As an engineer, I’ve heard it a million times: You can’t measure something without changing it.
Upon reflection, it appears to be true; every measurement I can think of will (in some way) “load down” the source, and thus add error. This applies to temperature, length, force, pressure, etc.
So does this rule always apply? Or are there exceptions, i.e. are there measurements that do not add any error?
(Note that I’m referring to continuous-type measurements, not discrete or “counting” measurements.)
I might be dense, but I can’t see how using reflected/emitted light to measure the length of a brick changes the brick or loads down the source. The light had already been reflected/emitted. It would have been intercepted by something else if it hadn’t been intercepted by my clinometer (as an example). No matter what actually intercepts it there isn’t any feedback by that light which will affect the original source.
I can see how a measurement that involves applying energy to the brick will change the brick. But a great many measurements solely involve capturing energy that has already been lost from the brick. How could such measurements change the brick?
I guess you could stretch it and say that the mere presence of a clinometer or theodolite will alter the albedo of the room (or planet for outdoor bricks, or galaxy for bricks on other planets) and thus alter the temperature of the brick in that way. But that’s pretty damn spurious and is simply a complex way of demonstrating out that the universe itself is a closed system and all events influence one another eventually.
If you can’t determine how much change is introduced by the measurement of a property, how do you know if there has been a change in that property or not?
Did you hear about the fellow who went to a nit pickin picnic and there were no nits to pic.
Ah, but the brick has already been affected. The action of the light hitting the brick changes the brick in small ways. When the light reflectas back, you are really seeing the brick the way it used to be, before the light hit it, and not the way it is now.
Of course the light hitting th brick changed the brick. But the question wasn’t whether it is possible to measure the state of an object the way it is ‘now’. The question was whether it is possible to measure it at all without changing it. In this instance the act of measuring hasn’t changed the brick. Those changes would have occured whether the brick was measured or not. We can measure the brick without changing the brick. That is not the same as saying that a brick that never changes can be measured. A brick that never cahanges can’t be meausred because a brick that never changes can’t exist in this universe.
More fundamentally “the way it is now” is a meaningless phrase when we are talking about light. As old Albert would have told you ‘now’ is a place, not simply a time. It’s all relative, and what it’s relative to is the speed of light. Now is simply the place where light is arriving from our reference point, to someone else somewhere else ‘now’ is ‘then’. Or more simply put, if we are measuring the condition of the light form that object then we must be measuring the conditon of that object ‘now’ if ‘now’ is to have any meaning at all.
I was interested by this article I saw recently. Doesn’t this mean that quantum encryption is screwed? I thought QE relied upon not being able to measure bits without collapsing the waveform.
As an engineer who is not too prone to worrying about the quantum effects that occur in my day-to-day life, I would say that what you are offering up is *much ado about nothing * when considered in the macro (real) world. If splitting hairs is important then by all means go for it.
Why? Heisenberg never said that you can’t measure things without effecting them AFAIK, he said that the accuracy of the measurement of velocity was inversely proportion to the accuracy of mesurement of position when applied to extemely small objects. IOW there’s nothing that says that something can’t be measured exactly WRT to, say, length so long as you don’t expect it to be able to simultaneously measure its speed with infinte accuracy. The OP never said he wanted infinite accuracy of measurement of any parameter, much less simultaneous infinite measurement of all parameters.
Can you explain why anything Heisenberg said would rule out me being able to measure the height of a brick, to the nearest mm, with 99.9% accuracy using a clinometer without actually changing the brick?
Oy…like, the brick doesn’t really exist until you observe (measure) it? Maybe that wasn’t him though. I’m but a cave man and know nothing of such things.
No. The measurement is still projective (that is, the waveform “collapses” onto the eigenbasis of the measurement operator), as is required by the axioms of quantum mechanics. What they’ve been able to do is to make a measurement without destroying the qubit entirely–i.e., without resetting the qubit to the ground state, or to some random noise state. This avoids having to reload or recool the qubit for later use.
