Why quantum mechanics says the past isn’t real

The following is an excerpt from our newsletter, Lost in Spacetime. Every month we dive into fascinating ideas from across the universe. You can register for Lost in Spacetime here.

Adolf Hitler died on April 30, 1945. At least that's the official story. But a handful of historians disputed this evidence and insisted that the Fuhrer fled war-torn Berlin and was living somewhere in hiding. Although the latter version is now widely dismissed as a baseless conspiracy theory, no reasonable historian would doubt that, whatever the disputed evidence, there was at least a “fact of the matter.” Hitler either died that day or he didn't. It would be meaningless to say that Hitler was both alive and dead on May 2, 1945. However, replace Adolf Hitler with Erwin Schrödinger's famous catand the historical “facts of the case” become seriously unclear.

Schrödinger was one of the founders quantum mechanicsthe most successful scientific theory in history. It underlies all chemistry, particle physics, materials science, molecular biology and much of astronomy and has given us dazzling technological marvels, from lasers to smartphones. The problem is that, despite all its triumphs, quantum mechanics at its lowest point seems meaningless.

In everyday life we ​​assume that there is a real world “out there” in which objects such as tables and chairs have specific, well-defined properties.how to have an attitude and orientation whether anyone is looking or not. When we observe an object in the macroscopic world, we are simply discovering a pre-existing reality. But quantum mechanics deals with the microcosm of atoms and subatomic particles, where reality turns into uncertainty and fuzziness.

Quantum uncertainty implies that the future is not entirely determined by the present. For example, if an electron is shot at a thin barrier at a known speed, it may bounce back or tunnel through the barrier and escape to the far side. Or, if an atom is brought into an excited state, then after a microsecond it can still be excited, or it can decay and emit a photon. In both cases, we cannot predict with certainty exactly what will happen; Only betting odds can be specified.

And most people are comfortable admitting that the future is somewhat open. But quantum fuzziness also implies that past It's not a done deal yet. Look on a large enough scale and history dissolves into a mixture of alternate realities, technically called superposition.

blur of the quantum microworld sharply focused during measurement. For example, you can measure the position of an electron and find that it is in a certain location. But, according to quantum mechanics, this does not mean that the electron was already there before measurement, and observation simply shows where exactly. Rather, the dimension is projected onto the electron in position from a prior state of no position.

If this is true, then how should we think about the electron before it was discovered? Imagine many semi-real “ghost electrons” distributed throughout space, each representing a different object. potential a reality suspended in a state of uncertainty. This is sometimes described by saying that the electron is in many places at the same time. Then – bam! – a measurement is carried out that serves to promote a specific “winning ghost” into a specific reality, destroying competitors.

Does the experimenter have any choice about the outcome? Not when it comes to choosing the winning ghost – it's random. But nevertheless, there is an element of choice here, and it is crucial to understanding quantum reality. If, instead of measuring position, the experimenter decides to measure the speed of the electron, then the fuzzy previous state again leads to a clear result – but this time creating not an electron in place, but an electron with speed. And it was discovered that an electron behaves like a wave with speed. This is not the same as an electron in place, which is a particle. Apparently electrons somehow both waves and particles; which aspect they exhibit depends on how one chooses to interrogate them.

Bottom line: What happens to an electron—whether it behaves like a wave or like a particle moving forward—depends on what type of measurement the experimenter decides to make to observe it. Strange, of course, but this is where it gets really strange: it also happens that has happened to the atom to measurement depends on the experimenter's decision! That is, the nature of the electron in the past – wave or particle – is determined by this choice. It seems like something is going back in time and affecting the way the world is “out there” wasbefore measurement.

Is this time travel? Retrocausation? Telepathy? All these words appear in popular articles on quantum physics, but the most apt description comes from John Wheeler, the physicist who coined the term “black hole”: “The past does not exist except what is recorded in the present,” he said.

Wheeler's description sounds profound, but is there an actual experiment to back it up? Indeed there is, as I first learned from Wheeler himself when we met for breakfast at the Baltimore Hilton in 1980. The dinner began with a cryptic question, typical of a man: “How do you hold the ghost of a photon?” he asked. Seeing my confusion, Wheeler continued to explain the new twist he had come up with for a classic quantum experiment. This is easiest to do with light, although it can be done just as well with electrons or even whole atoms.

The experiment, first performed by the English polymath Thomas Young in 1801, is an attempt to demonstrate the wave nature of light. Young placed a screen with two narrow slits close together and illuminated it with a spot light. Light passes through the slits and falls on a second screen located slightly further from the light source. What did Yang see? Not two fuzzy streaks of light as you might imagine, but a series of bright and dark streaks called interference fringes. They arise because the light waves passing through each slit spread out, and where they arrive in steps – from peak to peak, from trough to trough – they are amplified, creating a bright area, and where they arrive out of step, they are neutralized and create a dark area.

Quantum mechanics arose when physicists debated whether light was made of waves or particles called photons. We now know that, as with electrons, the answer is both. And with the help of modern technology you can do Young's experiment one photon at a time. Each photon forms a small dot on a second screen, and over time, the many dots form a motley pattern that displays the characteristic stripes that Young discovered. This seems strange: if a photon is a tiny particle, it must necessarily pass through or one slot or another. But both The slits are needed to create an interference pattern.

What then happens if a cunning experimenter decides to see through which slit a particular photon passes? This can be easily achieved by placing the detector close to the slits. When this is done, the interference pattern will disappear. The detection of interference essentially caused the photon to manifest itself as a particle, thereby eliminating its wave nature. You can do the same with electrons – figure out which slit they went through and find no stripe pattern, or leave each electron's path ambiguous and watch for stripes (after many electrons have formed a pattern). So the experimenter has to decide, photon by photon or electron by electron, whether it will behave like a wave or like a particle when it hits the image screen.

Now we get to the Wheeler turn. The decision to watch or not to watch does not have to be made in advance. In fact, it can be left until the photon (or electron) passes through the slits and is on its way to the image screen. Essentially, the experimenter can choose to look back and see which slit the photon came out of or not. This setup, which was understandably called a delayed choice experiment, was carried out and the results were as expected. When the experimenter decides to look, the photons do not form bands; when they go undetected, they do. Conclusion? Reality that was Whether the light behaved as a wave passing through both slits, or as a particle passing through one, is determined by the subsequent choice of the experimenter. I should mention that in a real experiment the “choice” is automated and randomized to avoid bias that could skew the results and because it all happens faster than human reaction time.

The delayed choice experiment does not change the past. Rather, in the absence of experimentation, there are multiple pasts—many mixed realities. When choices are made about what to measure, some of these stories are discarded. The effect of this choice is to reduce some of the past quantum fuzziness and, if not to identify a unique history, then at least to narrow down the number of contenders. This is why it is sometimes called the quantum eraser experiment.

In a real experiment, the backtracking time is only a nanosecond or so, but in principle it could extend all the way back to the origin of the universe. Indeed, this was precisely the meaning of Wheeler's mysterious question about how to contain the ghost of a photon. He imagined a distant cosmic light source. gravitational lens from our point of view, an intermediate black hole, with two light paths going around opposite sides of the black hole and then converging on Earth, a bit like a double slit experiment on a cosmic scale. A photon ghost might come along one route, while another ghost, taking a different, perhaps longer route, might not get here for another month. To do such a cosmic interference experiment, you would have to somehow store or “delay” the first ghost to wait for the second one to arrive before combining them so that the waves overlap at the same time, as happens in Young's original experiment.

Einstein once wrote that the past, present and future are just illusions. He was wrong about this. The error lies in the word “the”. A the past exists today in the historical record, but it consists of a vast number of mixed “ghost pasts” combined in such a way as to form a unique narrative on a macroscopic scale. However, at the quantum level, it becomes a mixture of blurred parts of reality that lies beyond human experience.

Paul Davis is a theoretical physicist, cosmologist, astrobiologist, and best-selling author. His book Quantum 2.0published by Penguin in November 2025.

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