This long-held perspective is outdated. Some physicists - like Nobel-winner John Wheeler and SUNY-New Paltz professor Tarun Biswas - say that nothing is real until it is observed.
Now a crashing tree is merely producing air-pressure variations: tiny, rapid puffs of wind. There is no sound attached to them. If a person is nearby, the puffs physically cause her ear's tympanic membrane to vibrate, which then stimulates nerves only if the air is pulsing between 20 and 20,000 times a second - with an upper limit more like 10,000 for people over 40, and even less for those of us whose misspent youth included earsplitting rock concerts. Nerves stimulated by the moving eardrum send electrical signals to the brain, resulting in the cognition of a noise.
This experience, then, is symbiotic. The pulses of air by themselves do not constitute any sort of sound. The ear's neural architecture and a brain conjure the noise experience, and are every bit as necessary for sound as are the air pulses. In other words, the external world and human consciousness are correlative.
When someone dismissively answers, "Of course a tree makes a sound if no one's nearby," they are merely demonstrating an inability to ponder an event that nobody attended. They're finding it too difficult to take themselves out of the equation. They somehow continue to imagine themselves present when they are absent.
Now consider a lit candle: The flame is a hot gas that emits photons: tiny packets of electromagnetic energy waves. Each consists of electrical and magnetic pulses. Neither electricity nor magnetism has visual properties. So there is nothing inherently visual, nothing bright or colored about a candle flame.
But if these same invisible electromagnetic waves strike a human retina, and if the waves happen each to measure between 400 and 700 nanometers from crest to crest, then their energy is just right to deliver a stimulus to the eight million cone-shaped cells in the retina. Each in turn sends an electrical pulse to a neighbor neuron at 250 miles per hour until it reaches the warm, wet occipital lobe of the brain, in the back of the head. There, the firing of a cascading complex of billions of neurons creates the subjective experience of a yellow brightness occurring in a place that we have been conditioned to call "the external world."
So there isn't a "bright yellow" light "out there" at all. At most, there are invisible electrical and magnetic pulses. We are totally necessary for the "yellow flame." Again, it's correlative.
Consider rainbows: This one's easy, since it's obvious that we are a fundamental part of a rainbow's existence. When nobody's there, there simply is no rainbow. (Rainbows have such a low intrinsic reality that they don't even cast reflections).
Three components are necessary for a rainbow. There must be sun, there must be raindrops and there must be a conscious eye (or its surrogate film) at the correct geometric location. A person next to you will complete his or her own 42-degree geometry from the sun's antisolar point and will be at the apex of a cone for an entirely different set of raindrops, and will therefore see a separate rainbow, which needn't even look like yours. If the sunlit droplets are nearby - as from a lawn sprinkler - your companion may not see a rainbow at all. Your rainbow is yours alone.
But what if no one's there? Answer: no rainbow. An eye/brain system must be present to complete the geometry. A rainbow requires your presence just as much as it requires sun and rain.
Few would dispute the subjective nature of rainbows, which figure so prominently in fairytales that they seem only marginally to belong to our world in the first place. It is when we fully grasp that a skyscraper is just as dependent on the observer that we have made the first required leap to understanding the true nature of things.
The above is taken from a chapter in my new book Biocentrism, co-authored with Robert Lanza, MD, to be published next month.