# How does something age slower while moving fast in space

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I’ve seen people explain it before but I have absolutely no idea what they are talking about. An example of what I’m saying would be the movie interstellar or the old planet of the apes movie. It makes absolutely no sense.

In: Physics

Time dilation. Faster you move thru space, slower you move thru time. Don’t think ELI5 would be best place for even introductory relativity

They don’t age slower it just the relative passing of time is different it’s like this sunrise it’s the same sun for everyone but but relative to each other it arrives at different times hence we have time zones

First, consider how things move relative to one another.

If I’m standing still and a train passes me, from my point of view I’m stationary and the train is moving. What about a passenger on the train watching a second passenger walk down the aisle? From the first passenger’s POV, within the train they themselves are stationary, while the second passenger is moving.

Point is, movement is measured differently from different perspectives, and there’s **no one true perspective**. This is classical relativity. Objects can be described mathematically as moving at different speeds depending on who is looking (i.e. the frame of reference).

BUT scientists discovered that **light has a constant speed** no matter what perspective it is observed and measured from. How can that be when speed depends on the observer?

Einstein realised that light can remain constant if time itself moves at different speeds. He proved that the faster something moves through space, the slower it moves through time. This allows the speed of light to remain constant, no matter the reference frame / observer.

It’s weird stuff to wrap your head around, but hope that helps! [This video](https://youtu.be/ajhFNcUTJI0) might help.

Like someone else mentioned already, every object’s speed through space-time = the speed of light (299,792,458 m / s). So, that means:
Speed through space + speed through time = speed of light. Given the speed of which the earth hurtles through space, you and I experience the same speed through time, which we have come to know as the normal passage of time.

Now if you increase the speed at which you travel through space, the speed that you travel through time decreases because the sum of those two still equals the speed of light. In Interstellar, when they visit Miller’s planet, it is so close to Gargantua, that it is moving extremely fast, much faster (relatively) than earth, such that their speed through time is drastically reduced. When they spend what feels like only 3.5 hours to them on that planet, it turns out to be almost 27 years for people on earth. The astronauts bodies only experienced 3.5 hours of time while the people on earth experienced 27 years in the same time frame, meaning the astronauts were moving much slower through time.

I have a B.S. in biophysics, and one of my favorite college classes was “modern physics” where we learned about this. This is a special relativity concept known as “time dilation”. The ELI5 version of special relativity is: nothing can go faster than the speed of light, and the laws of physics do unintuitive things when you go really really fast (near the speed of light). The faster you go, the slower time moves for you with respect to someone moving slowly (time dilation).

You seem to be curious about the actual mechanism of this, which is really hard to explain in an ELI5 version, but I’ll try to go step-by-step.

On Earth, we perceive speeds “relatively” in a pretty simple way: if I am going 70mph in a car and there are cars whizzing past me going 70mph the opposite direction, they appear (from my POV or “reference frame” in physics terms) to be going 140mph straight at me. That’s makes decent enough sense. However, in 1887, physicists Michelson and Morley found something extraordinary: the speed of light is constant, whether you are moving towards it or away from it. In other words, the speed of light (c) is constant in all reference frames.

This result shocked the world of physics because it seemed irreconcilable with the world we know. Imagine you are in a car going 90% the speed of light (0.9c) and someone outside the car shone a laser pointer back at you, you would expect to measure the speed as additive: 0.9c you are racing toward it + 1.0c the light travels at you = 1.9c the light appears to be traveling from your frame of reference. However, this is not the case. You will still measure the light as traveling 1.0c. More than this, physicists found that nothing with mass can ever go faster than the speed of light (or even appear to go faster than the speed of light) in any frame of reference.

So now what? What if you are going 0.9c in one lane and a car was traveling 0.9c in the other lane as you… how could the math work out that you not perceive them as going 1.8c??? Well Hendrik Lorentz worked out the math to properly “transform” frames of reference going at very high speeds–“the Lorentz transformations”. They are equations that “fix” the math so that at even in frames of reference going high speeds, we will still perceive the speed of light as c, and even if two people are going high speeds on a collision course, neither will perceive each other’s speed as higher than c. One natural consequence of the Lorentz transformations is time dilation. When changing high speed frames of reference, there is a “Lorentz factor” that needs to be taken into account: 1/sqrt(1- v^2 / c^2 )

As crazy as this is, it is absolutely true. In 1941, Rossi and Hall showed that muons (heavy electrons) in cosmic rays decayed much much slower than they do when they are stationary because they were traveling at 0.99c, and the difference in their lifetime was exactly predicted by the Lorentz equations, showing that time is slower for extremely fast things. In 1979, Bailey et al. redid this experiment with even better controls in the CERN particle accelerator and got the exact same results.

The Lorentz transformations were essential to Einstein correctly theorizing Special Relativity in 1905. Then in 1907, he also described that time dilation can also occur due to gravitational effects of large bodies–what you see accurately depicted in *Interstellar*.

I know that was a lot, but I find it fascinating and this it is a great question as well. If you want to read further, the history of special relativity Wikipedia page is a good start: https://en.wikipedia.org/wiki/History_of_special_relativity

Everything is always moving though “spacetime” at the cosmic speed limit (or “speed of light”). Imagine driving a car at 50 mph north. Now the road turns and is heading northwest. You’re still going 50mph, but you’re not going 50 mph north. Think of time is another dimension, so just like East-West is perpendicular to North-South, time is perpendicular to all three spacial dimensions. Since everything travels at a constant speed, the faster you move through space, the slower you move through time. Photons, or particles of light, actually don’t age at all because they travel at the cosmic speed limit through space.

I made a more detailed video about Special Relativity if you’re interested: [https://youtu.be/y4JvayFQKD8](https://youtu.be/y4JvayFQKD8)