# eli5 how is time on earth different to time on other planets?

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Starting off my first question, how is time in space or on other planets different to earth is it just that it takes longer for that planet to do a full rotation meaning longer or more hours in a “day”?

then if Space/planet time is different to time on earth, how dose someone age slower in space than they do on earth, wouldnt your body still be replacing cells at the same rate and be “aging” at the same time?

is the “1 hour here is 7 hours on earth” just some movie gimmick for drama

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These are typically two separate considerations as you’ve already alluded to.

The first deals with the physical properties of the orbit, the tilt, the speed in which the planet spins on its axis.

The second deals with relativity as it pertains to the speed of light.

A person in space will not age slower than someone stationary, not to any appreciable degree. Satellites move at about 17k mph, they lose [about 7 microseconds a day](https://www.astronomy.ohio-state.edu/pogge.1/Ast162/Unit5/gps.html) to relativistic effects.

So you’d have to be moving much faster, much closer to the speed of light.

And that’s because time isn’t constant. The speed of light is. Space and Time will both warp itself to meet the constraints of that.

It’s not intuitive, and it’s not eli5, not easily. It’s Special Relativity, and even very smart people have spent more than a century trying to comprehend it.

Just know, time isn’t constant.

edit: Just because.. if you were moving at 17k mph, going away from Earth. If you travelled in one direction for 35 years, turned around and came back. Your twin would have aged 178.5 *seconds* more than you. That’s based on 7ms * 365 * 70. About 3 minutes over an average lifetime.

The *Interstellar* planet was orbiting a massive black hole, and traveling at such incredible speeds that relativistic time dilation begins to become significant and it actually experienced time at a significantly slower rate than “normal.”

This is a real thing, but you have to move very fast for this to happen, appreciable fractions of the speed of light to make a significant difference.

We’ve never actually observed a planet in such a gravitationally extreme environment, and it’s probably not possible to remain in a stable orbit under such conditions.

The planets that orbit normal stars in normal orbits aren’t moving anywhere near fast enough for this exotic time dilation effect to become noticeable. Their “day” and “year” are different because their orbits are different but the actual rate you’d experience time is the same.

Now the very minor differences in orbital speed do produce a very slight time dilation effect, enough to throw off precision timing in positioning equipment. GPS satellites have to correct for the fact that their clocks run slightly slower than clocks on Earth.

For those “one day on mars is 40 minutes longer” is just (roughly) how long it takes for it to spin around once. This is called sideral day if it measures according to the stars or solar day if it measures according to the sun. They vary because the sun is so close and sometimes when planets spin slowly they move quite a lot in one rotation.

The thing you see in movies where one minute here is x minutes on earth is usually time dilation and is often caused by gravitiy (in those scenes). Usually we do not care about this, but at high gravitiy differences (usually black holes in movies) time starts to pass differently for two observers. It also happens when two observers are at different speeds, which is why an astronaut and their twin on earth will be of different age when they meet again. For most of what we humans experience this is so small we never notice. But if we were to actually move near a black hole (or at near speed of light speeds) it would start to be noticable.

We did do an experiment where we sent one atomic clock on a high speed plane and another was sitting still. They became out of sync ever so slightly afterwards. Talking less than seconds tho. So nothing that impacts us in any daily scenarios.

The first part is true. If you were measuring days and years on the rotations of the planet you were on then they would be different.
I think what you might be confusing it with though is the fact that when you’re travelling very quickly, or standing on or next to something very heavy (with a lot of mass and a large gravitational field) then your time itself will be slower, compared to somebody who isn’t. Space is the most commonly referred to example of this, as it’s only in space where we have travelled the sorts of speeds that make this effect at all measurable, and/or where you will find things (like black holes) with enough mass to make a measurable difference.

You know how people always refer to Albert Einstein as a genius?

It’s because he figured this out.

It’s known as the Theory of Relativity, which is really two related theories: Special Relativity and General Relativity.

These are really hard to explain in an ELI5 fashion — I mean, Einstein *was* considered a genius for a reason — but I’ll take shot.

We usually think of space (up/down, foward/backward, left/right) as being separate from time (past/present/future).

What Einstein theorized — and what was confirmed through experiments over the last 125 years — is that space and time aren’t really separate things: they’re one single thing that Einstein called “spacetime.”

As objects move through spacetime they experience what is called “time dilation” — the appearance that clocks (time) operate at a different pace based on their positions *relative* to one another.

It’s really hard to see this at human scale, though; the effects aren’t really that dramatic until you get to *cosmic* scales.

But it’s literally true that someone moving faster is experiencing time at a different rate relative to someone moving more slowly than them. So astronauts in a space station whizzing around the Earth are moving more quickly relative to a person sitting at their desk typing away on Reddit. If you had super-accurate synchronized clocks on your desk and in the space station they might get out of sync by a few billionths of a second. You both experience an hour passing, but when you *compare* your super-accurate clocks, you’ll find that they are out of sync by a few billionths of a second.

There are two kinds of time dilation: “kinetic” (based on relative speed — or, more precisely, acceleration) and “gravitational” (based on relative experience of gravity). They’re really kind of the same thing, but we’ll get to that in a bit.

So time dilation can be experienced by going *really* fast relative to someone else: if I’m in am (imaginary) spaceship traveling at 1/4 the speed of light an hour still feels like an hour *to me* and I age as if an hour passed, but when I compare my super-accurate synchronized clock to yours sitting on your desk back on earth it will tell us that they’re *way* off — like weeks or years apart! That’s kinetic time dilation.

Then there’s gravitational time dilation. What Einstein’s theories show is that the mass of an object bends spacetime.

So think about a string that is hung loosely between two poles. If you attach a bowling ball to the string, that string is going to bend/droop because of the mass of that bowling ball.

That’s kind of like what mass does to spacetime.

That string is just one dimensions though (left/right, let’s say, because one end is on the left and the other on the right), but it bends in a *second* dimension (up/down) because of the mass of the bowling ball.

So space is three dimensions (up/down, left/right, forward/back), but mass bends it in a *fourth* dimension: *time*! Stronger gravity stretches spacetime and results in time dilation. (Weird side note: because space and time are really spacetime, *space bends too* — so as you go faster, your length changes, too! But let’s stick to time because that’s enough for now!)

So the closer you get to a massive object — like the black hole in the movie *Interstellar* — the more spacetime bends. And the more spacetime bends, the more time dilation someone close to the black hole experiences relative to someone far away from the black hole (where spacetime hasn’t been bent as much).

The kicker is that “kinetic” and “gravitational” time dilation are *kinda* the same thing, because when you speed up (acceleration) you experience the same thing that you would experience if you were experiencing more gravity.

You know how people talk about “G force” or “On the curve on that roller coaster you experience three G’s”? That’s because the experience of acceleration and gravity are basically the same thing. If you’re on a roller coaster curve where you experience 3 G’s, then you are (just *momentarily*) experiencing what the gravity on, say, Jupiter is like (Earth gravity is 1 G; Jupiter is about 2.5 G’s).

So because acceleration and gravity are *kinda* the same, you experience time dilation with both acceleration (speeding up relative to someone not accelerating as quickly as you) or with gravity (which is like speeding up because spacetime is all bendy, stretchy, or squished when you get near something massive like a black hole).

Whew. That’s just all the background to answer your questions!

> how is time on earth different to time on other planets?

Time dilation doesn’t have anything to do with “different planets.” There’s not going to be a lot of time dilation between, say, Earth and Mars. There will be *some*, but it’s so small — billionths of a second — that it’s basically imperceptible to the human mind.

(Engineers do have to be careful with things like communication satellites and GPS satellites, though, because even the billionths of a seconds of time dilation between a comm satellite whizzing around the Earth and the receiving station on the ground can throw communications out of whack.)

Time dilation doesn’t really become something that would affect *human perception of time* until you’re accelerating super, *super* quickly (like fractions of the speed of light — far beyond the capability of humans to achieve today) or you’re near something really, *really* massive (like a black hole).

> how dose someone age slower in space than they do on earth, wouldnt your body still be replacing cells at the same rate and be “aging” at the same time?

They don’t age slower; *time itself is different*.

Also, it’s not just *being* out in space that causes the time dilation; it’s *accelerating* faster, relative to the person still on Earth. That acceleration is necessary to get out into space and to get anywhere interesting in a lifetime.

The speed of light is, IIRC, around 670 million miles per hour. So let’s say I’m in starship going one-quarter the speed of light, around 165 million miles per hour (eat *that*, Tesla ludicrous mode). I’m moving really fast relative to how you’re moving, so time is way more stretchy on my spaceship compared to you, back at your desk reading Reddit.

When I’m accelerating fast, time gets all bendy, stretchy, or squishy for me compared to how time is where you are at your desk. So I go away on my spaceship for a year — or what feels *to me* like a year — and when I get back to Earth it turns out *decades* have passed for you because time was not as stretchy *for you* as time was *for me* on the spaceship

Time itself moved differently on my fast-accelerating spaceship (165,000,000 MPH) than it moved for you back on ol’ slow Earth just chugging around the sun like a turtle (67,000 MPH).

> is the “1 hour here is 7 hours on earth” just some movie gimmick for drama

Nope. It’s actually the *opposite*.

All the space travel tech you usually see in movies and TV and science fiction novels — subspace, hyperspace, warp drive, the TARDIS, etc. etc. — are the gimmicks that writers use to *get around* Einstein’s Special Relativity and General Relativity. If you see a movie — like *Interstellar* — where the characters experience time dilation and age differently, then that’s actually *more* scientifically accurate.

So yeah, time dilation is real. Hyperspace and warp drive are the gimmicks.