What is reality? Mainstream science describes reality as "the state of things as they actually exist". One simple interpretation of this very broad definition is this: reality is everything we observe to be real.
"Reality is merely an illusion, albeit a very persistent one"
Albert Einstein
But hang on - I consciously observe the lucid dream world, so does that make it a genuine reality?
Just how many realities are there - yours, mine, his, hers? As Einstein suggested, is every form of reality merely an illusion? Is nothing real?
Let's start by looking at how the human brain perceives reality, and how this can give way to subjective experience.
Human Perception
The human brain is split into two distinct halves: the right brain and the left brain. They have completely separate roles and agendas. Some would even say they have separate personalities. However, in order to function, the two halves of the human brain must communicate as one via the corpus callosum.
The right brain is all about the present moment; right here, right now. It thinks entirely in pictures and learns through the kinesthetic movement of your body. It absorbs energy from the world around you and translates that into information for your sensory systems. It does not know the difference between your individual consciousness and the world around you. The right brain only sees one universal energy field of awareness.
The left brain is a very different place. It thinks linearly and methodically. It picks out countless details from the events in the past and makes calculated predictions about the future. The left hemisphere thinks in language, which creates your internal voice. Crucially, it makes you aware of your existence, as a separate being from the mass energy field perceived by the right brain.
Imagine if the human brain had evolved with only the functions of the right hemisphere. Your perception of reality would be completely different. You would be drifting around in a universe filled with energy in the here and now, with no perception of the past and future. You would not know where your body ended and the ground began, or the difference between you and me.
This is a very different perception of the world. But would it be a more accurate representation of reality? Knowing this about the human brain, the question "what is reality?" changes form. It now hinges on your individual perception. This has led to multiple theories of reality by various philosophers and scientists.
Types of Reality
Phenomenological reality is based on subjective experience. Whatever you observe is instantly real to you. This theory of reality means that unreality is non-existent. Therefore lucid dreams, hallucinations, spiritual experiences, and astral travel are all forms of one subjective reality.
Consensus reality is based on the opinions and observations made by a group of people. A few individuals may decide on an interpretation of an event, which spreads across entire societies and becomes a consensual truth. Religion is a good example of a socially constructed reality.
Non-reality simply means that there is no such thing as objective reality. Every possible observation or interpretation is tainted by subjectivity and therefore does not constitute truth. Nothing is real! This is supported by quantum theory, which states that prior to observation, nothing can be said about a physical system (read In Search of Schrodinger's Cat by John Gribbin for an excellent introduction to this topic). This theory is further backed by the Double Slit Experiment, which suggests our mere observation changes the outcome.
The Double Slit Experiment
When quantum physicists stumbled upon the Double Slit Experiment, they were in for a big shock. This infamous quantum experiment proves how tiny particles behave differently when they are being measured. It's as if they know they are being watched. Check out this video synopsis to learn more.
As Niels Bohr once said, "Those who are not shocked when they first come across quantum theory cannot possibly have understood it." Another quote by Richard Feynman goes "It is safe to say that nobody understands quantum mechanics."
We are all just guessing - albeit using all the scientific evidence gathered to date - and the universe could well be unthinkably bizarre. If we are on the right track with current theories, it could be terrifyingly bizarre. We just don't know yet.
This particular experiment, which remains one of the most famous and tantalizing experiments of quantum mechanics to date, implies that there are multiple realities where every possible outcome is played out in a parallel universe. Each scientist in each universe observed a different outcome, throwing our original question - what is reality? - into more chaos, as now we have infinite realities.
If the idea of parallel worlds isn't your cup of tea, how about The Simulation Argument? It adequately explains what caused the Big Bang, how our reality is affected by observation, and whether there is intelligent life beyond Earth...
The Simulation Argument
What is reality? It is an elaborate computer game created by smart human beings in the future. That is the unfailing conclusion of The Simulation Argument put forward by philosopher Nick Bostrom of Oxford University.
By process of elimination, Bostrom says it is extremely likely that we are living inside a virtual reality world. His probability equations conclude that at least one of the following statements are true:
The human race will become extinct before we develop the technology to create a convincing virtual reality computer simulation.
Any race with such technological ability would choose not to run a large number of simulations of their evolutionary history.
We are almost certainly living in a computer simulation.
The reason why the Simulation Argument is compelling is because we can virtually eliminate the first and second conclusions above. But how?
Issue #1: Technological Barriers
Humans have been evolving for millions of years. We have survived meteor strikes, ice ages, diseases, droughts, famines, wars and more. Now we have developed the technological ability to create basic virtual life simulations like The Sims and Spore - and where next? Analysts predict that we will reach the point of technological singularity in the year 2045, when Artificial Intelligence will become self aware and surpass human intelligence.
The chances of the entire human race becoming extinct before that point are small. Soon we will have sufficient computer memory to support a computer simulation with millions of years of human evolution. This would finally prove that such a virtual reality simulation is technologically possible, and therefore could already exist on another planet in this universe.
Issue#2: Ethical Barriers
If you could simulate creatures that appeared to make their own choices and evolve according to set rules - does that mean you would?
Bostrom admits that there could be ethical barriers to The Simulation Argument. Yet the likelihood is still extremely low. Take The Sims computer game as an example. Characters in The Sims are assigned specific personalities, appearances, work schedules, and even romantic relationships. We are already playing God, but because of AI limitations, they are not self aware... yet.
However, it's quite believable that if we had the ability to build a simulation with conscious entities, we would not have any ethical concerns. After all, it could be argued that the simulated creatures are merely robots, designed with pre-set laws in mind. We can't relate to such simple entities, even if they do eventually evolve into a complex forms, demonstrated by John Conway's Game of Life.
The Undeniable Conclusion
The conclusion of Bostrom's Simulation Argument is inescapable. If we have the technology and aren't afraid to use it, then the third statement must be true: we must be living in a computer simulated world. But why?
It is a game of numbers. A single computer simulation would involve a universe containing billions of organisms. As each civilization in that universe became technologically advanced, they too would begin making AI simulations. And in those virtual worlds, there would eventually be even more virtual worlds.
So, we have a single true reality, containing the creators of the very first computer simulation. They are the oldest civilization of all. Then we have the trillions of offspring simulations that arose from their virtual creations.
Statistically speaking, the likelihood of you existing in that original universe are infinitesimal. You are most likely to be the construct of a computer game.
At least, that's the theory :)
Final Thoughts
So what is reality? A computer simulation? A multiverse? Or is our human brain perceiving just one possible interpretation of the ultimate reality?
I think philosophers will be stuck on this one for a while. Our understanding of the universe is very much limited to our own capacity for understanding... The truth is out there - and it is probably much crazier than we can imagine.
The right brain is all about the present moment; right here, right now. It thinks entirely in pictures and learns through the kinesthetic movement of your body. It absorbs energy from the world around you and translates that into information for your sensory systems. It does not know the difference between your individual consciousness and the world around you. The right brain only sees one universal energy field of awareness. 
