Updated: Sep 5, 2020
Time travel has been the focus of many sci-fi stories over the ages. So much so, that everyone probably has a good idea about the rules of time travel. Time is fourth dimensional and relative to space, if you alter the past the future will change, causing a paradox will destroy the universe, to make the jump you have to go at least 88 miles per hour, etc. Even outside of science fiction we have pondered, and experimented, with time in practical science. We know that gravity greatly affects time so much that we must adjust clock settings to match up with global positioning satellites that are in orbit around the earth. Special relativity tells us that the closer to a gravity source that you are, the faster time moves. This causes an effect called time dilation. We have even managed to alter time at the quantum level in some isolated experiments using quantum computing. But are time travel devices ever going to be achievable?
Let’s take a look at the problem from the perspective of an engineer. At the core of the debate, we are really asking two questions. A. Is time travel possible? And B. Will humans be able to travel through time to see events that have already taken place or that have not happened yet?
The question of if time travel being possible may be less of a debate than you might think. Most physicists agree that time is not constant. We know that time is affected by gravity waves and therefore can be manipulated. In most instances, we have only been able to observe time slowing down. While we do not know exactly what causes gravity, but we do know that the greater mass and density an object has the stronger the pull. Currently, singularities (black holes), are the greatest known sources of gravitational pull. This is because you have the entire mass of a star condensed into a single subatomic point in time-space. It is like the head of a pin weighing as much as an elephant, but way worse. The only thing that can escape a black hole is Hawking Radiation which is a byproduct of light being drawn in at speeds that exceed its speed. Anything approaching such a dense mass will experience the effects of time dilation. The closer it gets; the faster time will be moving. However, to an observer at any distance, time will be slower. Observing an object in this manner will make it appear to be moving slower. This is an example of the matter being propelled forward through time. If the observer stays stationary, the matter will continually appear to never change distance moving very slowly in a state of falling through time-space. If the observer were to follow the object it would perpetually appear to be in a slower state and constantly stay at a distance that could never be closed. Since the object farther away could never reach a velocity exceeding the first, the two objects would be eternally locked in this struggle until the gravitational source was cut off. So, while we know that objects can move forward through time at an accelerated rate to our own, because of relativity, it cannot be properly observed as it is happening. Currently, photons (light) are the fastest moving objects in the known universe. They move at 186000 miles per second or the speed of light. No object in the universe would be able to observe anything at, or past, those speeds because light could not travel fast enough to create an image to observe. While conventional relativity states that no object can exceed this speed, special relativity states that objects exceeding the speed of light would appear to be moving backward through time.
We know that objects can forward in time at an accelerated rate. But, what about backward? As I stated before, the only known way to physically move backward in time is to exceed the unexpendable speed of light. Until recently, most physicists would answer this question with a resounding, no. However recent experiments have upended this verdict a bit as we have managed to successfully play with time at a quantum level. Researchers from the Moscow Institute of Physics and Technology teamed up with colleagues from the U.S. and Switzerland and returned the state of a quantum computer a fraction of a second into the past. They also calculated the probability that an electron in empty interstellar space will spontaneously travel back into its recent past. The full study is published in Scientific Reports. Quantum physicists from MIPT decided to check if time could spontaneously reverse itself at least for an individual particle and for a tiny fraction of a second. They examined a solitary electron in an empty interstellar space.
We know due to the second law of thermodynamics that observing the electron’s evolution through a period of time is governed by Schrödinger's equation. For those who don’t follow, this means that as time moves forward the universe will evolve toward chaos rather than order. Most laws of physics make no distinction between the future and the past. For example, let an equation describe the collision and rebound of two identical billiard balls. If a close-up of that event is recorded with a camera and played in reverse, it can still be represented by the same equation. Moreover, it is not possible to distinguish from the recording if it has been doctored. Both versions look plausible. It would appear that the billiard balls defy the intuitive sense of time.
However, imagine recording a cue ball breaking the pyramid, the billiard balls scattering in all directions. In that case, it is easy to distinguish the real-life scenario from reverse playback. What makes the latter look so absurd is our intuitive understanding of the second law of thermodynamics—an isolated system either remains static or evolves toward a state of chaos rather than order.
Most other laws of physics do not prevent rolling billiard balls from assembling into a pyramid, infused tea from flowing back into the tea bag, or a volcano from "erupting" in reverse. But these phenomena are not observed, because they would require an isolated system to assume a more ordered state without any outside intervention, which runs contrary to the second law. The nature of that law has not been explained in full detail, but researchers have made great headway in understanding the basic principles behind it.
Keeping the second law in mind, the evolution of the electron state is governed by Schrödinger's equation. Although it makes no distinction between the future and the past, the region of space containing the electron will spread out very quickly over a period of time. We can observe an electron moving forward through time because it will maintain an ever-increasing space in which it moves in. This is called a “smeared” electron. However, if time were reversed, instead of smearing the electron would instead localize. Mathematically, it means that under a certain transformation called complex conjugation, the equation will describe a 'smeared' electron localizing back into a small region of space over the same time period." Although this phenomenon is not observed in nature, it could theoretically happen due to a random fluctuation in the cosmic microwave background permeating the universe.
The researchers then attempted to reverse time in a four-stage experiment. Instead of an electron, they observed the state of a quantum computer made of two and later three basic elements called superconducting qubits.
Stage 1: Order. Each qubit is initialized in the ground state, denoted as zero. This highly ordered configuration corresponds to an electron localized in a small region, or a rack of billiard balls before the break.
Stage 2: Degradation. The order is lost. Just like the electron is smeared out over an increasingly large region of space, or the rack is broken on the pool table, the state of the qubits becomes an ever more complex changing pattern of zeros and ones. This is achieved by briefly launching the evolution program on the quantum computer. Actually, a similar degradation would occur by itself due to interactions with the environment. However, the controlled program of autonomous evolution will enable the last stage of the experiment.
Stage 3: Time-reversal. A special program modifies the state of the quantum computer in such a way that it would then evolve "backward," from chaos toward order. This operation is akin to the random microwave background fluctuation in the case of the electron, but this time, it is deliberately induced. An obviously far-fetched analogy for the billiards example would be someone giving the table a perfectly calculated kick.
Stage 4: Regeneration. The evolution program from the second stage is launched again. Provided that the "kick" has been delivered successfully, the program does not result in more chaos but rather rewinds the state of the qubits back into the past, the way a smeared electron would be localized or the billiard balls would retrace their trajectories in reverse playback, eventually forming a triangle.
The researchers found that in 85 percent of the cases, the two-qubit quantum computer returned back into the initial state. When three qubits were involved, more errors happened, resulting in a roughly 50 percent success rate. According to the authors, these errors are due to imperfections in the actual quantum computer. As more sophisticated devices are designed, the error rate is expected to drop.
In light of this study, we have proven that “technically” time travel in reverse is possible, but moving particles in predictive patterns is a long way off from reversing time itself. But, we do have an answer to our first question. Can objects travel through time? The answer is a resounding yes. However, we still need to apply this science to answer our second question: Will humans be able to travel through time to see events that have already taken place or that have not happened yet?
Engineering a device would be the first hurdle. And, as ridiculous as it sounds, given enough time humanity could potentially develop devices capable of moving and withstanding light speeds or large amounts of gravity. Too many outside factors prevent physical objects from being able to do the things required to move through time, but in the end, these are just engineering challenges. Throwing all mechanics aside, could a machine capable of moving through time take a person to past and future events?
In my mind, it depends on how much faith you have in engineers. To start with, the device would have to protect itself from also suffering the consequences of aging and regressing. Traveling backward in time would be great, but not if it turns you into a baby or your machine back into ore. In theory, this could be fixed by creating a gravity-well, but let us save that for another discussion. Also, in most depictions of time travel, the machine moves through time but remains static in space. However, this is a gross misunderstanding of space. Unfortunately, what most people do not realize is that like time, space is also relative. (This is why they are connected as time-space.) Space is not a constant, everything is constantly moving. The planet rotates at 1000 miles per hour, it revolves around the sun, which also revolves around the center of the galaxy, which also moves through the universe in undetectable ways. Think of it this way: When dinosaurs were roaming the planet, our entire solar system was located on the opposite end of our galaxy. We cannot measure and conclude where our galaxy was in the universe at that time. We can only calculate its relative distance to other known objects. Typically, distance is calculated relative to the position of Earth. So, this poses two problems for time travelers and their devices.
First off for physical time travel to be possible, your time machine would have to also move through space. Coordinates would have to be calculated to offset the movement of the entire universe. This would mean we would have to understand and calculate space as a constant, not as relative to ourselves. If we didn’t, we would simply end our journey in an empty or occupied unit of space. The reason you could not calculate relative to our own location is that we could not calculate the shift of the entire universe which is homogenous. Moving through time is much different than moving in time. It’s possible that short jumps could be calculated and achieved, but the further a jump, the more complicated and riskier the math would become. However, it should be noted that after a number of short jumps (provided they can be survived), by calculating the errors in position calculation, we could in theory learn if there is a universal constant for a position in the universe.
In my mind, the mechanics alone will prohibit our species from ever achieving any form of true time travel. But, even if the physics were solved, there are so many challenges that would keep us from being able to utilize it in any practical matter. But, knowing that subatomic particles can achieve movement through time, I think the possibility of being able to send messages in either direction via waves could be entirely possible as long as the recipients had a way to receive them. This has long been a debate around a theoretical particle known as the Tachyon. This theoretical particle moves faster than the speed of light. So, to any observer, it would be moving backward through time. If a message could be sent as a wave of tachyon particles, humans could send messages backward through time.
So, can people travel freely through time to visit events? I do not think on any timeline that this would be possible at all. I always laugh when I see stories of time travelers from our near future coming back to warn us of some impending doom. The thought experiment is exhausting enough without getting into the reality of everything related to the mechanics getting in the way. I will not even get started on singular vs. multiple timelines and how time would work if it could be altered.
Maybe I’m completely wrong, but while science will forever be exciting to me and I would love to see it advance. In my mind, I cannot see time travel as a conceivable possibility. So until we are able to send messages to our past selves through our time radios, I won’t hold my breath.