At a physics meeting last October, Nobel laureate David Gross outlined 25 questions in science that he thought physics might help answer. Nestled among queries about black holes and the nature of dark matter and dark energy were questions that wandered beyond the traditional bounds of physics to venture into areas typically associated with the life sciences.
One of the Gross's questions involved human consciousness.
He wondered whether scientists would ever be able to measure the onset consciousness in infants and speculated that consciousness might be similar to what physicists call a "phase transition," an abrupt and sudden large-scale transformation resulting from several microscopic changes. The emergence of superconductivity in certain metals when cooled below a critical temperature is an example of a phase transition.
In a recent email interview, Gross said he figures there are probably many different levels of consciousness, but he believes that language is a crucial factor distinguishing the human variety from that of animals.
Gross isn't the only physicist with ideas about consciousness.
Beyond the mystics
Roger Penrose, a mathematical physicist at Oxford University, believes that if a "theory of everything" is ever developed in physics to explain all the known phenomena in the universe, it should at least partially account for consciousness.
Penrose also believes that quantum mechanics, the rules governing the physical world at the subatomic level, might play an important role in consciousness.
It wasn't that long ago that the study of consciousness was considered to be too abstract, too subjective or too difficult to study scientifically. But in recent years, it has emerged as one of the hottest new fields in biology, similar to string theory in physics or the search for extraterrestrial life in astronomy.
No longer the sole purview of philosophers and mystics, consciousness is now attracting the attention of scientists from across a variety of different fields, each, it seems, with their own theories about what consciousness is and how it arises from the brain.
In many religions, consciousness is closely tied to the ancient notion of the soul, the idea that in each of us, there exists an immaterial essence that survives death and perhaps even predates birth. It was believed that the soul was what allowed us to think and feel, remember and reason.
Our personality, our individuality and our humanity were all believed to originate from the soul.
Nowadays, these things are generally attributed to physical processes in the brain, but exactly how chemical and electrical signals between trillions of brain cells called neurons are transformed into thoughts, emotions and a sense of self is still unknown.
"Almost everyone agrees that there will be very strong correlations between what's in the brain and consciousness," says David Chalmers, a philosophy professor and Director of the Center for Consciousness at the Australian National University. "The question is what kind of explanation that will give you. We want more than correlation, we want explanation -- how and why do brain process give rise to consciousness? That's the big mystery."
Just accept it
Chalmers is best known for distinguishing between the 'easy' problems of consciousness and the 'hard' problem.
The easy problems are those that deal with functions and behaviors associated with consciousness and include questions such as these: How does perception occur? How does the brain bind different kinds of sensory information together to produce the illusion of a seamless experience?
"Those are what I call the easy problems, not because they're trivial, but because they fall within the standard methods of the cognitive sciences," Chalmers says.
The hard problem for Chalmers is that of subjective experience.
"You have a different kind of experience -- a different quality of experience -- when you see red, when you see green, when you hear middle C, when you taste chocolate," Chalmers told LiveScience. "Whenever you're conscious, whenever you have a subjective experience, it feels like something."
According to Chalmers, the subjective nature of consciousness prevents it from being explained in terms of simpler components, a method used to great success in other areas of science. He believes that unlike most of the physical world, which can be broken down into individual atoms, or organisms, which can be understood in terms of cells, consciousness is an irreducible aspect of the universe, like space and time and mass.
"Those things in a way didn't need to evolve," said Chalmers. "They were part of the fundamental furniture of the world all along."
Instead of trying to reduce consciousness to something else, Chalmers believes consciousness should simply be taken for granted, the way that space and time and mass are in physics. According to this view, a theory of consciousness would not explain what consciousness is or how it arose; instead, it would try to explain the relationship between consciousness and everything else in the world.
Not everyone is enthusiastic about this idea, however.
'Not very helpful'
"It's not very helpful," said Susan Greenfield, a professor of pharmacology at Oxford University.
"You can't do very much with it," Greenfield points out. "It's the last resort, because what can you possibly do with that idea? You can't prove it or disprove it, and you can't test it. It doesn't offer an explanation, or any enlightenment, or any answers about why people feel the way they feel."
Greenfield's own theory of consciousness is influenced by her experience working with drugs and mental diseases. Unlike some other scientists -- most notably the late Francis Crick, co-discoverer of the structure of DNA, and his colleague David Koch, a professor of computation and neural systems at Caltech -- who believed that different aspects of consciousness like visual awareness are encoded by specific neurons, Greenfield thinks that consciousness involves large groups of nonspecialized neurons scattered throughout the brain.
Important for Greenfield's theory is a distinction between 'consciousness' and 'mind,' terms that she says many of her colleagues use interchangeably, but which she believes are two entirely different concepts.
"You talk about losing your mind or blowing your mind or being out of your mind, but those things don't necessarily entail a loss of consciousness," Greenfield said in a telephone interview. "Similarly, when you lose your consciousness, when you go to sleep at night or when you're anesthetized, you don't really think that you're really going to be losing your mind."
Like the wetness of water
According to Greenfield, the mind is made up of the physical connections between neurons. These connections evolve slowly and are influenced by our past experiences and therefore, everyone's brain is unique.
But whereas the mind is rooted in the physical connections between neurons, Greenfield believes that consciousness is an emergent property of the brain, similar to the 'wetness' of water or the 'transparency' of glass, both of which are properties that are the result of -- that is, they emerge from -- the actions of individual molecules.
For Greenfield, a conscious experience occurs when a stimulus -- either external, like a sensation, or internal, like a thought or a memory -- triggers a chain reaction within the brain. Like in an earthquake, each conscious experience has an epicenter, and ripples from that epicenter travels across the brain, recruiting neurons as they go.
Mind and consciousness are connected in Greenfield's theory because the strength of a conscious experience is determined by the mind and the strength of its existing neuronal connections -- connections forged from past experiences.
Part of the mystery and excitement about consciousness is that scientists don't know what form the final answer will take.
"If I said to you I'd solved the hard problem, you wouldn't be able to guess whether it would be a formula, a model, a sensation, or a drug," said Greenfield. "What would I be giving you?"