Astronomy hangups - Knowledge vs. comprehension

Palomar · 2003-04-14 10:29:44

Unassembled I'd save about $250 on a 10 inch scope, but then I'd have to figure out how to construct the tube, etc. Not sure if I'm really up to that. And if it took me more than a work week to figure out (ie, 40 hours thinking it over, constructing theoreitical mounts, etc), I would've been able to pay for the $250 tube and accessories.
Am I making sense?
What kind of scope do you have, Cindy?

*Yes, Josh, you're making sense. I have 4" telescope (Astroscan 2001, reflector). I'm looking at an 8" Dobsonian; very simple...mounted on a wheeled base, easy swerving maneuverability, light weight and fairly easy to transport. No fancy gadgets such as timed tracking/coordinate devices...just a practical, nice 'scope.

--Cindy

Josh Cryer · 2003-04-14 23:04:52

I saw one of those 8" ones for under $500, delivered, at the site I was looking around fo prices.

Might wanna check it out if you're interested: http://www.e-scopes.cc/

(Feel free to let me know where you've been looking, though I know we're just speculating here.)

I think automative gadgetry takes away from the joy of observing, myself. I mean, I can see how it'd be useful, just type in a number and voila, you're there. That's great for the professional who needs to make the best use of their time, or someone who just wants to look. You don't really learn about where you're looking, though, and I think there's a certain joy in knowing where constellations are, etc.

Palomar · 2003-04-15 06:01:42

Josh: "Might wanna check it out if you're interested: http://www.e-scopes.cc/

(Feel free to let me know where you've been looking, though I know we're just speculating here.)"

*Thanks for the link. I've been looking in my issues of "Astronomy" magazine.

Josh: "I think automative gadgetry takes away from the joy of observing, myself."

*I agree. It's a must for astrophotography, of course; but otherwise it's simple enough to just gently nudge the 'scope yourself when the object is moving out of the field of view and many people (like myself) like to view many things in the course of an evening or simply do some slow and steady "sky sweeping"...don't need gadgets for that.

--Cindy

Ian · 2003-04-15 11:48:56

Well, for number 1, doesnt gravity do exactly that? It's a force with no mass, yet it exerts a pull on mass. its hard to understand quite how it works-only that it does.
i have a "hangup" with the wormhole theories. if they existed, as sagan said, as black holes, how could we use them to travel through? as soon as you passed through the center, even assuming you could do this, you would be sucked right back to the center by gravity. thus, there is no way of exploiting the wormhole to shorten travel, in my understanding.

How can you tell if gravity has no mass?

dicktice · 2003-04-17 08:32:58

I hate to be tedius, but awhile back I submitted a hangup regarding light-sailing, but no reply ... so here goes, again:

If sunlight imparts thrust to a perfectly reflecting light-sail, normal to the radiation, so that total reflection back towards the Sun occurs, where does the energy to produce thrust come from...?

Shaun Barrett · 2003-04-17 20:10:44

Hi Dicktice!
The energy to produce thrust comes from the change of momentum of the photons. They have zero rest mass but they do have momentum.
If you're interested, there was a discussion of this last year. I posted a brief and simplistic layman's guide to the mathematics of how light can have no mass and yet still have momentum.
Go to Science and Technology (Page 3), Humans and Relativity (Page 2), dated Sep. 06 2002.

A post by Preston gave us the mathematical inspiration and erudite comments by Pat Galea really fleshed the whole thing out! Between us, I think we got the whole 'science of light-sailing' thing pretty well nailed down ... a great team effort!

Josh Cryer · 2003-04-17 21:00:23

Hmm, I wanted to read it out of curiosity, so I decided to come back and share a link to make things easier.

I recall that thread, good stuff.

Shaun Barrett · 2003-04-18 06:03:09

O.K. While Dicktice is digesting our ruminations about the momentum of photons - and thank you muchly, Josh, for the helpful link, which is much more convenient than my clumsy 'address' - I have a problem of my own. (No, Phobos, I'm not talking about my metal helmet problem!! )

Many of you will have been following the construction of a new cosmological observatory - LIGO. The acronym stands for Laser Interferometer Gravitational-Wave Observatory. For a brief description of it, click here.

The point is, gravitational waves from far away in the universe are expected to distort mirrors and cause measurable changes in the interference pattern of a split beam of coherent light.
Now, as I understand it (and I use the word 'understand' rather loosely! ), gravity is a distortion of space-time and space-time is the fabric of the universe in which we live. A gravity wave is like a ripple in that fabric, such that space-time compresses then relaxes as the wave passes. Am I right so far?

What I don't understand is this: If the very fabric of the universe in which we live and perform measurements is distorting, should it not be impossible for us to measure such a distortion? As a section of space-time in which the laser beam is situated compresses due to the gravity wave, the beam itself will compress also and the number of wavelengths of the light between the mirrors will remain the same. Indeed, shouldn't any and every tool we use to measure the distortion fail to detect it because it, too, distorts in precisely the same manner and proportion as that which it tries to measure? ???

I know there's a major flaw in my logic somewhere because internationally renowned scientists are not spending billions of dollars on an observatory that can't possibly work!!!
But I just can't seem to get a handle on how it's supposed to work, when the ruler we're using must alter in its dimensions as much as the space-time we're measuring!

Can anyone point out where my mental picture of all this is incorrect? (Knowing me, I'm looking for too much complexity in a simple physics problem.)

Shaun Barrett · 2003-04-18 20:31:28

My question about the LIGO thing wasn't actually directed specifically towards Josh. It was really an open question, so if anyone can help, please feel free to jump in! (I usually need all the help I can get! )
How gravitons fit into this problem is a mystery to me, I'm afraid. I have enough trouble with the wave/particle duality of light ... never mind the proposed similar duality of gravity!! (So no I won't bother to thank you for your suggestion, since it's only added to my confusion! )

In response to your question, Dicktice, I don't think it's possible to get total reflectance of light from a light-sail. Imperfections in the surface must result in some absorption.
The photons which reflect impart twice the momentum to the sail, I would think, as the photons which are merely absorbed. This is because, with reflectance, you have a complete reversal of the sign for the momentum from +ve to -ve. With absorption, the momentum of the photon is merely reduced to zero.
The light reflected from one light-sail will still impart momentum to a second light-sail in accordance with the equation for momentum.

Shaun Barrett · 2003-04-20 02:11:03

I'm not quite sure I follow this question: "-- would you get thrust increase per sail...?"
You should theoretically get thrust at each sail, if I understand your scenario correctly. The amount would presumably decrease from sail to sail as energy losses due to incomplete reflection took their toll. If you've ever stood between two not-quite-parallel mirrors and seen the multiple reflections of yourself receding into the distance, you may have noticed that each successive reflection becomes dimmer and less distinct. Same principle.
Even in a theoretically near-perfect reflectance at each mirrored surface, the reflected light may have its wavelength altered by the encounter. Longer wavelength photons carry less momentum - so perhaps there will be energy losses in this way too.
The angle at which light strikes the sails will clearly affect the amount of momentum imparted also. A vector analysis of the forces involved would give you the appropriate figures.
Whichever way you go to work, you won't get any net energy gain, obviously (see the work of one Isaac Newton if you don't believe me! ), and your scenario, in a closed system, must conform to the law of increasing entropy. If I've said anything which seems to contradict that, I apologise!

Shaun Barrett · 2003-04-23 18:57:37

Hi Dicktice!
In an idealised thought-experiment, I believe the answer is yes (if I understand correctly).

dicktice · 2003-04-24 16:10:47

Shaun: What patience! My previous three posts were thrown out in hopes of getting a photon discussion going, but the examples I used were foolish .. so I've deleted 'em, and your replies are still just a'dangling there. Sorry about that.
I shall now try to do better, i.e.: Regarding total reflection (no absorption) of monochromatic laser light directed at, and normal to, the perfectly flat surface of a rigid light-sail of some mass.... If the reflected light is unchanged except for direction reversal, where does any thrust imparted to the idealized light-sail come from...?

Shaun Barrett · 2003-04-24 21:01:03

Dicktice, have you found time yet to browse through the discussion about photon momentum which Josh provided the link to? If not, I feel it would be a big help to us in this exchange to do so, in order to avoid needless repetition.

It may help to imagine your single wavelength photons as perfectly elastic rubber balls, all of precisely the same mass, striking a perfectly rigid, flat surface normally (as you specify).
Each rubber ball has momentum - a certain 'energy of movement' in a certain direction - which we can designate positive momentum. When it strikes the surface in question, it is momentarily halted and then rebounds. It is perfectly elastic and the surface perfectly rigid, so it rebounds at exactly the same speed but in the opposite direction. The surface has not only absorbed all the positive momentum of the rubber ball, but has endowed it with an equal amount of negative momentum (momentum at 180 degrees to the original momentum).
The surface is consequently given positive momentum equal to twice the original positive momentum of the rubber ball.

The situation with photons is exactly the same. As discussed previously, a photon has momentum too. With a solid object the momentum can be calculated using this simple equation:-
p = mv (.... where p is momentum
m is mass
v is velocity.)
This equation holds for all but relativistic velocities.

For photons, it has been shown that momentum can be calculated using this equation:-
p = h/L (.... where p is momentum
h is Planck's constant
L is wavelength.)

Thus, the idealised light-sail is propelled forward courtesy of the imparted momentum.
The wavelength of visible light is very small, but Planck's constant is excruciatingly tiny!! So, the momentum of each photon is also extremely small.
But at Earth's distance from the Sun, the number of photons per second striking a light-sail is vast and therefore we get a measurable thrust which we can utilise.

You may be interested to know that The Planetary Society, of which I'm proud to say I've been a member since 1989, will stage a demonstration of light-sail propulsion later this year.
This first ever solar sail mission is called 'Cosmos 1' and you can read more at this site.

dicktice · 2003-04-25 19:55:39

Shaun: Whilst I should be chagrined perhaps for not "doing my homework," my purpose was to ask childishly ... something I'm in the habit of doing ... having been cursed with a poor math algorithm retention together with a terrific ambition to "know all." Especially regarding orbital mechanics of tethered, light-sailing and most recently, plasma-cloud sailing--or any other alternatives to fueled-rocket-propulsion for interplanetary space transportation. (Thinks: Steam-jet-propelled water-ice shielded space vehicles excepted.) Anyway, I'll peruse the sources you indicated with interest, and then come back at'cha.
But, a'fore I go ... I can't resist reminding you of the billiard ball which collides with an identical, stationary ball, and stops dead, having imparted its total momentum to the other ball, which (I assume) moves off with the first ball's velocity at the instant of impact. These balls while hard surfaced, can be thought of as being perfectly elastic, as well. Rubber in your analogy may be a trifle overdoing the "elastic/distortion" aspect in the thought problem. I would assume hard elasticity for the sail. Now I see multitudinous tiny photon "billiard-balls" as stopping dead (I hope you're not going to bring up "no rest mass" again!), causing a flattened "billiard ball" sail of some mass to move off in response to the impacts. (I probably should have made my earlier stationary ball a bowling ball, to more resemble the light-sail.)
I'd like to return to the gravity wave/particle duality suggestion to help resolve your original gravity wave detection hangup, but if I can't overcome my light-sail wave/particle ditto, how can I hope to contribute?
I leave you now--not grinding your teeth, I hope?

Shaun Barrett · 2003-04-25 23:55:48

No, Dicktice, not grinding my teeth at all!

Any small amount of mathematics and physics I know (and it doesn't really amount to much! ), I gleaned with more perspiration than inspiration! So I think I understand very well how someone feels when they find a concept difficult to assimilate and I tend to empathise with their situation.

By the same token, I hope others will empathise with me over problems I'm having ... like this LIGO thing which has me perplexed at the moment.
Which reminds me, I've just learned that the first test of the detector has failed to come up with any evidence of gravity waves. The scientists running the show are unperturbed by this development, saying the sensitivity of the instrument will improve as it is fine tuned over the next few years. For now, they report they're delighted that the first crude data has been collected and the project is, at last, up and running.
Of course, according to my logic, they can fine tune LIGO 'til they're blue in the face but they're never going to see a gravity wave!!! :laugh:
(I'm laughing at me, by the way, not at the scientists!)

[PS Maybe you're right about the 'rubber'. If you're more comfortable with billiard balls, all well and good. The situation you mention, with the first billiard ball stopping dead as it imparts all its momentum to the second ball, is how I imagine the scenario with a photon which is absorbed by the solar sail instead of reflected.
The actual mechanism by which the momentum of the photon is transferred to the sail is not clear to me. I know that a photon striking an atom can raise one of the atom's electrons to a higher energy level. And after a time, that electron can spontaneously drop back down to its customary level and emit a photon in the process. Whether some variation on this phenomenon can explain the difference between absorption and reflection, I haven't investigated.
My 'broad understanding' of how a light-sail works is enough to satisfy me at this point. I suspect too close an inspection of the mechanics of momentum transfer at the atomic level will lead me into ever deeper water ... water I'd rather leave to better swimmers than I'll ever be!! )

dicktice · 2003-04-26 09:30:38

Shaun: What are you saying--that reflected light involves electrons being "raised and and dropped" to produce a reflection (as in a mirror)...? I never thought we'd have to conceptualize image reflection before understanding space (light) sailing.
My conception of phosphorescence involves electrons being raised by randomly energetic photons, then lowered spontaneously to produce fixed energy photons of specific wavelength ... nothing to do with "phosphorescent imaging" if you get my meaning.
So, right back to the totally reflecting sail material being propelled by Solar light, with the totally reflected light retaining its ability to propel equally another same sized sail ... etc. Not all that important, but conceptualizing this may lead to something useful further along. Like conceptualizing gravity propagation (which I understand takes place at lightspeed)....

Shaun Barrett · 2003-05-31 02:33:14

I'm sorry, Dicktice, but I'm not sure I can go any further in answering your questions. As I mentioned, I've never really given much thought to the actual quantum mechanism whereby a photon imparts momentum to matter.

Adding to my April 24th post in this thread, wherein I mentioned the solar sail project of The Planetary Society, here is some more good news from a recent TPS newsletter - and I quote:-

SOLAR SAIL NEWS
Hardware is on the move. The engineering model of Cosmos 1, our solar sail project, has now traveled from Moscow to Miass, home to the Makeev Rocket Design Bureau, builder of the Volna launch vehicle. At Makeev, the the engineering model will be tested with the Volna's payload separation system, which will sever the spacecraft from the booster after launch. This is a critical milestone in the certification of flight readiness for our mission.

I'm hopeful that The Planetary Society, of which I'm a proud member, will soon become the first organisation to demonstrate the principle of solar sailing!
You too could become a member of this great enterprise for a modest annual subscription!

dicktice · 2003-06-01 13:50:55

No problem, Shaun. I just had to press-on until we reached some kind of plateau of awareness/understanding. A great deal of "current knowledge" turns out to be new jargon for previously taught phenomena, don't you agree? You've given me hope. Thanks, and now let's get on with the more practical aspects of space sailing ... such as remotely piloted solar-sailing models, controlled from the ISS by members of the crew, for example? We could deveop the kits down here, and then ship 'em up to the ISS in a Progress or Soyuz.

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#26 2003-04-14 10:29:44

Re: Astronomy hangups - Knowledge vs. comprehension

#27 2003-04-14 23:04:52

Re: Astronomy hangups - Knowledge vs. comprehension

#28 2003-04-15 06:01:42

Re: Astronomy hangups - Knowledge vs. comprehension

#29 2003-04-15 11:48:56

Re: Astronomy hangups - Knowledge vs. comprehension

#30 2003-04-17 08:32:58

Re: Astronomy hangups - Knowledge vs. comprehension

#31 2003-04-17 20:10:44

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#32 2003-04-17 21:00:23

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#33 2003-04-18 06:03:09

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#34 2003-04-18 20:31:28

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#35 2003-04-20 02:11:03

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#36 2003-04-23 18:57:37

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#37 2003-04-24 16:10:47

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#38 2003-04-24 21:01:03

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#39 2003-04-25 19:55:39

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#40 2003-04-25 23:55:48

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#41 2003-04-26 09:30:38

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#42 2003-05-31 02:33:14

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#43 2003-06-01 13:50:55

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