<p>Your question, if related to general relativity theory, needs a few explainations. First of all GR describes gravity as a geometry of spacetime; very many geometries are possible - the GR does not tell us which one has been realized in the Universe. While some, higly symmetric solutions of the equations of GR exist, and are thought to describe our universe &quot;well&quot; in the macro scale, e.g. <a href="https://en.wikipedia.org/wiki/Friedmann%E2%80%93Lema%C3%AEtre%E2%80%93Robertson%E2%80%93Walker_metric" rel="noreferrer">https://en.wikipedia.org/wiki/Friedmann%E2%80%93Lema%C3%AEtre%E2%80%93Robertson%E2%80%93Walker_metric</a>, these solutions are obviously inapplicable to &quot;local inhomogenities&quot; such as galaxies, rotating compact objects, etc. Furthermore &quot;time&quot;, or &quot;global time&quot;, is not, in general, well defined: at every point of the spacetime, (call it &quot;event&quot;), we can set up the future and past light cones - the &quot;interior&quot; of the future light cone is the set of events that can be reached from current event by a &quot;causal curve&quot; (light ray, or worldline of an observer). Hence, for instance, every observer has his &quot;proper time&quot; (<a href="https://en.wikipedia.org/wiki/Proper_time" rel="noreferrer">https://en.wikipedia.org/wiki/Proper_time</a>), as measured on the &quot;watch&quot; he carries on his &quot;wrist&quot;. Given that &quot;inhomogenities&quot; (galaxies, etc) tend to collapse over time, it is quite generic to expect that large classes of worldlines hit an &quot;end&quot; in finite proper time - hence their time is &quot;finite&quot; in the future direction. In fact, there are general theorems to the effect that such situation is generic in GR - under general conditions there will be a &quot;geodesic incompleteness&quot; in the spacetime (meaning: worldlines cannot be extended indefinitely, see <a href="https://en.wikipedia.org/wiki/Penrose%E2%80%93Hawking_singularity_theorems" rel="noreferrer">https://en.wikipedia.org/wiki/Penrose%E2%80%93Hawking_singularity_theorems</a>).</p>
<p>A small note on &quot;energy&quot;: in general in physics energy is a constant of motion present once the system has time-translation symmetry; in GR this means that there is such a notion (of energy) if the spacetime is at least &quot;stationary&quot; (<a href="https://en.wikipedia.org/wiki/Stationary_spacetime" rel="noreferrer">https://en.wikipedia.org/wiki/Stationary_spacetime</a>); since the spacetime of our Universe is not stationary, there is no such thing as &quot;energy&quot; not to mention its &quot;conservation&quot;.</p>

CC BY-SA 4.0
2022-07-19T22:03:05.077Z
Your answer certainly is interesting in contrast to the above answer. If it is not much to ask and if you have time, would it be possible to accompany your answer with crude drawings/figures to further understand the concept.
Ed_Gravy
CC BY-SA 4.0
2022-07-19T13:25:21.970Z
2022-07-19T13:25:21.970Z
P Marecki
<p>Time is relative so it changes by location and eon, i.e. towards black holes and at the beginning and end of the universe. The speed depends on matter-gravity states. At some places there are dimensional states which are not compatible with our notion of time, just as it was prior to the Big Bang, so we could say &quot;Times&quot; and that every place works on clocks at different speeds. Sometimes those clocks just melt and divide into many clocks.</p>

CC BY-SA 4.0
2022-07-19T05:00:09.880Z
2022-07-21T08:00:01.190Z
2022-07-21T08:00:01.190Z
bandybabboon
<p>As far as we can tell, the four-dimensional space-time continuum is unbounded in the space directions (space is infinite) the time dimension seems to have a singularity (about 13.8 billion years before now) and so it is impossible to talk in a sensible way about events before that time.  We would normally say that the universe and time itself started at that point, and so any statement about &quot;before the big bang&quot; is completely meaningless. There are other possibilities, such as &quot;eternal inflation&quot;, but they are rather speculative, with little positive evidence.</p>
<p>In the other direction, into the future, time seems to be unbounded, though this is less certain, it depends very precisely on several cosmological parameters, whose values are not yet certainly known.</p>
<p>So our current best hypothesis is that time is finite into the past, but infinite into the future.</p>
<p>Energy is locally conserved within the universe, but don't apply such reasoning to the formation of the universe.  Indeed the whole concept of physics ceases to operate at that singularity.  The basis of nature as cause-and-effect cannot operate at a singularity.</p>
<p>If space is indeed infinite, and the universe is homogenous and isotropic, then the total mass-energy of the universe could be infinite.  However the potential gravitational energy might be negative infinite, and so the total energy of the universe might be zero, leading to Hawking's observation that &quot;the universe is the ultimate free lunch&quot;.</p>

CC BY-SA 4.0
2022-07-19T10:48:09.753Z
"time is finite into the past, but infinite into the future." This is such an interesting statement.
Ed_Gravy
CC BY-SA 4.0
2022-07-19T13:36:43.267Z
@Ed_Gravy this means each one of us was not born yet a finite time but will be dead infinitetly :(
nerdess
CC BY-SA 4.0
2022-07-19T13:41:29.077Z
@nerdess, or may be our energy transitions to somewhere else once our bodies (organic machines) are dead. I mean who's to say death/after life is the most certain and mysterious thing at the same time.
Ed_Gravy
CC BY-SA 4.0
2022-07-19T15:46:27.943Z
@Ed_Gravy I wonder if the concept of the [Big Crunch](https://en.wikipedia.org/wiki/Big_Crunch) sets an upper boundary to the time dimension.
Mindwin Remember Monica
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2022-07-19T15:49:03.727Z
@Mindwin, philosophically speaking, I actually think the same way as the `Big Crunch` hypothesis. I mean scientific knowledge, a million years from now would be very very different that it is now.
Ed_Gravy
CC BY-SA 4.0
2022-07-19T15:52:24.243Z
@Ed_Gravy make that a million minutes.
Mindwin Remember Monica
CC BY-SA 4.0
2022-07-19T16:38:42.033Z
Actually, James, thinking about what came before the Big Bang is incredibly important. You say it's meaningless to ponder such things, which is such an orthodox astrophysicist thing to say.
White Prime
CC BY-SA 4.0
2022-07-19T18:33:55.133Z
It **may** be meaningless.  Feel free to ponder!  But ultimately science is not pondering, it's not speculation, it needs actual evidence. At the moment there is no evidence for "existence" prior to  13.8 billion years ago.  By the way,  while I'm not an astrophysicist (not by a long shot) I do find the fact that you use "orthodox" as an insult very amusing!
James K
CC BY-SA 4.0
2022-07-19T18:36:29.957Z
@Mindwin  Re "big crunch"  Yes, it would set a limit, but the evidence is that, as a result of dark energy, the universe won't have a big crunch.  But that is an empirically derived observation rather than a well supported theory, so I'd say its not yet "settled science".
James K
CC BY-SA 4.0
2022-07-19T19:12:23.327Z
@nerdess, I mean according to Donald Hoffman, our reality is just selective information filtered by our brain (like Matrix). So, who knows (maybe one day we will) what reality actually is since in sensical terms, reality might seem subjective. So, who's to say what comes after death. I mean hey I might end up in Valhalla (pun intended)
Ed_Gravy
CC BY-SA 4.0
2022-07-19T22:37:19.233Z
The usual term for this is that the universe is "semi-infinite" in the time direction: that it, it's bounded on one side and infinite on the other.
Mark
CC BY-SA 4.0
2022-07-20T07:16:25.420Z
"our current best hypothesis" +1. *We* can neither create nor destroy energy, which is unfortunately why religion is unassailable. Also 'time started' at 3/4 of a (mili?)second *after* the Big Bang. Prior to that, *we don't know, and likely never will*. That's why it's meaningless.
Mazura
CC BY-SA 4.0
2022-07-20T13:18:16.050Z
*Energy is conserved within the universe* Not quite. In a non-constant spacetime metric it is not. Which is how CMB photons lose energy.
Jens
CC BY-SA 4.0
2022-07-20T14:43:08.593Z
I don't think this is right, or at least it is not universally accepted, that the current universe began at a singu;lar point in time 13.7 Gyr ago. NB. This is not the same as saying that there wasn''t a big bang etc - but actually, saying anything about time before the inflationary epoch is quite unconstrained (e.g. ideas like eternal inflation remain acceptable).
ProfRob
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2022-07-20T16:06:15.540Z
@ProfRob, if it's okay can you please add your answer too?
Ed_Gravy
CC BY-SA 4.0
2022-07-19T08:32:56.843Z
2022-07-20T18:43:04.053Z
2022-07-20T18:43:04.053Z
James K
<p>I am not a scientist nor do I have a degree in Astrophysics, but I do like to learn new things by asking questions. With that being said, I have read that time is relative to space which began after the big bang. Moreover, before or right at the moment before the big bang, energy was confined to a very very small scale like the tip of the needle or something.</p>
<p>But, I wonder since energy can neither be created nor destroyed, that means energy is infinity with no beginning.</p>
<p>So, when it is said time started with the big bang, is that in reference to our universe? Because in reference to energy, shouldn't time be infinite too? Or time has to be relative to space as well and so there is no concept of time prior to the big bang?</p>

CC BY-SA 4.0
2022-07-19T04:52:30.030Z
There’s a logical jump in the statement “I wonder since energy can neither be created nor destroyed, that means energy is infinity with no beginning” that is losing me; what I think you’re getting at is that since energy can’t be created how did the Big Bang come to be, but I’m not sure what that has to with the conclusion that energy is infinite. Could you elaborate?
Justin T
CC BY-SA 4.0
2022-07-19T10:41:53.717Z
@JustinT well since energy can't be created or destroyed, so wouldn't that mean that it has no beginning and an end translating to infinity?
Ed_Gravy
CC BY-SA 4.0
2022-07-19T15:44:06.280Z
@Ed_Gravy Energy being infinite would mean that there's unlimited energy. The energy exists forever (as far as we know, and we have good reason to think this is the case), but we have no evidence that it's unlimited. You seem to be confusing the idea of "infinite" with the idea of "eternal".
Hearth
CC BY-SA 4.0
2022-07-19T15:46:01.933Z
@Hearth and here I thought `eternal` and `infinity` mean the same thing or are similar, so your comment is indeed interesting.
Ed_Gravy
CC BY-SA 4.0
2022-07-19T15:48:48.883Z
@Ed_Gravy Infinite means that there's an unlimited amount of something, whereas with something that's eternal, there may only be a very tiny amount of it, but there will always be that same amount. In principle, something that's infinite now doesn't even have to be infinite tomorrow--there could be as much of it as you want right now, but none to be found tomorrow.
Hearth
CC BY-SA 4.0
2022-07-19T20:25:52.100Z
I wish Sir Roger Penrose was a user on this website and could've read my question. His response to this question would've been very very interesting.
Ed_Gravy
CC BY-SA 4.0
2022-07-19T20:56:43.920Z
Sidenote: unlimited and infinite are _also_ not the same. There can be an infinite amount of something, but limits in how its accessed or where it is.
Mooing Duck
CC BY-SA 4.0
2023-12-13T09:09:05.480Z
Zathras says there is infinite time... but this, this is wrong tool.
hobbs
<universe><time>
Is time finite or infinite?
Ed_Gravy
# Is time finite or infinite?

(Asked by: username_0 on 2022-07-19)

I am not a scientist nor do I have a degree in Astrophysics, but I do like to learn new things by asking questions. With that being said, I have read that time is relative to space which began after the big bang. Moreover, before or right at the moment before the big bang, energy was confined to a very very small scale like the tip of the needle or something.

But, I wonder since energy can neither be created nor destroyed, that means energy is infinity with no beginning.

So, when it is said time started with the big bang, is that in reference to our universe? Because in reference to energy, shouldn't time be infinite too? Or time has to be relative to space as well and so there is no concept of time prior to the big bang?

**Tags:** universe, time

> **Comments:**
> **username_1**: There’s a logical jump in the statement “I wonder since energy can neither be created nor destroyed, that means energy is infinity with no beginning” that is losing me; what I think you’re getting at is that since energy can’t be created how did the Big Bang come to be, but I’m not sure what that has to with the conclusion that energy is infinite. Could you elaborate?  
> **username_0**: @JustinT well since energy can't be created or destroyed, so wouldn't that mean that it has no beginning and an end translating to infinity?  
> **username_2**: @username_0 Energy being infinite would mean that there's unlimited energy. The energy exists forever (as far as we know, and we have good reason to think this is the case), but we have no evidence that it's unlimited. You seem to be confusing the idea of "infinite" with the idea of "eternal".  
> **username_0**: @username_2 and here I thought `eternal` and `infinity` mean the same thing or are similar, so your comment is indeed interesting.  
> **username_2**: @username_0 Infinite means that there's an unlimited amount of something, whereas with something that's eternal, there may only be a very tiny amount of it, but there will always be that same amount. In principle, something that's infinite now doesn't even have to be infinite tomorrow--there could be as much of it as you want right now, but none to be found tomorrow.  
> **username_0**: I wish Sir Roger Penrose was a user on this website and could've read my question. His response to this question would've been very very interesting.  
> **username_3**: Sidenote: unlimited and infinite are _also_ not the same. There can be an infinite amount of something, but limits in how its accessed or where it is.  
> **username_4**: Zathras says there is infinite time... but this, this is wrong tool.  

---

## 3 answers

### Answer by username_7 on 2022-07-19. Score: 13 | Accepted answer

As far as we can tell, the four-dimensional space-time continuum is unbounded in the space directions (space is infinite) the time dimension seems to have a singularity (about 13.8 billion years before now) and so it is impossible to talk in a sensible way about events before that time. We would normally say that the universe and time itself started at that point, and so any statement about "before the big bang" is completely meaningless. There are other possibilities, such as "eternal inflation", but they are rather speculative, with little positive evidence.

In the other direction, into the future, time seems to be unbounded, though this is less certain, it depends very precisely on several cosmological parameters, whose values are not yet certainly known.

So our current best hypothesis is that time is finite into the past, but infinite into the future.

Energy is locally conserved within the universe, but don't apply such reasoning to the formation of the universe. Indeed the whole concept of physics ceases to operate at that singularity. The basis of nature as cause-and-effect cannot operate at a singularity.

If space is indeed infinite, and the universe is homogenous and isotropic, then the total mass-energy of the universe could be infinite. However the potential gravitational energy might be negative infinite, and so the total energy of the universe might be zero, leading to Hawking's observation that "the universe is the ultimate free lunch".

> **Comments:**
> **username_0**: "time is finite into the past, but infinite into the future." This is such an interesting statement.  
> **username_8**: @username_0 this means each one of us was not born yet a finite time but will be dead infinitetly :(  
> **username_0**: @username_8, or may be our energy transitions to somewhere else once our bodies (organic machines) are dead. I mean who's to say death/after life is the most certain and mysterious thing at the same time.  
> **username_9**: @username_0 I wonder if the concept of the [Big Crunch](https://en.wikipedia.org/wiki/Big_Crunch) sets an upper boundary to the time dimension.  
> **username_0**: @Mindwin, philosophically speaking, I actually think the same way as the `Big Crunch` hypothesis. I mean scientific knowledge, a million years from now would be very very different that it is now.  
> **username_9**: @username_0 make that a million minutes.  
> **username_10**: Actually, James, thinking about what came before the Big Bang is incredibly important. You say it's meaningless to ponder such things, which is such an orthodox astrophysicist thing to say.  
> **username_7**: It **may** be meaningless.  Feel free to ponder!  But ultimately science is not pondering, it's not speculation, it needs actual evidence. At the moment there is no evidence for "existence" prior to  13.8 billion years ago.  By the way,  while I'm not an astrophysicist (not by a long shot) I do find the fact that you use "orthodox" as an insult very amusing!  
> **username_7**: @Mindwin  Re "big crunch"  Yes, it would set a limit, but the evidence is that, as a result of dark energy, the universe won't have a big crunch.  But that is an empirically derived observation rather than a well supported theory, so I'd say its not yet "settled science".  
> **username_0**: @username_8, I mean according to Donald Hoffman, our reality is just selective information filtered by our brain (like Matrix). So, who knows (maybe one day we will) what reality actually is since in sensical terms, reality might seem subjective. So, who's to say what comes after death. I mean hey I might end up in Valhalla (pun intended)  
> **username_11**: The usual term for this is that the universe is "semi-infinite" in the time direction: that it, it's bounded on one side and infinite on the other.  
> **username_12**: "our current best hypothesis" +1. *We* can neither create nor destroy energy, which is unfortunately why religion is unassailable. Also 'time started' at 3/4 of a (mili?)second *after* the Big Bang. Prior to that, *we don't know, and likely never will*. That's why it's meaningless.  
> **username_13**: *Energy is conserved within the universe* Not quite. In a non-constant spacetime metric it is not. Which is how CMB photons lose energy.  
> **username_14**: I don't think this is right, or at least it is not universally accepted, that the current universe began at a singu;lar point in time 13.7 Gyr ago. NB. This is not the same as saying that there wasn''t a big bang etc - but actually, saying anything about time before the inflationary epoch is quite unconstrained (e.g. ideas like eternal inflation remain acceptable).  
> **username_0**: @username_14, if it's okay can you please add your answer too?  

---

### Answer by username_5 on 2022-07-19. Score: 5

Your question, if related to general relativity theory, needs a few explainations. First of all GR describes gravity as a geometry of spacetime; very many geometries are possible - the GR does not tell us which one has been realized in the Universe. While some, higly symmetric solutions of the equations of GR exist, and are thought to describe our universe "well" in the macro scale, e.g. <https://en.wikipedia.org/wiki/Friedmann%E2%80%93Lema%C3%AEtre%E2%80%93Robertson%E2%80%93Walker_metric>, these solutions are obviously inapplicable to "local inhomogenities" such as galaxies, rotating compact objects, etc. Furthermore "time", or "global time", is not, in general, well defined: at every point of the spacetime, (call it "event"), we can set up the future and past light cones - the "interior" of the future light cone is the set of events that can be reached from current event by a "causal curve" (light ray, or worldline of an observer). Hence, for instance, every observer has his "proper time" (<https://en.wikipedia.org/wiki/Proper_time>), as measured on the "watch" he carries on his "wrist". Given that "inhomogenities" (galaxies, etc) tend to collapse over time, it is quite generic to expect that large classes of worldlines hit an "end" in finite proper time - hence their time is "finite" in the future direction. In fact, there are general theorems to the effect that such situation is generic in GR - under general conditions there will be a "geodesic incompleteness" in the spacetime (meaning: worldlines cannot be extended indefinitely, see <https://en.wikipedia.org/wiki/Penrose%E2%80%93Hawking_singularity_theorems>).

A small note on "energy": in general in physics energy is a constant of motion present once the system has time-translation symmetry; in GR this means that there is such a notion (of energy) if the spacetime is at least "stationary" (<https://en.wikipedia.org/wiki/Stationary_spacetime>); since the spacetime of our Universe is not stationary, there is no such thing as "energy" not to mention its "conservation".

> **Comments:**
> **username_0**: Your answer certainly is interesting in contrast to the above answer. If it is not much to ask and if you have time, would it be possible to accompany your answer with crude drawings/figures to further understand the concept.  

---

### Answer by username_6 on 2022-07-19. Score: 1

Time is relative so it changes by location and eon, i.e. towards black holes and at the beginning and end of the universe. The speed depends on matter-gravity states. At some places there are dimensional states which are not compatible with our notion of time, just as it was prior to the Big Bang, so we could say "Times" and that every place works on clocks at different speeds. Sometimes those clocks just melt and divide into many clocks.


---

