Thread: expantion, accellertion, all relative

Could it be? As our universe expands and becomes less dense. Gravity has less effect on the relative passage of time. . . . Im try'n to wrap my head around this. if a ship is sailing away from shore at a constant speed but time on shore is passing slower and slower relative to time within the space between ship and shore. The ship would appear, as viewed from shore, to be accelerating from shore.
This thought occurred to me years ago. Would SOMEONE please tell me why Im wrong about this. Cause there's noway a painter from New Jersey solved this riddle.
It is my understanding that the galaxies is for the most part stable and not flying apart themselves. The gravity within them could be considered a constant for the purpose of this discussion. Galaxies are flying away from each other so the gravity between them is fleeting. Just want to make it clear. In my analogy the shore is our galaxy. The ship is other galaxy or galaxies and the sea is space.
I welcome all responses.
thank you,
Tom
expantion? I mean expanding!

Tom, yes - all relative, and not intuitive and so hard for everyone to get their head around. But also relativity has made some predictions about behavior that have been confirmed by observation (and are different behavior than classical physics predicts).

A long response below, but I think that you are merging two or three things that are not really related.

The time effect that you mention is real (but not quite what you describe - see below. The effect is called time dilation). But it is not what makes it look like the galaxies are moving apart from each other ie the expansion of the universe. They really are moving apart - as a consequence of the Big Bang (think of debris from an explosion flying away from the site of the explosion).
You're thinking that the galaxies are accelerating apart (or appear to be). I don't think that's true. They are moving apart at a constant speed (or something very close to that for our timescales). And that's how they appear to be moving too. Again, think of debris from an explosion.

There's a big question in cosmology about whether the galaxies will
a) continue to fly apart forever, or
b) eventually reverse their flight and fall back together because of gravitational attraction.

Again, this is not to do with time dilation (the time effect on the ship that you describe). It's more like a question of whether the initial explosion was violent enough that the debris (galaxies) reached escape velocity and will continue to travel outwards forever, or whether the explosion was not violent enough so that gravitation attraction eventually pulls everything back together.
Kind of like throwing a ball into the air on Earth - mostly b) is what applies: the ball and Earth move apart for a while, but their relative speed decreases and eventually they move back together because of gravitational attraction. But if you could throw a ball into the air fast enough it would escape Earth's garavity and move away from Earth forever. That's a) above.

Although there's a huge amount of research being done on whether a) or b) is what will happen there's no definitive answer yet. And the research is raising some other interesting questions because the observed data seems inconsistent.

I don't know if that helps you.
There's a bunch more detail about some of the details in your post below. The thing to bear in mind is that there's no link between the galaxies flying apart and the time dilation effect.

--- More details ---

In your analogy I believe you haven't got everything quite right. Plus, this is an area where Einstein saw something peculiar in Special Relativity and which gave rise to new insights soon after Special Relativity was first introduced. I referring to the Twins Paradox. So, no surprises that this subject is not obvious.

Anyway.
In a ship sailing away from shore at a constant speed time passes more slowly for those on the ship than it does for those on the shore. This is called time dilation.
However
a) the ship does not appear to be accelerating to those on the shore. It's travelling at a constant speed. (The people on the ship and the people on the shore may not be able to agree what that speed is, but they will agree that it's constant). Time dilation is a function only of velocity, not of distance apart.
b) there is no continuous time shift in the space between the shore and the ship. Time passes at different rates on shore and in the ship, and at any point between the shore and the ship time passes at a rate that only depends on how that point is moving - not on where the point is. So if the point is stationary with respect to the shore time will pass at the same rate as on the shore regardless of how close to the ship or to the shore the point is. Equally if the point is stationary with respect to the ship time will pass at the same rate as on the ship.

This effect has been demonstrated many times in the last hundred years, so to some extent you don't have to understand why it is, but you should accept that this is the way it is (even though it seems strange).

So, on to the galaxies.
Gravity within a galaxy is constant.
Not totally - every body within a galaxy is acted on mostly by its nearest neighbors, so locally gravity's strength is set by what is around you. Lots of variation there.
But also on the large scale throughout a galaxy there is a gravitational attraction towards the center of the galaxy. That's what holds the galaxy together. And the strength of that attraction towards the center is weaker nearer to the center.
But, as you say - that might be irrelevant for this discussion.

Galaxies are flying away from each other so the gravity between them is fleeting.
Not true - the gravitational attraction between them is not small.
You can't assume that just because galaxies are moving apart that the gravitational attraction is small.
Consider a ball thrown upwards from Earth.
If you observe the ball for a short while while it's rising you might conclude that gravity is a small effect - the ball is moving away from the Earth quite quickly and continues to do so. It's only when you observe the ball for a longer time you notice first of all that it moves away from Earth more and more slowly. And if you observe for longer you see that it reverses direction and falls back to the Earth.
We haven't observed the galaxies for long enough and accurately enough to be able to tell if the gravitational attraction between them will ultimately bring them back together. (But there is very active research into cosmological models that seek to predict what will happen based on other measurements).