GIVEN the remarkable degree of understanding that science now provides about the physical world, it is easy to lose sight of the numerous, enormous unsolved mysteries that still haunt the dreams of scientists. Below is a list of some of the greatest of these unanswered questions which perhaps, with luck, will be answered (in part or in full) in the next hundred years. This list is by no means all inclusive, as there are important enigmas that remain in nearly every branch of science.
1. Dark Matter
We still have little understanding of dark matter, the strange and non-interacting material that seems to make up about 22% of the energy of our entire universe. We can infer its existence through its gravitational effects (which, for example, alter the properties of galaxies), but we have yet to detect it directly (though there are currently attempts underway to do so). If scientists are correct about dark matter’s existence, then it seems our own milky way galaxy has much more dark matter than other types of matter. There are a variety of theories now competing to describe what dark matter is composed of, but whatever it is it interacts so little with its environment that it can easily pass through our planet and our bodies without us being able to tell the difference.
2. Dark Energy
We still have very little idea about the nature of dark energy (which, despite its name, is unrelated to dark matter), a form of energy thought to permeate all of space. Dark energy appears to be required by theory in order to account for the idea that our universe has an accelerating rate of expansion (contradicting the old belief that gravity should be causing our universe’s growth to slow).
This mysterious stuff is estimated to make up about 74% of the energy in our universe, and hence it seems that the vast majority of energy is accounted for by things that we do not yet really understand! At this point we cannot rule out the possibility that one day new theories will be able to explain astronomical observations in ways that make the ideas of dark energy (and perhaps also dark matter) unnecessary.
3. Quantum Theory + General Relativity
No one yet knows how to combine quantum theory (which deals with the physics of the very small) and general relativity (which is Einstein’s theory that describes gravity) into a single consistent theory of the universe. Some people believe that the relatively new field of string theory may lead to a consistent “theory of everything”, but since it has not yet produced any predictions that scientists have been able to test, there remains ample reason to doubt that the theory is true or even useful. Interestingly, even if we do manage one day to explain all of the known forces of physics in a single, consistent theory, that theory could lead us to still deeper riddles or new unexplained forces that we are now unaware of.
4. The Beginning of the Universe
We do not have a good understanding of the very, very early universe (say, from the moment the big bang is believed to have occurred until one trillionth of a second afterward). One aspect of the big bang that makes it so troublesome to analyze is that it is one of those rare occasions when the equations of quantum theory and general relativity must be used simultaneously, and no one knows precisely how this is to be done. The equations of relativity applied on their own predict that the universe may once have had infinite density at the moment of the big bang, but it is possible that quantum effects could alter our understanding of this scenario.
5. The End of the Universe
It is not yet understood what will happen to our universe in the long run. Will it expand forever (a “big freeze”)? Collapse into a point (a “big crunch”)? Collapse and expand in endless cycles (a “big bounce”)? Tear all matter apart due to ever increasing acceleration (a “big rip”)? Or will it come to a sort of equilibrium where repulsive and attractive forces balance? Knowledge of which of these possibilities is going to come true depends on some currently unknown facts about the universe, such as its geometry (which relates to the curvature of space) and the precise amount of matter and dark energy that it contains.
6. Memory and Learning
There are still many mysteries remaining about how the brain stores memories and information. One thing that does seem to be known is that the formation of memories occurs in many different areas throughout the brain, depending on the type of the experience, rather than being localized in a single region. A related unsolved problem is that we still lack an understanding of what sort of algorithms the brain uses to learn and to make predictions from existing information. Perhaps one day knowledge of the process of human learning could allow scientists to build machines that posses what we would recognize as true intelligence.
7. Consciousness
We don’t yet have a good sense of how consciousness (or, at least, what feels like consciousness to us) arises in the brain. Some researchers believe that consciousness is a property of any sufficiently complex system which might one day be realized in silicon chips much as it is today in carbon based brains. Others suggest that consciousness may have evolved through mutation and natural selection in order to increase the chance that the organism has of survival. Still others think that consciousness may be a fiction, a mere “trick” that our brain plays on us.
Another point of uncertainty is the extent to which animals other than humans have consciousness (however we are to define it), including chimpanzees, dogs, mice, fish, birds, reptiles, insects, and microscopic organisms. Some philosophers and scientists view consciousness as a discrete property (you either have it or you don’t) whereas many think that it can come in continuous degrees (ranging from weak forms to the strong form that we have).
8. Embryogenesis
There are many remaining uncertainties as to how a fertilized egg accomplishes the creation of an entire organism. The zygote, consisting of just a single cell, divides again and again and again, eventually producing every cell in the body. Each cell that is produced follows a set of locally applied rules (many of which are unknown or poorly understood) that lead to the construction of every organ in the body.
9. The Origin of Life
The origin of life on our planet is still shrouded in much mystery. Darwinian evolution provides a theoretical explanation for how life likely began. However, the transition from dead matter to living organisms seems to have occurred relatively rapidly more than 3.5 billion years ago, during a time when we do not fully understand the conditions on earth. Furthermore, the events during that period left no fossil record behind. Hence, it is hardly surprising that there are still many things that we do not understand about how life got its start. It is even theoretically possible (though there is currently little if any evidence to support this idea) that microscopic life on earth actually originated on another planet, but spread to earth via an asteroid or comet.
10. Alien Life
If life was able to spring into existence from organic (or perhaps even inorganic) compounds on earth, then it seems extremely likely that a similar process has occurred on at least one planet orbiting one of the billions of trillions of stars in one of the many tens of billions of galaxies that are thought to exist. What’s more, if life exists on a great many planets throughout the universe, it seems probable that in at least some locations it will have evolved to be more intelligent than human beings, therefore likely possessing technology that is vastly superior to our own. However, until life is actually identified on another planet, we cannot completely rule out the possibility that we are alone in this staggeringly wide and ancient universe. Questions relating to how common life is in our universe, what conditions are necessary for life to form, and how similar life on different planets would be to us (e.g. does all life use something similar to DNA?) remain deep mysteries.
11. Interpreting Quantum Mechanics
There is currently a deep mystery relating to quantum mechanics, that branch of physics that describes the world of extremely small particles like protons, neutrons, electrons and quarks. Quantum mechanics tells us that particles do not truly have intrinsic fixed positions and velocities as they seem to in our macroscopic world. Instead, they seem to possess (or, perhaps, consist of) probability waves (often called “wave functions”) that describe their potential to be in different positions at different times. Only when we perform a measurement of some property of a particle do these probability waves collapse into a particular “choice”, at which point we can say the particle was actually in a given location or had a certain velocity. Unfortunately, the reason for this wave function collapse is not yet understood. Current proposed explanations for this phenomenon are varied, including special properties caused by the interaction between macroscopic and microscopic systems, a “multiverse” theory where there are many different “universes” of events that only agree with each other when they become entangled, a “global wave” theory where the universe is really deterministic but has local behavior which can be effected by particles arbitrarily far away, or even an explanation involving a special role for the consciousness of sentient beings (which is, of course, popular in new age circles, but lacks any concrete evidence).
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