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 05-20-2012, 02:38 AM #1 veteran     Join Date: Oct 2010 Posts: 2,617 What does it mean to say that the Universe is 13.7 B years old? There is a confusing element in all this. Since Einstein, we know that there is no such thing as absolute time. And yet, when we talk about the age of the universe, which is pretty much all the time that has elapsed, it sounds like we're talking about an absolute age. What about an object or a particle which since the Big Bang has been moving with an average speed of 0.4c? If that object/particle "were to measure" the time since the Big Bang, would it get a different number than 13.7 (i.e., a much smaller number)?
 05-20-2012, 07:58 PM #2 newbie     Join Date: May 2012 Posts: 20 Re: What does it mean to say that the Universe is 13.7 B years old? I think it just means we cant get any (useful) information further back than 13.7 billion years ago.
 05-20-2012, 08:11 PM #3 centurion   Join Date: May 2012 Posts: 172 Re: What does it mean to say that the Universe is 13.7 B years old? Agreed, I think it's like, the max time a particle has experienced in our observable Universe. I would imagine that all molecules in our solar system have experienced almost this full amount of time. But OP is right in that some particles have experienced significantly less, if they are traveling close to c, or orbiting a black hole or something
 05-21-2012, 12:07 AM #4 veteran     Join Date: Jun 2009 Location: Live from StL It's Sat Night Dead! Posts: 2,890 Re: What does it mean to say that the Universe is 13.7 B years old? So, perhaps some particles could be younger, but no particles could be older than 13.7 B right?
 05-21-2012, 12:35 AM #5 centurion   Join Date: May 2012 Posts: 172 Re: What does it mean to say that the Universe is 13.7 B years old? yep
05-21-2012, 03:02 AM   #6

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Re: What does it mean to say that the Universe is 13.7 B years old?

Quote:
 Originally Posted by la6ki There is a confusing element in all this. Since Einstein, we know that there is no such thing as absolute time. And yet, when we talk about the age of the universe, which is pretty much all the time that has elapsed, it sounds like we're talking about an absolute age.
This is true for general solutions to Einstein's equations but cosmological solutions are a bit special. They're assumed to be isotropic and homogenous and that allows a natural choice of time which is the "proper time" measured by observers that are stationary. This isn't possible in most spacetimes.

Quote:
 Originally Posted by la6ki What about an object or a particle which since the Big Bang has been moving with an average speed of 0.4c? If that object/particle "were to measure" the time since the Big Bang, would it get a different number than 13.7 (i.e., a much smaller number)?
Yes, in general a clock traveling with the object would measure a different proper time http://en.wikipedia.org/wiki/Proper_time

 05-21-2012, 04:06 AM #7 veteran     Join Date: Oct 2010 Posts: 2,617 Re: What does it mean to say that the Universe is 13.7 B years old? Makes sense. Thanks guys. But all these things should be explicitly stated when talking about the age of the Universe imo, it may lead to confusion in other people as well, I'd imagine.
 05-21-2012, 04:54 AM #8 veteran     Join Date: Aug 2009 Location: Stanford, CA USA Posts: 3,320 Re: What does it mean to say that the Universe is 13.7 B years old? Be careful however because the term particle and age are not exactly ok together. A particle doesnt have a clock to tell it how old it is. Ie an electron doesnt know how old it is or it cant answer that question by any experiment. Same with protons etc. You cannot distinguish between an electron produced at CERN in collisions of protons (shower decay of other particles including that electron, pair production etc) and one that was orbiting a proton since the first formation of that atom soon after Big Bang and which now was energetically excited (ionization) for the first time in billions of years to extract that electron. Expect that this simple understanding about time will soon play a key role in a new theory among many other features. Also look cosmological time as roughly ~ 1/H ~ 14 bil y (H Hubble's constant in the v=d*H law projection of distance becoming real small ie big bang era) Better look here also; (comoving time, cosmological time) http://en.wikipedia.org/wiki/Cosmological_time http://en.wikipedia.org/wiki/Comovin...ng_coordinates http://en.wikipedia.org/wiki/Friedma...3Walker_metric (check also Einstein's radius of the Universe as a rough concept etc) Its tempting to claim that no timelike geodesic has proper time larger than T (the cosmological age say) or what classically a clock on that trajectory would show as having passed after the initial singularity and that other ones do exist that are smaller. The fact is we do not have complete very realistic solutions of typical astrophysical objects or even particles in various spacetimes superimposed on a global expanding spacetime (like a combination metric of the real thing which is both due to local gravity but also due to the expansion of the universe, as its ultra tough to solve or ever write down the field equations and proper boundary conditions describing the entire universe. What we have is approximations of the real thing either cosmologically (FLRW http://en.wikipedia.org/wiki/Friedma...-Walker_metric ) or locally (Kerr Newman for rotating charged black holes http://en.wikipedia.org/wiki/Kerr%E2%80%93Newman_metric) derived using various symmetry arguments and some rare special exact solutions of both together (ie some Schwarzschild solution in an FLRW metric). So the argument isnt very clean although it seems intuitively correct. However expect those classical ideas to become irrelevant eventually. They are mere classical restrictions on how to view the world imposed by our classical senses and resulting philosophy of science that have survived even QM revolution but not for long more. Last edited by masque de Z; 05-21-2012 at 05:08 AM.
 05-21-2012, 02:11 PM #9 adept   Join Date: Feb 2008 Location: \$2.20/180mans Posts: 731 Re: What does it mean to say that the Universe is 13.7 B years old? It's true that our universe is only approximately FRW but when people talk about cosmology that's usually what they mean, because as you said the situations without much symmetry are very hard (even for numerical relativists).
05-21-2012, 11:24 PM   #10
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Re: What does it mean to say that the Universe is 13.7 B years old?

Quote:
 Originally Posted by la6ki Makes sense. Thanks guys. But all these things should be explicitly stated when talking about the age of the Universe imo, it may lead to confusion in other people as well, I'd imagine.
Well it's kind of tricky, because most people don't understand special relativity, so having a different age for 2 different objects is mind exploding enough. I'd say you are the minority here who knows too much, but not enough, in a way

Where as telling some one the Universe is 13,700,000,000 years old is pretty easy to comprehend and is pretty much the truth if you make the right clarifications/defintions

 05-22-2012, 07:09 AM #11 old hand     Join Date: Jul 2007 Location: Losing at Omahaha Posts: 1,483 Re: What does it mean to say that the Universe is 13.7 B years old? Can't we just define the age of the universe in terms of the speed of light, which is the same in all reference frames? I.e. a photon created in the Big Bang (or a little bit after the Big Bang - whenever photons was created) has travelled a distance x since then. We know it travels at the speed c, so the age of the universe is x/c.
05-22-2012, 09:15 AM   #12

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Re: What does it mean to say that the Universe is 13.7 B years old?

Quote:
 Originally Posted by acehole60 Can't we just define the age of the universe in terms of the speed of light, which is the same in all reference frames? I.e. a photon created in the Big Bang (or a little bit after the Big Bang - whenever photons was created) has travelled a distance x since then. We know it travels at the speed c, so the age of the universe is x/c.
therefore when somebody says "the universe is 13.7 B yrs old" they mean that the perceived beginning point is 13.7 B light years away?

05-22-2012, 09:55 AM   #13
old hand

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Re: What does it mean to say that the Universe is 13.7 B years old?

Quote:
 Originally Posted by neverfoldthe1outer therefore when somebody says "the universe is 13.7 B yrs old" they mean that the perceived beginning point is 13.7 B light years away?
I'm not an expert, and my last post was more a question than a statement, but I do not think it makes sense to talk of a "beginning point", since space itself is expanding and has been doing so since the Big Bang. I.e. there is no particular point where it all began; rather the universe has expanded from that starting singularity to what it is today (EDIT: and in this sense, the "starting point" is everywhere).

Last edited by acehole60; 05-22-2012 at 10:07 AM.

05-22-2012, 10:11 AM   #14

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Re: What does it mean to say that the Universe is 13.7 B years old?

Quote:
 Originally Posted by acehole60 I'm not an expert, and my last post was more a question than a statement, but I do not think it makes sense to talk of a "beginning point", since space itself is expanding and has been doing so since the Big Bang. I.e. there is no particular point where it all began; rather the universe has expanded from that starting singularity to what it is today.
I'm pretty sure the notion of space expanding derives from matter traveling past the farthest previous observation point, since "space" itself cannot be measured without something (some kind of matter) inside of it to create reference points. so, the 'expanding universe' is not like some container holding all matter expanding, but rather the distance between observable matter increasing. so, space itself is not expanding. also, this "starting singularity" you speak of is the "beginning point"....which means perhaps the center of the universe?

05-22-2012, 01:26 PM   #15
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Re: What does it mean to say that the Universe is 13.7 B years old?

Quote:
 Originally Posted by neverfoldthe1outer I'm pretty sure the notion of space expanding derives from matter traveling past the farthest previous observation point, since "space" itself cannot be measured without something (some kind of matter) inside of it to create reference points. so, the 'expanding universe' is not like some container holding all matter expanding, but rather the distance between observable matter increasing. so, space itself is not expanding. also, this "starting singularity" you speak of is the "beginning point"....which means perhaps the center of the universe?
I don't think this is true. Maybe somebody more knowledgeable can correct me, but I think space is extending independent of matter. Also there is no such thing as the center of the universe, since it's kind of a 4 dimensional sphere?

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