The Madness of Time Travel

Time travel is a staple in science fiction stories. Marty McFly traveled into the future and back into the past by means of a flux capacitor designed by the eminent Dr. Emmett Brown. Until he surrendered it to Thanos, Dr. Strange used the power of the time stone to control time. And Dr. Who cavorts merrily through time and space in his TARDIS with the simple flip of a lever. It all seems so easy. Humans have invented all manner of dazzling wonders, from pottery to ships to steel to microchips to orbiting telescopes. Surely it is just a matter of time before some genius working in a garage builds a variant of Dr. Brown’s flux capacitor and is thereby able to zap himself into the future, whether with the flip of a lever or by racing through a mall parking lot at 88 miles per hour.

But is time travel actually possible? Well, certainly it is. With no energy expenditure at all everything and everyone in the universe moves inexorably forward into the future. And it is certainly possible to move into the future at a faster rate than other observers. Special relativity says that two observers moving relative to one another experience the flow of time at different rates, and that difference depends on their relative velocity. This prediction of relativity has been confirmed in experiment many times. For relative velocities that are a small fraction of the speed of light, the difference is quite small. But even so, the Global Positioning Satellite System is so time dependent and so accurate that it had to be designed to account for this and other relativistic phenomena.

Just how extreme can the difference between the clocks of two observers get? Well, the most extreme case concerns one observer at rest and another moving at the speed of light (in their mutual reference frame). In this case the moving observer’s clock actually stops while the clock of the at rest observer continues at its usual pace. If the moving observer travels at the speed of light for a million years, then returns to the physical position of the at rest observer, the at rest observer would be long since dead though the traveling observer would not have aged a single second.

Okay, so travel into the future is certainly possible. What about travel into the past? For an answer to this question we must turn to an astounding result due to Kurt Godel. In 1949 he constructed a solution to the field equations of Einstein’s General Theory of General Relativity that allows an observer to travel to any point in space and time– present, future, or past! This particular solution of Einstein’s theory is a fascinating and instructive study in its own right, but it is decidedly not a solution that corresponds to our own universe. The universe of the Godel solution has an intrinsic rotation about an axis. Our universe has no such rotation. The following video demonstrates how the closed time-like paths of Godel’s solution would enable one to travel into one’s own past: https://www.youtube.com/watch?v=078jOiaevAQ

Let us assume for the moment that such minor difficulties can be overcome in the grand cathedral of future human knowledge. Time travel still presents many practical difficulties that must be considered. Imagine that you are sitting in the driver’s seat of Dr. Brown’s DeLorean, and that you set the time control device for six months in the future. Now you stomp on the accelerator, get the car up to 88 miles per hour, and fzap! You reappear six months in the future, in precisely the same physical location where you disappeared.

But the Earth moves. The Earth is currently revolving around the Sun. In six months the Earth will be on the other side of the Sun. So the DeLorean cannot simply move in a straight line through time and space to reach the point where the future Earth will be in six months. It must move along an arc that exactly follows the path that the Earth will take.

And more than that, the Sun itself is moving. The entire solar system is revolving around the center of our galaxy at the rate of one complete revolution about every 225 million years. So six months in the future, the solar system would have moved a considerable distance around the galactic center from its present location. Dr. Brown had better make corrections for that, or the DeLorean will reappear in interstellar space.

There are other motions to consider as well. The Earth rotates on its axis, and the axis itself has a precession– that is, a wobble. Earth’s axis makes one complete revolution about every 26,000 years. So the position from which the DeLorean departed will have moved, irrespective of the other motions we have discussed.

There are other influences as well. Johannes Kepler showed that the paths of the planets are ellipses, not circles. But that is only to a first approximation. The moon and the other planets exert gravitational forces on the Earth. Those forces distort Earth’s orbit from that of a perfect ellipse. So to ensure that the DeLorean returns to the exact point from which it departed, every detail of Earth’s orbit will have to be considered– including all of the influences due to other gravitational objects in the solar system.

And there are more mundane considerations as well. What if someone builds a cement wall just a few feet beyond the point from which the DeLorean disappeared. When it reappears, the DeLorean will travel just a few feet before smashing into a cement wall. Not good. 😦

An earthquake might thrust up a chunk of the Earth’s crust right into the DeLorean’s path on return. A river might change course, causing the DeLorean to plunge into a torrent of water. Someone could park a car right in the DeLorean’s future path. Ouch.

Time travel as a literary device is pretty ridiculous. If its purpose is to get the reader to think about the possible future course of events, it may have some value. But I have never encountered any science fiction story that makes a full accounting of all of the many considerations we have discussed. There is in fact little or no “science” involved in the way time travel is generally portrayed. And therefore time travel will have to remain fully in the province of fantasy, rather than science fiction. Wave a wand, utter magical incantations, discover an ancient artifact that will open a doorway to a time portal. But please don’t pretend that time travel has any basis in science. It’s just not possible.

Galactic Colonization

Galactic empires are common plot elements in science fiction writing. But as I showed in another post titled “Faster Than Light Travel,” the likelihood is that it will never be possible to maintain an empire stretching across the galaxy. The laws of physics, as we presently understand them, simply won’t support it.

But would it be at all feasible to colonize the galaxy? Yes, and in fact it’s almost inevitable– assuming our species and our culture can survive long enough. Let’s assume for the moment that it is possible to build a spacecraft that can accelerate to 1,000,000 miles per hour. That may sound extremely fast, but it’s only about 0.15% of the speed of light. Even so, it’s roughly three times faster than the fastest man made spacecraft ever built– the Parker Solar Probe. One light year is about 5.87×10¹² miles. At 1,000,000 miles per hour it would take about 6700 years to travel ten light years. That may seem like an extremely long time– and it certainly is as compared to the history of human civilization. But as compared to the 225 million years that it takes the Earth to make one complete orbit around the center of the galaxy, it’s hardly anything at all.

The Earth’s orbit around the galactic center is roughly 170,000 light years in circumference. At the rate of 1,000,000 miles per hour it would take roughly 114 million years to traverse the Earth’s orbit. That’s only about half the time it would take the Earth itself to travel the same distance.

But there are plenty of complications in this broad overview. Spacecraft carrying humans to distant planets for the purposes of colonization must accelerate and decelerate. And it may be necessary to refuel, which might require slowing down to orbit a planet. All of that will take additional travel time.

We would also need to consider how to design a spacecraft to support human life for an extended period. There are only three major possibilities. First, the spacecraft could be designed to contain all the comforts of home. There would be gardens for growing food, recycling plants to reprocess waste, living quarters for each person on board, and air and water sufficient to support every living thing aboard the spacecraft.

This option is the most difficult and most costly to implement, and it is the one most prone to catastrophic failure. Any leak in the air system, however minute, could result in a complete loss of atmosphere by the end of a 6700 year voyage. Any failure of the agricultural systems would mean starvation for the entire crew. And normal wear on the complex systems involved could mean that crucial equipment fails long before the spacecraft arrives at its required destination. One can provide spare parts, or the raw materials necessary to fabricate any part on the spacecraft– but all of that would add weight, and additional weight adds a requirement for additional fuel, and additional cost.

An important fact to bear in mind about this option is that a 6700 year voyage means that there will be hundreds of generations of people who live and die aboard the spacecraft. There will be no room aboard for cemeteries, so the bodies of those who die will have to be recycled back into the agricultural systems.

The second option is to put the members of the crew into some form of suspended animation. Ideally we should like the crew to be maintained in a state that requires no air, no water, and no nourishment to maintain their bodies for a 6700 year period of time. And of course we would want to revive each passenger after the voyage with no significant loss of physical or mental capabilities. No one has ever found a way to do that. Assuming that it becomes possible in some as-of-yet unforeseen future, this option would require far fewer supplies and complex systems than the first.

But it is not without its own risks. When the spacecraft finally reaches its destination, a site must be chosen for landing and disembarkation. The personnel could be awoken as the spacecraft approaches the selected target planet, thereby permitting the decision to be made by humans. But there remains the possibility that the chosen destination planet is not a good option for colonization, and the spacecraft must travel on to another planet or perhaps another star system. That would require putting those crew members who have been awoken back into suspended animation, and that may involve its own unpleasant side effects. Alternatively, the spacecraft could be designed to survey the planet, check for required living conditions, and select an optimal spot for landing– without human intervention. That would clearly require a highly sophisticated system of software– one which can run for thousands of years without a hitch.

The third option is less of a realistic option than it is a dream. The spacecraft would not be transporting humans, but rather only human gametes. Once the ship arrives at its destination the male and female gametes would be allowed to fertilize and grow.

This last method would appear to require the least resources of the three. It would afford fewer opportunities for catastrophic failure, and it would require less fuel. But it would also require a method for raising and educating the infants that would result. And who will do that?

There is only one possible answer to that question– robots. An army of robots would have to attend to the infants as they are born. That would require feeding, bathing, playing, teaching– not the sorts of activities one ordinarily associates with robots. The robots would have to behave very much like humans– though it isn’t necessarily the case that they would have to look like humans. And of course the robots would have to provide all of the background information necessary to help the children adapt to their new environment.

Once a spacecraft designed for this third option arrives at its destination, the children will have to grow up in an environment that supports all aspects of human life. There will need to be systems for agriculture, waste processing, air filtration, water reclamation– everything that is required to support human society. But that environment would have to be developed and maintained without human intervention, until such time as the children have matured to the point at which they can take over all aspects of operation. That interim environment would therefore have to be built and maintained by the robots.

In some respects this last option is the most complex. It would require a level of robotic sophistication far beyond anything that has thus far been developed. But over the course of the next several centuries, it just might be possible.

This highlights another important aspect of galactic colonization– the search for viable planets. Before embarking on a 6700 year mission it would be best to get a fair idea of which destination planets are likely to be most habitable. We would want to know that the planet has an atmosphere with plenty of oxygen, that it’s surface temperature falls within an acceptable range, that it has liquid water on its surface, that it gets plenty of light from its star, that it isn’t already occupied by a hostile species… All of these conditions are very difficult to assess from a distance of several light years. That means it is highly possible to travel for thousands of years only to find that the chosen planet is unsuitable.

It will therefore be necessary to send advance unmanned probes first. These probes should be small, but would be outfitted with a full array of sensors. They should be set off on their travel to the stars at a significant percentage of the speed of light. At 50% of the speed of light a probe could reach a star 10 light years distant in 20 years and could return its findings to Earth in 30 years. A 50% speed of light velocity might attainable via a slingshot route around a nearby star. Such a route would undoubtedly result in g forces too extreme for human passengers, but should do no harm to unmanned probe.

Ideally we would want each probe to land on a planet, take physical samples, and assess the planet’s suitability to human habitation. In a system with multiple potential planets we would want these probes to visit as many planets as possible. That means each probe will need to be independently maneuverable, which means more fuel, and therefore more weight, and therefore more complexity, and greater cost.

Another major problem with colonization concerns the problem of adapting the environment of the chosen planet to human life. We can carry with us a storehouse of knowledge as to how to smelt ores, build power plants, pump water, grow food, build houses. But what if the planet’s atmosphere has too little (or too much!) oxygen? Or too little carbon dioxide? What if the surface is too cold for growing crops? What if water is only available deep underground? What if there is a bacterium that is airborne and fatal to human life? What if there is an intelligent life form that is hostile to our intervention? The potential problems of living on a completely alien world are innumerable.

This suggests that the best option is a multi-phase process. First, exploratory probes evaluate each potentially habitable planet. To those which qualify, a team of robots is sent to establish a human habitation, with all the systems necessary for the operation of a human colony. Once habitations have been built, then humans can be placed on transports to carry them to the colonies.

There will be plenty of time to assess and address these problems. It may be that we will have to be extremely choosy in evaluating planets for habitation. We shouldn’t expect that suitable planets will always be available along our preferred routes through the galaxy.

Interstellar travel is certain to be much harder than science fiction writers have thus far described it to be. It will take time– quite a long time, I suspect– to develop a process for galactic colonization. The opportunity is undoubtedly immense. Billions of stars and planets, each with its own geology, biology, wonder, and possibility. But there is really no guarantee that any planet within a reasonable distance would be suitable to our habitation. There will undoubtedly be a great deal to learn.

Faster Than Light Travel

Humans have explored the globe, traveled faster than the speed of sound, and gone to the Moon. Humans have learned secrets of the universe that no other species of our planet could possibly comprehend. Surely it will be only a matter of years, or perhaps decades, before humanity will begin traveling to the stars.

How hard can it be? We were told in the early decades of the twentieth century that no aircraft would ever be able to exceed the speed of sound. Yet on October 14, 1947, Chuck Yeager became the first person to do exactly that. And now aircraft repeat that astounding feat with routine aplomb. Surely breaking the speed of light barrier will be no different. Once we learn how to do it, we’ll build spacecraft that will flip into faster-than-light mode (FTL) as readily as a car switches into overdrive.

Before we attempt to understand the notion of FTL, we should first try to understand just how vast our galaxy truly is. The Milky Way galaxy is somewhere between 100,000 and 200,000 light years in diameter, and the Earth is about 27,000 light years from its center. Hence the Earth traverses an orbit roughly 170,000 light years in circumference for each of its 225 million year revolutions about the galaxy’s center. A spacecraft traveling at the speed of light would therefore require 170,000 years to make one complete circumnavigation of the Earth’s orbit. And that allows no time at all for either acceleration or deceleration. There would therefore be no time in that 170,000 years to stop and smell the flowers on any of the millions of planets one might encounter along the way.

The speed of light is about 186,000 miles per second. That’s about 669,600,000 miles per hour. The fastest human created spacecraft as of this writing is the Parker Solar Probe, which has used the tremendous gravitational field of the Sun to accelerate to 330,000 miles per hour. That is less than 0.05% of the speed of light! At that rate it would take the Parker Solar Probe more than 340 million years to traverse Earth’s orbit. That’s actually longer than it takes the Earth to make the same circuit!

Those who dream of galactic empire must confront the hard realities of the sheer size of our galaxy. The first galactic explorers will certainly want to chart the star systems they encounter– taking note of the habitable planets they discover, as well as those which are already inhabited. And they will undoubtedly need to refuel along the way. To stop long enough to survey a planet will require deceleration and acceleration– all of which will cost fuel, and time. To conduct such a reconnaissance mission at the measly rate of the Parker Solar Probe would ensure that by the time the explorers return to Earth, human civilization would have evolved into something vastly different than what it was at the time of departure.

Information is key to maintaining an empire. Desperate events in distant quarters may require a speedy reallocation of resources. To simply know that there is a problem requiring attention at the far end of a galactic empire would require a messaging system that can traverse the intervening distance in a reasonable time. On a galactic scale, that means the signal must travel faster than light. If the message implies that resources must be reallocated to address the issue, then those resources must themselves be transported in a reasonable time. “Reasonable” in the context of a galactic scope means within minutes or hours, not millenia.

Let’s imagine that the Earth is the seat of a galactic government, and that on the opposite side of the galaxy, about 54,000 light years distant, the local governor of a planet calls for aid in putting down a rebellion. At the speed of light it would take 54,000 years for the governor’s call to reach Earth. That’s not an actionable time.

But even at 54,000 times the speed of light it would still take one full year for the governor’s call for aid to reach the seat of power. In most cases news that arrives a year after the fact is too late to be useful. To reduce the travel time to one hour, the message would have to travel 8,766 times faster still– or 473,364,000 times faster than light!

Science fiction stories of galactic empire routinely mention traveling at two, three, four, or even ten times the speed of light, as if that were so astonishingly fast that it should be possible to travel anywhere in the galaxy in just a matter of hours. But in fact it’s not even remotely fast enough to hold an empire of galactic dimensions together.

If the sound barrier could be broken, why can’t we break the speed-of-light barrier? The reason is that the two barriers are of two completely different categories. The sound “barrier” was a concern raised by materials engineers of the times that no airplane fuselage could be designed to withstand the terrible shock wave that would be created by exceeding the speed of sound. It was chiefly a problem of materials.

But the speed-of-light barrier is altogether different. The two foundational principles of Einstein’s Special Theory of Relativity are that (a) all signals exchanged throughout the universe propagate via electromagnetic radiation (including visible light); and that (b) the speed of light is constant for all observers, regardless of their relative velocities. These two seemingly innocuous assertions have tremendous ramifications– one of which is that no physical object can travel faster than the speed of light. More than that, it would take an infinite amount of energy to accelerate a physical object to the speed of light!

But the weirdness of Einstein’s Relativity doesn’t stop there. As an object accelerates, its internal clock slows down. And in fact the clock of an object traveling at the speed of light actually stops completely. A beam of light experiences no time! So even if you could accelerate to the speed of light, your clock would stop. You would never age– but you would also never have any more thoughts. And consequently you could never observe the stars or planets you pass by, never plan where to go next, never decide to slow down or stop.

These strange consequences of Einstein’s simple claims have been repeatedly tested. Relativistic principles even had to be considered in the design of the Global Positioning Satellite System. So even a cell phone provides daily proof of the fact that Einstein was right when it simply accesses the GPS system.

Is there any loophole anywhere in Einstein’s reasoning? Doesn’t Quantum Entanglement mean that messages can be transmitted at essentially an infinite speed? The inflationary period of the Big Bang theory is a time when the universe expanded at more than 10²¹ times the speed of light. Doesn’t that say that Einstein was wrong?

Quantum entanglement isn’t likely to provide a useful solution as it is only capable of propagating quantum states. Two particles are said to be entangled if their quantum states are strongly correlated. In this case knowledge of one particle’s state instantaneously conveys knowledge of the other’s. But if one particle is disturbed, information about that disturbance can only be conveyed from one particle to the other at the speed of light. So although entanglement seems to offer the promise of instantaneous transmission of information, it does not support the notion of instantaneous transport of a physical force at a speed faster than light. And therefore it doesn’t really provide a way to transport a physical object from one location to another at a faster-than-light velocity. At least, not as presently understood.

As for the theory of inflation, the mechanism that would have triggered inflation isn’t known. It has been hypothesized by the advocates of the inflationary theory that gravitational attraction could have been flipped to repulsion in the very first instant’s of the universe’s existence by the presence of an extremely small amount of “exotic matter.”

So all we have to do is just create some of this “exotic matter” and we should be able to go as fast as we want, right? Uh, well… The current model of inflation only requires an extremely minute amount of exotic matter (relative to the total amount of matter in the universe) to cause the entire universe to expand exponentially. It doesn’t seem like it would be a good idea to create such a volatile material without knowing exactly how to handle it– unless you don’t mind blowing up the entire universe as part of your FTL experiment.

The only way out of the dilemma posed by the Theory of Relativity, as I see it, is to reconsider the first of Einstein’s two pronouncements– that all signals throughout the universe are conveyed by forms of electromagnetic radiation. Consider the human body. Our bodies are comprised of materials that consist of molecules, which are built up from atoms held together by atomic bonds. Atomic bonds are based on electromagnetic attraction. The present day theory of electromagnetic interaction, Quantum Electrodynamics, holds that electromagnetism is the result of the exchange of photons between charged particles. That exchange of photons happens at the speed of light, c.

But what if there is some other type of physical signal that can travel at a speed much faster than that of light? Let us suppose, for example, that there is another type of matter, call it FTL Matter, that is able to travel at speeds much greater than that of light. Suppose further that interactions between particles of such matter are propagated by some type of radiation that also travels at a speed much faster than the speed of light– call it c’. Now let’s go back to the primary assertions of Special Relativity and reframe them in terms of FTL Matter:

(a) All signals exchanged between particles of FTL Matter travel at the velocity c’.

(b) The speed c’ is constant for all observers comprised of FTL Matter in the universe.

From these two fundamental assumptions a new set of Lorentz transformations can be derived that involve c’ rather than c, and in all other respects the physics of FTL Matter would parallel those of ordinary matter. And this would establish a new cosmic speed limit c’, rather than c, for all FTL Matter.

So can we use some of this FTL Matter for FTL travel to distant parts of the galaxy? Perhaps the method would be to build an engine that consumes FTL Matter fuel, using the laws of FTL Matter physics, to propel a spacecraft made of ordinary matter to velocities close to c’. Sounds enticing, but at present nobody knows if there is any such thing as FTL Matter, or if the idea of constructing an FTL Matter engine is even remotely feasible.

Breaking the speed-of-light barrier is a completely different category of problem from that of breaking the sound barrier. This isn’t simply a problem of materials engineering, though there may very well be a serious question as to what happens to ordinary matter when it is accelerated to a velocity greater than c. The real problem is at the most fundamental level of the physics of our universe. Thus far, the Special and General Theories of Relativity have survived every test to which they have been subjected– and so they represent the very best knowledge we presently have of how our universe works.

I realize that this isn’t what fans of science fiction want to hear. They want to believe that we will soon be exploring the length and breadth of the galaxy, and will soon be trying to figure out how to travel to other galaxies beyond our own. Given what we presently know about the way matter behaves in our universe, it seems extremely unlikely that FTL travel will ever be possible. And that means that exploration and colonization of the Milky Way galaxy will proceed at a slower-than-light speed and will therefore take millions of years.