The Science of Time Travel

time-machineLet us draw an arrow arbitrarily. If as we follow the arrow we find more and more of the random element in the state of the world, then the arrow is pointing towards the future; if the random element decreases the arrow points towards the past. That is the only distinction known to physics. This follows at once if our fundamental contention is admitted that the introduction of randomness is the only thing which cannot be undone. I shall use the phrase ‘time’s arrow’ to express this one-way property of time which has no analogue in space.

-Arthur Eddington. The Nature of the Physical World (1928)

Time travel features heavily in speculative fiction. It provides a useful means of foreshadowing and helps to heighten suspense as the characters try to avert a looming disaster or manipulate the future for their own ends. It appeals to all of us who have ever experienced guilt or loss and want to go back and fix it. It is rife with unintended consequences and can trigger exciting conflicts. However, it also provides a great source of frustration for writer and reader alike as they try to contend with the plot holes, paradoxes, and skewed logic associated with tampering with the fundamental laws of our universe.

In this post, I will address the most common problems and paradoxes associated with time travel, and then discuss the science that could make it possible.


Cause and effect. That is how the universe works. Nowhere in nature can an effect cause itself, which is to say that energy cannot spontaneously manifests itself to perform an action. Thermodynamics and all of Newton’s laws require a cause and effect, but time travel inevitably breaks these laws.

Like the Billy and Rubin comic above, if the Professor succeeded in going back in time to stop Billy from building a time machine, he would then have no time machine with which to make the journey. Traveling to the past, for even a few seconds, can violate causality and initiates all kinds of paradoxes.

Grandfather paradox.

There is no better example of a causality violation than the Grandfather Paradox. If a time traveler kills his own grandfather before he meets his grandmother, the traveler will have never been born. Most disturbing of all, are the implications for “free will.” If the traveler sees his grandfather, he will be physically incapable of killing him, for doing so will prevent his own existence. Imagine a knife that physically cannot interact with a person, because if it were to interact, it would prevent its own interaction. *Mind blown*.

Butterfly effect.

A term used in chaos theory, the Butterfly Effect is coined after the concept of a gentle disturbance in the air caused by a butterfly’s wings, which eventually leads to a hurricane.

Some writers insist that any disruption to the timeline will “heal,” and all will be set back on course, but this is unlikely. If the person went back just to witness an event, they talked to no one, and received no more than a passing glance by others and were quickly forgotten, then I could see the future not changing… much. But even if something small happens, like the traveler buys a slice of pie from a street vendor, it could initiate a chain of events that divert the future substantially. What about the person who was supposed to buy that slice? That person might then continue walking to find another vendor, and chat with friend he met on the street. If that friend subsequently misses a trolley and arrives late to work, failing to smile at the woman who would have been his future wife, then generations of people will have ceased to exist in the future, and all of their actions, and achievements, will have been erased… just because of a slice of pie. This is another example of causality, and every major and minor moment in our lives can be traced back to equally minuscule events.

Foresight and self-fulfilling prophecies.

Time travel isn’t the only thing that violates causality, it can also be violated with foresight. Having knowledge of a future event can allow the future to be changed, but is it really the future if it can be changed?

Prophecy is a common plot device in Fantasy novels. If a seer or prophet sees the hero’s future or reads their fortune, what will happen if that hero decides to do something completely different? If the hero changes the future, was it ever the future to begin with? What is to stop a person from just sitting down and not doing anything if they learn of their future? If that future depends on them performing an action, yet that person refuses to do anything, how can that future exist? This is the Idle (or Lazy) argument. For example, if a man learns he will die by being hit by a bus, that man can refuse to leave his house, thus preventing the future. I have seen authors stretch the limits of believability by having the hero walk into situations, saying and doing exactly what the prophecy says they will, even though they know exactly what fate awaits them.

This only works if the prophecy aligns with the main character’s own motivations, or if they are somehow duped into causing the situation they were hoping to avoid. We call these self-fulfilling prophecies, wherein the hero makes something happen because he or she believes there is no avoiding it, or because they want it to happen. For example, there is a prophecy that a castle will be invaded; so on the day of, the character leaves his guard post at the gates and flees the city. The enemy notices this new weak point in the castle’s defenses and decides to invade.

The science behind time travel:

Paradoxes aside, it should be noted that time is very strange. Some scientists suggest it is nothing more than a product of our minds trying to make sense of the universe. Time can go faster for some, and slower for others, all depending on how much gravity is around or how fast an object is travelling.

Black holes.

Time is inherently linked to the three dimensional fabric of space. Therefore, a force that can condense that fabric, can also affect time. Gravity is such a force, and a black hole is a near infinite supply of gravity. If it were possible to survive the spaghettification (gravity literally stretching you out) associated with entering a black hole, you would most certainly be crushed by the pressure of the mass surrounding you. There is a theory however, that a zone exists around a black hole where the centrifugal forces of its spin counteract the forces of its gravity. Thus, time would be slowed (possibly even reversed), but you would not be pulled into the center.

Special relativity.

Satellites in orbit are actually experiencing time a little slower than we are, largely because of the speed at which they circumnavigate the globe. Einstein introduced the concept of special relativity, which basically states that, while nothing can travel faster than light, light will still appear to travel at light speed, even if the light source is traveling at close to light speed. So, depending on your reference frame, time will move differently based on your speed. This time dilation can make a person’s 300 year journey near light speed feel like 20 years. This is probably the closest humanity will come to “traveling though time,” but it is a one-way ticket. Traveling faster than the speed of light, theoretically, would reverse the flow of time. Most scientists maintain this is impossible, because it would violate causality.

Quantum mechanics and the Many-Worlds interpretation.

Some writers have gotten around the causality argument by suggesting that time might be like a river. If a significant event disrupts the flow of time, it can branch off into another stream, parallel to the first, creating two different timelines of different pasts and different futures.

Based on observations of quantum entanglement, and particle-wave duality, it is clear that, at the quantum state, an object can be in two places at once, and doing different things. Physicists have since theorized that any and every action creates a parallel universe, in which the opposite action was taken. These infinite worlds can be very similar to our own or very different. While this concept doesn’t quite offer up a solution to time travel, if proven true, it can help eliminate many of the causality paradoxes associated with it.


Because there are so many theories regarding time, its nature, and how to travel through it, there is no correct way to portray it in speculative fiction. I would advise, however, to thoroughly outline your book if it contains elements of time travel. For many readers, time travel paradoxes are indistinguishable from plot holes.

What other considerations should writers take when writing about time travel? Did I miss a theory? Leave your comments below.

Rest assured, if time travel is possible, I will travel back in time to this very moment to ensure that I got everything right…

…nope. No Phil from the future. I’m a little disappointed, actually.

The science of gravity

gravityFor this week’s post, I’ve decided to talk about the thing that keeps us all grounded, makes us fall, and keeps us from venturing too high. It’s a very weighty subject, something I hope will draw you in, and it’s apparently a great source of puns. It is gravity.

Writers have gone to great lengths to circumvent this fundamental law of nature. When gravity can be eliminated or overcome, new and astounding opportunities arise. Our characters can strap themselves into rockets, dirigibles, and aircrafts to view our world from amazing heights, or visit entirely different worlds.

Unfortunately, many writers think their grasp of gravity is sufficient enough to excuse them from any research. In most cases this is true, but when glaring mistakes prevent readers from being immersed in the story, a little research would have been invaluable.

The science.

Gravity is many magnitudes weaker than electromagnetism at the microscopic level, allowing even a weak fridge magnet to resist the pull of our entire planet. What gives? Why isn’t it pulling its own weight? That was the last pun, I swear. The truth is, while electromagnetism predominates over the small, on the planetary scale, gravity always prevails. So for such a strong force, why do we have no idea how it works?

Unveiling the mysteries of gravity is the sole priority of many research labs around the world, and many theories have been proposed, but only a few have been found scientifically and mathematically sound (though they make many assumptions). The problem with all these theories is that they offer few testable hypotheses, and are based principally on math. Some, like Einstein’s theory of General Relativity and Loop Quantum Gravity (LQG), believe that gravity isn’t a force at all, but instead a warping or changing of the geometry of space-time (4 dimensions). While other theories, like Quantum Field Theory (QFT) and M-theory (string theory), believe the force of gravity is mediated by particles, the graviton, which propagate out like any other particle, not just in our 4 dimensions, but into other dimensions. There are pros and cons to each theory, and some go to extraordinary lengths to justify the strength of gravity relative to electromagnetism or the method by which it propagates. Some theories don’t try to explain everything at all (create a unifying theory) because it simply won’t work. Recently LIGO detected the previously hypothetical gravitational wave from two colliding black holes, giving researchers some clues as to how gravity propagates through space. This casts some doubts on those theories that suggest gravity has everything to do with space-time geometry, unless you somehow justify that ‘ripples in space’ are somehow to blame.

If you are creating a story line where gravity manipulation is a major plot point, some of these theories might make a good starting point for ‘anti-gravity’ technologies. Since no theory has been proven, there is a lot of room for creative license. I myself have absolutely no formal training in physics. In fact, I probably got away with taking fewer physics courses than the average science graduate. I also am pretty terrible at math. I have been looking into theories of gravity for several months now, and most of them gave me headaches. I applaud anyone who can make sense of them. I, with my limited understanding of physics, think gravity might be a combination of several theories. I theorize that perhaps gravity is a distortion of space-time, but that this distortion is the result of a particle, the graviton. If the Higgs boson (spin 0) is unable to move freely through space (or the Higgs field, whatever that is), imparting mass to objects, and the photon (spin 1) is able to move across space freely, what if the graviton (hypothetical spin 2), had the same speed as a photon, but also affected space, not by resistance, but repulsion. If the graviton were responsible for distorting space, this could explain its weakness (displacing space at the plank scale), its ability to affect time, and its ability to propagate like other particles. I have no idea if this theory has already been suggested or has already been debunked, but it’s what I am going with until someone decides to educate me. Seriously, if you are a physicist, we should chat.

Whether or not you came up with your own theory, gravity is still one of the most studied and characterized ‘forces.’ It follows certain rules. These rules are so defined, that we can take Newton’s equations from the 16th century, and use them to launch a tiny ship about 240,000 miles to land exactly where we want on the moon with zero to minimal course adjustment. So here are some considerations when writing your novel:


It is important to note that just because your characters are in space does not mean they will be weightless. If you take a balloon to the very edge of space, you will still feel the gravity pulling you down. It is only when you achieve angular momentum, momentum away from gravity, that weightlessness occurs. This is an orbit.

While orbits are relative, we tend to say one thing is orbiting another thing, when that second thing is the more massive of the two. Orbits are pretty simple to understand, and once understood, writers can avoid making some simple mistakes. A stable orbit is when a body has an angular momentum (outward force) that is equal to the inward force supplied by gravity. Because the force of gravity decreases according to the inverse square law, the further something is from the center of mass, the slower it has to travel to remain in orbit. For example, the International Space Station has to travel at nearly 17,500 miles an hour to remain in a low earth orbit of about 200 miles (orbits every 90 min), whereas a satellite in geosynchronous orbit, or about 22,000 miles from earth, only has to travel at about 7,000 miles an hour (orbits once 24 hours). At about 5 trillion miles from the sun, the objects in the Oort cloud barely need to move at all and only need to be nudged in order for them to come careening toward us as a comet. If you need a planet to revolve around another, but close enough to fill a quarter of the sky, they need to be revolving pretty fast around each other in order to counterbalance the pull of gravity, which will also cause some extreme tidal forces (to the core and oceans).


Like orbits, directions are relative. North on our planet is simply the magnetic north, where the magnetic field lines converge back on our planet (the place in the north pole where the compass starts to act a little erratic).  On other planets, this may not be the case. Venus for example, has an extremely weak magnetic field, perhaps due to its very slow spin, or the loss of convective forces due to a thick crust.

Gravity is also the only thing that differentiates up from down. The saccule and utricle of the inner ear contain grains of calcium carbonate that respond to gravity and momentum, tugging on hair cells (mechanoreceptors). This as well as visual stimuli, help you orient yourself to gravity and keep you from falling over.

In space, without gravity, this sensation is lost and many astronauts have to deal with a bit a vertigo and nausea as their eyes tell them something their ears are not. In space, people need to orient themselves to something besides gravity, like a feature of the galaxy (quadrant), the orientation of equipment or text on the spaceship, or they can learn to ignore directions all together. In my current work in progress, it is a struggle to describe motion and actions when lacking a directional cue. Are you really reaching up to flip a switch if you are upside down relative to everyone else? It is important to orient your reader to the character’s POV and sense of direction in order to prevent confusion.

Other effects of gravity.

As mentioned earlier, a world orbiting closely to a gas giant will likely have tremendous geological activity and a molten interior from tidal forces. A small world about the size of our moon that is not orbiting a gas giant will likely have very little atmosphere and no molten interior, since gravity is responsible for both.

Buoyancy is another ‘force’ that exists because of gravity. As Archimedes’ principle states, whether it is water or air, if something weighs less than the stuff around it, it will rise above it until it finds an equilibrium. It will sink when the object weighs more than the medium. This upward force is caused by the pressure differential in the medium, i.e. the pressure of the medium against the bottom of the object will be slightly greater than the pressure of the medium against the top of the object, causing it to rise. Neutral buoyancy occurs when the medium it displaces weighs the same as the object itself, and when the pressure difference between the top and bottom equals zero. The larger the volume of the object, the more medium it will displace, but it will also tend to weigh more. That is why ‘density,’ the weight of a certain volume of an object, is commonly used to estimate buoyancy. In orbit, the effect of gravity is canceled out and everything will be weightless. Thus there is no buoyancy in space. You can inject a drop of air into a sphere of water, and it will stay in place and not rise to the surface.

There are also many health effects associated with a long exposure to weightlessness, including muscle and bone loss as well as some neurological and visual problems. Rather than go into all of this, I will encourage you to read this post from Amber, a fellow sci-fi blogger and science nerd.

Difference between mass and weight.

This is a relatively minor point, but that should make it easy to remember. Mass is measured in grams, and weight is measured in newtons (gravitational force multiplied by mass). Most of the time weight and mass can be used interchangeably, unless there is space travel involved. Your character’s mass will be the same on earth as it is on mars, however, their weight will have changed. Weight is the measure of an objects gravitational attraction to another object, whereas mass is a physical property of the matter the object is made of. Mass is caused by the Higgs boson, but weight is caused by gravity (perhaps the graviton). While the incorrect use of this terminology will probably not dissuade many of your readers, it might cast doubt on your knowledge of the subject. Better safe than sorry.

Artificial gravity.

This encompasses any technique that is used to mimic the effect of gravity, and it is used in nearly every hard sci-fi story where astronauts are able to walk instead of float around on their space-ships. Creating an artificial gravity will be required for prolonged periods of weightlessness to prevent many of the adverse health effects. It it important to note that artificial gravity is not gravity at all.

Centrifugal ‘force’ is a method used to generate artificial gravity, whereby a torus, or another type of structure, is rotating around a central point. This has the effect of making all objects within the structure want to fly outward, but the structure itself is preventing that (centripetal force), thus allowing all the objects to be forced to the inside of the structure, with up being the center of rotation, and down being out into empty space. There are some pretty simple equations that will allow you to estimate the amount of rotations/min needed for a torus of a certain radius, to generate a certain amount of force (equivalent Gs).

Other continuous forms of acceleration can also apply a constant force, however, rockets will run out of thrust eventually, and when the people inside the rocket catch up to the rockets velocity, they will become weightless again.

It is up to the writer whether or not they want to address how gravity is simulated in their ship. I personally prefer there to be some mention of it to avoid logical inconsistencies. For example, if there is no torus or rocket used to apply this continuous force, then I will assume gravity has been mastered and replicated. If that is true, why would you need propulsion at all? What happens when the ship loses power (assuming generating gravity consumes power)? Where is the device that creates it? Where are all the floating cities, flying people, and gravity weapons? Conquering of gravity would of course result in all these and many other amazing things.


It does not exist (yet). Sadly, most contraptions that claim to be working by anti-gravity are in fact operating by buoyancy or propulsion. In order from something to be anti-gravity, it must ignore gravity, or perhaps reverse it, not just compensate for it. This probably won’t happen until we find out what gravity really is. Is it the curvature of space time by Mass? A change in the geometry of space-time?

But when we finally do unravel the mysteries of gravity, we may be able to redirect it, amplify it, or turn it off altogether. Exciting times are ahead, but as sci-fi writers, we don’t have to wait, we can bring that excitement to the here and now.


On that note, I think I will end with a blurb for my current work in progress. This has been extremely fun to write so far and I look forward to keeping you guys up to date on its progress.

Grounded (working title) blurb:

In the not-so-distance future, the graviton, the fundamental particle believed to carry the force of gravity, has been discovered. A government research facility wants to be the first to harness the power of gravity and reduce the cost of aerospace ventures. But when a high energy experiment goes awry, all matter within a mile radius is made weightless. Hundreds of lives are lost to the sky.

In the wake of the event, all further graviton experiments are banned in order to prevent a future disaster, one that could destroy the very thing that holds the earth together. The survivors are left with a choice: to leave the research facility and become grounded by the very foods they eat and air they breathe, or to be maintained in a weightless state and embark on a scientific expedition to the sky and beyond without gravity to hold them back.

But with nearly every commercial industry developing applications for weightless matter, its value has soared. Many of the survivors seeking to return to a normal life and weight have gone missing. When heavily armed men break into the facility, they do not steal the wealth of weightless materials around them. They are after something far more valuable: the graviton research that started it all. With the fate of the planet at stake, one lone researcher must stop them at any cost, even if it means casting off the moorings that hold the facility to the ground.