Starship Asterisk*

by **Ron-Astro Pharmacist** » Fri Jan 18, 2013 5:13 pm

It seems interesting to me that the spiral, coiling shape of the DNA molecule also generally resembles the turning, twisting movement our planet and moon form moving though space and time. Its helical structure seems to represent the "Movement that Defines Us". Just a coincidence??

Ron

by **scr33d** » Thu Aug 23, 2012 4:46 am

Chris Peterson wrote:
scr33d wrote:See chapter 27 of Gravitation, or a short exercise here:

http://www.astro.ufl.edu/~guzman/ast793 ... ect01.html

Cosmological curvature almost by definition can't be measured locally--it is dependent on the energy density of the universe and the cosmological constant. In principal you could do it with lasers and mirrors locally (imagine measuring the curvature of the earth with subatomic scale rulers), but not in practice.
The canceled NASA probe might have been a local spacetime geometry, supernova/distance, or CMB mission?
The problem is an engineering one, not a purely scientific one, and I think you are underestimating the engineering capabilities available. What in your references contradicts the assertion that cosmological curvature can't be measured if you can compensate for the fixed bias of local geometry? As I recall, the mission was vetted as scientifically sound, which suggests that there was at least reasonable confidence as to the methodology.

I'm motivated now to track down the project details. It wasn't that long ago... I think in the last 10 or 15 years at most.

The Riemann curvature tensor gives, for a homogeneous closed universe and a triangle base of 10^7 Km, an angle deviation of (10^-53)º from 180º. You'd need a millions-lightyear size triangle to have any chance (and you need to be far away from galactic masses too), so triangle-sum measurement of universe geometry belongs to thought-experiment/concept-explanation.

However, see this article by Krauss and Schramm (1993) to measure Ω:

http://articles.adsabs.harvard.edu//ful ... 3.000.html

Now, to implement the scheme of KS93, in Gravitation p.795 MTW proposed to use space-based long-baseline radio telescopes to better resolve the angular size of the objects at fiducial length. (This is in fact what WMAP does: deducing curvature from the power spectrum curve of angular size distribution of the CMB fluctuation.) So NASA, pre-WMAP, COBE, may have a mission with orbiting radio telescopes to do what WMAP did?

by **Chris Peterson** » Wed Aug 22, 2012 10:35 pm

scr33d wrote:See chapter 27 of Gravitation, or a short exercise here:

http://www.astro.ufl.edu/~guzman/ast793 ... ect01.html

Cosmological curvature almost by definition can't be measured locally--it is dependent on the energy density of the universe and the cosmological constant. In principal you could do it with lasers and mirrors locally (imagine measuring the curvature of the earth with subatomic scale rulers), but not in practice.
The canceled NASA probe might have been a local spacetime geometry, supernova/distance, or CMB mission?

The problem is an engineering one, not a purely scientific one, and I think you are underestimating the engineering capabilities available. What in your references contradicts the assertion that cosmological curvature can't be measured if you can compensate for the fixed bias of local geometry? As I recall, the mission was vetted as scientifically sound, which suggests that there was at least reasonable confidence as to the methodology.

I'm motivated now to track down the project details. It wasn't that long ago... I think in the last 10 or 15 years at most.

by **scr33d** » Wed Aug 22, 2012 10:21 pm

See chapter 27 of Gravitation, or a short exercise here:

http://www.astro.ufl.edu/~guzman/ast793 ... ect01.html

Cosmological curvature almost by definition can't be measured locally--it is dependent on the energy density of the universe and the cosmological constant. In principal you could do it with lasers and mirrors locally (imagine measuring the curvature of the earth with subatomic scale rulers), but not in practice.
The canceled NASA probe might have been a local spacetime geometry, supernova/distance, or CMB mission?

by **scr33d** » Wed Aug 22, 2012 8:14 pm

Chris Peterson wrote:
scr33d wrote:If you are thinking of LISA, that is a mission for gravity wave detection (unless there is a component of the mission I am not aware of).
No, not LISA, or any of the space-based gravity wave missions. The mission (I don't recall the name off-hand, or how far it got into the proposal process) was specifically for looking at the geometry of spacetime.

Cosmological flatness/curvature is too small a signal to measure directly given that you are around massive objects: I mean WMAP has to use CMB variation distribution to deduces omega--and even then it still (as you pointed out) can't rule out other geometries.
I don't think this is true. The local spacetime geometry can be compensated for, because it represents a fixed bias, not noise. For instance, simply by observing the changes with orientation it is possible to separate local and global components of the signal. The real problem is establishing a long enough baseline, not dealing with local effects.

Agh, Chris. Let me dust off and check with my copy of MTW's Gravitation!

by **Chris Peterson** » Wed Aug 22, 2012 6:21 pm

scr33d wrote:If you are thinking of LISA, that is a mission for gravity wave detection (unless there is a component of the mission I am not aware of).

No, not LISA, or any of the space-based gravity wave missions. The mission (I don't recall the name off-hand, or how far it got into the proposal process) was specifically for looking at the geometry of spacetime.

Cosmological flatness/curvature is too small a signal to measure directly given that you are around massive objects: I mean WMAP has to use CMB variation distribution to deduces omega--and even then it still (as you pointed out) can't rule out other geometries.

I don't think this is true. The local spacetime geometry can be compensated for, because it represents a fixed bias, not noise. For instance, simply by observing the changes with orientation it is possible to separate local and global components of the signal. The real problem is establishing a long enough baseline, not dealing with local effects.

by **scr33d** » Wed Aug 22, 2012 6:05 pm

Chris Peterson wrote:
scr33d wrote:Um, my tensor calculus is nowaday a little rusty but I think cosmological space curvature effects can't be measure around the solar neighborhood, you'd be swamp by local gravitational noise from the sun and planets.
The local gravitational field is a source of signal that has to be compensated for, not of noise (which is another word for uncertainty). The mission I referred to is real, although currently suspended for funding reasons.

WMAP measurement has good evidence that Ω = 1, i.e. we are in a flat universe.
It only suggests Ω = 1 with a 0.5% margin of error, which isn't good enough to rule out other geometries. It's a tricky problem, because if the Universe is flat, we may never know that for sure. That's because no matter how good your measurement, all you can tell is that you are very close to zero flatness- there will always be both a positive and negative sign within your error bars.

If you are thinking of LISA, that is a mission for gravity wave detection (unless there is a component of the mission I am not aware of).
Cosmological flatness/curvature is too small a signal to measure directly given that you are around massive objects: I mean WMAP has to use CMB variation distribution to deduces omega--and even then it still (as you pointed out) can't rule out other geometries.
Best evidence for flat universe? Flatness problem---> Ω = 1 to 10^30

by **Starswarm Magellan** » Wed Aug 22, 2012 4:20 pm

Frankly, I'm lately seeing far more evidence of Chaos Theory than of Natural Selection as these DNA processes relate to the human species. I can't tell if some are really smart and putting me on or idiots who mean it

by **Chris Peterson** » Wed Aug 22, 2012 3:46 pm

Ann wrote:For myself, I'm not at all assured that the shape of the universe can be compared in any way with the shape of the Earth. The Earth is spherical. If you go far enough in any direction on the Earth, you will eventually return to where you started. But for the same principle to be true about the universe, the universe has to be a "four-dimensional sphere". As far as I can understand, however, astronomers believe that the universe is "flat", so that it does not curve back on itself.

A flat universe can still curve back on itself. Topology is often rather nonintuitive. While a sphere is not topologically flat, a torus is. Both are compact and finite. Using a sphere to describe the Universe is still useful, because the concept is the same as for flat, bounded manifolds as well... and a lot easier to visualize.

by **Chris Peterson** » Wed Aug 22, 2012 3:41 pm

scr33d wrote:Um, my tensor calculus is nowaday a little rusty but I think cosmological space curvature effects can't be measure around the solar neighborhood, you'd be swamp by local gravitational noise from the sun and planets.

The local gravitational field is a source of signal that has to be compensated for, not of noise (which is another word for uncertainty). The mission I referred to is real, although currently suspended for funding reasons.

WMAP measurement has good evidence that Ω = 1, i.e. we are in a flat universe.

It only suggests Ω = 1 with a 0.5% margin of error, which isn't good enough to rule out other geometries. It's a tricky problem, because if the Universe is flat, we may never know that for sure. That's because no matter how good your measurement, all you can tell is that you are very close to zero flatness- there will always be both a positive and negative sign within your error bars.

by **flash** » Wed Aug 22, 2012 3:28 pm

Ann wrote:
flash wrote:
GBDB wrote:REST ASSURED: The universe is infinite.
Thanks for the assurance. But you can probably tell that I'm not all that assured.

Finite but unbounded is the generally accepted belief. Just as the 2-dimensional surface of a sphere is finite in area (but has no end or boundary), the 3-dimensional volume of the universe is finite (but it too has no end or boundary). I find this analogy most satisfying. Just as flatlanders living on a two-dimensional surface of a sphere, if you go far enough in any direction you will eventually return to where you started.

Having no boundary is not the same thing as infinite.
For myself, I'm not at all assured that the shape of the universe can be compared in any way with the shape of the Earth. The Earth is spherical. If you go far enough in any direction on the Earth, you will eventually return to where you started. But for the same principle to be true about the universe, the universe has to be a "four-dimensional sphere". As far as I can understand, however, astronomers believe that the universe is "flat", so that it does not curve back on itself.

That said, I agree with you that the universe may not be infinite.

Ann

It seems to me that If it is not infinite, then either it is has a boundary, or it curves back on itself. (Is there any other alternative?) I can't understand the boundary at all. It's similar to the flat earth concept that if you get too close to the edge you'll fall off. (Or, "What's beyond the edge?") Therefore it must curve back on itself.

Another analogy that sways me to believe in the "four dimensional sphere" model is the commonly used explanation of how the universe is expanding, with the distance between galaxies (at least currently) increasing over time. The analogy is between the galaxies in 3-dimensional space and points on a 2-dimensional surface of an inflating 3-dimensional balloon. The points are all increasing (or decreasing) in distance from each other with inflation or deflation, in the same manner as galaxies.

I know this isn't proof, but I think it is convincing.

by **Beyond** » Wed Aug 22, 2012 4:08 am

: emo30.gif (11.09 KiB) Viewed 1816 times

by **bystander** » Wed Aug 22, 2012 3:14 am

BMAONE23 wrote:It would seem to me that Finite but unbounded would be untestable at least to the limits of current technology. Using the 2d model in a 3d reference frame would essentially say that we could in fact travel in any 1 of 3 spacial directions and eventually wind up back here. How could this hypothesis ever be tested?

I guess we could wait for V'Ger to return.

by **Ann** » Wed Aug 22, 2012 2:53 am

A part of the Hubble Deep Field.

Thanks to our DNA, we are the part of the universe that is looking back on itself - filtered through that incredibly tiny part of it that is us, trying to take in the whole of it.

Ann

by **scr33d** » Wed Aug 22, 2012 2:04 am

Chris Peterson wrote:
BMAONE23 wrote:It would seem to me that Finite but unbounded would be untestable at least to the limits of current technology. Using the 2d model in a 3d reference frame would essentially say that we could in fact travel in any 1 of 3 spacial directions and eventually wind up back here. How could this hypothesis ever be tested?
It can be tested by studying the geometry of space. It's technologically tricky, but perfectly feasible. If space is flat (for instance, the Universe is a hypersphere of infinite radius) then the internal angles of a triangle add up to 180°. If space is concave or convex, the sum of the angles will be slightly more or less than 180°. This is just like you can do on the surface of Earth. If you draw a triangle on the ground, its angles add up to more than 180°, demonstrating that the Earth isn't flat, but is convex. Of course, you need to draw a really big triangle for the difference to be detectible. To test the shape of the Universe, you send out three probes that connect to each other with lasers, and measure the angles between them. They might be millions of kilometers apart- a nice big triangle.

Um, my tensor calculus is nowaday a little rusty but I think cosmological space curvature effects can't be measure around the solar neighborhood, you'd be swamp by local gravitational noise from the sun and planets.
WMAP measurement has good evidence that Ω = 1, i.e. we are in a flat universe.

by **Chris Peterson** » Wed Aug 22, 2012 1:26 am

BMAONE23 wrote:It would seem to me that Finite but unbounded would be untestable at least to the limits of current technology. Using the 2d model in a 3d reference frame would essentially say that we could in fact travel in any 1 of 3 spacial directions and eventually wind up back here. How could this hypothesis ever be tested?

It can be tested by studying the geometry of space. It's technologically tricky, but perfectly feasible. If space is flat (for instance, the Universe is a hypersphere of infinite radius) then the internal angles of a triangle add up to 180°. If space is concave or convex, the sum of the angles will be slightly more or less than 180°. This is just like you can do on the surface of Earth. If you draw a triangle on the ground, its angles add up to more than 180°, demonstrating that the Earth isn't flat, but is convex. Of course, you need to draw a really big triangle for the difference to be detectible. To test the shape of the Universe, you send out three probes that connect to each other with lasers, and measure the angles between them. They might be millions of kilometers apart- a nice big triangle.

by **Ann** » Wed Aug 22, 2012 12:37 am

flash wrote:
GBDB wrote:REST ASSURED: The universe is infinite.
Thanks for the assurance. But you can probably tell that I'm not all that assured.

Finite but unbounded is the generally accepted belief. Just as the 2-dimensional surface of a sphere is finite in area (but has no end or boundary), the 3-dimensional volume of the universe is finite (but it too has no end or boundary). I find this analogy most satisfying. Just as flatlanders living on a two-dimensional surface of a sphere, if you go far enough in any direction you will eventually return to where you started.

Having no boundary is not the same thing as infinite.

For myself, I'm not at all assured that the shape of the universe can be compared in any way with the shape of the Earth. The Earth is spherical. If you go far enough in any direction on the Earth, you will eventually return to where you started. But for the same principle to be true about the universe, the universe has to be a "four-dimensional sphere". As far as I can understand, however, astronomers believe that the universe is "flat", so that it does not curve back on itself.

That said, I agree with you that the universe may not be infinite.

Ann

by **scr33d** » Wed Aug 22, 2012 12:31 am

No more opinion on ID, please! Can someone elucidate on the protein-folding, the entropy of molecular machines and the helicase motor mechanism instead?

by **kreigor426** » Wed Aug 22, 2012 12:20 am

.
i can bearly imagine, 6 meters of data/info in every living cell...
.
it's also hard to comprehend a million milleniums...
.
each time i learn some new fact re: my body, earth, or the universe,,,
i begin to feel very small, and quite unimportant...
.
PLEASE, APOD, continue this great resource for all of us knowledge starved readers...
.
and as always,,, thanks to everyone...

by **BMAONE23** » Tue Aug 21, 2012 11:45 pm

It would seem to me that Finite but unbounded would be untestable at least to the limits of current technology. Using the 2d model in a 3d reference frame would essentially say that we could in fact travel in any 1 of 3 spacial directions and eventually wind up back here. How could this hypothesis ever be tested?

by **flash** » Tue Aug 21, 2012 11:17 pm

GBDB wrote:REST ASSURED: The universe is infinite.

Thanks for the assurance. But you can probably tell that I'm not all that assured.

Finite but unbounded is the generally accepted belief. Just as the 2-dimensional surface of a sphere is finite in area (but has no end or boundary), the 3-dimensional volume of the universe is finite (but it too has no end or boundary). I find this analogy most satisfying. Just as flatlanders living on a two-dimensional surface of a sphere, if you go far enough in any direction you will eventually return to where you started.

Having no boundary is not the same thing as infinite.

What has convinced you otherwise?

by **Chris Peterson** » Tue Aug 21, 2012 11:08 pm

GBDB wrote:REST ASSURED: The universe is infinite.

That is far from certain. And without doubt, the accessible universe is finite.

by **GBDB** » Tue Aug 21, 2012 10:39 pm

Ann wrote:
Guest76 wrote:

The only way it could be inevitable is if the universe is infinite, and then you could say, 'life had to happen somewhere, and we're it'. But the universe does not seem to be infinite.
How do you know that the universe isn't infinite?

Ann

REST ASSURED: The universe is infinite.

by **ems57fcva** » Tue Aug 21, 2012 10:35 pm

zorts wrote:This is, indeed, a cool video clip - but I like it better when APOD sticks to straightahead astronomy.

I will second that.

by **flash** » Tue Aug 21, 2012 10:29 pm

neufer wrote:The skeptic poses a complex question of how, even given billions of years, a single cell could develop into a complicated human body that thinks and feels.

In my view the leap from no cells to one cell is greater than the leap from one cell to a human. But that doesn't prevent me from believing that that's how it all evolved.

just for fun:
http://doonesbury.slate.com/strip/archive/2005/12/18
http://doonesbury.slate.com/strip/archive/2011/07/10

Starship Asterisk*

APOD: DNA: The Molecule that Defines You (2012 Aug 21)

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