Have you ever imagined the solution to a logic problem instead of calculating it?

Such “thought experiments” are carried out using one’s imagination rather than doing actual research. Typically delivered in narrative form with accompanying diagrams, thought experiments are used in wide-ranging disciplines for “entertainment, education, conceptual analysis, exploration, hypothesizing, theory selection” and more, according to the Stanford Encyclopedia of Philosophy.

But no one knows when the first thought experiments took place, according to The Decision Lab, an applied research firm that specializes in behavioral science. But we do know the first written evidence of thought experiments comes from ancient Greece, where pre-Socratic philosophers used them to solve math equations.

✅ A thought experiment doesn’t really have an answer—and that’s the whole point; these sometimes strange, open-ended questions use hypothetical scenarios to let your creativity and problem solving run wild, according to The Decision Lab. And hopefully, through the process of unpacking circular reasoning (when you use evidence to support a claim that is just repeating the claim itself) and rhetorical logic (the art of persuasion), the thought experiment will illustrate some sort of big idea. No wonder these exercises are so popular in philosophy.

From an Einstein puzzler developed while writing up his special theory of relativity to a brain-twisting exercise that could prove computers don’t really understand language, here are four modern thought experiments that will test your mental mettle (and probably fire up some heated debates at your next family dinner).

Einstein’s Train-and-Embankment Thought Experiment

In the 20th century, thought experiments played a key role in defining a physics revolution. When Albert Einstein, a physics professor at the Humboldt University of Berlin, was writing Relativity: The Special and General Theory, he created a thought experiment that unraveled outdated concepts of what time is.

Before Einstein’s book was published in 1920, people assumed that time was universally constant across all frames of reference. Einstein showed that events are not simultaneous in different physical frames of reference if one frame is traveling relative to the other. In other words, time is actually relative.

To illustrate this concept, Einstein described a scenario in which a long train is traveling relative to an embankment with the velocity v. If lightning strikes at two locations simultaneously, as perceived from the railway embankment, these lightning strikes will not happen at the same time from the vantage point of someone in the train.

In the diagram below, the lighting strikes occur at points A and B. The two rays of light from points A and B meet at the midpoint, M, on the embankment. Meanwhile, a traveler on the train will see one flash very slightly before the other because he is located at point M’ and is traveling to the right; so, he will see the flash from point B before he sees the flash from point A. This will result in him thinking that the flash from point B took place first.

Based on this thought experiment, Einstein concluded that time varies depending on what frame of reference one has.

The theories of relativity have had profound consequences, changing our ground rules for how we expect the universe’s geometry and operation to work. According to the Encyclopedia Britannica, special and general relativity “overthrew many assumptions underlying earlier physical theories, redefining in the process the fundamental concepts of space, time, matter, energy, and gravity.”

The Twin Paradox

When Einstein wrote about the theory of relativity in a 1905 paper, he was curious about a problem that arose: if there are two clocks, and one of them travels, the clock that is traveling will record less time passing. He wrote, “the clock that moved from A to B lags behind the other which has remained at B by ½tv²/c² sec … where t is the time required by the clock to travel from A to B.”

In 1911, Paul Langevin, professor of physics at the Collège de France, expanded this example to describe human twins who age differently because one of them has taken a space flight and the other has not.

The paradox is that from the perspective of the twin in the spacecraft, the twin on the planet has accelerated away; so, the reverse should be true, since according to special relativity, their frames of reference obey equivalent physical laws if they are not accelerating.

This paradox has been hotly debated on the Q&A board StackExchange and in other physics discussions. According to a 2021 paper from the Journal of Applied Mathematics and Physics, it has still not been resolved.

“Many attempts have been made to explain the twin paradox, which fall in two categories; one based on asymmetry and the other on acceleration,” wrote Pirooz Mohazzabi and Qinghua Luo, the two professors at the University of Wisconsin-Parkside who were the co-authors of the paper. “To resolve the twin paradox, some authors resort to [an] asymmetry argument. They argue that twin B does not remain in a single inertial frame of reference during the entire process; traveling toward the star, she is in one frame of reference, while coming back, she is in another one.

“In a second school of thought, some authors argue that the twin who leaves the Earth undergoes acceleration whereas the one who stays on Earth is not accelerating,” Mohazzabi and Luo wrote. “Some even argue that [the] time dilation equation is not valid if the reference system accelerates.”

However, the paper says the explanations based on asymmetry and acceleration do not hold if the twins are both accelerating away from each other and have both left the planet, for example. There are other exceptions as well.

This slowing down of time due to travel, which is known as time dilation, has been verified through multiple experiments. So although we don’t know why it’s happening, we do know it is happening.

The Chinese Room Argument

An intriguing thought experiment known as the “Chinese Room Argument” describes how computers can imitate human language without understanding it.

According to the Stanford Encyclopedia of Philosophy, John Searle, a philosophy professor at the University of California-Berkeley, wrote that he could imagine himself alone in a room, following a computer program that tells him how to respond to Chinese characters that someone slips under the door. He could do this without understanding Chinese.

Essentially, Searle would be communicating in Chinese the same way that artificial intelligence knows how to respond to strings of characters without understanding them.

This thought experiment shows evidence that the “Turing Test” does not provide evidence of real artificial intelligence. According to the Turing Test, a computer cannot be considered intelligent unless it can produce responses that a human observer could view as regular human responses. However, a computer can use language in a convincing way without understanding it. According to the Stanford website, this means that human minds are more than information-processing systems. Human minds come from biological processes; computers simulate them.

This thought experiment piques the curiosity of researchers who study language and computing. Other academics have critiqued some of its assumptions and conclusions. It is considered an argument against what is known as “Strong AI,” or artificial intelligence that mimics the human brain.

Anyone who has experimented for a while with online tools such as ChatGPT will notice some of the comical and surprising behavior that AI can engage in due to its lack of comprehension of human language and cultural and social contexts.

Mary’s Room (the “Knowledge Argument”)

Data may not be able to describe the colorful nuances of real-world experiences. In 1982, Frank Jackson, who is now an emeritus professor of philosophy at the Australian National University, proposed a thought experiment that he at first believed proved this was true. According to a TED video, this thought experiment has been used since then to describe why computers cannot have human experiences.

In this thought experiment, Jackson imagined that a brilliant neuroscientist named Mary lived in a black-and-white room and had never seen color, but knew the theoretical and practical science behind color vision. For example, the video said, someone like this would know that within the eye, three different types of light stimulate cone cells that send electrical signals along the optic nerve to the brain to allow us to perceive color.

Suddenly, Jackson said, Mary’s computer began to display color—or she left the black-and-white room. She now had a new experience that her previous scientific knowledge did not encompass.

This shows there are nonphysical properties and knowledge that can only be discovered through experiences. As the video said, the experience of color transcends the knowledge of color; this implies that abstract knowledge cannot capture the full zest of real life.

Philosophers call this experiment the “Knowledge Argument.” They say experiences have subjective qualities called “Qualia” that can be experienced, but not fully described. Some experiences cannot be described in words.

Jackson changed his mind later and said that the experience of viewing a colored image on a screen could be described in terms of an event in the brain. According to the open-access journal Philosophical Investigations: “He came to believe that there was nothing apart from redness’s physical description, of which Mary was fully aware. This time, he concluded that first-hand experiences, too, are scientifically objective, fully measurable events in the brain and thus knowable by someone with Mary’s comprehension and expertise.”