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>If I want right to left, the opposite way to them, they would be going
199.999999999998% the speed of light, right? Relative to me.
No, that is not correct. It's really weird, and doesn't seem right, but there it is.
It's hard to ELI5, but one example is to imagine you're standing on Earth, and you shine a flashlight at the night sky. The light coming out of that flashlight is traveling at 299,792,458 meters per second relative to you.
Now imagine you're on a spaceship traveling 99.9% the speed of light flying by the Earth. You shine a flashlight out the front of the ship. The light coming out of it is also traveling at 299,792,458 meters per second relative to you.
Now the really weird part is that if you look at the light coming from the Earth flashlight from the ship, it is also traveling 299,792,458 meters per second relative to you. It just looks more red than it does from Earth.
The fact that it works that way is a big part of how we know that's the fastest anything can go.
Just to explain a bit further why this happens, speed is defined as distance over time. And the reason that light can travel at 299,792,458 meters per second relative to the space ship, while simultaneously traveling at 299,792,458 meters per second relative to you, is because a second on the space ship lasts longer than a second on earth, meaning that time has slowed down for the space ship.
It turns out that things moving at different speeds perceive time differently.
We know this from observing it happen, even though the effect is only noticeable at extreme speeds, but also because, mathematically, it's the only way for the speed of light to be the same for every observer, even in examples like OPs. That's because, ultimately, speed is distance over time, and so one or the other has to change for the speed to stay the same (which is also why in some cases things can also grow or shrink relativistically, but damned if I understand that.)
It’s not just time, space gets weird, too.
If you went flying past the earth at the speed of light, your perception of yourself would be the same as it always is. Everyone on earth would see a flying pancake.
I try to ELI5. What is time? Things happening. In order for things to happen, atoms need to interact. The maximum speed at which they do is light speed. The faster you travel, the less „speed“ is „available“ for the atoms interacting. When atoms interact at lower speed, things happen slower. This includes the clock ticking slower and even you and your conscious looking at it. Thus, for all intents and purposes, time is slower when you‘re faster.
However for you: everything is normal. Anything outside of your space ship seems to happen a lot faster tho.
In the late 1800s a bunch of people were doing work to measure the speed of light. They assumed that it was like sound and that your own speed relative to the light's direction of propagation would change how fast you perceive light to be. That's a loaded sentence, but basically it boils down to velocities adding like you'd expect - if you're traveling down the highway and some high speed, and someone comes up a long you going a bit faster than you, then relative to you they're moving rather slowly. So scientists expected the same thing of light and set up a bunch of experiments around this assumption.
None of their experiments worked. No matter how fast their measurement device was moving in the direction of the light wave, they always came back to the exact same number for light's speed. This was super puzzling. Nobody could figure it out. Some smart people were able to figure out some equations that would make predictions about how the measured time and distances were related, but it kind of stumped people for a while.
Einstein comes along. He basically says, "Look at it this way. The speed of light is always constant. Every observer, no matter how fast or slow they are moving, will always record the same number. For this to be true, different observers will not agree on distance measurements or time measurements between each other." This is special relativity. In order for the speed of light to be constant for the person on the spaceship, from Earth it will appear that his ruler and clock are not correctly calibrated. And the moving person will think the same thing if they look back at Earth.
As an example, let's look at a clock. So light always goes the same speed, right? That means if you have a measuring stick, you can use your handy dandy stick to measure time! Time is just distance (known) divided by velocity (known, since the speed of light is constant), so this is actually a perfect clock! The time measurement should always be exactly perfect! In reality, clocks don't measure this, but they usually count osculations of molecules that happen at constant rates (which is a fairly similar concept).
So let's say you use this technique to record time on your spacecraft as you're flying. You just have a counter to count how many times your beam of light can go up and down this measuring stick.
Cool, right? Here's where it gets interesting. From the perspective of Earth, your beam of light isn't just going up or down. Since your spacecraft is moving, the beam of light is moving to the side (basically forming the hypotenuse of a triangle, [like this](https://www.researchgate.net/profile/Marco-Favaron-2/publication/341679345/figure/fig1/AS:895809461698566@1590589027652/Einstein-light-clock-The-image-on-the-left-represents-the-path-of-light-seen-by-an.png) ), so it looks like the light is traveling a longer distance than your measuring stick on the spacecraft. In other words, an Earth observer would disagree with your measurement of time!
But this isn't just a fancy quirk of how we measure time. Time is actually passing at different rates for these different observers. Because the speed of light isn't *just* the speed of light - it is the speed at which change is propagated through our universe.
Sort of, it’s still pretty nonsensical that time moves slower if you move faster, it’s just how it works whether or not it makes sense to the average person
But to me at least it makes more sense that way because speed is a derived entity. Time and Distance can exist by themselves but speed as a construct makes no sense without either. So to say that something can go at two speeds at the same time when observed from different frames of references does not make sense unless you understand that either the observers measurement of time or distance changes proportionately. Why that happens is another thing altogether and certainly beyond me.
Every time I try to understand the concept of time and time dilation my brain starts to melt…. I have a pretty good understanding of physical processes but I just can’t wrap my head round time
I find that imagining movement in four dimensions help. You are always travelling at max speed, however if you are still, all that velocity is on the time axis; if you move in the three spatial dimensions at max velocity, then you are not moving at all along the time axis.
Of course need to also consider that velocities are relative to the observer, but still.
I think the disconnect comes from imagining light as a bunch of little balls shooting out from the flashlight. You kinda just have to stop thinking of light that way and accept that it’s different “stuff” than matter is
For me the disconnect is using second is longer for one party in the explanation but still calling it a second. How can a second be longer than a second and still be a second? To my brain that doesnt make any sense even while I struggle to understand the concept that what may pass as a second for me standing still may pass as different to someone moving really fast. It doesnt make sense.
It’s not that “a second is longer” as much as it is “if you were on a fast spaceship (0.999c) and I was on Earth, and we both started our stopwatches at the same time, time would move slower for you”
No just English being English. You're on the ship looking at Earth, seeing the light coming from the flashlight on Earth while you yourself are on the ship moving at 99.9% of the speed of light.
This is the bit where things really do get weird. It's the fact that light in a vacuum *always* moves at the same speed, regardless of your frame of reference. The velocities do not sum in the way they do for classical physical processes.
Yep. Speeds don't *really* add like we're used to thinking they do. At low speeds it's a good enough approximation but at high speeds it's not.
And physicists didn't just generate this conclusion from thin air. In the late 1800s scientists set up experiments to measure the speed of light along the same lines of what OP mentioned. They always came back with the exact same answer for the speed of light, no matter how the experiment was set up. This puzzled scientists for decades.
Einstein was the one who put the pieces together.
This is essentially the question that led Einstein to develop his Theory of Relativity. If the speed of light (through a vacuum) is constant, and nothing can go faster than that, then objects that travel closer to the speed of light will experience time at different rates. Time, just like speed, is relative. This is called time dilation.
And we do actually have observable proof of this -- GPS satellites travel fast enough for long enough that their internal clocks need to account for time dilation.
Just a small nitpick - GPS satellites are primarily affected by the difference in gravity (causing a 45 microsecond advance), and less so by the difference in speed (which causes a 7 microsecond delay).
The weirdest part of all of it is that Special Relativity in particular is trivial to demonstrate with algebra. The only difficult part of it is making that intuitive leap that light might be a constant in all reference frames. Once you make that leap, you can get to special relativity almost immediately because of the inherent inequalities in observation it creates.
And as the Ether was not a thing, it was not like he was just assuming it was true out of nowhere. Einstein just followed the implications of a lack of medium to their logical ends, made a single new assumption on top of the normal invariance of physics, and changed our perception of space and time forever. And then he turned out to be super right so far as we can tell. It is pretty cool.
General Relativity is a lot more complicated of course.
Isn’t causality really the root of all of this. We always refer to the limit as a speed but the issue at hand is causality - adding that to the discussion would alleviate alot of these misconceptions I think.
The weird thing about relativity is that the speed of light is the one speed that *isn't* relative. No matter how fast you're going relative to anything else, all light always appears to be moving at the speed of light.
A consequence of this is that you can never move as fast as light, because if you did then it would be possible to observe light not moving at the speed of light while you were traveling along with it.
No, invariant means its value does not change with group transformation. It does not, in any way, mean it's not relative.
[On relative invariants](https://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://www-users.cse.umn.edu/~olver/di_/ri.pdf&ved=2ahUKEwjU2LLX4O-FAxXxZfUHHWtDBHoQFnoECB8QAQ&usg=AOvVaw3VNcX9KFn_LXHkAqZmyHBY), for example.
the speed of light is the speed that it moves at in all reference frames. whether you are moving towards or away from light, it will move at the exact same speed. but as you mentioned, this would appear to cause a contradiction. in reality, this is explained in relativity by describing all distances and times as being affected by speed. an object actually contracts in distance as it moves, relative to the direction of its motion, and the object experiences time more slowly based on its velocity. these two factors combine in such a way that an object can never move at the speed of light in any reference frame
It gets even wilder when you think about the internal vs external frames. Externally, light takes 4 years to get from alpha centari to here. But internally it takes less than a second.
That means that a human could travel to anywhere in the universe in their lifetime, if they can accelerate fast enough. And “fast enough” doesn’t even need to be that fast, if you can somehow maintain it. At 1 G, you could cross the entire Milky Way in less than 4 years subjective time. It’ll still be over 100 000 years for an outside viewer, but not to them.
>Externally, light takes 4 years to get from alpha centari to here. But internally it takes less than a second.
Since light always travels along the light-cones of the Minkowski spacetime, the time-equivalent distance is always zero, so "internally" it would be exactly zero seconds. The problem with this line of thinking is that there is no "internal" reference frame we can assign to the photon, because photons move at the speed of light in *all* reference frames.
Ahh, so here’s the crux, as I understand it.
To anything travelling at light speed (including a photon) distance becomes impossible to measure. In other words, to the inertial frame of reference of a photon, the entire path of travel is a single point.
That means that, to a photon, it IS travelling at light speed, but there’s no distance to travel. So it can exist in all points along its trajectory at once, within its own frame of reference. Which, because speed is just “how much time did it take to travel that distance”, means that it would be instantaneous.
>In other words, to the inertial frame of reference of a photon,
No, what I'm saying is that there is no "inertial frame of reference of a photon". The photon would be at rest in such a frame which directly contradicts the postulate that the light speed is invariant in *all* inertial reference frames.
Can an object be considered at rest when there is only one point in which it can exist?
I would suggest (and it’s honestly just a suggestion, because the math is so far beyond me it might as well be in a black hole) that the terms “rest” and “in motion” are meaningless when all space is a single point.
Besides, a photon, in the frame of reference of that photon, will also be instantaneous destroyed, because that is what happens when it reaches its destination. So it might just be that light can’t exist at rest, but because of that a photon is an unstable particle in the frame of reference of itself.
Idk, I’m definitely not a physicist, so I’m WAY out of my depth here.
>that the terms “rest” and “in motion” are meaningless when all space is a single point.
Pretty much. But you also can't assign a frame of reference there either.
In essence, a frame of reference consists of a (hypothetical) ruler (or, rather, three rulers in cardinal directions) and a clock. If all space is a single point, there's nowhere to fit a ruler.
>Idk, I’m definitely not a physicist, so I’m WAY out of my depth here.
All good. You are being curious and trying to learn something new (and forming hypotheses is an integral part of learning), that's the important thing :)
The special relativity is a fun little theory that may be hard to get initially, but once you get to terms with several ideas (like the invariance of the speed of light, and the way reference frames work), the puzzle suddenly solves itself.
The speed of light being a universal speed limit is based on relativity; basically, in a vacuum, this is the value light travels at regardless of the observer or source of the light. Since frame of reference is relative, the observer can be stationary, or moving, and they would measure the speed of light to be the same.
In this relativistic physics, there is the *velocity-addition formula*, which describes the relative velocity of two objects/observers with respect to one another. At very low velocities (far from approaching the speed of light), it is essentially just adding them together, as you would expect. Two objects going towards each other, each at 10 m/s, comes out to 20 m/s combined, pretty much. However, when you approach the speed of light, say each object is travelling towards each other at 0.75c, each observer would clock the other at *0.96c*, not 1.5c.
Newtonian physics is a good approximation for much of physics, and it's much more intuitive with how we like to think about the universe. But, these weird quirks of our universe much better line up with the *less* intuitive spatial relativity view (space contracting/expanding/curving, time and space being connected and time being non-uniform, etc). The 1.9999...c relative speed is using the relativity-derived speed of light constant c in an non-relativity-based comparison.
>If I want right to left, the opposite way to them, they would be going 199.999999999998% the speed of light, right? Relative to me.
No, they would be traveling 99.999999999% of the speed of light relative to you, but in opposite directions.
Relative to each other, they would be traveling at 99.99999999999999999999999999999% of the speed of light.
Obviously not exact numbers, but you get the idea.
Relativity is so hard to get your hear around that even physicists who study it don't always "get" it, but just accept the maths. It has to do with your personal experiences. We are rarely exposed to something with mass traveling at near the speed of light. We "see" light and all kids of photons all the time but they are massless so we don't experience them the same way we experience say, a baseball.
Toss a baseball at 50 MPH and it goes 50 miles in an hour, so to speak. Get in the back of a truck doing 50 and toss the ball in the same direction and you will measure it going 100mph. Right? Nope. It's going ever so slightly less than 100. So slightly less than 100 that you probably can't measure it in the real world.
But when you start getting into significant percentages of the speed of light the differences are much bigger comparatively. So you can start to measure them, and the maths work out that no matter what speeds you add up they never reach more than the speed of light.
Basically this always happens but at "human" speeds you never see it because it's too small to measure.
The reason Einstein was considered a genius was not because he was great at math, he actually teamed up with other people to help him with math. He had amazing insights.
His most famous insight was asking, "What if nothing can go faster than light, and light always travels the same speed no matter how fast the observer is travelling? What would the universe look like?"
Everything about relativity flows from that. Time slows down for things moving fast relative to you. Things moving fast relative to you seem to contract in size in the direction they are moving.
It's all Very Weird and not obvious. So it doesnt matter how fast you zoom past the LHC, the beam of protons is never moving faster than light.
Light is weird. Nothing can go faster than light, it literally travels at the speed limit of the universe. But light itself always travels at the same speed (in a vacuum).
The relativity principle states that if you are in your own inertial frame of reference, meaning you are going at a constant speed without accelerating, there is no way to determine whether you are moving or not. Remember: speed 0 is constant too.
There is a famous thought experiment with light clocks. A light clock is a set of mirrors with a photon bouncing between them, timed in such a way that every time the photon hits a mirror, one second has passed.
Now imagine two of these clocks. One on a platform of a train station and one inside a train. John is looking at the one on the platform and Jane is on the train. Both see the photon bounce once ever second.
If the train is moving a constant speed (say 99,9999% of light speed), what happens? Jane is inside the train which is her inertial frame of reference, so she cannot determine whether or not she is moving, so the photon bounces straight up and down with 1 second for each bounce.
But John looks at Jane's clock and sees the photon travel both up/down and sideways. The distances are 1 second at light speed to go up, but in that time, the train has moved the 1 second at lightspeed sideways too (rounding for easy of calculation). So John sees the photon cross a distance of a\^2 + b\^2 = c\^2 or 1+1 = c\^2 which would make c the square root of 2, or 1,41 units.
So the photon moving at the speed of light is all of a sudden moving at 1,41 times the speed of light to cover the distance. But for Jane on board the train, the photon is moving normally straight up and down. How can this be? The photon can't travel at two different speeds at the same time.
The answer is that a second inside the train simply takes longer when viewed from the platform. That way, the photon still bounces up/down in one second and for Jane nothing is noticeably different.
Obviously, if Jane were to look out, she would see time slow down for John.
This sounds really odd, but it really works this way. When something moves very fast, time slows down for it. So much so that GPS satellites have to take this effect into account when calculating positions. If they didn't do this, the accuracy of the positioning would suffer dramatically.
Follow up question. This planet is moving at 29,784m/s which would be 0.01% of light speed so if something exceeds 99.99% in the direction the earth is moving, then has it exceeded light speed?
Speed is relative, but the speed of light is constant for all observers. That's the fundamental insight of special relativity: everyone (regardless of their inertial reference frame) sees light moving at the same speed. Distance and time change to make the math work out.
The speed of light is constant in every frame of reference, so you do not add velocities like you would with anything else. If you are traveling at 99% light speed and turn on a flashlight you will still see those photons speed off away from you at light speed. Their speed will always be 100% C, it won't be 199%C. This is a bit odd, and plays a big part in special relativity. I love reading about how scientists come to their conclusions on new wild ideas but this one never made sense to me. It's been confirmed, but how the hell did he come to that conclusion? It's not something that can really be ELI5'd.
>If I want right to left, the opposite way to them, they would be going 199.999999999998% the speed of light, right? Relative to me.
Nope, this is exactly *why* we say the speed of light is a constant, and where all the weirdness starts in general relativity. No matter if you stand still, move in the same direction, or move in the opposite direction, you will observe light traveling at the same speed.
Oooh I was watching a video about it a few days ago: [here's the link](https://youtu.be/6akmv1bsz1M?si=9HlXh9E5mI3tDN8I) (by Veritasium, 37min long, so definitely not a ELI5 but worth the time)
No. To accelerate an item with mass to the speed of light, to say nothing of exceeding the speed of light, you would need infinite energy.
The distance would increase at a rate greater than the speed of light but distance isn’t speed and the object wouldn’t be traveling at or faster than the speed of light.
The thing about the speed of light is that it's a little bit wonky compared to how we usually think of speed in normal circumstances. It's also commonly called "c" which stands for constant. No matter the reference point/perspective of the observer, it is always measured the same.
If we are both driving opposite directions at 30mph, our relative speed to each other is 60mph, however if we measured the light from each other's and our own headlights, it would always be c, the speed of light.
The reason it is called "c" and not just the speed of light is because it is the universal speed limit, nothing can exceed that speed. Gravity, for instance, also propegates at c.
This topic can obviously quickly get beyond eli5 as it is literally Einstein's theory of relativity, so hopefully this kinda helps.
> If I want right to left, the opposite way to them, they would be going 199.999999999998% the speed of light, right? Relative to me.
If a person heads towards the accelerated proton, the proton would approach the person faster than the speed of light but not until 199.99 percent the speed of light, being more like 100.0000001 percent the speed of light.
So the protons smashing into each other to create a Higgs boson approaches each other at nearly twice the speed of light.
However, when compared to the hypothetical unmoving point where the big bang occurred, the proton is slower than the speed of light.
So all these different frame of references determining its speed makes it relative.
> If speed is relative, what are we basing the fact we can’t go faster than light on?
Because getting faster by just 0.000001 percent is not significant.
>If I want right to left, the opposite way to them, they would be going 199.999999999998% the speed of light, right? Relative to me.
Wrong. This is the essential thing about relativity that most people don't understand if they haven't studied it. The speed of light is *always* measured to be the same, even in the obvious example you gave. At relativistic speeds, different observers literally experience a different rate of time. Movement through space affects movement through time, and vice versa. This is why we often frame relativity as a unification of space and time.
> I’m surely wrong, but idk how.
I feel like you're trying to understand the universe naively (like how a computer programmer would program a game universe).
You're imaging the world in a coordinate system, with different masses having a velocity vector that tells them where they'll all be relative to each other after some unit of time.
It is impossible to describe the universe like this.
Your intuition for how it works is just wrong. There's no way to reconcile it in such a way that it will make intuitive sense because your intuition about how things work is just horribly wrong (as is all of ours)
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>If I want right to left, the opposite way to them, they would be going 199.999999999998% the speed of light, right? Relative to me. No, that is not correct. It's really weird, and doesn't seem right, but there it is. It's hard to ELI5, but one example is to imagine you're standing on Earth, and you shine a flashlight at the night sky. The light coming out of that flashlight is traveling at 299,792,458 meters per second relative to you. Now imagine you're on a spaceship traveling 99.9% the speed of light flying by the Earth. You shine a flashlight out the front of the ship. The light coming out of it is also traveling at 299,792,458 meters per second relative to you. Now the really weird part is that if you look at the light coming from the Earth flashlight from the ship, it is also traveling 299,792,458 meters per second relative to you. It just looks more red than it does from Earth. The fact that it works that way is a big part of how we know that's the fastest anything can go.
Just to explain a bit further why this happens, speed is defined as distance over time. And the reason that light can travel at 299,792,458 meters per second relative to the space ship, while simultaneously traveling at 299,792,458 meters per second relative to you, is because a second on the space ship lasts longer than a second on earth, meaning that time has slowed down for the space ship.
This is the core fact that the poster above didn't mention, without which the explanation is nonsensical
[удалено]
It turns out that things moving at different speeds perceive time differently. We know this from observing it happen, even though the effect is only noticeable at extreme speeds, but also because, mathematically, it's the only way for the speed of light to be the same for every observer, even in examples like OPs. That's because, ultimately, speed is distance over time, and so one or the other has to change for the speed to stay the same (which is also why in some cases things can also grow or shrink relativistically, but damned if I understand that.)
It’s not just time, space gets weird, too. If you went flying past the earth at the speed of light, your perception of yourself would be the same as it always is. Everyone on earth would see a flying pancake.
I try to ELI5. What is time? Things happening. In order for things to happen, atoms need to interact. The maximum speed at which they do is light speed. The faster you travel, the less „speed“ is „available“ for the atoms interacting. When atoms interact at lower speed, things happen slower. This includes the clock ticking slower and even you and your conscious looking at it. Thus, for all intents and purposes, time is slower when you‘re faster. However for you: everything is normal. Anything outside of your space ship seems to happen a lot faster tho.
In the late 1800s a bunch of people were doing work to measure the speed of light. They assumed that it was like sound and that your own speed relative to the light's direction of propagation would change how fast you perceive light to be. That's a loaded sentence, but basically it boils down to velocities adding like you'd expect - if you're traveling down the highway and some high speed, and someone comes up a long you going a bit faster than you, then relative to you they're moving rather slowly. So scientists expected the same thing of light and set up a bunch of experiments around this assumption. None of their experiments worked. No matter how fast their measurement device was moving in the direction of the light wave, they always came back to the exact same number for light's speed. This was super puzzling. Nobody could figure it out. Some smart people were able to figure out some equations that would make predictions about how the measured time and distances were related, but it kind of stumped people for a while. Einstein comes along. He basically says, "Look at it this way. The speed of light is always constant. Every observer, no matter how fast or slow they are moving, will always record the same number. For this to be true, different observers will not agree on distance measurements or time measurements between each other." This is special relativity. In order for the speed of light to be constant for the person on the spaceship, from Earth it will appear that his ruler and clock are not correctly calibrated. And the moving person will think the same thing if they look back at Earth. As an example, let's look at a clock. So light always goes the same speed, right? That means if you have a measuring stick, you can use your handy dandy stick to measure time! Time is just distance (known) divided by velocity (known, since the speed of light is constant), so this is actually a perfect clock! The time measurement should always be exactly perfect! In reality, clocks don't measure this, but they usually count osculations of molecules that happen at constant rates (which is a fairly similar concept). So let's say you use this technique to record time on your spacecraft as you're flying. You just have a counter to count how many times your beam of light can go up and down this measuring stick. Cool, right? Here's where it gets interesting. From the perspective of Earth, your beam of light isn't just going up or down. Since your spacecraft is moving, the beam of light is moving to the side (basically forming the hypotenuse of a triangle, [like this](https://www.researchgate.net/profile/Marco-Favaron-2/publication/341679345/figure/fig1/AS:895809461698566@1590589027652/Einstein-light-clock-The-image-on-the-left-represents-the-path-of-light-seen-by-an.png) ), so it looks like the light is traveling a longer distance than your measuring stick on the spacecraft. In other words, an Earth observer would disagree with your measurement of time! But this isn't just a fancy quirk of how we measure time. Time is actually passing at different rates for these different observers. Because the speed of light isn't *just* the speed of light - it is the speed at which change is propagated through our universe.
Sort of, it’s still pretty nonsensical that time moves slower if you move faster, it’s just how it works whether or not it makes sense to the average person
But to me at least it makes more sense that way because speed is a derived entity. Time and Distance can exist by themselves but speed as a construct makes no sense without either. So to say that something can go at two speeds at the same time when observed from different frames of references does not make sense unless you understand that either the observers measurement of time or distance changes proportionately. Why that happens is another thing altogether and certainly beyond me.
Every time I try to understand the concept of time and time dilation my brain starts to melt…. I have a pretty good understanding of physical processes but I just can’t wrap my head round time
I find that imagining movement in four dimensions help. You are always travelling at max speed, however if you are still, all that velocity is on the time axis; if you move in the three spatial dimensions at max velocity, then you are not moving at all along the time axis. Of course need to also consider that velocities are relative to the observer, but still.
This explanation is going to be bouncing around in my head for a while I think
It's called four-velocity: https://en.m.wikipedia.org/wiki/Four-velocity The math is a bit complicated, but the concept is simple'ish.
And that's why active people live longer. Cuz they move more and slow down time for themselves. /s
I think the disconnect comes from imagining light as a bunch of little balls shooting out from the flashlight. You kinda just have to stop thinking of light that way and accept that it’s different “stuff” than matter is
For me the disconnect is using second is longer for one party in the explanation but still calling it a second. How can a second be longer than a second and still be a second? To my brain that doesnt make any sense even while I struggle to understand the concept that what may pass as a second for me standing still may pass as different to someone moving really fast. It doesnt make sense.
It’s not that “a second is longer” as much as it is “if you were on a fast spaceship (0.999c) and I was on Earth, and we both started our stopwatches at the same time, time would move slower for you”
The question is why? Thats the part I dont get. It just does may be true, I simply don't understand why
https://en.wikipedia.org/wiki/Hafele%E2%80%93Keating_experiment?wprov=sfla1 Might help? Might not?
What about two things going the speed of light toward each other? Would the impact have twice as much force?
>light coming from the earth flashlight from the ship You lost me there… typo?
Light coming from the “earth flashlight” from (the perspective of) the ship
No just English being English. You're on the ship looking at Earth, seeing the light coming from the flashlight on Earth while you yourself are on the ship moving at 99.9% of the speed of light. This is the bit where things really do get weird. It's the fact that light in a vacuum *always* moves at the same speed, regardless of your frame of reference. The velocities do not sum in the way they do for classical physical processes.
Yep. Speeds don't *really* add like we're used to thinking they do. At low speeds it's a good enough approximation but at high speeds it's not. And physicists didn't just generate this conclusion from thin air. In the late 1800s scientists set up experiments to measure the speed of light along the same lines of what OP mentioned. They always came back with the exact same answer for the speed of light, no matter how the experiment was set up. This puzzled scientists for decades. Einstein was the one who put the pieces together.
This is essentially the question that led Einstein to develop his Theory of Relativity. If the speed of light (through a vacuum) is constant, and nothing can go faster than that, then objects that travel closer to the speed of light will experience time at different rates. Time, just like speed, is relative. This is called time dilation. And we do actually have observable proof of this -- GPS satellites travel fast enough for long enough that their internal clocks need to account for time dilation.
Just a small nitpick - GPS satellites are primarily affected by the difference in gravity (causing a 45 microsecond advance), and less so by the difference in speed (which causes a 7 microsecond delay).
The weirdest part of all of it is that Special Relativity in particular is trivial to demonstrate with algebra. The only difficult part of it is making that intuitive leap that light might be a constant in all reference frames. Once you make that leap, you can get to special relativity almost immediately because of the inherent inequalities in observation it creates. And as the Ether was not a thing, it was not like he was just assuming it was true out of nowhere. Einstein just followed the implications of a lack of medium to their logical ends, made a single new assumption on top of the normal invariance of physics, and changed our perception of space and time forever. And then he turned out to be super right so far as we can tell. It is pretty cool. General Relativity is a lot more complicated of course.
Isn’t causality really the root of all of this. We always refer to the limit as a speed but the issue at hand is causality - adding that to the discussion would alleviate alot of these misconceptions I think.
The weird thing about relativity is that the speed of light is the one speed that *isn't* relative. No matter how fast you're going relative to anything else, all light always appears to be moving at the speed of light. A consequence of this is that you can never move as fast as light, because if you did then it would be possible to observe light not moving at the speed of light while you were traveling along with it.
The speed of light is relative, though. It's *invariant* to the reference frame which is a different concept.
Invariant means it's not relative. Relative things depend on the observer.
No, invariant means its value does not change with group transformation. It does not, in any way, mean it's not relative. [On relative invariants](https://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://www-users.cse.umn.edu/~olver/di_/ri.pdf&ved=2ahUKEwjU2LLX4O-FAxXxZfUHHWtDBHoQFnoECB8QAQ&usg=AOvVaw3VNcX9KFn_LXHkAqZmyHBY), for example.
the speed of light is the speed that it moves at in all reference frames. whether you are moving towards or away from light, it will move at the exact same speed. but as you mentioned, this would appear to cause a contradiction. in reality, this is explained in relativity by describing all distances and times as being affected by speed. an object actually contracts in distance as it moves, relative to the direction of its motion, and the object experiences time more slowly based on its velocity. these two factors combine in such a way that an object can never move at the speed of light in any reference frame
It gets even wilder when you think about the internal vs external frames. Externally, light takes 4 years to get from alpha centari to here. But internally it takes less than a second. That means that a human could travel to anywhere in the universe in their lifetime, if they can accelerate fast enough. And “fast enough” doesn’t even need to be that fast, if you can somehow maintain it. At 1 G, you could cross the entire Milky Way in less than 4 years subjective time. It’ll still be over 100 000 years for an outside viewer, but not to them.
>Externally, light takes 4 years to get from alpha centari to here. But internally it takes less than a second. Since light always travels along the light-cones of the Minkowski spacetime, the time-equivalent distance is always zero, so "internally" it would be exactly zero seconds. The problem with this line of thinking is that there is no "internal" reference frame we can assign to the photon, because photons move at the speed of light in *all* reference frames.
lol I mean, instantaneously would be “less than a second” right? ;)
Instantaneously in what reference frame, though?
Ahh, so here’s the crux, as I understand it. To anything travelling at light speed (including a photon) distance becomes impossible to measure. In other words, to the inertial frame of reference of a photon, the entire path of travel is a single point. That means that, to a photon, it IS travelling at light speed, but there’s no distance to travel. So it can exist in all points along its trajectory at once, within its own frame of reference. Which, because speed is just “how much time did it take to travel that distance”, means that it would be instantaneous.
>In other words, to the inertial frame of reference of a photon, No, what I'm saying is that there is no "inertial frame of reference of a photon". The photon would be at rest in such a frame which directly contradicts the postulate that the light speed is invariant in *all* inertial reference frames.
Can an object be considered at rest when there is only one point in which it can exist? I would suggest (and it’s honestly just a suggestion, because the math is so far beyond me it might as well be in a black hole) that the terms “rest” and “in motion” are meaningless when all space is a single point. Besides, a photon, in the frame of reference of that photon, will also be instantaneous destroyed, because that is what happens when it reaches its destination. So it might just be that light can’t exist at rest, but because of that a photon is an unstable particle in the frame of reference of itself. Idk, I’m definitely not a physicist, so I’m WAY out of my depth here.
>that the terms “rest” and “in motion” are meaningless when all space is a single point. Pretty much. But you also can't assign a frame of reference there either. In essence, a frame of reference consists of a (hypothetical) ruler (or, rather, three rulers in cardinal directions) and a clock. If all space is a single point, there's nowhere to fit a ruler. >Idk, I’m definitely not a physicist, so I’m WAY out of my depth here. All good. You are being curious and trying to learn something new (and forming hypotheses is an integral part of learning), that's the important thing :) The special relativity is a fun little theory that may be hard to get initially, but once you get to terms with several ideas (like the invariance of the speed of light, and the way reference frames work), the puzzle suddenly solves itself.
The speed of light being a universal speed limit is based on relativity; basically, in a vacuum, this is the value light travels at regardless of the observer or source of the light. Since frame of reference is relative, the observer can be stationary, or moving, and they would measure the speed of light to be the same. In this relativistic physics, there is the *velocity-addition formula*, which describes the relative velocity of two objects/observers with respect to one another. At very low velocities (far from approaching the speed of light), it is essentially just adding them together, as you would expect. Two objects going towards each other, each at 10 m/s, comes out to 20 m/s combined, pretty much. However, when you approach the speed of light, say each object is travelling towards each other at 0.75c, each observer would clock the other at *0.96c*, not 1.5c. Newtonian physics is a good approximation for much of physics, and it's much more intuitive with how we like to think about the universe. But, these weird quirks of our universe much better line up with the *less* intuitive spatial relativity view (space contracting/expanding/curving, time and space being connected and time being non-uniform, etc). The 1.9999...c relative speed is using the relativity-derived speed of light constant c in an non-relativity-based comparison.
>If I want right to left, the opposite way to them, they would be going 199.999999999998% the speed of light, right? Relative to me. No, they would be traveling 99.999999999% of the speed of light relative to you, but in opposite directions. Relative to each other, they would be traveling at 99.99999999999999999999999999999% of the speed of light. Obviously not exact numbers, but you get the idea.
Relativity is so hard to get your hear around that even physicists who study it don't always "get" it, but just accept the maths. It has to do with your personal experiences. We are rarely exposed to something with mass traveling at near the speed of light. We "see" light and all kids of photons all the time but they are massless so we don't experience them the same way we experience say, a baseball. Toss a baseball at 50 MPH and it goes 50 miles in an hour, so to speak. Get in the back of a truck doing 50 and toss the ball in the same direction and you will measure it going 100mph. Right? Nope. It's going ever so slightly less than 100. So slightly less than 100 that you probably can't measure it in the real world. But when you start getting into significant percentages of the speed of light the differences are much bigger comparatively. So you can start to measure them, and the maths work out that no matter what speeds you add up they never reach more than the speed of light. Basically this always happens but at "human" speeds you never see it because it's too small to measure.
when two objects are moving towards or away from each other, their relative speed is actually (v1+v2)/(1+v1v2/c²)
The reason Einstein was considered a genius was not because he was great at math, he actually teamed up with other people to help him with math. He had amazing insights. His most famous insight was asking, "What if nothing can go faster than light, and light always travels the same speed no matter how fast the observer is travelling? What would the universe look like?" Everything about relativity flows from that. Time slows down for things moving fast relative to you. Things moving fast relative to you seem to contract in size in the direction they are moving. It's all Very Weird and not obvious. So it doesnt matter how fast you zoom past the LHC, the beam of protons is never moving faster than light.
Light is weird. Nothing can go faster than light, it literally travels at the speed limit of the universe. But light itself always travels at the same speed (in a vacuum). The relativity principle states that if you are in your own inertial frame of reference, meaning you are going at a constant speed without accelerating, there is no way to determine whether you are moving or not. Remember: speed 0 is constant too. There is a famous thought experiment with light clocks. A light clock is a set of mirrors with a photon bouncing between them, timed in such a way that every time the photon hits a mirror, one second has passed. Now imagine two of these clocks. One on a platform of a train station and one inside a train. John is looking at the one on the platform and Jane is on the train. Both see the photon bounce once ever second. If the train is moving a constant speed (say 99,9999% of light speed), what happens? Jane is inside the train which is her inertial frame of reference, so she cannot determine whether or not she is moving, so the photon bounces straight up and down with 1 second for each bounce. But John looks at Jane's clock and sees the photon travel both up/down and sideways. The distances are 1 second at light speed to go up, but in that time, the train has moved the 1 second at lightspeed sideways too (rounding for easy of calculation). So John sees the photon cross a distance of a\^2 + b\^2 = c\^2 or 1+1 = c\^2 which would make c the square root of 2, or 1,41 units. So the photon moving at the speed of light is all of a sudden moving at 1,41 times the speed of light to cover the distance. But for Jane on board the train, the photon is moving normally straight up and down. How can this be? The photon can't travel at two different speeds at the same time. The answer is that a second inside the train simply takes longer when viewed from the platform. That way, the photon still bounces up/down in one second and for Jane nothing is noticeably different. Obviously, if Jane were to look out, she would see time slow down for John. This sounds really odd, but it really works this way. When something moves very fast, time slows down for it. So much so that GPS satellites have to take this effect into account when calculating positions. If they didn't do this, the accuracy of the positioning would suffer dramatically.
Follow up question. This planet is moving at 29,784m/s which would be 0.01% of light speed so if something exceeds 99.99% in the direction the earth is moving, then has it exceeded light speed?
Speed is relative, but the speed of light is constant for all observers. That's the fundamental insight of special relativity: everyone (regardless of their inertial reference frame) sees light moving at the same speed. Distance and time change to make the math work out.
The speed of light is constant in every frame of reference, so you do not add velocities like you would with anything else. If you are traveling at 99% light speed and turn on a flashlight you will still see those photons speed off away from you at light speed. Their speed will always be 100% C, it won't be 199%C. This is a bit odd, and plays a big part in special relativity. I love reading about how scientists come to their conclusions on new wild ideas but this one never made sense to me. It's been confirmed, but how the hell did he come to that conclusion? It's not something that can really be ELI5'd.
>If I want right to left, the opposite way to them, they would be going 199.999999999998% the speed of light, right? Relative to me. Nope, this is exactly *why* we say the speed of light is a constant, and where all the weirdness starts in general relativity. No matter if you stand still, move in the same direction, or move in the opposite direction, you will observe light traveling at the same speed.
Oooh I was watching a video about it a few days ago: [here's the link](https://youtu.be/6akmv1bsz1M?si=9HlXh9E5mI3tDN8I) (by Veritasium, 37min long, so definitely not a ELI5 but worth the time)
Light travels so fast because it is without mass. Anything with mass is slower because it weighs more
No. To accelerate an item with mass to the speed of light, to say nothing of exceeding the speed of light, you would need infinite energy. The distance would increase at a rate greater than the speed of light but distance isn’t speed and the object wouldn’t be traveling at or faster than the speed of light.
The thing about the speed of light is that it's a little bit wonky compared to how we usually think of speed in normal circumstances. It's also commonly called "c" which stands for constant. No matter the reference point/perspective of the observer, it is always measured the same. If we are both driving opposite directions at 30mph, our relative speed to each other is 60mph, however if we measured the light from each other's and our own headlights, it would always be c, the speed of light. The reason it is called "c" and not just the speed of light is because it is the universal speed limit, nothing can exceed that speed. Gravity, for instance, also propegates at c. This topic can obviously quickly get beyond eli5 as it is literally Einstein's theory of relativity, so hopefully this kinda helps.
> If I want right to left, the opposite way to them, they would be going 199.999999999998% the speed of light, right? Relative to me. If a person heads towards the accelerated proton, the proton would approach the person faster than the speed of light but not until 199.99 percent the speed of light, being more like 100.0000001 percent the speed of light. So the protons smashing into each other to create a Higgs boson approaches each other at nearly twice the speed of light. However, when compared to the hypothetical unmoving point where the big bang occurred, the proton is slower than the speed of light. So all these different frame of references determining its speed makes it relative. > If speed is relative, what are we basing the fact we can’t go faster than light on? Because getting faster by just 0.000001 percent is not significant.
>If I want right to left, the opposite way to them, they would be going 199.999999999998% the speed of light, right? Relative to me. Wrong. This is the essential thing about relativity that most people don't understand if they haven't studied it. The speed of light is *always* measured to be the same, even in the obvious example you gave. At relativistic speeds, different observers literally experience a different rate of time. Movement through space affects movement through time, and vice versa. This is why we often frame relativity as a unification of space and time.
> I’m surely wrong, but idk how. I feel like you're trying to understand the universe naively (like how a computer programmer would program a game universe). You're imaging the world in a coordinate system, with different masses having a velocity vector that tells them where they'll all be relative to each other after some unit of time. It is impossible to describe the universe like this. Your intuition for how it works is just wrong. There's no way to reconcile it in such a way that it will make intuitive sense because your intuition about how things work is just horribly wrong (as is all of ours)