Why gravity?

From: "Jonathan Thornburg [remove -animal to reply]"
Subject: Why gravity?
Newsgroups: sci.astro.research
References:

jacob navia wrote:
Now, what would happen if we leave gravity and suppose that at
galactic scales new forces become relevant that we can't even suspect
given our minuscule size.

I have two (somewhat-related) comments.


First, Occam's razor suggests not hypothesing a completely new force
until we find phenomena which we can't explain with already-known
physics.

Dark matter doesn't require any new physics, it just requires that
there be some matter which early-21st-century humans can't easily
observe. (We use the adjective "dark", but all we really mean is
that it doesn't interact electromagnetically.) We already know of
other sorts of matter which behave that way and are really hard to
observe (e.g., neutrinos), so hypothisising some other
a-bit-harder-to-observe matter doesn't seem too implausible.

There are also the Bullet cluster results, which seem to be strongly
supportive of the dark matter hypothesis.


Second, suppose we go ahead and hypothesize a new force (NF) responsible
for flat galactic rotation curves. The problem is that we have very
little data to constraint the properties of this force. So, we're
basically left to speculate about such questions as
* how does the NF operate?
* what determines the magnitude & direction of the NF?
* what does NF act on?
- e.g., does it act on photons?
- e.g., does it act on the gravitational binding energy of a binary star?
* what are the strong-field or weak-field limits of NF?
* (how) is NF quantized?
* how does NF relate to Newtonian-gravity-as-the-linear-approximation-
-to-general-relativity?
* how does NF relate to the {strong,weak,electromagnetic} forces in the
context of gauge theories, elementary-particle physics, etc?

Another way to look at this is to compare NF with string theory.
String theory is often criticized for "unconstrained speculation".
[N.b. I am NOT making any statement here about the
validity (or lack thereof) of string theory as a
research area -- that's an interesting subject, but
(a) I think it's not relevant to the subject of this
discussion, and (b) it probably belongs over in our
sister newsgroup sci.physics.research, not here.]
or of string theory as a discipline in physics. That
But at least string theory has the constraints that it must be consistent
with the HUGE body of experimental evidence relating to flat-space
(non-relativistic and relativistic) quantum mechanics, quantum field
theory, Newtonian gravity, and general relativity in their respective
regimes of validity.

In contrast, about all we can say about NF is that it should reproduce
flat galaxy rotation curves, and that (by hypothesis) it should NOT be
observable in a whole bunch of other well-studied system in the universe.

So, it seems to me that all the "unconstrained speculation" critiques
of string theory apply far MORE to NF.


ciao,

--
-- "Jonathan Thornburg [remove -animal to reply]"
Dept of Astronomy & IUCSS, Indiana University, Bloomington, Indiana, USA
on sabbatical in Canada through late August 2013
"There was of course no way of knowing whether you were being watched
at any given moment. How often, or on what system, the Thought Police
plugged in on any individual wire was guesswork. It was even conceivable
that they watched everybody all the time." -- George Orwell, "1984"

On 7/24/2013 10:09 PM, Jonathan Thornburg wrote:
From: "Jonathan Thornburg [remove -animal to
Subject: Why gravity?
Newsgroups: sci.astro.research


jacob wrote:
Now, what would happen if we leave gravity and suppose that at
galactic scales new forces become relevant that we can't even suspect
given our minuscule size.

I have two (somewhat-related) comments.


First, Occam's razor suggests not hypothesing a completely new force
until we find phenomena which we can't explain with already-known
physics.

Dark matter doesn't require any new physics,

If it's non-baryonic and not consisting of
neutrino's (and aren't we pretty sure of both
these things?) then it must be new physics, I
would say..

it just requires that
there be some matter which early-21st-century humans can't easily
observe. (We use the adjective "dark", but all we really mean is
that it doesn't interact electromagnetically.) We already know of
other sorts of matter which behave that way and are really hard to
observe (e.g., neutrinos), so hypothisising some other
a-bit-harder-to-observe matter doesn't seem too implausible.

But it would be hypothesising a new matter
field instead of a new force field. In either
way you add new field components to the total
pool of the standard model. Would one way
really be less "new physics" than the other?

Now if neutrino masses were about right to
explain dark matter then we would have a much
better solution! The "previous" new physics
(i.e. the once unexpected neutrino masses)
would then nicely explain the problem. Only
it does not seem to be that way..

There are also the Bullet cluster results, which seem to be strongly
supportive of the dark matter hypothesis.

OK, new matter seems more plausible than new
force (because of the observations) but I
still think it would be equally, ehh.. new!

--
Jos

Thanks for your answer Mr Thornburg, I will reply shortly to your
objections but I think there is a misunderstanding.

I would like to know if we could (more or less) easily MEASURE
that force by carefully following some spacecraft.

For instance a force of the magnitude required could have
modified the path of the GRAIL spacecraft that recently
measured the gravity of the moon.

If that force exists, it should appear as an acceleration tangential to
the galaxy rotation...

For a professional here, it is a small time and the rewards could be
huge...

Just figuring out:

Where the galactic center is at all moments of the flight of the
twin probes.

Supposing that the distance differences are dwarfed by 30 thousand
light years (or so), we can ignore the spacecraft trajectory and keep in
mind that a force of vector tangential to the sun's rotation
around Sag. A* is pushing the spacecraft very gently...

Just 1e-10 m/sec*sec

This could make a small difference in the trajectory of the spacecraft
in a known direction.

Of course probably there is nothing there. A professional here would
have wasted some days, some calculations for nothing.

BUT...

Imagine you look AT THE DATA and a nobel prize is waiting for you just
around the corner?

:-)

WHAT IS SCIENCE?

Isn't it about seeking the unknown and figuring it out?

What could you loose?

Some wasted time, and ANYAWY you can publish always a
"lower limit" article, that adds to your scientific
CV: "Lower limit on the existence of a force pushing
us around."

(Of course with a better wording :-) )

Science is EXPLORING, catching precisely the UNKNOWN!

I hope somebody gets interested here :-)

jacob

Op woensdag 24 juli 2013 22:09:21 UTC+2 schreef Jonathan Thornburg:
From: "Jonathan Thornburg"

jacob navia wrote:
Now, what would happen if we leave gravity and suppose that at
galactic scales new forces become relevant that we can't even suspect
given our minuscule size.

I have two (somewhat-related) comments.

First, Occam's razor suggests not hypothesyzing a completely new force
until we find phenomena which we can't explain with already-known
physics.
You should only modify physical laws if the modifications results
in improved accuracy under the constraint "simplicity wins"

Dark matter doesn't require any new physics, it just requires that
there be some matter which early-21st-century humans can't easily
observe.
Dark matter requires that we make a clear distinction between
dark baryonic and dark non-baryonic matter.
It is also requires that we have an open mind that this dark baryonic
matter can be both in the bulge, in the disc and in a halo. Secondly
that the disc is much larger as observed using visible baryonic matter

(We use the adjective "dark", but all we really mean is
that it doesn't interact electromagnetically.)
What if, if this dark baryonic matter contains iron?

We already know of
other sorts of matter which behave that way and are really hard to
observe (e.g., neutrinos), so hypothesizing some other
a-bit-harder-to-observe matter doesn't seem too implausible.
As I said above simplicity wins.

There are also the Bullet cluster results, which seem to be strongly
supportive of the dark matter hypothesis.
In relation to the issue discussed read is this document:
http://arxiv.org/abs/1209.0384
"The Bullet Cluster revisited: New results from new constraints
and improved strong lensing modeling technique"
The whole document is challenging and a lust for the mind.
Only twice the word baryonic is used.
Dark matter very often. The issue is the meaning.
In paragraph 4.3 we read:
"Gas in galaxy clusters represent 10-15% of the total mass, which can
be fairly easily measured with X-rays "
The issue how much gas is there in each galaxy (on average)
and its distribution. A little further:
" Thus, including the gas mass as a separate component of the mass
model is essential for realistic modeling the total mass distribution
of the bullet cluster. "
I agree. In paragraph 4.1 at the end is written:
"(the galaxy mass component is including both stellar mass and
dark matter mass)"
What I really want is a distinction between:
stellar mass, planet sized mass, gas, and non-baryonic matter.

Second, suppose we go ahead and hypothesize a new force (NF) responsible
for flat galactic rotation curves.
MOND is such a new force.
Only go into such a direction if all the other options are excluded.

Nicolaas Vroom

Le 25/07/13 07:46, Jos Bergervoet a écrit :
OK, new matter seems more plausible than new
force (because of the observations) but I
still think it would be equally, ehh.. new!

?????

This mysterious "new matter" is not detectable after decades.

It is that after so much search no matter appears anywhere.


It could be that a new cohesion force maintains the body
of the galaxy spinning uniformly. Galaxy bodies are quite
big structures mind you. They could be ruled by galactic
scale forces, that are way too slow and feeble for us at
our scales to be observed UNLESS... you look for them.

They could "sense" forces that we can't ever feel in our
minuscule size and time scale...

But GRAIL has so much good position data of an almost
perfect inertial system...

The trajectory is an integration of the resultant of ALL
the forces acting. As any trajectory of course. This one
has the advantage of eliminating friction, vibrations, etc
that are inherent to an earth laboratory.

Is that at the precision of 1e-10?

Probably the exact distance to the spacecraft is measured
VERY precisely and there are a lot of seconds between
leaving earth and crashing in the moon... A lot of meters too,
high precision clocks...

That gives a hell of an inertial system that Newton would
have dreamed upon.

In article , Jos Bergervoet
writes:

Dark matter doesn't require any new physics,

If it's non-baryonic and not consisting of
neutrino's (and aren't we pretty sure of both
these things?) then it must be new physics, I
would say..

It depends on what one means by "new physics". New species of animals
and plants are still being discovered. When that happens, one doesn't
hear anyone say "that means new biology". In this sense, dark matter is
not new physics, just a new type of matter.

I think it is rather absurd that we should be surprised that there is
matter which is dark, i.e. which we can't detect via electromagnetic
radiation. Why should everything in the universe be directly detectable
by us?

On 7/28/2013 9:17 AM, jacob navia wrote:
Le 25/07/13 07:46, Jos Bergervoet a écrit :
OK, new matter seems more plausible than new
force (because of the observations) but I
still think it would be equally, ehh.. new!

?????

"New" here is meant as in "not currently in the
standard model of particle physics".

This mysterious "new matter" is not detectable after decades.

No mysterious "new force" has been detected
after exactly the same decades.

It is that after so much search no matter appears anywhere.

Nor any new force.. But the proof of Fermat's
last theorem required 358 years of search. And
exoplanets were only confirmed in 1988, whereas
Giordano Bruno was burned at the stake already in
1600 for believing in them. You should have more
patience, perhaps.

It could be that a new cohesion force maintains the body
of the galaxy spinning uniformly. Galaxy bodies are quite
big structures mind you. They could be ruled by galactic
scale forces, that are way too slow and feeble for us at
our scales to be observed UNLESS... you look for them.

Your claim seems to be that nobody looked for new
forces (as opposed to new matter) but I think the
subject has had quite some attention (e.g. related
to the search for extra space-time dimensions).

--
Jos

In article , Nicolaas Vroom
writes:

(We use the adjective "dark", but all we really mean is
that it doesn't interact electromagnetically.)

What if, if this dark baryonic matter contains iron?

Two points. First "electromagnetically" here means any sort of
electromagnetic interaction, in particular emitting or absorbing light
or other electromagnetic radiation. Second, the bulk of the dark matter
in the galaxy cannot be iron.

In article , Jos Bergervoet
writes:

On 7/28/2013 9:17 AM, jacob navia wrote:
Le 25/07/13 07:46, Jos Bergervoet a écrit :
OK, new matter seems more plausible than new
force (because of the observations) but I
still think it would be equally, ehh.. new!

?????

"New" here is meant as in "not currently in the
standard model of particle physics".

Discovering some new force is a bigger change than discovering some new
kind of particle. Note that the standard model of particle physics
doesn't PREDICT all of the (non-composite) particles; it ASSUMES them.
Whether one calls the discovery of a new particle "new physics" or not
is a matter of definition. The point is that dark matter, implying new
particles, is less of a change than a new force.

Le 28/07/13 11:56, Jos Bergervoet a écrit :
On 7/28/2013 9:17 AM, jacob navia wrote:
Le 25/07/13 07:46, Jos Bergervoet a écrit :
OK, new matter seems more plausible than new
force (because of the observations) but I
still think it would be equally, ehh.. new!

?????

"New" here is meant as in "not currently in the
standard model of particle physics".

This mysterious "new matter" is not detectable after decades.

No mysterious "new force" has been detected
after exactly the same decades.


The spaceship galileo,when coming back from Venus used the
earth gravitation to increase its speed in direction of jupiter.

There was an unexplained acceleration, very small.

I think that looking a GRAIL and Gravity probe A and B
we should be able to detect this acceleration if it exists.

I do not mention Pioneer since it is controversial.