So the Newtonian concept of absolute time has been discarded in favor of the relativistic concept of Time: it is not absolute, and flows at different rates for different observers. Both the frame of reference and the presence of a gravitational well influences the time you experience passing.
So when scientists say “the universe is 13 billion years old”, are they not making an absolute statement about how much time has passed since the Big Bang?
If you think of each galaxy as an accelerating frame of reference, they will all disagree with this statement, as their clocks read differently. In fact, no two measurements are likely to agree exactly on how much time has passed since the BB.
I think the OP is correct. Beings in other galaxies will have other measurements. But for everyone on THIS planet, it’s the same 13 or 14 billion years.
The OP is right: Time elapsed depends on your reference frame, and the laws of physics don’t distinguish between different reference frames. However, according to our current best models of the Universe, the Universe does in fact have a set of reference frames associated with it which are, in a global sense, preferred.
This calls for some further elaboration. Here on Earth, I could work in any of a multitude of different reference frames. Any local experiment I ran in any of those reference frames would work out the same way, and give the same results. In any of those reference frames, I could also measure the age of the Universe, and I would get a different answer in different reference frames. This is not a contradiction, because a measurement of the age of the Universe is inherently not a local measurement. In no reference frame would I find an age greater than 13.798 billion years (to within the error bars), and I would find that age in exactly one reference frame. That one reference frame is therefore, in some sense, special, and also has some other nice properties: For instance, in that frame and that frame alone the Universe will appear to be isotropic (the same in every direction).
If I were in some other galaxy, billions of lightyears away from Earth, I could make the same statements: Local experiments won’t care about reference frames, but cosmic-scale measurements would see differences in different reference frames, and there would be exactly one reference frame where the age of the Universe was a maximum, at 13.798 billion years. However, that reference frame would not be the same reference frame as the one that’s favored at the Earth. Every point in the Universe has a different preferred reference frame associated with that point.
By way of analogy, consider different points on the surface of the Earth. I can draw planes in 3D space anywhere on the Earth, but at any given point, there’s one plane that’s special, because it’s horizontal (i.e., parallel to the surface of the Earth). But the horizontal planes at two different points on the Earth are not parallel to each other.
Does the Cosmic Microwave Background radiation not give an absolute answer? Aliens on another world might measure it locally as 20 billion blurgs, but that would be equal to our 13.7 billion years.
The reference frame in which the CMB is isotropic is exactly the reference frame Chronos means. It’s just that the reference frame that makes the CMB isotropic is different for every point is space.
The cosmic microwave background is a bit overhyped-- It’s not inherent to the Universe itself, but just to some stuff in the Universe. That said, though, the material which emitted the CMB was, by and large, all in its locally-preferred frame (which means, as said above, that different particles of it in different locations were in different frames), so it can be used as a simple and convenient proxy for those frames. In particular, at any given location the reference frame where one sees the CMB as being approximately isotropic is very close to the reference frame which is preferred for that location.
One of my continuous, ongoing intellectual shortcomings has been inability to understand the concept of the frame of reference, so please excuse any naive questions.
Does Earth’s one reference frame which provides the maximum age of the universe have some sort of geographical location, or is it located in a certain class of objects which might be anywhere on Earth geographically, or does it possess any other kind of physically identifying characteristic?
The 3D plane analogy is helpful. Can you perhaps provide another analogy to help guide the perplexed to a better understanding of the one maximal reference frame on Earth?
Forget about trying to localize things down to some specific point on the Earth. We’re talking about cosmological distance scales, here: For practical purposes, any point in the same galaxy is “the same place”. Everywhere I wrote “Earth” above, replace it with “the Milky Way”.
It’s also worth pointing out that the age of the Universe for at any given event could be defined as the maximal proper time along any past-directed timelike curve originating at that event. Which physically would represent the maximum age of any observer arriving at that event. This is coordinate-independent, but would give you the same age of the Universe for an event as would be given by the standard definition. I see Chronos has hinted at this in an earlier post.
Tbh the whole concept of reference frames can be misleading, it’s applied to several different theoretical constructs, not all of which have a clear physical meaning all of the time.
To carry on from what I said in my last post: if you like the age of Universe would be the maximum time displayed on any theoretical clock (that keeps the ‘correct’ time) arriving at my precise location right now. The time such a theoretical clcok displays would be dependent upon the complete history of its motion in the Universe.
I would like to know more about the properties of the frame of reference where the maximum age of the universe is observed. Is there anything you can tell us besides that its location, for us, consists of the Milky Way galaxy? It might become clearer with a description some of the frames of reference where the age of the universe is found, for us, to be less than the maximum.
I also wonder if there is a frame of reference giving a minimum age. Is there, and if there is, what is that minimum age?
I think it is better to talk about observers than frames of reference, the problem being with the latter that you need to define exactly what you mean, whereas an observer has a natural definition in terms of a timelike curve. An observer who starts at the big bang and sees an isotropic Universe (this of course assumes the Universe can always appear isotropic by selecting the right observer) until he arrives at present-day Earth will experience the maximum amount of time possible between the big bang and the present-day Earth.
There is no minimum age. That is you can always pick an observer for which the amount of time experienced by that observer starting at the big bang and arriving at present-day Earth is as small as you like (but non-zero).
Frames of reference, in the sense being used here, don’t have a location, just a velocity. Each of these special frames of reference is associated with one location, but it does not exist only in that location. If someone in that far-distant galaxy started running really fast in this direction, they’d be in our frame of reference, even though they’re very far away.
I can also say that most objects in the Universe tend to have velocities very close to that of their local “special” frame of reference. This is true simply because if your velocity isn’t close to that, you’ll be moving towards the place in the Universe where it is. Or to turn this around, the frame of reference associated with a particular galaxy will be close to the frame of reference where that galaxy is at rest.
Nope, you can have an age that’s arbitrarily small.
I thought that there was no preferred “center” of the universe. That is, I thought that we would see pretty much the same thing as we do here from any galaxy in the universe.
(That is, assuming the universe is isotropic. Or does that assumption merely imply the preceding statement?)
Now that I review the statements of Asymp. Fat, I see that he requires the preferred observer to live a life where he always sees the universe as isotropic. Is this another (more precise) way of saying that the observer is at rest with respect to the ‘fixed stars’, as they say in basic general relativity texts (or even better, at rest with the CMB)? I think Chronos and Pasta said the same thing a bit differently.
I suppose this is what it would be like to live in a galaxy with no relevant gravitational interactions with the rest of the universe (that is, no local group, no supercluster, no “Great Attractor”, etc).
All three of us are agreeing. And Asymptotically Fat’s hypothetical observer could make a claim at being “in the center of the Universe”, but so could a multitude of other hypothetical observers: There’s one such hypothetical observer for every point in the Universe.
The property you’re looking for is homogeneous, but they’re related. If the Universe is isotropic from at least two different points, then it’s isotropic and homogeneous everywhere. The reverse does not necessarily hold: You could have a Universe that is everywhere homogeneous, but nowhere isotropic.
I thought that the universe is anything but homogenous. All clumpy with stars, planets, cluods, and vacuum. Even on the biggest scales it’s all “bubbly”.
Observationally, homogeneity is a statistical concept. Physically, it says that the laws of physics are the same everywhere, even if those laws can give rise to observable differences. These differences appear more starkly on small scales.
As an analogy, imagine you had an infinite chess board for which the color of each square was decided by a random process, say a fair coin flip. After coloring the board this way, you will find all sorts of clumps and apparent patterns, but it is still homogeneous in the cosmological sense. Statistically, any region of the board is the same as any other region. The rate of, say, clumps of four black squares surrounded by white squares should be independent of where you look.
On the other hand, if your process were “heads and this square is the same color as the one immediately above it, tails it’s opposite”, and your coin turned out to be biased, then your chessboard would still be homogeneous, but it would be anisotropic: You’d tend to see more vertically-aligned features than horizontal ones. Actually, even with a fair coin, it’s still be somewhat anisotropic, just by virtue of the fact that it’s a grid, though that doesn’t show up on scales significantly larger than the grid size.
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