Most of us intuitively feel that reality ought to exist just fine on its own when we aren’t looking. If a tree falls in a forest when no one is around to hear the crash, the air still vibrates with sound waves, right? Yet it is a tricky proposition to prove and one that gets more slippery when you consider things that seem to exist, but that we will never be able to observe. Grappling with the question of how to measure the immeasurable can, however, help us see what reality is truly like.
credit: Jamie MillsThere are a few realms where the laws of nature themselves forbid us from treading. Nothing can travel faster than the speed of light, which means we will never see beyond the edge of the observable universe – the maximum distance that light can have traversed to reach our telescopes since the beginning of the universe. General relativity rules that nothing within a black hole can escape, so that is another no-go zone. (see “What is inside a black hole?” at the end of this article)
But perhaps the most fundamental limit to what we can measure comes from the laws of quantum physics. These tell us that if we measure some property of a quantum particle today, it is impossible to know if we will get the same result when measuring it with an identical set-up tomorrow. In this sense, the laws of quantum mechanics aren’t like Isaac Newton’s classical laws of motion, which give definite predictions (see “When things are outrageously complicated” ). Instead, they can only predict how stuff behaves on average.
The measurement problem
The traditional interpretation of these facts is that particles exist in a cloud of many possible states at once, described by a mathematical construct known as a wave function. The idea is that the wave function only collapses into a single state, to certainty, upon measurement. If so, before we look at it, reality is a kind of fog of possibilities and our knowledge of it is blurry at best.
But not everyone agrees with that. For Vlatko Vedral, a physicist at the University of Oxford, it is a mistake to make a distinction between a particle that adheres to the rules of quantum theory and the observer or measurement apparatus that follows the laws of classical physics. He reckons that, ultimately, everything is quantum and we should see reality as one gigantic, universal wave function.
If we accept that wave functions are the essence of reality, this casts quantum physics in a new light. Everything has a wave function and all of them are quantum entangled with each other, meaning a measurement of one affects the others. So we can’t think about measuring isolated objects in the traditional scientific sense because the measurement apparatus and the object being measured always interact. In other words, reality as we see it is a product of both the observer and the object under scrutiny, rather than some stand-alone real view of that object, which seems beyond us. “You can only ever isolate systems imperfectly,” says Noson Yanofsky at Brooklyn College in New York. “That leads to a lack of knowledge.” Whether this is really a limitation, though, depends on your point of view. “It’s only a limitation if you’re thinking in terms of these old concepts,” says Vedral.
Relational quantum mechanics
Many physicists would agree that drawing a clear line between small quantum objects and larger classical ones is problematic. But opinions differ on precisely how to interpret the quantum realm. Carlo Rovelli at Aix-Marseille University in France doesn’t think the wave function is a real object. He has been working on an alternative idea known as the relational interpretation of quantum mechanics. In this view, everything in existence exists only in relation to other things, including you. A particle is there when you measure it, but doesn’t exist at all times. “To ask what an electron’s momentum is can simply be a meaningless question,” he says.
We may never prove which interpretation is right, because our observations of the quantum world seem to change it. But there may be ways to progress. Rovelli argues that we can use the different interpretations to clarify existing puzzles in physics. For instance, he suspects that his relational view could help align the two famously incompatible pillars of modern physics: quantum theory and general relativity. If it does, that is a good reason to favour it.
Then there is the possibility – albeit a slim one – that one day we will find a way of seeing into the quantum fog without collapsing the wave function. If the laws of physics say something is impossible, that is only valid as long as the laws themselves hold. A deeper version of quantum theory could come along and make the impossible possible. But Vedral isn’t holding his breath that a definite, predictable world will reappear. “It’s probably going to be even weirder,” he says.
WHAT IS INSIDE A BLACK HOLE?
If you fall into a black hole, you aren’t coming back. That goes not just for people, but any measuring device or unfortunate craft we might send into the void. Past the black hole’s iconic event horizon, gravity becomes so strong that objects are “spaghettified”. Still, there might be a loophole by which something can come back out.
Decades ago, Stephen Hawking figured out that black holes slowly evaporate by emitting radiation. If a black hole evaporates entirely, then the information it had absorbed over the aeons would have been obliterated – but a key law of nature is that information can’t be destroyed. This is the essence of the black hole information paradox.
Physicists remain sharply divided on what to make of it. We have photographed black holes, modelled them using exotic fluids here on Earth and chipped away at the problem theoretically. Still, the goal is that by studying what happens right at the edge of the event horizon, we can inch closer to solving it. In the process, many hope we might get a glimpse of a new theory of quantum gravity, one that surpasses both quantum mechanics and general relativity.
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