The theory is if you go near the speed of light then the time will move much slower than someone that is stationary but it is also stated that speed is relative, which seems to contradict this theory. To the person moving near speed of light in space will see the stationary person moving near the speed of light and vice versa, so how does time change happen between them?

In: Physics

Someone goes passed you moving at 3/5ths the speed of light.

Special Relativity tells us that their times (and lengths) are altered by that (by a factor of 4/5, at that speed). For every minute you experience, they only experience 48 seconds. If they should be 1m long (in the direction of relative motion), they are only 80cm long.

But as you say, from *their* perspective they are stopped and it is *you* who are going past them (backwards) at 3/5ths the speed of light.

From their point of view, for every minute they experience you should only experience 48 seconds, and you should be squished by a factor of 4/5ths in the direction of relative motion.

So who is right?

The answer – counter-intuitively – is both of you!

This is a version of [the twin paradox](https://en.wikipedia.org/wiki/Twin_paradox) – a famous thought experiment (that has been performed for real, just not with twins) in Special Relativity.

It turns out that each perspective is perfectly valid and fine. From each point of view it is the other person who experiences these effects, but crucially they cannot get back to each other to compare notes without accelerating, and accelerating messes with the effects (and the numbers all end up working out).

As for how? “How” and “why” questions are a bit tricky in physics, but roughly speaking, when something accelerates it twists space and time around it. We know that when something changes speed its idea of “here” shifts (because it is “moving” compared with how it was before), and it turns out things’ ideas of “now” also shift. Two events can happen “now” for you, but minutes apart for someone else.

Which can help explain things like length contraction. Famously you can fit a 5m long ladder inside a 4m long barn if you make it travel fast enough (at least 3/5ths the speed of light). But you can only do that if the barn is open at both ends; from the barn’s point of view when the front of the ladder gets to the back of the barn, and when the back of the ladder finally enters the front of the barn happen at the same time. But from the ladder’s point of view these two events happen at different times – the back of the ladder doesn’t enter the barn until the front of the ladder is nearly 2m out past the back!

Time is relative too! If you’ve seen the movie “Interstellar”, you might remember a scene where the astronauts spend a few hours on a planet, but when they return to their ship, the crew mate they left behind has aged over a decade. This is an exaggerated example, but that’s the concept.

So different objects (people) can literally move through time at different speeds.

The reason *why* this happens is very complicated, but at a fundamental level, it turns out “space” (the thing we move through when we talk about the speed of light) and “time” (the thing we *usually* move through at a roughly constant rate when we talk about getting older) are *the same thing*: spacetime. So when you change the way (i.e., the speed) you’re moving through space, you’re really changing the way you’re moving through *spacetime*. Make a big enough change, like moving close to the cosmic speed limit (speed of light), and you’ll start to notice changes in the way you move through time.

You don’t even need to think about this in terms of moving at the speed of light. If you were to have two identical clocks and set them to the same time, then place one where it’s stationary and the other in your vehicle, then drive as you would normally day to day, you’ll find that the over time the clock in the car will lose time as compared to the stationary clock. It’s an example of this effect that many people experience.

If you throw a ball on a plane, from your point of view the ball is traveling at less than 100mph. But observed from outside the plane, the ball is traveling at that speed plus the speed of the plane. Potentially 600 mph in total on a commercial airliner.

Speed is relative to your position. Time works the same way. Your watch is moving at 1 second per second from your point of view. But to somebody traveling much slower, your watch would appear to be moving much faster.

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