[ExI] Dark energy = (anti)gravity?
John Clark
johnkclark at gmail.com
Mon Nov 6 17:31:04 UTC 2017
On Sat, Nov 4, 2017 at 1:47 AM, Stuart LaForge <avant at sollegro.com> wrote:
> All paths leading up to the first instant of time are equally invalid
>>
>> because there were none.
>
>
> How do you know that?
Are you asking how I know the first is the first?
>
>> in the case of the universe, all those stop watches started at the
>>
>> same time and in the same place.
>> But
>> to measure a time interval both a start and stop point is needed and
>>
>> all those stopwatches stopped at different times because there is no
>>
>> universal agreement on simultaneity, so there is no agreement on if
>> watch X stopped before watch Y or watch Y stopped before watch X.
>
>
> >
> Ah but there is a universal agreement on simultaneity - a precise
>
> mathematical definition given by special relativity
That is incorrect. There is
universal agreement
on the spacetime distance between 2 events but there is no
universal agreement
on the spacial distance between the 2 events nor is there agreement on
the time interval between the 2 events or even agreement on if event X
happened before Y or Y happened before X.
> >
> All events equidistant from the observer that lie in the hyperplane will
>
> occur simultaneously
That is not universal agreement. Yes you can always find 2 observers that
agree, but I can always find one that doesn't, in fact I can find an
infinite number of them.
>
>> >
>> And to make things even
>>
>> worse the stopwatches are running at different rates. So there is no
>>
>> universal agreement on when the Big Bang happened; right here right now
>>
>> we say it was 13.8 billion years ago, but others would disagree
>
>
> >
> Under the assumption that we are the stationary observer,
Would an observer in the Coma Cluster be willing to make that assumption?
Why are we more important than him even from his point of view?
> >
> then our
>
> measurement of 13.8 billion years is the maximum observable age of the
>
> universe
Perhaps so if the universe were to come to an end today.
> >
> and all the moving observers will measure a younger universe
>
> whose clock is moving slower than theirs.
>
If you and I are moving relative to each other I will see your clock
moving slower than my clock and you will see my clock running slower than
your clock. There is no contradiction in this because when we try to
measure how long it takes for one of us to move between 2 points we don't
agree on when to start and stop the stopwatch. It's a good thing there
isn't universal agreement on simultaneity, if there were physics would be
logically inconsistent.
>
> Of course, we can't actually be stationary relative to the big bang
>
We can't be moving away from the spot where the Big Bang happened because
it happened right here, it's just that the spot keeps getting bigger with
time.
>
> because at the very least we are moving away from it in time. This
>
> suggests the universe could be far older than we measure it to be.
>
I don't know what you mean by that.
>
> Furthermore the universe as whole is significantly larger than the
>
> observable part of it, then
>
> there are parts of the universe for which our
>
> "big bang" has not yet happened.
I don't understand that either.
> >
>> >
>> There are a infinite number of ways a bunch of distant clocks can be
>>
>> brought together in a expanding universe, and I don't see how everyone
>>
>> could agree on how to do it. Should distant clock X be brought to clock Y
>>
>> or should clock Y be brought to clock X It makes a difference because
>>
>> one clock would be accelerated and the other clock would not and
>> a accelerated clock runs slower than one in a inertial frame of reference.
>
>
> >
> Why would you bring the clocks back together again? Just read the distant
>
> clocks through a telescope and adjust for relative velocity and distance.
>
I don't see what the point of that would be, we already know what we'd see
through our telescopes, we'd both see a stopwatch running slower than our
own. And if I am going south at half the speed of light and you are going
north at half the speed of light and I watch you through my telescope
measure the time it takes you to pass mile markers on the road I will see
you write in your notebook the same figure that I do even though I see your
stopwatch running slow, because from my viewpoint you started and stopped
your stopwatch at the wrong times. We don't agree on simultaneity, we don't
agree on the instant you passed the 2 mile markers.
If you bring the clocks together to find out which clock was *really*
running slow the symmetry of the situation will be broken because one must
accelerate and one must not. If you accelerate back toward me you will see
my clock running FASTER than your clock and I will see your clock running
SLOWER than my clock. When you get back to the same place I'm at and we're
moving at zero speed relative to each other we will both observe both
clocks again running at the same rate but they will no longer
be synchronized, we will agree that much more time has passed on my clock
than your on clock because your clock was accelerating but mine was not.
The same thing would happen if instead of accelerating you were in a
stronger gravitational field than me.
> >
> The way I am hypothesizing it happens is that everything we can see is
>
> inside a hollow sphere of infinite thickness and constant density and thus
>
> infinite mass. And so everything in the sphere is being gravitationally
>
> pulled to the closest part of the sphere because the infinite mass of the
>
> universe is just a little closer in that direction.
>
Infinite
thickness? Be careful, there Be Dragons. If I'm standing on the inner or
outer surface of a sphere with an
i
nfinite radius
the surfaces
will both look the same to me,
they
will look like I'm standing on a infinitely flat infinity large plane, and
the gravitational acceleration produced by such a plane is 2*π*G*D*H,
where G is the gravitational constant, D is the density and H is the
thickness of the plane. A derivation of this formula can be found in volume
1 of The Feynman Lectures On Physics, in section 13-4 called "Gravitational
field of large objects".
There are 2 interesting things to see from that formula:
1) Unlike a point source or a finite sphere the gravitational field does
not decrease with the square of the distance, in fact for a infinite sphere
or infinite plane the gravity does not decrease at all for any finite
distance.
2) If it's infinitely thick then H is infinite so if D, the density, is
not zero then the g force on the surface will be infinite and not just on
the surface, it will be infinite everywhere. Needless to say we do not
observe that.
And even if the sphere is not infinite but just astronomically large
I don't see how
that idea
can explain why the universe was decelerating for the first 9 billion years
of its existence and only started accelerating 5 billion years ago. It's
also odd that the Earth happens to be at a very untypical place, the center
of a infinite sphere. It all seems a bit too Ptolemaic.
>
>> >
>> And that percentage will increase as time passes because both normal
>> matter
>>
>> and dark matter will keep getting diluted but dark energy will not, the
>>
>> more space that space itself creates
>>
>> he more dark energy there is
>> but
>> the amount of matter in the universe
>> will be constant.
>
>
> > why
> would you insist that matter be conserved but that energy is not?
>
I don't insist on matter being conserved but not energy, but for the last
20 years we've have lots of empirical evidence that the universe does
insist on exactly that. As to why the universe prefers things that way
you'll have to ask the universe not me.
>
> Both Hawking radiation and radioactive decay are adding particles to the
>
> universe.
So what? That in no way changes the fact that as the universe expands the
mass of matter in the universe, both dark and regular, becomes more and
more diluted but Dark Energy does not because unlike matter Dark Energy is
a property of space itself.
> While it would seem at first glance that fusion decreases the particle
>
> number of the universe,
>
> when one considers photons and neutrinos as
>
> particles, it is clear that fusion too increases the particle number ofthe
> universe.
True
, when 3 protons fuse they produce 4 particles, a positron a neutrino a
gamma ray and a Helium-3 nucleus, but the number of particles is irrelevant
from a cosmological perspective, but the total mass/energy is not and the
fusion process does not change that.
> when one considers photons and neutrinos as
>
> particles, it is clear that fusion too increases the particle number of
> the universe.
>
Fusion or no fusion the mass/energy content of the hydrogen remans
constant, Hawking radiation or no Hawking radiation the mass/energy
content of a Black Hole remans constant; but as the universe expands
the mass/energy
content of Dark Energy does NOT remain constant.
>
> So if the number of particles is increasing and they have relative
>
> velocities and therefore kinetic energy, it is clear that the mass of the
> universe must be increasing.
No that is not clear at all. The number of neutrinos is certainly
increasing but as they have AT LEAST 45 billion times less mass than a
proton that fact is not very important.
The mass of matter is not increasing,
the amount of Dark Energy is.
>
> But if energy is conserved
> [...]
>
On a cosmological scale energy is not conserved in a expanding
accelerating
Einsteinian
universe.
>
> Obviously, as the particle number and volume of the universe increases, so
>
> too will the entropy. Since those particles will have more degrees of
>
> freedom available to them.
>
I agree.
>
> It is not the conservation laws that are the contortions but the
>
> cosmological constant, dark energy and dark matter. They are like
> epicycles. No need to make the universe more complicated than it already
>
> is.
>
Einstein said theories should be as simple as possible but
not
simpler. I admit the physical theories we have today are a bit of a mess
but if somebody can come up with a simpler idea that can explain the rather
bizarre
observations we've been seeing
recently
I'm sure it would be embraced with enthusiasm, but that won't be easy. For
example, some have proposed getting rid of Dark Matter by modifying the
laws of gravity, but no modification can explain galaxy
Dragonfly 44. From the motion of stars and the orbits of globular clusters
in orbit around
it we know that
Dragonfly 44
has about the same mass as our galaxy, a trillion solar masses, and yet
Dragonfly 44
has little gas or dust and less than 1% as many stars as the Milky Way
does, it is one of a new class of objects called "dark galaxies". Unless
there is no universal law of gravity at all because it works differently in
dark galaxies than in normal galaxies like ours the only conclusion is that
99.99% of the matter in
Dragonfly 44
must be dark verses only 88% for the Milky Way.
> >
> The acceleration of the universe is just the curvature of spacetime in the
>
> outward direction caused by the mass density in all the other causal
>
> cells. Come on, acceleration, curvature, and gravity are all synonomous.
>
But we now know that mass alone can not explain the large scale shape of
spacetime, and even mass and pressure is not sufficient to do so because
tension caused by Dark Energy is also a factor, and in fact the most
important factor.
> >
> It's just the equivalence principle and Einstein came up with that. Not
>
> me.
>
The equivalence principle is still true, spacetime still tells matter how
to move, its just that we now know that matter is not the only thing that
tells spacetime how to curve, Dark Energy also has a say in how spacetime
should curve.
>
>> >
>> The density of normal matter and Dark Matter has decreased over time but
>>
>> it has become clear that the gravitation caused by matter alone (not
>>
>> even with the help of dark Matter) is insufficient to explain the
>>
>> evolution of the universe. For a very long time the expansion of the
>>
>> universe was slowing down just as you'd expect, but about 5 billion years
>>
>> (and nearly 9 billion year after the Big Bang) the deceleration
>>
>> stopped and things started to accelerate. This can only be because the
>>
>> matter became diluted and so did the gravitational force trying to slow
>>
>> things down but some property of space itself called Dark Energy which
>>
>> nobody understands causes things to speed up, so whatever it is when there
>>
>> is more space there is more Dark Energy
>
>
> >
> Where is the evidence that the universe has been decreasing in density?
The evidence has been known since 2003. For the first 9 Billion years of
its existence the expansion of the universe was slowing down, but about 5
billion years ago that changed and it started to speed up because the
amount of matter (both dark and non-dark) that was trying to slow thins
down became more dilute as space expanded but the amount of Dark Energy
trying to speed things up did not become more dilute due to the fact that
Dark Energy is a property of space itself.
http://iopscience.iop.org/article/10.1086/383612/meta
http://www.nytimes.com/2003/10/11/us/a-cosmic-jerk-that-reve
rsed-the-universe.html
> >
> If
>
> space-time is expanding and the particle number of the universe is
> increasing, then density could be going up.
>
The number of particles is irrelevant, the total mass of all those
particles is not and that total mass of matter remains constant, but the
amount of space those particles can be in is not constant if space is
expanding so mass is becoming more dilute. And if Dark Energy is a property
of space itself then it is not becoming more dilute as space expands.
Matter tries to slow the expansion and Dark Energy tries to speed it up,
for 9 billion years matter won that tug of war but then it just got too
dilute 5 billion years ago and Dark Energy started to win.
> >
> What I am saying is that spacetime cannot curve away from a
>
> mass without curving toward some other mass somewhere.
>
>
Curved 4D spacetime can be defined without the need for a fifth dimension
or anything else for it to curve into. Curved spacetime just means the
Euclidian distance formula, aka the Pythagorean Theorem, will not work
there but Einstein provided a distance formula that will.
>
> >
> What's outside of our light-cone? Literally almost *everything*.
>
Maybe everything maybe nothing, it's outside our light-cone so we can
never know.
John K Clark
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.extropy.org/pipermail/extropy-chat/attachments/20171106/dcb68ac1/attachment.html>
More information about the extropy-chat
mailing list