Hard to answer this very well; it lies at the very heart of what interacting fields do. Collisions of ripples in one type of field can generate ripples in another type of field with which they interact.
These are the simplest calculations in quantum field theory, but they do require understanding what quanta are and how they can behave. Yes, we call this muon-antimuon scattering. Very well done! If energy is conserved after a collision, then the particles have not annihilated; they have changed. Holding still? In the same place? Not spinning? Is there such a thing? This is semantics. For instance: suppose an electron and positron annihilate into three photons, or four, or five.
This can happen. Who changed into who? Practically, stationary tends to mean: moving much slower than I can measure. How slow that is depends on context.
Question 2 In the case of proton to proton collisions above an energy level of about 6 Gev, could a possible explanation of the occasionally observed outcome:.
As far as I can tell, the mass energy equation would remain valid. Question 3 Finally, could this possibly be the remarkably puny type of Dark Matter particle discovered some years ago by the DAMA group?
I have a doubt. When light waves opposite in phase meet there is destruction of waves. Is this similar to a photon and an antiphoton meeting each other? What happens to the energy when they meet? No, this is simply interference. A photon can interfere with itself; two photons can interfere with each other. The cancellation is not indeed cannot be, thanks to energy conservation perfect; the energy is lost in some places but gained elsewhere.
There is no such thing as anti-photons; an anti-photon is just a photon. All types of particles have anti-particles, but in some cases the anti-particle is the same type of particle as the particle you started with. Thanks for your answer. Though I am still not very clear. When two waves in opposite phase interact the amplitude of the wave becomes zero which according to wave theory means zero energy. Is it not? While according to the theory of conservation of mass and energy the net mass-energy has to remain the same what actually happens in this case?
The amplitude of the wave was zero to start with and it remains so. If you create two waves that for a brief moment are EXACTLY in opposite phase but are moving in opposite directions, then the amplitude of the wave will not be zero most of the time — it will be zero only for a moment, and then it will be large, and then zero again for a moment, and then large — so there is definitely something there, and lots of energy.
Another excellent article. I know photons are their own antiparticles, in a sense at least. So is it possible for photons to not interfere but to scatter off each other?
Two photons meeting, and producing two photons moving in a different direction? And if so, would their energies have to be equal? Yes, two photons can scatter — though because they do not interact directly it can only happen through an indirect quantum mechanical effect.
If they meet with equal energy from your point of view , then — as is true for any two particles of equal mass and equal energy that collide — they will exit the collision from your point of view with equal energy.
Is there any way that the direct result of anihilation can be a photon of visible light, especially in electron-positron anihilation? Or can such a photon be a result of immediate decays of paticles that are the result of anihilation? If an electron and positron annihilate, the total amount of energy available from the point of view of someone who is at rest with respect to the electron-positron pair is about one billion electron volts.
The energy in a photon of visible light is about electron volts. So if the electron and positron annihilate to make two and only two photons, the two photons each have hundreds of millions of times too much energy for your eye to detect them. However, it is possible for the annihilation to produce 3 photons, or 4, or 10, or , etc. Most of the energy will still be in invisible photons far too energetic to be visible.
The likelihood that the majority of the energy of the annihilation comes out in a few hundred million visible-light photons is incredibly tiny and has probably never happened in the history of the universe, which has had a LOT of electron-positron annihilations!!!
Two more questions: I understand that only one photon cannot be created in annihilation because it momentum has to be conserved yet the photon has to move. But what about other bosons that are their own antiparticles? Is it possible that more than one pair annihilates? If we we arrange it so that 4 electrons and 4 positrons meet at the same point in time, do we treat this like one event where the total energy of resulting particles is equal to that of those 4 electrons and 4 positrons or is such a thing impossible according to our theories and in the experiments of this kind no particles were observed with energies greater than in a single annihilation of electron and positron?
The only constraints on the energies of the photons are those of energy and momentum conservation. A single photon is forbidden by those considerations. Two photons are allowed, but they must have equal energy and and opposite momenta in the rest frame of the annihilation. Once the number of photons is greater than 2, then the number of constraints is too small, and so the energies can be anything; two could be very high energy and one could be very low.
The sum of the energies still has to be the total energy that the annihilation created. If a particle has a mass M, then the energy required to make it is M c-squared. And it may be impossible, because conservation laws of other sorts also have to be obeyed. If you have two electrons and two positrons, the annihilation process occurs pairwise with extremely high probability.
First, the pairwise annihilation is intrinsically more likely; fewer electromagnetic interactions are required. Second, it is technically difficult to get four particles into the same location think about how much harder it is to arrange a meeting with three extremely busy friends rather than just with one. The reverse process — where two photons annihilate to make two electron-positron pairs rather than one, does not suffer from the second problem, though it still suffers from the first.
The probability of having a second pair can be as small as times smaller than just having one pair though the probability grows slowly as you increase the energy of the annihilating photons. This wild conjecture is premised upon my own simple geometric imagery that likely only flies in the bastion of my ignorance but… The transmutation of particles and energy described above suggests that all of the particles and energy itself are just different configurations of the same elementary particle, presumably gravitons.
The physical particles seen in colliders are the most stable graviton configurations, think Bucky Balls of different sizes , that garner energy and momentum from attachment of gravitons in the interstitial spaces on the surface of the particle at specific locations to generate vector. Strong, weak and electromagnetic forces may just be the manifestation of graviton interaction when physical particles are excited to specific resonant field orbits that form characteristic force carriers when disrupted in that particular orbital field state.
Dark Matter and Dark Energy could be most easily explained as accelerated gravitons emanating from disassociated matter in Super Massive Black Holes that must decay before reaching a lower energy that allows attachment, absorption cross section , to baryons and photons at some distance from their source dictated by the emission acceleration that is proportional to the size of the SMBH and the galaxy itself, yields consistent rotational speeds in galaxies of all sizes and disassociated DM fields, Bullet Cluster, Abel could still have an unseen SMBH source.
For some reason, once these accelerated gravitons decay further, beyond the DM halo and below the photon absorption cross section , they manage to attach to baryons on the opposite side to accelerate that baryon in the direction of initial momentum and manifest as DE, function of wavelength or gravitons could be initially clustered upon emission from the SMBH to affect absorption characteristics upon further disassociation?
Too simple? I could speculate about anti-particles, anihilation, BECs, dipole effect, electromagnetism, superconductivity, etc. Which one of the four forces is played in anihilation? This is rather like asking which of the fundamental forces are involved in light waves interfering.
Annihilation is not related to any of the four fundamental forces, it is something that waves in a field do. If you have a friend and a long rope you can send an up wave and down wave from either end of the rope and watch them cancel in the middle, annihilating their energy being converted to heat.
In a similar way a particle and anti-particle will annihilate when they meet, in the same way they can reflect and interfere. Antiparticle and particle annihilation is far from any phenomena that can be explained using some intuitive pictures.
I like intuitive pictures, double slits experiment is a good example. The fact that a single photo or a single particle can produce an interference pattern has become one of our intuitive notions about nature. Two photons or multiple photons can also produce the interference pattern, but they have to be twins or born in coherence.
Antiparticle and particle annihilation does look like two local vibrations cancelling each other, and it may well be the case, but how can we understand this? A pair of antiparticle and particle, say an antineutron and a neutron, is created in an accelerator.
It may not be a far fetched speculation to say that these two local vibrations are opposite in phase, off by degrees in time. But we are not talking about this pair of particles that may annihilate each other, annihilation occurs when the antineutron hit anywhere in the accelerator, the walls, molecules in the accelerator, anything that contains neutrons, that is everything. The antineutron will seek out a neutron from the matter, and annihilate it. The particles do not have a grudge against each other.
They are two independent vibrations travelling in space-time. There is no obvious reason we would expect that they just happened to be vibrating at opposite phase. This is confusing particle-antiparticle annihilation with interference, and represents a serious conceptual error. You can see the error as follows; the cancellation you just referred to will not occur for more than an instant. Instead the two waves will pass right through each other. The equations for waves on a string are nearly linear equations, even accounting for fraction, and they nearly satisfy superposition; there is nowhere near enough interaction to cause the two waves to annihilate into friction-related heat.
All four forces can participate in annihilation. There need not be a pair of particles; I can create a neutron-antineutron pair, then let the antineutron decay to an antiproton and positron. The two antiparticles can then annihilate independently.
I think there is a much simple model to explain particle and antiparticle. It is like two phase electric generator. We place two coils opposite to each other, connect one end of each coil to ground, and place a magnet on the shaft. If we crank the shaft in a full circle, one of the coils, coil A will generate a full cycle of electron oscillation.
The other coil, coil B will also generate a full cycle of electron oscillation. As we have grounded one end of each coils. We can say that coil A is 90 degree relative to the ground and coil B is degree relative to the ground.
In this model, the frequency of electron oscillations in the coils are determined by how fast we crank the generator, but it does not need to be. One can turn the oscillation in coil A into anything one wants using some electric circuit, a square pulse for example. One can do the same thing for the oscillation in coil B, or even split oscillation into smaller oscillations in time. However, no matter what you do to the initial oscillations, their relative phase will stay the same.
We can think a high energy photon as local vibration travelling in space-time. If this photon is the one to create a pair of particle and antiparticle at a particular event, the last cycle of its vibration before its demise is used to crank the generator we have described.
Just like the pulse A and pulse B generated in our model, the pair of particles generated in this way will always have a fixed phase relative to the ground, in this case, the gravity field. One can expand the simple model, there does not have to be just two coils, we may put three or four coils, and coils themselves may have complicated structures. It looks like that particles and antiparticles are annihilated because they are vibrations in opposite phase.
Previously I though their relative phase is kept by the vibrations of gravity field. This may not be the case. Like in a two phased electricity generator, we have to ground one end of each coils, then we can say the electrons in one coil are vibrations 90 degrees ahead relative to the ground and vise versa about the other coil.
The vibration phase is referenced to the time in the space-time event point. Yes, both of them are time travelers, both can travel to future and past, just not far. An interesting question is how do they keep their distinctive birth phase while travelling in space-time? I think the answer is simple yet amazing. It is simple because they have mass, they are bound to gravity. It is amazing because I never thought about the space-time in this way, they are not even visible, yet the binding of the particles to a space-time event point is so strong that even the particles are vibrating at billions of times per second, they never miss a step.
Image that a particle was born some billions of years ago. On its path to our present time, it might have been fused together with other particle in a star, and later was blown off when the star became a supernova and exploded. After this enduring journey, it still bind to its space-time event point, and by chance, it ended up on the walls of a particle accelerator. By another chance, an antiparticle created in the accelerator happened to hit the particle on the wall, their paths merges together become one event point.
Since both particles are bound to their event point so tight that they know they are vibrations on opposite phase, they annihilate each other. Maybe this graviton-graviton annihilation process is why we see the skewed galactic gravitational effect we call Dark Matter? The above annihilation equation also suggests a massive graviton, how big is it? Gluons are also their own anti-particles and also self-annihilate, for instance inside protons and in the proton-proton collisions of the LHC.
The above annihilation equation only suggests gravitons have energy. Two massive particles Electron-positron can annihilate to two massless ones, so long as the energy balances. A photon is a photon.
Some day there may be a photon-photon collider that can make Higgs particles in its collisions. So does this make photon-photon collisions different from particle antiparticle collisions, or is it just a case of symmetry? And if I have two narrow beams of light say commercial laser pointers at right angles to one another would their be enough collisions for me to detect their beams interacting?
Antimatter curves space exactly the same way matter does. This is obvious from the fact that some particles are their own anti-particles; it would be impossible for photons to curve space at all if particles and anti-particles curved space differently. I am not sure if this way of thinking is compatible with the second law of thermodynamics but I has a nice symmetry to it. Many members of an entire class of particles, the mesons, are massive and their own antiparticle, being composed of a quark and antiquark.
On a more fundamental level the Z boson is its own antiparticle. First let me correct a misconception. The Z particle has a mass and is its own anti-particle; the same is true of the Higgs particle, and of a number of hadrons.
It may be true of dark matter. What Feynman said and he is the source for this common notion is that the math describing anti-particles moving forward in time is the same as the math describing particles moving backward in time.
This is a statement about the math. Anything you actually measure involves measuring how a starting point evolves in time to an ending point. It is not as though experiments involving anti-particles would go backwards, with a result known in advance that is forgotten by the time the experiment is over. Furthermore, and very important, there is no unique definition as to which is particle and which is anti-particle.
This is where the true symmetry lies. On the subject of Feynman and past directed time travel; I believe that in the s he and Wheeler did much to popularise the concept of advanced and retarded fields, in which radiation propagated in both directions through time, creating a web of cause and effect in both directions.
Anti-particles are not theoretical speculation, they are established fact. And of course we use them in particle physics all the time; for instance, the Tevatron accelerator, which operated for several decades, generated collisions of protons with anti-protons. Anti-matter particles do not have anti-energy, or negative energy; they have positive energy, just like particles do.
This is well-verified in particle physics experiments. And so they should generate the same gravitational effects as particles. An earth made from anti-matter atoms would generate the same gravitational effects as our the earth does.
The point is that nature does not create antimatter. Not in our galaxy at least. To think that matter somehow ends up in one place and antimatter ends up somewhere else violates the second law of thermodynamics. These experiments are just another solid proof that mass can convert to energy and vise versa but mass is not matter. A spinning top and an oscillating string when their motions are extremely fast may appear like solid objects and are much heavier than they are at rest.
Mass is a notion of motion not a notion of matter. We would also observe a red-shift of light coming from other icebergs even if all the floating icebergs were in some equilibrium distances relative to each other. This is simply an optical effect as light travels slower in water than in ice. There may not be such a thing as vacuum but only a limitation of our ability to detect what is in the space. If we dismiss the possibility that matter is created this way by nature, the only conclusion left is that matter is there all along in the space we call vacuum.
In the photon scattering process, the photon energizes the matter into different modes of motions of the same fundamental particle. Electrons and quarks are from the same particles that are motionless when they are in bound state in space. Mass, charge and spin are not intrinsic properties of elemental particles but different modes of motions of a more fundamental form of matter. We all expect to find a more fundamental form of matter but why do we expect to see that fundamental form of matter behaves the same as the forms of matter we know?
The ordered phase is transforming into galaxies. All galaxies are in a macroscopic single state, much like super fluid He3 with no temperature. Outside galaxies, it is the disordered phase with a bone cracking cold temperature of 2. With this thought, that there is a perfect plan wave running in the background of every galaxies, some deep puzzles in physics can be explained. Like the entanglement experiment of two particles, where the oscillatory state of each particle in no way can be determined, but when the relative phase of their oscillatory states is checked entangled , and afterwards they have been separated thousands of miles apart, at that time, if the oscillatory state of one particle is checked, the state of the other particle can be determined with absolute certainty.
Recently, I went ocean fishing, where I saw speed boats running on ocean surface wave in a harbor. At certain speed, the up and down motion of speed boat became harmonic with the ocean wave and it started skipping the wave with a rhythm of its own.
A particle as big as a speed boat can behave like a wave just by riding one, and it can do so at any speed, but for a particle to behave like a plan wave, it has to move at a speed that is an integer times the speed of the plan wave that it is riding on. It occurred to me that I had seen electrons doing just like that. LEED is a simple device for checking surface crystal structure of a material. It has an electron gun mounted at the center of a half spherical screen and a voltage control for changing electron speed.
The electron gun shoots electrons perpendicularly at a sample mounted in front of the gun, and the back scattered electrons form a diffract pattern on the screen. I was puzzled then by the fact that a sharp diffraction pattern is not observed with a continuous voltage change, but only at a set of discrete voltages within a range of volts. The voltage of the first diffraction pattern observed is around 10 Volt.
For electrons to act like a plane wave again at a higher speed, they have to double, triple or quadruple this minimum speed. As the voltage is proportional to the square of velocity, the set of voltages that diffraction pattern can be observed is, 10, 40, 90, in the V range. I remember that the voltage settings I used to use were around this set of values, but the settings varied from time to time.
When I search online, there are also different set of voltages used. Actually, the variation of this set voltages from one LEED experiment to another is a better evidence that the electrons are riding on the galaxy wave. The 1. Three factors may affect the speed of electrons in the galaxy frame. Sun orbital velocity on the Milky Way plane is 0.
This variation should be observable from one experiment lab to another, as the orientation of the electron gun is likely different from one lab to another. Even in the same lab, as earth rotates, the orientation of the electron gun with respect to the sun orbital direction changes during a day. If one takes a set of measurements of voltages that diffraction patterns are observed in the morning, and takes another set of measurements in the afternoon, and if a variation on the two sets of measurements is observed, one can be sure that one is seeing the galaxy wave in motion.
Do you think that this is genuinely an explanation of the existence of antiparticles? If so, what sort of explanation is this? How would you characterize how it works, if it does indeed explain why there are antiparticles. But this knowledge does raise another question—why is there such a predominance of matter and so little antimatter? Possible explanations emerge later in this and the next module. Particles can also be revealingly grouped according to what forces they feel between them.
All particles even those that are massless are affected by gravity, since gravity affects the space and time in which particles exist. All charged particles are affected by the electromagnetic force, as are neutral particles that have an internal distribution of charge such as the neutron with its magnetic moment. Special names are given to particles that feel the strong and weak nuclear forces. Hadrons are particles that feel the strong nuclear force, whereas leptons are particles that do not.
The proton, neutron, and the pions are examples of hadrons. The electron, positron, muons, and neutrinos are examples of leptons, the name meaning low mass. Leptons feel the weak nuclear force. In fact, all particles feel the weak nuclear force. This means that hadrons are distinguished by being able to feel both the strong and weak nuclear forces.
Table 1 lists the characteristics of some of the most important subatomic particles, including the directly observed carrier particles for the electromagnetic and weak nuclear forces, all leptons, and some hadrons. Several hints related to an underlying substructure emerge from an examination of these particle characteristics. Note that the carrier particles are called gauge bosons. Fermions obey the Pauli exclusion principle whereas bosons do not. All the known and conjectured carrier particles are bosons.
Figure 2. When a particle encounters its antiparticle, they annihilate, often producing pure energy in the form of photons. In this case, an electron and a positron convert all their mass into two identical energy rays, which move away in opposite directions to keep total momentum zero as it was before. Similar annihilations occur for other combinations of a particle with its antiparticle, sometimes producing more particles while obeying all conservation laws.
All known leptons are listed in the table given above. There are only six leptons and their antiparticles , and they seem to be fundamental in that they have no apparent underlying structure. Leptons have no discernible size other than their wavelength, so that we know they are pointlike down to about 10 m. The leptons fall into three families, implying three conservation laws for three quantum numbers. Once the muon was discovered in cosmic rays, its decay mode was found to be.
One principal decay mode is. Now, note that the hadrons in the table given above are divided into two subgroups, called mesons originally for medium mass and baryons the name originally meaning large mass. The division between mesons and baryons is actually based on their observed decay modes and is not strictly associated with their masses. Mesons are hadrons that can decay to leptons and leave no hadrons, which implies that mesons are not conserved in number.
Baryons are hadrons that always decay to another baryon. A new physical quantity called baryon number B seems to always be conserved in nature and is listed for the various particles in the table given above. The conservation of total baryon number is a more general rule than first noted in nuclear physics, where it was observed that the total number of nucleons was always conserved in nuclear reactions and decays. That rule in nuclear physics is just one consequence of the conservation of the total baryon number.
The forces that act between particles regulate how they interact with other particles. For example, pions feel the strong force and do not penetrate as far in matter as do muons, which do not feel the strong force.
This was the way those who discovered the muon knew it could not be the particle that carries the strong force—its penetration or range was too great for it to be feeling the strong force. Similarly, reactions that create other particles, like cosmic rays interacting with nuclei in the atmosphere, have greater probability if they are caused by the strong force than if they are caused by the weak force.
Such knowledge has been useful to physicists while analyzing the particles produced by various accelerators. The forces experienced by particles also govern how particles interact with themselves if they are unstable and decay.
For example, the stronger the force, the faster they decay and the shorter is their lifetime. The neutron is a good example of decay via the weak force. None would be created if the strong force was responsible, just as no leptons are created in the decay of 8 Be.
The systematics of particle lifetimes is a little simpler than nuclear lifetimes when hundreds of particles are examined not just the ones in the table given above. Turning this around, if we measure the lifetime of a particle, we can tell if it decays via the weak or strong force. No known physical processes violate charge conservation. In the next section, we describe three less-familiar conservation laws: baryon number, lepton number, and strangeness. These are by no means the only conservation laws in particle physics.
No conservation law considered thus far prevents a neutron from decaying via a reaction such as. This process conserves charge, energy, and momentum. However, it does not occur because it violates the law of baryon number conservation. This law requires that the total baryon number of a reaction is the same before and after the reaction occurs. To determine the total baryon number, every elementary particle is assigned a baryon number B.
Thus, the decay does not occur because the total baryon number changes from 1 to 0. However, the proton-antiproton collision process. Based on the law of conservation of baryon number, which of the following reactions can occur? Determine the total baryon number for the reactants and products, and require that this value does not change in the reaction. Since the net baryon numbers of the reactants and products are equal, this reaction is allowed on the basis of the baryon number conservation law.
Since the net baryon numbers of the reactants and proposed products are not equal, this reaction cannot occur. Baryon number is conserved in the first reaction, but not in the second. Baryon number conservation constrains what reactions can and cannot occur in nature.
Lepton number conservation states that the sum of lepton numbers before and after the interaction must be the same. In any interaction, each of these quantities must be conserved separately. Lepton number conservation guarantees that the number of electrons and positrons in the universe stays relatively constant.
Note: The total lepton number is, as far as we know, conserved in nature. To illustrate the lepton number conservation law, consider the following known two-step decay process:.
Therefore, muon-lepton number is conserved. Thus, electron-lepton and tau-lepton numbers are also conserved.
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