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 🙂
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It’s something like that, but not quite because of what you said. “Right now”, if such a thing existed with precision, people from millions of light years away would see our sun as it was millions of years ago.
The real reason there is no true universal timeline is that time stretches or contracts (speeds up or slows down) depending on how fast you’re going compared to whatever exerts some force upon you (usually gravity). Everything travels at different relative speeds, which means there’s no true universal clock.
Even here on earth, time passes differently for astronauts in the ISS than it does for us. People on top of mountains don’t see time at the same rythm as people on the bottom of ocean, one of them gains something like a trillionth of a second per year on the other.
Here on earth, at the local level, establishing a timeline is easy because everything has a frame of reference that references the same things: The Earth, sun and moon. Get wider than this, and the idea of a universal timeline starts looking like a wild simplification or approximation at best.
Hi /u/Almeidaboo!
>[T]the stars we see are light from millions of light years away
The distance of stars visible to us with the naked eye is one of the exceedingly rare instances where laypeople **overestimate** astronomical distances. The furthest star we can see unaided is V762 Cassiopeiae, which is 16,300 lightyears away from us. Thus, all the starlight we see is less than 17,000 years “old”.
>is the whole universe “existing” in the same time?
Sort of. Not really.
The theory of relativity does indeed tell us, that two events which occur simultaneously from the point of view of one observer O do not generally occur simultaneously for another observer O’ moving at a high speed relative to O.
Furthermore, two events A and B which occur in the order A, B from the POV of observer O may occur in the order B, A from the POV of O’.
**However**, the order of two events A, B can only be different to two observers, if they are not causally connected. That is, if event A can theoretically influence event B under the constraint of traveling at or below the speed of light, then the order of events is the same for all observers.
Best way I can think of explaining it like this. Tom and Jane are living in different parts of the world. Tom sends a letter to Jane, that says, I’m having pizza for lunch! The letter takes a week to arrive through snail mail.
When Jane receives the letter, she is now informed, that Tom is having pizza for lunch. But she knows that this was a week ago, because that’s how long the letter takes to gets delivered. Tom could be having something else for lunch at the present moment, but she will never know what he is having at the exact time she is, because information via mail will always take a week to arrive.
The letter in this example is light. The speed of the letter being sent is light speed. Tom and Jane can be sending letters to each other, but they will always only know what the other person was doing a week ago in real time. But the present time passes universally for both of them.
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.
Spacetime. Space and time are one in the same. Every different “location” is a different “time”.
So … the whole universe is only existing in the same time as everything else as everything else is existing in the same space as everything else.
It becomes a matter of interpretation at this point.
Lets think of this like having penpals.
You send letters to a friend across the world, they send them back. Lets say it takes 5 days to reach them.
They describe their days, their challenges, their triumphs. You can’t celebrate on the same day they got a promotion, nor console them the day they lost a loved one.
But their life happens at the same time yours does. They got a promotion the same day you found a new favorite pizza place. Those two events aren’t connected at all, but they occurred on the same day. These event’s *can’t* be connected, as you won’t learn about their promotion for 5 days.
However, if you learn of the promotion, then decide to send them a bottle of Champaign to celebrate their promotion and they will receive this 10 days after their promotion. 5 days for you to learn about it, 5 days for them to receive the Champaign. These events can be (and are) linked.
We, and distant stars, exist at the same time. However what happens to the star can’t impact us until a long time has passed. And our reaction to it, cannot effect the star for a similarly long time.
Most interactions are so small there is no real impact.
However things like orbits are significant enough. Our star is shifted due the gravitation of other stars. Not much, but some. However we are shifted towards where they were when they ‘sent’ their message, not where they are now.
> 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?
Yes.
> 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?
They might be, but there’s no way for us (or them) to know.
The five-year old explanation:
Bobby lives on one side of town. Alice lives on the other side of town. Neither family has phones or Internet or chimneys for smoke signals (it’s a very poor town).
The only way Bobby and Alice can talk to each other is to ride a bicycle across town — about a 30 minute bike ride — and deliver the message in person.
Alice’s little brother is shooting baskets. He’s up to ten consecutive baskets! This is big news (it’s also a very boring town), so Alice hops on her bike and rides 30 minutes across town to tell Bobby the news.
She arrives at Bobby’s house. “Bobby, Bobby,” she yells, “my dumbass little brother Toby hit ten free throws in a row!”
Pete — being appropriately amazed — says “THAT’S AMAZING! How many freethrows is he up to *right now*?!?”
So. Can any of these hypothetical people (Bobby, Pete, or Alice) know the answer to this question? Of course not.
We can make guesses about what Li’l Dumbass Toby is doing right now: maybe he’s up to 300 free throws, maybe he missed at #12, maybe he quit and went inside for a sandwich … or maybe the *entire house was swallowed by a sinkhole* killing Toby and Alice’s parents and Alice is now an orphan.
There are many things that could be happening with Toby *at the same time* Bobby and Alice are having this conversation, but there is *no way for Bobby and Alice to know for sure* because **information** from that side of the town can only travel to the other side of the town at the speed of bicycle.
Now let’s replace this with astronomy.
Star 1 is on one side of the galaxy. Star 2 is on the other side of the galaxy.
The only way information from Star 2 to get to Star 1 is for electromagnetic radiation (e.g. light or radio waves or whatever) to travel at the speed of light across the galaxy — about 100,000 years from Star 2 to Star 1.
Star 2 is unstable and the wavelength and intensity of the light it is giving off indicates it might explode as a supernova. This is big news! That light that indicates “possible supernova” starts traveling across space, across the entire galaxy at the speed of light, as one does if you’re light.
100,000 years later that light hits a telescope on a tiny blue marble of a planet. And the astronomer looking at that light (whose name is Bobby) says “Dude, dude, look at this light — this star looks like it might explode!”
The other astronmer, Pete — being appropriately amazed — says “THAT’S AMAZING! Has it exploded or is it still there *right now*?!?”
As above, we can only make guesses about whether Star 2 exploded alread or hasn’t yet.
There are many things that could be happening with Star 2 *at the same time* Astronomer Bobby and Astronomer Pete are having this conversation, but there is *no way for them to know for sure* because **information** from that side of the galaxy can only travel to the other side of the galaxy at the speed of light.
Actually, this is not a stupid question at all. You’re very close to understanding Special Relativity. There is no universal “now” but it’s not because of how distant things are, it’s because they are moving relative to each other.
Thing of time and space as a loaf of bread, with each slice being a “moment” in time and the bread as all the things that happened at that moment. If we are moving relative to each other, I will experience the universe as if it were sliced at an angle compared to you. In other words, you might see two things as happening at the same time while I see one as happening before the other. And both viewpoints are perfectly valid (because in Relativity there is no way to say that one of us is “really” moving and the other is stationary).
So that’s why there is no “universal time”.
By the way, this always bothers me in science-fiction movies or in the Marvel movies when they say something on some distant planet is happening “at the same time” as something else on Earth: what “the same time” means depends on how you travel between the two places.
>If the light we see from stars in the sky are actually “the past” as they’ve left their source light years ago
Light year is a measure of distance, not time, and thus has no ‘ago’.
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