What separates a good idea from a bad one? It’s not always easy to say. Take the invention of vaccination, for example. Extracting pus from a woman infected with cowpox and injecting it into an 8-year-old boy seems utterly reckless, but 18th-century physician Edward Jenner found a way to combat the deadly scourge of smallpox.
Only with hindsight can we see that Jenner did something: a principle that has now saved millions of lives. That’s why, at this quarter of a century, we decided to look back and celebrate the ideas that really mattered in the last 25 years – the ones that are already changing the way we behave, think or understand what’s around us.
While compiling our list of the 21 best ideas of the 21st century, there was a lot of heated debate within the editorial team. Our first hurdle was the unexpectedly cryptic question of whether the first quarter of the 21st century ended at the beginning of 2025 or at its end. We chose the end to be safe. Then it was down to the ideas themselves and further discussion about what should really count, from whether the microbiome is really a 21st century concept (we decided it was) to whether social media was a good or bad idea (after a while we decided it was bad. What makes a good or bad idea is, after all, subjective.
In the end, we came up with a strict set of criteria. To make this list, a concept must already have a transformative impact—whether on our understanding of ourselves, our health, or the wider universe. It must have an idea at its core, even if it has been backed up by scientific discoveries. And finally, it had to happen in the last 25 years.
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Rather than trying to predict the future, it is worth spending time reflecting on the past
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You might think the last criterion would be easy to manage, but there were many designs that didn’t quite meet it. Gravitational waves were discovered in the 21st century and brought us a whole new way of looking at the universe, but were first predicted by Albert Einstein 100 years earlier. Other ideas like weight loss drugs, personalized medicine and mRNA vaccines show a lot of promise, but haven’t had time to shine yet. Maybe they’ll make our 2050 list.
When coming up with our picks, we couldn’t ignore many ideas that seemed great at first but ended up falling flat. That’s why we’ve also compiled a list of the five worst ideas of the century so far. Sometimes it’s surprisingly hard to draw a line between the best and the worst, which is why some of our best-of-the-best list choices can seem controversial—eg. smartphones, for example, which many people would rather see removed from the planet, but on balance we see this as a positive. Or the 1.5°C global warming target that could be considered a failure: a new report found that the three-year average of global temperatures just exceeded 1.5°C for the first time. Despite this, we argue that changing the threshold from 2°C remains one of the best ideas of the century, setting the benchmark for global climate ambition.
Continuing to make a real transition away from fossil fuels is certainly a good idea, and one perhaps surprising hero we’ve come to know in this area is Elon Musk. In 2016, before dabbling in social media and politics, Musk’s automaker Tesla opened its first “gigafactory” in Nevada, marking a breakthrough in the energy transition by using economies of scale to electrify our transportation and energy systems. Other attempts to combat climate change, such as alternative fuels and carbon offsets, have made our naughty list, causing more harm than good.
One thing we learned in compiling our selection is the extent to which ideas come by chance. For most of us, finding a working socket on a long train journey allows little more than a few extra minutes to scroll through our smartphone. But for two physicists in 2005, it changed the entire world’s decarbonization strategy. Similarly, it was a eureka discovery that revealed the origin of our most complex thought processes. We learned that brain regions do not work independently, but coordinate with each other to form a powerful and complex network. Since then, these neural networks have changed our understanding of the brain, as well as how we diagnose and treat its problems.
Looking back a quarter of a century, the world was a very different place. We’ve avoided the millennium bug, the first draft of the human genome has just been completed, and the first crew has arrived on the International Space Station. We didn’t know what a Denisovan was and the word “microbiome” wasn’t in our vocabulary. On the website The new scientistwe celebrated new technologies like wireless communication, marveling at a computer chip no bigger than an aspirin tablet that would make it possible. “At its heart is a device called a Bluetooth chip,” we wrote, “and scientists are tipping it to be the next big thing.” Reasonable guess, but headphones that you don’t have to plug in are more of a nice touch than a world-changing thing, so we called it wrong.
This reflects the fact that while predictions can be appealing, they are all too often wrong and forgotten in our rush to move on to the next shiny thing. Rather than trying to predict the future, this exercise taught us that it is worth taking time to reflect on the past. Advances in health, technology, and environmentalism have undoubtedly made the world a better place this century, and we hope—if not predict—that they will continue to do so.
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