Thought Experiments and the Progress of Physics

Thought experiments are among the most applied tools in the process of scientific investigation, and in fact, they are among the most crucial factors that have historically aided the advancement of physics.

To simply put it: A thought experiment (German: Gedankenexperiment, Gedanken-Experiment, or Gedankenerfahrung) is an experiment carried out in the mind where the physicist could possibly derive the farthest logical conclusion.

Thought experiments are not only applied by physicists but also by the common man; it is an inseparable part of human logic and existence. We most times apply thought experiments when we want to analyze the outcome of a particular future situation.

And surprisingly, we make our decision based on what we think about it. For instance, you are not likely to meet someone who you think dislikes you for assistance, and this is because you have concluded that he or she won’t help you.

You could only have deduced this based on a thought experiment that presents to you a mental picture of the unwanted outcome if you proceed to ask for that assistance. This example which relates to how we handle and approach rejection is one of the many examples of thought experiments in our daily lives.

There are many other examples of thought experiment in our day-to-day life. Another example is in how we plan a journey. One may, for instance, take another route to a particular place because after performing a thought experiment one concluded that the other route to that place may be longer and more tedious.

The person has succeeded in making the journey in his mind, taking the two considered routes, and having derived the possible outcomes, proceeded to decide based on them. This is an example of thought experiment applied in our daily lives.

Thought experiments are so integrated with critical thinking such that we most times don’t know that we are performing a thought experiment when we make our daily decisions.

However, thought experiments have taken a special place in physics and we consciously apply and make reference to them in order to rightly present a problem, resolve a problem, understand a new phenomenon, and to probably make a scientific discovery.

When a physicist is performing a thought experiment, he arranges and puts together all the parts and components necessary for the experiment in his mind, and as a physicist, he or she employs the assistance of known laws of physics and possibly intuition, through which the physicist is able to describe the outcome of the thought experiment and the many consequences that may follow.

This is how thought experiment has assisted in the progress of physics, such that it can even be said that thought experiments aided the founding of classical and modern physics, especially the latter, where it is a popular component of modern scientific discourse.

To prove the point that thought experiment played a crucial role in establishing classical and modern physics, one has to make reference to the Galileo’s Gravity Experiment and also to Einstein’s quite a number of thought experiments, among many others, I believe.

The Galileo’s Gravity Experiment is one thought experiment that helped found classical physics as it was what proved at a purely logical level that the Aristotelian logic or conclusion concerning the relationship between mass and gravity was wrong.

Aristotle had taught us that a heavy object falls faster than a lighter one. But Galileo performed a thought experiment to refute it. The refutation of this Aristotelian logic by Galileo is presented thus:

According to Aristotle, if a light object and a heavy object were tied together and dropped off a tower (Galileo considered the Pisa tower), then the heavier object would fall faster than the lighter one, and the rope between the two objects would become taut.

This taut or tension would cause the lighter object to create a drag and slow down the heavy object. But Galileo reasoned that once this occurs, the weight of the two objects together should be heavier than the weight of either one by itself, thus making the system as a whole fall faster.

So, the question becomes: how could the system fall faster despite the drag or tension created between the two objects? This is a contradiction which Galileo noticed and went ahead to prove Aristotle’s hypothesis wrong.

Many refer to this experiment as the Pisa Experiment which is actually believed to have been performed by Galileo where he dropped two objects of different masses from a height in the Pisa tower. But according to reports, it could simply have been a thought experiment as have been discussed in this article.

Galileo Galilei

Now, if we think of this experiment as a thought experiment, which is very likely, though I still refer to the experiment as the Pisa Experiment, we are drawn to see how thought experiment helped lay the foundation of classical physics and ensured the progress of physics.

This is because the basic understanding of the independence of mass from the rate of free fall is crucial to the foundation of classical physics; it was what unlocked the further understanding of gravity that culminated in Newton’s Principia.

So, in the 17th century, a thought experiment was applied by Galileo to disprove Aristotle and to lay the foundation of classical physics. It would be about three centuries later before thought experiments would be applied in a more radical manner to establish modern physics. We really have an inseparable history with thought experiments.

There are quite a number of points to trace the beginning of modern physics, like from the introduction of Mach’s principle to the first publication of Planck’s constant, but one of such points is to a thought experiment performed by young Albert Einstein at the age of sixteen, when he imagined what it would be like to run alongside a beam of light.

What would he see? Would he see a standing wave of light as classical relativity implies or would he see light zap past him at 299792458 m/s as Maxwell’s electrodynamics asserts?

This simple and beautiful thought experiment exposed the contradiction inherent in physics before the modern era, and young Einstein accepted the challenge and pursued this experiment till he arrived at the logical conclusion, which was to come later in 1905.

In 1905, Einstein, who was then 26 years old, published the scientific article titled: “On the Electrodynamics of Moving Bodies”. It was in this seminal paper which is popularly referred to as “special relativity” that Einstein presented what would be the result or logical conclusion of running alongside a beam of light, as he had imagined when he was 16 years old.

The theory proved Maxwell right about the nature of light and this was with a profound modification of classical relativity in a manner which showed that space and time were not absolutes, but that they transform according to the laws of modern relativity.

This simple thought experiment which we may refer to as the Beam of Light Experiment was an important part of what laid the foundation of modern physics. It led to the disapproval of the classical aether and the launch of a physics that espouses the independent nature of light.

Again, it wasn’t long, in 1907, that Einstein conceived another thought experiment that led to the discovery of a new scientific theory and that finally consolidated the foundation of modern physics. This new theory is what is popularly referred to as “general relativity”.

This thought experiment, which Einstein referred to as the happiest thought of his life, describes the experience of an observer inside an elevator moving far away from the Earth’s gravitational field at 9.81 m/s2.

Einstein reasoned that in such a scenario the observer will not be able to distinguish if he is accelerating or simply at rest on the Earth’s gravitational field. Einstein elevated this reasoning or logical conclusion to a principle which he referred to as the strong equivalence principle, to distinguish it from what he referred to as the weak equivalence principle of classical physics.

This strong equivalence principle, which states that gravitation and acceleration are indistinguishable, was the logical conclusion of Einstein’s simple thought experiment, which I like to call Einstein’s Elevator Experiment (EEE).

The Einstein’s Elevator Experiment turned out to be crucial for modern physics after general relativity became formally accepted. And till today, physicists all over the world still refers to this thought experiment when explaining general relativity or the strong equivalence principle.

This is also the case in post-modern physics where the Einstein’s Elevator Experiment finds a more profound application in the understanding of the universal equivalence principle.

Modern physics became characterized by a number of thought experiment, some of which were applied in the historical debates between the two giants of modern physics, Albert Einstein and Niels Bohr, concerning the central nature of quantum mechanics.

And concerning quantum mechanics, we have a famous thought experiment referred to as the Schrödinger’s Cat. The popular Schrödinger’s Cat is a paradox relating to quantum mechanics and that was first proposed by the physicist Erwin Schrödinger.

“It concerns a cat that is sealed inside a box for one hour along with a radioactive element and a vial of deadly poison. There is a 50/50 chance that the radioactive element will decay over the course of the hour. If it does, then a hammer connected to a Geiger counter will trigger, break the vial, release the poison, and kill the cat.

Since there is an equal chance that this will or will not happen, Schrödinger argued that before the box is opened the cat is simultaneously both alive and dead.”

The Schrödinger Cat is a thought experiment that focuses on the weirdness of quantum mechanics. It also represents the concept of quantum superposition, where a quantum object cannot be said to be in any particular state except when observed.

Thought Experiments
Schrödinger Cat: Both Dead and Alive

How observation leads to quantum collapse is a modern mystery that has followed quantum mechanics since its inception. So, even quantum mechanics is not without its own thought experiments.

This article therefore shows us the crucial role thought experiments have played in the discovery of scientific theories and in the progress of physics.

And in post-modern physics, we still refer to these historical thought experiments but with a better understanding which we now have of them. This is the great input post-modern physics brings into all of this.

In post-modern physics, all the thought experiments of physics are brought to a new understanding that shows us the unity of all things. And I want to conclude this article by saying that thought experiments are fascinating and they will always have immense importance in the progress of physics.

Until next time,

I will be here.

– M. V. Echa


M. V. Echa

M. V. Echa

My message is the universe, my truth is the universe, and this blog contains all you need to know about the universe, from the true nature of reality to the long-sought unity of the cosmos — which is the big picture!