My sweetheart got sick on my birthday (yesterday), and as a result we couldn't share the special birthday cake he had made. So today I made a time machine out of a cardboard box while he was at work. I drew on pretty spirals and dots, and I put yarny, stringy, hanging bits in it. I made sure to write "Time Machine" on it, so everyone would know what it was. It isn't done yet, but soon I will put an arrow on that can point to "forward" or "backward" (that way you can be in charge of whether you go forward or back in time). When he comes home, we can use the cardboard box time machine in order to go back to yesterday and share cake on my birthday.

I wouldn't want him to miss my birthday cake. It only comes once a year, and really, when you think about it, this specific birthday cake will only happen once, ever.

Maybe I can use it to go forward in time before he gets home and make sure it goes over well.

There will be, at most, two types of time machine. The first one, and the best one. This is because anyone dedicated enough to actually build a time machine can simply go into the future and find the best form of time travel ever made.

The first set of specs on how to build a real time machine were drafted in the 1970's by an Engineering student. They involved creating a closed loop of time (which, while tricky, is theoretically possible according to relativity) by using an infinitely long cylinder hung in space. Of course, the only purpose for building a time machine out of an infinitely long cylinder would be to go back in time and tell yourself not to waste the effort.

In a fit of logic worthy of a Douglas Adams novel, physicists then announced that it might be possible to use a cylinder that was only 1 kilometer in length, if it was made out of a superdense material. Material so dense, in fact, that the cylinder would collapse along its long axis before it could ever be completed.

And so the world continued to wait for a viable time machine, which may be the only thing really worth waiting for. It would, if you think about it, be the last thing you ever would have to wait for. This is why television has been so popular since the 1970's.

Thankfully, just such an alternative has appeared on the horizon thanks to the research of one Ronald Mallett.

Despite not having any mass, light still bends space. Mallett explained how, were light to be refracted and reflected to form a ring, it would create a spacial vortex within the circle. Then came the eureka moment: Time could be bent in the same way.

Time, being somewhat stickier than space, would require a second light beam, sent around in the opposite direction to the first. However, if the light were intense enough, time and space would swap places. This means that what an observer would sense as time would actually appear as a spacial direction to one inside the ring of effect. Walking in the correct direction, it would be possible to step out of the ring before one had actually entered it.

The titanic amounts of energy required for such a feat are daunting, but, as luck would have it, not actually that tricky. As light is slowed down, it gains inertia, and thus energy. Conveniently, Lene Hau of Harvard University has managed to slow light down to a few meters-per-second (a bit slower than the usual 300,000 kilometers-per-second light prefers to move at).

Of course, slowing light requires firing it through an ultra-cold bath of atoms called a "Bose-Einstein condensate," just a few degrees over absolute zero. Still, figuring out how to deal with temperatures that low will probably prove easier than punching wormholes through the fabric of the universe.

The current plan is to test this whole mess by constructing a ring and putting a single particle in the middle. Hopefully, as was the case in Back to the Future II, if the particle moves through time and meets itself, the universe will not end.

Further, none of this takes quantum mechanics into account. Quantum theory suggests that a time machine would magnify quantum flux to the point where it would form an intense beam of radiation, destroying whatever was passing through the ring, the ring itself, or both. However, this may not happen at all, since proving it would require a theory of quantum gravity, which would unite quantum physics with relativistic physics, which would be pretty sweet in and of itself.

The only other problem with the "temporal loop" method of time travel is that, using a given ring, it would only be possible to move back in time as far as when the ring was first turned on. This means you couldn't go back in time and hang with your favorite religious figure, kill Hitler, Beethoven or Stalin, save Jim Henson, Edward Gorey or Douglas Adams, or adopt a pet dinosaur. It also means that were we to build a fixed gate, we would immediately be bombarded by an uncountable number of time tourists, wanting to see how it all began.

New Scientist magazine for 19 May 2001 (ironically, the day after I made this writeup).
Continuum: Role Playing in the Yet (Yes, it's a game, but they researched time travel theory well).
The Back to the Future trilogy from Amblin Entertainment and MCA/Universal

Since no one has seen fit to include the obvious..

A likely first time machine, and perhaps the only possible variety, will look more like a television than a car or a telephone booth.

It is much more practical to view the past and the future than to visit. Any correct theory of the Universe, including the Big Bang, that describes a beginning with definable parameters allows the creation of such a time machine.

If you can determine the state of all matter in the Universe at any one moment (such as the beginning), and you understand how all matter interacts (gravity, fusion, lepton decay, etc.), then you can input this information into a supercomputer, press the Play button, and watch the Universe progress from the very start.

You would, naturally, be able to jump around from any time to any other time. Watch the dinosaurs go extinct, see who killed JFK, and what happened to those Anasazi again?

A big plus to a viewing time machine is the lack of paradoxes. The machine is naturally self-aware and aware of the viewer: their matter is subject to the same interactions as all other matter. The past is not influenced at all by the viewing. The future that the machine shows you is unchangeable-- that you used the machine to view the future, and any actions you took as a result of that, have already been included in the output you're viewing.

If while using the machine you had seen your best friend killed in a car accident, for example, and told them not to drive that day, the machine would show you watching it, seeing your friend die, telling your friend not to drive, and your friend surviving. There is no paradox there at all.

A simpler and more accurate example, however, would be a vase sitting next to the machine. In 10 seconds a car is going to backfire outside, causing the vase to fall and break. You are using the machine to view yourself 10 seconds in the future, so you see the vase fall and decide to catch it as soon as you hear the car backfire. The catch here is that you didn't just see the vase fall, you see yourself watching the vase fall, deciding to catch the vase, and catching the vase. You can take this to tens of levels of depth in your mind and on the monitor (like looking into a mirror with another mirror facing it on the opposite wall), but at the end of the 10 seconds you will have made a decision whether to save the vase or not-- and this is what the machine will show. You can see then, how events 10 years in the future cannot be changed either; all of your actions from now until that point 10 years in the future have already been accounted for.

So you can, in fact, alter the future that would have happened if you hadn't used the time machine, but you cannot alter the future the time machine shows you. Complex, yes. Paradoxical, no.

How, exactly, can a supercomputer, composed of a tiny percentage of the total number of particles in the Universe, keep track of all of the particles (including photons, gravitrons, etc.)? The principles for such a feat are already in use in today's programming: shortcuts, workarounds, and dirty hacks. You could, for example, mark off entire cubed light years as "vacuum" or entire cubed meters as "hydrogen". You could treat most atoms as indivisible most of the time, and only track subatomic particles when needed. While there would undoubtedly need to be thousands or hundreds of thousands of such hacks, it is definitely within reason to assume such a machine could be built.

And of course, with the right VR equipment and enough computing power, you could even play along. Let's get started.

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