Quantum Theory: If a tree falls in forest... | OUPblog

Oxford University Press's
Academic Insights for the Thinking World


Quantum Theory: If a tree falls in forest…

If a tree falls in the forest, and there’s nobody around to hear, does it make a sound?

For centuries philosophers have been teasing our intellects with such questions. Of course, the answer depends on how we choose to interpret the use of the word ‘sound’. If by sound we mean compressions and rarefactions in the air which result from the physical disturbances caused by the falling tree and which propagate through the air with audio frequencies, then we might not hesitate to answer in the affirmative.

Here the word ‘sound’ is used to describe a physical phenomenon – the wave disturbance. But sound is also a human experience, the result of physical signals delivered by human sense organs which are synthesized in the mind as a form of perception.

Now, to a large extent, we can interpret the actions of human sense organs in much the same way we interpret mechanical measuring devices. The human auditory apparatus simply translates one set of physical phenomena into another, leading eventually to stimulation of those parts of the brain cortex responsible for the perception of sound. It is here that the distinction comes. Everything to this point is explicable in terms of physics and chemistry, but the process by which we turn electrical signals in the brain into human perception and experience in the mind remains, at present, unfathomable.

Philosophers have long argued that sound, colour, taste, smell and touch are all secondary qualities which exist only in our minds. We have no basis for our common-sense assumption that these secondary qualities reflect or represent reality as it really is. So, if we interpret the word ‘sound’ to mean a human experience rather than a physical phenomenon, then when there is nobody around there is a sense in which the falling tree makes no sound at all.

This business about the distinction between ‘things-in-themselves’ and ‘things-as-they-appear’ has troubled philosophers for as long as the subject has existed, but what does it have to do with modern physics, specifically the story of quantum theory? In fact, such questions have dogged the theory almost from the moment of its inception in the 1920s. Ever since it was discovered that atomic and sub-atomic particles exhibit both localised, particle-like properties and delocalised, wave-like properties physicists have become ravelled in a debate about what we can and can’t know about the ‘true’ nature of physical reality.

Albert Einstein once famously declared that God does not play dice. In essence, a quantum particle such as an electron may be described in terms of a delocalized ‘wavefunction’, with probabilities for appearing ‘here’ or ‘there’. When we look to see where the electron actually is, the wavefunction is said to ‘collapse’ instantaneously, and appears ‘here’ with a frequency consistent with the probability predicted by quantum theory. But there is no predicting precisely where an individual electron will be found. Chance is inherent in the collapse of the wavefunction, and it was this feature of quantum theory that got Einstein so upset. To make matters worse, if the collapse is instantaneous then this implies what Einstein called a ‘spooky action-at-a-distance’ which, he argued, appeared to violate a key postulate of his own special theory of relativity.

So what evidence do we have for this mysterious collapse of the wavefunction? Well, none actually. We postulate the collapse in an attempt to explain how a quantum system with many different possible outcomes before measurement transforms into a system with one and only one result after measurement. To Irish physicist John Bell this seemed to be at best a confidence-trick, at worst a fraud. ‘A theory founded in this way on arguments of manifestly approximate character,’ he wrote some years later, ‘however good the approximation, is surely of provisional nature.’

When Bell devised his famous inequality in 1964 these questions returned with a vengeance. Bell sought a way to discriminate between conventional quantum theory and a whole class of alternative, so-called local hidden variable theories which do not need to assume a collapse of the wavefunction. He deduced a mathematical relationship in which local hidden variable theories predict results that are manifestly contradicted by the predictions of conventional quantum theory, providing a direct laboratory test. Some exquisite experiments performed subsequently proved beyond any doubt that quantum theory, with all its apparent ‘spookiness’, is correct.

In 2003, English physicist Tony Leggett took the debate to another level. Local hidden variable theories are characterized by a couple of key assumptions. In one of these, it is assumed that the outcome of a measurement on a quantum particle can in no way be affected by the setting of the device used to make measurements on a second particle with which it is ‘entangled’ (in other words, both particles are described by a single wavefunction). Leggett chose to drop this assumption to see what would happen.

He went on to deduce a further inequality. For a specific combination of measurement settings, quantum theory predicts results which violate this inequality, implying that the outcomes of measurements on distant particles can be affected by some unspecified non-local influence of the device settings. The result we get depends on how we set up another device, even though this may be halfway across the universe. Spooky, indeed.

The results of experiments to test Leggett’s inequality were reported in 2007. Once again, quantum theory was proved to be correct. This kind of result cannot be reconciled with any theory which ascribes fixed properties to the particles prior to measurement. This means that we can no longer assume that the properties we measure necessarily reflect or represent the properties of the particles as they really are. These properties are like secondary qualities – they exist only in relation to our measuring devices. This does not mean that quantum particles are not real. What it does mean is that we can ascribe to them only an empirical reality, a reality that depends on our method of questioning.

Without a measuring device to record it, there is a sense in which the recognisable properties of quantum particles such as electrons do not exist, just as the falling tree makes no sound at all.

‘Reality is merely an illusion,’ Einstein once admitted, ‘albeit a very persistent one.’

Headline image credit: Explosion Illustration. CC0 via Pixabay

Recent Comments

  1. M Stone-richard

    I think my brain just exploded. But I’m not really sure . . . it may have been an illusion — caused by my perception that it exploded. Can a brain witness its own demise?

    Anyway . . . great article.

  2. Random

    I am obsessed with this. It kind of throws your whole way of life out of order. If reality is and illusion, then can we alter the illusion to better suit us? For example, if I wanted to, could I be a squirrel? I don’t know if it mentioned anything about that in there, I couldn’t catch all of it. But if so, then maybe the insane aren’t really insane after all. Perhaps that is their reality. If Big Brother says 2+2=5, is it?

  3. M Stone-richard

    LOL! I just finished re-reading 1984. Right now, I’m reading The Tao of Physics, and the chapter that I’m on correlates to this article. Astute observation (if you’re aware of something, you’re more likely to see that something)? Coincidence? Synchronicity? Quantum entanglement? Coo-coo-ca-choob.

  4. Scott de B.

    I am not a physicist, but my understanding is that the Leggett-Garg equality doesn’t violate macrorealism per se, but only macrorealism combined with noninvasive measurability. So macrorealism could still be true.

  5. Zathras

    The dichotomy between physical properties as real and physical properties as illusion is a false one. I keep coming back to Kant for this: “If we treat the physical world as appearance, we must investigate the reality behind the appearance.” Instead of being an ultimate reality or an illusion, consider physical properties as incomplete information about the thing-in-itself. Just as a shadow conveys some but not all information about the dimensions of a 3-d object, physical properties convey some but not all information.

    The best mathematical tool to work with here are projection operators. A shadow is a projection of an object, or a projection operator acting on the dimensions of an object. When we do a double-slit experiment and find the wave nature of the electrons, we are observing a projection of the thing-in-itself. We get the wave nature in the projection, but other information is lost, like the position of the physical particle.

  6. troy

    @Zathras – Considering physical properties as incomplete information presumes that they are in fact able to be complete. Isn’t this substituting one dichotomy for another?

    I’d suggest that the investigation itself is all that’s needed …that is; the investigation ‘is’ our reality.

  7. Carl

    Such questions only boggle the minds of anthropoids because we tend to be so thoroughly anthropocentric in a Universe which is anything but anthropocentric.

    Why should the existence or form of anything rely on detection by a human, whether directly or indirectly through some mechanical extension of limited human capabilities? The sound of a falling tree is experienced in countless ways, regardless of whether the sound is defined as vibration of matter or by some form of detection of such vibration by human or by non-human “detectors,” whether living or non-living (by our standard concepts).

    Likewise, all manifestations of matter (and anti-matter) – form, non-form, presence, influence – are surely experienced, “known” and “understood” in countless ways by various “beings” (ways of being) of the Universe, whether known or unknown to us.

    It is true that we, both individually and collectively, are incredibly significant in the scheme of things in this Universe. Yet we simultaneously are also incredibly insignificant, and any attachment to our significance is a delusion that arises from our perceived illusions.

    This doesn’t mean we can’t or shouldn’t bother to count, measure and describe all that we can “get our hands on” and try to work with what we think we’ve got hold of. But it does mean we need to constantly be aware that the minute portions of the Universe we think we have a grip on offer us precious little knowledge, let alone understanding, of the enormous “creature” whose tail hair we’ve barely touched. There are, however, means other than through our curious minds by which we can learn to “listen” to the “purrs” and “growls” and experience the “feel” of a presence that reveals truths that can open the way to greater understanding and wisdom.


    How do we know the tree in question actually fell if no one was around to see it?

  9. […] mean nothing to them. “What do you mean by a one-way street? Quantum physics has given us new ways to look at the observer and the […]

  10. Larry L

    This article still did not answer the question succinctly. Here would be my way of addressing this question. If no one is in a part of Siberia during a particular day, does that mean there was no daylight in that part of Siberia?
    Or, if we look at the Weather Channel and notice that satellite info shows it to be raining over Mt. Whitney, but no one is on Mt. Whitney, does that mean that there is no rain on Mt. Whitney?
    The problem with the question is not that someone is available to hear a sound, or see a sight, but whether humanity is so central to the events of the natural world. The hubris would be to say it is so.

  11. peter machado

    If a tree falls (if I have an idea) and nobody hears it (and nobody listens/no forum exists for me to express it/I don’t express it) does it make a sound? (do I make a difference?).

    Of course it makes a sound, we are so arrogant a species as to think that the whole universe revolves around our body parts; We perceive the sound with our ears, but whether we hear it or not it still makes a sound. That’s how avalanches can start, that’s how a sonic boom can shatter windows.

    The whole thesis is allegorical. If I have an idea and don’t express it, or cannot express it, or am forbidden to express it, then of course it’s as if I’ve never spoken at all, and that’s what it’s all about. If I’m ignored, then even if I shout it from the rooftops, it’s as if I’ve said nothing. But if I’m noticed for what I’ve said, good or bad, then I’ve made a sound. If the impact of what I’ve said is earth shattering, also good or bad, then I’ve made a very great sound, enough to “shatter the earth”.

  12. peter machado

    the question being asked is, if a tree falls and nobody hears it, does anybody hear it? (duh! we just said Nobody hears it, so why ask the question?) The reasoning is that the sound is defined as the action of vibrations on the cochlea of the ear, so if there is no cochlea around to hear it, then of course nobody hears it. This is one of the most stupid questions – it’s been pondered as a great philosophical conundrum of the ages, I think originally it was asked tongue-in-cheek, in jest!

  13. […] distinguishing between if human eyes need to see render output for render to be complete. (Answers: not at the quantum level, and […]

  14. David

    The question is not a rational question and anyone who approaches it with mechanical explanations (which conclude the tree does make a sound), do not understand where the question is coming from. Consider this – if there was no conscious life, would the physical universe still exist? I have real problems in answering that question but tend to the belief that the physical universe would not exist in those circumstances.

  15. Bob Spencer

    if a radio station transmits from the forest and no one has their radio turned on is there still sound?

  16. Ethan

    This question all depends on your definition of this type of sound.

  17. Steve

    Compression waves wafting through the air do not become “sound” until they react on an ear drum (human or animal) which converts the waves into electrical impulses that the brain interprets as a sound. Until that happens, you’ve got nothing but waves. The sound is not out there, it’s in the recorder, the brain. The wave itself is not sound. So, the falling tree does NOT make a sound, only a compression wave.

Leave a Comment

Your email address will not be published. Required fields are marked *