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If the nearest star is over 4 lightyears away from earth,...

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Question by Frieghttrain
Submitted on 8/6/2003
Related FAQ: [sci.astro,sci.astro.seti] Contents (Astronomy Frequently Asked Questions) (0/9)
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If the nearest star is over 4 lightyears away from earth, how do we know that any stars exist presently?

Answer by Truthassassin
Submitted on 9/16/2003
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There is no way to know that. In fact, there is no reason to know that, either!

All effects that the star exercises over the Earth, including its gravitational pull (however small it is at such a distance), also travels at the speed of light. So even though the star ceases to exist, it will stop affecting distant objects only after a while.

If you think that actually if a star appears somewhere, we will know about this also only after a while. We are talking only about knowing things a bit later. This is a problem of concurrency.

Now, imagine a pathminder crawling on Mars. The radiowaves travel for about three minutes from there. Now imagine if NASA had to control it in a way you control your radio-controlled toy-car. Try to run your car in a room and imagine if you saw the car running towards a chair only three minutes after it actually started moving there, and the car swerved to avoid collision only after another 3 minutes after you pulled a lever... Perhaps, by that time there would be no car to swerve away from an obstacle!

The car should, therefore, be either very clever (which the Pathfinder is, to some extent), or very slow (like the first automatic Moon-lander). But why should it hurry anyway? Who cares? It's only us who are so impatient! :-)

How that happens? Well, we are simply used to getting news instantly. Do you remember a Greek warrior who had to run 42 kilometres to Marathon to bring good news of victory to the citizens? The battle was already won, but the citizens still lived in fear and uncertainty, until the messenger arrived (after a while!).


Answer by Truthassassin
Submitted on 9/16/2003
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More on concurrency and knowing things right in time. There lived a dinosaur as huge as 30 metres from its head to the end of its tail (brontosaur, if I am not mistaken).

When do you think the creature will know what happened to its tail? Neural signals travel slower, than the speed of light (because it is not only electricity, but chemistry as well). Funny enough, it could have taken a second or two for the dinosaur to get the message and decide what to do, plus overheads for the signal getting to the tail or its hind leg(!). Someone hungry and cruel could have bitten a chunk off it and run away before the dinosaur could decide to move its leg to squash the teaser!

What did the nature do? It created a local decision centre! The dinosaur's spine evolved so that its bottom part (yeah, where the bum is :-)) became swollen (reads: contains more neural cells than the rest of the spine). Thus the head brain got rid of some minor decisions about balancing when walking and kicking when it hurts.

That's about what the Pathminder is: it is the brain in the bum that makes simple decisions about avoiding cracks in the ground and steep slopes, and the brain in the head is NASA engineers watching it from the laboratory on Earth.

Hey, what if people had two brains, like the dinosaurs did - one in the head and one in the bum? :-) Everyone would be schizophrenic, everyone would have split personality! ;-) That's a psychiatric theory why the dinosaurs became extinct: the head and bum just quarrelled a lot and finally torn all the dinosaurs apart, the head and front legs going for weed, and the hind legs and bum going for a poo.

Mind that this all is not only a joke, but the thing everyone must realise: there must be someone who makes ultimate decisions in a largely distributed system. It is the head, and not the spine or bum; it is the lieutenant, and not the sergeant (though the latter co-ordinates troops in the actual combat to achieve what lieutenant wants!); it is the NASA engineers, and not the Pathminder. But it is also important to give some freedom to these remote and scattered pieces of equipment and organs, so they will be robust and quick. The balance between autonomy and total control is something for the experts to decide.


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