The act of measuring reality alters what you’ll observe

When we divide up matter into the smallest possible chunks that it’s made of — into the stuff that can be divided or split no further — those indivisible things we arrive at are known as fundamental particles: the elementary quanta that compose our Universe. But unlike the macroscopic, familiar objects that make up our world, we can’t simply assign definitive properties to them like “position” and “momentum” in an unambiguous fashion. Instead, the story gets complicated each time we ask the question: how does each individual quantum behave? Do they behave like particles? Or do they behave like waves?

The most puzzling fact about quantum mechanics is that the answer you get depends on how, or even whether, you look at the individual quanta that are part of the experiment. If you make certain classes of measurements and observations, they behave and interact like particles; if you make other choices, they behave and propagate like waves. Whether and how you observe your own experiment really does change the outcome, and the double-slit experiment is the perfect way to illustrate how, even more than 200 years after it was first performed.

Back in the late 1790s and early 1800s, the first double-slit experiments were performed by Thomas Young, who was seeking to investigate whether light behaved as a wave or as a particle. Newton had famously claimed that light must be a particle, or corpuscle, and was able to explain a number of phenomena with this idea. Reflection, transmission, refraction, and any ray-based optical phenomena were perfectly consistent with Newton’s particle-like view for how light should behave.

But other phenomena seemed to require a wave-like interpretation in order to explain them: interference and diffraction in particular. When you passed light through a double slit, it behaved in a similar fashion to how water waves behave, producing a familiar set of interference patterns in both cases. Instead of the crests and troughs that appear in water waves, light passed through a double slit would instead exhibit alternative light-and-dark bands, which appeared on the screen behind the double slit mask. These bands corresponded to constructive-and-destructive interference, indicating that — at least under the right circumstances — light indeed behaves just as a wave does.

If you placed your two slits in very close proximity to one another, with only a tiny separation between them, it would stand to reason that any individual quantum of energy would have to go through either one slit or the other. Like many others before you had thought, you might think that the reason light produces this interference pattern is because you have lots of different quanta of light — photons — all going through the various slits together, and the quanta going through the first slit must then interfere with the quanta that go through the second slit, and vice versa.

This is no longer a question that’s up for interpretation, however; it’s a question that can be settled by continued experimentation. Instead of light, you can take a different set of quantum objects, like electrons, and fire them at the double slit. Once again, you’ll get an interference pattern, but then you come up with a brilliant tweak: you’re going to fire the electrons one-at-a-time through the slits.

With each new electron, you record a new data point for where it landed. After thousands upon thousands of electrons, you finally look at the pattern that emerges. And what do you see? The same characteristic patterns associated with interference.

Somehow, each electron must actually be interfering with itself, rather than with the other electrons that surround them. The individual electrons must be acting fundamentally like a wave. (We’ve since repeated the experiment with single photons, one-at-a-time, and obtain an identical result.)

For many decades since, throughout all of the 20th and 21st centuries so far, physicists have puzzled and argued over what this means must really be going on. Is the electron going through both slits at once, and interfering with itself somehow? This seems counterintuitive and physically impossible, but we have a way to tell whether this is true or not: we can measure it.

We can set up the same experiment as before: passing electrons through the slits one-at-a-time and measuring the pattern that accumulates on the screen. However, this time, we have a little light we shine across each of the two slits. When the electron goes through, the light is slightly perturbed, so we can “flag” which one of the two slits it passed through. With each electron that goes through, we’ll record a signal coming from one of the two slits, telling us which slit the electron passed through before landing on the screen. At last, after each electron has been counted, we now know which slit every one went through.

And now, at the end, when we look at our screen, this is what we see.

The interference pattern that had appeared previously? It’s gone. Instead, it’s replaced by just two piles of electrons: the paths you’d expect each electron to take if there were no interference at all.

What’s going on here? It’s as though the electrons “know” whether you’re watching them or not. The very act of observing this setup — of asking, “Which slit did each electron pass through?” — changes the outcome of the experiment from the behavior it would have exhibited had you not made those observations.

This isn’t to suggest that the electron itself is conscious, and making decisions based on whether you make the decision to observe it or not. Rather, we can only describe the electron’s behavior based on the physical setup of the experiments we can conduct. If you measure which slit the quantum passes through, it behaves as though it passes through one and only one slit with no interference or wave-like behavior: it acts like a classical particle. If you don’t measure which slit the quantum passes through, it behaves as a wave, acting like it passed through both slits simultaneously and with an individual electron’s final position determined by a probabilistic wavefunction.

Can we figure out what’s fully going on here? To get closer to the answer, we have to perform more experiments.

One experiment you can conduct is to put a movable mask in front of both slits, while still firing electrons through them one-at-a-time. Practically, this has now been accomplished in the following fashion:

• a movable mask with a hole in it starts off by blocking both slits,
• it moves to the side so that the first slit is then unmasked,
• it continues moving so that the second slit is also unmasked (along with the first),
• the mask continues its motion until the first slit is once again covered (but the second is still unmasked),
• and finally both slits are covered again.

As we continuously move through all five of these conditions, with both slits initially obscured, then the first slit gradually being unmasked, then the second slit also being gradually unmasked until both slits are fully open, then with the mask gradually covering up the first slit and finally to cover up the second slit again, we can ask the all-important question, from an observational perspective, of “How does the pattern that we observe change?”

Exactly as you might have expected:

• you see a one-slit (mostly non-interfering, except for single-slit interference) pattern if only one slit is open,
• the standard wave-like, two-slit (interference) pattern if both slits are open,
• and a hybrid of the two during the in-between times, where one slit is fully open and the other is only partially open.

It’s as though if both paths are there as available options simultaneously, without restriction, you’re destined to get interference, and to see the wave-like behavior we associate with constructive and destructive interference. But if you only have one path available, or if either path is restricted somehow, you won’t get interference and will get particle-like behavior.

So we go back to having both slits in the “open” position, and shining light across both slits to measure which slit an object passes through, while you simultaneously pass electrons one-at-a-time through these double slits.

If your light is both:

• energetic (high energy per photon),
• and intense (a large number of total photons),

you won’t get an interference pattern at all. 100% of your electrons will be measured at the slits themselves, and you’ll get the results that you’d expect for classical particles alone: just a pile of electrons that went through the first slit and a second pile of electrons that went through the second slit.

However, if you steadily lower the energy-per-photon, there will come a point where the “two pile” pattern starts to change. You’ll discover that, when you drop below a certain energy threshold, your photons don’t interact with every electron that passes through the slits. Some electrons will pass through the slits without registering which slit they went through, and you’ll start to get the interference pattern back as you lower your energy.

Same thing with intensity: as you lower it, the “two pile” pattern will slowly disappear, replaced with the interference pattern, while if you dial up the intensity, all traces of interference once again disappear. It’s as though energy and intensity of the “measuring tool” you’re using both can play a role in whether the electron going through the double slit behaves as a wave or a particle.

And then, you decide to go a step further still, and get the brilliant idea to use photons to measure which slit each electron goes through, but then to destroy that information before looking at the screen.

This last idea is known as a quantum eraser experiment, and it produces the fascinating result that if you destroy the information sufficiently, even after measuring which slit the particles went through with an energetic, intense photon, you’ll still see an interference pattern on the screen.

Somehow, nature knows whether we possess the information that “marks” which slit a quantum particle passed through. If the particle is marked in some fashion, you will not observe an interference pattern when you look at the screen; if the particle is not marked (or was measured and then unmarked by destroying the marking information), then you will observe an interference pattern when you look at the screen.

We’ve even tried doing the experiment with quantum particles that have had their quantum state “squeezed” to be narrower than normal, and they not only exhibit this same quantum weirdness, but the interference pattern that comes out is also squeezed relative to the standard double slit pattern.

The squeezed quantum state result was only determined in 2018, showcasing how active of a research topic the double slit experiment still remains, more than 200 years after the earliest double slit experiments were first performed. It’s then extremely tempting, in light of all of this information, to ask what thousands upon thousands of scientists and physics students have asked upon learning it: what does it all mean about the nature of reality?

Does it mean that nature is inherently non-deterministic?

Does it mean that what we keep or destroy today can affect the outcomes of events that should already be determined in the past?

That the observer plays a fundamental role in determining what “reality” really is?

The answer, disconcertingly, is that we cannot conclude very much about any one particular property of nature at all. We still do not know whether nature:

• is deterministic or not,
• is local or non-local,
• or whether the wavefunction is real or not real.

What the double slit experiment reveals is, perhaps maddeningly, the most complete description of reality as we know how to obtain. Physics is an experimental science, and to that end, knowing the results of any experiment we can perform is as far as physics can take us. The rest — including our musings about what it might mean for the ultimate nature of reality — is no more than what we call it: an interpretation.

If your interpretation of quantum physics can successfully explain what the experiments reveal to us, it is valid; all the ones that cannot are invalid. Everything else is aesthetics, and while people are free to argue over their favorite interpretation, none of the valid interpretations can lay any more claim to being “real” than any other: not Copenhagen, not de Broglie-Bohm, not the ensemble interpretation, not the many-world interpretation, etc. However, the very heart of quantum physics can be found in these experimental results. Although many gleefully impose their own preferences on the Universe when choosing an interpretation, we must beware that we make such choices at our own peril. Adding a personally or intuitively satisfying layer atop our measured reality doesn’t make that extraneous layer real in any way. The only successful path we can take towards understanding the Universe is to simply listen to the experimental results. That’s what the Universe tells us about itself.

This article was first published in April of 2023. It was updated in August of 2026.

This article is featured on Big Think.