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.

---

Sources:
New Scientist magazine for 19 May 2001 (ironically, the day after I made this writeup).
also: http://www.newscientist.com
Continuum: Role Playing in the Yet (Yes, it's a game, but they researched time travel theory well).
also: http://www.aetherco.com
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.

Dr. Johnson pumped his fist in the air and shouted out a loud 'WAHOO!' to no one in particular. The noise echoed off the walls of the office; Johnson couldn't help himself from imagining the air molecules vibrating and bouncing and careening around the room. He was just that kind of guy. Some of those tiny particles would no doubt find their way into coworkers' eardrums, leading to surprise (short term) and gossip (long term). But Johnson didn't care. The calculations were in. Johnson feverishly triple-checked everything, and...yes. There could be no mistake. None at all.

He had just invented a time machine.

Another 'WAHOO!', louder this time, full of the pure glee Johnson hadn't felt in decades. He heard, but didn't comprehend, any words said to him the rest of that day. He couldn't sleep that night. The far-off realm of science-fiction and daydreams, was here. It was real.

The only thing missing was a suitable test subject. But Johnson already knew who that would be.

---

Johnson trembled like a bridegroom walking to the marriage bed. He couldn't unlock the door to his lab. His fingers kept slipping and dropping the keys, or jiggling them and losing grip. He was deathly afraid that they'd snap off in the lock. That would be a treat: a call to the locksmith and then a call from his fearsome boss about just what the fucking hell was Johnson doing, sneaking across The Field and into the laboratory on a weekend like a goddamn undergraduate trying to cheat before the practical exam. Behavior unbecoming of a professor, goddamnit man, are you a scientist or not? But Johnson would not be so unlucky. The key turned: the door opened. He stepped inside, threw his coat on the hanger, and stepped onto the laboratory floor.

The time machine was a briefcase. Even smaller: it fit inside Johnson's briefcase. As Johnson ran some final tests he tried to calm his weak hands and trembling knees. All the calculations and simulations worked out right, he told himself. There wasn't even a chance of a Grandfather paradox or mucking about with the timeline. I proved the inability to change history, he told himself. Johnson's Temporal Reconvergence Law. It was in all the theoretical physics textbooks now, as an interesting, if useless bunch of splendid mathematics.

Kind of funny how damned important useless mathematics can become, thought Johnson. He dialed the date into the side of the machine and waited. Just a few more minutes now...

All three lights on the top of the briefcase's handle turned green. This was it. His index finger hovered over the 'Activate' button. He wavered. Wait, shouldn't he do some more tests, maybe a final run-through before...fuck it.

Johnson's world exploded into a thousand points of light, streaming out into space, streaming out into the unknown.

---

Johnson blinked. Then he looked forward. There was the school, alright. But that didn't prove anything; the school still stood in Johnson's time, and the main building looked exactly the same back then. He jogged over to the right and collapsed on his knees. There it was. The playground they'd knocked down three decades ago for that extra middle-school wing off to the side. It stood there, glistening in the morning dew. Pristine. He saw the slide, the three crawl-tubes, the climbing-net, the best swing ever made by man (third from the right, of course, just as he remembered...). And suddenly, Johnson lost it. Right there, on that playground, the 50-year-old scientist cried and cried and cried until his face was full of sweet tears.

"Hey, mister, what are you doing here?"

Johnson peaked his head up suddenly at the voice. The speaker was behind him, maybe ten feet away. He turned and looked to discover a frail-looking six year old boy, clearly perplexed by the sight of a bawling old man on his playground.

The scientist stood up and looked at him. "I'm sorry for the little show you saw here. Sometimes grownups get sad and cry, just like you do. What's your name?"

"Jason." Johnson already knew that, of course.

"Well, Jason, you look like a bright kid. This your playground?"

The kid looked up at him and smiled. "Yes! Yes-yes! I always sneak out early and go play here before the big kids come. They always push me around and don't let me get my favorite swing and I get sad and mad but if I go here and swing before them, I don't get sad and mad anymore." The words tumbled out of him at machine-gun velocity.

Johnson smiled. "Well, don't worry about them. They're stupid and not cool like you anyway, you remember that, alright?"

Jason smiled. Johnson wanted to tell him so much, wanted to tell him that it's alright, you'll get through this, you'll end up this on the other side with most of your dreams intact. Not all, and you'll surely suffer along the way, but you'll change the world! You'll make your sick mother proud before she pours out her life with your tears in that damned hospital bed!

But he didn't. Johnson's Temporal Reconvergence Law meant it wouldn't matter. So he just smiled.

Johnson said: "I have to go now."

"Hey, mister, before you leave, want to know how smart I am?"

Johnson smiled again. "Sure."

Jason gave a mischievous wink and, staring the scientist in the eye, said, "I bet you don't know how sound works, mister."

Johnson shuddered inside, but kept the smile on his face as tight as he could. "I surely don't. How does it work?"

Jason broke into a wild grin and the words started tumbling out of him again. "Well you see there's these little pieces of air, called molecules, and they bounce and vibrate around when you talk, and then eventually these air molecules hit your ear, and your ear hears them because they're vibrating!"

The three lights were green. Jason and the rest of the playground burst outward and sliced themselves off, trailing into wisps. Johnson's body moved impossibly fast towards the horizon, and all was blackness.

---

Johnson looked up and saw the interior of the dusty lab. He looked to the back entrance and saw the safety glass. 'In case of fire, Break!' He shoved his fist through it, took the axe out, and threw the briefcase on the floor. He didn't stop until he tore that wonderful time machine into twisted pieces of metal and plastic. For a split-second, Johnson could see the shards reflecting a thousand points of light, weaving a tapestry that connected past to future. But maybe he only imagined it.