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Albert Einstein once said, to journey into the future, we must approach the speed of light. To journey into the past, we must exceed the speed of light.
The current time-travel record holder is Sergei Krikalev, who has traveled approximately 337 million miles in orbit at speeds of around 17,450 mph, reaching a total of 0.02 seconds into the future. This means he takes a step two hundredths of a second before you even notice. Time travel to the future is possible, as you’ll soon learn.
However, no one has yet traveled into the past. This will remain impossible unless we break the light barrier, as the following evidence will demonstrate.
10. The Bootstrap Paradox
The term originates from Robert Heinlein’s short story “By His Bootstraps.” The phrase “pull yourself up by your bootstraps” refers to a self-sustaining process that doesn’t require any external influence to function.
Alternate history is a common theme in time travel, focused on the idea of changing history, whether by accident or intention, while traveling back in time. A counterargument to this idea suggests that any change a time traveler makes is exactly what was always meant to happen (see #3).
However, a key element of the paradox this counterargument doesn’t address is the fact that any object traveling through time continues to age normally. A person cannot travel faster than light to stay eternally young; even if they return to Earth after 10 years while 1,000 years have passed for Earth, they are still 10 years older and will eventually die. The same principle applies to inanimate objects. For example, if you misplace your Oscar acceptance speech and travel back 30 minutes to retrieve it, you still age normally and can return to the future just in time to deliver the speech for your “Lincoln” performance. This is #3 on the list.
But the crucial point is this: any object traveling through time, during its journey, no longer influences history. After 100 million years, the sheet of paper, along with the traveler, will have disintegrated into dust. But the show must go on, and the Oscar is awarded to the same person, now accepting it without the speech, because it no longer exists to return to them in the future.
Let’s now explore the idea of information traveling backward through time. Imagine you invent a time machine and use it to travel back 1,000 years. You share your knowledge of time travel with the people of that era, and they go on to invent the time machine. A thousand years later, you invent the time machine, travel back, and the cycle continues. Here lies a dilemma: since there cannot be multiple origins for something, the invention of time travel ultimately has no origin, and this paradox is just as illogical as dividing by zero.
9. Weak Cosmic Censorship Hypothesis
Stephen Hawking dedicated much of his career to studying black holes, and much of our understanding of them comes from his work. The surface of a black hole is known as the event horizon, and once an object crosses it, it no longer exists within our spacetime continuum. It is drawn by immense gravity into an infinitely thin strand of energy called a singularity.
Hawking’s research posits that only the incredible energy of a black hole can generate a singularity. The weak cosmic censorship hypothesis suggests that no singularity can exist without being hidden by a black hole, meaning a singularity can never be directly observed. The singularity is a key focus in cosmology, as one theory of black holes describes them as gravitational forces so powerful that they can accelerate any object entering them to faster-than-light speeds. The singularity is the driving force behind a black hole’s gravity.
If a spacecraft wanted to break the light barrier, it would only need to travel through a black hole. Upon emerging on the other side, it would still be traveling at this speed—effectively jump-starting the spacecraft past light speed and allowing it to return to Earth at some point in the past.
However, no object can survive the singularity of a black hole. Here, matter might actually be destroyed, which appears to contradict the law of conservation of mass. Therefore, until it is proven that singularities can exist outside black holes, this method of time travel remains impossible.
8. The Chronology Protection Conjecture
This conjecture was proposed by Hawking himself, and it involves a lot of complex mathematics without actual numbers. Simply put, it suggests that there can be no such thing as a closed timelike curve (CTC). A CTC is a closed path that an object follows through 4-dimensional spacetime; if the path brings the object back to its starting point, it is considered closed.
No mathematical theory has yet been able to predict whether CTCs exist. If they are proven to exist, Hawking’s conjecture would be proven false, making time travel to the past possible—likely through the method described in the next entry. If CTCs do not exist, then the conjecture holds true, and as Hawking states, 'historians throughout the Universe are protected.'
Our best chance of discovering whether CTCs exist lies in quantum gravity, a branch of mathematics focused on unifying all four fundamental forces of the Universe into a single framework that can describe all physical laws across both macroscopic and subatomic scales. These four forces are: the weak force, which binds electrons to atomic nuclei, triggers hydrogen fusion in stars, and causes radioactive decay of subatomic particles; the strong force, which holds protons and neutrons together in atomic nuclei; electromagnetism; and gravity. The General Theory of Relativity reconciles all forces except electromagnetism; quantum gravity, using a different approach, reconciles all forces except gravity. Until quantum gravity is fully understood, CTCs remain hypothetical, and time travel into the past remains impossible.
7. Wormholes Violate the Laws of Physics
Our understanding of time travel is built on what we know about the physical properties and interactions within the Universe. We’ve developed a branch of mathematics, distinct from physics, to describe the laws that govern the microscopic scale, and this is called quantum physics. This field strongly suggests the existence of Einstein-Rosen Bridges, named after the two scientists who contributed most significantly to our understanding of them.
These are more commonly referred to as wormholes, which are tears in the fabric of spacetime. If we could harness them, the shortest distance between two points wouldn’t be a straight line, but rather zero, achieved by puncturing spacetime at both the starting and destination points, much like poking holes in a piece of paper. This effectively folds spacetime until the two points coincide, allowing the traveler to move from A to B without physically moving, and spacetime is then unfolded back to its original form. No physical movement takes place, yet the destination could be anywhere in the known Universe, and the spacecraft wouldn't need to surpass or even approach the speed of light, but instead would simply teleport.
This idea suggests that time travel to the past may be possible without needing to surpass the speed of light, but it overlooks what actually happens inside a wormhole. The truth is that physics has no clear explanation for this. The laws of physics as we know them do not seem to apply, or may not even exist, within a wormhole. If we try to understand wormhole travel through the lens of our current understanding of physics, we’re missing the point entirely and haven’t advanced beyond square one.
6. No Tourists from the Future
Let’s take a brief break from the math. A theory strongly supported by many in the higher mathematics community, including Stephen Hawking, suggests that we already have concrete evidence that faster-than-light travel is impossible: there are no visitors from the future walking among us right now, at least not as far as we can tell. Academics and even casual science fiction enthusiasts have gathered in meetings to discuss this theory while eagerly waiting for time travelers to show up. The idea is that, in the future, people will know about these meetings the way we know about World War II—it will be history to them. So, if time travel ever becomes a reality, travelers should eventually return to present-day participants and prove it.
Up to now, this hasn’t happened, and considering we’re talking about the entire future from now until the end of time, there should be countless travelers from various points in time visiting numerous moments in their past. A fun critique of this idea is the question: 'Why would anyone want to visit our time? September 1st, 1939, sure, but today? If they had a message to deliver, what could it be? Would they return with some profound philosophy on achieving world peace?'
Imagine being able to travel to any point in the past. What would you want to see? Most potential time travelers would likely want to find out whether Jesus Christ truly existed. But would you take the opportunity to return to the present and stop an imminent war between Israel and Hamas? So far, no one has.
5. The Twin Paradox
This paradox primarily concerns time travel to the future. It involves two identical newborn twins, one remaining on Earth while the other travels to Proxima Centauri, the nearest star, located 4 light years away. If the spacecraft travels at 80% of the speed of light—an amusingly more plausible scenario—the round trip will take 10 years. This means the twin on Earth will be 10 years old by the time his sibling returns.
However, on the spacecraft, the crew observes that both Proxima Centauri and Earth appear to move in relation to the craft, effectively shortening the distance between Points A and B to 2.4 light years, rather than 4. With the spacecraft traveling at 80% of the speed of light, each leg of the journey will take 2.4 light years divided by the speed, which equals 3 years for one leg, and 6 years for the round trip. Thus, the twin aboard the spacecraft will have aged 6 years, while the time span for the twin on Earth is the same. This scenario isn’t logically impossible.
The impossible part comes when one twin travels at 101% or more of the speed of light. According to our understanding of physics, this would send the twin into the past and effectively erase his existence—he would vanish from the spacecraft and not return to his sibling on Earth.
4. E = MC Squared
This is arguably the most famous equation in the history of mathematics, and it reveals the relationship between energy and mass. In 1942, it was unfortunately seized upon as the basis for a powerful new weapon. Albert Einstein, who had no idea his theory could be used to build a more destructive bomb, wept when Enrico Fermi and Robert Oppenheimer explained to him what was happening at Oak Ridge, Tennessee.
In addition to explaining the vast amount of energy contained within any size of matter, this equation also delves into the effects of mass as it moves faster. The faster an object travels, the more energy is needed to maintain that speed. As the object nears the speed of light, its mass grows infinitely, requiring infinite energy to keep moving forward.
This doesn’t completely rule out time travel to the future, since all it takes is for an object to approach the light barrier. For instance, when you walk to the kitchen to grab a beer, you are technically traveling into the future, but the distance is so minuscule that it doesn’t matter. However, as you move, you gain a negligible amount of mass. The energy required to propel a large object, like a spacecraft, any significant distance into the future—relative to our frame of reference—would surpass the energy contained in VY Canis Majoris, the largest star known to us.
However, attempting to break the light barrier would send the traveler into the past, demanding infinite and even greater-than-infinite amounts of energy. Achieving this is, as we know, impossible.
3. No Unified Field Theory
To be honest, all the points made earlier are more grounded in logic than in pure mathematics, as we can only speculate on time travel based on our very limited understanding of it. Albert Einstein dedicated his life to the theory of Relativity. He proposed two key theories, but the next critical step is to combine the General Theory of Relativity with electromagnetism. Einstein passed away while working on this, and although progress has been slow, today’s scientists have made only modest advances. The most advanced mathematical theory today is known as 'M Theory,' which is still not fully defined. To some mathematicians, it's almost like a religion, as there is so little clarity about it that some doubt its validity.
M Theory suggests there are 11 dimensions in the Universe, not just 4, and its supporters believe it can unify the five previous string theories and potentially take the final step needed: unifying the four fundamental forces of the Universe. M Theory aims to bridge the gap between General Relativity and Quantum Gravity, combining them into one unified framework. It involves a mathematical examination of how the Universe originated and functioned when it was an infinitely small point, containing all the matter and energy present today. Mastering such a theory would allow us to manipulate spacetime itself, enabling travel into the future or back to the past. Until someone can combine all four forces into a single physical constant for every point in spacetime, we will remain stuck in the present.
2. Temporal Paradox
This concept is essentially the inverse of #3 and is often referred to as the Grandfather Paradox. The idea that traveling into the past could be possible is paradoxical because it would allow you to go back and eliminate your own existence. But if you cease to exist, how could you possibly travel from the future to kill yourself? Critics, including science fiction enthusiasts, argue that our understanding of mathematics grows daily, thanks to figures like Newton, Einstein, Hawking, and Michio Kaku, and that with this expanding knowledge, we may eventually resolve the logical puzzles involved in time travel.
A leading theory that challenges the temporal paradox is the concept of the Multiverse, which suggests there are countless versions of yourself, each living different lives at countless points in time. You might be fatally wounded in a bar brawl at the age of 100 in one Universe, while in another, you could succumb to childhood cancer. Picture a Universe where Mytour never existed. Our current grasp of quantum mechanics provides compelling evidence that the Multiverse could be real, thus nullifying the temporal paradox and others, offering you a future even after you’ve 'killed' yourself. However, until a fully formed theory of the Multiverse comes to light, this paradox remains unresolved.
1. Temporal Causality Loop
This paradox is centered around a particular scenario: the creation of the first time machine. The inventor attempts to go back in time to make his grandfather and grandmother fall in love, but accidentally ends up killing his grandfather (see #2). In a desperate attempt to ensure his own future existence, he then sleeps with his future grandmother and fathers his own father, thus ensuring that he will eventually be able to go back in time and father his father again, perpetuating the loop.
This paradox is inherently flawed because it describes a situation where an effect in the future precedes its cause in the past. For instance, imagine you travel back to a time before the Big Bang, and somehow trigger the event, thereby creating the Universe. According to fate, this sequence of events would happen to ensure that, 1 billion years later, you would invent the time machine, use it to travel back, and create the Universe, allowing for the invention of the time machine. This circular reasoning is fundamentally nonsensical.
