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If a tree falls in the woods

  1. Dec 31, 2008 #1
    and no one is around to hear it does it make a sound?...

    my real question is, does the tree even exist if no one is around? and what qualifies an "observer". do wave functions collapse only in the presence of humans? why can't schroedinger's cat tell if it is dead?

    I guess I'm asking for a general overview so that I can contribute CORRECT information the next time conversation of this type starts. Nothing worse than asserting incorrect facts about physics, or anything for that matter.
     
  2. jcsd
  3. Dec 31, 2008 #2

    olgranpappy

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    yes
    yes
    no
    something dead does not know it is dead since it is dead.
    This thread should probably be moved to the philosophy section since the answers I gave are highly unsatisfactory.
     
  4. Dec 31, 2008 #3
    Of course not!

    However, the behaviour of the system is exactly the same as if all the "observations" made by the tree, the grass, the wildlife, and so on were instead made by you when you stumbled into the forest a year later.
     
  5. Dec 31, 2008 #4
    jeez, way to take the question out of context and make it seem ridiculous. maybe someone else can give a less arrogant answer.

    it's not a philosophical question, it's a question about wave function collapses and observation.
     
  6. Dec 31, 2008 #5

    hey, my last comment wasn't aimed at you. i'm having a tough time wrapping my head around your answer. is there a way you can explain it further? I've heard people reply to the question, "if a tree falls in the woods does it make a sound" with "well, actually quantum theory tells us the tree doesn't even exist if we aren't around to observe it, therefore no sound can be made from a non-existent tree". comments on that?
     
  7. Dec 31, 2008 #6
    if the fall leaks information out into the surrounding environment (limbs break, wildlife runs away, ...) then I guess it's real to your mind. couldn't you go measure things afterward and see that it broke?
     
  8. Dec 31, 2008 #7
    If any of the people who told you "the tree doesn't even exist if we aren't around to observe it" are physicists, please smack them for me. :)

    To expand on what I meant...

    In quantum computing, it's called the "principle of deferred measurement." I believe it to be the fundamental paradox in quantum mechanics, and the root of all the trouble people have in understanding it.

    The evolution of a quantum system can be modeled by a series of continuous operations U, interspersed with discontinuous, "jumpy" measurements R of the whole or part of the system R. So you can conceptually think of the evolution of a quantum system as a continuous graph with a bunch of discontinuous jumps. (I take this picture directly out of the Road to Reality by Penrose.)

    [​IMG]

    The principle of deferred measurement states that such an evolution can ALWAYS be replaced by one with a single, continuous U, and a single measurement R performed at the very end.

    In computational terms, whenever you want to measure the value of a qubit and perform a routine like, "if I measure a 0, perform operation A; if I measure a 1, perform operation B," you can replace that measurement and procedure by a purely quantum operation.

    In other words, you can always push back or defer an observation until the very end of your "computation."

    In terms of a tree in the forest, you can reasonably say that the tree was in a "Schrodinger's Tree" state until you entered the forest and peeked at it. But you can just as reasonably say that when the tree fell, the ground measured its position, and so on, and so on, and by the time you finally peaked at it, you simply saw an already collapsed quantum system.

    (The same principle made it so difficult for me to understand entanglement! The trouble with entanglement goes away if all the interactions are purely quantum ones until the two entangled particles "meet" and get measured, but it's incredible that any number of local measurements before the meeting, when the particles are miles apart, can be replaced by quantum operations. It seems as though a local measurement has an effect on the pair particle, but this effect is in a sense virtual.)

    You might instead ask, what if I never observe the tree, and in fact no one ever goes inside it and no one observes whether the tree fell over or not?

    It's in the answer to that question that you may be tempted to say, it simply doesn't exist, in the sense that it is a completely isolated system, and so anything that happens in it has no effect on us. However, completely isolated systems don't exist in reality. The universe is one, big quantum system where everything is connected on some level.

    You can ask the same question outside of any quantum theory, if, say, you're trying to calculate the acceleration of a falling object on earth. The force of gravity exerted by the moon is so negligible that we can consider the earth an isolated system. But it's absurd to say the moon doesn't exist because of this.
     
    Last edited: Dec 31, 2008
  9. Dec 31, 2008 #8
    Sorry for repeating it again
    There is no such thing as wavefunction collapse.
    There is no R process.
    http://en.wikipedia.org/wiki/Quantum_decoherence

    Can we finally put Copenhagen interpretation to a place where it belongs - ether etc.
     
  10. Dec 31, 2008 #9

    Ivan Seeking

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    From the wiki link

    http://arxiv.org/abs/quant-ph/0112095

    The Role of Decoherence in Quantum Mechanics
    http://plato.stanford.edu/entries/qm-decoherence/
     
  11. Dec 31, 2008 #10
    Decoherence solves all problems only when coupled with the Multi-world interpretation of course. Multi-world is a consequence of quatum decoherence, not an axiom like in early theories.

    The above is definitely written by the person who does not believe in many-worlds, so he talks about a 'single one' - not even leaving a room for 2 cats and 2 observers asking 'how the wavefuctions had collapsed into an alive/dead cat???'. These 2 copies of the same observer also claim that QM is random because they can not tell in advance if cat is alive while in fact QM is deterministic
     
  12. Dec 31, 2008 #11

    mgb_phys

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    There was once a man who said `God
    Must think it exceedingly odd
    If he finds that this tree
    Continues to be
    When there's no one about in the Quad.'

    Sir, Your astonishment's odd:
    I am always about in the Quad.
    And that's why the tree
    Will continue to be,
    Since observed by Yours faithfully, God.
     
  13. Dec 31, 2008 #12

    DrChinese

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    Welcome to PhysicsForums!

    It is generally agreed that the tree exists independently of the act of observation.

    However, some observable attributes of the tree - say its color, shape, location - are dependent on the act of observation. This view is not shared by all, but is certainly consistent with experimental evidence. In this view, those attributes are said to be contextual (or non-realistic).
     
  14. Dec 31, 2008 #13
    If a tree falls in the woods...
    and no one is around to hear it does it make a sound?...
    my real question is, does the tree even exist if no one is around?


    Looking at it philosophically:

    Let us put your questions in more formal terms. It can therefore be summarized as follows:

    1. There exists a tree (T) in the woods
    2. The tree (T) falls
    3. There is no human around to hear it (T)

    Q1. Does T it make a sound?
    Q2. Does T exist?

    As you can see, The answer of Q2 is obvious, it is one of your premises (1). The tree must exist for your question to make sense.

    Now the answer to Q1 is also obvious. It is implied by your third premise. If the tree does not make a sound, then the act of "hearing it" is a logical impossibility which would imply that your third premise is false. Therefore for your question to make sense, the tree must exist and must make a sound when it falls.

    Looking at it physically, a falling tree produces vibrations in air molecules and ground molecules within 1000 and 6000Hz, the range of frequencies detectable by human ears (ie sound). So long as molecules are present, they will vibrate. Humans ears hear sound, they do not create sound. The sound must exist for ears to be able to hear them not the reverse. In other words, vibrating air molecules transfer energy to the molecules of the ear-drum which start vibrating as well so the vibrating air molecules must be vibrating BEFORE ear molecules start vibrating.
     
  15. Dec 31, 2008 #14
    I am an adept of Multi-Worlds+Quantum Decoherence

    When tree falls and you dont know about it, the state of your consciousness is the same no matter if a tree fell or not. So different branches of reality (tree fell or not) can interfere before decoherencing finally in your mind.
     
  16. Dec 31, 2008 #15
    If you intend your interpretation to be predictive in the slightest, you still need to apply the usual probabilistic rules of quantum mechanics at some point or you end up simply engaging in philosophical onanism.

    And the fact will remain that what you call Nature probabilistically choosing a universe for us to travel down will look exactly like what I call a measurement resulting in collapse.

    The latter is also useful. But if practical use is not your concern, there are plenty of string theorists who are all-ears to such ideas.
     
  17. Dec 31, 2008 #16

    olgranpappy

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    yes
    no. maybe.
     
  18. Dec 31, 2008 #17
    This question can easily be run around in circles depending on how "sound" is defined, but in general you can say that a falling tree on earth will produce "sound waves" upon falling even if these waves do not carry to a humans ear for a human to become aware that they were made. Note that this assumes the tree is in an environment/earth where "sound" is produced...if you placed the tree on the moon and it fell it would not make a sound even if an observer was five feet away watching it fall (because there is no medium in space to carry the sound). Some people define "sound" as the sensation we experience only when those sound waves interact with the receptors in our ear drums. In this case, "sound" is not produced if a human is not around because there are no ear drums to receive the waves. And so on. Like much of philosophy these arguments boil down to how sound is defined. Once that is sorted out, you can answer the question reasonably enough.
     
  19. Jan 1, 2009 #18
    You are misunderstandig the multi-world.
    There are many 'us', so we are travelling in all possible paths
    But as we remember only the Past, we perceive Time as linear, not like a tree.

    If some interpretations include Collapse and others do not, shouldnt we cut the Collapse using occams razor?

    Also, if you believe in collapse you need to answer all sorts of questions like 'what exactly makes a measurement apparatus so magical that it causes a collapse while other collections of QM particles do not, deal with a dead/alive cat and Wigners'friend etc.
     
  20. Jan 1, 2009 #19
    "Multi-world" is by itself an extreme violation of Ockham's Razor.

    Same questions apply to multi-world, What exactly makes a particular decomposition preferred over others in a measurement? In other words, if the cat is alive in one world and dead in another, what causes the observer to perceive one state and not the other on opening the box?
     
  21. Jan 1, 2009 #20
    1 Why?
    At first, it is not an axiom but a consequence of Quantum Decoherence
    I understand that for some reasons it is easy to imagine and to believe into an infinite time or infinite space. But for the infinite number of parralel options is 'weird'.
    For some reason, some infinities are much easier to imagine then others.

    2 If I understood your question correctly,
    As soon as box is opened the observer is decoherenced with a cat. So it splits into 2 copies: one copy observing dead cat, another copy observing a live one. Both copies are asking the same question "what causes me to perceive one state and not the other on opening the box?" :) So the full picture is deterministic.

    There are some questions which in fact are not addressed by the Multi-world, but you dont ask them :)
     
    Last edited: Jan 1, 2009
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