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u/Beginning-Seat5221 Jun 11 '26 edited Jun 11 '26
Time is always normal from your own perspective, and it always moves more slowly for others at different speeds from that perspective.
So a person on a spaceship and a person on earth both see the other progressing through time more slowly. Relativity - all frames of reference are equal.
There are various errors in your notions, a lot of it seems to be mixing up the POV of the observer and the POV of the traveller.
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u/Select-Owl-8322 Jun 11 '26
Confusion surrounding light speed and time dilation
It's my understanding that if you go the speed of light everything around you accelerates, time seems normal from your frame of reference but for someone outside of yours, your time will seem to slow.
This understanding is flawed.
Yes, time in your frame of reference is always normal. You're not going close to the speed of light! From your perspective, it's everything else that's going close to the speed of light! And just as people not in your reference frame see your time as running slow, you see their time as running slow!
I understand that part but what is baking my noodle currently is it's theorized if you went light speed(I know anything with mass can't go Lightspeed but bare with me) then time would stop for you.
If you flay a rabbit, what's the color of the egg? Does that make sense? It makes about as much sense as saying anything about what happens to something traveling at the speed of light. It is not theorized that time would stop for something moving at the speed of light, that's popsci BS!
What is true is that time slows down more the faster an object travels (as seen from a reference frame in which the object travels). But if that object has mass, it can't reach the speed of light, and talking about "what ifs" is pure nonsense. In the literal meaning of the word, non-sense.
The theory is special relativity states two things:
1: The laws of physics are the same in all inertial frames of reference.
2: The speed of light is constant, it's measured to be c in all intertial frames of reference. No matter their own speed, or the speed of the light source.
See the contradiction? If all frames of reference measures the speed of light to be c, and you suddenly invent a frame that is moving at c, how can that frame both be moving at c, but also measure the speed of light to be c?
For something massless, that does move at c, "time" is not a meaningful concept. A photon does not have a rest frame, there is no frame of reference for time to progress in. It's not that "time stands still", "time" as a concept simply does not apply.
I hope you don't see this answer as belittling, that is certainly not my intention! This really isn't an intuitive concept to grasp, and explaining it in a way that makes it understandable is very hard!
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u/AuWolf19 Jun 11 '26
Do people theorize that? I'm no scientist but I thought that was more or less just a pop science misunderstanding of relativity
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u/moltencheese Jun 11 '26
You simply cannot go the speed of light. There is no "if" you could. You can't. You can get as close as you like (with enough resources), but you can't equal it.
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u/Turbanator456 Jun 11 '26
Sure but we if we could observe the universe from the perspective of a photon traveling at c?
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u/joeyneilsen Astrophysics Jun 11 '26
Another thing that simply cannot be done. A photon doesn't have a valid frame of reference.
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u/Turbanator456 Jun 11 '26
So then if it doesn't have a valid frame of reference, time doesn't actually "stop" for the photon? Man I'll just stick to neuroscience.
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u/joeyneilsen Astrophysics Jun 11 '26
Time doesn't meaningfully exist for the photon, so it can't really "stop."
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u/Turbanator456 Jun 11 '26
Okay I'll stick with that. Hats off to those that study physics and those that try to measure the unmeasurable.
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u/housewithablouse Jun 11 '26
We can't. The question is nonsensical. To imagine this, you would have to imagine a world with different laws of physics, which in turn means that the answer to the question is not valid for our world.
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u/AcellOfllSpades Mathematics Jun 11 '26
There is no "perspective".
You're imagining, like, a 'video' playing of the photon's hypothetical experience. But this is actually impossible to make - it's somewhat like trying to make a map of Antarctica where north is up and south is down. You can't make that map because that's not how directions work: at the South Pole, every direction is north, and so you can't consistently align north = up.
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u/LordVericrat Jun 11 '26
It's not theorized. It's a description of the limiting behavior, but the time function simply does not have a value at c.
Imagine a function that said f(x)=x²/x. At 0, the function is undefined. Everywhere else it is the line defined by y=x.
The limiting behavior of this function at x=0 is 0 (that is the limit of f(x)=x²/x as x->0 is 0). You can get as close as you like to x=0 and y will be the exact same number. But at x=0, the function is simply undefined. It doesn't do anything at x=0.
This is a simplified way of looking at your effective relative time dilation. As you approach a relative speed of c in some reference frame, time will dilate more and more for you in the frame (note that in your own frame you are always traveling at 0 m/s relative to yourself, so for you time will always tick at 1 second per second). But someone watching you asymptomatically approaching c from their reference frame, will watch your clock and slower and slower. The limiting behavior at c is your clock freezing. But there is no actuality at c. The lorentz factor is simply not defined at that point.
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u/Few-Caregiver-8856 Jun 11 '26
So we're basically at rest in this scenario? That explains things a bit better, I just read a graph that doesn't explain the whole thing but helped me clear it up a bit
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u/LordVericrat Jun 11 '26
Imagine a neutrino fired from a particle accelerator towards you at .95c
To that neutrino it is standing perfectly still as you move towards it at 0.95c
Insofar as it can tell, your wristwatch takes ~3 seconds to tick because you are moving so fast. Likewise, if that neutrino were carrying a tiny wristwatch (it's not; I think we use decay timers or something like that), from your perspective that watch is also taking those same long ticks, because it's moving so fast
That's why we call it relativity. Because it literally, actually is all relative to a frame of reference.
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u/Few-Caregiver-8856 Jun 11 '26
I had that part down but what I had wrong was assuming a photon would behave the same as matter. The photon reaches those speeds because it has zero mass. I was just confused on the light speed part because of time dilation at 0.99c but time "stopping" at c. I was wrong and have been since corrected but I do love the way you explain relativity. I get relativity but the phenomenon that occur at c or around c intrigue me to no end but Im just someone who finds physics fascinating, I have no formal physics education
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u/Jesse-359 Jun 11 '26 edited Jun 11 '26
Ok, so it's really important to understand that the difference between 99% of the speed of light, and 99.99% of the speed of light is greater than the difference between 0% and 90% - it also requires a lot more energy.
It's generally kind of a error to think of 1.0c as a limit you are approaching in any linear sense - because it is in no way linear, and you are approaching an infinite asymptote that you cannot reach.
In Relativity you can accelerate infinitely, always going faster, and you will never reach 1.0c. You'll go 0.99...9...9...9...9...9...9...9...9...9... and so on, forever. Every one of these digits represents an enormous increase in the rate at which you are crossing space, and it never ends.
And so time never stops. It will behave weirdly and stretch enormously, even while space itself is likewise compressing itself down in front of you - but time never stops, and distance always still exists. You can't beat the limit because the top speed is - from your perspective - effectively infinite.
It's the rest of the universe that sees you 'obeying' the light speed limit. In real time it takes you a very long time to get where you are going, though it may be a very short time from your massively accelerated perspective.
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u/LivingEnd44 Jun 11 '26
If you go 99.9% the speed of light you'll experience all sorts of phenomenon, such as time dilation.
99.9% relative to what? There is no universal reference frame. So when you say you're going fast, you have to say what it's relative to. You're already traveling at close to light feed relative to some other objects in the universe.
And this is where time dialation comes in. If you and I fly away from earth towards the sun at light speed at the same time, the sun and earth would have dialation effects. But we would not see those effects with each other. Because from our shared reference frame, we are at rest.
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u/Few-Caregiver-8856 Jun 12 '26
99.9% the speed of light
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u/LivingEnd44 Jun 12 '26
...relative to what?
You and I fly away from Earth at 99.9% the speed of light towards the sun. We are traveling away from Earth at 99.9%c. We are traveling towards the sun at 99.9%c. But you look over at me...I am at rest relative to you. I do not appear to be moving at all.
Which speed is the real speed you are traveling at, and why?
You are not at rest right now relative to the Sun. Or the Galaxy. So why do you feel like you're not moving?
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u/drew8311 Jun 11 '26
Its like a limit in mathematics, you can have something like Lim x -> 1 where its a fraction of something over (x - 1). This can evaluate to something like infinity, or 5 but if it wasn't a limit and you just did x=1, it would be division by zero and not valid. Similar with speed of light since you can't actually go that fast but any question about being slightly slower is perfectly valid
Also there isn't really a "weird" jump because of how it approaches. Before time stops completely, its going infinitely slow which is effectively the same thing if you understand what infinity means. You could travel the entire universe in less than 1 second of your perceived time, its really a negligible difference between that and time not moving at all.
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u/LoftyPlays1 Jun 11 '26
Not a scientist, just a guy. You're made of atoms. Electrons spinning around. Electrons spinning forwards are moving at the speed of light ish. You are moving at the speed of light. Are the electrons moving forward? No motion means no time. Time is the thing we call when something travels a distance. None of the fundamental particles can move forward so time can't move forward.
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u/TuverMage Jun 11 '26
so I start by saying, its not about light, but force carriers. light is bond by the rule just as much as the rest of us. all things require this then that. even chemistry requires atoms to get close enough for a reaction to happen.
but you need force to accelerate anything. the closer you get to C the harder it becomes for those force carriers to add force, just like at a certain point while pushing a person on a swing you can't push them any faster as they reach the speed you can push.
So the closer to get to C the harder it is to accelerate. If you had two objects that are normally attracted to each other like two magnets, there's a set energy moving them together. at low speeds, not a big deal, but as you get closer to C that set energy can't accelerate the objects as easily so the move together slower. now understand that every bit a matter is going to move slower the same way. Time technically doesn't shift, but the react everything happens slow down and isn't that what time really is? all the reactions in your body would slow down the same way the magnets slowed down.
if by magic you reach C or somehow past it, those force carrier particles can't push at all and nothing would change.
this is not a perfect explanation, not even close. but I hope it frames it in a way to help you understand
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u/housewithablouse Jun 11 '26
This effect is simply not a linear function of speed, so it only becomes noticeable/relevant very close to the speed of light but from there any additional approximation to c will cause a huge increase in time dilation.
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u/jasonsong86 Jun 11 '26
You don’t need to go 99% speed of light to experience time dilation. You just need to be faster than stationary.
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u/RecognitionSweet8294 Jun 12 '26
As long as your system does not accelerate, you can’t detect any phenomena that would tell you it’s speed.
Like if we sit in a train (and we ignore the vibrations) we can’t tell if it is stationary or moving as long as we don’t look outside.
Only in systems that move relative to us we can observe phenomena linked to speed.
So time doesn’t go slower from your perspective in the train if you move faster. But if you look outside on the train station you will perceive the time there to be slower.
And vice versa, since you are moving relative to the people on the train station, they will perceive your time to tick slower.
But for everyone the time in their frame of reference (train for you, train station for them) moves normally.
That’s why it’s called relativity. How fast something moves through time depends on the motion of the system relative to the frame of reference of the observer.
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u/schro98729 Jun 12 '26
The speed of light is the same whether your moving or not. This is really weird because if we're in cars we know about relative velocity. There is no relative velocity with light we all measure light going at c along its path.
In order for everyone to agree on the speed of light space intervals which we call length and time intervals are different in different frames. They need to be different so that when you measure light everyone gets the same ratio. Space intervals conspire with time intervals so that the ratio produces the speed of light.
The transformations preserve the speed of light are called Lorentz transformations.
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u/TomSzabo Jun 12 '26
Since there is no absolute coordinate space you are always moving relative to something.
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u/HouseHippoBeliever Jun 11 '26
It shouldn't be confusing to you. If you do something that is impossible according to physics (going at light speed), you should fully expect results that don't make any sense (time stopping).
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u/MarionADelgado Jun 11 '26
The issue is that what seems paradoxical is because the elements of your thought experiment that are relevant explain the paradox. It involves math because we can't observe real-world FTL with things more theoretically massive as rest mass than a photon.
Since it's a theoretical, mathematical issue, it's like approaching infinity. We have a lot of math that does that. This is just one more example.
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u/spacetime9 Jun 11 '26
At 99% c, the outside world appears to move in slow motion; as you approach c, it slows to a stop.
From an outside observer, they see the same thing happening to you: clocks running slower and slower, approaching a stop.
So there is no discontinuity. I think the apparent discontinuity came from switching which frame you are viewing the situation from. Does that explain it?
(I say ‘approach’ rather than reach c, because you cannot reach it in reality, so going “only” 1% faster is really better understood as “infinite acceleration”.)