by Bradley Knockel
There are the usual mysteries of science: Can we build more efficient engines? Can we make materials that are superconductors at even higher temperatures? Can we figure out the specific identity of dark matter? Are there more planets or near-Earth asteroids in our Solar System? Can we better understand chaotic motions such as turbulence? Science is well equipped to explore these questions by trying to gather more data, and the Internet is full of accessible information on these topics.
There is another common type of mystery in science, but it only exists on the personal subjective level. As we try to wrap our heads around proven ideas such as Einstein's relativity, the universe can seem beautifully mysterious. We objectively understand the equations and math behind ideas like relativity in that we can "teach" computers the math so that they can do very precise calculations, but half the fun of science is trying to personally understand these ideas. As we first begin to learn about the world, we are mystified by: the complexity of a living cell, how the atmosphere pushes on every square inch of our skin with 15 pounds, how the Earth goes around the Sun rather than the Sun going around the Earth, how both ends of a tug-of-war rope have the same force regardless of who is winning, etc.
However, I want to explore the deeper mysteries that may never be solved and that are not always talked about. In doing this, we may begin to understand the limits of science. Deeper more fundamental mysteries have to do with humans being ignorant of the most basic laws of physics (everything we have now is only an approximation!). The following list of three mysteries is put in order from what I consider to be least mysterious to the most mysterious.
A fundamental assumption of science is that the objective world exists with consistent universal laws. If this assumption is lifted, a person begins to wonder things such as whether or not their whole life has been a dream or simulation. So that I don't have to think about such weird things, let me keep this basic assumption of science in all the discussion that follows.
We know that quantum mechanics is correct, but it only works when there are not huge curvatures of spacetime that create strong gravity, so it does not work inside black holes or during the first fraction of a second after the Big Bang. We know that our understanding of gravity as curvatures of spacetime, Einstein's general relativity, is correct, but it only works when the matter in stars is not denser than the nucleus of an atom (that is, when quantum mechanical effects become significant), so general relativity also does not work inside black holes or during the first fraction of a second after the Big Bang. Physicists consider quantum mechanics to be modern physics and Einstein's relativity to be classical physics, and neither can take us into the most exotic locations in time and space! They are incomplete.
We do not know the correct theory that allows general relativity and quantum mechanics to coexist! String theory is our only complete guess. However, string theory currently makes no predictions that we could test, so it should not really yet be called a scientific theory. String theory is important because it could be the laws that the universe has to obey. Perhaps we will never completely understand black holes, the Big Bang, or why the expansion of the universe is accelerating without fully working out this larger theory.
More importantly, we may never know all the interesting things that this new theory would show us. For example, string theory would inform us that there could be many compactified extra spatial dimensions that are too small to notice, or maybe our universe is a 3D "slice" of a much higher-dimensional place.
We only see two aspects of the full picture (that is, we see quantum mechanics and general relativity), but who knows how amazing the nature of the actual complete laws of physics are!
Infinities. Regardless of how improbable, given infinite chances to occur, any possible event will happen. In fact, it will happen infinite times. This is an inescapable mathematical fact.
Boltzmann brains are the random assembly of a working brain in the coldness of space. While unimaginably unlikely, there is some chance that the dust and gas in space can collide in just the right way! Because entropy will eventually prevent stable complex life in the universe, thoughts from Boltzmann brains in the entire infinite future vastly outnumber thoughts from normal (stable) brains in regards to amount of conscious thought (assuming certain cosmological situations), so why are you a stable person rather than a Boltzmann brain? Or maybe you are a Boltzmann brain that simply believes it has your memories and will be destroyed by the harshness of space momentarily.
A possible situation is that our universe is an inflationary bubble that is part of a multiverse of infinite inflationary bubbles. Our universe would be one of the inflationary bubbles that have conditions suitable for life. Even without needing infinite time, in this speculative model, there is the problem with Boltzmann brains outnumbering us in the likely-to-be-far-more-common messier universes of the multiverse where stable life and planets cannot form unless some specific inflationary mechanism prevents these messy universes. Or it could be that there is no multiverse, but our universe is spatially infinite and the density of galaxies is constant, which could cause similar problems with Boltzmann brains dominating conscious thought. In an infinite universe or multiverse, the mystery is not why seemingly unlikely things such as the origin of life can occur. Instead, the mystery is whether or not there is infinite life primarily in bizarre configurations in the universe.
These mysteries arise when there is infinite (or very large) time and space, and we do not know if this is the case. Science does not know our far future, if there is a multiverse, or if our own universe is infinite.
A similar type of mystery involves the microscopic infinities. That is, even a fraction of a second for a tiny atom seems to contain an infinite amount of calculation for the infinite amount of decimal places of space, time, etc. The problem is multiplied by the perhaps infinite quantum states of the atom all existing and interacting with each other. If the universe were a computer, it seems to need to be infinitely large to calculate just the smallest part of itself. Maybe the universe is not microscopically infinite in that laws of physics may be like a game of cards with finite pieces and rules, and maybe understanding this game of cards could help solve the other mysteries in this list.
We know that quantum mechanics is correct, but we do not know how to interpret it.
First, I must explain that light is a wave. When shining light through two slits, it can bend and interfere like a wave...
A fantastic video showing how unexpected it is that light is a wave.
An experiment you can personally do if you have a laser.
Quantum mechanics becomes relevant when the intensity of light is so low that only a single photon passes through the slits at any time.
Shining single photons through two slits.
We observe that the single particle, in this case a photon, goes through both slits and interferes with itself at the screen. Yes, a single photon goes through both slits. This is as if the photon were a wave when going through the slits then remembering that it is a particle at the screen where it hits an exact location.
A fundamental idea of quantum mechanics is that particles move in all possible ways at the same time (our photon going through both slits for example). These possibilities can interfere with each other giving the appearance of a wave (for example, bright bands on our screen). However, when a measurement is made such as at the screen, the particle must finally hit a single location. That is, it must hit a single location on the screen because it is a particle after all.
The mathematics of quantum mechanics very accurately predicts the probabilities of where the photons will strike the screen. We cannot predict where a single photon will collide, but, after looking at many photons, we can confirm our predictions of the probabilities. The deep unanswered question that now arises is: how did each single photon know which part of the screen to hit if it was simultaneously traveling towards all parts of the screen? Interference explains why the photon is more likely to hit certain parts of the screen, the bright bands, but how then does the photon "choose" which bright band to hit? No one knows. On a practical level, answering the question is not very important because the mathematics of quantum mechanics work just fine without anyone needing to answer this question. But the question is philosophically very interesting, and may become practically important as we continue to further develop quantum computers.
Electrons and photons are very similar in that they both travel as waves and interact with detectors as particles. Electrons form their electron clouds around atoms because of their wave nature just how photons can produce light waves. Here, Brian Greene explains everything we have been talking about but with electrons. With good science, we must be able to make correct predictions. Because electrons have electric charge, if we send them through two slits, we can put detectors by the slits that detect the electric fields created by the electrons as they pass through the slits to see which slit the electrons actually go through. Quantum mechanics predicts that the measurement at the slits (being an interaction with a macroscopic object/detector) causes the electron to act like a particle again and "choose" a slit. That is, when the detector is present at the slits, there will only be two bands of electrons on the screen (one band for each of the two slits). When the detector is removed, the interference pattern of many bands should reappear. This is what we observe! Quantum mechanics truly is the correct theory!
So what determines where the photon or electron hits? It could be that the universe is fundamentally random (nondeterministic). It could be that some supernatural guidance or guidance by human consciousness occurs. It could be that the photon carries with it some information regarding what choice it will make, but theories like this must be exotic to conform to experiments (Bohmian mechanics is an example of a possible theory). It could be that the photon hits every part of the screen in many parallel universes that pop into existence during the measurement—each universe containing one of the possibilities—which would lead to an ever growing number of universes in which all variations and possibilities are realized. Yikes! None of these interpretations are easy to swallow, but one of these interpretations may be correct. There may be no way to ever find out because these interpretation do not make observable predictions.
However, other simpler possibilities exists that we may be able to observe in the far future. Let's discuss the Everett interpretation of quantum mechanics. Measurements are defined to be when a particle such as a photon interacts with a much larger (macroscopic) system of particles such as a screen. If our computational ability is ever good enough, we may be able to simulate the quantum mechanics of the entire measurement (photon and entire screen) and unambiguously see that the appearance of a measurement is just an emergent result of the complexity of the system. For example, it may easily be the case that the photon always keeps trying to simultaneously travel in all possible paths, so it hits all parts of the screen, but this interaction with the macroscopic screen and possibly any human observers separates ("decoheres") the possible outcomes of the now combined ("entangled") screen&observer&photon system so that all outcomes of this combined system simultaneously exist (just like how the photon went through every possible slit simultaneously) but the "decoherence" prevents these outcomes from interfering creating effectively isolated new worlds having every possible variation of the screen&you&photon.
While the Everett interpretation seems bizarre and makes me feel very insignificant (I'm just one of many versions of me), it likely requires no new physics (decoherence is becoming ever more well understood to be a simple expected occurrence), and actually requires fewer postulates than traditional quantum mechanics, so it passes Occam's razor. As for me feeling insignificant, science has always had this trend from discovering the center of the Solar System to be the Sun to discovering that there are billions of billions (perhaps infinite) planets in the universe, and our galaxy is not the center of anything. Perhaps there are infinite simultaneous realities that have decohered and continue to decohere from each other! Could it be that there are realities where I never exist or am completely miserable? Sometimes, these thoughts fill me with more awe than dread.
But where do these new versions of ourselves come from? Well, the multiple paths were always present in the photon (and the emitter of the photon before it), and the environment gets mixed up with the photon's various paths as the photon interacts with the screen. This interaction causes the environment to differ in the different versions of reality that already existed.
However, considering that we know that our understanding of quantum mechanics is incomplete (see above discussion on quantum gravity), I cannot shake the idea that these multiple versions of ourselves may not be able to fully decohere, and may somehow interfere to eventually select only one these paths as being possible. This may be a simple emergent property of what we already understand, or there could be some new physics that causes it. To speculate much further, perhaps then our consciousness could be somehow related to the sum of all these soon-to-interfere quantum realities just as the universe is the sum of quantum realities. Or it could be that, in the case of a soul, our soul may follow only a particular version of ourselves. Or, our standard ideas of math may not apply to the universe!
We cannot scientifically test these interpretations of quantum mechanics in the near future, so just please understand that all of these interpretations are possible. However, the interpretation can affect philosophical pondering such as that in this list. For example, the Everett interpretation is a fundamentally deterministic system (even though it does not seem this way relative to any observer). This gives a "smoother" description of how quantum systems evolve in time (compared to the case where measurements abruptly change the system). I have read that this smoother description may be used to remove various forms of the Boltzmann-brain problem.
2020 Update: Check out this great video explaining everything above!
It's almost miraculous that we can understand the universe as much as we do. To expect this progress to continue forever may not be correct. The first mysteries I mentioned—how to make faster engines, how to understand Einstein, etc.—were solid science. We then moved into more philosophical grounds by speculating about the fundamental laws of physics. Finally, the greatest mysteries of this universe are almost completely philosophical.
Existence. Try to imagine nothingness: the universe does not and never existed (no space, time, or matter). This is easy to imagine if you find yourself in some other place such as heaven. But now imagine that this heaven and any other place never existed and that you never existed. No task is harder for me than trying to imagine nothingness because the only thing that I truly know or can know while I exist is (my) existence. The fact that something exists is actually quite bizarre. Why should a universe exist that is governed by such interesting laws? It could be that matter and energy arise naturally from the laws of physics, but the question then becomes: where did the laws of physics (or logic itself) come from? A common answer is that God or gods created the laws of physics. But this answers very little because why then does that supernatural world exist? Perhaps, just like trying to imagine non-existence, trying to answer questions about a supernatural world or about the origin of physical law does not make sense. Certainly science can have no say on the issue.
Consciousness. Every cell in your body is alive and acting independently determined by its internal chemical structure and determined by outside stimuli from other cells or from the external world. Each of these cells is unimaginably complex. Interestingly, the majority of cells in your body are not even human because the foreign bacteria living in your gut outnumber your human cells. What is unique about the brain that makes this collection of cells—which are each just a collection of atoms—conscious? No one knows! Why does cutting down a tree (which certainly distresses the cells and life of the tree) seem OK but distressing a person's brain by chopping off their arm seems to injure something that is conscious of the distress?
The science known as neuroscience is learning about what types of things create consciousness. For example, we can be awake and have no consciousness (by being "vegetables"), and you can be asleep and have consciousness (dreams during REM sleep). General anesthesia seems to completely pause consciousness without removing life. We are not conscious of sensory signals but instead are conscious of what the brain believes the sensory signals to be (hence hallucinations). Consciousness is correlated to when the brain uses many different parts of itself for a single task. This sort of description comes no where near to answering the hard question: how does consciousness arise from (or in spite of) physical reality?
There are many other questions that arise when thinking about this topic. Do you perceive red in the same way that I do? Is there a soul that exists in addition to physical existence and what aspects of the brain (memory, emotion, etc.) would it share/access? That is, does consciousness arise from a soul interacting with the brain, or does it emerge from just a brain? Is there a difference between duplicating and restarting a brain or are both cases creating a new consciousness? What would happen if all Chinese were to create the China brain? If a computer simulates a working brain, is it killing to stop it? When does consciousness begin? Is human consciousness significantly different from animal consciousness especially if considering very young humans?
Answering these questions becomes quite hard especially when we realize that our bodies (and brains) almost completely replenish all the atoms over several years. Though, important exceptions occur: neurons, while sometimes can form later in life, are not replaced, and DNA atoms are often not replaced.
A mystery that I ponder involves why consciousness experiences time. In physics, time is a dimension just like any of the 3 spatial dimensions. Time only has a seemingly favored direction because of the low-entropy initial conditions of our universe (the fundamental laws of physics have a CPT symmetry, so there should otherwise be no seemingly favored direction). If the universe is deterministic, then everything that will be and has been are on equal footing, all set in stone. Assuming that my consciousness is somehow "attached" to the 4-dimensional worldline of my brain in the quantum version of the universe I inhabit, why is it only at one point of the worldline and then the next always in the same temporal direction? Or is it? When I die, what is so special about time that my consciousness should stop existing in any meaningful way? My consciousness is part of the fixed 4D spacetime, and nothing can change that. A related mystery is: in the so-called "twin paradox", a twin can become younger than the other twin by taking a high-speed journey away and back, and, during the away journey, relative to either twin, the other twin is younger. To describe this in physics, we say that time is relative. This means that every consciousness is fundamentally isolated from every other consciousness not just by space but by time. Honestly, I don't know if any of these ponderings even make any sense, and I have certainly made many assumptions. Perhaps the universe is not deterministic or our consciousness somehow selects the quantum version of reality to follow moment by moment. Truly, consciousness is a great mystery.
While scientific words are used in this discussion, I believe that science will never help much in truly understanding consciousness, which may be the most complex phenomenon to exist in the entire universe.
In school and life, we learn so many things, and many of these things are known quite well, so we often forget that the things that we humans don't know outnumber what we do know. We especially do not usually consider many of the deepest mysteries because they are so strange compared to more practical concerns. I feel that they are at the heart of curiosity and the spirit of science.