Hi all, I didn’t know how to formulate the question in a non stupid way so I’ll explain.
If the light we see from stars in the sky are actually “the past” as they’ve left their source light years ago, from another point in the universe another planet sees our Sun’s light the same way, correct?
If that’s the case, if there was an “universal year” or an “Universe’s current year”, would all the stars and planets be living in the same year?
Maybe I am 5, I feel 5 right now.
Thanks 🙂
In: 466
There is no such thing as “right now” in physics.
Time is relative. Two things can happen at the same time for one observer but at different times for another. With both perspectives being equally valid. Fortunately most of these effects can be corrected for with enough information, and it smooths out over large scales. But it does mean that we have to be careful.
Ideas about distance and time in cosmology get a bit awkward, and terms have to be defined carefully. For example, cosmology uses both “proper” distance and what it calls “comoving” distance where it factors out universal expansion. Similarly there is a “comoving time”, which measures time from the Big Bang based on the perspective of a “comoving observer” who is at a fixed “comoving” position (only “moving” based on universal expansion).
When we talk about the age of the universe we mean the time from the start of the universe to our local “now”, from the perspective of one of these “comoving observers” (someone who is “fixed” in space, with disclaimers as to what this actually means). We measure the age based on cosmic microwave background radiation, with some assumptions and other measurements throw in. The maths used to do this comes from things called the Friedmann equations, which are derived from General Relativity. The maths is awkward, and getting all the right numbers is very difficult, but it gives us a number of about 13.8 billion years.
Cosmology is an area of physics that gets deep very quickly.
Latest Answers