Or are they just a region of space where the forces are acting around its source, for example a magnet? But if they are just regions, and not physical objects, then how can the Earth's magnetic field be reshaped to a "teardrop" by the solar wind?
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3$\begingroup$ I think it might be necessary to clarify what is meant my "physically exist". Most physicists would probably say something exists if it can be measured, but on some topics (such as virtual particles, for example) the distinction is not so clear. It's a deep question, but IMO only partially related to physics. $\endgroup$– psitaeCommented Dec 12, 2016 at 12:35
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$\begingroup$ I agree with psitae. This is a philosophical question more than a physical one, and it is not a simple question to answer. See also "*Does the electromagnetic field physically exist?" over at the Philosophy of Science StackExchange. $\endgroup$– PirxCommented Dec 12, 2016 at 12:37
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$\begingroup$ A key phrase in anna v's answer is "extending over all space", i.e., a field (in the modern quantum sense) isn't merely something that happens in a local region: by definition, it fills the whole of space. So (with reference to electromagnetism) don't think of space as containing a bunch of separate electromagnetic fields at various locations. Instead, when you make an electromagnetic observation you are performing a measurement on the electromagnetic field at a particular spacetime event. $\endgroup$– PM 2RingCommented Dec 12, 2016 at 13:18
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$\begingroup$ The magnetosphere is "tear drop" shaped due to a combination of forces (i.e., electromagnetic and fluid-like). It somewhat looks like what one might expect for a semi-spherical object sitting in a wind tunnel in a collisional media because the hyperbolic shape is largely dominated by dynamic pressure and shock features. As for the "force field" part, I defer to the answer by @annav. $\endgroup$– honeste_vivereCommented Dec 13, 2016 at 2:12
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4 Answers
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A force field by definition is a vector field, i.e. a region of space where to each point a vector (=force) is assigned. As such it is not a physical object, however you can of course probe/measure the field with a suitable test object (mass in case of gravitational field, charge in case of electrical field...).
The solar wind, being a plasma, carries the sun's magnetic field to the earth (interplanetary magnetic field). So at the earth you will have two contributions to the magnetic field, the magnetic dipole from earth (which is more or less symmetric) and the field from the solar wind. A test probe would be affected by the sum of these two fields.
Now, due to the Lorentz force, particles from the solar wind get deflected and travel around the earth. This leads to similar effects as in hydrodynamic when you have an object in a stream of fluid (see Magnetohydrodynamics), making the total field asymmetric (and different/reshaped from the magnetic dipole field).
answered Dec 12, 2016 at 11:10
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Physics uses fields, a mathematical concept, both at the classical framework and at the the quantum mechanical framework, to model observations and predict outcomes of experiments. The answer of user1583209 is within the classical framework for the use of 'fields' as you are asking about the magnetic field.
A field can be classified as a scalar field, a vector field, a spinor field or a tensor field according to whether the represented physical quantity is a scalar, a vector, a spinor or a tensor, respectively. A field has a unique tensorial character in every point where it is defined: i.e. a field cannot be a scalar field somewhere and a vector field somewhere else. For example, the Newtonian gravitational field is a vector field: specifying its value at a point in spacetime requires three numbers, the components of the gravitational field vector at that point. Moreover, within each category (scalar, vector, tensor), a field can be either a classical field or a quantum field, depending on whether it is characterized by numbers or quantum operators respectively.
Italics mine.
As far as classical fields go, they are a mathematical construct dependent in most cases centered on sources and their kinematic behavior. An exception is the electromagnetic wave which propagates independent of the source, but due its its motion it cannot be considered as "regions in space"
The mathematical theory which successfully describes quantum mechanical effects in the microcosm of atoms and particles is quantum field theory. These are fields that are quantum operators operating on a quantum ground state. All particles in the standard model of particle physics are assigned a quantum field , extending over all space , and is assigned a ground state on which a creation operator for the field, the electron for example,operating on the ground state will create an electron, and an annihilation operator will destroy it. The ground state is zero, if there are no particles created/annihilated so in this sense also the concept is mathematical to allow for calculating the behavior of interacting particles.
For people interested on how classical fields emerge from quantum mechanical fields this link explains it but it needs mathematical tools.
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$\begingroup$ Are those quantum fields, "force fields" (as in the question)? $\endgroup$ Commented Dec 12, 2016 at 13:27
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$\begingroup$ @user1583209 they build up the classical force fields. The classical fields emerge from the quantum ones as can be seen in the link I provided at the end $\endgroup$– anna vCommented Dec 12, 2016 at 13:47
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$\begingroup$ No doubt about it, but I was wondering how relevant quantum field theory is to the original question which as far as I can see can be solved in classical physics or philosophy. $\endgroup$ Commented Dec 12, 2016 at 14:09
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$\begingroup$ @user1583209 my answer is within the philosophy of the site, as partly a repository for answering random searches with similar words. I referered to your answer which is more complete in the sense of answering the magnetic field of the earth question $\endgroup$– anna vCommented Dec 12, 2016 at 14:58
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Fields are genuine physical objects, which carry energy, momentum, and sometimes charge (as in Yang-Mills theory and General Relativity). Fields can exist without any sources, as, for example, electromagnetic waves or hypothetical glueballs.
answered Dec 12, 2016 at 11:15
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1$\begingroup$ You are confusing two frameworks, the classical as in "electromagnetic field" and"glueballs" which are predicted quantum phenomena i $\endgroup$– anna vCommented Dec 12, 2016 at 11:41
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$\begingroup$ @annav There is a one reality which may be described in one way or another. Some objects can be described by classical physics, while for the other ones the quantum description is needed. For proper description of electromagnetic field one also has to take into account quantum effects. $\endgroup$ Commented Dec 12, 2016 at 12:15
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$\begingroup$ I agree, it is confusing for a newbie though when no distinction is made $\endgroup$– anna vCommented Dec 12, 2016 at 12:47
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My gut feeling is that the question originates from a mental model of a world full of physical bodies, i.e. things. A drawing in this world depicts a physical object: A chair, a table, a house. Planets. I can manipulate the smaller ones: Take them elsewhere, turn them over. The bigger ones influence each other: The sun attracts the planets which orbit around it.
I can only draw things which exist, in this palpable sense.
Now somebody draws a magnetic field. I interpret your question in this sense: Does this drawing depict something that really, physically, bodily exists? The answer, unfortunately, goes in the opposite direction of what your question implies. The physical objects we see are actually systems of fields. The chair is a collection of nuclei and electrons held in place by electrostatic and -dynamic interaction; and on a closer look we cannot maintain the "bodily existence" of any of these "particles" in any "solid-matter" sense. They are waves, quantum states, probabilities, processes, which in the particular case of the chair I'm sitting in right now congeal to an overwhelmingly probable interaction with my behind resting on it. Phew.
The answer therefore is that a magnetic field like the earth's is exactly as real as anything else around you; not more, and not less. But the reason is that the air of "reality" our daily surroundings emanate is rather deceiving.
answered Dec 12, 2016 at 16:20