If you are the type of person who wants clear, solid answers, then thought experiments may drive you a little bit crazy. When considering thought experiments, sometimes you will have to become comfortable with a paradox, where two seemingly contradictory things may be true at the same time. Other times you will end up with a thought experiment that is not solvable because the answer lies beyond human understanding.

No matter the frustrations with these issues, thought experiments are useful to stretch the mind in ways that most people rarely do and geniuses do all the time.

When Albert Einstein was a lowly patent clerk, the job was so easy for him to do. It was so boring that he finished his work quickly each day and then spent the rest of the time staring out the window and considering his thought experiments. This effort led him to a whole new way of seeing the universe, a deeper understanding of the relationship between time and space, and ultimately the creation of his Special Theory of Relativity. He became a renowned genius for his original thinking, which sometimes took many decades for scientific proof to become available that supported his theories.

If you fancy yourself a kind of genius or if you just want to have some fun thinking about strange things; here are fifteen types of thought experiments that are mind-blowing. If you come up with a new unique answer then you may be the next Einstein!

15. The Zen Koan

Zen is a form of Buddhism. A koan is a type of question or statement that allows you to ponder it and yet it does not have a definitive answer. One of the most famous ones is “If a tree falls in a forest and there is no one there to hear it. Does it make any sound?”
If you consider sound as the vibration of air molecules then the tree falling would definitely vibrate the air molecules surrounding it.

However, if you consider a sound more deeply the concept becomes more complex. The sound comes from the interpretation of the vibration of the air molecules that has a vibratory effect on the tiny parts of the ear, the little hairs, the small bones, and the eardrum. This vibration then causes a signal to be sent to the brain, which the brain interprets as a sound. In this analysis of a sound, it requires a participant for the sound to exist.

To further explore the nature of a sound is to consider the question of where exactly is the sound? Is it near the tree, in the surrounding air, in the ear, or in the brain of the person perceiving it? A completely deaf person standing nearby a tree that falls would not hear any sound, although he or she might feel the vibration of the tree as it struck the ground. A sound cannot simply be a vibration because even though a deaf person can feel possibly feel it, they cannot hear it.

14. The Origin Question

This is one of the oldest thought experiments that humankind has pondered for ages since the time of Ancient Greece. It is “Which came first, the chicken or the egg?” This question goes far beyond the origin of poultry because it could just as easily be asked about the universe. If the universe was created from the “Big Bang,” where did the singularity that created the Big Bang come from?

The reason why this is so perplexing it that it comes from our experience. We see that something always arises from something else. A chicken must come from an egg. It does not just spontaneously burst into existence. A religious person might say that God created the chicken and every other animal on Earth all at once and out of nothing. A Darwinian scientist would say all living creatures developed from some primordial ooze.

In 2016, R&D magazine reported that a team of scientists led by Craig Venter, Ph.D. were able to create a simple synthetic life form by using DNA created in a test tube, proving that it is possible. Dr. Venter and his colleagues have been working on this technique since 2010. This simple microbe had 473 genes as compared to a human that has over 20,000. This means maybe a bacterium came first before the egg that eventually became a chicken.

Also during 2016, the Financial Times reported that other scientists at Bath University created a viable offspring without needing to fertilize an egg. This means it is possible to have a chicken without first needing the egg. So, we are back to the same dilemma, which came first?

13. Are You Real or a Copy of Yourself?

Plutarch from Ancient Greece is noted as the one who first proposed this thought experiment. There was a ship from Theseus. Over many years of use, parts of the ship began to wear out. The people of Athens kept the ship in good repair. As each part wore out, it was replaced by a new part. Over time, all the parts of the ship had been completely replaced. Is this still the ship from Theseus? If all the old parts were found in a junkyard and assembled into a ship would it be the ship of Theseus?

Let’s take this a step further. Over a period of approximately seven years’ time, every cell in the human body has been replaced by a new one serving the same function as the previous one. If every cell in your body is different, are you still the same person?

Imagine there was a transporting device, like the kind featured on “Star Trek,” with one big difference. This transporter does not send your particles from one part of space to another. Instead, it takes the information about your body down to the tiniest, exact detail and makes a copy, including your memory and consciousness. This device builds an exact copy of you at the distant location. Everyone who encounters this new you thinks it is exactly the same as the old you. You also feel the same. In order to do this transportation, the original you is destroyed in the process. Is the new you the same person? If you looked, acted, and felt exactly the same, would you still be you?

12. Could a Monkey Write a Shakespeare Play?

The infinite monkey theorem is a thought experiment that says, if you had an infinite amount of monkeys that were trained to type on a keyboard, given an infinite length of time, eventually, one of them would type a Shakespeare play by typing at random. In fact, one would type all of Shakespeare’s work. Moreover, many would type copies of any finite work like a Shakespeare play an infinite number of times.

Even though the probability of a monkey typing Shakespeare is extremely low, it is not zero. Given enough time of say, hundreds of trillions of times the age of the universe, a monkey would randomly type Shakespeare. Even if the chance is only one out of a googolplex (this is a number of 1 followed by one hundred zeros), infinity is forever and larger.

There is mathematical proof of this theorem. The probability of two completely independent events occurring at the same time is the combination of the probability of both separate events occurring. For example, if the chance of it raining in one place today is 0.5 and the chance of having an earthquake in another place is 0.000005. Then, the probability of rain in one place combined with an earthquake in the other place is 0.5 times 0.000005, which equals 0.0000025.

If a keyboard has 50 keys, the chance of typing a single letter is one out of fifty. If a monkey is typing at random the chance of typing the word “banana” is (1/50) × (1/50) × (1/50) × (1/50) × (1/50) × (1/50). This equals the chance of one out of 15,625,000,000 for the monkey to type banana. The chance is small, but no matter how small a chance is, if infinite time is applied the chance is never zero.

11. Brain in a Jar

Before the “Matrix” films, there was a thought experiment called the “brain in a jar” (sometimes called the “brain in a vat”). This experiment removes a human brain and keeps it alive in a jar. The scientist uses a computer to provide electrical/chemical impulses to the neurons of the brain in the jar that simulate what a brain normally experiences while inside a living human body.

If the computer was sophisticated enough to both provide stimulus to the brain in the jar and react to the brain’s activity in the same ways as if it was still inside the skull of a living human being, then the brain, from its perspective, would not be able to tell that it was in a jar and would have experiences that to the brain seem exactly like reality.

This kind of philosophical examination of our definition of reality brings up the possibility that we are all existing in some form of an illusion that we have no true way of knowing whether it is real or not. We may only be able to determine that we are real; however, everything else is our perception of reality and may not have a true independent existence. At least, there is no sure way to prove the independent existence of anything else to ourselves, since our perceptions are all we have available to use.

10. Time Travel Paradox

If a time machine existed that allowed a person to travel back in time and they used it to go back to kill their own grandfather, how could that person still exist? This is called the grandfather paradox or an alternative version is the Hitler paradox.

In the Hitler Paradox, a person using a time machine to travel back in time to kill Hitler before he rises to power would have then removed the reason that made the time travel necessary, since Hitler would no longer exist.

One way that quantum scientists used to explain how to avoid the time travel paradox is by using an infinite number of multiple universes. Gong back in time and killing your own grandfather would only kill him in a parallel universe that is identical to the one you come from, with the sole exception that there is no grandfather and therefore no you in it.
By the way, the photo is Hitler as a baby. If you could go back in time, would you be able to kill a baby who was at that time just an innocent child?

9. As Above, So Below

The very largest things in the universe, called the “macrocosm”, are very similar to the very small, called the “microcosm.” This idea was first proposed by Hermes Trismegistus. The macrocosm is the universe. The microcosm is oneself. As the microcosm compares to something larger, it also becomes the macrocosm to something smaller. From the level of atomic particles, a person is a macrocosm.

Scientists, who study particle physicists, continue to look for the fundamental building blocks of the universe. Until the splitting of the atom was possible, it was assumed that the atom was the smallest unit of the universe. After the atom was split, so many new particles were discovered. In contemporary times, the large Hadron collider in Switzerland continues to find new particles of smaller sizes and with different configurations. The tiniest building blocks of the universe have yet to be discovered and perhaps they do not even exist, if everything can be both infinitely small and infinitely large at the same time.

8. Holographic Universe and Fractals

Related to the “As Above, So Below” thought experiment, is the idea of a holographic universe. The thought experiment about the holographic universe notes that everything in the universe is contained and replicated on a different scale in any smaller piece of it. Just like each part of a laser-made hologram contains the entire image, the holographic universe theorem says that everything is simply a fully contained copy of everything else. The only difference is scale.

Another way to think about this is the idea of fractals. Fractals are mathematical expressions that repeat themselves based on simple rules. As you zoom in on a fractal you see the same pattern repeated, just the same as if you zoom out. The pattern is the same no matter what scale is used. A thought experiment from fractal mathematics is that it is impossible to measure a coastline with 100% accuracy. As the scale of measurement becomes smaller, the edges of the coastline become more varied, thereby increasing its overall length. The only way to measure a coastline is by using an approximation at a certain scale.

7. Unexpected Hanging Paradox

In this thought experiment, a prisoner on death row is told he will be hanged one day next week between Monday and Friday and he will not know the day of the hanging in advance. This means that he cannot be hanged on Friday because if he is alive on Thursday he will know in advance of his hanging on Friday, which is the last day of the week it is possible for the hanging.

He cannot be hanged on Thursday because if he is alive on Wednesday he will know that since he cannot be hanged on Friday, he will be hanged on Thursday. Since he cannot know this in advance Thursday is also not a day he can be hanged.

This same logic continues to show he cannot be hanged on any day because he will know in advance when he will be hanged. This even applies to Monday because if all the other days are not possible then Monday is the only possible day, and if Monday is the only day left he knows he will be hanged on that day in advance.

The prisoner feels confident in his logic and knows there is no day of the week that is possible for him to be hanged. Monday comes and he is not hanged; however on Wednesday at noon, to his surprise, he is hanged.

6. Time Dilation

In Einstein’s Special Theory of Relativity, he predicted the phenomena of time dilation. Time dilation is an effect on time as one accelerates to move closer to the speed of light. More time passes for something moving at a slower speed than for something moving at higher speeds. There is also a gravitational effect on time. Things move slower for those closer to a gravitational force like the Earth compared to those further away from the gravitational force.

The strange thing about this phenomena is that Einstein imagined that “time was relative” before there was any way to prove it.

In Einstein’s famous scientific paper that he published on special relativity during 1905, he concluded that when two synchronized clocks that keep perfect time were used and one was taken away from Earth and then brought back, the one that stayed on Earth would have moved at a faster rate of speed and the one brought back would be lagging behind in time.

Decades later, this theory was tested using atomic clocks, which are extremely accurate and the theory was proven correct. One clock stayed on Earth and the other was taken into orbit around the Earth. The one that returned was slightly behind in time when compared to the one that remained on Earth. Einstein’s theory has been proven many times. Atomic clocks on satellites run slightly slower than the same atomic clocks on Earth, so they have to be adjusted for the difference.

5. Runaway Trolley Dilemma

In this thought experiment, developed by Philippa Foot in 1967, you come across a set of train tracks. On one set of tracks that the runaway trolley is going down there are five people tied to the tracks who are unable to move. On another set of connected side-tracks, there is one person tied to the tracks. A fast trolley is approaching. It is going too fast to stop in time. There is a switch that will divert the trolley onto one set of tracks or the other. There is no time to do anything but either throw the switch or do nothing. What do you do?

Do you let the trolley kill the five people or do you throw the switch to kill only one to save the five others? What if the one person tied to the tracks alone is your own child?

Another variant has a fat man standing nearby who is large enough to stop the trolley before it hits the five people. Do you push the fat man into the way to save the five that are tied to the tracks? Surprisingly, most would pull the switch to save a net of five of the six lives, but few would push the fat man onto the tracks to save four others. There is a perceived moral difference between the two acts, even though the net number of deaths is the same. However, this decision is reversed if the fat man is the villain who is responsible for tying the people to the tracks.

4. Deterministic Universe

It the theory of the deterministic universe everything has a cause and an effect. There is nothing that happens without a related cause that creates it. Another way to think about this is the idea of fate or karma. Fate is something that happens, which cannot be prevented by the person it happens to. Karma is the effect from something done in the past, Buddhists believe in reincarnation, so for them, karma can last for more than a single lifetime.

From a Buddhist point of view, this explains what bad things happen to seemingly good people or innocent children. They may not have done anything to deserve the bad consequences in this particular life, but they must have done something bad in a past life to bring the bad karma in this life.

A deterministic universe contradicts with the idea of free will. If fate is pre-determined then there is no such thing as making any choice. Choices are just an illusion, which brings the person to the same end result, no matter what they do.

3. Allegory of the Cave

This was a thought experiment developed by the Greek philosopher Plato and presented in Plato’s work as a conversation between Plato’s brother and his mentor Socrates. In this thought experiment, Plato has Socrates explain that some people have lived all their lives in a cave held in place by chains and facing a blank wall. They watch the shadows on the wall of things passing by the front of a fire, which is behind them. They create names for the shadows. Some of the shadows’ appear at the same time as when they get water and food. For them, the shadows are the reality. They do not even have a desire to leave the cave because they have not known any other way of life.

In the dialogue written by Plato, Socrates says that a philosopher is like a person freed from the cave. The philosopher sees the true nature of reality, not the manufactured reality of the shadows that are thought to be the reality by those held in the cave.

2. Flatland

Similar to the allegory of the cave is the thought experiment of Flatland. Flatland was a satirical novel written by an English school teacher named Edwin Abbott. In Flatland, everyone lives in two dimensions. The leader of Flatland is a Square. The Square tries to convince the King of the one-dimensional Lineland, which is a group of lustrous dots, that there is more than one dimension. The King tries to kill the Square rather than listen to what he considers nonsense. The Square escapes back to Flatland.

When a three-dimensional object passes through Flatland, the Sphere, it first appears in Flatland as a dot, then changes into a circle, which widens and then shrinks back from a circle to a dot and then disappears. This leads the Square to discover the third-dimension of Spaceland, which he visits.

The Square has dreams of other dimensions, including Pointland, which is a single dot who believes there is nothing in the universe except him. The Square also dreams of higher dimensions and he tells the Sphere and others in Spaceland about his dreams. He is forced to return to Flatland when he is thrown out of Spaceland for trying to spread his crazy ideas about other dimensions.

In Flatland, he is imprisoned for his heretical beliefs. After seven years in prison, the Square writes a book of his memoirs and experiences in other dimensions hoping those in future generations will read it and be able to see beyond their two-dimensional existence.

1. Schrödinger’s Cat

This is one of the most famous thought experiments in physics invented during 1935 by the Austrian physicist Erwin Schrödinger as his argument against the theory of Quantum Superposition that was proposed by other scientists at that time.

Quantum Superposition theory states that at a quantum level, a particle may be in an undetermined state that is sort of in-between states of existence, which collapses into a certain state only upon being observed.

Schrödinger thought this Quantum Superposition proposition was ridiculous and used his cat thought experiment to demonstrate his reasoning.

In the Schrödinger’s Cat thought experiment, you think of putting a cat in a sealed box along with a vial that contains a tiny bit of radioactive material that has a 50% chance of decaying within one hour’s time. Also, you put a Geiger counter in the box that is connected to a hammer and a vial of poison gas. If the Geiger counter detects radioactive decay, then the hammer falls, breaking the glass vial with the poison gas and the cat is killed. Regardless of the macabre nature of this thought experiment, there is no way to know if the cat is dead or alive without opening the box.

Under the Quantum Superposition theory, the cat would be in an undetermined state of dead/alive or alive/dead until the researcher opened the box after one hour had passed. This is what Schrödinger thought was absurd. Clearly, the cat is either dead or alive and because the poison gas kills almost instantly. The cat is never in a state of both being dead and alive at the same time.

Schrödinger’s logic is correct for larger objects such as a cat; however, he was dead wrong when it comes to actions of particles at a quantum level. Research since then has shown quantum particles have the ability to fade in and out of states of reality and simultaneously maintain the probabilistic state of two contrary conditions at the same time until an observation is made.

The observation causes the quantum field to collapse and the state of the particle can then be determined by the researchers. In spite of Schrödinger’s skepticism, quantum research is uncovering strange and unusual things happening at a quantum level, which do not exist in larger forms. Luckily, no cats had to be killed to prove this.

Sources: alternativephysics.org, rdmag.com, ft.com, iflscience.com