When I got into science, my goal was ambitious but simple: to devise a theory that could explain “everything,” at least everything about the physical world. I wanted to know The Truth. But alas: Decades spent practicing science taught me a lesson that was both wonderful and humbling: We can’t know everything. We can’t even know what “everything” is or means. If there is a final truth out there, it’s beyond us. Science works under strict boundaries, and as hard as we may try, we can’t go beyond them. To know all answers, we need to start by knowing all questions. And that is simply impossible. Our view of the world will always be incomplete.
This pronouncement may seem strange coming from a practicing theoretical physicist, especially a tireless promoter of the STEM (science, technology, engineering and mathematics) fields to the general public and to today’s students. But it’s time for science to be presented for what it is and not for what we would like it to be. Science, as a very human endeavor, shares many of our most endearing virtues, including our fallibility.
There are two main reasons why science has essential limits. The first comes from the tools we build to investigate the world. We use all sorts of instruments to amplify our vision of nature, to define what we call reality. And tools do serve us well, revealing invisible and unsuspected parts of nature. They bring us galaxies that are billions of light years away, microbes that can kill us, atoms that make up matter. But tools only work within a certain range and precision. We are always going to be partially myopic to the full action. For example, the Large Hadron Collider, the machine at the European Center for Particle Physics that discovered the Higgs boson, can probe energies about 10,000 times that stored in a proton mass. This is an incredible achievement! But our current theories of the universe run all the way to energies 1,000 trillion times that value. Even if we could cover this enormous range, we couldn’t catch everything on the way. Much remains outside our grasp.
The second source of limits is even more problematic. Nature itself offers insurmountable barriers to how much we can know. The speed of light, the fastest possible speed we’ve observed, is still not instantaneous. That means that all information that we collect about the world is in the past. You are seeing this article about one billionth of a second ago, the time it takes for light to travel from the screen to your eyes. You see the sun as it was about eight minutes ago. To look at the sky is to look at the past. Given that the universe is 13.8 billion years old, light traveled a finite distance since the beginning of time. Like fish in a bowl, we live inside a cosmic bubble of information with a radius of about 46 billion light years. (Not 13.8, because the expansion of space stretches the reach of light.) The universe may well continue beyond, just as the ocean continues beyond the horizon we see from the beach. But we can’t see what’s out there. We can’t ever be sure if the universe is infinitely big or infinitely old. We can’t measure “infinite.” We can only answer questions within the range of what we can measure.
There is no “absolutely sure” in science, although we can reach compelling conclusions. We can obtain data with a certain amount of precision and then extrapolate from it using statistics. The age of the universe, the composition of matter and global warming are good examples. Models that we develop to explain the data change as we gather more details. “Reality” is a construction carefully put together from a combination of what our senses and instruments can catch of nature. As our instruments change, what we call reality changes. The universe will be a very different place in 100 years, just as it was 100 years ago. Science is an incredibly successful narrative that we patch together the best way we can. But it should not be sold as perfect or all-powerful.
This is not a defeatist view of science. Quite the contrary, it’s liberating. Science is the only way to make quantitative sense of the physical world. As scientific knowledge advances, we find new and unpredictable ways to understand nature; we can ask questions we couldn’t even have anticipated before; we find new goals to pursue instead of the same old ones. Knowledge is the endless frontier.