object that is actually at rest?

[David Hess]

As for the universe "expanding", is all matter in the universe expanding equally meaning, are the planets expanding, us humans, my computer, car, etc... or is just the boundaries of the universe expanding and everything (planets, stars, etc...) remaining the same distance from each other?

Objects which are gravitationally bound, including galaxies and galaxy clusters, are not expanding within themselves.  So the space between distant objects which are not gavitationally bound expands such that those objects move away from each other, but objects which are closer are gravitationally bound and are not expanding away from each other.

[bostonman]

So the "walls" of the universe are expanding, nothing else?

In other words, if we wanted to travel to the edge of the universe, it will always be escaping us due to expanding (at the speed of light?), but, everything else (galaxies, stars, planets, etc...) will always keep the same distance?

My understanding is our sun doesn't move but moves as our Milky Way galaxy moves. So all eight (or nine) planets, moons, and the stars, rotate around the sun and the sun remains "still" but the galaxy is moving through the universe.

I know things like our moon is slowly pulling away from Earth, but, regardless, does the sun move relative to our Milky Way?

[SiliconWizard]

I've always wanted to take a sip, sitting on the edge of the universe, but this very thought keeps expanding.

[coppercone2]

if the walls are just moving you can still stay in the same place IMO. even if ratiometrically between the measured distance to each wall

[IanB]

if the walls are just moving you can still stay in the same place IMO. even if ratiometrically between the measured distance to each wall

Motion is relative. So if you can see something moving, then you are moving relative to it. The only way to be stationary is for nothing in your observable horizon to be moving (including distant stars and galaxies). Since it is effectively impossible for this ever to happen, it is effectively impossible to be stationary in our present universe.

[Bud]

You guys need a girlfriend. Return to the reality will be quick.

[coppercone2]

ooga oooga

[Microdoser]

You guys need a girlfriend. Return to the reality will be quick.

Sounds like you need a wife

[bostonman]

Motion is relative. So if you can see something moving, then you are moving relative to it.

How can this be? If I'm sitting on my front lawn (i.e. at rest) and a car drives by, I can see it moving.

In any case, ignoring that our solar system is moving through space, the sun remain still, correct?

[IanB]

Motion is relative. So if you can see something moving, then you are moving relative to it.

How can this be? If I'm sitting on my front lawn (i.e. at rest) and a car drives by, I can see it moving.

But if someone is sitting in the car and looking out the window, then they see you moving past the window as the car goes past. From their perspective the car is stationary and everything outside the car is moving past. Physics says that either perspective is valid.

In any case, ignoring that our solar system is moving through space, the sun remain still, correct?

No, even within the solar system the the Earth and the sun are orbiting around each other. If you think of an imaginary string between the center of the sun and the center of the Earth, then the Earth and the sun are revolving around a point somewhere along that string (which is somewhere inside the sun, but not at the center of the sun).

[David Hess]

So the "walls" of the universe are expanding, nothing else?

The "walls" are the boundary beyond which nothing can have an effect on us because beyond that point space is expanding away from us faster than the speed of light.  Since light from that area can never reach us, that area can have no effect on us.

In other words, if we wanted to travel to the edge of the universe, it will always be escaping us due to expanding (at the speed of light?), but, everything else (galaxies, stars, planets, etc...) will always keep the same distance?

Objects which are far enough away not to be gravitationally bound are also moving away from us.  If you were to travel 100s of millions of lightyears to a point outside of the Virgo Supercluster, or maybe the Laniakea Supercluster which includes the Virgo Supercluster, then the Milky Way and Sun, and entire Virgo Supercluster, would gradually recede with the expansion of space.

My understanding is our sun doesn't move but moves as our Milky Way galaxy moves. So all eight (or nine) planets, moons, and the stars, rotate around the sun and the sun remains "still" but the galaxy is moving through the universe.

I know things like our moon is slowly pulling away from Earth, but, regardless, does the sun move relative to our Milky Way?

The sun, and objects gravitationally bound to the sun like its planets and moons, is gravitationally bound to the Milky Way so orbits the gravitational center of the Milky Way.

[David Hess]

Motion is relative. So if you can see something moving, then you are moving relative to it.

How can this be? If I'm sitting on my front lawn (i.e. at rest) and a car drives by, I can see it moving.

You cannot actually establish when you are at rest.  Albert A. Michelson and Edward W. Morley tried that by measuring the speed of light in different directions.  When at rest, the speed of light should be the same in all directions, right?

Well, it does not work that way.  What they measured showed that the speed of light was the same in every direction no matter what your velocity is.

https://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_experiment

We can be "at rest" in comparison to the microwave background radiation, but so can every other point in the universe and because the universe is expanding, those points are moving away from us.  So no points are at rest and all points are at rest.  "At rest" has no meaning when comparing observers at different points.

[TimFox]

What you can do, however, is determine if you are being accelerated:  an example would be a rotating reference frame.
One of Einstein's important postulates is that the effect of a gravitational field is equivalent to being in an accelerating reference frame.

[bostonman]

No, even within the solar system the the Earth and the sun are orbiting around each other. If you think of an imaginary string between the center of the sun and the center of the Earth, then the Earth and the sun are revolving around a point somewhere along that string (which is somewhere inside the sun, but not at the center of the sun).

Maybe I'm confusing the terminology. I'm thinking the sun doesn't move meaning, unlike (let's just use one planet) Earth that is rotating and orbiting the sun. I assume the sun rotates, but does it orbit (again, ignoring that our Milky Way is moving through the universe)?

I (much like I assume many) picture the sun as a sold non moving object with our eight (or nine) planets circuling it. This is also ignoring the miniscule distances things like our moon is pulling away from Earth, and, I assume, planets are not sitting in perfect orbit and are slowly drifting closer/further.

[TimFox]

When you work out the orbital mechanics of the earth orbiting the sun, it is convenient to reduce the problem mathematically.
If you look in any elementary book on classical mechanics, the two positions are replaced by an equivalent radius at which an equivalent mass orbits around an effective center:  see textbook for details.
Since the sun is far heavier than the earth, the effective center is well within the body of the sun, so the correction is small.
When you deal with binary systems where the masses are not very different, this is a large correction.

[IanB]

No, even within the solar system the the Earth and the sun are orbiting around each other. If you think of an imaginary string between the center of the sun and the center of the Earth, then the Earth and the sun are revolving around a point somewhere along that string (which is somewhere inside the sun, but not at the center of the sun).

Maybe I'm confusing the terminology. I'm thinking the sun doesn't move meaning, unlike (let's just use one planet) Earth that is rotating and orbiting the sun. I assume the sun rotates, but does it orbit (again, ignoring that our Milky Way is moving through the universe)?

I (much like I assume many) picture the sun as a sold non moving object with our eight (or nine) planets circuling it. This is also ignoring the miniscule distances things like our moon is pulling away from Earth, and, I assume, planets are not sitting in perfect orbit and are slowly drifting closer/further.

No, the sun is not a solid, non-moving object. If two objects are orbiting each other in space, like the sun and a planet, then they are each orbiting around a common center point, called the barycenter. So the sun is not fixed, it is experiencing orbital movement of its own.

See here for more info:

https://spaceplace.nasa.gov/barycenter/en/

[TimFox]

That useful article points out that the "barycenter" for the Sun-Earth system is close to the center of the Sun, but for Sun-Jupiter it is approximately at the surface of the Sun.

[Microdoser]

Albert A. Michelson and Edward W. Morley tried that by measuring the speed of light in different directions.  When at rest, the speed of light should be the same in all directions, right?

Well, it does not work that way.  What they measured showed that the speed of light was the same in every direction no matter what your velocity is.

You can't measure the speed of light from one point to another one way. You can only time how long it takes for light to do a round trip, so you'll never know if it took less time to get there than it did to get back, only the total time. Even if you get two perfect clocks to measure the time the light arrives and compare to the first clock, you have to match them up next to each other, otherwise the potential difference in the speed of light affects the synchronisation, and special relativity says that moving a clock changes what time it says, and the change is dependent on the speed of light, so if the speed of light is different going this way compared to that then your clocks will be wrong by a proportional amount.

Michelson and Morley did their experiments before we knew about relativity, special or general, or they would have known this.

[coppercone2]

well I think the reference frame for still must be at zero velocity at the start of time / motion. A velocity at which it looks like everything is moving away from you.

If you can put a ship in the middle of the big bang or matter incursion or whatever and measure the distance to the interface/surface of the expanding thing, then you can stay still, if the rates measured on all the sensors are equal. That is like hovering in the middle of a expanding balloon. There seems to be a very specific 'front' that you can use as a reference in this case.

Would it not be like the center of mass point kind of?

But its still confusing because how do you treat it, at first its a ball of high energy expanding quickly. It seems feasible to stay in the middle of that. But when it starts condensing and you have orbits between matter condensation, even if you stay in the middle based on some kind of boundary, it seems poorly defined on what is still at that point. The only thing that kind of makes sense to me is center of mass or net force cancelation, but its very confusing... some averaging comes to mind to remain statistically motionless despite their being complex minor motion.

So it seems to me at least for a while you can stay still if you know the shape of the wave? front of the big bang/rip/whatever.

[SiliconWizard]

well I think the reference frame for still must be at zero velocity at the start of time / motion. A velocity at which it looks like everything is moving away from you.

What is time?

[coppercone2]

WELL in that case it seems like when you can measure velocity

[David Hess]

well I think the reference frame for still must be at zero velocity at the start of time / motion. A velocity at which it looks like everything is moving away from you.

As I pointed out, the problem with what you suggest is that if I do the same thing for a another point at a distance, then both points by your measurement are at rest, but they are also moving away from each other.

If you can put a ship in the middle of the big bang or matter incursion or whatever and measure the distance to the interface/surface of the expanding thing, then you can stay still, if the rates measured on all the sensors are equal. That is like hovering in the middle of a expanding balloon.

The ship is on the surface of the balloon and not in the middle of the balloon.

[Microdoser]

well I think the reference frame for still must be at zero velocity at the start of time / motion. A velocity at which it looks like everything is moving away from you.

What is time?

Some people think, based on the fact that when you fall into a black hole space becomes timelike, and inwards becomes forwards in time, that this creates a new spacetime where 'inwards' is the future, and the past leads to the event horizon. It neatly explains why there is a limit on the speed of light, for example.
 
Anyway, this would mean that the answer to 'What is time' would be, 'it is the physical manifestation resulting from us falling towards a singularity that exists at the end of time'. We do this at the speed of light.