I know this, too. I also know the “balloon analogy” is a useful shorthand, but technically inadequate to describe what happened.
But let’s stick with the balloon analogy for a moment in order to explain how it’s wrong here: all of the objects on the surface of the balloon are rushing away from each other as it inflates. Likewise, if we go to a 3-dimensional model, all the points within the balloon are rushing away from each other as it inflates. In both instances, there’s still a central point from this expansion.
Why wouldn’t the same apply to a 4- (or more) dimensional model?
(Actually, just writing this down, I see an interesting conundrum with the balloon model-- the 2d model easily shows how there is no central point on the surface of the balloon, yet within the ballon there is clearly a central point. To a 2d observer, everything is moving away from everything else, there is no central point to the surface of the balloon. But at the 3d level, there clearly is. So, are we in the same situation with 3d versus 4d? We see everything moving away from us in 3d, but as we are “on the surface of the balloon” we are unable to discover the central point that resides in 4d?)
To make the balloon analogy work, you have to understand that the two-dimensional surface of the balloon is everything. There is no inside to the balloon, no three-dimensional perspective in which to see it. Just the dots, each one exactly equivalent to one another, without a center possible even in theory.
Most people never look at the balloon that way, which is why it has passed out of favor as an analogy.
In our 3D world, there is no 4D outside perspective because there is no fourth spatial dimension. (I exclude the hypothetical dimensions of string theory. They don’t apply here.) You can’t get outside the universe to peer inside.
If you do the balloon analogy properly, then in both cases there is no center, no way to find or determine a center, no center that ever existed. (Which is equivalent to saying that if you start at any particular point, it looks like the center of everything, just as every other point does because everything always looks the same in every direction [over large enough distances] and always has.)
The important thing to realize is that once you’re far enough away from the Earth (or any gravitational body), your frame of reference is with respect to the center, not any point on its surface.
This assumes you came out of hyperspace or something and hadn’t been watching those stars all along the trip. Except for the stars you were traveling right towards, you’d have a huge parallax base for most of the stars already and know which were close.
But I don’t know you’d even need that. You should be able to distinguish sun-like stars (e.g. type G stars) from their color and spectrum. The only bright G stars in the vicinity would be the sun and Alpha Centauri, and maybe Tau Ceti and Epsilon Eridani but I’m not even sure those are G’s.
Okay, I know that I’m rapidly outpacing my ability to comprehend the discussion here-- and this wasn’t the topic of the thread-- and I appreciate that laymen get fed a lot of analogizing in cosmology, but here’s what I don’t get about this discussion.
The “Big Bang” wasn’t necessarily an explosion, but it was an expansion, right? Infinite density and energy one moment, then the next, inflation and then expansion. This implies a single point (how else is such density achieved?). Everything that exists existed within that single point, we’re not all racing away from each other. I get that our feeble minds can’t readily imagine what kind of shape our universe looks like, but it has a shape, right?
My question being, inflation/expansion implies that objects in space were once closer to other objects in space which implies that at one time everything was once next to each other. It wasn’t a case of everything appearing all at once at some far away position, THEN racing away from each other… or was it? Was the Big Bang really just the Big Poof?
Anyway… I hate to lose the balloon/bubble analogy, especially since every single popular science magazine still relies on it when talking about this. If that’s not a good analogy for us laypeople, then what is?
Okay, thank you! Still, there has to be a more-or-less-gradual transition from your frame of reference being a particular spot on the Earth’s surface, to its being the Earth itself (as best represented by its center), as you make you way up through and beyond the stratosphere. Maybe that’s a topic for another thread…but if one of you folks has a thought on that, I’d be much obliged.
Think of space as an infinite rubber sheet. The galaxies are dots painted on the sheet. Over time time the sheet stretches, and all the galaxies move farther and farther apart. The density of the universe is decreasing.
If we run the movie backwards, if we look backwards in time, the sheet contracts. The galaxy-dots move closer together. The density of the universe increases.
But the sheet is still infinite. It still stretches off unbroken in all directions.
If you run time backwards long enough eventually you reach a point where the density of the sheet at any point becomes so extreme that the laws of physics as we know it cease to apply. The corner of the universe we can currently observe has been compressed into a tiny speck.
But the sheet is still infinite. It still stretches off unbroken in all directions.
It’s not really correct to say that the universe started small and got large. It’s more appropriate to say that the universe started dense and got less dense.
To be a bit picky, the center of gravity of the solar system is not exactly at the center of the sun, so the sun actually rotates around the center of gravity of the solar system (as do the planets) and thus wobbles. Then you get to deal with the other stuff people have said.
You don’t actually leave a frame of reference; you use the one most useful to you at the moment, since they are all equivalent. When the astronauts started they were in the frame of reference of Florida. In orbit what is under you is not so important. In computing where you are and where you are going to wind up you must include a vector composed of velocity from rocket firing and from the rotation of the Earth, and from gravity of both the Earth and the Moon (and the Sun, I suspect.) When you are heading for the moon you need to use it as a reference, also taking account its movement around the earth. Remember also the moon does rotate - it is just the same amount of time as a revolution.
This is true. The problem is that nobody can imagine an infinite rubber sheet. They imagine one with edges. And anything with edges has a center.
There really isn’t a good, easily visualizable analogy for the universe. The universe is everything. All of our experience is inside, and therefore partial and bounded. Nothing matches up. Both the 2D balloon surface and the infinite sheet work mathematically but neither exist. All other analogies - the infinite raisin cake, e.g. - fail the same way.
You have to think the problem through. There’s nothing to point to and say - that.
New, semi-related question. Given that everything is moving as discussed above, how likely is it that we encounter previously undiscovered comets/asteroids/planet X/other large objects in a potentially conflicting orbit? Given the timeframes involved, we’ve only been around to see a tiny fraction of the sun’s “great loop” for want of a better term…
The sun has what, 99% of the gravitation attraction in our system. If a small piece of junk comes along it will just get swept into the sun or batted away. The odds of our planet encountering anything from the outside are tiny.
It’s far more likely that we would hit a sun, or rather, come close enough to a system to disrupt orbits. Suns are massive and can do damage from a distance.
But even then, remember the lesson from Hitchhiker about how space is really BIG. You’re asking that if we put a few marbles on the surface of the earth whether they’d randomly hit one another. Not for a really long time.
In a really long time, the Andromeda galaxy may be on a collision course with the Milky Way galaxy, long being over the next few billion years. And that may still not actually bother us because the spaces between stars are too big.
Okay, that helps, thanks. But I’m still uncertain about the gradualness/suddenness at which one frame of reference (Florida) will cease to be useful as one goes skyward.
I guess this partly depends on your eventual goal. If your goal is to get into orbit (or beyond), then what land is below you is unimportant from the start (except in the sense that the Earth’s turning can give you a boost if you fly with an eastward component, as all space-bound craft which take off in the northern hemisphere do). **You take off straight into the air (i.e., the Earth’s turning doesn’t noticeably change your course at first) not because the air column is holding you there, but because you started out as a part of that land surface – you were stuck to it entirely, just as the air column is stuck to it a little less firmly – and it takes a few minutes of rapidly accelerating motion to start to “shake off” the state of moving along with the surface of the earth, and start “doing your own thing” more and more. ** (PLEASE correct me if I’m misunderstanding the bolded part.)
If you’re an airplane, you need to make your calculations based on two frames of reference: the surface of the earth (so you go to where you want to land; and, just as with the rocket, you started out by “riding along with” that surface, stuck to it on the airport tarmac), and the column(s) of air you pass through on our way there (basically, the prevailing wind direction and speed).
If you’re a hot-air balloon, I guess you’re stuck just with just worrying about the surface of the earth, since your position is always equivalent to the latter frame of reference from the airplane example.
Place a cantaloupe on the beach in Los Angeles. Place another one in the middle of Central Park in New York. That’s approximately the relative size and distance of the sun and Alpha Centauri, our NEAREST neighbor. Collisions are really unlikely.