Warp Speed: How Fast do you have to go to get the Stars to Sail by?

In a star cluster orbiting the  milky way (Space Engine)
In a star cluster orbiting the milky way (Space Engine)

John Pyle’s post on Light Speed, Not so Much, reminded me about something I wanted to look into based on my Space Engine experience. In this wonderful simulator you can go anywhere in the universe with the engine kindly accelerating you to fantastic speeds, as necessary. As part of the experience, you get to experience the Star Trek warp effect: stars go sailing by all around you as a readout at the bottom of the screen shows your current speed. It might be km/s over a planet or AU/s in a planetary system. Between stars, you typically get to go parsecs per second (a parsec is 3.26 light years). Hop over to the next galaxy and you’ll be travelling at hundreds of kilo-parsecs per second. This is a good thing, because otherwise travel would be tediously slow.

One of the neat things about Space Engine is that it doesn’t just pop you over to your destination, you travel to it, accelerating and decelerating. You fly through dusty arms of galaxies, through nebula, globular clusters. As I mentioned stars zip by a la Star Trek. This got me to wondering: I think Space Engine has a pretty solid model of spatial relationships and velocity. Therefore, I can use it to gauge what Star Trek warp factor I’d need to be traveling to get the same visual effect of star motion that we’ve seen since the original series. Of course, I had a feeling the TV show was way off but it is fun to do the numbers.

First, I’ll use the newer series’ definition of warp. You can find a great definition of what warp factors mean at Star-fleet.com’s engineering department: Warp Factors Defined. At warp 5, you’re going 215 times the speed of light and at warp 9 you’re going 1516 times the speed of light. That’s pretty fast, right? Indeed it is but as the old cliché goes, space is really big. At warp 5, it’s 40 hours per light year. Not bad for the local neighborhood but not too zippy. At warp 9, it’s only 5.9 hours per light year, so several light years per day.

So what would the star motion look like? Fire up Space Engine, play around with your speed and… you’ve got to be going around a light year per second to start to notice much star motion. Pick a densely populated region like the heart of a star cluster and maybe you can get away with 0.1 light years per second. But at those speeds, you are just starting to notice star motion. If you really want to be seeing stars zip by, like in the opening credits,  you might be looking at 1 to 5 or more light years per second.

Even at warp 9.99, at 7912 times the speed of light, you’re crawling by at one light year per hour+, or ~4000 times slower than the speed you need to get the stars to zoom by. I’m not sure how to turn that back into a warp factor but since warp 10 is the max, it is something like warp 9.9999, perhaps?

This means that if you intend to use that cool “star gliding” effect in your own stories, you better be having your star ships travel much, much faster than they do in Star Trek, so fast that your ship could get to the Andromeda galaxy in a few days. That’s probably too fast for most science fiction stories, unfortunately. Put another way, the Star Trek apparent star motion isn’t really practical, although it sure does look cool.

Just goes to show that space is really, really big.

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Semi-colons; No Place for Them in Fiction?

Kurt Vonnegut speaking at Case Western Reserve...
Kurt Vonnegut speaking at Case Western Reserve University (Photo credit: Wikipedia)

This week, as part of a critters critique, I passed on some comments I’ve received from many critters and from a few professionals as well: don’t use semi-colons in fiction. Per critters guidelines, I didn’t present it as a rule, more as a “here’s what response I get when use them.”

You see, I love semi-colons. They are a natural form of expression for me.

Here’s what Kurt Vonnegut had to say about them in A Man without a Country, according to The Quotations Page. I’ve seen it referenced many times so it’s probably an accurate quote.

Here is a lesson in creative writing. First rule: Do not use semicolons. They are transvestite hermaphrodites representing absolutely nothing. All they do is show you’ve been to college.

Kurt was entitled to his opinions and he was no doubt on firmer ground than I am but that seems overly dismissive to me. Semi-colons have a recognized role in non-fiction. While they may not be the convention in fiction, they clearly mean “soft end of sentence, following sentence related to previous one.” Or sometimes, they are used in place of a comma between clauses if the clauses contain a lot commas for other reasons (and-lists, multiple adjectives, etc.)

After passing on my wisdom, such as it is, in my critique, I thought I’d double-check my current novel manuscript for them. Again, I’m kind of fond of them, but why buck convention? It’s a style thing that seems to rub editors the wrong way. It’s not like they are absolutely necessary (the semi-colons, not the editors).

So I fire up Scrivener and find that ‘fond’ doesn’t really begin to describe my preference for them. They were everywhere, not every scene but probably two-thirds of them, and in many scenes, multiple places. And this is after I had already made a decision to avoid them before I started my last revision. Ugh. Imagine if I hadn’t decide to purge them already 🙂

For the record, Scrivener’s global search is kind of lame: you get a folder of the scenes containing the search phrase. Within a scene, you can search for next occurrence but to go to the next scene, you need to click on it in the search results folder. Worse, if you do a ‘next’ in the search pop-up and there are no more instances in the current scene, you have to clear an annoying pop-up warning before you can do anything else.

That makes an irritating task all the more tedious because removing semi-colons is not trivial. Sometimes it is no more than replacing with a period and capitalizing the next letter, which doesn’t exactly roll off my fingers on the keyboard. But often, more substantial re-work is required to avoid the construct: turning the sentence pair into a compound sentence or even re-writing the pair entirely.

I’m about two-thirds finished after several hours. A funny thing has happened as I purge semi-colons, though. While I don’t agree with Kurt that they have no meaning, I am starting to truly appreciate why they aren’t typically used in fiction because I think my semi-colon-less prose is better than it’s predecessor. I’ve removed a few unnecessary sentences in some places. In more, I’ve reworked the sentence into something that seems more interesting to me.

Now, if I can only avoid putting them in my drafts in the first place, I’ll be all set. It’s tough: I don’t notice them any more than a comma or period these days. It’s probably a manifestation of my engineer-think.

How about you? What’s your opinion on semi-colons?

Using Human Nature and the Rule of Three

Fire hydrant in Charlottesville, Virginia, USA
Who knew these were lethal: Fire hydrant in Charlottesville, Virginia, USA (Photo credit: Wikipedia)

Nila’s post on a kerfuffle over a scathing video review is a reminder that we’re all human. In fact, it’s our human foibles that make the best hooks for stories because it is easy to see ourselves in the failings of others. In the case of the two ill-behaved authors, neither should have acted the way they did but help me if I can’t see myself in both of them.

So, for the post-Thanksgiving stupor, here’s some musings on human nature and the way our brains work.

Hours of careful research watching fail videos with my son have left me with a number of insights. One, of course, is never let your child ride anything wheeled without a helmet, nor try any stunt without a cup. But better for the writer, never underestimate a young male’s thirst for glory. It may manifest in the 21st century as really stupid parkour attempts but it has to go back to before humans could speak. Glory-seeking is a proven motivation for young heroes, especially if a woman is watching. The popularity of these videos is also a testament to the enduring appeal of schadenfreude, although in our defense, my son and I prefer the silly ones to the ones where someone gets hurt.

Parkour-pivot
Parkour-pivot (Photo credit: Wikipedia) Stickman doesn’t have any breakable bones. And where’s his helmet!

There are a few other tidbits to be gleaned from these videos. Do you know that infants and very young toddlers go stiff when they fall? It must be an evolutionary advantage, I’m guessing because it makes it easier for a parent to catch a slipping child. I vaguely recall that from my own sons’ infant days: it’s easier to catch a board than a pile of jelly, not that I ever dropped my child.

Today, while in Old Navy, I had the chance to coach my son on the finer points of life, another tradition dating back to pre-humanity (most mammals train their offspring in someway, from a lioness teaching a cub to hunt, to a mare nudging a colt into the herd.) Of course, these days, it’s not so much about where to find prey or how to make a fire but more important things like, “Son, when your girlfriend or wife is shopping, make her carry the stuff she wants to buy. That way, the shopping trip will end sooner.” My wife stuck her tongue out at me but I was simply furthering an ancient father-son tradition, really, it was more of a duty than anything else.

Something even older than mammals: organic brains are wonderful pattern-matchers. Even pre-brain ganglia do this to some extent as they desensitize to repeated stimulus. A simple fish will learn the signs of when food is around. Any animal will stay away from something that makes it sick. The first grazer by the water hole probably swiftly learned to watch out for those floating logs that might be a crocodile.

Pattern-matching, may, in fact, be the primary purpose of brains beyond the basic control of body, feeding, reproduction, etc. Every mammal learns the basic patterns related to season, time of day, and various warning signs. I remember a mouse trap experience: I bought a fancy electronic one, supposedly a swift kill. I didn’t mind the mice myself so much but when one surprised my wife in the bathroom at night, they had to go. So I smeared peanut butter, as recommended, as the bait and zap: caught one the first night. Never caught one after that. I bet it’s because the mouse voided some “this place is bad” scent as it died and the trap no longer smelled good to other mice, despite the peanut butter.

Fast forward to humans and marvel at the patterns we find. Many are real. Grumpy lady at the DMV counter? Tread carefully because she can and will make your life miserable. A bunch of young men drinking from paper bags on a street corner after dark? Maybe there’s a better way to get to where I’m going. With our mammalian brains, we are excellent at gleaning patterns, even if we don’t realize what we’re doing. With our human intellect, we are outstanding at it. Of course, this can get us into trouble.

One place is stereotypes. As someone who recognizes the pattern-matchers in our heads, I’m not actually hostile to stereotypes. They can sometimes be useful. Of course, in contemporary life, they can be also be troublesome. They can even be immoral if a stereotypes keeps someone from doing something they are well suited to do. But we all make use of them. They are a natural response of our organic brain to the available stimulus so no point pretending they don’t exist. Acknowledging them can help you bypass your evolutionary wiring and do the right thing, when necessary.

As a writer, stereotypes are a mixed blessing. If your reader shares the same stereotypes, which they often do, you can make use of them to shorten a character sketch. Some stereotypes are likely to be accepted without much issue, such as a little girl playing with a doll, but many cause a negative reaction in your reader, sometimes enough to lose them. Even the doll example might lose some readers, although hopefully fewer in this post-feminist age. It’s a gray line because one person’s stereotype is another person’s archetype. In the end, better to steer clear of them or at least be conscious of how you are using them.

Another aspect of our pattern matching is superstition. I’m not personally superstitious, except when it comes to D&D dice:  hey, I need some excuse to have so many dice because I really don’t need a pound of them for any other reason.  Even so, I don’t really begrudge the superstitious because that’s what you get when you give a mammalian brain the awesome computing power of a human brain. Especially when you look to superstition in the medieval or ancient world, it’s my opinion that for most people, it made some sense of a crazy world.

A well-educated Roman might (and did, we have many of their writings), scoff at the superstitious but for most in that day, raised on only stories and what they could observe with their eyes, it was easy to attribute misfortune, disease, war, famine, etc., to the violation of some arcane ritual or the ill-will of a displeased god.

For one, they had few valid data points to extrapolate on. Saw two different people run over by a cart while wearing a striped shirt? Maybe I just won’t wear striped shirts anymore. I warn my children of the peril. Generations later, there may be a prohibition against wearing striped shirts, for reasons unknown. (In the Imperial period there were many customs, even ritual phrases, whose meanings were lost in the mists of time.)

Those who study how people react to bad news, do so in part to understand our fascination with gruesome events. We’re fascinated by things like the following, even though the odds of it happening to you are miniscule:

Humberto Hernandez, a 24-year-old Oakland, California resident, was killed after being struck in the face by an airborne fire hydrant while walking. A passing car had struck the fire hydrant and the water pressure shot the hydrant at Hernandez with enough force to kill him.

(Wikipedia has an article for everything! More disturbing is how many of the stories I already knew because of my own fascination with bizarre deaths.)

Once upon a time, our ancestors lived as hunter-gathers in small groups. Even with annual gatherings of larger groups, they might have only known of 2000 people in their entire lifetime. If something bad happens to 1 in 2000 people, it isn’t that unreasonable to worry that it might happen to you. If it happens to in 1 in 6 billion people, maybe you really shouldn’t lose any sleep over it. Great advice but our brains are wired to worry about anything we hear about. Good for hunter-gathers, bad for media-connected 21st century humans.

It’s like the move to soft baseballs. I can see the reason for it as a parent but as an engineer, I can’t help but note that probably more kids die in car accidents on the way to baseball games than actually die from a hard baseball to the chest. Still, we do it because it’s all about control: we can’t control the car accidents (or maybe fool ourselves that we can, since we are the driver) but we can control the baseball, so we do.

Back to superstitions: they allow(ed) people to feel they had some control over what otherwise seemed a chaotic world. Superstition is just another way of organizing and categorizing the world, another way to allow a human to predict outcomes. If I don’t wear a striped shirt, a wagon won’t run me over.

It’s not so different in our age. If our infant’s binkie falls on the floor, we clean it. Of course, we know there are actually microbes that might make them sick, but how do we know this? Did we culture the floor and examine it under the microscope? No, unless you happen to be a microbiologist, you know this because someone told you, in person, through a book, a documentary, whatever. So, in the end, like the advice to not wear a striped shirt, it is just something we learned from others. We may have more convincing reasons behind it but this idea of questioning received wisdom is something rather recent, and for most of human history, perhaps not really a good trait. Maybe the advice to not wear a striped shirt is not too valuable but the advice to not swim in a waterhole that herd animals are avoiding might be really good advice. Chomp, says the crocodile.

As to those dropped binkies? For the first few months with the first baby, they get washed in the dishwasher on the anti-bacterial cycle. By the second child, he’s lucky if it gets wiped off before it’s stuck back in his month. Bad parenting you say? Not at all: it’s more pattern matching. You see, by the second child, we parents have observed all the things our children put in their mouths and they didn’t get sick! So, we skip the binkie sterilization. What’s the point? He puts everything else on the floor in his mouth. Hey, he even licks the floor.

This makes me recall a friend’s tale of his toddler coming from the backyard with half a slug in her hand.

“What did you do?” I asked.

“I fished the rest out of her mouth.”

“Why? It wasn’t going to hurt her.”

“Yeah but it was really gross.”

True. One of the perils of life in the northwest. Here’s another and it’s a pattern I learned from only a single incident, and one you can benefit from without having to suffer it yourself: when walking on decking at night or early morning in Oregon, wear shoes. Smushing a fat slug between your toes is really no fun at all.

Back to writing, there are many ways to make use of your readers’ pattern matching but one of the most powerful is the rule of the three. Myself, I love it both as a reader and a writer. It’s basically, two events set a pattern, a third incident (that violates it) breaks it. It’s a wonderful tool from D&D games to books to movies because humans pick up on the pattern without even realizing it, which means they develop a visceral expectation of what comes next. So when you break the pattern, they are surprised and entranced at a core level.

What crazy patterns have you observed among your fellows, or your own behavior? Do you make use of patterns in your own writing?

Don Maass’ thoughts on Setting as Character

Donald Maass - The Breakout Novelist 2011
Donald Maass – The Breakout Novelist 2011 (Photo credit: jwordsmith)

If you have a chance, catch a workshop by Donald Maass. He’s advice is cutting and he’s funny to boot. Time will tell but I think he’ll turn out to be one of the few writing coaches whose advice will prove to be formative for me. My main take-away from when I heard him earlier this year was to use contrary (unexpected) emotions. Aside from its rather important value of surprising the reader, it’s simply a fantastic way to break through writer’s doldrums: don’t feel like writing a scene? Try it from a different emotional slant.

Writer’s Digest recently posted his advice on setting: treat it like a character. It’s old advice but it is especially useful for a fantasy writer and Don Maass conveys it in his usual effective and engaging way. If you like how he puts things, WD has all his books on sale for $39 (price is all of them together and there’s no shipping) which is a good deal.

Climbing the Next Hill

Pixar: For the birds
Pixar: For the birds (Photo credit: Colin ZHU) Maybe they are scared of their next revision.

My revision of Shadow of the Archon finished, I updated my progress bar to show 100% this morning. I even left it there for a whole 30 minutes while I started on the next revision. Now, it’s back to 1%.

As I set out on another revision, I’m reminded of Nila’s post on Pixar Rule #18: “You have to know yourself: the Difference between doing your best and fussing. Story is testing, not refining.” Of all the rules so far, I think that one has had the widest range of interpretations in the comments section. I don’t think anyone really gets the ‘testing’ part but clearly there’s a worry about spending too much time on a project. Sometimes you have to move on.

I’m trying to be better with this project. After the first draft, as planned, I did a major revision and polish, then had some readers look at it as an ‘alpha stage’. Post that feedback (and some soak time while I worked on other things), the next part of the plan was to mark up a printed copy, revise it, do one more pass on-screen to ‘polish it’, before having another set of eyes on it. After that, we’ll see. I like the story but we’ll see what the feedback looks like.

As I reset my progress bar this morning, I was reminded of my school days: my fellow students and I worked up from 1st grade to 6th grade in elementary, from 7th to 8th in Junior high, from freshman to senior in High School and so on into college. It was bottom rung to top rung, with the reward being getting to start at the bottom again. I actually loved school so I don’t mean that in a negative way but it was funny going from high to low at each major step. Sort of like finishing a revision only to start another one (or a new first draft).

How to Make Time Travel Work

Cover of "Back to the Future"
Cover of Back to the Future

I’m talking about how to make it work in your stories, not how to build a time machine. Apologies if you are looking for engineering schematics.

Time travel has got to be one of the oldest tropes in fantasy and science fiction. Who wouldn’t want to go back and fix a mistake or just observe the past? Trouble is, basic time travel has, as we all know, fundamental paradoxes. What happens if you kill your grandfather or commit one of the endless variations? Yet, there doesn’t seem to be an end to time travel stories in sight, and, why not? They can be fun. Plus for TV shows, there’s the added bonus of getting to use all those cool historical outfits the studios have lying around.

While I usually groan at time travel, I have to confess one of my favorite Star Trek: Next Generation episodes is Yesterday’s Enterprise. And I still get a chuckle out of Back to the Future.

As with Teleporters or Lasers or the like, an author can simply ignore any issues with time travel. However, in this case, if you are too blatant with the impossibilities it is likely to kill your story with editors and agents, not to mention readers. This is one trope where some amount of hand-waving is required. Let’s look through the methods.

First off, I’m just talking about going back in time. Going forward is easy.

Don’t Pollute the Timeline

This is a favorite of Star Trek but shows up in many other places. It’s basically a way to say you are dealing with time travel paradoxes without actually dealing with them, which makes it fine for certain stories but, for me, anyway, not very satisfying.

Why doesn’t it solve the problem? Because there is truly no way to avoid polluting the timeline. Even if you avoided making contact with any sentient beings, what about that fly that changed course, causing the frog to be elsewhere when the snake wanted to eat, causing the snake to search farther afield for food, biting the naturalist causing her to spend a week in the hospital and miss meeting the man with whom she would have mothered the next Einstein? Or leave out organisms, what about changes in gravitational or electromagnetic fields? You might not think that speck of dust floating in space is all that important but maybe it’s trace as a meteor inspires a poet who writes a poem that changes the life of a person who… You get the point. History is a very chaotic system and tiny perturbations can have dramatic effect. Oh, and don’t forget the microbes time travelers will leave behind.

Echo Park Time Travel Mart
Echo Park Time Travel Mart (Photo credit: Scott Beale)

Destiny

One way to deal with this is to take the approach that the time travel event was always part of how things were meant to be. There was never an earlier timeline where the time traveler did not appear. This sounds weird but Hawkings has noted that, so far, anyway, we don’t have a good physics reason for why time runs forward. And maybe the universe is cyclical: it runs forward to a certain point then runs backward undoing everything in exactly the same way. If we were living on the backward progressing arc, supposedly we wouldn’t even realize it (we would still think as if we were going forward). Or something like that.

This method has neither aesthetic nor intellectual appeal to me. I have to confess it doesn’t even make a lot of sense to me but I’m willing to concede that smarter people than me think it does so there you go. While your story need not posit the backward replay of the universe to make use of this method, it has similar problems: that of freewill. If things can run backwards as well as forwards, then there is nothing unpredictable in the universe. Using destiny to make your time travel work isn’t any different: something must occur, therefore the characters have no choice in what they do.

There are some physicists who think freewill may be an illusion. But do you really want your story to be about characters who can’t actually make any choices? Seems to remove the entire point of a story. If you use this one, you may want to be very sparing with it and be careful of nudging the reader into the conclusion that your characters’ desires are irrelevant to what occurs because if that’s the case, why care about what happens to them?

Observability Horizon

Time travel involves a violation of known laws of physics. This is not a problem because your time travel story is speculative fiction. But that doesn’t mean we can’t take advantage of other laws of physics: if the time travel is to a place beyond the temporal event horizon from earth, there is no possibility of a paradox, at least for a single time travel event.

As any astronomy buff knows, looking into space is looking back into time. If you look at the sun, you see the sun as it was about 500 seconds ago. Look at a star and you are looking back at least a few years, maybe thousands. Galaxies are millions of years away. So, for something that is X light seconds away, if you travel back in time less than X seconds, anything you do can’t change anything that has happened on Earth. It’s harmless.

For instance, lets say you had a 10-year-old child. Let’s also say you travel back in time 11 years. If you do that on earth, who is to say something you did might not change whether your child was even born, let alone the course of his life? But instead, let’s say you travel 10 years back in time to Gliese 832, which is 16 light years away. Now, even if the first thing you did was construct a planet-destroying death beam and fired it at earth, it would not arrive until after your ‘present.’ There is no way for your time travel to affect your child or anything else on earth, at least in the present. When that death beam pulse finally gets here, it’s another matter.

Of course, you might be wondering, what good is time travel if it can’t actually affect the past? Well, obviously, this method doesn’t allow you to change events in your local space. But it could still make an interesting story. Imagine if humans are losing a war to an alien race in 2113 but they have the ability to do this sort of time travel. They send humans back in time 10,000 years to several habitable planets 10,000 light years away with the hope that some of them proceed to the point where they can arrive at the home-world in 2113 with the technology necessary to save the world. Kind of a deus ex machina ending but there’s a story there. Or maybe there is no intent for the time traveling humans to save the present, instead, this is an ark-in-time: humanity dies on Earth but lives on, propagating through time and space.

This falls apart if your time travelers turn around and go back in time to the originating world. However, just like it takes infinite energy to go the speed of light, maybe it takes infinite energy to violate the laws of temporal observability: you can try but it won’t work. It’s hard for me to imagine a mechanism that would actually cause that to be but (a) I’m not a cosmologist and (b) this might be obscure enough that this method might satisfy most readers.

Forked Universes

Another way to deal with time travel is to say that each time travel event “clones” a universe: when you travel back in time, you create a duplicate of the universe at the time you arrive. Anything you do occurs in a separate universe. You can still kill your grandfather but only the grandfather in this duplicate universe, not the one who sired your father who then made you. No paradox.

Of course, universes do tend to contain a fair amount of energy, as in mind-bogglingly vast amounts. But according to current theory, the universe sprang from perturbations in a vacuum so I am not sure this is a really a problem. The energy would come from outside the universe, maybe it doesn’t really matter in the grand scheme of things. Or maybe it’s the old “how many angels can dance on the head of a pin” conundrum. The answer in the middle ages was an infinite number because god could put an angel down on half of the pin, make a half-sized angel, put it down in a quarter of the remaining space, make a quarter size angel, put it down in an eighth of the remaining space, and so on without end. Maybe when you duplicate a universe through time travel you actually make two halves, like a fertilized egg dividing: same mass but now two half-sized cells. To someone in the smaller universe, things might still look the same.

The main issues with this approach for your readers is how to make the universe cloning seem plausible. Some may also wonder just what can cause a cloning event. For instance, does it have to be a sentient being, because if physical processes can cause it there would be a near infinite and ever-increasing number of universes. Even if it has to be a sentient being, you might soon have a vast number of clones out there. Of course, any person in any given clone would only be aware of his or her universe. And maybe the plethora of many universes provides a setting in itself: your characters might hop between possibilities. While they could not return to their own universe in the past, nothing says they couldn’t go from universe A to B and back to A, as long as they arrived in A later than their departure.

As a side note, you could use this for the issue of freewill instead of time travel: maybe every decision results in a universe fork. While it is true that all possible decisions exist in the set of all universes, any given individual is experiencing the results of his decisions. Not sure it really solves the freewill problem but it’s an interesting thought.

Superposition

Quantum mechanics tells us that an electron in a hydrogen atom isn’t orbiting the proton like a planet around a sun. It’s actually in a probability cloud, sort of smeared out in time and space. I think QM would say even Jupiter is in a probably cloud in its orbit around the sun, just that given the mass of the sun and Jupiter, and the distances involved, the probability envelope is really tiny: i.e., for all practical purposes it inhabits a point in a space.

What if time travel works this way? Any given time travel event adds to a cloud of timelines. What happens in the now is the sum of timelines in the past. You can go back and kill your grandfather but that will either not really happen in the present because that event is improbable or you will change the now so that you aren’t present.

Maybe to change the past, you have to go back multiple times and nudges events so that the probability sum results in your desired outcome, or at least a different outcome: this sort of time travel seems rife with unintended consequences. This is actually at the core of my next short story (see, I take my own advice to heart. While I find most time travel stories paradoxical, how would I make this impossibility possible? I’m going with superposition.)

Where in the Past Art Thou?

That’s it for ways I can think of to make time travel work but before I wrap, let’s touch on the where of time travel. Let’s say you do find a way to travel back in time. Where do you go? Not only is everything moving in space, it is moving relative to other things. There isn’t a universal coordinate system. If you want to travel back to Earth one hundred years ago, how do you specify where the earth was then, both in its relationship to the sun (and remember our orbit is not perfect, it does vary due to interactions with the other planets, even with passing asteroids) and the sun’s position in the galaxy and the galaxy’s position in the local cluster, etc., etc. Not only would it be very difficult to compute where the earth was with respect to where it is now, how does your time machine spit you out in the intended spot, and at a velocity that allows you to interact with the Earth (rather than, say, impact it at relativistic speeds?)

Let’s say you can do all that: you can put your time traveler in the exact spot and with the precise velocity vector to avoid smacking into something. Do you know where the surface of the Earth was 100 years ago? Things move, not just down but up. Continents drift: 100 years ago America was about 8 feet closer to Europe. Trees grow. It would suck to appear in the middle of an oak tree, wouldn’t it? For all those reasons, time travel might be best executed in a spaceship with enough delta-v to match the destination.

It’s all handwaving

While the Observability Horizon and the Universe Forking methods avoid time travel paradoxes, they have their own problems. In the end, all of these methods simply constitute more obscure ways to explain time travel. Even so, they may satisfy many readers’ suspension of disbelief better than “avoid polluting the timeline”. And for those it doesn’t satisfy, you might get points for trying or at least amuse them as they think through the puzzle you posed.