Decoherence, or If a Tree Falls In the Forest...?

One of the basic unsettled questions of quantum physics is why we don't see quantum superposition in everyday objects. At the quantum level – and before being “measured” – mass and energy exist simultaneously as both wave and particle. The classic examples are light and electrons. Photons exist as both wave and particle and manifest as either depending on how it is observed. Similarly, electrons do not exist, in reality, as tiny “planets” circling the nucleus in neat orbits but in clouds of probabilities that may be “found” as a particle in a particular “place” only when measured. Everything that exists at the quantum level – the realm of the very tiny – shares this dual nature as wave and particle. It can be more accurately described as a wave function.

If everything were to remain in quantum superposition in the macro-world we inhabit, Schrödinger's cat – and everyone else's – would be both alive and dead at the same time. We don't see in that way because superposition seems to breakdown when things get large. The wave function has collapsed and we see either waves or particles, i.e., individual, unconnected, single state things. Why?

The easiest answer might be that we don't see quantum superposition at the macro level because when we look at the world, we as conscious observers collapse the wave function. Light, sound, touch, smell, taste all enter our perceptual mechanisms and, interacting with brain and mind, are perceived. The world is there when we observe it because the act of observation collapses the wave functions around us even if nothing else did. But does this mean that if a tree falls in a forest with no one there to hear it, it doesn't make a sound?

One answer might be yes, the unobserved falling tree makes no sound. The basic reality of the universe may be thought of as one all-inclusive wave function in which everything is entangled. The universe is one big cloud of probabilities. Nothing exists per se until observed. But that verges on solipsism. So, science has considered a variety of other mechanisms for decoherence of quantum superposition – collapsing the wave function of anything tiny before it can get very big. It may happen simply because as things get bigger, they get more complex. They interfere with each other, fall out of phase, or vibrate at different frequencies. The latest theory posits that as mass slows down – dilates – time, even the gravity of earth would be enough to pull entangled particles into different time streams.

But at least some aspects of the macro-world do work through quantum effects. The efficiency of photosynthesis arises from quantum mechanical effects. Quantum mechanics may explain how birds use magnetic fields to navigate and our sense of smell. It may be that the cosmos is an entangled universal wave function that decoheres only at the boundary of individual acts of “observation.” But the observers would not simply be conscious human beings but any living thing interacting with its environment? Might the definition of life be that which breaks wave functions?

Gravity, Mass and Time II

I recently noted that mass, gravity and time may be essential features – givens – of our universe, that gravity is something that slows time and that at the speed of light, time stops. Actually time doesn't stop at the speed of light but becomes instantaneous. At that speed, everything happens at once. It's at an event horizon that time actually just stops passing. As whatever it is that is “falling” into a black hole passes the event horizon, the time that it may be experiencing cannot escape. Beyond that, at the singularity, anything/everything disappears from this universe (leaving aside the mechanism by which black holes “evaporate” over time). The mass and energy falling into the singularity is converted into the very warping of space that is the black hole.

How long does it take to fall from the event horizon into the singularity? Has time there stopped or has it become instantaneous? Apparently, if you could survive passing through the event horizon, you would still experience your own personal time. The length of time you'd experience would be very short but it would pass. As under general relativity there is no absolute standard of time, that would be all that counts for you. Indeed, time may be thought of as something entirely a matter of perspective. As I would be falling through the event horizon experiencing my own usual passage of time – it would not slow down or stop – it would appear to be doing so only to an outside observer experiencing his own usual passage of time.

Our human sense of the passage of time may be an entirely arbitrary experience defined by our nature as biological mechanisms (with mass) operating according to physical laws as elaborated by the evolution of life on our particular planet. One defining process may be the rate at which ribosomes add amino acids to the protein it is building (called translation). In all life on earth this process proceeds at the same speed of 10-20 additions per second. A “second” is a human unit of time but not an entirely arbitrary one as at the most fundamental level it is related to two apparent givens: the ability of our consciousness to hold just 2-3 seconds as our now and the existence of a basic unit – the Planck time – of 5.39x10 to the -44^th seconds. Or perhaps we might simply say that our human, species experience of time is one heart beat. That, however, might speed up a bit as we crossed the horizon.

Light tricks: The Delayed Choice Experiment

Physical Review A reports a recent "experimental observation of simultaneous wave and particle behavior in a narrowband single-photon wave packet."  This is also covered in a more accessible form in Science News.  The experiment is a variation on the delayed choice model that submits a photon to being observed (measured) after it has already been through a double beam splitter setup.  This essentially is a way of forcing the photon to behave first as a particle (by passing it through a beam splitter) and then after having made that "choice" having it behave like a wave again, as predicted by quantum physics.  The recent experiment takes this one step further by first stretching out a single photon so that it takes a small but measurable period of time to pass through the second beam splitter.  With the splitter in place, the photon acts like a wave.  With it removed while the photon is still passing through it, the photon manifests as a particle.  The very same photon during one single act of observation -- in two parts -- is both particle and wave.  This does not violate quantum physics but, as a scientist quoted by Science News suggests:  "‘Wave’ and ‘particle’ are just words.  In quantum physics, those words are imprecise at best."

This beautifully done experiment offers a window into the nature of not only light but the universe.  As noted before, at the speed of light, time does not exist.  Therefore, every photon is everywhere it will ever be at the same instant. The speed of light measures the degree of departure of our existence as mass affected by gravity from that cosmic external moment in which light exists.  When we measure light we seek to capture in time that which exists without time.  Wave and particle are the way we perceive its timeless nature as we move at our own pace through time and space.