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Re: [Phys-L] gravitational waves

Regarding the use of the trampoline model: I’ll be honest the model we use depends on the audience we are addressing and the depth of understanding we want to impart. I think it’s a bad idea to assume that one model will work for all audiences. When I explain via analogy to someone in 5-10 minutes versus a class, and when I explain to a 6 year old versus a physics graduate I use different means of explanation. I believe Einstein’s Messengers ( einsteinsmessengers.org ) used a 3-d grid explanation within their graphics - and that is a better graphic (IMHO) for a graphical interpretation.

David Bowman said: But my biggest quibble with the analogy/model is that it makes it look like *Newtonian* gravitational forces are something that fall out of the *spatial* curvature caused distortions of space (when presented as a distorted map in flat space).
Agreed - I think that points to a fundamental weakness of the model that people can quickly grasp. I point that out when we use that model, I also mention that it’s 4-d space-time. I don’t use this model if the person has heard of a Lagrangian.

You can say that we shouldn’t even attempt to explain gravity or a gravitational wave, to someone in 5 minutes unless they already have a background. I would disagree - I think the idea of saying: sorry I’m not going to try to give you a sense of what we’ve found unless you can participate in a class of activities with me, is a poor response to public interest. However we may disagree - this disagreement makes some sense since we reach out to vastly different audiences. Our press conference has both been criticized and lauded for its approach (full disclosure: I had little to do with the press conference, but thought that it was a good approach).

-William


William Katzman Program Leader
wkatzman@ligo-la.caltech.edu LIGO Science Education Center
225-686-3134 "Inspiring Science"





Thank you for replying, William.

1) As you certainly know, most elementary physics textbooks do not
introduce special relativity before introducing gravity.

2) Gravity is explained in terms of Newton's laws

3) That seems to be a pedagogical barrier, for most high school and
college
teachers. What do you think about this?

I think you are right.


4) What do other EPO people think about the need to overcome the barrier?

Ludwik Kowalski (See Wikipedia )

==========================================================
==

On Apr 18, 2016, at 12:03 AM, William Katzman wrote:

A few clarifications..

1) Below is the link to FAQ about gravitational waves.

http://www.ligo.org/science/faq.php#what-are-gw
This is one of several FAQs. This FAQ is compiled by LIGO Scientific
Collaboration (LSC) Education and Public Outreach (EPO) members on an ad-
hoc basis. This collaboration involves over 1000 people, but only a
handful of
them actively work on EPO.
Another FAQ is: https://www.ligo.caltech.edu/LA/page/faq
And a primer of sorts is at: https://www.ligo.caltech.edu/LA/page/learn-
more as well as: http://www.tapir.caltech.edu/~teviet/Waves/gwave.html
and http://ligo.org/students_teachers_public/read.php .


2) The answers were given by "LIGO Science Education Center," whose
director is William Katzman.

I had almost nothing to do with the FAQ at ligo.org, as most of it
predated
me. I work at the Livingston Observatory (which is only one small portion
of
the LSC) , managing their Education and Public Outreach. I am also in the
handful of LSC-EPO folks. At Livingston we keep the explanation of
gravitational waves generally non-quantitative since it doesn't seem to
add
to understanding. Gravitational waves are transverse, although I find
this to
be a less critical distinction than many of my colleagues. We do use the
trampoline model of the universe to explain gravitational waves. It is
flawed
- as the mathematics don't work well, but it does provide several apt
analogies - including the analogy that it actually can stretch - like
space.
Saying that gravitational waves are ripples or waves on that trampoline
like
model seems to get across the basic idea to a wide range of people, but it
doesn't get into the hairy details of it being a quadrupole wave. I think
it's
smart to introduce gravitational waves in a general manner, because
they're
in the news and therefore can serve to inspire a subset of people to study
science more, ut for introductory students I wouldn't approach it
quantitatively since that requires an understanding of GR.


3) He posted a Phys-L message (on 4/13/2013). Is he a PHYS-L su
bscriber?
I assume so.

Yes, I (William Katzman) am a subscriber in digest format.

Cheers,

-William


_______________________________________________
Forum for Physics Educators
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_______________________________________________
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Phys-l@www.phys-l.org
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From: John Denker <jsd@av8n.com>
Subject: Re: [Phys-L] gravitational waves
Date: April 18, 2016 at 1:17:03 PM CDT
To: Phys-L@Phys-L.org


All analogies are imperfect. That's why we call them analogies
rather than copies or instances.

It helps to know in what ways the analogy is faithful to reality,
and in what ways not. Consider the blank space in the following
chart:

* ** *** ****
x xx xxx xxxx
+ ++ ++++
o oo ooo oooo
v vv vvv vvvv

Here the rows are faithful as to shape, while the columns are
faithful as to number. In this way we can communicate the idea
of "three plus signs" even if it is expressed directly.

This analogy is itself imperfect, insofar as "three plus signs"
could have been expressed directly. However, you can extrapolate
to physics, where a great many ideas cannot be so easily expressed,
and instead must be built up via a series of approximations and hints.

Still the point is that it is important to know in what ways
each analogy is faithful to reality, or not.

On 04/17/2016 09:03 PM, William Katzman wrote:

We do use the trampoline model of the universe to explain
gravitational waves. It is flawed - as the mathematics don’t work
well, but it does provide several apt analogies - including the
analogy that it actually can stretch - like space.

I'm not convinced.

It seems to me that the trampoline model qualitatively represents
some sort of field that mediates an interaction between the
particles. This field is a real physical thing with its own
dynamics. So far so good.

However, things go downhill rapidly after that.

*) Consider a region that is locally cylindrical, like
the bottom of a long shallow trough. There is "curvature"
in the high-school sense, i.e. extrinsic curvature, but no
intrinsic curvature, no Gaussian curvature. The paths of
free particles are not straight and do not even behave the
same as one another. However, the geodesics remain straight,
i.e. there is no geodesic deviation.

Therefore it seems that the trapoline model has conceptual
problems (not just math that "doesn't work well"). It seems
there are multiple problems at the most fundamental, qualitative,
conceptual level:

*) In the trampoline model, free particle trajectories don't
follow geodesics. In the real world, they do.
*) The aforementioned "stretch" is not an "apt" analogy.
-- In the trampoline model, the crucial idea is the deflection
in the /off-universe/ direction, i.e. the deflection into
the embedding dimension. The aforementioned "stretch" is a
second-order effect. It can be made arbitrarily small by
suitable engineering, and the model still works.
-- In contrast, in the real world, there is no need for an
embedding dimension. Everything happens /within/ the universe.
The stretch is pretty much the whole story.
*) The model doesn't represent the polarization correctly.
*) It is a model of space, and it's not the least bit obvious
how to generalize it to spacetime.
-- In particular, AFAICT the model predicts zero gravitational
redshift.
-- In contrast, general relativity predicts a redshift.
Forsooth, special relativity gets this more-or-less right
(hint: traveling twins).
*) The model can represent repulsion just as easily as
attraction, which is yet another sign that whatever is
being exhibited is not really gravitation.
*) The model depends on an "ether" that carries the disturbances.
Special relativity takes a dim view of such things.

Overall, I'm not convinced this model is particularly apt.
Better models are available. If I wanted to learn (or teach)
about gravitation, I wouldn't start with this.




From: David Bowman <David_Bowman@georgetowncollege.edu>
Subject: Re: [Phys-L] gravitational waves
Date: April 18, 2016 at 2:50:17 PM CDT
To: "Phys-L@Phys-L.org" <Phys-L@Phys-L.org>


Regarding JD's quarrel with the trampoline model/analogy for GR.

I'm not convinced.

It seems to me that the trampoline model qualitatively
represents some sort of field that mediates an interaction
between the particles. This field is a real physical thing with
its own dynamics. So far so good.

However, things go downhill rapidly after that.

I think I'm mostly on JD's side in quarrel. Over and above the problems JD already mentioned in his post there are other serious ones as well. For instance, often diagrams of the trampoline model show gravitating bodies sitting on top of the trampoline and having them locally distort the fabric there. Such diagrams can be very confusing to the uninitiated who don't already know what's going on. Such diagrams make it look like there is some sort of background gravitational field orthogonal to the spatial fabric pulling the gravitating bodies down into the fabric and causing the distortions. Also, putting such 3-d objects outside of the 2-d fabric into the 3rd embedding dimension is a good way to confuse people who are already uncertain as to which dimensions are which in the analogy, i.e. what is a 2-d model for space or spacetime & what is the significance of the orthogonal embedding dimension in the analogy.

But my biggest quibble with the analogy/model is that it makes it look like *Newtonian* gravitational forces are something that fall out of the *spatial* curvature caused distortions of space (when presented as a distorted map in flat space). In fact, in the Newtonian limit, the curvature of space, per se, is quite irrelevant because it is so tiny. Rather the curvature in the way the *time-like* dimension of space-time is incorporated into the manifold in a spatially locally differential manner is what is responsible for Newtonian gravitation. IOW, the Newtonian gravitational forces come from locally varying amounts of gravitational *time* dilation. But the model doesn't even describe or address the issue of the gravitational time dilation at all as it completely suppresses the time-like dimension. Admittedly the absolute amount of distortion due to curvature in the time-like dimension (i.e. gravitational time-dilation) is comparable in intrinsic magnitude to the spatial distortions due to purely spatial curvature, but those distortions appear to higher order in 1/c^2 in the slow motion Newtonian limit where gravitating massive bodies never move at speeds approaching an appreciable fraction of c. So it is only the effect of gravitational time dilation that is responsible for Newtonian gravitational effects.

BTW, a nice way to extract Newtonian gravitational effects on a freely falling object (with non-zero mass) is to realize that the Newtonian Lagrangian functional for it is simply -m*c^2 times the differential amount of local relative gravitational time dilation (from the sqrt of the coefficient of the time-like coordinate in the GR line element for the square proper time for the object) relative to time kept by a clock at spatial infinity (assuming all gravitating masses are spatially localized). The factor of c^2 in the unit conversion factor between relative dilation and the Lagrangian's energy units is what amplifies the tiny amounts of time dilation into something that has significant Newtonian forces in that limit. Since the spatial contribution to the line element is of higher order in 1/c^2 than the temporal one the spatial distortion/curvature effects tend to drop out in the slow motion regime.

Note if one wants to see how GR makes a *light ray* deflect as it passes a gravitating body then *both* the local time dilation along the path *and* the spatial distortions show up with equal contributions to the overall effect. This is because the light ray is sampling equal amounts of space and time as it moves on a null geodesic at 45 deg. But the slow speed Newtonian bodies are hardly sampling any space for the amount of time their world lines cover as they are directed almost completely parallel to the time axis, and they are far more sensitive to just the temporal distortions. If one only included the temporal effects on the light ray path one would find a predicted deflection of 1/2 of the actual amount predicted by GR and the experimentally observed results.

David Bowman



From: "LaMontagne, Bob" <RLAMONT@providence.edu>
Subject: Re: [Phys-L] gravitational waves
Date: April 18, 2016 at 3:28:04 PM CDT
To: "Phys-L@Phys-L.org" <Phys-L@Phys-L.org>


To emphasize you point about the curvature of the 'time-like' dimension being the important factor, it is the one that explains why an apple falls to the ground when it is released. Since the rigid surface of the earth cannot follow that curvature, the apple and earth meet each other.

Bob
________________________________________
From: Phys-l <phys-l-bounces@www.phys-l.org> on behalf of David Bowman <David_Bowman@georgetowncollege.edu>
Sent: Monday, April 18, 2016 3:50 PM
To: Phys-L@Phys-L.org
Subject: Re: [Phys-L] gravitational waves

Regarding JD's quarrel with the trampoline model/analogy for GR.

I'm not convinced.

It seems to me that the trampoline model qualitatively
represents some sort of field that mediates an interaction
between the particles. This field is a real physical thing with
its own dynamics. So far so good.

However, things go downhill rapidly after that.

I think I'm mostly on JD's side in quarrel. Over and above the problems JD already mentioned in his post there are other serious ones as well. For instance, often diagrams of the trampoline model show gravitating bodies sitting on top of the trampoline and having them locally distort the fabric there. Such diagrams can be very confusing to the uninitiated who don't already know what's going on. Such diagrams make it look like there is some sort of background gravitational field orthogonal to the spatial fabric pulling the gravitating bodies down into the fabric and causing the distortions. Also, putting such 3-d objects outside of the 2-d fabric into the 3rd embedding dimension is a good way to confuse people who are already uncertain as to which dimensions are which in the analogy, i.e. what is a 2-d model for space or spacetime & what is the significance of the orthogonal embedding dimension in the analogy.

But my biggest quibble with the analogy/model is that it makes it look like *Newtonian* gravitational forces are something that fall out of the *spatial* curvature caused distortions of space (when presented as a distorted map in flat space). In fact, in the Newtonian limit, the curvature of space, per se, is quite irrelevant because it is so tiny. Rather the curvature in the way the *time-like* dimension of space-time is incorporated into the manifold in a spatially locally differential manner is what is responsible for Newtonian gravitation. IOW, the Newtonian gravitational forces come from locally varying amounts of gravitational *time* dilation. But the model doesn't even describe or address the issue of the gravitational time dilation at all as it completely suppresses the time-like dimension. Admittedly the absolute amount of distortion due to curvature in the time-like dimension (i.e. gravitational time-dilation) is comparable in intrinsic magnitude to the spatial distort
ions due to purely spatial curvature, but those distortions appear to higher order in 1/c^2 in the slow motion Newtonian limit where gravitating massive bodies never move at speeds approaching an appreciable fraction of c. So it is only the effect of gravitational time dilation that is responsible for Newtonian gravitational effects.

BTW, a nice way to extract Newtonian gravitational effects on a freely falling object (with non-zero mass) is to realize that the Newtonian Lagrangian functional for it is simply -m*c^2 times the differential amount of local relative gravitational time dilation (from the sqrt of the coefficient of the time-like coordinate in the GR line element for the square proper time for the object) relative to time kept by a clock at spatial infinity (assuming all gravitating masses are spatially localized). The factor of c^2 in the unit conversion factor between relative dilation and the Lagrangian's energy units is what amplifies the tiny amounts of time dilation into something that has significant Newtonian forces in that limit. Since the spatial contribution to the line element is of higher order in 1/c^2 than the temporal one the spatial distortion/curvature effects tend to drop out in the slow motion regime.

Note if one wants to see how GR makes a *light ray* deflect as it passes a gravitating body then *both* the local time dilation along the path *and* the spatial distortions show up with equal contributions to the overall effect. This is because the light ray is sampling equal amounts of space and time as it moves on a null geodesic at 45 deg. But the slow speed Newtonian bodies are hardly sampling any space for the amount of time their world lines cover as they are directed almost completely parallel to the time axis, and they are far more sensitive to just the temporal distortions. If one only included the temporal effects on the light ray path one would find a predicted deflection of 1/2 of the actual amount predicted by GR and the experimentally observed results.

David Bowman
_______________________________________________
Forum for Physics Educators
Phys-l@www.phys-l.org
http://www.phys-l.org/mailman/listinfo/phys-l




From: "Bob Sciamanda" <treborsci@verizon.net>
Subject: Re: [Phys-L] gravitational waves
Date: April 18, 2016 at 4:44:40 PM CDT
To: <Phys-L@Phys-L.org>


My main quarrel with the trampoline model is that it assumes what it claims to explain. It assumes some a priori (Newtonian?) gravity to give the mother body the weight which will depress the trampoline and thus produce the curvature which is gravity!!!

Bob Sciamanda
Physics, Edinboro Univ of PA (Em)
treborsci@verizon.net
www.sciamanda.com

-----Original Message----- From: John Denker
Sent: Monday, April 18, 2016 2:17 PM
To: Phys-L@Phys-L.org
Subject: Re: [Phys-L] gravitational waves

All analogies are imperfect. That's why we call them analogies
rather than copies or instances. . . .




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