Higgs field
- This is an introduction to the Higgs field; for more technical material see Higgs boson and Higgs mechanism
The Higgs field is a possibly discovered, ubiquitous quantum field supposed to be responsible for giving elementary particles their masses.
All quantum fields have a fundamental particle associated with them. The particle associated with the Higgs field is the Higgs boson.[1] In this context, the word "field" is used in the sense used in physics and not in the everyday sense.
Overview
In quantum field theory the fundamental entities are not particles but fields, like the electromagnetic field. Particles are represented by oscillations or persistent changes in these fields. The oscillations in the electromagnetic field are called photons; those in the Higgs field are called Higgs bosons.[2]
Some quantum fields represent the known elementary particles, while the existence of others enables spontaneous symmetry breaking, the cause of the differences between different kinds of particles and forces. For example, in the electroweak theory the Higgs field was introduced to explain why, at low temperatures, electromagnetism and the weak nuclear interaction have such different characteristics, i.e. a broken symmetry.
The Standard Model of particle physics incorporates a mechanism that endows particles with mass. Known as the Higgs mechanism, it was developed by Peter Higgs in 1964 to introduce mass into Yang-Mills theory.[3] Abdus Salam and independently Steven Weinberg recognized its importance for unifying the theories of the weak force and the electromagnetic force into a unified gauge theory of a single electroweak force.
Using the Higgs mechanism they found that the carriers of the weak interaction, the W and Z bosons, have large masses, whereas the corresponding carriers of the electromagnetic force have no mass. Therefore, the Higgs mechanism is often credited with explaining the "origin" or "genesis" of mass.[4] But there is some doubt as to whether the Higgs mechanism provides sufficient insight into the actual nature of mass. As Max Jammer puts it, "if a process 'generates' mass it may reasonably be expected to provide information about the nature of what it 'generates' as well".[5] But in the Higgs mechanism, mass is not "generated" in the particle by a miraculous creatio ex nihilo, it is transferred to the particle from the Higgs field, which contained that mass in the form of energy, and "neither the Higgs mechanism nor its elaborations...contribute to our understanding of the nature of mass".[6]
The "machinery" of the Higgs mechanism, the procedure by which spontaneous symmetry breaking endows gauge fields of zero mass with mass, is based on the assumption of the existence of a scalar field, the "Higgs field", which permeates all of space. By coupling with this field a massless particle acquires potential energy and, by the mass–energy relation, mass. The stronger the coupling, the more massive the particle.
The way particles acquire mass through interacting with the Higgs field is analogous to blotting paper absorbing ink.[7] Pieces of blotting paper represent individual particles and the ink represents energy. Different particles "soak up" different amounts of energy, depending on "energy absorbing" ability and the strength of the Higgs field.
Inflation
The Higgs field has been proposed as the energy of the vacuum from which all else came. In the first instant of time, it had the featureless symmetry of an undifferentiated energy that was all the universe was. In successive symmetry breakings at phase transitions occurring at discrete, lowering temperatures and densities, it gave rise to the universe. The last was the breaking of the electroweak force that liberated the weak and electromagnetic forces, and is now in reach of experiment. Well out of reach is the phase transition that separated the electroweak from the strong force. But the Higgs field, the proposed origin of all rest mass, is as central to investigation of the strong force as the weak.[8] The Higgs field has been postulated as a cause for inflation.[9] This is not part of the standard inflationary model, where the cause of inflation is left open. The name "generic" inflation has been suggested. The Higgs field is a "nonthermal" field, a field whose energy does not decrease as the universe expands. The higher the energy density, the faster the universe expands. So the large Higgs field is postulated as the cause of inflation.
Above unification temperatures it is suggested that there was a single electronuclear force, and the bosons of the electroweak and strong forces were indistinguishable. As the universe's temperature dropped, it is thought the Higgs field caused the electroweakstrong force to fragment into the electroweak and strong forces and give separate identities to the electroweak bosons (photons, W and Z bosons) and the strong-force bosons (gluons).
Eventually, even the energy of the Higgs field dropped to zero, marking the end of inflation.
Dark energy is postulated as an energy of the vacuum welling from the Higgs field.[10]
Standard model
In the standard inflationary model the energy source for the geometrical fields in Einstein's equations are taken to be the physical vacuum energy-given by virtual particle-antiparticle pairs and radiation using quantum field theory. This 'vacuum energy' is taken to be the cause of the original expansion of the universe.
"Generic" model
The gravitational force between ordinary matter is attractive. It is postulated that out of the initial quantum vacuum emerged a new sort of matter, different from ordinary matter in that it repels ordinary matter. This is postulated to be the Higgs field. The Higgs field decayed into ordinary matter, leaving the ordinary matter to continue in its expansion. This is the scenario that is meant to explain the presently observed expansion of the universe.
Criticism
There are several criticisms that have been applied to the generic inflation model, some of which apply to the standard model of inflation.[11]
The Higgs field is one of the leading, theoretical explanations for the observed expansion of the universe. But there is no empirically or mathematically conclusive evidence for this cosmological model, nor for the existence of the 10-dimensional strings.
The negative mass concept already predicts repulsive gravitational forces.
Motivation
One of the main motivations for postulating the Higgs field comes from the quest to find simple, symmetrical laws of nature.[12] Things fall down, not sideways. It required considerable effort to realize that the three dimensions of space are equivalent. Not until Galileo did people learn to "blame the earth" for hiding the simplicity of the principle of inertia. It was a good idea to formulate the basic laws of physics in empty space.
Physicists are now convinced that empty space itself is a complicated environment. They "blame the vacuum" for many complications. Background fields permeate empty space. These fields hide the full simplicity and symmetry of physical laws.
Almost all electrons emitted in neutron decay have left-handed spin, meaning the spin and the momentum of the electron point in opposite directions. Imagine what looks like a left-handed electron moving north (and thus, with spin pointed south). To an observer moving north even faster than the electron, the electron appears right-handed. The principle of relativity provides that observers moving with any constant velocity must see the same laws of physics, but "left-handed electron" is not a concept all such observers agree on. So, if the principle of relativity is correct, it cannot be strictly true that only left-handed electrons emerge from weak decays.
In a world where electrons had zero mass the problem in identifying left-handed electrons would not arise. In such a world, electrons would always travel at the speed of light, and observers cannot move more rapidly. All observers moving with different velocities would agree on the handedness of electrons. In this case, it would be a possible law of physics—consistent with the principle of relativity—that only left-handed electrons are emitted in weak decays.
Nature has provided a hint that the universe is closer to this simpler one--the vacuum itself may be the cause of the difference. One can consider the possibility that there is a background field permeating all space responsible for giving the electron mass, and slowing them down. This mass-generating field is the Higgs field.
The most direct tests of the idea would be to compare physics with and without the background Higgs field. Unfortunately it is not a practical proposition to turn off the background Higgs field. It may soon be possible, however, to break off and observe tiny chips of the Higgs field. Even without direct experimental evidence most physicists are convinced that the concept of the Higgs field is here to stay, for it allows one to envision more perfect worlds and relate them systematically to the world in which we live.
The weak interaction tends to favor the left-handed form of all fundamental fermions. If the Higgs field vanished all fermions would be massless; it could be an exact law that only the left-handed particles participate in the weak interaction. Now, suppose that the universe differs from this imaginary one only in the presence of an all-pervasive background Higgs field. While the interaction of particles with this background field gives them mass, it does not alter their weak interactions. The weak interactions still favor the left-handed form. Calculations suggest that the background Higgs field can occasionally cause the spin to flip. So the approximate rule that the weak interactions favour left-handed particles encodes an exact law.
The background Higgs field must have the same value throughout the universe because the light from distant galaxies contains the same spectral lines we find on Earth, showing that electrons have the same mass throughout the universe. If electrons are acquiring mass from the Higgs field, the field is implied to have the same strength everywhere.
Heat up a magnet and it becomes demagnetised. Its electrons do not recognize any special direction in space; the system is perfectly symmetric. But cool it down and the electrons align their spin axes due to forces between their spins. The perfect symmetry between the directions of space is destroyed through spontaneous symmetry breaking. Symmetric forces enforce an asymmetric solution. Physical laws are more symmetric than any stable realization of them.
Physicists suspect that a similar effect is responsible for the background Higgs field permeating the universe. The answer to the question "Why isn't our vacuum more empty?" is that emptiness is unstable.
Just as the electromagnetic field is higher near heavily charged particles, the Higgs field should be higher near heavy particles. For instance, near a Z boson—an object that accelerators should be able to produce in great abundance in the near future—the Higgs field is changed. The Z boson is unstable. When it decays into lighter particles, the disturbance in the Higgs field must take on another form. It might become a travelling disturbance in the Higgs field itself—a packet of energy propagating outward-a Higgs boson. The Higgs particle is to the pervasive mass-generating Higgs field what the photon is to electromagnetic fields.
Early in the history of the universe the fundamental particles were massless, and the equations describing the world were simpler and more symmetric.
References
- ^ "What exactly is the Higgs boson?", Stephen Reucroft, Scientific American, October 21, 1999
- ^ John D. Barrow, The Routledge Companion to the New Cosmology, ed. Peter Coles (London: Routledge, 2001) 300
- ^ P. W. Higgs, "Broken Symmetry, Massless Particles and Gauge Fields", Physics Letters 12, 132–133 (1964); "Spontaneous Symmetry Breakdown Without Massless Bosons", Physical Review1 45, 1156–1163 (1966).
- ^ R. Castmore and C. Sutton, "The Origin of Mass", New Scientist 145, 35–39 (1992). Y. Nambu, "A Matter of Symmetry: Elementary Particles and the Origin of Mass", The Sciences 32 (May/June), 37–43 (1992). J. LaChapelle, "Generating Mass Without the Higgs Particle", Journal of Mathematical Physics 35, pp. 2199–2209 (1994).
- ^ Max Jammer, Concepts of Mass in Contemporary Physics and Philosophy (Princeton, NJ: Princeton University Press, 2000) 162
- ^ Jammer 163, who provides many references in support of this statement.
- ^ M.J.G. Veltman, "The Higgs Boson", Scientific American 255 (November), 88–94 (1986).
- ^ Gerard Piel, The Age of Science: What Scientists Learned in the 20th Century (New York: Basic Books, 2001) 160
- ^ Tony Rothman, and George Sudarshan, Doubt and Certainty: (Cambridge, MA: Perseus Publishing, 1998) 238, Questia, Web, 13 Jan. 2012.
- ^ Piel 180
- ^ Mendel Sachs, Relativity in Our Time: From Physics to Human Relations (London: Taylor & Francis, 1993) 155-156
- ^ Frank Wilczek, and Betsy Devine, Longing for the Harmonies: Themes and Variations from Modern Physics (New York: W. W. Norton, 1988) 240-246