If every part of the universe has aged differently owing to time running differently for each part, why do we say the universe is 13.8 billion years old?

Question

For some parts relative to us, only a billion years would have passed, for others maybe 20?

In: Physics

Answer 1

Man, i just happen to be almost done reading Einsteins booklet on relativity. Think of it this way… if we view the spacetime continuum, or just, the universe, as a long sheet of rubber, like bubble gum flattened and stretched out as a big sheet, then we can also imagine that gravitational fields that exist on that sheet can cause some parts of it to ‘bubble up’ a bit and stretch, others to contract. If you were to stand on one area of the sheet of bubble gum from the estimated start of the universe, your own gravitational field would allow you to perceive the length of “time” that has passed for you until now. Remember however, that this big sheet of bubblegum is connected everywhere, and those ‘distortions’ are all connected to in the curves of spacetime. But here’s the big problem with our conception of this. You immediately want to think about how being in a different part of the universe would affect you and how you would age vs your friends on earth, etc… this makes things hard to conceptualize. If we don’t call time ‘time’ anymore, and instead just perceive it as another dimension, just another numerical value, you can then start to understand… Let’s now think of the universe as a 2D finite plane. With the third dimension as time, we can start to piece things together. If we look at the plane just from directly above, it appears perfectly flat; however, looking at this from the edge reveals something odd. The plane has bumps and differences in height in different places. We can now see ‘valleys and peaks and mountains’ of varying heights. Let’s consider this depth, the third dimension in a seemingly 2D universe ‘time2D’. For the beings that live on that universe, ‘time2D’ affects their perceptions of life and rates of change from different perspectives. We however only see it as different heights on what looks like the aforementioned stretched bubblegum sheet. We know, that regardless of how those beings have perceived ‘time2D’ in different locations, the seemingly 2D universe has existed for a certain period of our human earth time which we know is true and measurable. To them, it is only measurable by relative time from each individual perspective because of the bumps and stretches that define ‘time2D’ as their third dimension. Just apply that to our perceptions of the 3 dimensional world. We can only measure the universe’s age from this location in the universe, oherwise we might perceive a different measure.