Throughout history, even before the formal development of the scientific method, scientists debating theories and changing their minds once existing ideas were disproven has given us an ever-greater understanding of the world around us. And that process continues today. Within the scientific community, lively debates continue about questions from whether amyloid plaques cause dementia to what a healthy diet really looks like. Outside the scientific community, debates continue on manmade climate change, vaccination and evolution, even though the scientific consensus on these topics is clear.
But some of the ideas that have been hugely controversial in history might well surprise you today. Here are some of the hardest-fought scientific battles of history, and how they were resolved.

1. Heliocentrism versus geocentrism

There are two great scientific debates from history that might have come to mind when you read the title of this article. One is about whether or not the world is flat. The other is about whether the Earth orbits the sun or vice versa. The fun thing about this is that the first of those two debates is a myth; no one has seriously doubted the fact that the Earth is round since the first ships were seen to go over the horizon. But the second of those debates was quite the opposite: it raged for some decades, and saw scientists persecuted for their arguments in favour of the heliocentric model.
While various scientists throughout the ages have proposed that perhaps the Earth orbits the sun rather than vice versa, the person usually credited with producing the first plausible version of the idea was Nicolaus Copernicus in the 1530s. Copernicus’ explanation wasn’t entirely right either – his model had the planets orbiting the sun in perfect circular obits – but it was a lot closer than mainstream opinion at the time. The trouble was, that Copernicus’ model neither fit the mathematical data well enough to persuade all of his fellow astronomers (given its flaws) nor pleased people who disliked the idea that humanity might not be at the centre of the universe. Galileo Galilei, who championed Copernicus’ theory, was even subject to investigation by the Roman Inquisition and after being found guilty of heresy, spent the rest of his life under house arrest. Only from the 1800s was it generally accepted that the Earth orbited the sun, and not the other way around.

2. The existence of aether

One of the great mysteries of medieval and renaissance science was how forces such as gravity could operate in the vacuum of space – not to mention how light could be transmitted with nothing to carry it. The solution proposed by scientists including Isaac Newton was “aether”, the name borrowed from a classical element in the Ancient Greek understanding of the world (though the late-renaissance aether and the classical aether are quite different in concept and suggested function; the separate use of ‘ether’ for a class of organic compounds also adds to the confusion).
Throughout the nineteenth century, various experiments were carried out to try to detect aether or the effects of aether on other objects. One such experiment was the Michelson–Morley experiment, which tried to detect the impact of aether on the speed of light, seeing whether this varied depending on the direction of measurement. They found that there was no difference, which started a chain of scientific progress that ultimately resulted in special relativity, ruling out the idea of aether that was stationary relative to the movement of the Earth.
This isn’t to suggest that the idea of a substance such as aether has been completely ruled out. Its existence has now been hypothesized at the sub-quantum level – however, at this point its supposed role becomes rather more complicated than the earlier concept of a medium that was believed to exist because it was needed to make the universe comprehensible according to the physics of the time, and would require an advanced level of physics knowledge to understand, let alone explain.

3. The existence of phlogiston

Aether wasn’t the only substance believed to exist because our understanding of the world didn’t make sense without it. The issue in question was why things burn and why things rust, and the theory, formulated by Johann Joachim Becher in 1667 and developed by Georg Ernst Stahl and JH Pott forty-five years later, was that some substances contain “phlogiston”. Aside from being completely wrong, as a theory it’s remarkably coherent. The idea goes that some substances contain phlogiston, which is released when they burn; the more phlogiston, the more effectively they burn. The air then absorbs the released phlogiston, but can only absorb so much; this is why when a burning object is deprived of air, the fire goes out. Phlogiston in the air is reabsorbed by plants, which is why plant matter burns especially well.
The theory was logical and based on observation, but as a result, further observation could disprove it. With more precise measurements, chemists discovered that some metals grew heavier when they burned, even though they lost phlogiston. In the 1780s, French chemist Antoine Lavoisier identified and named oxygen and hydrogen, correctly identifying the role that oxygen plays in combustion and thereby opposing phlogiston theory (and simultaneously disproving the millennia-old belief that water was an element in its own right). Supporters of phlogiston didn’t give up, though, with the theory becoming ever more complicated to allow for the new evidence that was steadily disproving it. Eventually it became so convoluted that it was clear that the alternative oxygen theory, marked by its simplicity, was correct.

4. Whether nuclear bombs could power spaceships

As we’ve discussed before, it takes a lot of energy to get something into space, and even more to keep it there and propel it any reasonable distance. One thing that produces a lot of energy is a nuclear bomb – so why not use a series of explosions of atomic bombs to propel a spaceship?
The very long list of possible answers to that question were not enough to stop it being seriously proposed, from initial ideas in 1946 after the development of atomic bombs, all the way through to a project funded by the US government to the tune of $1 million per year from 1958 to 1963, called Project Orion. It isn’t (quite) as mad as it sounds. Chemical rockets, which were NASA’s preferred option and which remain our main means of getting things into space, are inefficient, using a lot of fuel relative to the amount of thrust produced. A nuclear bomb is much more efficient and, if mass-produced, potentially cheaper. It may be that nuclear fission – or, if we ever manage to develop it, nuclear fusion – is the only technology that could enable interplanetary or even intergalactic spaceflight of larger payloads, rather than being able to travel only to the Moon or Mars.
But the potential downsides are painfully obvious. The fallout from the series of explosions that it would take to get a rocket into space would be considerable. And the development of rockets has been beset by accidents and near misses. Accidents involving chemical rockets have resulted in tragedies, but to date there have been just 31 fatalities during spaceflight, training or testing. An accident involving a spaceship loaded with atomic bombs could kill magnitudes more. Not that this was the reason why the idea was abandoned. Instead, it was the Partial Nuclear Test Ban Treaty of 1963, by prohibiting any test detonations of nuclear weapons except those carried out underground – making Project Orion impossible. The idea of powering a spaceship with nuclear bombs hasn’t been completely abandoned, though – but only in the setting of the depths of space, where the consequences of fallout or an accident would be much less severe.

5. How continents move

Have you ever noticed how neatly the continents of Africa and South America seem to fit together? In 1596, Flemish cartographer and geographer Abraham Ortelius was the first to spot this and suggest that perhaps at one point they formed a single continent, and have since drifted apart. The idea wasn’t much developed until a couple of centuries later, when more evidence had accumulated, such as fossils that were found on the edge of one continent and the edge of another, separated by thousands of miles of ocean with no evidence that a land bridge might have connected them, and similarly uncanny resemblances of rocks that certainly couldn’t have made their way between continents under their own steam.
By 1912, enough such evidence had accumulated for meteorologist Alfred Wegener to put forward a thorough explanation of what he referred to as ‘continental drift’, sparking a fierce debate that lasted until the late 1960s, when the theory finally gained acceptance by the last holdouts of the ‘anti-drifters’. The anti-drifters were fuelled by errors and omissions in Wegener’s work (his estimate of the speed of drift was much too high, and he couldn’t work out the mechanism – plate tectonics – by which continental drift occurred) and argued in favour of a “fixist” model instead. They not only opposed the idea, but were disdainful of it. The naturalist David Attenborough, while still a student at Cambridge in the 1940s, recounts that he asked a geology professor why he was not teaching them about continental drift. He said, “I was told, sneeringly, that if I could I prove there was a force that could move continents, then he might think about it. The idea was moonshine, I was informed.”
But with more research, the evidence for continental drift caused a series of anti-drifters to change their mind until continental drift became accepted scientific fact. In particular, the discovery of plate tectonics provided the force that could move continents that had previously been lacking from the theory. Scientific tests such as magnetometer readings of sediment cores and geomagnetic reversal provided further evidence and were the last nails in the coffin of anti-drifter opposition.

6. Whether there was such a thing as the Big Bang

The question of where the universe came from is one of the most fundamental that humans have ever set out to solve. As well as being the basis of most of the world’s religions, it’s also been hotly debated among its scientists.
From the early 20th century onwards, it’s been established that the universe is expanding, both on the basis of observation of galaxies moving away from us and on the basis of equations derived from Einstein’s theory of general relativity. In 1931, Belgian physicist Georges Lemaître suggested that if the universe is steadily expanding, then we can infer that at some point in the past all of its mass was concentrated in a single point. This idea was developed by Russian-American cosmologist George Gamow into what we now think of as the Big Bang theory; the name was originally intended to be mocking, given to the theory by Fred Hoyle, the astronomer who led the opposition to the idea. He instead supported the steady state model, which held that as the universe expanded, matter was continuously created, resulting in a universe that has no beginning and no end, and does not change its appearance over time.
Though a brilliant astronomer and a talented science fiction writer as well, Hoyle is now remembered first and foremost for his dogged defence of the steady state theory even as the evidence for the Big Bang became overwhelming. In particular, the discovery of cosmic microwave background radiation, which Gamow and his research associates had predicted, provided a boost to those who supported the Big Bang theory. The Big Bang is now generally accepted by the scientific community, though that’s not to say that all of the issues that made people doubt the theory have been resolved, and a few more have arisen along the way – for instance, why the universe has more matter than antimatter. The process by which the theory is formulated, debated, refined, debated some more and finally has all its inconsistencies ironed out is still ongoing in the case of the Big Bang, and it will undoubtedly be fascinating to see how it develops.

Image credit: globe; gravity; bonfire; spaceship; volcano; Hubble Space Telescope; Galileo.