Viewing a single comment thread. View all comments

Aseyhe t1_jb0odnt wrote

When we say that the universe is 13.7 billion years old, this is actually in the rest frame of the cosmic microwave background, not that of the Earth. However, the difference due to gravitational time dilation (mostly due to the galactic potential) and kinematic time dilation (since we're moving at ~370 km/s with respect to the cosmic microwave background) is of order one part in a million, so any ambiguity in the age of the universe due to time dilation is much smaller than the measurement uncertainty in the "13.7 billion years" value.

More generally, the question of whether the age of the universe depends on where you are depends entirely on what convention you adopt. There is no such thing as a universal "now". If you wanted, you could define that "now" means the elapsed time, in the cosmic microwave background frame, is 13.7 billion years. This convention is called "synchronous gauge" and is commonly used in cosmology calculations. Under this convention, the age of the universe does not depend on position.

For other conventions, like the "Newtonian gauge" that is also commonly used in calculations, the age of the universe does depend on position.


7eggert t1_jb0vkzk wrote

>There is such thing as a universal "now".

Is this missing a negation or did I miss something?


Aseyhe t1_jb0x01j wrote

yes, fixed, thanks!


pepe_silvia_12 t1_jb151yf wrote

I’m sorry could you explain that more please? There is no universal “now”?


Anonymous_Otters t1_jb1crzq wrote

The flow of time is relative, so time is flowing at different rates at different places depending on the curvature of the universe in respective locations and on relative velocity. Only when you get into the same gravity well in the same place can two objects be considered to be more or less at the same time. As soon as they leave the same gravity well or travel at different relative velocities, they are technically now in different times until they meet up again. That said, every single molecule, atom, subatomic particle is technically running at slightly different times even within the same object. Space and time are, under general relativity, considered inextricably linked, so can be thought of as the same thing. Changing space changes time, so being in even a slightly different spaces means you are in a slightly different timr. This is why the cosmic microwave background radiation is used as a universal resting position to make some sort of standard time.


Melkor15 t1_jb2286m wrote

This is truly amazing, thanks for your time explaining it.


JasonDJ t1_jb30ffx wrote

So, wait a tick…

If you’ve got two watches in perfect sync on earth, and one leaves at a low rate of speed (well below speed of light), where is “out” of our gravity well that they are significantly losing sync, and would that still hold true with atomic clocks?

Assuming we had some sort of hypothetical instantaneous radio communication, would the communications be distorted once one side were in a different gravity well?


Anonymous_Otters t1_jb313dn wrote

Atomic clocks are routinely used to measure the time difference between things on the surface of earth and things in orbit. The difference of the flow of time needs to be calibrated or else things like GPS wouldn't work right.

If communication were instant, how it would work would I guess depend on what you were using, since that isn't supposed to be possible.


Hapankaali t1_jb70cm0 wrote

Instantaneous communication leads to quite a few problems of the "grandfather paradox" type. For example, the relativity of simultaneity means that according to some observers, the response to a query will be given before the query itself.


Grimyak t1_jb26yje wrote

I think of the "now" as a kind of unbroken fabric of causality. Although I guess with the universe expansion and stuff there are places that will float past the horizon where anything connected to our specific causality cannot interact.

Anyways in my mind "now" is more tied to cause and effect than it is strictly a time based measurement. As in "now" is simply the period where the "cause" side of cause and effect can be manipulated.


Anonymous_Otters t1_jb27sbx wrote

Causality is the reason for there being different nows since if causality existed irrespective of time then things outside of now, from the perspective of an observer, could causes changes too far away to have actually been caused by the observer if, say, information could propagate faster than light. The reason light speed is what it is is because light speed in a vacuum is the speed of causality.

Your definition doesn't make sense as the period where the causes can be manipulated since, for example, my observation of the light from a distant galaxy is completely unaffected by anything happening in my now since the "now" of the galaxy I'm observing occured billions of years ago from my now. Now is entirely relative. The only way I can see the now of the galaxy I'm looking at would be to go there, and by then the now I want to be part of would have passed.


Grimyak t1_jb2evbg wrote

I apologize for my poor explanation. I understand that there are regions in our universe where causality cannot apply due to distance and time limitations.

My intention was not to suggest that there is a single shared "now" across the universe. Rather, I meant that the local "now" we experience is the time period where object interactions and state changes occur.

In hindsight, my use of the phrase "unbroken fabric of causality" may have been misleading. What I intended to convey is that within its sphere of influence, causality remains unbroken and could be considered to have one "now" that bends and conforms to the fabric of space/time in that region. However, beyond a certain distance, causality no longer applies, as even light emitted from our location will never reach those areas. In my mind that place would have its own separate discreet "now" to ours.


criminally_inane t1_jb2sb76 wrote

But then there is a place in between here and there that shares a "now" with both.


Waste_Bin t1_jb3i7wb wrote

I prefer to think of it as three distinct points with three different horizons dependant on relative position and acceleration.

The "now" in-between the two is an artifact.


Grimyak t1_jb30k5j wrote

From a technical standpoint, every location has its own distinct area of influence, even if they have points that overlap each other. However, this doesn't contradict my earlier point as far as I can tell.

To illustrate this concept, imagine three circles that overlap in a linear fashion. The outer two circles extend inward, but do not touch. While an outer circle can interact with the portion of the inner circle that it overlaps with, the inner circle cannot transmit this information beyond the point where it is overlapped by the outer circle. Essentially, although each circle has the potential to overlap with another's influence, it is still unable to communicate or affect regions outside of its own radius.


OberonsGhost t1_jb3i8rp wrote

Can't causes be interconnected instantaneously due to quantum entanglement?


SewFine69420 t1_jb3yev6 wrote

This time stuff has been gnawing on my brain for years, as a fiction writer. In my writing I have beings that were born in different parts of the galaxy, some born on things that weren’t even planets or near a star, so I have a hard time trying to put forth how old they are, except to say they are ancient or ageless or something vague like that. If I try to put a number of years on them I immediately think “okay well they aren’t from earth nor are they on earth, so what exactly is a year in this context”. I have yet to settle on a solution.


andreasbeer1981 t1_jb24q7l wrote

So if a tree falls and there is noone to hear, it doesn't make a sound.


Anonymous_Otters t1_jb274k3 wrote

So, what I described it the opposite of that assertion. No one has to be around to observe difference in time, it is inherent to the fabric of reality.

Also, if a tree falls in the woods and there is no one to hear it, it still makes a sound as sound is the propagation of a fluid compression wave, which occurs regardless of an observer. What it doesn't make is a noise, since noise is the conscious experience constructed by the brain using the stimulus of sound.


Aseyhe t1_jb17c9w wrote

Perhaps I should say there's no unique choice for a universal "now". "Now" would be a 3D surface in 4D spacetime that intersects the point "earth in the year 2023". But there are lots of possible choices you could make in defining this surface, and none of these choices are more correct than the others.


Teo_Filin t1_jb1dzzk wrote

What about simultaneity of entangled particles? Same "now"?


Aseyhe t1_jb1pypu wrote

Entanglement doesn't have any connection to time. It just means that the outcome of a measurement of one particle is correlated with the outcome of a measurement of the other. Those measurements could be performed at any times.


Teo_Filin t1_jb1quc6 wrote

And entangled particles are "connected" via their past moment of interaction?


[deleted] t1_jb1tzq5 wrote



Aseyhe t1_jb1vjul wrote

This is a good way to think of entanglement, as long as you also keep in mind that not every configuration of entangled particles corresponds to a set of local hidden variables in this way (those would be the spooky outcomes you're referring to). But most entanglement scenarios people think of are indeed equivalent to local hidden variables.


OldWolf2 t1_jb3un0o wrote

Your scenario is not entangled, it's just a lack of knowledge on the part of the other person .

Lack of knowledge is NOT the explanation of entangled particles.


PercussiveRussel t1_jb1kli4 wrote

I don't know a lot about cosmology, but I do know about quantum and relativity.

This is a very interesting question! The best way I 'mangle' entangled particles into relativity is to avoid it all together. This is based on the fact that, in order to know about the simultaneity of entangled particles, you'd need a classical channel to transfer information.

Take this experiment: you and I have an entangled particle pair (that's either in AA or BB) and we're lightyears apart. I measure my particle and measure it to be in state A. This means I know yours is in state A too. However, I don't know whether mine is in state A because you collapsed it to be in A, or whether I was the first and have therefore collapsed yours to A. There is no "now your particle is in A" because it could've been there for years (depending on how long ago we separated the particles). The only thing that my experiment proves is that now I know that your particle is in state A. In this way, it's safe to say there is no real simultaneity occurring. This is one of the more prevalent explanations at the moment and one that I personally believe is the most robust since it doesn't require us to completely rip up either relativity or quantum (which are both among the most verified theories we have)


In other words: to me relativity describes the speed of information. Entangled particles don't break this speed as no information is changing hands. This means there is no shared now between the particles.

(The math also shows this, the order of observations is symmetric (they commute), so they have no impact with regards to causality. No causality = no "now")

Edit: To clear something up, for those who don't live and breathe this stuff: entangled particled are two or more particles that share a common set of states. A "quantum object" (which is not a scientific term, but I'll use it anyway) can occupy a quantized (meaning discrete) set of states. When the object is said to be in a superposition, this object is in an undefined state, where it has an x% chance to be in state A, a y% chance to be in state B etc. When we measure the state of this object, it "chooses" one of these states at random and will be that state.

An entangled state means that two seperate objects share a combined superposition. This means that there are less possible states than just all possible permutations. Eg: two objects who can each be in state A or B and are not entangled if they're in the following (equally likely) superposition: AA, AB, BA and BB. Knowing that the first one is A leaves us with the following possible equally likely states: AA and AB. We know precisely nothing about the second object from knowing the first.

Now an entangled object removes some of these possible states. For example the particles are in a superposition of AA, AB or BB. Now knowing that the first one is B tells us that the second one has to be B too. Knowing the first one is A leaves us with no knowledge on the second one. (they're still entangled though, even if the AA and AB states are really useless)

This is "all" entanglement is. It's not something whereby moving the first from state A to B automatically moves the second one from A to B too.

Since you can't control the outcome of a random measurement, there is no way to encode any information in the entanglement itself that is transported on observation. It's pure probabilistics. I mean, you could make the states in a "superposition" of AA and BA so you know the second person always gets a state A after measuring, but you need to send them the particle which goes with locality, so that's just sending an email with extra steps.

The reason that so called "quantum communication" is so interesting though, is that no one can listen in on your conversation. Take the following scheme:

We agree on the following protocol: We have an entangled pair that's in a superposition of AA and BB. I measure mine, then send you a phone call with my measurement. Then you measure your particle (and therefore verify my phone call, beceause our measurements have to be the same).

If I lie, I mean to transmit a 0, if I speak the truth I mean to transmit a 1

No one knows what we are talking about, because they don't know what our entangled pairs are doing. For them, when I tell you "A", it's either a 0 or a 1, with 50% chance each. However notice I do need to say something to you, because I can't affect the outcome of the observation.

I think this explanation clears up a lot about what entanglement actually is. Some people imagine it's like two switches where if you flip one, you instantaneously flip the other one. It's not.


BedrockFarmer t1_jb22lez wrote

This made sense to me, a non mathematician and non physicist. So basically the entangled particles will behave identically when observed. So there is no information linking the particles across spacetime.

So like if I had two cans and two six sided die and I “entangled” the die and closed the lid. I could then send one can to the moon and keep the other and when opened, both die will show the same result because of entanglement when normally there would be a 1/6 probability of what is observed for a single die.


PercussiveRussel t1_jb25fro wrote

Bingo! This is effectively the same thing.

However, I have to be a bit pedantic here, in your example the dice might always have been "destined" to be the same, becauase a simple explanation could be that I glued the dice to the bottom of the can, both facing the number 3 up, and that you and a friend measure the same thing because the dice were always going to show 3. This is what we'd call a 'hidden variable theory' and is almost surely not how quantum probability works.

But yeah, entanglement simply means that knowing the outcome of 1 of the experiments gives you some sort of knowledge about the other experiment (like I said, this could be knowing the exact outcome of the other, or just give you better odds than pure luck for guessing the other experiment). The key concept is that you can't control the outcome of the experiments, you just improve your chances of guessing the other experiment correctly, which is exactly what happens in your dice example.


Teo_Filin t1_jb1o0zq wrote

Cool! So no way for instant info exchange? Such a pity.

Though I can't accept "information" as physical measure at macro-level (it's our description of some properties), just at quantum-level (numbers defining a particle).

I even doubt that time is a coordinate meaning some "future" and "past" do exist as places we can visit (present is real with some intensity of processes influenced by gravity and speed; future is our anticipation, past is our memory).

I'm not deep in advanced physics and higher mathematics, just evolving my perception and checking cosmology with common sense sometimes.


PercussiveRussel t1_jb240js wrote

>So no way for instant info exchange?

Reddit was being reddit, so while I could see your reply in my replies, I couldn't find it to reply to. Reply.

I've added a bit at the bottom of my post to explain why. This isn't just aimed at you, I don't know your understanding of quantum mechanics, but I notice that entanglement is a pretty misunderstood topic. And to be honest, actual quantum researchers talking calling their incredibly cutting edge cool research "quantum teleportation" isn't exactly helping this.

Not accepting or even understanding information at a macro level is perfectly understandable. I'm a physicist and I can't marry my knowledge of quantum with the macro world, I don't understand "information" as a quantum concept in macro scale either. (to be honest, I don't think I fully accept "energy" on the various scales)

I think the most simple way to accept "the speed of information" is to just imagine someone transmitting "Hello world", with laser beams through the vacuum of outer space. Since we know that the speed of light is finite, and furthermore light is the fastest thing in the world, no message can get to us faster than a message sent with light. There are lots of caveats to this of course (there always are), but this conceptually at least made sense to me when I first started to learn about this. Once you start to trust this concept, just flip it on its head: the speed of information is 3.00e8 m/s, so there is no way light is faster than information.

(this is a way to get to grips with the concept, this is in no way a proof nor meant as such)


IAmEnteepee t1_jb2ca9j wrote

In simpler terms, time on earth and on Mars will never be in sync. People traveling back and forth will experience time differently as well, relative to their counterparts on Earth or Mars.


mywan t1_jb2d1v2 wrote

One way to see this is to consider the clock paradox. Spaceships A and B are moving about 86% the speed of light relative o each other. Spaceship A look at B and says the clock at B has slowed down by half. Time slows down for moving objects. But B says no, they are not moving, A is moving. So B says the clock that the clock at A is the one that has slowed down by half.

So how do we test who is right? If A goes and parks next to B to prove they are right then B would appear to be right. But if B goes and parks next to A to prove they are right then A would appear to be right. “Now” is not any more universal than up and down.


sirgog t1_jb3yhjl wrote

> I’m sorry could you explain that more please? There is no universal “now”?

The technical term is "relativity of simultaneity"

Consider two events A and B that happen at "about" the same time, separated by a great distance. They are close enough to simultaneous that light from A cannot get to the location of B until after B has happened, and light from B arrives at A after A has happened.

Let's assume that A occurs at 1230 GMT on 11-Mar-2023, and is a political speech given worldwide coverage made in Scotland. And let's assume that B occurs at 1500 GMT on 11-Mar-2023 by Earth reckoning, and is a critical failure of the Voyager-1 probe which is currently 21 light hours away from Earth.

Observers of both events can calculate the time of each, and deduce which was 'first'.

However, these observers will not necessarily agree with each other.

An observer on Earth who notes both events will conclude, after light arrives 21 hours after Voyager's incident, that the speech was made 2½ hours before the probe failed.

But an alien probe moving through the solar system at 99.98% of lightspeed which saw both events might well conclude that the speech occurred 7 hours after the probe failed.

Both would be correct.

It's a completely counterintuitive mess, but the math checks out on it.


LazyLizzy t1_jb1m5vr wrote

But when will then be now?


TransparentMastering t1_jb0ya8q wrote

How is the rest frame of the CMBR determined if light travels at the same speed in all reference frames? Is it to do with the red and blue shift?


Aseyhe t1_jb0yzuq wrote

It's the center-of-momentum frame for all of the CMB photons (within some volume of space), not the frame of individual photons. It's also the frame in which the CMB temperature is the same in all directions. If you're moving with respect to that frame, you'll find that the CMB is hotter (blueshifted) in the direction of your motion and colder (redshifted) in the opposite direction. That's what we find, and the magnitude of this effect tells us that we are moving at 370 km/s with respect to the CMB.


Void_vix t1_jb2ssty wrote

So accelerating with respect to any other object causes the CMB to Doppler shift for the observer?

Wouldn’t that mean that a truly non inertial frame would have to be at the center of a mass?


azkedar_ t1_jb19cn0 wrote

Is the age of the universe in the CMB frame the maximum it can be in any reference frame? If not, in what sort of reference frame could it be older?


Aseyhe t1_jb1y8p1 wrote

I'll first note that when thinking about the possible trajectories that reach a given spacetime point (e.g. earth in 2023), there is a trajectory that maximizes the elapsed time since the beginning of the universe.

Does that trajectory correspond to the CMB frame? If the universe is homogeneous, then yes. In the presence of peculiar gravitational fields, like those of our galaxy, that doesn't generally remain true, although I think you could pick a "time threading" such that it does. That's another gauge freedom I didn't mention. Along with the freedom to pick the "different positions at the same time" surfaces in spacetime, there's also freedom to pick the "same position at different time" lines.


Ralph_Shepard t1_jb1xfya wrote

Wait, you can really use the cosmic microwave background as a sort of "absolute" reference frame? (Since it is certainly more universal than the Sun or even galactic core). Wow, that is interesting.


SharkFart86 t1_jb203b1 wrote

What you’re asking is kind of a weird question, you can use anything you want as your reference frame. The CMB is often used as the reference frame because it has some convenient qualities that make it more a “fair” reference in cosmology, but there isn’t anything inherent about it that makes it more absolute than any other frame of reference.

And keep in mind that the reference frame matters within the context of the subject. Using CMB as a reference in regards to the orbit of the planets in our solar system makes way less sense than using the sun as the frame of reference. The frame of reference should be whatever makes the model clearer to accurately understand. The CMB works great when looking at the movement or rotation of galaxies, because otherwise those values are very difficult to describe in a clear way.


TaiVat t1_jb2cwhd wrote

I think the question there in terms of "absolute" is that the CMB is "special" in the sense of being the most distant observable "object", and thus is kind of "absolute" in the sense that its the most encompassing of all possible frames. I.e. all of the rest of the observable universe is within it. Kinda like a skybox in a video game.


Ralph_Shepard t1_jb2epci wrote

That is why I used the quotation marks and "better than", but I can't think of any other better reference point


CarolBaskinDidntDoit t1_jb1985y wrote

In a similar (and possibly dumb) question: When we say that the expansion of the universe is accelerating, is it possible that the objects at the far edges of the universe are actually travelling at constant velocity from their reference, but as they get further away from the gravitational center of the universe, the time dilation reduces what a second is and therefore appears from our reference to be accelerating?

E.g. if a=dv/dt. If we measure the velocity of the distant object for an arbitrary amount of time - say 10,000 seconds. And let’s say the object is travelling at constant velocity, but as the object gets away from the gravitational center, a delta 10,000 seconds from the object’s reference would be shorter than a delta 10,000 seconds from our reference. So therefore we perceive this as a change in velocity?


bluesam3 t1_jb1txud wrote

The universe expanding isn't anything to do with objects having velocities: it's the space between them getting bigger.


Howrus t1_jb2mmeo wrote

> objects at the far edges of the universe are actually travelling at constant velocity from their reference

That's the neat trick - they are not traveling at all! Expansion of Universe is happening because space between everything is growing, not because something is flying away.

Closest analogue would be - draw two points on a balloon and start inflating it. Distance between this two points would start to increase without them moving by themselves.


ViralVortex t1_jb2gy10 wrote

You’re looking at now, sir. Everything that happens now is happening now.

What happened to then?!

We passed then.


Just now.


clocks212 t1_jb109x2 wrote

Does the cosmic background radiation have a different velocity relative to the earth in different directions, or as far as we can tell we can consider all of the CMB in every direction to be identically “stationary”.


Aseyhe t1_jb12c8v wrote

The CMB frame is different in different places. It's also the frame of a comoving observer -- that is, one who is moving only due to the expansion of the universe and does not have further ("peculiar") motion. So if we consider distant galaxies in some direction, they are receding in that direction, and so is their CMB frame.


Drops-of-Q t1_jb1gjma wrote

Can you also explain why the age of the universe is dependant on position with the Newtonian gauge?


Aseyhe t1_jb1tjn1 wrote

Gravitational time dilation, due to the -(1+2Φ)dt^2 term in the metric. (In synchronous gauge, that term is just -dt^(2)). The result is that the elapsed time is shortened by a factor of about 1-Φ, where Φ is the Newtonian gravitational potential, of order 10^-6 in our galaxy.


lGrayFoxl t1_jb1h2mn wrote

Is it possible to misinterpret a small detail that made us arrive at 13.7 billion years old that would make us be billion of years off or is it 13.7 give or take a few million for sure.


Aseyhe t1_jb1qtgd wrote

A billion years off is likely possible, given the Hubble tension. There's about a 7% difference between the present-day expansion rate favored by the CMB (~68 km/s/Mpc) and the present-day expansion rate favored by supernovae (~73 km/s/Mpc). 7% of 13.7 billion years is a billion years.


fizzbish t1_jb1rjan wrote

Thanks for this response. But I've always been confused about the CMB frame of reference. Isn't the CMB moving at the speed of light as microwave radiation? How is it the "rest frame" that is always used? Wouldn't its frame be the same as that of a random photon from a star?


bluesam3 t1_jb1v0ev wrote

It's something of a misnomer: it's not the rest frame of the radiation itself, but the rest frame in which the CMB appears the same in all directions: in most rest frames, you'll see it redshifted in one direction, and blueshifted in the other (this is what this looks like for us, for example: the overall hot/cold spots (NB: on this diagram, red is blue-shifted and blue is red-shifted, because humans like red to be hot, even though blue is hotter) are due to our velocity reshifting it, the funky lumps are local effects. If you adjust that to account for shifting the velocity of the observer, you can get it to the point at which that looks almost exactly flat (this famous image scales up the differences by orders of magnitude in comparison to the previous one - actual differences are on the order of one part in 100,000). The reference frame where that image is flattest (modulo a few adjustments for local effects) is the CMB reference frame.


fizzbish t1_jb63gbx wrote

Thanks this clears it up a whole lot for me!


Aseyhe t1_jb1txf9 wrote

An individual photon doesn't have a rest frame, but the CMB rest frame is the center-of-momentum frame of a collection of photons. Since the photons have all different momenta, their center of momentum doesn't move at the speed of light.


Atlein_069 t1_jb2pmzt wrote

How different are the two ages? Like is we consider position does it change significantly if we don’t use Newtonian gauge?


Aseyhe t1_jb2qrs7 wrote

Gravitational time dilation is at most of order one part in a million in most contexts. It's only significantly larger near black holes, neutron stars, and white dwarfs.


DrMaxwellEdison t1_jb34z1i wrote

A follow up question, if I may.

Is there some consensus on a more precise age for the universe than "13.7 billion"? Certainly it's not something that matters in the context of our own life spans, but at some point that 13.7 has to go up. Like the joke of a guard working in a museum who says a dinosaur is 65 million and 4 years old, because it was 65 million when they started working there 4 years ago.

So, is there some agreed-upon starting point we can count up from?


Aseyhe t1_jb47564 wrote

The Planck 2018 paper gives 13.787 ± 0.020 billion years, so that's an uncertainty of 20 million years. That's from the cosmic microwave background.

However, supernova-based measurements of cosmic expansion favor about a 7% higher expansion rate, which could imply the universe is younger by of order a billion years. This discrepancy is the "Hubble tension", a major current research topic.


PvtDeth t1_jb3eo5n wrote

How can you use the CMB as a reference frame if it is roughly uniform in every direction?


Aseyhe t1_jb46ate wrote

It's only isotropic (the same in every direction) if you're in the CMB rest frame. Otherwise, due to the Doppler effect, one direction will be blueshifted and the other redshifted.