I’m sure some version of this question has been asked on these boards, but here it goes…
I’m having trouble reconciling two theories in physics that most experts appear to agree on, at least according to the PBS Space Time series and the SDMB in-house contributors:
The age of the observable universe is measured to be 13.8 billion years based on the measurement of red shift from the light of the furthest stars. Anywhere you go in the universe, this is the age you would measure.
The Twin paradox tells us that an astronaut moving away from earth at close to the speed of light, and then returning, will be significantly younger than her twin upon arriving. If both of them set their watches to the exact same time prior to the astronaut’s departure, the watches would actually prove this upon her arrival.
But then wouldn’t their measurement of the age of the universe also differ by the same amount of years? What if the two watches were actually set to zero at the time of the Big Bang?
I’m sure I’m missing something or mis-interpreted something. Or both, but I’d appreciate help in understanding.
Different observers experience different amount of times, but there is a certain set of observers who maximize the time between the big bang and the present and it is these observers who define the age of the Universe.
To help understand why this is, look at this schematic spacetime
Time is left to right and one dimension of space is represented on the vertical axis (but using a concept of distance such that the distance between galaxies remain constant, rather than increases due to expansion). There are white, red and green observers who start somewhere near the big bang and meet at the present time. The white and red observers both travel with constant velocity, whereas the green observer accelerates in their journey.
You should be able to see that the line representing the white observer is the shortest line and is in fact the shortest possible line between the big bang and the point where the observers meet. Due to how spacetime is represented on the diagram, this means the white observer experiences the maximum amount possible amount of time.
Of course you have to take care a little as an MS Paint diagram doesn’t prove the existence of a maximal time observer in cosmological spacetime, but it is an imprecise visual representation of the proof.
I meant to add, the maximal time observer is the non-accelerating observer who observes the Universe around them to be isotropic (the same in all directions). The red observer is non-accelerating, but will observe a red shift and blue shift with a directional bias.
Thanks, the diagram really helps. I assume this means that we are following a path that is non-white and thus not maximal time observers. And if we had wristwatches on since the Big Bang, they would show something less than 13.8 billion years. Right?
Another way to look at it is that the universe is another watch that keeps ticking along at the same rate. While you are gone, both the watch of your twin back home and the universe will show the same amount of time has passed.
I believe that the explanations have answered my question. But to put my question another way, let’s assume that you and me were both given identical clocks exactly 1 second after the big bang. I was placed on a solid mass that eventually became planet earth, so my path to now and here involved some collisions and explosions and such. Meanwhile, you were placed on a spaceship that could travel at 99.9% of the speed of light, so you were able zip around the universe for a long long time before finally meeting back up with me for breakfast this morning. My clock would have shown time having elapsed longer than yours. My clock might read 10 billion years have elapsed, so that’s how old I would believe the universe to be. Yours might read 8 billion years, so you would think that it is much younger. Then ** Asympotically Fat** would stroll by and laugh at us and say “No you morons, it’s actually 13.8 billion years because you are not accounting for the relativistic effects on time based on your path to get here”
Except a clock sitting on a mass like planet Earth won’t experience a 30% time dilation. The mass of the Earth, and of a star like the Sun for that matter, are pretty small. You won’t get any decent time dilation unless you’re really close to a really massive object, and a regular old star isn’t massive enough. And nevermind that the Earth isn’t 10 billion years old, since that doesn’t matter for your hypothetical.
Yes, if you had a super-accurate clock and kept one on Earth and one in interplanetary space, when you bring the clocks back together you’ll find that the clock that spent time in Earth’s gravity field will be slower than the clock that didn’t. But it’s going to be a very very small difference. Luckily we have clocks that are really really accurate, and so we really can measure that difference of some fraction of a microsecond.
But we don’t measure the age of the universe by inspecting the internal clocks of objects or matter in the universe. We measure the age by observing the universe as a whole, and deducing how long it has existed - e.g. how long it has been expanding for.
Can you help me with this? It seems to my eyeballs that the white observer, the red observer, and the green observer experience the exact same amount of time. The white observer traveled less distance but they end up at the same “when”. What am I missing?
The diagram represents spacetime in a fashion and the arrows are akin to the “time axes” of each observer and the shorter the arrow the more time the observer experiences
There’s no point getting too hung up on “maximal time observers” (more usually called comoving observers or isotropic observers), they are purely hypothetical observers which occur in certain models that happen to be very good at explaining the Universe on a large scale, though we know these models are simplifications. However if we are going to define an age for the Universe it better match up with the time experienced by some observers, even if they are purely theoretical, and given that time experienced is observer-dependent we should have some justification for preferring to use those observers to define the age.
Yes, the “end up” at the same when. When all three step out of the cockpits and shake hands, they’re at the same time. If all they did was take out their watches and compare, they’d find their watches didn’t agree on how much time had passed since the Big Bang. But if all three of them set up three astronomy departments to try to figure out the age of the universe, they’re going to come up with the same answer. And that answer is going to agree with the white guy’s clock.
We don’t have a watch to consult that would measure how many subjective billion years have passed in our stretch of space-time since the beginning of the universe. So all we can do is take the second approach, and observe the universe, and we come up with the white guy’s answer. If a clock were physically possible it would actually be like the red guy’s clock. But…the slope of the line wouldn’t be the same as in the image. It would actually be really really really close to the white line, and only off by a pixel or something, which is what Chronos was saying.
I should’ve added what my diagram is in technical terms in amateurish 2D representation of 4D cosmological spacetime conformally mapped to 4D Minkowski (flat) spacetime and there are certain features that I have glossed over or not mentioned at all.
And to tie it back into special relativity: SR says that no one reference frame is preferred over others. But it’s not any one special reference frame that says what the age of the Universe is. It’s a different reference frame in every point in the Universe. You can refer to “the reference frame that’s co-moving right here”, or “the reference frame that’s co-moving over at that galaxy over there”, but those won’t be the same reference frame.
This sounds analogous to the fact that there’s a local level reference frame at every point on the Earth’s surface, but they are all different from each other.
"The age of the observable universe is measured to be 13.8 billion years based on the measurement of red shift from the light of the furthest stars. Anywhere you go in the universe, this is the age you would measure. "
Note the ‘observable’ ; Can there be other stars, more than 13.8 billion LY away, whose light just hasn’t reached us yet, but may someday?
Even the furthest stars that we see are more than 13.8 billion lightyears away. Their light has traveled 13.8 billion lightyears, but in that time, the stars themselves have gotten further away yet.
And there’s no reason to believe that there aren’t stars even further away that we can’t see yet, or even which we’ll never be able to see.
