"It would, but it won't", replied her father a little distractedly, and somewhat unnecessarily brusquely, as he seemed more interested in how his various plant samples were accustoming themselves to the dank and dark conditions in this, the most remote corner of the cave.
"But, how do you know it won't, Daddy?" Jessica persisted.
"That boulder's probably been there for many thousands of years. It's not going to come down just when we're here."
Jessica was not at all happy with this. "surely, if it's going to fall down sometime, then the longer it's been there, the more likely it's going to fall down now?"
Jessica's father stopped prodding at his plants and looked at Jessica, with a faint smile on his face. "No, it's not like that at all." His smile became more noticeable, but now more inward. "Actually, you could even say that the longer that it's been there, the less likely that it's going to fall down when we're here." No further explanation was evidently forthcoming, and he turned his attentions back to his plants.
Jessica hated her daddy when he was in these moods--no she didn't; she always loved her daddy, more than anything or anybody, but she still wished he didn't have moods like that. She knew they were something to do with him being a scientist, but she still didn't understand. She even hoped that she might someday be a scientist herself, but if she did, she'd make sure she never had moods like that.
At least she'd stopped worrying that the boulder might actually fall down and block the cave. She could see that her daddy wasn't afraid that it might, and her daddy's confidence made her feel confident too. She didn't understand her daddy's explanation, but she knew that her daddy was always right about that kind of thing--or at least he almost always was. There was that argument about the clocks in New Zealand, when mummy said one thing but daddy insisted that the opposite thing was true. Then three hours later daddy came down from his study and said that he was sorry, and he was wrong, and she'd been right all along! That was funny! "I bet that mummy could have been a scientist too if she'd wanted to be", she thought to herself, "and she wouldn't have had strange moods like daddy has".
Jessica was more careful to put her next question at a moment when her daddy had just finished what he had just been doing and hadn't quite started what he was just going to do. "Daddy? I know that the boulder isn't going to fall down. But let's just imagine that it did, and we're trapped here for the rest of our lives. Would the whole cave get very dark? Would we be able to breathe?"
"What an unpleasant thought!" answered Jessica's father. Then he looked carefully at the shape and size of the boulder, and at the opening in the cave. "Mmmm", he said, "yes, I think that the boulder would fill the entrance-hole very tightly. There would certainly be some space for air to get in and out, so we wouldn't suffocate. As for light, well I think that there would be a small roundish crack at the top that would let some light in, but it would be very dark--much darker than it is now. But I'm pretty sure that we could see all right, once we'd got used to it. It wouldn't be very nice, I'm afraid! But I can tell you one thing: if I had to live here the rest of my life with anyone, then I'd rather it was with my wonderful Jessica than with anyone else in the whole wide world--and with mummy too, of course."
Jessica remembered why she loved her daddy so much! "I want mummy in here too, in my next question, because I'm going to suppose that the boulder fell down before I was even born and you and mummy had me here in the cave, and I grew up with you here . . . and we could keep alive by eating all your funny plants."
Her father looked at her a little oddly, but said nothing.
"Then I would never have known any life at all except this here in the cave. How could I know what the real world outside was like? Could I know that there are trees in it, and birds, and rabbits and other things? Of course you could tell me these things, because you'd known them yourselves before you got trapped, but how would I know--I mean how would I really know myself, rather than just having to believe what you said?"
Her father stopped and thought for a few minutes. Then he said: "Well, I suppose that every now and again, on a sunny day, a bird might fly exactly in line between the sun and the crack, and then we could see its shadow on the cave wall behind. Of course its shape would be distorted somewhat, on the rather irregular-shaped wall, but we could learn how to correct for that. If the crack was small and round enough, then I the bird might cast quite a clearly defined shadow, but if not, then we might have to make other kinds of corrections also. Then, if the same bird were to fly across many times, we might begin to get quite a good picture of what it actually looks like, and how it flies, and so on, just from its shadow. Again, when the sun was low in the sky, there might happen to be a tree, suitably positioned between the sun and our crack, with its leaves waving, so that we could begin to get a picture of this tree, too, from its shadow. And perhaps from time to time a rabbit might jump up into the way of our crack, so we could begin to picture it from its shadow too."
"That's interesting", said Jessica. She paused for a few moments, and then said: "Do you think that it would be possible for us to make a real scientific discovery, while stuck down here in the cave? Imagine that we had made a big discovery about the outside world, and then we were in here having one of your big conferences, trying to persuade everyone else that we were right--of course all the other people at the conference (and you as well) would have to have been brought up in the cave too, otherwise it's cheating. But it's all right for them to be brought up in the cave too, because you've got lots and lots of funny plants and we could all live off them!"
This time, Jessica's father visibly winced, but still he said nothing. He looked pensive for several minutes. Then: "Yes, I think it would be possible. But, you see, the hardest thing would be to try to persuade them that any outside world existed at all. All that they would know about would be the shadows, and how they moved about and changed from time to time. To them, the complicated wiggling shadows and things on the cave wall would be all that there was to the world. So, part of our task would be to convince people that there actually is an outside world that our theory refers to. In fact, these two things would go together. Having a good theory of the outside world would be an important part of making people accept that it was really there!"
"OK Daddy, what's our theory?"
"Not so fast . . . just a minute . . . here it is: the earth goes round the sun!"
"That's not a very new theory."
"No--it's actually nearly twenty-three centuries old--nearly as old as the length of time that boulder's been wedged there near the entrance! But in our imagination we've all spent our entire lives in the cave, and the people wouldn't have ever heard of such an idea before. We'd have to convince them that there was really such a thing as the sun--and even as the earth, for that matter. The idea is that the simple elegance of our theory in explaining all sorts of fine details of the movement of the light and shadows would eventually persuade most of the people at the conference that not only is there actually a very bright thing out there--that we're calling the 'sun'--but that the earth is in continual motion around it, spinning on its axis all the time."
"Would it be very difficult to persuade them?"
"It certainly would! In fact we'd have to do two quite different kinds of thing. First, we'd need to show how our simple theory explains in a very accurate way an awful lot of very detailed data concerning how the bright spot, with its shadows, moves about on the cave wall. Now, some people might be persuaded by this, but others would point out that there's a much more common-sense" picture in which the sun moves around the earth. In detail, that picture would be more complicated than the one that we are putting forward. But these people would prefer to stick to their complicated one--reasonably enough--because they simply couldn't accept the possibility that the cave was moving around at about a hundred thousand kilometres per hour, as our theory would require."
"Gosh, is it really doing that?"
"Yes, that sort of thing. So for the second part of our argument, we'd have to change tack completely and do something that a lot of people at the conference would think was completely irrelevant. We'd be rolling balls down tracks and swinging pendulums and that sort of thing--just to show that the physical laws that govern the behaviour of things in the cave would be unaffected if the whole contents of the cave were moving in any direction you like, at any speed you like. This would show them that they wouldn't actually feel anything if the cave moves around at an enormous speed. That's one of the important things that Galileo had to show--you remember about him from that book I got you?"
"Of course I do! Oh dear, the whole thing sounds awfully complicated. I bet that lots of people at our conference will go off to sleep, just as I've seen them do at real conferences when you're giving a lecture."
Jessica's father reddened just noticeably. "I expect you're right! Yes, but I'm afraid that this is what science is often like: lots and lots of details, most of which can seem very boring and sometimes almost completely irrelevant to the picture that you're trying to get across, even if that final picture might have a striking simplicity to it, as with our idea that the earth spins as it goes round a thing called the sun. Some people might not feel that they need to bother with all the boring details because they find the idea plausible enough anyway. But the real sceptics would want to check through everything, looking for possible loopholes."
"Thank you daddy! I always like it when you talk to me about things like this, when you sometimes get all red and excited. But can we go back now? It's getting dark and I'm tired and hungry--and a bit cold."
"Come on then." Jessica's father put his jacket over her shoulders, collected up his things and put his arm around her, to guide her out of the now darkening cave entrance. As they made their exit, Jessica looked up at the boulder again.
"You know, I think I agree with you daddy. That boulder's going to stay up there more than another twenty-three centuries!"