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koti

Spacetime is doomed.

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We are at wonderous times right now (yeah right, every generation before us said the same thing), we’ve managed to advance in physics and math tools an incredible amount since we’ve come into existence 160K years ago (or 2mln depending how we look at it) We’ve reached energies at LHC probing distances a million times smaller than the atomic nucleous, we’ve managed to build machines that land on other moons & planets thanks to physics, material and software advancements and various other branches of science. We can clone organisms, develop software that wins at „Go” with humans and we’ve managed to achieve thousands of other awesome things in science. Yet we have no idea what time is or what gravity is in its true nature...we have tools that describe virtually everything around us including black holes, first nanoseconds after the BB but we can’t predict where the smoke from my cigarette will go when I light it up and let several seconds pass. We are also sure to a point of absolute certainty that GR is not giving us the underlying view of nature, its a a gem in physics which gave us so much insight into where/how we and stuff exists but its still just an aproximation like Newton before it and the Greeks before him.

We also know a few other things...when we used to sail the seas before we knew that the north pole exists - it existed. Then we discovered it and we charted every centimeter of Earth with satellite technology. Why would the Universe be different from our Earth, is there a rational reason to believe that we can’t discover everything one day? Why is there such a dissonance between us knowing the underlying laws governing satellite orbits or alleles in cells and not knowing seemingly simple stuff like cigarette smoke and other fluid turbulance? Every profound theory seems simple and beautiful when it gets discovered and tested, the beauty is in the fact that its not falsifable whatever complex experiment we throw at it. The fact that we are not doing any mainstream experiments that I know of, trying to experiment and make out  what makes ‚time’ tick is a disappointment to me. Every physicist I read and listened to that had something predictive to say about the nature of gravity, said that it either „disolves” or „leaks” in a way we only detect part of its a action. As I see it, we know jack s*** about time besides that its got an arrow and its glued to space by GR. Are there any theories and/or experiments we can run on „time” that could help us get at least some idea of what makes it tick?

 

Edited by koti

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I actually think we know as much about time as we do space. That which came into existence in there current form at t+10-43 seconds...or perhaps both are just simple mundane requirements of the universe and its existence. One separates everything, the other stops everything from happening in an instant and both can be interchanged. and are the same thing as per Minkowski. 

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24 minutes ago, beecee said:

I actually think we know as much about time as we do space. That which came into existence in there current form at t+10-43 seconds...

Is the origin of time predicted by the BB Theory? I was under the impression that was unknown.

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38 minutes ago, zapatos said:

Is the origin of time predicted by the BB Theory? I was under the impression that was unknown.

I'm reasonably sure that space and time [as we know them] evolved from t+10-43 seconds. So a tentative yes to your question. :)

An important question that needs to be asked is that can space and time [or spacetime] exist without the matter/energy within it. An answer to that is from Sten Odenwald...

https://einstein.stanford.edu/content/relativity/a11332.html

Special & General Relativity Questions and Answers

Can space exist by itself without matter or energy around?

No. Experiments continue to show that there is no 'space' that stands apart from space-time itself...no arena in which matter, energy and gravity operate which is not affected by matter, energy and gravity. General relativity tells us that what we call space is just another feature of the gravitational field of the universe, so space and space-time can and do not exist apart from the matter and energy that creates the gravitational field. This is not speculation, but sound observation.

:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::

Now obviously that sounds contradictory as the two words I have highlighted. So I E-Mailed Sten Odenwald about this and he apologised and did say it was an error and the highlighted part should not be there.....

Does that answer the question in the OP? 

Edited by beecee

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1 hour ago, beecee said:

I'm reasonably sure that space and time [as we know them] evolved from t+10-43 seconds. So a tentative yes to your question. :)

An important question that needs to be asked is that can space and time [or spacetime] exist without the matter/energy within it. An answer to that is from Sten Odenwald...

https://einstein.stanford.edu/content/relativity/a11332.html

Special & General Relativity Questions and Answers

Can space exist by itself without matter or energy around?

No. Experiments continue to show that there is no 'space' that stands apart from space-time itself...no arena in which matter, energy and gravity operate which is not affected by matter, energy and gravity. General relativity tells us that what we call space is just another feature of the gravitational field of the universe, so space and space-time can and do not exist apart from the matter and energy that creates the gravitational field. This is not speculation, but sound observation.

:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::

Now obviously that sounds contradictory as the two words I have highlighted. So I E-Mailed Sten Odenwald about this and he apologised and did say it was an error and the highlighted part should not be there.....

Does that answer the question in the OP? 

I'm not sure what you mean by the 2 higlighted "can & and" words beecee and I think both you and Odenwald don't want to admit it - we know jack sh** about time (no shame, every physicist does that in some way when trying to digg into time)
Every theory in physics I know of and every prediction in physics I know of does not elaborate on time in any useful way. I don't think its accurate to say that we have the same problem with time as with space...we have a bunch of working theories on how matter/energy behaves in different manners within space, we even have hypotheses that pockets of distant universe can behave differently with matter from what we see here. Time is just so bizzare that as far as I know, we got nothing remotely concrete on it. 

 

1 hour ago, zapatos said:

Is the origin of time predicted by the BB Theory? I was under the impression that was unknown.

The origin of anything is not predicted by the BB theory. We know really a lot about what happened after but nothing about what was at t=0 or before. The before part is in my opinion not a valid question at all...I think time has to be a „thing” of some sort describable in some way and not just a wigi board or an ectoplasma or a some other mindless explanation like - its what it is.

Edited by koti

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What makes time tick is a matter of philosophy. Is experimental philosophy a thing?

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21 minutes ago, swansont said:

What makes time tick is a matter of philosophy. Is experimental philosophy a thing?

I don’t know but it sure would be interesting. Comon, everyone is taking swings at quantizing everything.

I’m ready to be moved into speculations or the trash :P 

Edited by koti

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1 hour ago, koti said:

I'm not sure what you mean by the 2 higlighted "can & and" words beecee and I think both you and Odenwald don't want to admit it - we know jack sh** about time (no shame, every physicist does that in some way when trying to digg into time)
Every theory in physics I know of and every prediction in physics I know of does not elaborate on time in any useful way. I don't think its accurate to say that we have the same problem with time as with space...we have a bunch of working theories on how matter/energy behaves in different manners within space, we even have hypotheses that pockets of distant universe can behave differently with matter from what we see here. Time is just so bizzare that as far as I know, we got nothing remotely concrete on it. His answer [at least to me]  seems to explain it quite reasonably.

The origin of anything is not predicted by the BB theory. We know really a lot about what happened after but nothing about what was at t=0 or before. The before part is in my opinion not a valid question at all...I think time has to be a „thing” of some sort describable in some way and not just a wigi board or an ectoplasma or a some other mindless explanation like - its what it is.

The two highlighted words should not be there. 

Look, I am the first to agree that at best the defining of time is controversial and there are plenty of opinions on that. And obviously as swansont has also said, it is more a philosophical question then anything else. But in support of the Odenwald quote, we are reasonably confident that matter/energy did first arise from the evolution of space and time and the "Superforce" in that first 10-43 second. As that superforce started to decouple, and phase transitions  created, excesses of energy and fundamental particles evolved. So as Odenwald claims, and confidently, without the time and space, we have no matter or energy and subsequently no universe.

Hint: Read Odenwald's answer again.

and https://en.wikipedia.org/wiki/Superforce

 

Disclaimer: The further back in time we go and the closer to the BB, the less certainty applies to what we believed happened. 

But they are the best answers we have at this time, according to experimental and observational data.

Edited by beecee

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Just to clarify some things:

The BB is a solution of GR.  GR it itself is not a theory which allows to describe the creation of the universe out of nothing, even in principle.  The GR BB solution is singular, that means, it makes some physical variables (like the density) infinite at the singularity, and arbitrary large near the singularity.  This is obviously nonsense, so that we certainly know that GR has to be replaced by some better, yet unknown theory which does not have such a nonsensical singularity.  

We don't know up to which density the solution makes sense as an approximation.  But we can make reasonable guesses about this.  Say, we have studied in particle accelerators sufficiently well matter with a density of atomic nuclei.  We have also observations and theory about neutron stars, which have the same density.  So, it makes sense to assume that our theories are fine for such densities.  Optimists may think our theories are much more reliable, and become problematic only where quantum gravity effects become important.  

Whatever, if we look at this and compare it with the GR solution, we can identify a moment of time  after the singularity in the GR solution when the GR solution possibly starts to become a reasonable approximation of reality.  This leads to expressions like "[math]10^{-43}[/math] seconds after the BB".  Such expressions implicitly suggest that there is something meaningful we can say about the world before those "[math]10^{-43}[/math] seconds after the BB", namely that this period lasts only [math]10^{-43}[/math] seconds and starts with some moment of creation of the universe as we know it.  In reality, we know exactly nothing about the time before. The Big Bang may be as well a Big Bounce with an infinite history of time in the past. 

The other thing which requires clarification is that GR is only a theory about what clocks and rulers measure.  If the clocks really measure time or are, instead, distorted by the gravitational field (or the ether) so that it is not time what they measure, remains a question of interpretation.  In fact, once two similar clocks following different trajectories between two events where they are together and can be compared immediately show different numbers, it is quite clear for common sense that what the clocks have measured is not time in the common sense meaning of time.  Thus, the naive common sense interpretation would be that GR proper time is simply what clocks measure, and has no relation to time.  Common sense time would have to be something like a time coordinate: Between two events there can be only one time difference, defined by the difference of the time coordinates of the two events. GR simply does not make any claims which of the many imaginable time coordinates is what is our naive common sense time.  It is unobservable, in the sense that we have no reliable clocks to measure it.  

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6 hours ago, Schmelzer said:

 The other thing which requires clarification is that GR is only a theory about what clocks and rulers measure.  If the clocks really measure time or are, instead, distorted by the gravitational field (or the ether) so that it is not time what they measure, remains a question of interpretation.  

You misspelled "GR is a well-tested (and the only workable) theory about what clocks and rulers measure.

And no, it's not a matter of interpretation — that's what the theory says. If you have some other hypothesis, you are free to make a model and gather experimental evidence in support of it.

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39 minutes ago, swansont said:

You misspelled "GR is a well-tested (and the only workable) theory about what clocks and rulers measure.

And no, it's not a matter of interpretation — that's what the theory says. If you have some other hypothesis, you are free to make a model and gather experimental evidence in support of it.

There are other workable theories, all one needs for a workable theory is that the equations have the Einstein equations as a limit, in the same way as NT is the limit of GR.  But to discuss now such alternatives would be off-topic.  Given that "well-tested" has not been questioned here, I have seen no necessity to write this. 

But if what the clocks measure is worth to be named "time" is a matter of interpretation.  The only thing which one can imagine as a proof that it is not time has been done, and shows that it is not time - namely that the same clocks show for the same pair of events different results, while the time difference should be the same.  So, if one names what the clocks show "time", one invents a new notion of time, different from the classical one, and this is certainly a question of interpretation.  

If I have another interpretation of the GR equations, why do you think this would lead to differences in the empirical predictions?  A prerequisite for gathering experimental evidence would be that it is not an interpretation, but a different theory.  The theory says, in its empirical predictions, what the clocks and rulers show.  It  does not say if this is some strange curved four-dimensional spacetime or simply a distortion of clocks and rulers by the gravitational field.  

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22 minutes ago, Schmelzer said:

There are other workable theories, all one needs for a workable theory is that the equations have the Einstein equations as a limit, in the same way as NT is the limit of GR.  But to discuss now such alternatives would be off-topic.  Given that "well-tested" has not been questioned here, I have seen no necessity to write this. 

But if what the clocks measure is worth to be named "time" is a matter of interpretation.

No, I disagree. If you do this, you have to invent a new notion of time, and you need new physics to go along with it.  

 

Quote

  The only thing which one can imagine as a proof that it is not time has been done, and shows that it is not time - namely that the same clocks show for the same pair of events different results, while the time difference should be the same.  So, if one names what the clocks show "time", one invents a new notion of time, different from the classical one, and this is certainly a question of interpretation.  

What is the reference for this happening?

 

Quote

If I have another interpretation of the GR equations, why do you think this would lead to differences in the empirical predictions?

I don't. I think if you expect different answers you need a different model. What I am not getting is why you think there is a different interpretation to be had here.

 

Quote

  A prerequisite for gathering experimental evidence would be that it is not an interpretation, but a different theory.  The theory says, in its empirical predictions, what the clocks and rulers show.  It  does not say if this is some strange curved four-dimensional spacetime or simply a distortion of clocks and rulers by the gravitational field.  

Yeah, actually it does say it's a curved four-dimensional spacetime, and not a physical distortion of a device (see the equivalence principle). And if you think it's a physical effect you would need to explain kinematic dilation. The burden of proof here is yours.

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8 hours ago, Schmelzer said:

The other thing which requires clarification is that GR is only a theory about what clocks and rulers measure.

Dir you mean SR or GR ?

The equations of GR refer to other properties than elapsed time and length  (what clocks and rulers measure)

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9 hours ago, Schmelzer said:

The other thing which adequires clarification is that GR is only a theory about what clocks and rulers measure.

who knew a yard stick could be a variable.

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15 hours ago, Schmelzer said:

In fact, once two similar clocks following different trajectories between two events where they are together and can be compared immediately show different numbers, it is quite clear for common sense that what the clocks have measured is not time in the common sense meaning of time. 

My understanding is that at any instant, they are measuring local time, which varies depending on the local conditions. Or to put it another way, they are measuring time, in the location in which they exist, and in the frame in which they are stationary and not accelerating. 

And since you can use that time to calculate how time behaves in any other set of circumstances, when you measure time in one place, aren't you effectively measuring time for the whole Universe? 

 

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1 hour ago, mistermack said:

My understanding is that at any instant, they are measuring local time, which varies depending on the local conditions. Or to put it another way, they are measuring time, in the location in which they exist, and in the frame in which they are stationary and not accelerating. 

And since you can use that time to calculate how time behaves in any other set of circumstances, when you measure time in one place, aren't you effectively measuring time for the whole Universe? 

 

What your last is referring to is called cosmological time which uses time as per the mean average density. Which also sets the fundamental observer. (Don't confuse with a priveleged observer.)

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1 hour ago, Mordred said:

What your last is referring to is called cosmological time which uses time as per the mean average density. Which also sets the fundamental observer. (Don't confuse with a priveleged observer.)

I wasn't really making that point. What I meant was, if you know that A=10 x B , then if you measure A, then you are in effect measuring B as well. So if you have a formula for how the time on the surface of Jupiter is related to the time on Earth, then when you measure time on Earth, you are also measuring time on Jupiter. And the same principle would apply to anywhere in the Universe. 

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Rather than type out my views, I'll let M O'Dowd explain in this PBS video, which agrees rather closely with my understanding...

https://www.youtube.com/watch?v=K8gV05nS7mc&vl=en

He basically goes on to explain that as we travel backwards in time, geodesics converge to a singular point ( IE become un-physical ), just as they converge to a singular point ( in the future, this time ) in a Black Hole.
The geodesics cannot continue on the other side of the 'singularity' in either the Big Bang's case, nor the Black Hole's case, so you cannot ask "What came BEFORE the Big Bang ?" or "What happens AFTER reaching the center of a Black Hole ?"
That would be the same as North of the North Pole or South of the South Pole.

He does go on to explain ways around this like bounces and eternal inflation, and while there could be 'time' before a bounce or our own specific Big Bang ( of many ), there is no guarantee that it would be the same 'time'.

Edited by MigL

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14 hours ago, studiot said:

Dir you mean SR or GR ?

The equations of GR refer to other properties than elapsed time and length  (what clocks and rulers measure)

The claim holds for both.  The proper time formula in GR is [math]\tau = \int \sqrt{g_{\mu\nu} (x) \frac{dx^\mu}{dt}\frac{dx^\nu}{dt}} dt [/math], and the formula for proper time in SR is simply the particular case of the Minkowski metric [math]g_{\mu\nu}(x)=\eta_{\mu\nu}[/math].  And "proper time" is the time as measured by a clock following a particular trajectory.  If you know proper time for all trajectories, you can define [math]g_{\mu\nu}(x)[/math] uniquely.  So, the gravitational field in GR is simply the field defined by what the clocks measure.  

 

4 hours ago, mistermack said:

I wasn't really making that point. What I meant was, if you know that A=10 x B , then if you measure A, then you are in effect measuring B as well. So if you have a formula for how the time on the surface of Jupiter is related to the time on Earth, then when you measure time on Earth, you are also measuring time on Jupiter. And the same principle would apply to anywhere in the Universe. 

Unfortunately, length measurements are also distorted, and there is also no undistorted way to identify contemporaneity.  All these distortions taken together make your proposal impossible. 

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9 hours ago, Schmelzer said:

The claim holds for both.  The proper time formula in GR is τ=gμν(x)dxμdtdxνdtdt , and the formula for proper time in SR is simply the particular case of the Minkowski metric gμν(x)=ημν .  And "proper time" is the time as measured by a clock following a particular trajectory.  If you know proper time for all trajectories, you can define gμν(x) uniquely.  So, the gravitational field in GR is simply the field defined by what the clocks measure.  

 

Thank you for the reply,

I think it is more complicated than this, and in particular gravitation was not the only variable I had in mind.

 

The metric tensor you mention is a 4 x 4 array with 16 coefficients (in 4 dimensions) and describes the coordinate frame being used to measure the interval between events.

This is because it is based on the general quadratic

ds2 = g11(dx1)2 + g22(dx2)2 + g33(dx3)2 + g44(dx4)2 + 2g12dx1dx2+ 2g13dx1dx3+ 2g14dx1dx4+ 2g23dx2dx3+ 2g24dx2dx4+ 2g34dx3dx

Which you will note has only 10 terms.

Thus we have 10 equations and 16 coefficients.
So there are many ways to make the reduction to 10 coefficients.

Of course a general equation of a higher power could have been chosen, resulting in even more coefficients.
But we have no evidence (or even perhaps the means to make supporting observations) for doing this.
Higher powers seem to have negligeable effect.

But this is the Remanian geometry of GR and the g coefficients themselves are not real or imaginary constants, they are themselves functions of the coordinate axes.
This geometry applies globally.

In SR we can reduce this further to the 'flat' Euclidian geometry of the four squares.
This geometry only applies locally.

Having set the scene let us look at the Field equation(s) where this tensor is used.

The metric tensor premultiplies the 'Energy-Stress' tensor which contains elements describing the distribution of energy, mass and momentum.

However there is yet another term in the Field equation - the Lambda tensor, which is not affected by the metric tensor.
So even if you reduced all the g coefficients to zero, reducing that term to zero, the Lambda term would remain.

Edited by studiot

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12 hours ago, Schmelzer said:

The claim holds for both.  The proper time formula in GR is τ=gμν(x)dxμdtdxνdtdt , and the formula for proper time in SR is simply the particular case of the Minkowski metric gμν(x)=ημν .  And "proper time" is the time as measured by a clock following a particular trajectory.  If you know proper time for all trajectories, you can define gμν(x) uniquely.  So, the gravitational field in GR is simply the field defined by what the clocks measure.  

 

Addendum

I think you are working in so called Natural Units when you post these formulae and say that the interval ds = proper time so that c = 1

Is this the case please?

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Hm. 

First of all, the metric tensor has from the start only ten coefficients, and what reduces it from 16 to 10 is simply the requirement that the tensor has to be symmetric. There is no other way in GR, 

Then, [math]\Lambda[/math] is simply a single real constant, which appears in the Einstein equations.  There is no such thing as a  [math]\Lambda[/math] tensor. 

Then, in SR the Minkowski metric is a global object.  (It is "local" only if one talks about applying it in GR locally as an approximation.) The Minkowski spacetime is Lorentzian, not Euclidean. 

If I say nothing about it, I use c=1

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1 hour ago, Schmelzer said:

Hm. 

1) First of all, the metric tensor has from the start only ten coefficients, and what reduces it from 16 to 10 is simply the requirement that the tensor has to be symmetric. There is no other way in GR, 

2) Then, Λ is simply a single real constant, which appears in the Einstein equations.  There is no such thing as a  Λ tensor. 

3) Then, in SR the Minkowski metric is a global object.  (It is "local" only if one talks about applying it in GR locally as an approximation.) The Minkowski spacetime is Lorentzian, not Euclidean. 

4) If I say nothing about it, I use c=1

Once again, thank you for the honest answers.

1)

No, just because some of the entries are zero, does not mean they don't exist.
Can you demonstrate a rectangular matrix with only 10 entries that is also a tensor?

2) So are you telling me that you can add a single real number to a tensor or are you telling me that when you multiply all the elements of a tensor by a single real number you don't get another tensor?
If you want to use a 'cosmological' constant you have to apply it everywhere, which means you multiply gυν by it.

3. SR is only global in the absence of all mass and energy in the system.

4. Thank you for that confirmation. So can you reference a clock that can directly read time units in this system, and what does it read? (perhaps swansont will help here - this is his field not mine.)
I would also respectfully suggest that adopting this practice whilst saying nothing is baffling for many members here who may be unfamiliar with natural units in general and geometrised ones in particular.

https://en.wikipedia.org/wiki/Natural_units

https://en.wikipedia.org/wiki/Geometrized_unit_system

Edited by studiot

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