# eli5: What does it mean when people say that time works differently in space?

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I’ve heard people talk about how people living on a space station their whole lives would age at different rates to people on Earth but…how?

In: Physics

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Essentially here on earth we have an environment. We have gravity, oxygen, heat and lack there of.

On a space station there is little of that. No real gravity to effect the strength of your bones. You have no weight in space so your bones don’t really need to grow stronger or denser. This would also effect muscle growth. As one of the biggest factors in growing up is how much your body needs to grow to accommodate gravity.

You have oxygen in the space station but it’s different than the air your breathe. There’s less in it and your body won’t experience the different pathogens and dust and other particles being in the air. This effects how resilient your lungs would be.

Heat or the lack of heat would mean your body would be slow to adjust to different temperatures. Your skin would more than likely be paler becuase there is no direct sunlight which causes skin to get darker.

The environment in space is not very friendly to our biology that has adapted to live on this planet.

The effect is very, very minor, but it *does* happen!

You’re probably familiar with the name *Albert Einstein* from his famous equation E=mc^2. If you’re particularly geeky, “Hey, Einstein, I’m on your side!”

Something else he’s known for is the theory of *relativity* (two of them, actually) which say that a lot of things are *relative to* other things — that is, they can’t be objectively measured. Specifically, we’re interested in the components of both theories that talk about time.

The theory of *special relativity* says that a moving clock ticks slower than a stationary clock. That is, a clock that’s moving very fast will measure time slower ***relative to*** a clock that’s not moving. It won’t see its own seconds passing any slower, but it will see a stationary clock’s seconds passing *faster.*

The other theory, about *general* relativity, says that a clock deeper in a gravity well ticks slower than one farther away. Conversely, a clock further away ticks faster.

Put these two together, and someone living on a space station actually experiences the flow of time very slightly differently: after spending six months aboard the ISS, Einstein’s theories predict that an astronaut would age about 0.005 seconds *less* than us schlubs on Earth.

This doesn’t really have anything to do with being in space per se, but instead has to do with relative movement and gravity.

It all comes back to the concept that there is a maximum speed limit for things to move. This is the speed of light; light moves at the maximum possible speed for anything in our universe. But movement is relative so there is no underlying frame of reference upon which this is based. You cannot for example say that an object is moving at half the speed of light in one direction and therefore cannot accelerate beyond another half of light speed.

As a consequence of this, everyone sees light as moving at the same speed regardless of their frame of reference. Shoot a beam of light out in front of you and it recedes at “c” (short for light speed), and if you accelerate to 90% of c then shoot a beam of light… it still seems to recede at c!

This seems pretty weird and it results in all sorts of strange consequences. Observers in different frames of reference will always agree on the maximum speed limit of c, but they won’t agree on either the distances between locations or the time which passes between different reference frames. If someone is moving very quickly they will start to see the universe compress in the direction of their travel, distances along their path seeming to be shorter than a “stationary” observer. The traveler will also experience less time passing than the observer that doesn’t move.

It is important to understand that this impact on aging has nothing to do with exposure to radiation, or some subtle influence on how our bodies grow and change. It has everything to do with the fundamental passage of time; a very fast traveler might experience only 24 hours while others experience 1000 years! Both of them are “correct” about how much time passed for them, our expectation that time and distance are the same for everyone is just untrue.

Getting back to people in space, our astronauts are orbiting Earth at about 7,700 meter per second. This means that over the course of a year they experience about 0.01 seconds less than people on Earth. Obviously this isn’t a huge difference, but it is there and significant enough that GPS satellites need to take it into account in order to function.

Hafele–Keating experiment actually showed this, they put a clock on board commercial aircrafts and fly around the world, then compare to a clock at a fixed position.

The effects is minimal, but it’s technically there, and more pronounced for people flying around in orbit.

As for how could that happen. You have to abandon the naive idea of time and space. The naive idea is that space that time is like 1 coordinate, and at each instant in time, everything happen in a 3 dimensional Euclidean space; the totality of space and time formed a 4-dimensional Euclidean hyperspace, and how much time something happened is just the difference between 2 time coordinate at the beginning and the end. But the more correct idea is that spacetime is combined into a 4-dimensional hyperspace, without separated time nor space coordinate, and the amount of time is actually the “length” of the path in that space, and between 2 points on spacetime there are no fixed time difference; different object moving between 2 points can actually move in different path and thus experience different amount of time.

I hope that’s an understandable ELI5. Different paths between the same 2 points can have different length.

It seems everyone is coming at the question from a different angle but usually when people talk about aging in space they are talking about relativity.

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The way relativity works is that things that are moving faster experience time more slowly. If you take two clocks and send one up in a rocket to spin round earth a few million times when you put both clocks together again the clock that went up in the rocket will actually be delayed by a few seconds.

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Similarly, an astronaut travelling at about a hundred +- miles a second round the earth on a space station will age slower.

Technically the heavier the gravity the slower (or is it faster) the time.

So hypothetically if you sat next to a black hole for 2 days and went home back on earth years might have passed.

Since space has less gravitation pull time moved differently than earth. It shouldn’t be major especially for people but electronics like to be in sync with each other so NASA actually has to make sure it’s equipment is set to run a tiny bit faster in space.