jack

I posted a short version of this, but I feel a longer version may get more comprehensive answers. I'm sure I remember a conversation that came to a conclusion once long ago, but not the details.

Suppose you have a stationary wormhole allowing FTL travel. If you can move one end of the wormhole to the other, you get plenty of time travel paradoxes, because the two ends will no longer be synchronised in time -- the one that's travelled will have experienced a shorter journey time, so when they're next to each other, someone entering the other one will exit from that one earlier, opening the question of "can they prevent themselves entering the wormhole".

But suppose you *don't* move the ends of the wormhole. I *think* that FTL + general relativity[1] must mean there's a time travel paradox somewhere for someone. Maybe someone travelling very fast relative to your wormholes? But for whom?

[1] I believe Stellaris DOES have time travel hidden somewhere but in general doesn't try to stay faithful to general relativity :)

Crossposts: https://cartesiandaemon.livejournal.com/1075149.html

Does anyone understand pilot wave theory even a little bit?

Prodded by several recent articles, I've been trying to follow what it says, and am still quite unsure of the realities.

The analogy usually presented is, if you have a small oil drop on the surface of water, and the water container is subject to a regular pattern of vibration, the water forms standing waves in shapes affected by the edges of the container and any obstructions in the surface of the water. And the oil drop tends to move across the surface of the water following the paths in those waves.

If you look solely at the oil drop, you can't tell which of two equal paths it would follow, but you can predict it will take one of them with equal probability, and predict its motion probabilistically. And if you couldn't see the standing water waves, you could deduce something in that shape exists.

You can even get some analogies for weird quantum behaviour like the an electron passing through two parallel slits and experiencing interference with itself: the water waves form possible channels for the oil drop, and the oil drop goes through one slit or the other, but ends up only at certain places on the far side.

However, the analogy to actual quantum physics is still unclear to me. Not whether it's true, but even what people are suggesting might happen.

Are people suggesting there's some underlying medium like the water? In that case, isn't there some propagation speed? The water waves exist in a steady state once all the obstructions are set up, but they don't respond to changes instantly. If the water trough were miles long, the oil drop would set off following water wave paths that existed at the point it passes through, not the paths corresponding to the obstructions that are going to be in place when the oil drop passes through them.

And yet, as I understand it, no-one expects a propagation delay in quantum experiments. People keep checking it out, but there never is: it always acts like an electron propagates just like it is itself a wave.

I agree, if there WERE some delay, if you changed the slits at this time, and got one result, and changed them at another time, and got another result, that would be massive, massive, evidence of something, possibly of something like pilot wave theory. But AFAIK proponents of pilot wave theory aren't advocating looking for such delays, and don't expect to find any.

Contrariwise, if this is just an analogy, and the quantum equivalent of the water waves (equivalent to the wave function in other interpretations of quantum mechanics) propagates at "infinite" speed, then... that is undetectable, indistinguishable from other interpretations of quantum mechanics. But it raises red-flag philosophical questions about what "infinite speed" means when all the intuition from special (or general) relativity indicates that all physical phenomena are local, and are influenced only by physics of nearby things, and "the same time" is a human illusion like the earth being stationary. Even if you don't expect to detect the pilot wave, can you write down what it should be in a universe where physics is local? Does that in fact provide a way to make QM deterministic and independent of observers, even if you change the reference frame? Because it doesn't sound like it will work.

FWIW, those are very superficial objections, I don't understand what it's saying enough to actually evaluate in depth. But I don't understand why these don't show up on lists of "common objections and rebuttals". Common objections have confident rebuttals in several places, and I've *seen* articles about them, but not understood well enough. Can anyone explain better?

Digression

I do agree, the idea that QM equations are an emergent property of something else, ideally a statistical interpretation of a deterministic underlying reality, would be very nice in clearing up a lot of confusion. But AFAIK, the closest candidate to that is Many Worlds, which doesn't appeal to many people who want to get away from QM unpleasantness.

Crossposts: http://cartesiandaemon.livejournal.com/1035554.html

I've been trying to get this straight in my mind by asking what the world would look like if relativity wasn't true.

It's probably true that "everywhere in the universe is the same time and everything has speed and position that's the same wherever you measure it from" is simpler and more natural than "the time between two events and the distance between two objects depends how fast you're going". But when you dig into it, it doesn't really hold together.

Lots of explanations try to tell you the details about how the world is different with relativity than if it was all newtonian. I don't understand it well enough to talk about how. But rather than glossing it over, I'd rather tackle head on, why I should think something weird is going on. It's like, explaining calculus without understanding what problems there are that are worth solving, and are hard to solve, but trivial with calculus, just feels like a pointless arbitrary set of rules. But once you get what it's for, you understand it in an equally important way as if you know how to do it.

I think the key question is, how fast does light move? Lots of people know, c, or 300 million m/s, or 1 foot per nanosecond. But relative to whom? When we describe speeds, we normally mean "relative to the Earth's surface", or "to the Sun" for things in the solar system. That's normally obvious, but it's only obvious because it's the same -- you have to pick the right scale, or else you say you can jog at the speed the earth orbits.

What are the possible answers?

1. Like normal objects, light travels at the speed of the object that emitted it, plus c. If you throw a ball on a train, within the train, it travels at the speed of your throw, but relative to the ground, it travels at that speed plus the speed of the train.

2. Like waves, it travels at c relative to a fixed stationary... something. If you drop a big rock into the sea from a fast plane, the waves spread outward at the same speed, however fast the plane is going. If you have a siren on a vehicle, the sound travels at the same speed in the air, however fast the vehicle is going (as you can tell, because if you fast enough you catch up with the sound -- a sonic boom).

3. You measure light as travelling at c, and everyone else measures light travelling at c, even if you're going in different directions at hundreds of thousands of miles per hour. Yes, this is ridiculous, how can different people measure the same thing and get different results? But -- we've measured light in all sorts of ways, and whatever we do we ALWAYS see it going at c, just like maxwell's equations say it should. (Well, slower in atmosphere, but a known amount.) I think if relativity were explained like this, "we measure light going at the same speed everywhere, how come?" it would make more sense than explaining the historical order.

4. Light travels at c relative to the nearest large planet, slowing down or speeding up as it moves from the neighbourhood of one planet to another.

Well, which makes sense? #1 sounds plausible. But we receive the light from stars, and can measure its speed. Stars go VERY fast, so those light beams should be at very different speeds depending what star it came from. But no, they all go at c.

#2 also sounds plausible to start with. But where is this invisible stationary air or water or something? The earth orbits the sun, and the sun orbits the galaxy, etc, etc, at very very high speeds, so we should never be stationary relative to the medium. Which means if you measure the speed of light in the direction of the earth's orbit, and perpendicular to that, you should get very very different answers. But no, the speed is always measured at c.

Alternatively, this medium is always exactly aligned with the Earth specifically. That should sound dodgy. From an orbital mechanics perspective the earth doesn't look at all special. It's hard to disprove this until you get to the fancier experiments in the footnotes, but it should sound like "a bodge", not "the answer".Ironically, of the three wrong explanations, this comes really close -- it works perfectly, it's just that in the real world, it's not just the Earth that has a special "stationary" aether, every point/velocity in the universe does.

What about #4? Again, this should sound dodgy, but is hard to actually disprove. Until relativity was accepted, this was a good theory, that the earth "dragged" the invisible aether with it, so it was always moving at the same speed. I can't remember what disproves this, but it shouldn't sound good. You'd also expect weird effects different to relativity as light moves near other planets and stars. And maybe weird red/blue shift as light moves from one speed of aether to another.

That leaves #3. Which is very counter-intuitive, but actually predicts something very like what we see -- both "at everyday speeds, everything acts like newtonian mechanics" and "but for everyone wherever they are, maxwells equations and the speed of light work exactly the same." It wasn't obvious that those would conflict, but they do, unless you accept relativity.

Footnotes

I can't remember all the relevant experiments. A big one is https://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_experiment which measures the speed of light in the direction of the Earth's orbit, and perpendicular to that, and discovers they're the same. It doesn't "measure the speed", by waiting until the light gets from A to B, rather, it sends light down the two paths and then adjusts the path lengths until you get an interference pattern showing they're exactly in sync. Which is when the paths are exactly the same. Every other "measure the speed" is similar.

If there's holes in the above, are there *other* experiments that suggest the universe isn't straight-forwardly Newtonian? Yes, lots, wikipedia has a big list, although not all easy to understand. My favourites are:

* Produce light of a specific frequency (with some crystal that has a very precise frequency response?) Check that's it's re-absorbed by that same crystal. Yes, if the apparatus is horizontal. No, if its vertical. Why does that make a difference? In a non-relativity world, Newtonian gravity and Maxwell's electromagnetism should be completely separate. But relativity says, moving to a different height in a gravitational field will change the energy, and hence frequency, in the light -- which is does.

* The incredibly precise times from GPS satellites needing to be tuned to account for general relativity.

Crossposts: http://cartesiandaemon.livejournal.com/973979.html

A friend asked on twitter, if you could place two connected portals anywhere on earth (but not in space), what would you do with them. Personally, linking Cambridge and Liv's campus flat would be nicest! Geopolitically, probably linking two disparate regions might be most useful.

But of course, the question turned to free energy. Suppose the portal is 1mx2m, laid horizontally, one at ground level and one 100km up at the edge of space, and you diverted sufficiently much water into the lower one to get an endless waterfall. How much energy would you get out?

I'm not sure I have these equations right any more, but under those assumptions (and that the density of water and the gravitational constant for the whole height are rounded to the nearest power of ten and that the square root of 2 is 1.5), and assuming that extracting the energy from the falling water slows it to rest again, I tried to calculate the speed and the energy. I think freefall from that height (assuming you can somehow construct an airtight tube) lands at 1500m/s. And that translates to 3x10^12 Watts = 3TW. Can anyone confirm if I have that right?

It's hard to tell from wikipedia, but it looks like that world energy consumption was 15 TW the last time someone worked it out and updated that page. So this device would make a worthwhile dent in it, but not obsolete everything else.

Of course, that assumes there are sensible engineering solutions to "build an airtight tube to the edge of space" and "slow water from mach 5 safely without wasting any energy" which there probably aren't.

You could dig the tunnel _down_ instead, but you'd have to actually dig it, although you could put one portal down there and dig up through the other one, until the rock fell freely. And you'd have to be careful not to go down too far because if your experiment starts spewing pressurised magma you've invented your own personal volcano, and you have to hope that it solidifies and buries it well enough to withstand ten atmospheres of pressure.

Of course, if you did the above experiment with rock instead of water, the energy involved would be 10 times greater, although presumably the engineering challenge would still be the limiting factor.

However, leaving the practical impossibilities aside, something bothered me about the physics. It seems like, if you don't slow the falling substance completely but let it go through the portal already at high speed, you get correspondingly more energy out. I guess because the limiting factor for energy is that each atom going through a portal to 100km up creates a certain amount of potential energy, so you get the more energy the more you cause that to happen. It seems dodgy that the energy production could just keep on growing in that case, but I guess the assumptions violated physics, so there's no reason not to expect that to violate a wide number of other principles. Have I actually got that right?

Crossposts: http://cartesiandaemon.livejournal.com/903088.html

It only recently occurred to me that people who objected to the non-deterministic nature of quantum mechanics were *right* if you think many-worlds interpretation is correct.

It's like, since Newton, physicists had a tacit assumption:

#1. The laws of physics are deterministic.

But then we discovered lots of evidence for QM that we couldn't ignore, and many people adopted a different assumption:

#2. The evidence for QM

And these seemed contradictory, so people who had #1 were (rightly) suspicious of #2, but people who accepted #2 felt they had no choice but to reject #1.

But was also assuming without even realising:

#3: there is only one universe, not a giant number of parallel universes

And it turns out that if you drop #3, you can keep #1 and #2.

Now, I don't think that's sufficient reason by itself to assume MWI. There are lots of other paradoxes that disappear (if QM works the way we think it does, though many physicists still think that is premature). But it's interesting that we might have to drop #1 one day, but not yet.

And I knew all that _in theory_, but I'd not actually stopped to think about Einstein's "god plays dice" quip since I learned slightly more about MWI.

Crossposts: http://cartesiandaemon.livejournal.com/867586.html

I've heard quite a few people proposing that the many worlds interpretation of quantum mechanics is "obviously right": many physicists, and very vociferously Elizier Yudkowsky. Unfortunately, other respected physicists have derided the whole thing as bunk.

Fortunately, Scott Aaronson, who I generally trust to know what he's talking about on quantum physics has obligingly written about it: http://www.scottaaronson.com/blog/?p=1103 (via cyphergoth).

It definitely wades into physics that I can't follow. It wades into physics that he doesn't know, for that matter.

However, as best I could tell, a few highlights. I found the best quote in his first comment:

I understand the case for MWI and agree with part of it: yes, MWI really is just the “obvious” story you would tell if you wanted to apply quantum mechanics to the entire universe. (The zillions of other worlds aren’t “added” per se; rather, they seem unavoidable once you accept that the Schrödinger equation applies always and everywhere.) Furthermore, all of the concrete alternatives to MWI on the market today are contrived and unsatisfactory in various ways.

On the other hand, for the reasons explained in the post, I reject the further step some people take these days: of asserting that MWI is as obviously true as the Copernican system, and that anyone who refuses to see that is an idiot.

I think the first part of this is what MWI-advocates like Yudkowsky have been pushing, that the simplest interpretation of what we know about how wave functions work and how observations of them work, is that wavefunctions continue to evolve according to the usual equations all the time without exception, and what we call an "observation" is the observer becoming entangled with the experiment.

I feel the part of the MWI argument that I found important to have been settled with that comment: I know many physicists will still resist, but purely in terms of "does this sound true to a professional physicist", I trust Scott's interpretation.

Of course, that still leaves the meat of Scott's post, where he disagrees with MWI advocacy, but I didn't find that as important. He thinks MWI advocates are jumping the gun in saying it's obvious and settled. I mention a couple of his major points below, but I don't think I can do them justice (I'm not sure I understand or agree) so you're better off reading his post directly.

Turtles all the way up

The meat of the matter seems to be, photons experience decoherence. As do electrons. As do buckyballs. But what about cats? Human brains? Is there a size bigger than which things don't experience decoherence?

There seem to be three possibilities:

1. Yes, it works all the way up, just like it looks like it should
2. No, there's a physical effect which stops at a particular size. Scott makes the point that we've no idea how to apply superpositions to physics including non-trivial gravity fields, which he thinks may be relevant. I think this is his objection, though it doesn't make sense to me?
3. There's some other explanation, such as hidden variables, or magic powers of consciousness which make waveforms collapse, which have been expressed by many very venerable theories of physics and many of the best physicists from the 1930s onwards, but are pretty clearly Not True, unless we're prepared to accept dubious propositions like "consciousness has magic properties not explained by the rest of physics" or "spooky information travels faster than the speed of light in untestable ways". This is the sort of thing which looked more reasonable than not 50 years ago, but lots of mathematical proofs have proved lots of things are impossible (given "reasonable" assumptions), and MWI advocates have spent a lot of time refuting.

Scott agrees with MWI advocates that by now it pretty much has to be 1 or 2. He thinks there's still a reasonable possibility of 2, given experiments we'd like to do but haven't done yet, but I don't understand why.

Occams Razor

Scott obligingly demolishes a common flawed application of occam's razor, namely applying it in an extremely literal sense to minimise the number of physical objects, rather than the complexity of physical laws.

As he points out, tables are made of atoms. Are they made of atoms all the time, or only when you look at them closely enough? The first theory has fewer physical objects (since a lot of the time you just have "table", not a lot of separate atoms), but the second has much simpler physical laws (since you don't have to define what "someone's looking at it" means).

But most people would agree that it only makes sense to assume the table is made of atoms all the time. Ie. we should minimise the complexity of physical laws, not the number of physical objects. This is the interpretation of Occam's Razor which (a) is what scientists usually mean by it and (b) has worked historically -- for instance, the discovery that stars were stars was unlikely in the "number of objects" way, but fine in the "simple laws of physics" way, and was right.

Physicists who agree with the concepts of MWI but disagree with MWI

He points out that some physicists basically agree with the "there's a wavefunction, end of story" interpretation, but think "many worlds" is the wrong concept for it. I think that's fine, they're probably right. I am using MWI as a handy label, I'm happy to use something else. I don't endorse all the trappings of MWI as the best way of explaining stuff (they may be, or may not be), I intend to refer to the sort of things these people would agree with.

For the record, the idea is that there's a waveform, and "worlds" is a convenient way of viewing that waveform as overlaid copies of mostly-separate waveforms that only interact with themselves, not with each other.

So the question "when, exactly, does the universe split into two 'worlds'" has the answer "at some fuzzy point during the process when the split becomes irrecoverable, ie. the worlds no longer interfere". But the fuzziness is in the concept that things should be divided into worlds, not in the underlying physics. MWI advocates blame this fuzziness on MWI detractors and vice-versa, but it's primarily terminological.

Philosophical Quibbles

Scott raises an objection about how we perceive consciousness if our brains may be nearly-duplicated in adjacent "worlds". To me this sounds like "I don't like the conclusion so I don't want the argument to be true" wishful thinking. But Scott explicitly agrees that that sort of wishful thinking is pointless. So it seems like he's saying something else, but I'm not sure what.

Edit: Fix spelling mistake in name.

Crossposts: http://cartesiandaemon.livejournal.com/779049.html

One of the reasons Schlock Mercenary is one of my favourite stories ever, is that while presenting itself as space opera fluff, it detours into some really interesting worldbuilding[1].

For instance, suppose someone goes back in time and kills a famous dictator, and changes history. We've lots of books about what it looks like to them. But what does it look like to you? Do they just disappear, never to be seen hence, nor at the point in history they were aiming for? Do you just disappear? Does you history change to be what it would be in world where the dictator wasn't born, but you were? If so, what exactly does that mean for you to "become" someone else with absolutely no continuity of existence?[2]

Are there other examples of this? The closest I can think of is a GRR Martin short story where it comes up, but isn't resolved.

[1] Although I think like most ongoing works, it is a little hampered by retrofitting the universe as first conceived into a more consistent model.

[2] Subject to the obvious qualifications of "what does X mean in time travel" means something like "how do commonly conceived and written about versions of how time travel would work deal with this question, if it all? are there any relevant speculations on what it would mean? is the question meaningful or not?" not "dismiss the question out of hand because it's pointless to speculate about how fictional physics would work." If you think it's pointless to speculate about fictional physics then don't, but I hate to break it to you, there's a whole genre of fiction about it, and a respectable science too.

I enjoyed Temeraire very much. "Napoleonic war with dragons" says it all, if you don't want to read it now, you never will[1]. However, it is carried by this premise: it's of calibre of fan-fiction.

So what are some people smoking? Go on, I dare your blood-pressure, go and look at the amazon reviews. "For the above reasons [fantasy being an overcrowded field, with lots of Tolkien plagiarism and lots of juvenile nonsense about Elves and Wizards] I am not a fan of Robin Hobb, and to liken Novik to Hobb is to demean Novik in my opinion."

Now, all good/great books are heavily flawed, and as I said, I enjoyed Temeraire. And books that just aren't good I just ignore. But I don't see how you can pretend it's well-thought out. And Hobb isn't perfect either. And I'm very very far from perfect. But things that other, good, authors, including Hobb, do and Novik doesn't:

* Characters. People who have motivations and opinions, and you care about. I like Laurence and Temeraire, but it's not like they had any dimension, and the other characters barely existed. I often wanted to slap Fitz, but I liked him for the same reason, and all the characters in that world were interesting.

* Eg. Celeritas, the the dragon training master. Right, great idea. And I like him. But personality, anyone? If you lived in a world where dragons ruled, and most humans were partnered with a dragon to be useful in war, and humans were bread to love their partners regardless of abuse, and you were in charge of a training school, with a free hand and authority over everyone in it, and one dragon consistently neglected his human, would you

(a) provide basic care and cleanliness to the human, and forbid the dragon from preventing other humans and dragons from helping and making friends with him?
(b) not?

Fine, not allowing someone else to do (a) can make sense, military discipline, etc, but for God's sake, do something! Don't make the protagonist run round your back to do the right thing.

* Any sort of suspense or doubt. I'm sure it's a spoiler to tell you that Temeraire turns out to be the most bestest dragon anywhere. If it weren't obvious from the first chapter. All the various risks, problems, danger, etc, just go away in a few pages, and it's obvious they're going to do so.

* I *like* fluff, but fluff isn't where you easily save the world. It's where, with much difficulty, you win a talent contest.

* Science. An appendix has a stab at explaining how dragons work, and all the various breeds, for which I'm grateful, and is written very well. It's superficially plausible, in that if you squint hard, you could imagine it could be like that.

But anyone who attended school to the age of nine or beyond should be able to spot the hand-waving. It's asserted that a large dragon might have an average weight of 10 tonnes, but have more like 30 tonnes of flesh, offset by air sacks containing buoyant gases. Which is a good idea.

But that makes 20 tonnes of buoyancy. Flesh has a density ~ 1000kg/m3 and air of 1kg/m3, being the maximum possible buoyancy. A reasonable proportion for the volume of a dragon devoted to air sacks might be 50%, but that implied is 60,000%. You'd think someone would notice.

[1] That's hyperbole, but probably a fair and accurate description: some of you won't want to read it now, and won't ever want to read it. See the next paragraph.

This came up the other day and I realised I didn't know. Do birds tack into the wind as boats do? I assumed not because:

* I couldn't see why it would help. A boat tacks because a keel at an angle into the wind can grip the water and have the wind push it sideways, but into the wind. A bird doesn't. If the wind were completely steady the bird would be just like in still air but being translated. I think if a bird wants to go into the wind it's going to have to flap.
* That's not how I've seen birds. They wheel about, but generally when flocking, I assume to coordinate and to evade predators; they don't seem to zigzag.
* No-one ever said they do and I couldn't find any citation

But I thought I should check. Do you know?

Many science-fiction/fantasy stories have a ghost or two, and it's often unclear what they can do. I've attempted to research the basic laws which govern such beings so everyone can make use of them.

* Ghosts rest on flat surfaces. The most likely explanation for this is subconscious direction from the ghost. In the comic 1/0 (which has caught up Terry Pratchett as the story providing the most handy-metaphors) ghosts naturally assume the shape they had at death (or possibly the most representative point in their life) and hover over the ground, but with concentration can change their shape a bit or a lot and move about.

It also explains a ghost's body, which is invariably quite like a real body, but doesn't actually have (even ghost) chemistry and physics going on inside.

The theory is almost entirely consistent, the only problem being it doesn't feel like a satisfying, physics, explanation, as you can justify almost anything like that.

* Another hypothesis is that ghosts have inertia. If they remained stationary, it would be meaningless because stationary only has meaning relative to something. If they continued in a straight line they'd drift away, within 5m for about 10 minutes, and gone in an hour. If they followed gravity naturally, they'd fall into the earth. However, if they had a tinier attraction to gravity, it'd be basically undetectable, but they'd orbit along with the rotation of the earth.

I don't know how this would interact with artificial gravity, or warp drive -- Geordi La Forge going up in a Turbo-lift in StarTrek isn't particularly explained, nor even going up stairs.

* Or that other things have a normal effect on ghosts, but not ghosts on other things. This breaks "Every action has an equal and opposite reaction", which I'm not sure if it makes sense or not. But then ghosts would feel but not exert gravity. They could be supported by earth and car seats and things, and blown by winds, and hit by people, but not pick things up, nor walk through things.

* There is often some unreliable telepathy, with people picking up thoughts from the ghost. This sometimes has to do with emotional intensity, but basically, is at times unconnected apart from plot need.

* Ghosts can't affect other objects, except possibly as times of strong emotion. Sometimes time of day is important, too. They can generally only blow things about, often small things, but occasionally large things. Some ghosts can actually write, though normally only at the end of the film.

* However, they can't do this in the presence of anyone acquainted with scientific method or scepticism. In front of people who think it's the wind, or who see ghosts in every kitchen, they can do all sorts of things. In front of someone who says "Here are two candles. If you can blow out the one I designate first each time I light the pair, I'll believe in you with 99.9% confidence" they are entirely powerless, and merely gibber invisibly. (To be fair to Piers Anthony, he did treat this sensibly, if misogynistically.)

* There is normally an arbitrary cut off in sapience which decides who can become a ghost, of whom a random selection will. It's often dependent on state of mind, eg. unfinished business, etc.

To me this is evidence that sapient consciousness has a special place in that universe, but sometimes there is an unexplained rationalistic explanation.

In some canons, inanimate objects become ghosts but fade almost instantly (thank you, Terry Pratchett, for having more consistency in comic fantasy than in many 'serious' books :)) In Night's Dawn trilogy, it's interesting: you can have electronic copies of people, who count as separate people for the purposes of making ghosts!

* Go into the light!

There are many different conceptions of time travel (including of prophecy) in fiction. Most start with someone's gut reaction of how it should work, and then someone explains laws of physics which would produce that effect.

* There is one universe of which the future is fixed and normally but not always unknowable. This covers everything descended from Greek myths like Oedipus, that assumed prophecies were fixed and likely to come about due to efforts to prevent them.

* Whenever you change the past, the future evolves naturally from there, but you continue to exist from that point onward even if there's no past to explain your existence. This covers every story where someone goes back in time and treads on a butterfly, and discovers all society is different.

* You can change the past, but cannot make a paradox.

* You can change the past, but if you make a paradox you slowly fade out of existence. As in, Back to the Future. This doesn't entirely make sense to me -- why should the change propagate at a definite rate? And if you disappear, does the change remain? But I admit it's at least self-consistent.

* Self-causing loops can't exist (in any sort of universe).

* Self-causing loops can exist (in any sort of universe). Eg. Oedipus. If the universe is changeable, you must be able to switch from one to another. (Eg. someone in the far future events time-travel, goes back in time, and helps people invent it earlier and earlier, but it's always invented. Unless someone erases the loop and it's never invented.) There may or may not be loops which could never have evolved from a linear timeline.

* The past can be changed, but some events are very likely to happen anyway.

Question 0 These last two tend to describe the Terminator films. I don't know why everyone says their time-travel is crap, as far as I can tell: they assume anything with Arnie in is wrong about physics; they object to the fact that the characters think they learn more about time-travel physics as they experience more of it, rather than being right from the start; they object to self-causality; they object to moralising.

Two more questions:

Question 1 If you have good reason to believe that your future can't be changed, and you see [relative X] die, why do you always try to prevent it by making them be elsewhere, etc? If you set out to *fake* the death you saw, then you know what you're doing is at least possible.

Question 2 In much fiction, people's intuitive idea is that if you travel back in time beyond your own lifetime it's like normal, but if you go to your own lifetime, you appear in place of your body there. It obviously makes sense on some level, but can anyone give a physics explanation which would fit it?

1. What is wind chill temperature? A human in cold air in a wind feels as cold as a human in colder but still air. We have fitted some approximations to this and come up with an official scale, but it was originally based on subjective judgements, and depends on other conditions as well, eg. humidity, so isn't definitive, but is a useful measure.

2. How a human feels doesn't really have meaning when you get near *that* cold. Instead death feels much like instant death :)

3. However, you should be able to create a standardised measure, right? Have object X at temperature T K in medium Y at temperature 0.x K and pressure Z, flowing at speed v. Establish the rate of heat loss at that moment (which is at least theoretically calculable). Define "wind chill temperature" to be the temperature in still medium at which the rate of heat loss is the same.

4. Could X lose heat faster under some speed than in still near absolute zero medium? I don't see why not. Physics is weird down there, but it can still heat up the surroundings, etc.

5. Does that make a negative Kelvin wind chill? On the one hand, it implies a wind chill colder than absolute zero. On the other hand, it doesn't actually define a wind chill at all because there is no temperature to compare it to. If you have a nice non-asymptotic graph you could extend it, but does that have any meaning?

Last saturday, I helped at Crash Bang Squelch, the CHAOS science week fair.

It was fun! I was demonstrating the bubble column, a big tall vat of water with air pumped in at the bottom, where you can see how little bubbles are round and big bubbles are flattened on the bottom with tendrils like Jellyfish.

Unfortunately, the physics behind this is fluids, so I read up on why it happens, but I didn't truly understand it, in the sense of knowing what would make it so and would would make it not, and possibly no-one really would. Understanding something is developing intuition for it, and having stories to compare it to things you're used to. But sometimes there is not good comparison, and I think wings are a bit like that: there are explanations, but none really capture what's really going on.

The kids were lovely, however. They were generally forward and polite, or shy. But easily taking turns, and not suffering from anything worse than overenthusiasm. I adopt the strategy of asking the eager one to show the next one, as you can't get it very wrong, and many seemed marvellously eager to help :) And some did seem interested, I think we may have made some new converts to science.

The biggest problem was every two minutes someone shouts "JACK! Be careful!" and I look round, and then realise that I'm ahead of the trend, and everyone under 10 is called Jack, so no longer can I assume anyone saying it is talking to me.

After we went to the postparty, where I met many nice people, and there was much gin.

On sunday I relished having a bike, read a lot of Confusion (the second volume in the increasingly accurately named Baroque Cycle trilogy), played a few interesting shareware games, did a lot of washing and up, etc, had a lie in alternating sleeping with drinking lots of water, and did some organising of the cts fresher's fair.

Is there a name for genres that distinguishes between action puzzle games like Tetris, where everything is in the game rules, and action puzzle games like Repton, where the games rules share importance with level design[1].

Anyway, I was playing some old and some new interesting games of the latter sort, which is what I like to write. Though the former has the advantage that it's as fun for the programmer to play as anyone else.

There was also Gish. Chronic Logic made a number of interesting physics-moddelling games, like Bridge Builder/Pontifex, which are great fun to see things modelled well, but a bit lacking in *gameplay*.

Gish is great. You control a blob or tar which is genuinely malleable. You can make it sticky, or firm and heavy, or slick, and everything happens in a truly malleable fashion. In short, they obviously modelled things at a lower level than normal, and much of the control is emergent behaviour. There isn't a "throw" or "pick up" button: you roll over a block, go sticky, roll until it's on top, squish down, and expand.

[1] An early incident. In a history class, the teacher said foo wasn't the only effect, bar was also necessary for the thingy, and in driving this home -- most people would assume foo was the major one -- said the thingy was a sum of foo and bar. I immediately queried -- surely product would make more sense, since if either foo or bar were zero, nothing would happen.

OK, I tend to take metaphors too far. But on the other hand, it is unhelpful to choose metaphors deliberately bad in the way you wish to emphasise. Speaking of which, why is "How high?" considered a good response to "Jump!"? Surely that's way too peedantic? I suppose the message is there on another level -- don't say 'how high' just because Sarge said to say it, just do it, but it seems peverse. I am amused at the vision of having got your paratroopers lined up in the plane, passing over the landing field, and suddenly having a literal reply to "Jump!" :)

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