Gedanken experiments: the art of avoiding fork-in-hand situations

I want you all to do something for me. Don’t worry, this won’t be horrifyingly embarrassing; you can do this sitting down and in the comfort of your own home. I want you to imagine placing your hand onto the table in front of you and pressing down. The force needs to be equal and constant.

Ok you can stop now. (Take your hand away from the table – you’re supposed to be thinking, not doing).

I want you to imagine that you’re pressing down with the same hand and the same amount of force. This time, instead of the table, you’re pushing your hand onto a fork (bet you’re glad you’re imagining now right?). Which one are you reluctant to do? Which one hurts more? If you answered “the fork!” then you’re absolutely right and your sense of self-preservation is clearly working.

What I just asked you to do is what’s called a “Gedanken” experiment. It’s a common device used in science communications to illustrate an idea without, say, the fuss and bother of calling an ambulance to deal with any awkward fork-in-hand situations.

The term was coined by none other than Albert Einstein. In his first language, German, it means “thought experiment”. Einstein famously used this method to develop and communicate his theory of general relativity. His memoirs are riddled with stories from his imagination that led him, step-by-step, to his ground-breaking conclusions. Like this one:

I was sitting on a chair in my patent office in Bern. Suddenly a thought struck me: If a man falls freely, he would not feel his weight. I was taken aback. This simple thought experiment made a deep impression on me. This led me to the theory of gravity.

Einstein concluded, through further thought and relation to everyday situations like riding in a lift, that within a given system, it is impossible to tell the difference between effects relating to gravity and extra acceleration. General relativity. You may also have heard the story of a night spent imagining himself riding a light beam… It’s a little hard to do in real life so Einstein had to be creative and then come back and prove his thought excursions were legitimate in other ways.

So why the weird and slightly worrying fork experiment?

Well sometimes it’s hard to demonstrate a principle to an audience without involving ridiculously convoluted or horrifically dangerous demonstrations. The use of the table vs fork experiment to illustrate that pressure decreases with increasing surface area, gives an audience something to relate to. I’m sure you winced at the thought of pressing your hand onto something sharp? Yes?

Then I declare the thought experiment a success.


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