card
170

thought experiment

OLogy Series
physics
card
170

thought experiment

OLogy Series
physics

A thought experiment is an experiment that can be described but is not actually performed. Instead, it is carried out by the mind, using reasoning and logic. Thought experiments have been used by great thinkers since ancient times, and they are still used today to explain concepts such as quantum physics and relativity. These "virtual experiments" help us explore the world.

Famous Thought Experiments
Since ancient times, great thinkers have used thought experiments to understand our Universe. In the first century B.C.E., the philosopher and poet Lucretius used a thought experiment to show that space is infinite: if the Universe had a boundary, that would mean you could throw a spear at it. If it pierced the boundary, there is something on the other side. If it bounced back, there must be some kind of wall existing in further space. Either way, there could be no edge of the Universe. In the 17th century, the physicist and mathematician Isaac Newton wondered, if he shot a cannonball hard enough, would it circle the planet without touching the ground? Later, this thought experiment would be carried out by satellites. In orbit, things are "falling" towards the Earth, but at speeds fast enough so that they never reach the ground.

Einstein predicted the twin who traveled into space would be ??? than his brother.

younger

older

the same age

Are you right?

Correct!

The "twin paradox" is a famous thought experiment: one twin stays on Earth while the other takes a trip into space. The twin who traveled to space would be younger than his brother, since for him, time moved more slowly.

In one of Einstein's most famous thought experiments, he wondered what it would be like to ride a:

star

beam of light

motorcycle

Are you right?

Correct!

At the age of 16, Einstein wondered: "If I could ride a beam of light, how would the world look?" This question inspired one of the greatest thought experiments of all time, and helped Einstein figure out his Special Theory of Relativity.

A thought experiment is impossible to carry out for physical, technical, or practical reasons.

Fact
or
Fiction
?

Fiction

Some thought experiments can be done in reality but many are conducted using scientific reasoning and a little imagination.

thought experiment
What it is: a virtual experiment carried out by the mind
Famous "thought experimenters": Galileo, Newton, Einstein
When it's used: often in astronomy and physics to understand something about the Universe
How it's done: 1) Ask a question; 2) visualize the situation; 3) make a prediction based on logic; 4) develop a theory.

Image credits: Eric Hamilton.