Meanwhile, back at the batcave,…
[This Batcave completely unrelated to the 50th celebrations of the airing of Dr. Who. Do you remember the 23rd November 1963?]
For reasons that are pretty incoherent I have been thinking about time machines. I like time machines. I have a collection of time-travel movies on my laptop, which I may turn into a web site or something one day. But meanwhile, I have been thinking not so much how they work – just dig a few quantum tunnels through the wormholes in spacetime and Bob’s your uncle. And nephew. And grandson. No, I have been pondering about where the heck you land.
This is not as simple as it seems. It is all right for Christopher Lloyd to run a De Lorean along rail tracks in 1885 and expect it to translate through time to land on the same rails in 1985. Seems a bit implausible, though. At very least continental drift or earthquakes would have moved the rails round a bit. In ‘Primeval’ (a UK TV series on Channel 3 a little while back, that seems (ironically) to have run out of airtime) the intrepid Professor and his side-kicks come across a glowy sparkly portal thingy standing (stationary) in the middle of the Forest of Dean that gives a gateway back to a (stationary) spot in the Permian. You mean, no change in land level? Wales was moving West-to-East about 30 mph faster in the Permian than today, as the tides have slowed the Earth’s rotation in the intervening time. Wouldn’t the sparkly portal thingy deposit the (on-foot) travelers on the Permian Welsh hillside and then leg it over the horizon? 
Let’s find out!
Professor Cartwright stands at the helm of his time machine, built in his garden shed on the profits of his global exam plagiarism empire. He is loved by exam-takers the world over almost as much as by his beautiful daughter Jill, who sits in the co-pilot seat with her blond hair cascading inconveniently over the controls. But Jim, plucky quarterback with the muscles and stamina of an ox in the high-school team of whatever sport it is that has quarterbacks, only has eyes for her, which was fortunate as he also has the IQ of an ox and can only stand by and admire his brilliant paramour’s skills with all that complicated science stuff.
“Well, chaps, this is it!” said The Prof. “Strap in, all set for AD 3000. Let’s see the future!”
With strong, firm grip he slams home the defrogulator, Jill fine-tunes the scroop, and they are outside time, no longer interacting with the Universe.
The time machine is a complete nuisance to city planners for the next 1000 years, as, because it does not interact with time, it sits there as an immobile and impenetrable shed-shaped mirror. When time re-asserts itself in 3014 AD, the organic material inside has been metabolised by countless generations of intestinal bacteria to a thick, brown, extraordinarily smelly sludge. The Prof’s inimitable half-moon glasses, and the Time Helix of course, are the only solid matter left. The mirror flicks off, the sludge immediately pours out over the ground of Urban Complex 17, and the Prof’s great-to-the-32nd grandchild gets hit for the cleaning bill.
No, no, that is not what we mean is it? The whole point is that time travellers do not spend 1000 years inside the machine to travel 1000 years. Or rather, spend 10 minutes inside, then suffocate and have their own bugs fun it up for the rest 999 years, 364.9 days on their putrefying corpses. Try again, Prof.
“Well, chaps, this is it!” says the Prof, firing the Transvaeble Field. “Strap in, off we g”, and the time helix wraps itself round the 87th dimension, and takes the time machine out of communication with space-time. All movement stops, and the next thing the Prof knows it is 3017AD.
“o!”, just before impossibly steep gravitational waves rip him and everything around him to bits.
Oh, yes, the problem is one of gravity here, or logic. Basically, either the inside is isolated from the outside, or it is not. If it is isolated from the outside, you can get free energy out of putting a big lead weight on the floor, turning the time machine on, and then turning the frozen shed+weight+time machine upside down, then turning the machine off and letting the falling lead weight, say, run a dynamo or crush beetles or something. If that is a naughty breaking of the rules, then the outside can identify the distribution of mass of objects on the inside, in which case the outside must interact with the inside in the same timescale as movements on the outside. Tides do not mean much to you day-to-day, but 1000 years of tides crammed into one microsecond would probably turn you into Spam.
But anyway, that is not a time machine either. That is just a boring old humdrum stasis field. We want a machine that sort of lifts out of space and time and kind of puts itself down sometime else type of thing, not that just travels forward in time at a tedious one day per day. Let’s try again.
“Off we go!” cries The Prof, with slightly less enthusiasm, and open the valve to the pistons. Jill lets in the clutch with a delicate touch, and the garden shed vanishes! In a wild flash of light and exclamation marks!! Inside it is as if no time has passed at all, but in the outside world 1000 years go by. Urban Complex 17 rises (luckily leaving a space where The Prof’s shed used to be), and in 3017 the machine pop’s back into existence.
“Oh fuck” says Jill delicately as the air rushes out of the Time Machine spinning in the darkness of interstellar space, shortly before she expires a bit more than a light-year from Urban Complex 17.
Yes, the Prof’s garden shed is not actually stationary, is it? The Earth is spinning, orbiting the sun, which is orbiting the galaxy, which is moving in the general direction of the constellation of Leo at 369km/sec. The Prof takes his shed out of spacetime, and then puts it down again in the same place but a different time. Meanwhile, the Earth has moved.
But originally the time machine was moving with the Earth, wasn’t it? So it should carry on in its state of rest or uniform motion until it hits something. Let us skip lightly over what we mean by ‘uniform motion’ when you are outside time and try again.
“OK, strap in, let’s try not to die this time.” says the Prof, wearily, and pushes the Big Red Button on the Big Grey Machine. With the sound of a thousand plungers unblocking a thousand toilets the machine vanishes from the Universe and follows a straight line for what, for want of a better term, we will call 1000 years.
The explosion is really quite big this time, but as it is 6300 astronomical units away no-one notices
This is even worse on the ‘energy for nothing’ front than the stasis field. Orbital velocity is an amazing 65,000 mph (578 million miles in 365 days), and you just fly off like a raindrop off a bike wheel. You end up far, far away from the Earth and the Sun, and (to the point) far away from their gravitational fields. The only way to conserve energy is to punch your own sun+earth deep gravitational dip in space somewhere out where the comets live, turning yourself and your time machine into a small black hole. Thank you Stephen Hawking for the resulting flash of gamma rays.
Not logical. Shouldn’t the time machine follow the geodesic like anything else, but without interacting with anything? Passing quickly over what you mean by ‘follow’ in a time machine … .
“Sod it, let’s try just one more time.” says Prof.. Jill gives him a disgusted look, but slams home the huge bronze knife switch that sends current surging through the toaster, and they dematerialize into sub-over-hyper-space. In 3017 it appears 1000 miles underground and traveling – well it does not matter how fast it was traveling because of the annoying unwillingness of the mantle rock to get out of the way. A modest earthquake under Urban Complex 17 shakes the Cartwright Institute of Implausible Engineering.
Yes, well, if you just leave something in a gravitational field it likes to fall. And if it refuses to acknowledge that the Earth is there to stop it, it does fall, in this case back and forth through the gravitational well that the Earth creates as it swings around the Sun. It would take quite precise timing to make sure that you materialised just at the ‘top’ of your orbit, outside the Earth.
The Prof was quite upset when Jill said where he could stick his rocket, but Jim helped load the garden shed into the payload bay and get it into orbit. And withstood his muttered curses of “OK, one more fucking time” as he pressed the space-bar and send them into several another, rather dull dimensions. So – falling along a geodesic that conserves energy and does not intersect anything annoyingly solid. What could go wrong?
What a silly question. But this version is at least is not logically inconsistent. Exactly where the garden shed emerges is another matter, and what happens if, for example, the Sun’s galactic orbit is not perfectly circular, or the galaxy falls further towards the Great Attractor. But let us be grateful for at least some limited consistency. Accelerate to orbital velocity. Keep to the same gravitational potential. Hope the planetary defense systems in the year 3017 do not shoot you out of the sky when you re-appear in orbit.
Which just leaves the question of how the ‘travel to parallel worlds portal’ works ...
 Yes, of course that is an old joke, but it was brand new when H. G. Wells brought it back from the neolithic.
 And what was that little flying bugger meant to be? It had three pairs of limbs. Three! Two pairs of legs and a pair of wings. What the f*** sort of vertebrate has three pairs of limbs? Bah! (At least they did not have grass on the landscape – be thankful for small botanical mercies.)
 Yes, of course that is an old joke, but it was brand new when Prof Cartwright brought it back from the early Eocene.