Ask Dr. Faizal 1 – The Classical and Quantum Understandings of the World – News Intervention

ByDr. Mir FaizalandScott Douglas Jacobsen

Dr. Mir Faizal is an Adjunct Professor in Physics and Astronomy at the University of Lethbridge and aVisiting Professor inIrving K. Barber School of Arts and Sciencesat the University of British Columbia Okanagan.

Here we start the cosmology educational series on the differences between the classical and the quantum worlds.

Scott Douglas Jacobsen: We have heard terms like classical physics and quantum physics. What do these terms mean in simple words, and what is the difference between them?

Dr. Mir Faizal:We have evolved at a certain scale, and our intuitive understanding of the world is also limited to that scale. Now common sense is the expression of this intuitive understanding of the world in languages like English or French. If this intuitive understanding of the world is expressed in mathematics, we naturally will obtain a mathematical description of common sense. This mathematical description of our intuitive understanding is called classical physics. However, there is no fundamental reason why such a description will hold at a different scale. In fact, now we have known that the classical description does not hold at very small scales, and common sense seems also to break at such a scale. It is hard to accurately describe the world at such a small scale using languages like English or French, as these languages have not been evolved to describe the world at such a scale. However, it is still possible to mathematically describe the world at such a small scale, and this mathematical description of a small scale is called quantum physics. Even though it is not possible to describe the world at such a small scale in common language, it is possible to use analogies to understand physics at such small scales.

Jacobsen: We see the worldaround us, and know how it behaves, and this forms a basis for our commonsense. Youmentionedthat our common sense breaks in quantum mechanical. Canyou give some examples of such a breaking of common sense in quantummechanics?

Faizal: Let us start by a simple example, to understand how the common sense breaks in the quantum mechanism. If there are two paths between your home and your office, and you are travelling between them, you can take any one of these two path at one time. However, you will infer that it is impossible to take both these paths at the same time. Even if you are really tiny, you cannot take two paths at the same time. The main reason for this is that it is impossible for you to be present at two different places at the same time. This seems to be something that you know from common sense. However, this description of the world does not hold at much smaller scales. In quantum mechanics, you go to your office from both those paths. In fact, you will take all the possible paths between your home and office, and we have to mathematically sum these path to describe your behaviour of going between your home and office. This is actually how things are calculated for quantum mechanical particles. This description of quantum mechanics (where a particle takes all possible path between two points) is called the Feynman path integral approach.

Jacobsen: We have seenpeople commute between their home and office. In fact, as more simple system,we have seen a stone fall down, and it does not appear to take many pathsbetween two points. We have also never seen a particle present at two places atthe same time. How does the quantum mechanical fit with these observations?

Faizal:In quantum mechanics, as soon as someone makes ameasurement on some object, it instantaneously collapses to just one of thosepaths. Now it is possible to calculate the chance of an object to be collapseto a certain path in quantum mechanics. For large enough objects, this almostcoincides with the path that the object is expected to take based on classicalmechanics. However, as the objects gets smaller, the deviations between the twopaths becomes significant. It may be noted to calculate the position of anobject at any point in future, you need to know about two things. You need toknow where that object is present at a given time, and you need to know howfast it is travelling in a certain direction. If you know both these things,then you can know where that object will be present in future. However, in quantummechanics, it is impossible to measure both the position of a particle and howfast it is travelling, at the same time. Thus, in quantum mechanics it is notpossible to accurately measure the position of a particle in future. What wecan measure is the chance for a particle to be present at a certain point intime. So, in quantum mechanics causality is also only probabilistically true.As it is impossible to obtain certain knowledge of cause, the effects can beonly probabilistically predicted.

Jacobsen: It is possible to exactly predict the future position of a particle by improving our technology and inventing better devices?

Faizal:Technological development cannot be usedto predict the future position of a particle beyond what is allowed by quantummechanics. This is because for such quantum system certain knowledge isactually not present in nature, and so we can only get probabilistic knowledgeof such system. This is the main difference between the classical and quantumdescription of the world. In classical mechanics, at least in principle, it ispossible to know the behaviour of a particle with certainty. In other world,the world is totally deterministic in classical mechanics. It might bedifficult to exactly calculate such a behaviour, but such a knowledge exists innature. In fact, even in classical mechanics, we usually use probability todescribe the world. This is the basis of statistical mechanics. However, such ause of probability is epistemological as certain knowledge exists atanontological level in classical physics. It is just very difficult forus to obtain such knowledge accurately for many systems. However, in quantummechanics there is anontological use probability as certain knowledge isabsent at anontological level from nature.

Jacobsen: Can you give asimple analogy of this difference to make it easy to understand?

Faizal:Let us again use a simple example tounderstand this difference. Someone is going to a coffee shop, and he usuallylikes to drink coffee but sometime orders tea. As it is a coffee shop they keeprunning out of tea. Now if it is known that he takes tea about twenty times in hundreddays, then you can calculate the chance of him drinking tea of coffee. Youcannot predict accurately what he will take on a given day, as such a knowledgeis not present in this system. However, knowing what he is more likely toorder, you can predict his behaviour over a large number of visits. So, for thenext ten days you can save two tea bag for him. This is an example of anontological absence of knowledge, and this is how probabilities work in quantummechanics. Now consider another example, in a group of ten people, two of themlike tea and the rest like coffee. Also they have a rule that they will notvisit the coffee shop more than once in ten days. Now if you do not bother toask them who like tea and who likes coffee, and just know how they behave in agroup, you can again predict the probability of them drinking tea. However, inthis case, the knowledge exists in form a hidden variable, which you did notbother to measure. This is an example of anepistemological absence ofknowledge, and this is how probabilities work in statistical mechanics.

Jacobsen: I can understandthat certain knowledge of the particle is not present, but where is theparticle actually present.

Faizal:Theparticle is present at every possible point it can occupy, till it is measured.However, when it is measured, it instantaneously collapses to a single point,and we can measure the chance of it collapsing to a certain point. This is animportant feature of quantum mechanics. In classical mechanics, two different contradictionscannotbe simultaneously existing. In quantum mechanics, all possibilitiessimultaneously exist, till they are measured. However, when they are measured,only one of them is instantaneously observed, and the system ceases to exist inthe other possibilities. This principle has been illustrated by the famousthought experiment of Schrodingers cat, in which a cat is killed by a quantummechanical process. There are two possibilities, as the cat can be dead andalive. Now if the system is not observed, then the cat can exist in a statebeing dead and alive at the same time. As soon as an observation is made, thesysteminstantaneously collapses to one of the two possibilities, so thecat is actually observed to be dead or alive. However, if no observation ismade, the cat is in a state of being dead and alive at the same time.

Jacobsen:Can these quantum effects be observed in our daily life?

Faizal: A important requirement of quantum mechanics isthat it should coincide with the classical physics at our scale, for all thesystem that have been described using classical mechanics. This means thesequantum effects become so small at our scale that they can be neglected, andcannot be observed. There are few phenomena like superconductivity andsuperfluiditywhere quantum effects can change the behaviourofcertain system at large scale. However, most quantum mechanical effect, whichbreak common sense, can be neglected at our scale, and the world at our scalecan described by classical mechanics. It is possible that there are somesystems, where other quantum effects become important even at large scale, and theirbehaviouris very different from thebehaviourpredictedfrom classical mechanics.

Jacobsen: Thank you for theopportunity and your time, Dr. Faizal.Faizal:My pleasure.

Photo by Billy HuynhonUnsplash

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Scott Douglas Jacobsen is the Founder of In-Sight: Independent Interview-Based Journal and In-Sight Publishing. Jacobsen works for science and human rights, especially womens and childrens rights. He considers the modern scientific and technological world the foundation for the provision of the basics of human life throughout the world and advancement of human rights as the universal movement among peoples everywhere. You can contact Scott via email, his website, or Twitter.

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Ask Dr. Faizal 1 - The Classical and Quantum Understandings of the World - News Intervention