Why can't time be constant for everything in the universe?

[Iggy]

(When a<r<b the potential will be some linear function of r which keeps the potential function smooth, but I can't be bothered to work it out right now.)

I know it's tangent to the problem you were working, but I'm pretty sure one gets,

 

[math]\phi_{®} = - \frac{GM}{2r} \left( \frac{3rb^2-r^3-2a^3}{b^3-a^3} \right)[/math]

 

for, like you say, a<r<b

 

 

 

or... for density...

 

[math]\phi_{®} = - \frac{2}{3} G \frac{\pi \rho}{r} (3rb^2-r^3-2a^3)[/math]

 

---

 

 

So, events don't happen simultaneously because there is no way to measure two events as being simultaneous? And events cannot be measured as being simultaneous simply because different frames measure events differently?

Am I correct in stating that the concept of simultaneity is essentially meaningless given the universe we live in?

 

It seems everyone can agree to a duration that is defined by the Big Bang at one end and 'now' at the other. Since the universe is less than 14 billion years old, if there existed two stars that lasted a total of 7 billion years each, can't we say that for at least part of their lives they were 'burning' simultaneously? Were they burning simultaneously, even if we cannot agree on exactly when they were burning simultaneously? When I look at an example like this it seems like we simply cannot agree on simultaneity, not that it doesn't exist.

 

I suspect I am simply looking at what you mean by simultaneous in a non relativistic manner, and perhaps that is what is hindering me from fully understanding.

 

 

Here is an interesting quote on that...

 

 

The question that may be raised in philosophical cosmology is whether or not this cosmic time constitutes an “absolute time” in the sense that Einstein rejected in his special theory of relativity. “Absolute time” and “relative time” may be defined in terms of the relation of simultaneity. If time is absolute, then this relation is two-termed, and is expressed by sentences of the form “x is simultaneous with y.” If time is relative, then the simultaneity relation is three-termed and is expressed by “x is simultaneous with y relative to z,” where z is the reference frame relative to which x and y are simultaneous. This suggests that the cosmic time posited by big bang cosmology is not absolute time, since the time measurements are made relative to the 'privileged' reference frame. For example, the assertion that the age of the universe is about 15 billion years old is elliptical for the statement “relative to the 'privileged' reference frame, the universe is 15 billion years old.”

 

Encyclopedia of time

 

"Simultaneous" isn't absolute, just like "tall" and "left" aren't absolute.

 

An object can be tall and short at the same time. Tall relative to one thing and short relative to another. Your left could be my right. The same direction -- the direction we are both pointing at -- is both left and right. Same with "simultaneous". When you said...

 

I think that this feeling is correct. There are two things happening right now, at the same time, here and in the Andromeda Galaxy.

It seems to me that the problem is determining which two events are happening simultaneously.

You can't determine 'which two events happen simultaneously', because they are both simultaneous and not simultaneous relative to different frames. It would be like determining which direction in the universe is really "up".

[Delta1212]

So, events don't happen simultaneously because there is no way to measure two events as being simultaneous? And events cannot be measured as being simultaneous simply because different frames measure events differently?

Am I correct in stating that the concept of simultaneity is essentially meaningless given the universe we live in?

 

It seems everyone can agree to a duration that is defined by the Big Bang at one end and 'now' at the other. Since the universe is less than 14 billion years old, if there existed two stars that lasted a total of 7 billion years each, can't we say that for at least part of their lives they were 'burning' simultaneously? Were they burning simultaneously, even if we cannot agree on exactly when they were burning simultaneously? When I look at an example like this it seems like we simply cannot agree on simultaneity, not that it doesn't exist.

 

I suspect I am simply looking at what you mean by simultaneous in a non relativistic manner, and perhaps that is what is hindering me from fully understanding.

Whether we can say that the lives of the two stars overlap depends on how far apart they are.

 

Let's say that, in the frame where the stars burn for 7 billion years, they are 3 billion lightyears apart.

 

Light from the first star leaves toward the second star at the beginning of its lifespan. (Event A). Upon arriving at the second star, light from that star heads back to the first star. (Event B). Finally, the light from the second star reaches the first star before it dies. (Event C).

 

Now, no matter what frame you are in, A happened before B, which happened before C. Since the first star was "alive" for the time from event A to event C, and since event B happened between A and C and the second star was alive at B, you can say that both stars were alive at the same time, and every frame will agree that this is true. They will not, however, agree on what points in the lifetime of each star correspond to one another.

[MigL]

Its that darn common sense misleading us again zapatos.

How can two events be 'simultaneous' when they happen at differing times in different locations or frames ?

 

Common sense also tells us that even if we cannot know the position and momentum of an electron, it must be moving at a specific speed at an exact location. And that is also wrong.

 

we have to unlearn common sense when dealing with uncommon situations like extreme energies, speeds and scales ( large and small ).

[hypervalent_iodine]

!

Moderator Note

robomont,

 

I have removed your posts from this thread and would ask that you please cease the attempts to hijack this thread with speculative content.

 

 

[zapatos]

Thank you everyone. I believe I've got it! It helps a lot to have it explained in so many different ways.