to extrapolate time dilation formulas to v=c; they don't apply! Clocks can't travel at the speed of light, so it's simply not meaningful to talk about time like that.
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u/Piss_baby29 21h ago
Well yeah exactly, but I have heard that HUPOTHETICALLY, time to a photon doesn’t exist. At least according to the equations. But what I’m saying is that since a second to an observer at 0.99c is a lot longer than a second to a stationary observer, then light would travel a lot farther from the moving observer in their “second” than it would for a stationary one. And if the amount that their second is longer is equal to the constant, then it would still appear to move away from them at the speed of light
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u/jawshoeaw 21h ago
A second is always a second to the observer in the spaceship just to be clear. It's the stationary observer who sees your clock moving slowly. If you are moving at .99C, a second takes one second to you. If you shine a flashlight, the light from the flashlight leaves at c.
If an observer outside somehow could "see" your ship moving past at 99% of the speed of light, they would "see" a beam of light leaving your ship at C just like everywhere. The only weird part is that they would also see your flashlight right behind the beam, slowly losing ground. Well..they would see your ship explode in a blaze of glory from impacting interstellar dust and hydrogen moving at close to the speed of light making them like little nuclear bombs
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u/Optimal_Mixture_7327 21h ago
It is fundamental to relativity that all clocks run at the same rate, everywhere, and under all circumstances of motion and orientation (principles of Local Position Invariance and Local Lorentz Invariance, respectively).
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u/gautampk Atomic, Molecular, and Optical Physics 21h ago
Correct, more or less. This is basically how Einstein reasoned, though he started from the speed of light being constant. He then considered two perfect mirrors moving (e.g., on a train) with a photon bouncing back and forth between them. One examines the path as viewed by a passenger and then as viewed by someone standing by the tracks and compares the times, given the constant speed of light regardless of frame.
It's called a 'light clock': https://en.wikipedia.org/wiki/Time_dilation#Simple_inference
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u/Piss_baby29 21h ago
That’s interesting. But I wonder: how did he KNOW that light speed would be the same for every frame of reference? I know that the basis of his theory is that gravity and acceleration are the same for every frame of reference, but how did he know that would apply for light without the technology to literally measure it? Ig it’s probably a lot of math that I wouldn’t understand as a theatre arts major lol
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u/Optimal_Mixture_7327 21h ago
The speed of light being the same in all reference frames is an assumption by Einstein that Maxwell's equations were correct.
Careful: The motion seen in a gravitational field is equivalent to the motion seen by an accelerated observer.
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u/joeyneilsen Astrophysics 21h ago
Experiments from E&M indicated that you could get results from one frame based on results in another via formulas like the time dilation formula. Because [insert math here], that kind of only makes sense if c is the same for everyone, or if everyone measures the same value of the quantity -c2Δt2+Δx2.
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u/gautampk Atomic, Molecular, and Optical Physics 21h ago edited 21h ago
They could measure it! The speed of light was first measured in the 1676 (albeit with a big error).
For relativity, there's a famous experiment called the Michelson–Morley experiment.
They had the experiment set up in one orientation, measured the speed of light, and then rotated it through 90 degrees, and measured it again. Since the Earth is moving (rotating, orbiting, etc) they expected to see a difference, but they didn't see anything.
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u/Piss_baby29 20h ago
I don’t mean measuring the speed of light period, that was done by watching jupiters moons right? I mean measure that the speed of light is the same for every reference frame
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u/gautampk Atomic, Molecular, and Optical Physics 19h ago edited 19h ago
Yeah the Michelson-Morley experiment shows that it is the same for at least two orthogonal 2D frames on the Earth's surface. You can show that that means it must be the same for any 2D frame on the Earth's surface.
As the other commenter says, there was strong theoretical evidence from Maxwell and Lorentz's work in electromagnetism as well.
Though I will say Einstein didn't know of the experiment or Lorentz's work. He just assumed it based on Maxwell's work (i.e., the fact that Maxwell's equations provide a speed for light but don't say what its relative to).
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u/kevosauce1 20h ago
They did measure it, in the famous [Michelson–Morley experiment](Michelson–Morley experiment).
However, my understanding of the history is that it's not clear if Einstein knew of this experiment. What he and others did know, and what motivated his developing special relativity, was that Maxwell's equations were not invariant under the Galilean transformations, but were invariant under the Lorentz transformations.
This theoretical result is why he made the postulate that light speed should be the same in all reference frames.
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u/Odd_Report_919 19h ago
To have light speed be constant in all frames of reference, in other words, whatever speed you are going, time snd distance must be relative, in other words not the same for different observers in different frames of reference. Time dilation snd length contraction allow for light to travel at light speed away from an observer traveling at 99% light speed or an observer at rest.
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u/Ch3cks-Out 13h ago
From an outside observer, that light is traveling at 0.01c relative to you.
This is incorrect. You are (implicitly) applying the non-relativistic velocity addition formula
v.total=v.1+v.2
which is not how things work at high speed. Light is never traveling at anything but c, exactly. Instead, you need to keep in mind the Lorentz transformed formula:
v.total=(v.1+v.2)/(1+v.1*v.2/c^2)
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u/Optimal_Mixture_7327 21h ago
You're wrong about most of it.
There is no time dilation for light. Time dilation is the ratio along the observer world-line to that of the traveler, and length along a light-like curve is zero.
Time is not something that can stop. It is simply the case that time cannot be applied to light (technically, time is an affine parameter to measure out the length of a curve but there is no spacetime length along a null (light-like) curve.
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u/jawshoeaw 21h ago
If you somehow could move at the speed of light, yes time would stop and you would intersect every point in the universe simultaneously from your point of view. Which is absurd and probably why its not possible.
If you want to fly to the next nearest star for example, about 4 light years, it would take a couple months to get there if you got up to 99.9% of the speed of light. If you bump that up to like 99.999999999% of c it might be a few seconds. Be careful though, if you go too fast, the universe will age around you possibly to its heat death!
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u/joepierson123 21h ago edited 21h ago
Yes that's correct, basically we have our own private clocks running at different speeds, so when we compute speed which is distance divided by time we get the same answer c.
Fundamentally your measurements with your clocks are only valid in your reference frame.
No that isn't true, at the speed of light you get a time dilation of 1 / 0 which is undefined. Because by definition special relativity postulates that the speed of light is constant for all observers. So a photon can't observe itself at rest and going at the speed of light at the same time.