I love the movie Interstellar but I have never fully understood how time dilation works. More recently reading “Project Hail Mary” this term came up again and I went on a Wikipedia binge trying to understand how it works.
How can time be different based on how fast you travel? Isn’t one second, one second everywhere? (I’m guessing not otherwise there would be no time dilation) but I just don’t understand what causes it or how to wrap my head around it
In: Physics
Any physics you get from movies, take with a grain of salt.
So here’s the thing:
The speed of light in a vacuum is always constant no matter the reference frame.
This results in other values we believe are constant, distance and time, to be observed differently depending on their reference frame.
None of these effects are readily visualized because they only start to become significant at high speeds.
It’s a consequence of the speed of light being the same everywhere. At non-relativistic speeds, velocity is basically additive; if you are on a train going 50 mph and you throw a ball at 10 mph, for outside observers not on the train, the ball is going 60 mph.
It doesn’t work like that for light. If you are going on a train going 0.5c (c is the speed of light), shining a flashlight forward doesn’t make that light go at 1.5c for someone not on the train. It just *always* goes at c.
And if the distance / speed can’t be modified, then the only thing that can change is the time it takes. Because outside the train it looks like the light travels farther (but again, the speed of light is constant), it **has** to take more time than it does inside the train.
The experience of one second would be the same for a person on the train or not, but relative to each other it’s not the same.
I’ve found the easiest way to explain this is using a [light clock example.](https://www.youtube.com/watch?v=-O8lBIcHre0)
It certainly is tricky. You have to start by accepting that the universe does not always have to make sense. We humans are used to how certain things behave because we are quite big and quite slow. This does not mean that reality behaves the same way when something is really, really small or goes really, really fast.
There is a rule in physics called the principle of relativity. This states that there is no way to tell the difference between a thing standing still and a thing moving at a fixed speed from the inside. Like in an elevator: you feel it when the elevator speeds up or slows down (not fixed speed) but while it is traveling at a fixed speed, it is very hard to tell the difference between moving and not moving.
Okay, so given that this principle is true, imagine a light clock. This is a device with 2 mirrors facing each other and a photon bouncing up and down between them, forever. The clock is constructed so that every time the photon hits a mirror, one second has passed.
Now, someone puts that clock on a train and the train starts moving until it goes really, really fast, almost as fast as the speed of light. Remember that once it is moving at that speed, it is impossible to tell the difference INSIDE the train between moving and standing still. The photon just happily bounces up and down at 1 bounce per second.
Imagine standing on a platform watching that train go by in the distance. (imagine this is all possible). When you look inside of the train, you see the photon bouncing up and down, but also moving sideways through space (since the train is moving sideways). So from the perspective of the platform, the photon travels like this “/ / / “.
Pythagoras’ theorem tells us that the hypothenuse of a 90 degree triangle is a^2 +b^2 = c^2. So if the train is traveling at nearly the speed of light, in one second, the photon has traveled 1 light second vertically and one light second horizontally, so 1^2 + 1^2 = 2^2 or the square root of 2 or *1.41 light seconds per second.*
The photon covers a distance in 1 second that should have taken it 1.4 seconds. Remember that from the perspective inside train, the photon is just bouncing up and down like normal so it is traveling at 1 light second per second.
But how can a photon a) travel faster than the speed of light, and b) travel at different speeds at the same time? The answer to both questions is “it can’t”, so the only solution, no matter how unintuitive it seems to us, *is that a second simply takes longer when the train is moving.*
Again, this makes no sense to us who move at a few 100 km/hour but reality does not have to make sense. The conclusion is inescapable. Inside the train, a second still takes a second since it is defined by the photon bouncing, but outside the train looking in, we see that time in there moves slower. Just because the train is moving.
This effect is very real. GPS satellites have to compensate for this effect to remain accurate, for instance.
You are always moving through space-time at a constant rate.
The faster you move in one, the slower you move in the other. So to balance, as your speed increases the time you experience decreases leading to time dilation. Conversely, when you are at rest then you are moving through “time” as fast as possible.
You can imagine that time is just distance in the next dimension up from where we see things.
So you can draw a line where every inch is a minute.
Now imagine a map of mountainous terrain. You can draw a line straight across a mountain that’s 6 inches long.
Your can then draw that same line on flat land.
The top down view is normal time. Everybody’s line is equal to the same amount of time
Now imagine looking sideways at the line on the train. The flat terrain line stays the the same distance but the mountain terrain line gets way longer because it has to stretch up in the air and then back down.
This is line dilation and the line represents time. So from a top down view you had the same amount of time, but on the ground, one time took longer to go the same distance.
Mountains in the this analogy are gravity wells or relativistic effects
We are traveling through both space and time simultaneously. The basic idea is that the faster you are traveling through one, the slower you are traveling through the other. You perceive the passage of time from whatever frame of reference you are in as being the same. So a second would still feel like a second whether you are at a near standstill or at 90% of light speed and the watch you were wearing would still tick the same as far as you are concerned. It’s everything else outside of your frame of reference that appears to be ticking faster or slower relative to you.
It helps to not think of time as some universal unmaliable property. There is an reason why we call it “Space Time”, that is because space and time usually are connected together always, when you bend space (gravity) you also end up affecting time. This is why time is the 4th dimesion, with 3 different space dimensions.
Right now, no one knows why time actually warps when observers compare clocks. All we know is that if it didn’t work this way, the universe would be completely different place.
So thus when you move fast towards the speed of light, you experience time exponentially slower in the perspectives of whatever your measuring your speed to, while you always experince time at a normal rate to yourself.
Also no matter what reference frame you compare, the speed of light or the speed limit is the same.
When looking at things that goes at the speed of light such as a photon, we see them go at max space speed, but zero time speed.
Photons (light) also very rarely actually reaches the speed limit of the universe as it gets slowed down in many cases. Which is why scientists have calculated photons have an life span of on average of 3 years, but to us non speedsters, they last for a billion billion years.
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