For some parts relative to us, only a billion years would have passed, for others maybe 20?
In: Physics
I’m not smart enough to offer an answer, but this video on the [Twin Paradox](https://youtu.be/UInlBJ4UnoQ) might offer some additional insights.
The oldest anything in the universe could be is 13.8 billion years old. This would be a hypothetical object that came into existence at the big bang and has been stationary (called comoving) relative to the cosmic microwave background for its entire existence. You are correct that there is no universal time for the whole universe, and any reference frame is valid, but using the CMB makes the most sense since it’s the leftover radiation from the big bang.
It’s also important to note that most parts of the universe are pretty close to being comoving with the CMB, so most of the universe is pretty close to this age. The only places where you’d expect a large difference in the measure of elapsed time would be close to massive objects like black holes and things that have been moving at relativistic speeds for most of the existence of the universe.
No no, he’s got a point, it’s not even a “we are humans so we describe it in a way humans understand”
I get the whole “from our point of view, the universe is x age” but what do we gain from saying? Like, if we find a rock, and we know this rock has existed since the start of the universe, we cant say that its 13.8 billion years old because we dont know the speed at which its travelled for the last 13.8 billion of our years, it could be older if it travelled slower than us, or younger if it travelled faster, and thats ignoring the effect of gravity
I’m no expert but as I understand it everything comes down to frame of reference.
We are trying to measure the age of our part of the universe. The assumption is that physics works the same in every part of the universe so if we could instantly teleport to the farthest place we can see and take a measurement there we would get the same answer.
Of course time dilation, univers expansion and the like produced some interesting effects like Methuselah star:
And we’re not even so sure about the 13.8 billion years old part:
For 99.9% of the conversation, the only perspective that everyone understands is that of humans living on earth. So it makes sense to use that perspective as a measurement point. It would not make a whole lot of sense to say it is xx “years” old from the perspective of someone living on another planet since pretty much no one understands anything from that perspective.
We can say that a “year” on Jupiter is about 12 of earth years. Would it make much sense to say that someone is 2 Jupiter years old? It is perfectly definable and measurable, but such a measure is pretty meaningless to everyone. Science is about discovery and the communication of discovery. So, where possible, it is logical to choose a way of communication that is relatable to most people.
We’re simply judging by earth years not relativistic years. Sure technically, certain parts have advanced further in “time” due to the effect of gravity. But according to us here on earth, looking out. There has been approximately 13.8 billion years since the big bang. (A year being the time it takes for the earth to complete 1 full cycle around the sun)
DISCLAIMER: I am not a physicist,,, at all.
But iirc they say this because of the “cosmological constant” which is something in math that describes the rate to which the universe expands (because it is always expanding).
And by assuming everything started as 1 point in space (directly prior to the big bang) we can use that constant to determine when most of what we observed would be around the same point.
Again, not a physicist, so I apologize if that’s completely wrong.
Simple, because time is actually just a construct. It isn’t a physical thing, merely a label attached to relativistic perception.
So while the interaction between molecules is effected by their relativistic speed and thus “age” differently, the pure time of existence of the components is the same.
If a 20 year old human departs earth on a round trip spacecraft and returns 300 years later with the body of a 21 year old, it doesn’t matter that the human only perceived 1 year, his body still exists for those 300 years and so it is 320 years old from the earth standard.
A guy by the name of hubble figured out that some of the smudges in the nights sky were actually other galaxies and not just nebulea.
In figuring this out he also discovered that the ones which were further away from us were also moving away from us the fastest. Through plotting these galaxies on a graph of distance and speed he determined the rate of expansion called hubble’s constant.
Through a little mathematics (1/hubbles constant) we can determine how long the universe has been expanding for, approximately 14.4 billion years.
We measure the age of the universe by looking at the Oldest Light we can perceive: The Cosmic Microwave Background.
Light moves at a constant speed, and the expansion of the universe is also constant. The wavelength of Light traveling over such distance “expands” at a steady rate as a side effect of spatial expansion, producing a “red shift.” By measuring the amount of red shift, we know exactly how long the light has been traveling.
We have measured the age of the universe by measuring the amount of red shift in the oldest light we can see. There’s a possibility that there’s older light beyond that boundary, but it hasn’t reached us yet… and that means that we can say that the universe is *at least* as old as the light from the CMB.
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