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There are some questions that, if you look up the answer, might make you question the reliability of your brain.

Many other examples abound, from the color of different flavor packets of Walkers crisps to the spelling of Looney Tunes (vs. Looney Toons) and Febreze (vs. Febreeze) to whether the Monopoly Man has a monocle or not.

Perhaps the simplest explanation for all of these is simply that human memory is unreliable, and that as much as we trust our brains to remember what happened in our own lives, that our own minds are at fault. But theres another possibility based on quantum physics thats worth considering: could these truly have been the outcomes that occurred for us, but in a parallel Universe? Heres what the science has to say.

Visualization of a quantum field theory calculation showing virtual particles in the quantum vacuum. (Specifically, for the strong interactions.) Even in empty space, this vacuum energy is non-zero, and what appears to be the ground state in one region of curved space will look different from the perspective of an observer where the spatial curvature differs. As long as quantum fields are present, this vacuum energy (or a cosmological constant) must be present, too.

One of the biggest differences between the classical world and the quantum world is the notion of determinism. In the classical world which also defined all of physics, including mechanics, gravitation, and electromagnetism prior to the late 19th century the equations that govern the laws of nature are all completely deterministic. If you can give details about all of the particles in the Universe at any given moment in time, including their mass, charge, position, and momentum at that particular moment, then the equations that govern physics can tell you both where they were and where they will be at any moment in the past or future.

But in the quantum Universe, this simply isnt the case. No matter how accurately you measure certain properties of the Universe, theres a fundamental uncertainty that prevents you from knowing those properties arbitrarily well at the same time. In fact, the better you measure some of the properties that a particle or system of particles can have, the greater the inherent uncertainty becomes an uncertainty that you can not get rid of or reduce below a critical value in other properties. This fundamental relation, known as the Heisenberg uncertainty principle, cannot be worked around.

This diagram illustrates the inherent uncertainty relation between position and momentum. When one is known more accurately, the other is inherently less able to be known accurately. Every time you accurately measure one, you ensure a greater uncertainty in the corresponding complementary quantity.

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There are many other examples of uncertainty in quantum physics, and many of those uncertain measurements dont just have two possible outcomes, but a continuous spectrum of possibilities. Its only by measuring the Universe, or by causing an interaction of an inherently uncertain system with another quantum from the environment, that we discover which of the possible outcomes describes our reality.

The Many Worlds Interpretation of quantum mechanics holds that there are an infinite number of parallel Universes that exist, holding all possible outcomes of a quantum mechanical system, and that making an observation simply chooses one path. This interpretation is philosophically interesting, but may add nothing-of-value when it comes to actual physics.

One of the problems with quantum mechanics is the problem of, what does it mean for whats really going on in our Universe? We have this notion that there is some sort of objective reality a really real reality thats independent of any observer or external influence. That, in some way, the Universe exists as it does without regard for whether anyone or anything is watching or interacting with it.

This very notion is not something were certain is valid. Although its pretty much hard-wired into our brains and our intuitions, reality is under no obligation to conform to them.

What does that mean, then, when it comes to the question of whats truly going on when, for example, we perform the double-slit experiment? If you have two slits in a screen that are narrowly spaced, and you shine a light through it, the illuminated pattern that shows up behind the screen is an interference pattern: with multiple bright lines patterned after the shape of the slit, interspersed with dark lines between them. This is not what youd expect if you threw a series of tiny pebbles through that double slit; youd simply expect two piles of rocks, with each one corresponding to the rocks having gone through one slit or the other.

Results of a double-slit-experiment performed by Dr. Tonomura showing the build-up of an interference pattern of single electrons. If the path of which slit each electron passes through is measured, the interference pattern is destroyed, leading to two piles instead. The number of electrons in each panel are 11 (a), 200 (b), 6000 (c), 40000 (d), and 140000 (e).

The thing about this double slit experiment is this: as long as you dont measure which slit the light goes through, you will always get an interference pattern.

This remains true even if you send the light through one photon at a time, so that multiple photons arent interfering with one another. Somehow, its as though each individual photon is interfering with itself.

Its still true even if you replace the photon with an electron, or other massive quantum particles, whether fundamental or composite. Sending electrons through a double slit, even one at a time, gives you this interference pattern.

And it ceases to be true, immediately and completely, if you start measuring which slit each photon (or particle) went through.

But why? Why is this the case?

Thats one of the puzzles of quantum mechanics: it seems as though its open to interpretation. Is there an inherently uncertain distribution of possible outcomes, and does the act of measuring simply pick out which outcome it is that has occurred in this Universe?

Is it the case that everything is wave-like and uncertain, right up until the moment that a measurement is made, and that act of measuring a critical action that causes the quantum mechanical wavefunction to collapse?

When a quantum particle approaches a barrier, it will most frequently interact with it. But there is a finite probability of not only reflecting off of the barrier, but tunneling through it. The actual evolution of the particle is only determined by measurement and observation, and the wavefunction interpretation only applies to the unmeasured system; once its trajectory has been determined, the past is entirely classical in its behavior.

Or is it the case that each and every possible outcome that could occur actually does occur, but simply not in our Universe? Is it possible that there are an infinite number of parallel Universes out there, and that all possible outcomes occur infinitely many times in a variety of them, but it takes the act of measurement to know which one occurred in ours?

Although these might all seem like radically different possibilities, theyre all consistent (and not, by any means, an exhaustive list of) interpretations of quantum mechanics. At this point in time, the only differences between the Universe they describe are philosophical. From a physical point of view, they all predict the same exact results for any experiment we know how to perform at present.

However, if there are an infinite number of parallel Universes out there and not simply in a mathematical sense, but in a physically real one there needs to be a place for them to live. We need enough Universe to hold all of these possibilities, and to allow there to be somewhere within it where every possible outcome can be real. The only way this could work is if:

From a pre-existing state, inflation predicts that a series of universes will be spawned as inflation continues, with each one being completely disconnected from every other one, separated by more inflating space. One of these bubbles, where inflation ended, gave birth to our Universe some 13.8 billion years ago, where our entire visible Universe is just a tiny portion of that bubbles volume. Each individual bubble is disconnected from all of the others.

The Universe needs to be born infinite because the number of possible outcomes that can occur in a Universe that starts off like ours, 13.8 billion years ago, increases more quickly than the number of independent Universes that come to exist in even an eternally inflating Universe. Unless the Universe was born infinite in size a finite amount of time ago, or it was born finite in size an infinite amount of time ago, its simply not possible to have enough Universes to hold all possible outcomes.

But if the Universe was born infinite and cosmic inflation occurred, suddenly the Multiverse includes an infinite number of independent Universes that start with initial conditions identical to our own. In such a case, anything that could occur not only does occur, but occurs an infinite number of times. There would be an infinite number of copies of you, and me, and Earth, and the Milky Way, etc., that exist in an infinite number of independent Universe. And in some of them, reality unfolds identically to how it did here, right up until the moment when one particular quantum measurement takes place. For us in our Universe, it turned out one way; for the version of us in a parallel Universe, perhaps that outcome is the only difference in all of our cosmic histories.

The inherent width, or half the width of the peak in the above image when youre halfway to the crest of the peak, is measured to be 2.5 GeV: an inherent uncertainty of about +/- 3% of the total mass. The mass of the particle in question, the Z boson, is peaked at 91.187 GeV, but that mass is inherently uncertain by a significant amount.

But when we talk about uncertainty in quantum physics, were generally talking about an outcome whose results havent been measured or decided just yet. Whats uncertain in our Universe isnt past events that have already been determined, but only events whose possible outcomes have not yet been constrained by measurables.

If we think about a double slit experiment thats already occurred, once weve seen the interference pattern, its not possible to state whether a particular electron traveled through slit #1 or slit #2 in the past. That was a measurement we could have made but didnt, and the act of not making that measurement resulted in the interference pattern appearing, rather than simply two piles of electrons.

There is no Universe where the electron travels either through slit #1 or slit #2 and still makes an interference pattern by interfering with itself. Either the electron travels through both slits at once, allowing it to interfere with itself, and lands on the screen in such a way that thousands upon thousands of such electrons will expose the interference pattern, or some measurements occurs to force the electron to solely travel through slit #1 or slit #2 and no interference pattern is recovered.

Perhaps the spookiest of all quantum experiments is the double-slit experiment. When a particle passes through the double slit, it will land in a region whose probabilities are defined by an interference pattern. With many such observations plotted together, the interference pattern can be seen if the experiment is performed properly; if you retroactively ask which slit did each particle go through? you will find youre asking an ill-posed question.

What does this mean?

It means as was recognized by Heisenberg himself nearly a century ago that the wavefunction description of the Universe does not apply to the past. Right now, there are a great many things that are uncertain in the Universe, and thats because the critical measurement or interaction to determine what that things quantum state is has not yet been taken.

In other words, there is a boundary between the classical and quantum the definitive and the indeterminate and that the boundary between them is when things become real, and when the past becomes fixed. That boundary, according to physicist Lee Smolin, is what defines now in a physical sense: the moment where the things that were observing at this instant fixes certain observables to have definitively occurred in our past.

We can think about infinite parallel Universes as opening up before us as far as future possibilities go, in some sort of infinitely forward-branching tree of options, but this line of reasoning does not apply to the past. As far as the past goes, at least in our Universe, previously determined events have already been metaphorically written in stone.

This 1993 photo by Carol M. Highsmith shows the last president of apartheid-era South Africa, F.W. de Klerk, alongside president-elect Nelson Mandela, as both were about to receive Americas Liberty Medal for effecting the transition of power away from white minority rule and towards universal majority rule. This event definitively occurred in our Universe.

In a quantum mechanical sense, this boils down to two fundamental questions.

The answer seems to be no and no. To achieve a macroscopic difference from quantum mechanical outcomes means weve already crossed into the classical realm, and that means the past history is already determined to be different. There is no way back to a present where Nelson Mandela dies in 2013 if he already died in prison in the 1980s.

Furthermore, the only places where these parallel Universes can exist is beyond the limit of our observable Universe, where theyre completely causally disconnected from anything that happens here. Even if theres a quantum mechanical entanglement between the two, the only way information can be transferred between those Universes is limited by the speed of light. Any information about what occurred over there simply doesnt exist in our Universe.

We can imagine a very large number of possible outcomes that could have resulted from the conditions our Universe was born with, and a very large number of possible outcomes that could have occurred over our cosmic history as particles interact and time passes. If there were enough possible Universes out there, it would also be possible that the same set of outcomes happened in multiple places, leading to the scenario of infinite parallel Universes. Unfortunately, we only have the one Universe we inhabit to observe, and other Universes, even if they exist, are not causally connected to our own.

The truth is that there may well be parallel Universes out there in which all of these things did occur. Maybe there is a Berenstein Bears out there, along with Shazaam the movie and a Nelson Mandela who died in prison in the 1980s. But that has no bearing on our Universe; they never occurred here and no one who remembers otherwise is correct. Although the neuroscience of human memory is not fully understood, the physical science of quantum mechanics is well-enough understood that we know whats possible and what isnt. You do have a faulty memory, and parallel Universes arent the reason why.

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Could quantum mechanics explain the Mandela effect? - Big Think