Which way is 'up' in outer space?

Ender says: The enemy gate is down

This is probably not the best scientific answer,  but, very simply, where there is no gravity, there is no up.

If you remember the old Star Trek series, and just about any other "outer-space" based sci-fi, you have to have a universal coordinate system that takes into account extra vectors, as opposed to a map or planetary globe which, for the most part only takes into account North-South and East-West.  In the case of an open area of space you would have to add at least a 3rd set of vectors which would represent, for lack of a better term, up-down.

It's very confusing, as soon as you leave the boundaries of a planet with gravity, but you can compare it to floating in blackness. If you don't know where you are, and have no way to map your progress, there is for all intents, no direction.

Basically, "Up", ends as soon as gravity ceases to be a factor. However, to map an area, a vector to cover the area "up-down" must be used. (Brains fully fried, now??)          

Classically, "Up" is away from the action of gravity, or in other words, the opposite direction of the pull the gravitational body. In our case, the opposite direction of the ground, the Earth. Taking that to the extreme, "Up" in space would be in a direction from the central core of our Solar System, the Milky Way. But this is such a great distance, that effectively from a single person's perspective in space, you could point in basically any direction and have "up".

One of the ways our body perceives "up" is by the gravitational pull on sensors in our inner ear. That is why we can feel "upside down" if we are head towards the ground, as anyone who has been in a rollover trainer well knows. In space, we lose this gravitational sense in our middle ear and lose the ability to determine "up". So in that sense, there is no "up" in space. Because of this, Astronauts will often refer to something relative to their position. "the red button above your head" will tell the astronaut to look "above" his head, regardless of his orientation in the spacecraft.

if there is artificial gravitational forces, such as created by spinning a space station, then the concept of up and down will return, but relative to the spin axis of the space station.

Think about the horizon. It looks flat, right? But we know the Earth is curved. It's just that from out perspective (ground level) it looks flat.

You can go "up" from that perceived base, but the reality is that space (and time) are a giant bubble, expanding outward from the Big Bang. So "up" from where we stand on Earth may be headed into the interior of the bubble, or out into deep space. Really, once you get away from the gravity of the Sun, "up" is wherever you want it to be. Imagine that-- if your child were to grow up to be the captain of a spaceship that could travel such distances, he or she could decide which way is "up"! What a delightful feeling that would be for a developing young mind.

it is all relative to which reference frame you are in. People on the other side of the world would observe our up as there down, and so on,

There isn't a true "up" or "down" anywhere. Everything is defined by social norms. Here on earth we as a society define up as the sky and down as the ground. While that one is fairly easy to explain due to thr force of gravity, north and south is not. What defines the north and south poles? Nothing. It was an arbitrary designation applied to one side of the rotational axis over another. As a society we have applied many of those arbitrary designations to space. Humans in orbit around the earth, use earth as the reference point. Spacecraft use the point of Aries to describe orbital and ballistic characteristics. So the short answer, for a human reference, is there are only references that we define as a collective. The "north" direction of an orbital plane is "up" while the planet is down while in orbit.

Which WAY is up?

'Up' has relevance to two factors:

The point of origin at the beginning of the climb, first. That is why, from the equator no one jumps 'out', but 'up', else one could only go 'up' from the North Pole (assuming that really IS the 'top').

Then there is the fact that one's head is the top of the body. Once the initial factor is removed far enough to be considered relevant, above one's head is 'up', leaving 'seated' travel to be 'forward', if so facing, et cetera, until landing comes in to play, where in the first factor again becomes relevant and resumes circumstantial control.

In Ender's case, even in zero gravity inside the craft, 'up' and 'down' are relative to the commonly accepted sections of the ship. Note the corridor floor in the gates remained stationary, indicating 'down'.

Ender's Game - Orson Scott Card (USMC CMC Professional Reading List) addresses this. When there is a lack of gravity, the concepts of Up/Down/Left/Right etc are all relative. Up is the direction your head faces, while down is the direction your feet face. Think Port & Starboard. They are "absolute" directions relative to the position of a boat. Up & Down are "absolute" directions relative to the position of the Planet (or gravitational field) you are interacting with.

When you are no longer in a gravitational field, there is no "Up or Down" there is just the classic X,Y,Z axis.

So to answer your son's question, you can go Up until you are in Freefall or no longer affected by gravity.

To get even more complex, since EVERYTHING in the universe is spinning CONSTANTLY, you have to use absolute markers for direction. We use Earth (Prime Meridian, Equator = 0,0). In outer space it becomes Sol (our Sun). If we were to leave our Solar System, it would become the Galactic Core. You get the idea.

The only logical answer in my opinion is that up is determined from your point of reference. If I am in Antarctica and you are in the North Pole and we both go up, we would be headed in opposite directions whilst both going "up".

That is a simple but complicated question. If you go north of the sun then you are going up, right, well what is north? Our Galaxy is disc shapeded, it has a top and bottom, choose what is north and south. Our Galaxy is part of a cluster that also has an orbital plain. The universe is expaning, what do you measure that with?

Damn, I typed this then the page reset. UGh.... BTW, I am a physics major from UCSB, so here is how I would explain. Like blowing up a balloon, our universe is growing. So, if you were driving on a road, the road is being built as we drive on it, We run out of gas before we reach the "end" (which would be the end of time, not of disatnce. That said... our universe is 13.8 billion years old, but that isn't the "size" of the universe we can view inside this multi-dimensional manifold is about 46.5 billion light years (radius). So the answer is, you can go UP as long as time continues to exist.

Whichever way Geirge Lucas says is Up is Up.

No matter, thanks to Obama and the Democrats there will be no CPT Kirk or uss enterprise , more like a Col Ivan Russky and a MIG Lenningrag or whatever the Russians call their spacecraft

Chaplain, in the greater universe there aren’t really “cardinal” directions per se like on a MGRS map. Nor is there really an up/down, north/south. But there ARE units of reference for directions for things “out there…” In many cases when we are trying to “predict” where an object is because the Earth is moving as well as the object! Now, taking “out there” to closer to home, if we wanted to know what “direction” a satellite or the ISS was from our position some of the considerations of WHERE something is versus “up there” or North are:
Epoch, Orbital Inclination, Right Ascension of Ascending Node (R.A.A.N.), Argument of Perigee
Eccentricity, Mean Motion, Mean Anomaly, and Drag
If we wanted to go to Mars or the next star, it simply is “Head west young man…” There is a LOT TO CONSIDER~!!!!!!
Former FA 40 – Space Systems Operations Officer

There really is no up or down, its objects relative to each other. Space in itself is not infinite, its like a big container eventually there is an end, which is hard to mentally wrap ones mind around.

Every way and no way. I'd probably tell a child that once getting far enough from Earth's gravity to be "weightless", there si no longer an "up".

"Up" is based on gravity.
There is no gravity in space.
Just like there is not real "Sun Rise".

Spoiler alert: in Ender's Game that concept is essentially the key revelation (perhaps your kid might enjoy the book given that the topic came up). In space relativity changes and you have to redefine up down left and right. Ender does so by orienting on the enemy: the enemy is down and his fighters literal think of themselves as 'falling on the enemy.' It's a brilliant concept in multiple ways. Up and down and even now and then are relative terms and you need to pick a reference point. As mentioned under your time travel thread: time passes at varying speeds relative to Forces acting on the observer. Substantial Gravity and near light speed are two great examples.