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Cambridge University Press eBooks

Backwards and forwards

Alastair I. M. Rae

Published Mar 26, 2012• 0 citations
Publisher

Abstract

In the last chapter, we considered the possibility that an object such as the pointer of an apparatus designed to measure H/V polarisation (see Figure 7.1) might in principle be forbidden from being in a quantum superposition if it was large enough. A microscopic system, such as a photon polarised at 45 ° to the horizontal, would then collapse into an h or a v state as a consequence of such a measurement. To test this idea, we considered how we might detect a macroscopic object in such a superposition and we found that this was very difficult in practice. The oscillating pointer of Figure 7.1 is very sensitive to random thermal disturbances and as a result is almost certainly going to swing in one direction or the other, rather than being in a superposition of both. Macroscopic superpositions have indeed been observed in SQUIDs (see the last section in Chapter 7), but only after great care has been taken to eliminate similar thermal effects. In the context of the last chapter, the randomness associated with thermal motion was considered as a nuisance to be eliminated, but might it instead be just what we are looking for? Perhaps it is not that thermal effects prevent us observing quantum superpositions, but rather that such states are impossible in principle when thermal disturbances are present. This could provide another way out of the measurement problem.

DOI
10.1017/cbo9781107295605.011

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Backwards and forwards

Alastair I. M. Rae

In the last chapter, we considered the possibility that an object such as the pointer of an apparatus designed to measure H/V polarisation (see Figure 7.1) might in principle be forbidden from being in a quantum superposition if it was large enough. A microscopic system, such as a photon polarised at 45° to the horizontal, would then collapse into an h or a ν state as a consequence of such a measurement. To test this idea, we considered how we might detect a macroscopic object in such a superposition and we found that this was very difficult to do. The oscillating pointer of Figure 7.1 is very sensitive to random thermal disturbances and as a result is almost certainly going to swing in one direction or the other, rather than being in a superposition of both. Macroscopic superpositions have indeed been observed in SQUIDs (see the last section in Chapter 7), but only after great care has been taken to eliminate similar thermal effects. In the context of the last chapter, the randomness associated with thermal motion was considered as a nuisance to be eliminated, but might it instead be just what we are looking for? Perhaps it is not that thermal effects prevent us observing quantum superpositions, but rather that such states are impossible in principle when thermal disturbances are present. This could provide another way out of the measurement problem.

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In the last chapter we saw how the measurement problem in quantum theory arises when we try to treat the measurement apparatus as a quantum system. We need more apparatus to measure which state the first apparatus is in, and we have a measurement chain that seems to go on indefinitely. There is, however, one place where this apparently infinite sequence certainly seems to end and that is when the information reaches us. We know from experience that when we look at the photon detector we see that either it has recorded the passage of a photon or it hasn't. When we open the box and look at the cat either it is dead or it is alive; we never see it in the state of suspended animation that quantum physics alleges it should be in until its state is measured. It might follow, therefore, that human beings should be looked on as the ultimate measuring apparatus. If so, what aspect of human beings is it that gives them this apparently unique quality? It is this question and its implications that form the subject of the present chapter. Let us examine more closely what goes on when a human being observes the quantum state of a system. We return to the set-up described in the last chapter, where a 45° photon passes through a polarisation analyser that moves a pointer to one of two positions (H or V) depending on whether the photon is horizontally or vertically polarised. At least that is what would happen if the analyser and pointer behaved as a measuring apparatus. If however we treat them as part of the quantum system, the pointer is placed in a superposition of being at H and at V until its state is measured. We now add a human observer who looks at the pointer (it would be possible to imagine the observer using one of his other senses, for example hearing a particular sound caused by a change in the pointer’s position, but it is clearer if we think of a visual observation).

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In the last chapter we saw how the measurement problem in quantum theory arises when we try to treat the measurement apparatus as a quantum system. We need more apparatus to measure which state the first apparatus is in, and we have a measurement chain that seems to go on indefinitely. There is, however, one place where this apparently infinite sequence certainly seems to end and that is when the information reaches us. We know from experience that when we look at the photon detector we see that either it has recorded the passage of a photon or it hasn't. When we open the box and look at the cat either it is dead or it is alive; we never see it in the state of suspended animation that quantum physics alleges it should be in until its state is measured. It might follow, therefore, that human beings should be looked on as the ultimate measuring apparatus. If so, what aspect of human beings is it that gives them this apparently unique quality? It is this question and its implications that form the subject of the present chapter. Let us examine more closely what goes on when a human being observes the quantum state of a system. We return to the set-up described in the last chapter, where a 45° photon passes through a polarisation analyser that moves a pointer to one of two positions (H or V) depending on whether the photon is horizontally or vertically polarised.

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In the last chapter, we considered the possibility that an object such as the pointer of an apparatus designed to measure H/V polarisation (see Figure 7.1) might in principle be forbidden from being in a quantum superposition if it was large enough. A microscopic system, such as a photon polarised at 45 ° to the horizontal, would then collapse into an h or a v state as a consequence of such a measurement. To test this idea, we considered how we might detect a macroscopic object in such a superposition and we found that this was very difficult in practice. The oscillating pointer of Figure 7.1 is very sensitive to random thermal disturbances and as a result is almost certainly going to swing in one direction or the other, rather than being in a superposition of both. Macroscopic superpositions have indeed been observed in SQUIDs (see the last section in Chapter 7), but only after great care has been taken to eliminate similar thermal effects. In the context of the last chapter, the randomness associated with thermal motion was considered as a nuisance to be eliminated, but might it instead be just what we are looking for? Perhaps it is not that thermal effects prevent us observing quantum superpositions, but rather that such states are impossible in principle when thermal disturbances are present. This could provide another way out of the measurement problem.
In the last chapter, we considered the possibility that an object such as the pointer of an apparatus designed to measure H/V polarisation (see Figure 7.1) might in principle be forbidden from being in a quantum superposition if it was large enough. A microscopic system, such as a photon polarised at 45° to the horizontal, would then collapse into an h or a ν state as a consequence of such a measurement. To test this idea, we considered how we might detect a macroscopic object in such a superposition and we found that this was very difficult to do. The oscillating pointer of Figure 7.1 is very sensitive to random thermal disturbances and as a result is almost certainly going to swing in one direction or the other, rather than being in a superposition of both. Macroscopic superpositions have indeed been observed in SQUIDs (see the last section in Chapter 7), but only after great care has been taken to eliminate similar thermal effects. In the context of the last chapter, the randomness associated with thermal motion was considered as a nuisance to be eliminated, but might it instead be just what we are looking for? Perhaps it is not that thermal effects prevent us observing quantum superpositions, but rather that such states are impossible in principle when thermal disturbances are present. This could provide another way out of the measurement problem.
In the last chapter we saw how the measurement problem in quantum theory arises when we try to treat the measurement apparatus as a quantum system. We need more apparatus to measure which state the first apparatus is in, and we have a measurement chain that seems to go on indefinitely. There is, however, one place where this apparently infinite sequence certainly seems to end and that is when the information reaches us. We know from experience that when we look at the photon detector we see that either it has recorded the passage of a photon or it hasn't. When we open the box and look at the cat either it is dead or it is alive; we never see it in the state of suspended animation that quantum physics alleges it should be in until its state is measured. It might follow, therefore, that human beings should be looked on as the ultimate measuring apparatus. If so, what aspect of human beings is it that gives them this apparently unique quality? It is this question and its implications that form the subject of the present chapter. Let us examine more closely what goes on when a human being observes the quantum state of a system. We return to the set-up described in the last chapter, where a 45° photon passes through a polarisation analyser that moves a pointer to one of two positions (H or V) depending on whether the photon is horizontally or vertically polarised. At least that is what would happen if the analyser and pointer behaved as a measuring apparatus. If however we treat them as part of the quantum system, the pointer is placed in a superposition of being at H and at V until its state is measured. We now add a human observer who looks at the pointer (it would be possible to imagine the observer using one of his other senses, for example hearing a particular sound caused by a change in the pointer’s position, but it is clearer if we think of a visual observation).
In the last chapter we saw how the measurement problem in quantum theory arises when we try to treat the measurement apparatus as a quantum system. We need more apparatus to measure which state the first apparatus is in, and we have a measurement chain that seems to go on indefinitely. There is, however, one place where this apparently infinite sequence certainly seems to end and that is when the information reaches us. We know from experience that when we look at the photon detector we see that either it has recorded the passage of a photon or it hasn't. When we open the box and look at the cat either it is dead or it is alive; we never see it in the state of suspended animation that quantum physics alleges it should be in until its state is measured. It might follow, therefore, that human beings should be looked on as the ultimate measuring apparatus. If so, what aspect of human beings is it that gives them this apparently unique quality? It is this question and its implications that form the subject of the present chapter. Let us examine more closely what goes on when a human being observes the quantum state of a system. We return to the set-up described in the last chapter, where a 45° photon passes through a polarisation analyser that moves a pointer to one of two positions (H or V) depending on whether the photon is horizontally or vertically polarised.