10 Classic Paradoxes in Science

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Modern science is filled with puzzles that often defy common sense. We evolved to fit into our environment and understand the world around us from our everyday frame of reference. We can understand something like the distance to the nearest star, Alpha Centauri, on an intellectual level, but our brains just aren’t equipped to understand the actual mind-boggling distance of four light years. Many scientific paradoxes, like science itself, have their roots in mathematics and philosophy. Despite what our senses tell us, these conundrums have predictive power and tell us something about how the universe really works. Here, in no particular order, are 10 of the best-known scientific paradoxes. If you have a hard time grasping these concepts, don’t worry — even the greatest scientific minds have struggled to explain some of these.


10. Olbers’ Paradox

Olbers’ paradox deals with cosmology and is a simple observation of our universe that you can make tonight. If the universe is infinitely old and contains an infinite number of stars, then why isn’t the sky infinitely bright? The answer was actually proposed by horror writer Edgar Allan Poe almost a century before the birth of modern cosmology. The very fact that we’re not blinded by infinite brightness in every direction is because the universe has a finite age with a finite number of stars in it.


9. The Monty Hall Problem

A classic in statistics based on the popular game-show host of the same name. Your chances of winning actually increase if you change your door choice after the first round. The reason is that you are changing your sample space from 1-out-of-3 to 1-out-of-2. While it seems that your odds should be 50%, they’re actually 66% in your favor with this strategy. (Try it, it works!) Hall himself talked about the puzzle in a 1991 interview, noting that his interplay with each contestant on Let’s Make A Deal canceled out the statistical effect of the situation.


8. The Faint Sun Paradox

Current models of stellar evolution suggest that the Sun had to be much fainter early in Earth’s history … so how did we avoid an “Ice Ball Earth?” The climate puzzle of the early Earth has yet to be completely solved, but there are some interesting hypotheses. One idea is the volcanic greenhouse gasses were much more prevalent in the atmosphere of the early Earth. Another proposal is that the Sun was slightly more massive and energetic in its early history than proposed. Looking at Mars, we know that it once enjoyed the same warmth as well and was able to have flowing water on its surface, despite its distance from the Sun.


7. Zeno’s Dichotomy Paradox

This is an idea from the Greek philosopher Zeno that says if an infinite number of points exist from point A to B, you can never reach point B traveling half the distance at a time. Although more of a logic-based conundrum, Zeno’s dichotomy paradox anticipated the subatomic world of quantum particles. Zeno also proposed many other notable paradoxes, including the Arrow paradox, which maintains that an arrow in flight occupies a single volume of space at any given time, and is therefore motionless.


6. The Tea Leaf Paradox

Why do tea leaves gather in the center of the cup as its stirred? While one would think centrifugal force would force the tea leaves towards the edge of the cup, more is at play just below the surface of the fluid. Specifically, the vertical rotation of the fluid is faster at the top than the bottom, setting up an inward pressure gradient. Albert Einstein first proposed the correct solution for this in 1926.


5. Schrödinger’s Cat

From quantum physics, this thought experiment explains that at a sub-atomic level, state is determined by observation. Schrödinger’s Cat asks us to think of a vial of poison gas in a box with said cat. Rupturing of the vial depends on the decay of a radioactive particle, the state of which cannot be known without observation. Thus, the cat actually occupies two states (alive and dead) until the observation is made. While arcane, there is observational evidence that the world of subatomic particles actually behave in this manner.


4. The Duality of Light

How can light behave both as a particle and a wave? Again, although this contradicts common sense, there is real evidence that light does just that. For example, the photoelectric effect is firm evidence of light photons acting as particles (This is how solar panels work). However, the double-slit experiment demonstrates the interference and scattering pattern of light, a very wave-like thing to do.


3. The Twin Paradox of Relativity

What happens to time as you approach the speed of light? Rather than being separate, we know that time and space is actually linked. The closer you approach the speed of light, the slower time appears to flow from your own frame of reference. This sort of effect has actually been measured using satellites and ultra-precise clocks. While a theoretical journey around the galaxy at 99.9% the speed of light might only take a few years from your own perspective, you would return to Earth to find that your family and friends had been deceased for hundreds of thousands of years.


2. Maxwell’s Demon

This paradox with the strange name shows that while the 2nd Law of Thermodynamics is a statistical certainty, temporary violations can exist. Maxwell’s Demon envisions a valve between two chambers allowing faster moving molecules to pass from one to the other, seemingly in violation of the 2nd Law. Note however, that the net entropy decrease is only temporary; for a true violation to occur, slower moving molecules would have to be allowed back across the barrier in the reverse direction as well. Systems (such as living organisms) can do battle with the 2nd Law for a time, but in the end, entropy always wins. By the way, the name Maxwell’s Demon comes from 19th century Scottish theoretical physicist James Clerk Maxwell.


1. The French Paradox

Why do the French drink, smoke and overeat but remain relativity healthy? Medical researchers have puzzled over the French paradox for years. Many have cited the presence of resveratrol in red wine as a preventative for cancer and heart disease. Others indicate that the high level of vitamin K2 (menaquinone) prevents hardening of the arteries. Or perhaps the paradox is no more complex than lifestyle. Many European cultures still walk and bicycle daily, a habit almost unknown to many Americans.


Written by

David Dickinson is a backyard astronomer, science educator and retired military veteran. He lives in Hudson, Fla., with his wife, Myscha, and their dog, Maggie. He blogs about astronomy, science and science fiction at www.astroguyz.com.