Ok getting some real sciency answers which proves you are way better than I am. Can you guys and gals break some of it down like I am 5 or so?
Direction is all relative, my friend. There is no objective up or down or left or right or north or south or east or west. The best we ever have is landmarks and pointing.
On Earth, that’s pretty easy. We have some pretty stable landmarks: up is away from the gravity. North is toward the pole that’s closer to more people. East is toward the way the sun comes up.
We can also get some directions from figuring out one we already know and going the opposite way. Down is not-up. South is not-North. West is not-East.
Being able to see one another, to know which way we’re pointing, gives us more options. Left is that way, right is that way.
You’ve been given all that for free every day of your life (assuming you’re a human who has never spent any appreciable amount of time in space). You were probably taught about North and South and East and West in elementary school. You probably got left and right from your parents. Up and down is hard wired into your senses.
But in space, all of those landmarks are gone. You don’t have any gravity to define up and down, you don’t have any poles to define north and south, you don’t have a sun rising to define east and west. So as a species we’ll have to decide on new ones!
It’s useful to use the sun as our point of reference, since it’s pretty stable for our purposes. A lot of scientific discussion uses the ecliptic as a handy way to define a similar idea as the equator. The ecliptic is the plane that most orbiting bodies move along as they orbit the sun (think “eclipse”–planets and moons and stars will eclipse one another on this plane), and while it’s not exact it is close enough to be helpful. “Above the ecliptic,” then, is more or less analogous to “North,” with “below the ecliptic” being analogous to “South.” And actually, since Earth never changes its orientation to the ecliptic, “above the ecliptic” is also pretty close to Earth North and “below the ecliptic” is pretty close to Earth South. We also use the Earth’s March equinox as the zero point in this system; the “prime meridian,” you might say.
Since almost everything we do is Earth-based, this is usually good enough. All of our telescopes are in the Solar System, all of our spacecraft are in or near it, any navigation we’re going to do will be based on it. But if we’re talking to aliens who have never been here, how are we going to explain it to them? How could we direct a friendly extraterrestrial to our solar system in the first place?
Now things get tricky. There are some shortcuts; the Galaxy has an ecliptic, too (called the “galactic plane”). There are terms like “spinward” and “coreward” that we can use. There are also some steady landmarks that we can point to: specifically pulsars, pulsing black holes that blink in easy-to-measure patterns. Those are visible from pretty much anywhere in the observable universe, so anyone we’re talking to can probably see them. Defining our location by how far away we are from a few of those pulsars could work; kind of like saying, “yeah, I live halfway between Orlando and Cocoa Beach.” But as far as anyone outside the government knows, we’ve never tried to do that.
But in general, you just sort of figure it out based on your use case and the landmarks that are handy to you!
You pick a point of reference. Most things use the plane of the solar system or the Milkway Galaxy and the general direction the earth’s north faces as “up”. And then angle of inclination/declination from there.
Correct, it’s all about what’s closest to the scale of movement you’re needing. Close to a body, you’ll likely line up with its rotation if nothing else. Away from a body, you use the next frame of reference that makes sense. That could be the next closest body’s plane of rotation (which could be the same as the solar system, but not a given, i.e Uranus), the solar system itself, or if approaching another system, line up with its plane. You don’t have to do this, a polar orbit is just as valid as one within the plane.
This. There is, generally, no up or down, unless you specifically choose a frame of reference. That might be the earth, the Milky Way, or your own geosynchronous satellite.
Hate to bother. But can you break that down for a lamen?
You always have to define what “up” and “down” mean. On earth, we define “down” as the direction gravity pulls, and “up” as the opposite of that. But out in space, that definition obviously doesn’t make sense, which is why you’re asking.
So either,
- there is no up or down anymore, or
- we come up with a new definition
One of the common definitions astronauts come up with is relative to the “plane of our solar system”. If you can imagine a giant piece of paper cutting through the middle of the sun, and extending out so big that it also cuts through all all the planets that are orbiting it, that’s called the plane of the solar system. Then we say “up” is one side of that paper, and “down” is the other side.
But again, that only works as a convention in our solar system. Wherever you are, you have to define up and down, because they don’t mean anything by themselves.
Buckminster Fuller didn’t like to use “up” or “down”, preferring “in” (towards the center of gravity) and “out” (away from the center of gravity). “Downstairs” becomes “instairs”, “upstairs” becomes “outstairs”. A tad pedantic for casual conversations on Earth, but highlights the central issue.
Knowing that the nature of gravity is just the curvature of spacetime, it seems arbitrary either way. I But I hadn’t heard that, interesting. Like that other commenter, my first thought was “the enemy gate is down”.
it’s all relative to a mutually agreed reference point. Within our solar system you might relate an object’s position as “sunward” or “outward”, and motion tangent to the rotation of the planets as “spinward” or “trailward”. In galactic space, the same directions apply, but substitute “coreward” for “sunward”.
The enemy gate is down?
Hell yeah 🔫
Short answer, you can’t. E.g. on the international space station, you could have 1 person “standing” on 1 wall, while another is “standing” on the other. They would both see the other as upside down. Both would be right.
Long answer, we agree on common frames of reference. We already do this on earth. It’s just that in space, we don’t have gravity, and everything is moving around a lot already.
E.g. in a ship under thrust, thruswards is up. We could also use the plane of the solar system, or the direction of the local planet as a reference. All of them are arbitrary however.
Amusingly, if you continue down that rabbit hole, you encounter relativity. It says there are no prime frames of reference, EVERYTHING is relative.
Direction is relative. Gravity on earth it’s easy since we all have the same down. But in space you just orient on whatever you can to describe your frame of reference.
If you’re standing on a spaceship bridge with artificial gravity, you take your spaceship as a reference. Down is to everyone’s feet, forward is to the front of the spaceship and left/right will be to the left and to the right.
Without gravity, the concept of “down” just doesn’t make a lot of sense, does it?I always liked this picture. Suppose you’re in a space suit, orbiting around the Earth. Gravity keeps you continually free- falling ‘down’ towards the surface below you. But, in orbit, you also have a velocity that’s tangent to the Earth’s surface. So while you’re falling, because that velocity is just right, the surface is curving away from you at the same rate!
So you remain at the same distance from the surface, and the ‘pull’ of gravity remains the same … orbit!
the enemy’s door is down.
no idea but I know they have done some experiments with space gps. I sorta feel like everything should be in relation to the sun and its rotation and the planets rotations and such. like I would make z above and below the plane of rotation. not real sure though how to decide x and y but you likely want the common rotation to be clockwise in the graph.
This will help you understand very very deeply
Also: Noether deserves her place next to Faraday and Salk
Too much science here…
Maybe get off c/AskScience, then?
I lied! There’s no science
I have no idea what I’m talking about but I imagine you could try and measure the speed of light to determine a true fixed point, theoretically?
I don’t think space has any fixed points, regardless of the methods one uses to try and discover one. Everything is relative motion and relative acceleration for two or more objects.
That’s what I never really understood, if there’s a maximum / speed of light, could you not fire a beam in an arbitrary x/y/z axis, measure it somehow, and then use that info to determine what speed and direction you are going?
Speed of light is a constant. For everyone. Because it’s linked to time, which is part of 3D space.
Let’s say you are standing still. Shoot a laser straight out in front of you. Measure the speed, you get c, the speed of light.
Now, start running forward at 10 mph. Shoot the laser again. Measure the speed. You still get c. Not c + 10, but exactly c.
Run backwards shoot laser out in front of you. Still exactly c.
Because your personal time/clock has changed by moving so that you end up measuring the speed of light the same.
Anyone looking at you, regardless of their relative motion to you will also measure your laser at c.
It’s just how the universe is.
Also, speed of light isn’t really the best term. More like maximum speed of the universe. Light just happens to travel at that speed because it has no mass. Everything with no mass travels at that speed.
I just want to add that apart from time dilation, velocity also causes length contraction wherein an object will be measured as shorter when it’s moving relative to the observer versus from its own rest frame. I’ve always found it (and time dilation) to be very unintuitive to reason about with my monkey brain - relativity in general is just super weird.
It hurts my head and I kinda like it
