Time Travel Does Not Work Like That

Whenever a discussion thread turns to time travel, somebody will always raise the challenging question: If you jump forward in time, say by a week, shouldn't you reappear in outer space because the Earth has continued to move and is now seven days further along in its orbit?

And I always feel obliged to answer: Time travel does not work like that.

This conception of time travel typically imagines a time machine as a box of some kind that jumps in time but stays in the same place. And written into that conception are some assumptions that mislead. One is the assumption that you can "jump" discontinuously from one point in time to another. A second is the assumption that movement in time can be considered separately from movement in space. And a third is the implication that the expression "in the same place" is well-defined: as Relativity taught us, we have to ask "the same place relative to what? The Earth? The sun? The center of the galaxy?" (and once we answer that question, we realize that we have pretty much answered our own conundrum.) To properly understand what should happen in time travel, we need to overturn each of those assumptions.

So as far as we know* there is no possibility of "a box that travels through time but doesn't travel through space". It's not just hard, it's an oxymoron -- although in order to understand why, you have to teach yourself to think about spacetime properly, instead of thinking of space and time as separable.

So we have to take a few steps back, discard the presumption that such a box exists, and ask what an actual time machine could be like. And it turns out that (realistic) time travel is a characteristic of a region of spacetime, not a characteristic of a particular machine.

The first thing to understand is that nothing ever moves through space alone or through time alone**. Everything is moving through spacetime, all the time (so to speak). And that movement is always continuous. It simply doesn't make sense to say that a particle X moved from time t1 to time t2 without specifying space coordinates too. The only thing that makes sense is to say that X moved from spacetime position (x1, y1, z1, t1) to (x2, y2, z2, t2). This is so basic that, Relativity shows, even basic concepts like "before", "after", and "simultaneous" are not well defined if you don't specific both time and space coordinates.

The second thing to understand is that physicists have dreamed up lots of ways that time travel might be possible, and all of them involve a continuous path through spacetime, not a mysterious jump. The trick is, the spacetime curves in such a way that when you return to the same coordinates in space, you arrive at a different time from, say, an identical twin who stayed home. However, at no point do you perceive anything "weird" or discontinuous happening. Think of it as being like an extreme version of the famous Twins Paradox from Relativity. In that "paradox" you leave home with a clock that reads 2pm, travel in a big loop at almost the speed of light, come home, and your clock now reads 3pm while an identical clock that stayed home reads 4pm. (This happens all the time, on a smaller scale, right here on Earth: the GPS satellites you rely on for satnav have to correct for the fact that their atomic clocks drift slightly from identical clocks on the surface of the Earth). If the spacetime path you follow is sufficiently warped, when you get home your clock might read 3pm while the one on Earth reads 1pm -- congratulations, you've traveled an hour into your own past!

Of course, we should also note that when you return from your big loop you might be a little surprised to discover that the Earth isn't where you expected it to be, it's actually a little earlier in its orbit from where you would calculate according to the clock that traveled with you, so you have to adjust your path in spacetime a little to navigate back to it. But notice there was still no "jump" involved: you traveled away from Earth, you traveled back to Earth, and found something unexpected, because the paths you and Earth followed through spacetime put your clocks out of sync.

Note also that I pulled a little sleight of hand in that paragraph. When I said "where you expected the Earth to be", I should really specify relative to what for the sentence to have any meaning. And what I really mean is "relative to where you would expect it to be if you had traveled at low speed through flat spacetime and your clock had kept time with Earth clocks". This is kind of a subtle point, but it is at the heart of what's going on here. Your clock tells you where Earth "should" be, and relative to your clock Earth has moved "out from under you" -- but that is only because you assume that you and your clock traveled in flat spacetime. If you could look back with a telescope and watch the Earth the whole time you were on your trip you would see nothing odd happen, other than that it's position drifts slowly more and more backwards from where your clock says it should be. But at no point does Earth blink out of view and reappear further back in its orbit, the way a "time jump" is typically depicted in sci-fi.

So really, when I say "return to the same coordinates", I need to specify relative to what. If I "return to the same coordinates" relative to the Earth, it's right where I expect. If I choose some other point of reference relative to which the Earth is moving, it's not.

By the way, one of the first people to demonstrate rigorously that this kind of thing could happen in General Relativity was Einstein's good friend Kurt Godel. He showed that if the universe is rotating and sufficiently large, you could follow a very long loop around the universe and return to your starting point at an earlier time. Reportedly, Einstein was quite upset by this.

Now, when we see time travel in TV or movies, one way to think about it is to assume that the device is creating its own region of curved spacetime that is extremely small and very severely curved. I like  to pretend that the time travel in 7 Days works this way: the device creates a highly distorted region of spacetime around the capsule. The capsule travels in that highly distorted region which takes it both into space (relative to the Earth) and back in time (again, relative to the Earth). It then emerges from that distorted region and navigates back to Earth traveling in our (relatively) flat spacetime. But rather than "jumping 7 days into the past", I think of it as "traveling 7 days into the past and elsewhere in space".

Another way to  think about it is this: the most consistently realistic depiction of time travel in fiction is wormholes: a wormhole connects two different points in both space and time. If you had a pair of wormholes -- or a single wormhole that connects two points close to each other in space but distant in time -- you would effectively have the classic time machine. And in fact, the wormhole is simply an extreme*** example of curved spacetime. 

I have an even longer answer than this that also explains why we haven't met time travelers from the future yet, but it runs to several pages, and I'm saving it for the book I'm never going to write, "Physics for Smarties: an essential math-free guide for curious arts and humanities students".

Anyway, I hope that helps a little bit, and if you take away nothing else, remember "continuous path through spacetime" and "doesn't travel in space -- relative to what?"


Footnotes


*And yes, I readily concede that "as far as we know" is not very far, but the fundamental character of General Relativity is very, very suggestive on this point.

**OK, you could imagine such a thing if you wanted to, but we have no idea what physics would describe it, so basically you could decide how it behaves completely arbitrarily. It does whatever you want it to do. You just have to bear in mind that there are no absolute coordinates in spacetime, so if you define that your box "doesn't travel through space", it's up to you define what you mean by that: doesn't travel relative to what frame of reference? In short, your time box does whatever you choose it to do, because you are making it up.

***Insert your own Wormhole Extreme! reference here.

Fulfilling work

I've been struggling for a while to figure out what makes, for me at least, a fulfilling or satisfying job. Yesterday it struck me that the answer was more or less at my fingertips in the form of a book by my friend Tharon Howard called Design to Thrive. In that book, Tharon lays out a model for how to think about a successful online community around the handy mnemonic RIBS: Remuneration, Influence, Belonging, and Significance. It occurred to me that a tweak on that model would equally well describe what I would want from the ideal job:

Remuneration: Obviously this includes salary and benefits, but beyond a certain point salary doesn't make much difference (until you are talking about start up IPO levels of Walk Away money). But remuneration also comes in other form such as respect of colleagues and industry peers, thanks for a job well done, or the knowledge that you've made a difference. In fact, the R in RIBS could stand for a whole collection of Rewards including Respect and Recognition.

Influence: It's well established that people are happier in their work when they have more control over their environment rather than feeling like helpless drones working on one assignment after another. That can mean little things like personalizing your workspace, more substantive things like flex time, and ultimately having a significant say in what assignments you are working on. This concept can be seen in so-called self-managed teams, and in the development world it's a significant (if often overlooked) element of the Agile movement. Even something as simple as believing that your opinion is taken into consideration in your manager's decisions makes a difference.

Belonging: This is a huge element of the success of online communities, and includes the rituals and stories that people tell each other to reinforce their sense of belonging to something greater than themselves. Many companies have a "founding myth" that, factual or not, helps new employees adapt to the culture. The team I'm in right now has been fairly good at this kind of thing: we have ritualized expressions ("Yes, and..." instead of "Yes, but..." is one) and running jokes that we initiate new team members into.

Significance: This one is probably the hardest to achieve when you're in the depths of a large company. I've left jobs before even when the day to day work was interesting and satisfying ("remuneration") because it felt like what I produced made no more difference than throwing stones into the ocean, that nothing I did or said or wrote would matter to anybody a month from now. 

Everybody will weight these four elements differently, but for me significance is the biggest one. I get very restless when I feel that my job amounts to no more than shuffling paper, reviewing other people's proposals to evaluate considering investigating something, or spreadsheet engineering. To be satisfying to me, my work has to at least appear to matter to somebody's real life.

Flash Fiction: Of All Possible Worlds

[Note: this story was written for the New Scientist 2010 competition Forgotten Futures, which asked entrants to write a very short story -- 350 words or fewer -- about how things might have worked out if some scientific event or discovery had turned out differently. My entry wasn't shortlisted, so I'm sharing it here.]

Hugh Everett III eased himself into the oversized leather chair, taking care not to spill his cognac. Outside, he could hear the laughter of his children, grandchildren, and great-grandchildren, still loudly celebrating the 80^th birthday of the Grand Old Man of Physics, as he had become known. The door opened quietly and his son Mark slipped inside, cradling the glass display case that held Hugh’s Nobel Prize medal. Mark reverently returned the medal, the centerpiece of the party, to its accustomed place on the bookcase.

Hugh gestured to the chair next to him where another glass of cognac waited, and Mark settled in next to his worlds-famous father.

“It’s ironic,” said Hugh, toasting the medal with his glass. “The press called me ‘the new Einstein’, and just like him, they gave me the Prize for my second-best work.” Few people outside the physics community realized that Everett’s Prize had been awarded for his audacious doctoral thesis that introduced the Many Worlds interpretation of quantum mechanics, not his later quantum gravity insights that had led to the exploitation of Dark Energy, the basis of the peace and prosperity enjoyed by everybody on Earth, not to mention it’s colonies on Mars and Ganymede.

“I’ll tell you what else is ironic,” replied Mark with a mischievous smile. “If everything possible happens in some universe, then somewhere out there is a world where everything went as wrong as it possibly could. Imagine if nobody had paid attention to your thesis and you had abandoned physics. In that world you became bitter and disillusioned, turned to cigarettes and drink, and died long before your time, leaving a world still suffocating and sweltering in its own pollution.”

Hugh laughed warmly. “Mark, that story gets worse every time you tell it. Many Worlds requires everything possible to happen, but it doesn’t allow for the impossible.” He took another sip of cognac and pushed away the thought of a life and a world gone to ruin. What ever might be might be, he mused to himself, but whatever must be must be.

The worst romantic comedy in recent movie history

Somebody recently referred to "Something's gotta give" in my hearing as "charming". Hence this rant...

The essence of any rom-com is an ill-matched couple, kept apart by some apparently insurmountable barrier, destined to somehow be together by the end of the movie. "Sleepless in Seattle": they live on opposite coasts. "You've got mail": opposing political beliefs. etc. Age, culture, distance, class, political beliefs, disapproving families: something has to keep the couple apart for ninety minutes before we can all go home happy.

"Something's gotta give" offers us the unique sight of a romantic comedy about a couple who are kept apart by... absolutely nothing.

Harry and Erica are two independent, financially stable people of similar age, both well-established in life and successful in their careers. Both are single and unencumbered. Circumstances force them to live in the same house. Erica has no obligations as her adult daughter is self-sufficient. Everybody approves of their relationship, including Erica's daughter (who might have been expected to object since she had previously dated Harry) and Erica's ex-husband, with whom she has such a comfortable relationship that he drops by for coffee. Even Harry's rival in love, Julian, simply smiles and gracefully steps aside when the moment comes, accepting that Harry and Erica should be together. There's not a shred of conflict or difficulty or incompatibility in this entire setup.

The only reason they're not a couple after the first five minutes? "He only dates younger women", the other characters repeatedly tell us, although this essential fact is something that the movie completely fails to show. We don't even get to see all of these alleged younger girlfriends until almost the end of the movie, and none of them is dignified with a name, let alone a personality, a story, or a motivation for dating him. We have to take the word of the other characters, sitting around a dinner table, that he is a desirable, eligible, notorious bachelor who was featured as such on the cover of a New York magazine. (I believe that "featured on a magazine cover" is what movies go for to illustrate that somebody or something is well-known when they can't afford the obligatory scene of Jay Leno telling jokes about their movie's subject: "How about that Harry Sanborn? His latest girlfriend is so young, he has to leave her outside when he goes into a bar!". Or perhaps there are some things that even Jay Leno won't do?) Oh, and there's also some business about her being too busy/vulnerable for a relationship, although again there's no reason offered for why.

And since the movie provides no coherent explanation for why Harry collects notches in his headboard, we also get no explanation for what resistance he is trying to overcome nor why he chooses to change. At the start of the movie, he doesn't date women his own age; at the end of the movie, he does.

Even the title of the movie falls flat. "Something's gotta give" implies that both of them have deeply-held positions that are incompatible and that one of them will have to "give". Harry could accept that he should date women closer to his own age; or Erica could... I don't know, become a younger woman?

Once Erica and Harry have met, the rest of the movie is just marking time until it delivers its unearned and emotionally flat payoff and pairs them off. There's not a single moment in the entire film that rings true, emotionally or dramatically.

NFL Season Media Guide

With dress rehearsals for the NFL season well underway, it's time for my guide for what to expect from the networks' NFL coverage:

• Fox: The studio has been replaced by the high school jocks' lunchroom table. They are having much more fun than you are. Meanwhile, Joe Buck will talk in a bizarre singsong cadence about other programs coming up on Fox while a football game takes place in the background. 
• CBS: Two middle-aged men in blazers, ties and slacks are mildly annoyed to have been dragged away from a very good Sunday lunch at their country club and are embarrassed to find themselves at a football game rather than a golf tournament. 
• NFL Network: Two business travelers are killing time in the bar of a soulless chain hotel. They make polite but awkward conversation about the football game playing on the bar's TV set, even though neither of them really understands the game or cares about the teams involved. Everybody finishes the night even more depressed than when they started. 
• ESPN: Three frat boys have broken into the broadcast booth and are making each other laugh with fart jokes. They have no idea the microphones are on. Meanwhile back in the ESPN studio, Chris Berman and a dozen other people would like to tell you how awesome ESPN is. 
• NBC: Please don't interrupt. The adults are talking.

The oddness of Star Wars technology

I've observed in previous posts that the Star Wars saga would have been very different if the Rebel Alliance had access to e-mail or torrent so that they didn't need to drag those droids across the galaxy. (Also handrails...).

It struck me recently that this is actually one example of something much more general in the Star Wars universe. As far as I can recall, there is no new technology whatsoever in the entire "galaxy far, far away". Everything shown in the movies either existed on Earth at the time that Star Wars was made, or was a well-established element of other space operas. Space ships and lasers and blasters and flying cars and robots were more realistically rendered and executed better in Star Wars than they had even been before, but were far from new. And earthbound imagination shows up in the absence of technologies we take for granted, such as mobile phones, personal computers, or credit cards. The medical technology is also oddly primitive despite being administered by robots: how did nobody know in advance that Padme was carrying twins? What kind of prenatal care are they giving senators and queens out there?

The one original contribution of Lucas appears to be the lightsaber. Although there may be inspirations and echoes of some earlier story elements, the lightsaber as a complete, unified concept can, I think, be attributed to Lucas. He also imbued it with rich symbolic meaning in the original trilogy, rather than merely using it as a shiny prop. Lightsabers, and the duels they are used in, all mean something significant in Luke Skywalker's "hero's journey". (One of the tragedies of the prequel trilogy is that they somehow managed to make lightsaber duels dull by repetition.)

The other odd element of Star Wars technology is that Lucas appears to have been familiar with the fiction but not the science of science fiction. In other words, his script is littered with the vocabulary of existing sci-fi, but without any apparent understanding of established meanings. For example, robots are famously known as "droids", an apparent abbreviation of android. But as any sci-fi fan will tell you, an android is a robot designed to resemble a human being (and that resemblance is the jumping off point for rich explorations in the sci-fi literature of the nature of what it means to be human, whether an android can truly be alive, etc.). No droid in Star Wars looks remotely like a living creature of any species -- the closest perhaps is C3-PO, who at least is roughly human-shaped. Similarly, C3-PO's role is described as "human-cyborg relations"; a cyborg, of course, is a hybrid of a living creature enhanced with robotic technology, something that is never actually seen or even referred to ever again, raising the question of the need for a droid specializing in relations with them?

The same odd dissonance applies to space travel technology. The terms "jump to lightspeed" and "jump to hyperspace" are used more or less interchangeably, often when speaking about the same vessel (ruling out the possibility that both technologies exist side by side). These are two completely unrelated technologies, about as different as "drive faster" and "take a shortcut" as ways to get to your destination sooner. Similarly, references to the speed of travel make no sense. At one point the Millenium Falcon, famously one of the fastest ships out there, is described as being capable of "point-five above lightspeed"; it's hard to imagine any reasonable interpretation of that term that wouldn't still leave you requiring years to travel between star systems. Amusingly, when the Millenium Falcon escapes (again) from capture by Darth Vader, a crew member comments a few seconds later that "if they've made the jump to lightspeed, they could be on the other side of the galaxy by now". In reality, at 1.5x the speed of light, it would take the Falcon about 5 minutes to travel the distance from the Earth to our Sun, and assuming the Star Wars galaxy is typical of the ones we know, tens of thousands of years to get to the other side!

There are many more such internal inconsistencies -- for instance, why do large spacecraft fight like naval battleships at sea, all oriented the same way up, while the small craft dodge and roll like fighters using wings in an atmosphere for lift? -- and this kind of "World War II in space" transplant is exactly what leads fans of serious Sci-Fi to dismiss Star Wars as "space opera".

The overall effect is of Lucas as somebody who knows all of the notes and none of the music.

Asking for Trouble

A crowded room full of preschoolers wearing hoods so they can't see. Supervised by an 800-year old goblin with a limp. And they're waving FREAKIN' LIGHTSABERS around. And at your kindergarten, you were in trouble if you ran with scissors.

Bonus: the goblin speaks backwards, slowly. "Cut each others arms off... you must not!"