Things I learned in physics class

POSTED BY: BadgersHat
UPDATED: Friday, August 12, 2005 15:00
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QBEAM

Thursday, July 21, 2005 5:43 PM

Quote:
One of the best realistic SF solutions to both those problems is to have a ship accelerating at a constant 1G for half the trip, and then decellerate at 1G for the other half, giving the entire spaceship an Earthlike gravity, with Down being the direction the engines are pointed, (although they have a brief period of zero G and readjustment as up becomes down for the rest of the trip). This requires a lot of fuel, however.

Yes--in fact, as someone else pointed out, if you assume rocketary propulsion (i.e. thrust based on expelling mass out the back), you can't carry enough feul to achieve 1g for years on end.

Fortunately, nature has given a potential solution: pick up the feul on the way. Space isn't empty--there are stray hydrogen atoms floating around. The Bussard ram jet is a theoretical way to harness that feul. You create a huge magnetic field to scoop the hydrogen up, like a funnel, then concentrate them in a magnetic bottle at the rear of the ship to kick of a fussion reaction. The only trick is that you have to get the ship moving in the first place, so your hydrogen net can start scooping.

I like to think this is how Serenity works. Remember how the ship starts out slowly, and then there's that big flash at the rear, and the ship suddenly gets a big burst of speed? To me, that looks like the ship starts off with some lower-power propulsion, then kicks on the Bussard field, and POW, you're off.

Apparently, though, they don't have the technology to harness the fussion reaction very well, since, as we know, they need some other kind of fuel for the other ship functions. Apparently, that fuel runs whatever the start-up propulsion is, too. This all still makes a lot of sense to me, because running life support and such would be a tiny power drain, compared to interplanetary thrust at close to 1g. I think they need fuel mostly to start up and stop; during the long cruising periods, they're not using much.

GROUNDED

Thursday, July 21, 2005 5:58 PM

Quote:
Originally posted by QBeam:
If you approximate 1g as 10m/s, and c as 4*10^8m/s

Why would you approximate c to that? It's 3x10^8 m/s...

SKYDANCE

Thursday, July 21, 2005 6:05 PM

Quote:
Originally posted by zoid:
Would I also be correct in stating that since everything is relative, as one approaches infinite mass while accelerating to c, every other bit of matter one might encounter (even massive particles) could be seen as acquiring infinite mass instead, from the viewpoint of a traveler who felt stationary on the ship?

I don't think it works that way, no. Your observations are going to be royally screwed by the time dilation, anyway.

You are 100% correct about not wanting to come into contact with anything, though. If you did encounter a pea-sized object, then one of two things would happen: either (1) your ship would transfer enough energy to the pea to make it match your velocity -- all in the course of a billionth of a second, or (2) part of your ship would occupy the same physical location as the pea.

I'm thinking #2 isn't an option. You'd just end up with #1 -- a really, really huge transfer of energy. By way of reference, atomic weapons are a nearly-instantaneous "transfer of energy." And the closer you are to the speed of light, the more energy you need to transfer to that pea to make it match your velocity ... and the less time you have to do it in, from the pea's perspective.

Quote:
Again from Zoid:
I'm also not keen on the names 'Shiva' and 'Nova' for an earth-based reactor...

ROFL!

Just remember: we (the USA) have detonated Fusion devices (for testing purposes). The fears of the atmosphere catching on fire are greatly exaggerated. You also might be interested in [url] http://news.uns.purdue.edu/UNS/html4ever/2005/050712.Xu.fusion.html

__________________________
"They weren't cows inside. They were waiting to be, but they forgot. Now they see the sky and they remember what they are."

ZOID

Thursday, July 21, 2005 6:20 PM

FourSkys:

Thank you very much for contributing your 'pop-in' explanation. It is very useful for those who might not otherwise understand the distinction between the actual QM and the interpretive 'schools'.

Since I don't want to make statements that others might misinterpret as a claim of expertise, I'll try to phrase the following 'outsider' observations in the form of questions, in order to keep myself out of jeopardy (pun intended). My understanding is that QM predicts the probabilities that an investigated particle will be found to possess certain properties (i.e., position or momentum, spin, et cetera). Correct thus far? QM then predicts the odds that the particle will be found in a specific configuration. (How'm I doing?)

While QM doesn't tell you where each specific electron is going to strike a phosphor (in the classic two-slit example), the results of firing thousands of individual electrons results in a distribution that exactly matches the mathematical prediction. (Or am I misunderstanding or otherwise over/understating this point?) This predictive power makes QM the most successful theory ever devised by Man. (Everybody agrees on this point, so I'm not even going to make that a question.)

The problem is that nobody can say for certain How quantum mechanics works. That's why they call it 'mechanics', right? It's like being in a stranger's kitchen with a recipe for chocolate cake. In the kitchen are bags marked 'A', 'B', 'C' and so on. The recipe says add 2 cups of 'A', 1/2 cup of 'B', etc. When you follow the recipe ('Do the math') you wind up with a perfect chocolate cake. But you have no idea where the individual bags come from, what processes were involved in manufacturing their contents, and there may be some questions about the accuracy of thinking of two different bags of 'A' as being identical. Corollary questions: Are all bags of 'A' really the same bag of 'A'? Is one bag of 'A' actually an infinite stream of identical bags of 'A'? But there is no denying that if you leave the extraneous questions out of it and simply follow the recipe, you will make a perfect cake. (NB: This has been an analogy. Analogies are simplified and imperfect similitudes of the actual process being indirectly examined.)

As you said, the math is undeniably descriptive of a level of reality that is incredibly bizarre when compared to the 'macro' world of our existence (which corresponds more closely with Relativity), and I wouldn't be typing this on a computer were the predictive powers of QM 'untrue'.

Einstein and others -- even practitioners of QM -- were unsettled (would you use that word?) by a theory that didn't include the 'How', and that included uncertainty as a cornerstone ( http://en.wikipedia.org/wiki/Uncertainty_principle , especially the section entitled "Interpretations"). Scientists are humans, too. This unsatisfied need to know the 'How' (please note I never said, 'why'), I believe, has led to the different interpretive groups, including the "Just do the math" school (Copenhagen, right?). Among the many schools/groups -- none of which denies the awesome predictive power of The Math -- there are those that possess a more than a passing resemblance to Eastern religions, in particular. Maybe it had something to do with the '60s, hmmm?

More than the math, I'm fascinated by the picture of reality that QM paints and the human (scientists') reactions to that necessarily blurred picture. As you said, the math is cut and dried. There's no question about it. How the math works and the actual characteristics of reality that underlie the math are the unanswered -- and therefore compelling -- questions for me personally.

Thanks for understanding and -- I'm begging you -- please correct anything I've said that is either incorrect or misleading, not only for myself but for others who might be following this with some interest (like TiPpY). You can even call me an idiotic poser if you'd like, so long as you enumerate specifically wh

SKYDANCE

Thursday, July 21, 2005 6:39 PM

Quote:
Originally posted by zoid:
... ventured to comment that the differing viewpoints -- many of which have been duly pointed out by each author I have read -- as well as the fantastic basis of QM, reminded me of religion: 'Believing' in something totally unrelated to everyday experience, and which you have very little hope of ever actually observing (i.e, like 'God'), and then settling into 'interpretation groups' regarding the best guess of what is really going on at the most basic level (i.e., like differing sects of the Protestant faith).

I think the problem is that we, looking from the inside, can understand how you, looking from the outside, came to the similarity. However, there is a distinct difference between Physics and Religion.

If someone comes to us with an experiment which produces an unexpected result, we all scurry about like insects with the light turned on, and begin revising our theories to come up with something that explains the observed results. We also demand the ability to reproduce the experiment.

If someone goes to the Pope and says, "God told me X," the Pope smiles at him and looks for someone who can help the poor deluded fool.

Religion != Science. That you see a similarity is nice, but it's not appropriate.

__________________________
"They weren't cows inside. They were waiting to be, but they forgot. Now they see the sky and they remember what they are."

ZOID

Thursday, July 21, 2005 7:12 PM

Skydance:

Okay. I don't want to belabor the 'science resembles religion' thing, since it's outlived any usefulness and since the respondents in this thread have made their viewpoints clear (i.e., I'm dead wrong).

But... (*crap*)

It's not historically accurate to say that religion hasn't altered its dogma to match experimental results. It was once canon that the Earth was the center of the Universe and that the Sun, planets and other celestial bodies orbited the Earth. Organized religion vehemently resisted the non-Earth centric paradigm (they burned Dominican friar and astronomer Giordano Bruno at the stake for declaring that there were an infinite number of Earth-like planets orbiting infinite Suns), but The Church eventually changed its 'theory' to match scientific evidence. And that's not the only example of such adaptation. Please note that you and none of your colleagues have been scourged by the Inquisition, other than yearly on or before April 15th...

We now return you to your regularly scheduled programming.

Respectfully,

zoid

P.S.
I really am letting the 'science resembles religion' analogy die. Honestly. I was obviously way off base and unintentionally offended folks by even suggesting it. Next up: How politics are like team sports...
_________________________________________________

"Sure as I know anything, I know this: I aim to misbehave." -Capt. Mal Reynolds, Serenity, a.k.a. 'the BDM'

DAIKATH

Thursday, July 21, 2005 8:16 PM

ZOID: I have one more thing to add to the 'sciences as religion' debate.

Yes, poeple who disagreed with the holy roman church were burned at the stake even though now some of those poeple turned out to be right.

None of that has happened in the scientific community, but poeple are being shunned and looked down upon. If someone in the scientific community is openly religious he is often not taken seriously by other scientists.

Although this is not equal to the burning at a stake, it is a modern way of weeding out the poeple who do not fit the standard beliefs. They may agree that each and everyone of their theories are just theories and could be proven wrong at any moment. Just so long as that improvement doesn't include a God.

(My personal obversation of hearing that religion is bad because it was against the scientific community in medival times. Today you might say that those poeple are wrong and science is right but the Inquisition was convinced it was right too).

SKYDANCE

Thursday, July 21, 2005 8:18 PM

Quote:
Originally posted by zoid:
Next up: How politics are like team sports...

*laughs* I think that might be an appropriate followup ....

I'm not sure I agree that the Earth-centric Universe was a matter of Faith, modified because of others coming forth with new information from God (or Angels, or Prophets). It certainly was a belief that was held by the Catholic community, and claims to the contrary were pretty harshly dealt with. Was it a matter of faith, though?

I feel a better analogy would be claiming Jesus never performed any miracles -- and then producing a bible-type document which explained each of those events as a normal event (like, the water-to-wine thing was explained by one of the guests bringing a wagonload of wine as a wedding gift, unbeknownst to the host or to Jesus, and the servants passing it out to the guests). Peter, Luke, et. al. never got that part of the story, so they wrote it the way they had experienced it: with awe and reverance.

Similar physics scenario: a particle physics researcher in India finds that using neutrinos to make observations allows her to measure both the location and momentum of a beta particle, at a level which exceeds Hiesenberg's Uncertainty Principle. Both her results and the experimental process are documented, and other physicists are able to reproduce her work.

Religion is going to discard the gnostic bibles. Science is going to embrace the unexpected experimental results (after a period of healthy skepticism, and reproducing those results), and then race to explain them. These are very different reactions.

The major difference, though, is that Religion is founded on faith. It is based totally on (1) things that happened when no one alive could observe them, and (2) how those things make you feel. Science is based completely on observation. If you ran an experiment, and got results that don't agree with theory, then you need to be able to reproduce that experiment over and over. If you can't do that (and no one else can), then the results don't count. We can believe that something happened, but if we can't reproduce the data you reported, then we can't be certain exactly what did happen. Maybe your test tubes were dirty. [url] http://en.wikipedia.org/wiki/Cold_Fusion#Pons_and_Fleischmann.27s_expe
riment]

Philosophy & differences of opinion occur in Physics when interpreting the implications of experimental results, but the experiment itself determines what we believe in common. If someone has a radical theory, and they can produce an experiment that agrees with their theory but doesn't agree with the other guy ... well. That's why they give out Nobel prizes. Matters of Faith, on the other hand, are not subject to review and verification. By their very nature, they cannot be verified, only believed.

__________________________
"They weren't cows inside. They were waiting to be, but they forgot. Now they see the sky and they remember what they are."

FOURSKYS

Thursday, July 21, 2005 9:01 PM

Zoid,

So what you said made sense, and, to the best of my knowledge, was correct. The Cake analogy fits, too, to some extent. Scientists for the most part don't know how QM works. "Spooky action at a distance" and the like. There are unresolved problems and things that have yet to have their "how" explained. Like how an electron really isn't anywhere until you look at it. It's entirely meaningless to us non-quantum mechanical beings. At this point, the only way to truely describe the "how" is to resort to math. But really, on a fundamental level, everything gets reduced to that at some point. For example: How does the earth move around the sun? Well, the sun warps the fabric of space-time around is according to its mass, and the earth follows the trajectory. How does the sun warp spacetime? It's a property of mass such that it does. But How? Good question. We can go into gravitons and things, but the questions is always answered with another how. Yay, don't you just love infinite loops?

Quote:
Originally posted by zoid:
P.P.S.
Someone else want to tackle the reason why individual electrons fired through a two-slit screen build up a QM-predicted interference pattern? I've said enough crazy things before lunch today (obscure reference intended). For the readers: It has to do with Feynman's sum-over-histories. You're gonna love this.

Nah, I'll pass on that one :-)

QBEAM

Friday, July 22, 2005 2:03 AM

Quote:
Why would you approximate c to that? It's 3x10^8 m/s...

So true... many a slip, 'twixt the the mind and the lip, I suppose.

To be more precise, it's 2.998, and I usually approximate that at 3.000. I'd recommend using that figure here, too...

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