1

**Physics, Astronomy & Cosmology / Re: Where did the big bang come from?**

« **on:**

**Yesterday**at 06:27:16 »

Time elapse differently at different speeds, that part I already had. The question is how is this objective?Time dilation is due to speed. It is relative, abstract. It is not objective.

So for instance, if clock A and B are moving relative to each other at 0.99875c, clock B will be dilated by a factor of 20 in the frame in which A is stationary, and clock B will be dilated by a factor of 20 in the frame in which B is stationary. There is no objective 'stationary'. Each is simply moving relative to the other.

A very related (and probably most important part) is relativity of simultaneity. That means when comparing two events (say when clock A says 1 hour, and when clock B says one hour, relative one inertial frame the A event happens first, and relative some other inertial frame the B event happens first, and relative to some third inertial frame, the two events are simultaneous.

Simultaneity, being frame dependent, is also completely abstract, not objective. It is established by convention, usually a convention that is based on a choice of frame.

Differential aging is something else. If clock A and B were together at some point, they can be compared, or they can be set to the same value then. If, later on, the are reunited (very much like the airplane going around the world and back to the airport clock), the times can be compared and one might be found to show more or less elapsed time, and is an example of differential aging. This is objective, physical, and not abstract. Relative to any frame (inertial or not), that comparison will be the same. One clock has taken a shorter path (called an interval) through spacetime, and clocks measure the spacetime interval of the path it takes. Spacetime intervals are frame invariant.

The only way two clocks can be together, move apart, and then be together again later on, is for one or both of them to accelerate at some point. If a clock accelerates, then there is no inertial frame in which that clock is always stationary, so the dilation of that clock relative to some frame gets a little more complicated.

If you're completely unfamiliar with these concepts, then all this can be pretty hard to swallow, but I assure you it is accurate. I applaud your willingness to read the posts and ask questions about the unclear parts.