I assume my non-science-guy status is evident from the question. If I had any talent for science, I would not have gone to law school.
From what little I think I know, the universe is always expanding. Does that mean that everything in the universe is moving away from the center/ground zero for the Big Bang? Does our entire solar system orbit the center, or move outward, or stay the same distance from the center? Put another way, if I come back in a hundred million years, will the Sun still be where I left it, or would I need a science guy to tell me which way to turn my [del]heavily armed armada[/del] peaceful science vessels?
Relativity means there is no privileged frame of reference – that there is no single point, anywhere in the Universe, that is stationary relative to everything else. Everything is expanding away from everything else, barring “local” motion (local being up to and including galaxy clusters, so it’s a pretty broad definition of ‘local’) bringing specific things closer together.
Relative to the Earth (or any of the planets), the Sun is at a fixed location at one focus of a very nearly circular ellipse. Relative to the core of the Galaxy, the Sun is in orbit around it, with a roughly 300 million year period. Relative to neighboring stars, the Sun is in motion, in a way resembling dodge cars on a vast desert. Relative to the Andromeda Galaxy, the Sun, along with the entire Milky Way Galaxy, is on a convergent course that will bring them together in several billion years.
One of the central tenets of the Theory of Relativity is that you can’t call anything a fixed point in space. Or, equivalently, you can call any point a fixed point, with equal validity.
That said, while the laws of physics can’t define a reference frame for you, the objects in the Universe can define such a frame (or many such frames, depending on what objects you choose). The Sun, for instance, is orbiting around the center of our Galaxy, and our Galaxy and the Andromeda galaxy are orbiting around each other, and so on. Ultimately, you can measure speeds relative to the average speed of the material that emitted the cosmic microwave background radiation, and we’re moving relative to that, too.
As for the Big Bang, you can set any point you like as the center, and the expansion looks the same from all of them. An observer anywhere in the Universe would see everything moving away from them in all directions.
Note to self: When you form your armada for galactic conquest, hire science guys. Pay them well.
So in my rephrasing with the peaceful science vessels returning from…elsewhere… in 100 million years–assuming I leave now–to find Earth upon my return, I need to compute where the Sun should be in its orbit around the core of the Milky Way?
You probably don’t need to do much computation, since the ships presumably start off in the Sun’s frame of reference, or a close approximation to it. It’s like when you use the bathroom on an airplane: Your seat has moved many miles since you left it, but all you have to remember is 17 rows forward and on the left.
If I understand Chronos correctly, that doesn’t matter all that much. As long as I don’t leave the universe, I have the same vector/velocity that earth has, in addition to whatever relative distance I travel from earth. I should be able to go 50 million years outbound, turn around and come 50 million years inbound, and wind up [del]in my backyard[/del]…approximately where I started.
This can’t be quite right, can it? Doesn’t a “frame of reference” have rather fuzzy boundaries? For example, when would you say the Apollo astronauts left the Earth’s “frame of reference”, and had to start taking into account the rotation of the Earth going on below them? As they gradually left the atmosphere, perhaps, which “locks in” balloons and such to the column of air which is more or less rooted by friction to the Earth’s surface? (“More or less”, because continental-scale weather patterns really are pushed to the side by the Earth’s rotation.)
(Or does their frame of reference have to do with gravitational forces? No, that can’t be it – the Apollo astronauts, for the first half-day, are just in free-fall as they orbit the Earth; and even as they approach the Moon, the Earth’s gravitational tug is still pretty strong…yet, once again, if they just turn around, they WILL land in a different place, because of the Earth’s having rotated.)
I expect you’d have trouble finding the sun after a 100 million years.
The orbits of stars around galactic center tend to be chaotic, so after one third an orbit, you might only be able to predict the sun’s location to within 10 light years: (typical open cluster lifetime being of the order of. 200 Myr (Wielen 1971))*
There’ll be a lot of sunlike stars to check in a 1000 cubic lightyear volume.
*Yes, it’s a roundabout method of showing chaos, but I’m not an astronomer, and don’t know the literature well.
No there won’t (probably). First off, I take within ten light years to be within a sphere with a radius of 10 light years. That’s a volume of a little more than 4000 cubic light years not 1000. A volume of 1000 light years is inside a sphere of radius 6.2 light years.
Using the larger radius of ten light years, there are currently 7 star systems excluding the Sun. Of these probably only Alpha Centauri A and B could be mistaken for the sun with decent astronomical techniques. And those are part of a multiple star system so you should be able to eliminate them. I suspect the two closest stars you might mistake for the sun are Epsilon Eridani at 10.5 light years and Tau Ceti at 11.9 light years.
If you go with the smaller number of 6.2 light years, there are only two other systems: the triple star Alpha Centauri and Barnard’s Star. The latter would be quite hard to mistake for the sun.
I understand what you’re saying here, but was there an actual center at one point? I.e., even though today there’s no way of telling now where that center once was, if all creation was once packed into a single spot, that spot had to occupy a center, no?
No. We think that way because all of everyday experience occurs within space.
However, the expansion of the universe was an expansion of space. We can’t really visualize what that means, but the evidence is clear that there was no center that everything is moving away from. Everything is moving away from everything else.
That probably looms mighty large. Being a chaotic system, we could easily be talking a volume 30 light years on a side. At that distance, the sun’ll just be another magnitude 4.8 point of light.
You might get lucky, and find home right where your model predicts it should be, then again you might not.
At a minimum, you’ll be spending a while doing parallax measurements to determine which stars are within your search volume.
