How Quantum Field Theory Describes the World
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Quantum field theory brings together quantum mechanics and relativity. It is the mathematical language in which the standard model of particle physics is written.
What is a quantum field? Most people will be familiar with the concept of classical fields, for example electromagnetic fields. Fields are used to explain how a force can be transmitted over a distance. For instance, how does the magnet in the picture 'push' the iron filings into that pattern without touching them? The answer is that a field exists around the magnet, which exerts a force on the iron filings causing them to line up along the field lines.
In particle physics, we use fields to describe particles. In quantum physics, particles are not thought of as point-like objects. The uncertainty principle of quantum mechanics tells us that we cannot know the exact location of a particle. The location of the particle is described using a probability field, which is strongest at the points where the particle is most likely to be found and weakest where we are unlikely to see the particle.
Path Integrals
This quantum uncertainty means that we cannot be sure what a particle is doing when we are not looking at it. If we observe a particle at point A, and some time later observe it at point B, we do not know what path it has taken to get from A to B. Quantum mechanics tells us that in order to calculate the probability of observing the particle at B after having seen it at A, we must add up the probabilities of all the paths it could possibly have taken from A to B. Feynman showed that this summing up of paths can be approximated by an integration over all the paths known as a path integral.
This expression means: "add up the “probability” exp(S[q]) of each of the paths q1, q2, etc... to get the probability of finding the particle at B". The classical path is the most likely so it has the largest contribution to the integral. When we consider everyday objects, which are much larger than elementary particles, the contribution from the classical path is overwhelmingly larger than from any of the other paths, which is why we do not see everyday objects behaving in a quantum way.
These integrals are rather complicated and difficult to evaluate. Fortunately, Feynman invented a pictorial way of representing them, known as Feynman diagrams.
Quantum field theory is a powerful mathematical tool which underpins the whole of modern particle physics theory. It is also used in statistical physics, cosmology, and theories of quantum gravity, which aim to find a “theory of everything” that is free from the inconsistencies that currently exist between general relativity and quantum physics. Resources for learning more are listed below.
Lecture Notes
- David Tong: Quantum Field Theory
A Cambridge University course with lecture notes, covering the canonical quantization of scalar fields, Dirac fields and QED.
Books
 | Peskin and Schroeder is the ultimate quantum field theory textbook. I found this book invaluable during my degree. Amazon Price: $64.16 List Price: $81.00 |
 | User-friendly book which keeps the math to a minimum and explains the physics clearly. Amazon Price: $46.95 List Price: $80.00 |
 | A useful book which explains the theory well and puts it in context. Amazon Price: $39.87 List Price: $70.00 |
 | Another useful quantum field theory book by Mark Srednicki. Amazon Price: $61.29 List Price: $85.00 |
Other physics hubs by topquark
- Feynman Diagrams: An Introduction
A Feynman diagram is a representation of an interaction between elementary particles. Feynmann diagrams are incredibly useful in calculating the probability of a reaction occurring. - Where is all the antimatter? Mystery of matter-antimatter asymmetry
This article is an introduction to the unsolved problem of matter-antimatter asymmetry in particle physics and cosmology. Why is the amount of matter in the universe so much greater than the amount of antimatter? - The Evidence for Dark Matter
Dark matter is the name given to the matter that holds together the Milky Way and other galaxies. It cannot be seen using telescopes as it does not emit light or other radiation. However, there is significant evidence for its existence.
topquark works as a researcher in theoretical particle physics and blogs about research at The Particle Pen.
CommentsLoading...
I think that's been done (although I'm failing to find the original reference) and the result was that the interference pattern didn't appear when you know what slit the photon goes through - i.e. it acts as a particle not a wave.
A human observer does observe something when he looks at the results of the experiment as recorded by the sensor. So you could view that as the action that makes the wavefunction collapse. It's like the Schrodinger's cat situation.
Interpreting quantum physics is baffling (and that's why it's so interesting, right?). The maths works beautifully at predicting outcomes, but it leads to some wierd interpretations when you try to think about what's going on physically.
Thanks for your thought-provoking comments :)
Yes. I am sure it was done. ;) but I do not know the details of how. Was the sensor completely passive? Can we rule out any electromagnetic interference from it? What kind of sensor was it exactly? Have we done the experiment with different types?
You say the observer observed the results but did we observe it after the fact or as it was happening?
I also know Feynman said there was no wave particle duality and that in the double split experiment the particle takes all possible paths creating interference with itself which emerges as a wave pattern. But I can not find his ideas on the sensor experiment or wave function collapse, which I know many scientists have doubt actually exists.
Can you enlighten me on any of these questions? I think it would make a great hub, actually.
You have written another very easy to follow hub that explains the concept in a way that anyone can understand. I also think that the resources you provide for further reading are excellent.
Have you considered writing a series for A-Level students as I feel your treatment of the concepts would help so many of them develop a much better understanding and appreciation for the material they will be tested on. Just an idea.
Slarty O'Brian Level 3 Commenter 11 months ago
Another interesting hub. What are your ideas on the double slit experiment with a sensor?
Personally I can't see how we can consider the sensor as an observer. To say that the observer collapses the wave function in this experiment seems like nonsense to me considering no human observer observed anything.