how big is universe ?

The Twin Paradox:

https://en.wikipedia.org/wiki/Twin_paradox

The paradox is not that one twin staying home ages more quickly than the other who goes off somewhere and returns. It's that each twin sees the other as moving away and then coming back, so why does only one of them age more quickly?

As the article mentions, however, the experience of the twins are not identical. One of them experiences acceleration to get up to speed, get somewhere, stop, and then return. However, the degree of time dilation depends only on how close the traveling twin gets to c and/or how long the traveling twin travels. These parameters are independent of acceleration and gravity issues. More fundamentally, however, the traveling twin occupies two inertial frames during his trek: an outbound frame and an inbound frame. The resting twin does not do this.

That's all there is to that. But there is also this from the article concerning the "leaving" and "returning" phases of the traveling twin's trip:

Explanations put forth by Albert Einstein and Max Born invoked gravitational time dilation to explain the aging as a direct effect of acceleration.[8] However, it has been proven that neither general relativity,[9][10][11][12][13] nor even acceleration, are necessary to explain the effect, as the effect still applies to a theoretical observer that can invert the direction of motion instantly, maintaining constant speed all through the two phases of the trip. Such observer can be thought of as a pair of observers, one travelling away from the starting point and another travelling toward it, passing by each other where the turnaround point would be. At this moment, the clock reading in the first observer is transferred to the second one, both maintaining constant speed, with both trip times being added at the end of their journey.

@Soothfast
I am not sure if you are agreeing with me or disagreeing with me.

@metal-brain said
@Soothfast
I am not sure if you are agreeing with me or disagreeing with me.

You're just not paying attention.

Your "equivalence principle" that "time dilation is gravity" is only valid in the context of general relativity, not special relativity. Ergo it is not a principle that applies to the universe at large.

The PBS Nova programs have not deceived you, but you have misconstrued their message.

I've said it before, but you didn't believe it. You still won't believe it, but maybe what I've said will be of interest to others.

@soothfast said
Oh, you need a one-line summary. Very well.

Your "equivalence principle" that "time dilation is gravity" is only valid in the context of general relativity, not special relativity.

I've said it before, but you don't believe it. You still won't believe it, but I'm not here for your benefit.

I never said otherwise. I meant time dilation from mass, not speed. I simply was not referring to SR, which is why I asked why you keep bringing it up. It is not useful in the context I am talking about because I was not talking about speed. You are, but you are just digressing unnecessarily into SR. I was not talking about that, so why are you? You did not prove anything by bringing it up.

The "equivalence principle" points out the link between GR and SR though. You can get time dilation from mass and speed. One does not disprove the other. If you think it does, that is where you went wrong. You can have both and we do. Our sun is traveling at a pretty good speed around the galaxy. That means we are too. We are experiencing time dilation from both mass and speed. More from mass though.

@metal-brain said

Our sun is traveling at a pretty good speed around the galaxy. That means we are too. We are experiencing time dilation from both mass and speed. More from mass though.

Our Solar System is traveling 446,400 mph around the center of the Milky Way... It will take approx. 230 Million years to make one orbit.

@ogb said
Our Solar System is traveling 446,400 mph around the center of the Milky Way... It will take approx. 230 Million years to make one orbit.

The vast distance makes that pretty slow relatively speaking. A lot slower than the speed of light anyway. The reality is that a relatively slow speed like that would cause very little time dilation, but any speed causes some.

Mass causes time dilation. That time dilation is the force that causes gravity, but I am using the word cause loosely. Time dilation is gravity. Things are attracted to where time passes slower. As Brian Greene put it "things do not like to age".

The big question is "why does mass cause time dilation"?
It is a mystery. Maybe it has something to do with matter displacing virtual particles. Maybe not. Hard to say.

@metal-brain said
I never said otherwise. I meant time dilation from mass, not speed. I simply was not referring to SR, which is why I asked why you keep bringing it up. It is not useful in the context I am talking about because I was not talking about speed. You are, but you are just digressing unnecessarily into SR. I was not talking about that, so why are you? You did not prove anything ...[text shortened]... means we are too. We are experiencing time dilation from both mass and speed. More from mass though.

I'll take the matter as settled, then.

It's irrelevant where you move your goalposts next.

@soothfast said
I'll take the matter as settled, then.

It's irrelevant where you move your goalposts next.

YouTube&t=470s

@Metal-Brain
The thing you cannot accept is time dilation is not the fundamental of physics since there are two separate affects causing it. There is more going on in the universe than you or most other folks do not understand yet. Which is why we don't have a theory of everything yet, because we have not combined relativity and quantum physics under one umbrella, not yet anyway, some future genius might do it though.

@sonhouse said
@Metal-Brain
The thing you cannot accept is time dilation is not the fundamental of physics since there are two separate affects causing it. There is more going on in the universe than you or most other folks do not understand yet. Which is why we don't have a theory of everything yet, because we have not combined relativity and quantum physics under one umbrella, not yet anyway, some future genius might do it though.

You are wrong. Time is fundamental to the universe and your existence. Without time nothing would exist.

@metal-brain said
You are wrong. Time is fundamental to the universe and your existence. Without time nothing would exist.

That may be, but now you're moving the goal posts again by substituting "time" for "time dilation."

@soothfast said
That may be, but now you're moving the goal posts again by substituting "time" for "time dilation."

Are you trying to argue time dilation means nothing?

@metal-brain said
The vast distance makes that pretty slow relatively speaking. A lot slower than the speed of light anyway. The reality is that a relatively slow speed like that would cause very little time dilation, but any speed causes some.

Mass causes time dilation. That time dilation is the force that causes gravity, but I am using the word cause loosely. Time dilation is gravity ...[text shortened]... tery. Maybe it has something to do with matter displacing virtual particles. Maybe not. Hard to say.

“ Things are attracted to where time passes slower. As Brian Greene put it "things do not like to age". ”
😂😂

@kevcvs57
Time passes noticeably slower when I’m at work but I’m not here because I’m in any way attracted to it.
Before you decided to screw this forum it was a great place to pop in for a read and get a heads up on the latest interesting stuff about reality but your constant misinterpretation of “life, the universe and everything” makes the threads hard to follow.
You are a complete misanthrope.

@soothfast said
That may be, but now you're moving the goal posts again by substituting "time" for "time dilation."

Are you suggesting there can be time dilation without time?
So silly.