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Are there limits to human/devices perception?

  1. Dec 15, 2014 #1
    As far as i know, measurement devices present measurements based on something that affects the device's particles, for instance, forces, heat, tension, voltage...

    My question is, given that every change of position of any particle may affect the particles of the measurement device, why cant we design devices that can distinguish between any state of the universe?

    Consider for instance our human body. If i have a car behind a Wall, i cannot distinguish this situation from one where there isn't a car behind the Wall, since i only see the Wall. Still, having a car behind the Wall surely affects something in our particles, maybe the gravity pull from the car, or some change in the overall environment light, or even a change in the trajectory of the light that arrives in our eyes (since we perceive light), even if the effect is minimal.

    Is there some law that limits measument and devices so that we cannot have a device that can distinguish from every state of the universe?

    Thanks in advance
  2. jcsd
  3. Dec 15, 2014 #2


    Staff: Mentor

    I would compare this to trying hear one voice from all the voices in the world. The sounds would be so entangled that there'd be no way to pull out a clear signal from the noise.
  4. Dec 15, 2014 #3


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    I'd say the limit depends on the sensitivity of our equipment. Detecting some things, such as very low frequency EM waves, is so difficult that the required measuring device becomes horribly impractical to build.
  5. Dec 15, 2014 #4


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    There's a basic problem that goes with all measurements and that is Noise. Random fluctuations are everywhere, due to temperature, grittiness of very small currents due to individual electrons being involved and other effects. Also there can be interfering signals from other sources. However 'sensitive' your equipment is -i.e. however much you can turn up the gain, there will be randomness. The wider the bandwidth (aka response time and good frequency response) the more noise gets in. To counter this, you can narrow the bandwidth, which means that you need to take longer and longer over your measurement (averaging out the noise) to reach the desired accuracy.
    The idea that superman could hear someone crying "help" is daft because there would be millions of other people shouting and playing music and he would be hearing the individual air molecules hitting his eardrums. Same thing goes for detecting distant stars in the presence of background light and the heat of the light detector.

    Taking long enough and using, say, very high power Radar, you could detect the car behind the wall - it's all down to Signal to Noise Ratio.
  6. Dec 15, 2014 #5


    Staff: Mentor

    A device that could distinguish every possible state of the universe would itself have to have at least as many possible states as the universe has. Since such a device would necessarily be a subset of the universe then it would either have to be the universe or it would have to have fewer states than the universe. Therefore, any such device would be the universe.
  7. Dec 16, 2014 #6
    There are as many numbers between 0 and 1 as there are in the whole real number range. Why is it impossible??
  8. Dec 16, 2014 #7


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    Because this is real life, not pure mathematics. Recording a state of a system requires another system to use a state to do the recording.
  9. Dec 16, 2014 #8
    But space is continuous, right? So particles can have an infinity of positions, hence the analogy works. Am i correct?
  10. Dec 16, 2014 #9


    Staff: Mentor

    Even with continuous space and continuous time, the possible states of a finite apparatus are still discrete.
  11. Dec 16, 2014 #10


    Staff: Mentor

    Nobody has mentioned quantum mechanics yet. When the scale of things are small enough to be in "the quantum realm", then there are very definite limitations on what can be measured.

    The quantum realm can be defined as the scale where things are so small that no measurement can be sensitive enough to observe Anyang without changing the thing observed.
  12. Dec 16, 2014 #11
    How is that possible? Let's consider a device composed of two atoms. These could be any distance D ∈ ℝ apart from one another, even if some of these values represent unstable situations.
  13. Dec 16, 2014 #12
    There are quantum limits and there are thermodynamic limits to the measurement. The thermodynamics limits come into play because by making a measurement, we are decreasing the entropy of the thing we are measuring. This must be accompanied by an increase in entropy in our measurement system or surroundings. We cannot measure the state of the entire universe because that would reduce the entropy of the universe, which is not allowed. Anytime you make a measurement, you have to entangle the system under measurement with something external to the system (such as a probe, detector, computer, yourself...).
  14. Dec 16, 2014 #13
    DarkFalz, one of the first problems studied by almost all students of quantum mechanics is called the particle in a box. If you haven't yet, I suggest you study it.

    In quantum mechanics, particles (such as atoms) do not have definite positions, so your classical view of a state does not apply. Rather, particles must occupy certain quantum states (or superpositions of them). For example, an electron in a hydrogen atom is only allowed to exist in certain orbitals. The distance between the electron and nucleus is indeterminate in any of the orbitals (every time you measure it you will get a different result, even if the "state" is the same) although you can define an average distance, which is only allowed to take on certain discrete values. (You can get other values by taking superpositions of orbital states, but this is ultimately limited by the size of the apparatus.)

    Note that DaleSpam said "finite apparatus". Finite is the key word here. For a particle in a box, the larger the box, the more states are available for a given energy. It takes up to infinite energy to create a state of two atoms with infinitely precise positions.
  15. Dec 16, 2014 #14
    What if the position of the whole atom itself in the universe represents the state?
  16. Dec 16, 2014 #15


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    Its position is not precisely defined in QM.
  17. Dec 16, 2014 #16


    Staff: Mentor

    States are represented by wavefunctions. The position is an operator on the wavefunction, not the state itself.

    Khashishi's comments above are right on. You should look into the particle in a box and the Bekenstein bound. Any measuring device would necessarily be finite, even if the universe were infinite.
    Last edited: Dec 16, 2014
  18. Dec 17, 2014 #17
    Let's wrap it up then.

    A device can't perform infinitely precise measurements because that would require infinite energy, which we do not have. Hence, when two very similar lights arrive to our eyes (or a camera), these are interpreted as the same color because the measuring device has finite precision when analyzing light, therefore if the RGB red component of a red light is, for instance, 1.3443666346 or 1.344365677 it does not matter because both are interpreted as 1.34436 (5 decimal places for example). Is this correct?

    What i find the most weird in all of this is how we humans kind of perform a "round" of our measurements, because looking at a rock with 100000000000 atoms and one with 100000000000 -1 atoms is definitely different, but still we perceive it as exactly the same!

    The same goes with digital measurement devices. If a digital device displays the temperature as 35,59ºC (only has 2 decimal places) but the temperature is actually 35,595466ºC or 35,76575ºC, but appear as PRECISELY the same, even though both represent different states of the device's particles. Is it that the device does not actually display the same but we humans interpret it as the same? I mean, if the temprature is different, even though the device displays the same digits, something inside of it has to be different, it is hotter in one situation than the other, and that affects particles. Are we humans the reason why we can't perceive the different states?
  19. Dec 17, 2014 #18


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    In some cases it is a limitation on our measuring instruments. In some cases we cannot define the quantity that we are measuring precisely enough to make a precise measurement possible (for instance, time of death accurate to the microsecond). In some cases it goes deeper than that -- the quantity that we are trying to measure simply does not exist (for instance, the precise position of a sub-atomic particle between measurements).

    I would put temperature into that final category. Temperature is a statistical measurement. Focus down to the very small and the very precise and it stops existing at all.

    In my view, that's not a human failure. That's just the way things are.
  20. Dec 17, 2014 #19


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    Many times finite measurement precision it is also a result of how we sample and store the information. For example, a sensor may have theoretically infinite precision in the sense that it is an analog instrument outputting a voltage, but modern computers store information digitally. In sampling the sensor voltage, we can only do this to finite precision, often 16 bits, so any information that cannot be measured within the constraints of the given bit depth is lost.
  21. Dec 17, 2014 #20
    DarkFalz, I think you are missing the point in your last post. It's not just the measurement device that is finite. The item under observation is also finite. Since the item itself is finite, it doesn't have exact values of certain observable values, even if you use an infinite measuring device.
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