There is a law called the law of conservation of mass and energy under
QM for short periods of time this law can be broken but for the periods
we are talking about it must be conserved.

In the scenario where a time traveller suddenly arrives in this time an
equivalent energy to him must go back to his time.

On 17 veebr, 11:27, bernardZ <Berna...@nospam.com> wrote:
> There is a law called the law of conservation of mass and energy under
> QM for short periods of time this law can be broken but for the periods
> we are talking about it must be conserved.
>
> In the scenario where a time traveller suddenly arrives in this time an
> equivalent energy to him must go back to his time.

But time travel is in and of itself a grievous violation of homogenity
of time.

Just why should the conservation of energy be observed in the specific
time? Why is two-way travel required?

On Sun, 17 Feb 2008 20:27:24 +1100, bernardZ <BernardZ@nospam.com>
wrote:

:There is a law called the law of conservation of mass and energy under
:QM for short periods of time this law can be broken but for the periods
:we are talking about it must be conserved.
:
:In the scenario where a time traveller suddenly arrives in this time an
:equivalent energy to him must go back to his time.
:
Time travels implies a space-time topology
that is not simply-connected - that is, there can be
two paths connecting two points in space-time that
cannot be continuously transformed from one to the
other. For example, the surface of a sphere is simply
connected - any path between two points can be
smoothly transformed into any other - but the surface
of a torus is not - a path going through the 'hole' of
the torus can't be smoothly transformed to a path
going around the outside of the torus.

Say there's a time-travel event from Los
Angeles in 2029 to Los Angeles in 1984. If you're in
Los Angeles in 1982, you can get to 1984 LA by just
staying where you are for two years, or you can move
to Sacramento, wait 40 years, move back to LA, wait
seven more years, and then travel back to 1984 LA.
But there's no smooth transition from one path to the other.

Global conservation of mass/energy requires a
simply connected space-time topology. To measure
global energy, you have to establish an approximation of
universal simultaneity. Again,. picture space-time as a
sphere, with the beginning of the universe at one point
and the end of it at its antipode. Global conservation of
mass/energy means that along any latitude between these
poles will have the same net mass/energy along it (think of
Cauchy's Integral Formula). It doesn't even matter if these
latitudes aren't perfectly 'horizontal', so long as no point in
the latitude is in the light-cone of any other point. But, if
space-time isn't simply connected, but instead like a torus,
then you can't really define latitudes between the beginning
and end of time, because these latitudes cannot smoothly
transform from one to the other. Not only is global
mass/energy not conserved in a not-simply-connected
space-time topology, it isn't even well-defined, because
there are points that are in each other's light-cones.
--
e^(i*pi)+1=0

George W. Harris For actual email address, replace each 'u' with an 'i'.

On Sun, 17 Feb 2008 20:27:24 +1100, bernardZ <BernardZ@nospam.com>
wrote:

>There is a law called the law of conservation of mass and energy under
>QM for short periods of time this law can be broken but for the periods
>we are talking about it must be conserved.
>
>In the scenario where a time traveller suddenly arrives in this time an
>equivalent energy to him must go back to his time.
>

If you wait long enough...it will.

George W Harris wrote:

> Global conservation of mass/energy requires a
> simply connected space-time topology.

Actually, it's worse than that; even in a simply-connected spacetime
topology, the concept of the total amount of mass, energy, momentum,
angular momentum of the Universe is problematic in general relativity.
You can talk about one, but those familiar with general relativity know
that they're basically cheating.

But more importantly, global conservation of energy is not in any
meaningful sense true in general relativity. Indeed, our models of the
Universe require that it be violated (namely, with cosmological redshift
-- photons traversing an expanding spacetime are losing energy, but the
energy isn't going anywhere).

In general relativity, one can only talk about _local_ (that is, in the
vicinity of a point) conservation laws. There are no global
conservation laws in our best theories that deal with the large-scale
structure and evolution of the Universe, as strange as that sounds.

--
Erik Max Francis && max@alcyone.com && http://www.alcyone.com/max/
San Jose, CA, USA && 37 18 N 121 57 W && AIM, Y!M erikmaxfrancis
More than eight I'll call you / Less I'll call it a day
-- Anggun

On Feb 17, 1:19 pm, Erik Max Francis <m...@alcyone.com> wrote:
> George W Harris wrote:
> > Global conservation of mass/energy requires a
> > simply connected space-time topology.
>
> Actually, it's worse than that; even in a simply-connected spacetime
> topology, the concept of the total amount of mass, energy, momentum,
> angular momentum of the Universe is problematic in general relativity.
> You can talk about one, but those familiar with general relativity know
> that they're basically cheating.
>
> But more importantly, global conservation of energy is not in any
> meaningful sense true in general relativity. Indeed, our models of the
> Universe require that it be violated (namely, with cosmological redshift
> -- photons traversing an expanding spacetime are losing energy, but the
> energy isn't going anywhere).
>
> In general relativity, one can only talk about _local_ (that is, in the
> vicinity of a point) conservation laws. There are no global
> conservation laws in our best theories that deal with the large-scale
> structure and evolution of the Universe, as strange as that sounds.

Just to go into a little more detail - the conservation of dynamical
quantities (energy, momentum, angular momentum, and some weird stuff
without convenient Newtonian analogues) must hold in any space-time
that is asymptotically flat. That is, if you go far enough away and
you reach areas where gravity reaches its weak field (Newtonian) limit
and space-time is approximately given by the Minkowski metric (that
is, it is flat and not warped, curved, spindled, or mutilated), and
this is true no matter what direction you choose to go far away in,
then you are guaranteed that energy and momentum and stuff will be
conserved (although the conservation might not be local while within
the region where space-time is not flat). If this condition is not
true then you can probably find ways of violating these conservation
laws.

Note that this holds even for non-simply connected topologies. By
now, most readers of this newsgroup are probably familiar with the way
a wormhole mouth gains the conserved quantities of things that go
through them, and lose said quantities of things that come out of
them. This is an example of a non-simply connected topology where the
conservation of dynamical variables (as well as electric charge) still
holds (assuming that if you go far enough away from the wormhole you
reach asymptotically flat space-time, of course - but that is usually
part of the definition of a wormhole).

Also note that the space-time surrounding Earth, or solar system, and
our galaxy is, to a very good approximation, flat. Thus time
travelers to and from Earth will need to worry about their mass being
conserved.

I think the conservation of electric charge holds in any space-time
geometry, since it arises from gauge symmetries rather than
geometrical symmetries. If someone knows more detail about this,
though, feel free to correct me (and explain why, of course - this
stuff is interesting to learn about).

Luke

In article <oYednTNsGYJJOSXanZ2dnUVZ_qWtnZ2d@speakeasy.net>,
max@alcyone.com says...
> George W Harris wrote:
>
> > Global conservation of mass/energy requires a
> > simply connected space-time topology.
>
> Actually, it's worse than that; even in a simply-connected spacetime
> topology, the concept of the total amount of mass, energy, momentum,
> angular momentum of the Universe is problematic in general relativity.
> You can talk about one, but those familiar with general relativity know
> that they're basically cheating.
>
> But more importantly, global conservation of energy is not in any
> meaningful sense true in general relativity. Indeed, our models of the
> Universe require that it be violated (namely, with cosmological redshift
> -- photons traversing an expanding spacetime are losing energy, but the
> energy isn't going anywhere).
>


General relativity was published by Albert Einstein in 1915/16. Albert
Einstein only adopted the big bang theory long after trying several
other alternatives first.

Clearly Einstein never saw that concept of an expanding spacetime is
necessary for GR to be correct.

> In general relativity, one can only talk about _local_ (that is, in the
> vicinity of a point) conservation laws. There are no global
> conservation laws in our best theories that deal with the large-scale
> structure and evolution of the Universe, as strange as that sounds.
>
>


I don't see how that changes my argument whether local or global.

In article <2bf42b5b-c631-4665-8884-5129e1db6dd6
@v3g2000hsc.googlegroups.com>, chornedsnorkack@hush.ai says...
> On 17 veebr, 11:27, bernardZ <Berna...@nospam.com> wrote:
> > There is a law called the law of conservation of mass and energy under
> > QM for short periods of time this law can be broken but for the periods
> > we are talking about it must be conserved.
> >
> > In the scenario where a time traveller suddenly arrives in this time an
> > equivalent energy to him must go back to his time.
>
> But time travel is in and of itself a grievous violation of homogenity
> of time.
>
> Just why should the conservation of energy be observed in the specific
> time? Why is two-way travel required?
>


This law of conservation of mass and energy is hardly a minor physical
statement, it is called a law not a theory for the reason that we are
extremely confident that it is true. Ultimately I suppose it depends how
much you are willing to give up of physics

bernardZ wrote:

> General relativity was published by Albert Einstein in 1915/16. Albert
> Einstein only adopted the big bang theory long after trying several
> other alternatives first.

This is incorrect on several levels.

Hubble first discovered the redshift of galaxies in 1918, and the Hubble
law relating redshift to distance in 1929. Those resulted in the
development of the Big Bang theory, so there was no such thing at the
time that Einstein was developing general relativity.

Indeed, at the time, the conventional wisdom was that the Universe was
finite in size and static and had been around forever. However, when
applying general relativity to cosmology, Einstein discovered that his
new theory predicted dynamic cosmologies: expanding or contracting
ones, but not static ones. Instead of going with what his theory
predicted, Einstein added a fudge factor, the cosmological constant, so
that his theory could result in static universes. Not paying attention
to what his theory was telling him and introducing the fudge factor was
what he called the greatest "blunder" of his life. (Though now we think
the cosmological constant is still relevant for other reasons, due to
dark energy.)

> Clearly Einstein never saw that concept of an expanding spacetime is
> necessary for GR to be correct.

No one said it did. But it is certainly true that general relativity
predicts dynamic cosmologies, and Einstein tried to shoe-horn it into a
static version, instead of being able to triumphantly declare Hubble
expansion as the first and foremost prediction of his new theory.

--
Erik Max Francis && max@alcyone.com && http://www.alcyone.com/max/
San Jose, CA, USA && 37 18 N 121 57 W && AIM, Y!M erikmaxfrancis
God will forgive me. It's his job.
-- Heinrich Heine

In article <ze-dndVfFKFidSTanZ2dnUVZ_vCknZ2d@speakeasy.net>,
max@alcyone.com says...
> bernardZ wrote:
>
> > General relativity was published by Albert Einstein in 1915/16. Albert
> > Einstein only adopted the big bang theory long after trying several
> > other alternatives first.
>
> This is incorrect on several levels.
>
> Hubble first discovered the redshift of galaxies in 1918, and the Hubble
> law relating redshift to distance in 1929.

This is a bit of a myth. Hubble already knew of the redshift earlier. He
that announce that his results confirmed it. They did but not greatly.


> Those resulted in the
> development of the Big Bang theory, so there was no such thing at the
> time that Einstein was developing general relativity.
???

> Indeed, at the time, the conventional wisdom was that the Universe was
> finite in size and static and had been around forever. However, when
> applying general relativity to cosmology, Einstein discovered that his
> new theory predicted dynamic cosmologies: expanding or contracting
> ones, but not static ones. Instead of going with what his theory
> predicted, Einstein added a fudge factor, the cosmological constant, so
> that his theory could result in static universes. Not paying attention
> to what his theory was telling him and introducing the fudge factor was
> what he called the greatest "blunder" of his life. (Though now we think
> the cosmological constant is still relevant for other reasons, due to
> dark energy.)
>

Agree. At the beginning of GR Einstein was committed to the static
universe. He found a dubious solution in the cosmological constant. It
was after that really with Friedman that an expanding universe starts to
appear in GR.


> > Clearly Einstein never saw that concept of an expanding spacetime is
> > necessary for GR to be correct.
>
> No one said it did. But it is certainly true that general relativity
> predicts dynamic cosmologies, and Einstein tried to shoe-horn it into a
> static version, instead of being able to triumphantly declare Hubble
> expansion as the first and foremost prediction of his new theory.
>

I doubt he even knew of Hubble work then although he probably knew of
the early works on the red shift.

Interestingly one problem that bugged Newton was that his model of the
universe was unstable too. Under Newtonian mechanics the universe should
collapse. To overcome this problem he also theorized a cosmological
constant which he identified with God.


--
Note change of name. The former owner of my sig Bernard Z wants it back.

Reckons his posts and mine are getting mixed up.

On 17 veebr, 23:19, Erik Max Francis <m...@alcyone.com> wrote:
> George W Harris wrote:
> > Global conservation of mass/energy requires a
> > simply connected space-time topology.
>
> Actually, it's worse than that; even in a simply-connected spacetime
> topology, the concept of the total amount of mass, energy, momentum,
> angular momentum of the Universe is problematic in general relativity.
> You can talk about one, but those familiar with general relativity know
> that they're basically cheating.
>
> But more importantly, global conservation of energy is not in any
> meaningful sense true in general relativity. Indeed, our models of the
> Universe require that it be violated (namely, with cosmological redshift
> -- photons traversing an expanding spacetime are losing energy, but the
> energy isn't going anywhere).
>
Photons encounter moving matter. Surely changing energy when changing
frames is a common phenomenon?

Crown-Horned Snorkack wrote:

> On 17 veebr, 23:19, Erik Max Francis <m...@alcyone.com> wrote:
>> George W Harris wrote:
>>> Global conservation of mass/energy requires a
>>> simply connected space-time topology.
>> Actually, it's worse than that; even in a simply-connected spacetime
>> topology, the concept of the total amount of mass, energy, momentum,
>> angular momentum of the Universe is problematic in general relativity.
>> You can talk about one, but those familiar with general relativity know
>> that they're basically cheating.
>>
>> But more importantly, global conservation of energy is not in any
>> meaningful sense true in general relativity. Indeed, our models of the
>> Universe require that it be violated (namely, with cosmological redshift
>> -- photons traversing an expanding spacetime are losing energy, but the
>> energy isn't going anywhere).
>
> Photons encounter moving matter. Surely changing energy when changing
> frames is a common phenomenon?

With cosmological redshift, they've already lost the energy by the time
that happens.

--
Erik Max Francis && max@alcyone.com && http://www.alcyone.com/max/
San Jose, CA, USA && 37 18 N 121 57 W && AIM, Y!M erikmaxfrancis
Why can't love be / Like molasses rain
-- Sandra St. Victor