ELIF: how is time relative?
In: Physics
Okay, this is a hard question to answer in a satisfactory way since the idea of time being relative is against all of our intuition. However the conclusion came from some rather intuitive assumptions – those assumptions just lead to some wild conclusions. Let me explain.
Special relativity (the branch of science that explains how time can differ for non accelerating reference frames without strong gravity) was highly motivated by some weird results that physicists got in their study of electricity and magnetism. Today we use 4 equations to describe the laws of electromagnetism. We call these Maxwell’s equations. These equations describe how electric and magnetic fields behave in the presence of charge. If you combine them in a certain way, you get something called a wave equation.
Now a wave equation has certain properties. It showed that electric and magnetic waves can be created, and that they travel at a certain speed that we’ll call C. It turns out that this speed matched closely with light. Thus we concluded that light was an electromagnetic wave. However, and this part is important, one peculiar thing about the equation is a speed popped up without there ever being a reference frame explicitly. That is, the equation said the waves traveled at speed C, but it wasn’t clear who or what was measuring that speed. In other wave equations, it was always obvious from context – such as waves in the surface of water moved relative to the still surface of the water – the median through which the wave propagated. No such median was known to exist for light.
Now scientists resolved this by claiming light traveled through some unseen median called the ether. They tried to look for this ether in experiment but failed. Instead, Einstein came in and claimed something else. He claimed that if you were still, or if you were moving in a car at a constant velocity, the laws of physics would be the same. Now, we intuitively can see this. Throw a ball up in a car, and it will come back down the same way as if you were still. But Einstein extended this idea to light. Measure the speed of a beam of light, then you will get the same answer whether you are in the car or not.
However, that conclusion immediately contradicts are intuition. Say, I’m traveling on a spaceship going half the speed of light and I shine a flashlight in front of me. For me, the light would be going 1C. For someone outside, we would intuitively think they would see the beam of light traveling at 1.5C, since it’s in a spaceship that is travelling at .5C. Einstein says that’s wrong. He says that the beam of light looks like it’s traveling at 1C to both users.
Say for the person in the spaceship it takes one second for the light beam to hit the front. How does that look for someone in the outside? The light moves the same distance in that time, but the front of the spaceship also moves. So to them, the light takes longer to reach the front. IThis is weird to us. Because we insisted that there speed of light is the same for all viewers, something else has to budge. And since velocity is distance over time, the thing that budges is how we measure both distance and time.
It turns out to reconcile the reference frames, things that are moving relative to you appear to slow down. Moving things appear to shrink as well.
This is not an easy concept to grasp, because it is not evident in our day to day lives. Hopefully this gives you motivation to understand why time has to budge.
Your combined speed in space and time is constant, so if you move really fast through space time slows down for you.
Easiest method, listen to a few heavy metal songs (or other high tempo music) until you can comfortably keep the tune in your head and follow along, almost natural and not fast at all you would think.
now, listen to classical music or other low tempo music, it will seem dramatically slow. keep listening until it feels regular speed and comfortable to follow.
Now, listen to the exact same heavy metal/high tempo music, it will be retardedly fast.
Time hasn’t changed, your perception of it has. Time is relative to your perception of it, your brain can speed up that perception if there are too many things coming at you (high tempo) and you only need to take in the important parts (melody of a song) making time appear fast. Likewise, it slows if there are very little things coming at you (low tempo) and your brain has enough time to analyze every detail of seemingly detail-less things. Making time appear slow.
This can also be viewed as how older generations feel like their time is fleeting and just whizzing by. They arent open to learning new things, experiencing the finer details of life because they’ve seen and done a lot of it already, so time appears to move by them at a fast pace.
When you are younger, everything is new, your brain is constantly learning even if you don’t know it is, new school? new people, new smells, new lighting, new everything. Yout brain is on overtime trying to analyze it all, which makes time appear dreadfully slow. Ever wonder why summer vacation as a kid felt like a small eternity but as an adult it just passed by like a missed opportunity?
This is why.
So, the speed of light is a constant. I’m not sure that ELI5-ing the reason for this is possible, beyond just saying that photons have a mass of zero. Anyway, say you build a clock that abuses this. It’s a tube with mirrors on the ends with a single photon inside of it, bouncing up and down. It measures an exact time based on the amount of bounces of the photon, since you know how long it is. This is all well and good.
Now, say you’ve got 2 people. One of them stands still, and the other puts the clock in his truck and speeds away. To the stationary observer, each time the light bounces, it has to move at a diagonal to reach where the other mirror will be as it moves. Diagonals are longer than straight lines, and the speed of light is constant, so the clock is now runny slow to the stationary observer. However, to the observer in the truck, the clock is relatively at rest, and the rest of the world is moving. For him, the light doesn’t have to take a diagonal, so his clock reads like it’s still correct, not slow. So, when the driving observer experiences as a second, the stationary observer will experience more than a second. In general, an observer that is moving and eventually stops will have experienced less time than someone who was still during this period.
Another way to think of it is that your *perception* of time is relative. Similar to how, if we’re both sitting in a moving car that’s moving at 60mph, we would appear to be sitting still.
I don’t know.. it all seems kinda off to me. Time dilation dictates that because of higher gravetational forces, the core of the earth is two years younger than the crust. But their both the same age. All gravity seems to do in my mind if affect rate of change, movement and reaction. Time is just a concept born of perception. It to me is a fixed dimension. It is always now, universally, everywhere. If you approached a time dilation phenomenon, then it exists in the same “time frame” as we do. But go inside it and your in the past or future (depending on which way you swing the gravity, higher slower, less future?) So now you have have mass and energy existing in two time lines at once, doubling matter and energy, and existing simultaneously in one overall timeline. Bs. The only thing that changes is perception, rate of change by way of movement or reaction.
I also am no genius, and am surely wrong and confused, but it’s what makes sense to me.. and it doesn’t matter a lick to my life if I have it wrong or right.
Imagine a space ship is about to leave on a voyage. It sets its clock to Earth time and by magic (or via a radio transmitter) the captain can always see both the earth clock and the ship clock. As the ship gets further from earth the captains can tell the difference between the clocks since light travels at a finite speed. At slow speeds the clock on Earth lags a little behind the ship clock, but they still go at the same pace. So far so sensible. The captain can tell how far away the ship is from Earth too.
Now lets say the ship is travelling away from Earth at a reasonable fraction of the speed of light. Things are a bit different now. The time the light takes to travel from the earth clock to the captain is increasing as the ship gets further from earth. Not only is the Earth clock lagging behind the ship clock but the increase in lag (over time) is noticeable. To the captain Earth’s time has slowed down. The impossible limit is when the light from the Earth clock never catches up with ship because the ship is going at the speed of light. To the captain the earth clock has stopped.
And then there is general relativity to complement special relativity (above) when gravity slows time down (give or take)…
think of spacetime as an X/Y graph
x is space, y is time.
all objects move through spacetime at C (the speed of causality) which is the maximum velocity that the effects of a cause can propagate through spacetime.
if an object is only moving through space, then its velocity through the X axis is C: this is a photon,
if an object isnt moving through space at all, then all of its velocity is through the time axis. (this is you sitting at home bored, time just flies)
but an object that is moving rapidly through the x axis necessarily has to subtract velocity from the Y axis, and the faster you move through space, the slower you move through time, because your “actual” velocity through spacetime is fixed at C.
A lot of the answers here are explaining why time is relative in an observation of events sense, which isn’t what the OP is going for. I assume they want to know about the theory of relativity, so I’ll give it a go:
Imagine you’re driving a car at 30mph, and another car overtakes you at 60mph. Common experience tells us that someone standing stationary at the side of the road will observe the overtaking car going at 60mph, whereas from your perspective it is going at 30mph.
Now imagine that you are driving a car at 30mph (13.4m/s) and your shine a light. From your perspective the light is travelling at the speed of light ( 299 792 458 m/s). Common experience tells us that the person at the side of the road will see that light travelling at 30 mph + the speed of light (299 792 471 m/s). But that doesn’t happen, both observers see the light travelling at the exact same speed (299 792 458 m/s). Einstein’s theory says that the speed of light can never be broken no matter what frame of reference you measure it from. That means if someone is another car travelling at half the speed of light, the light shone is still travelling at 299792458 m/s relative to them.
So how does this work? Well, speed is distance over time – so for speed to be fixed distance or time have to change within your frame of reference. Turns out both do depending on your reference frame. But for the car, time slows down relative to the guy who is stationary. The effect is very negligible until you reach speeds close to the speed of light.
It’s not just a theory, it’s been verified using atomic clocks.
As to how/why this works? I have no clue, I’m not sure anyone does.
Basically the faster you move, the slower time moves around you. To achieve any real noticeable time dilation you need to achieve massive speeds. As an example if we take the earth moving through space as a constant. a person at the equator will be moving, due to earths rotation, at 1000 miles per hour. The ISS travels around the earth at about 17,000 mile per hour. This 17x speed increase creates a time loss of .007ths of a second every 6 months. This dilation grows exponentially until you get up to lightspeed, where time no longer moves.
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