One of the interesting things about a photon is that it does not have a separate antiparticle (such as an electron versus a positron). Basically, a photon is its own antiparticle.

Back in the 80's, Photon was a moderately popular diversion. Based off of (ripped off of) lazer tag, Photon was a more team-oriented sport. Two teams, a red and a green team would descend into an arena, and attempt to play a game that could only be described as capture the flag, without the flags.

Your objective was to shoot your enemies as many times as possible, which was a given in any lazer tag-esque game. For big points, you'd could also go and to shoot the enemy's power base thingie 3 times, located deep within their base. It was usually well protected, as well, so it wasn't as easy as just running in, pulling the trigger 3 times, and running back out.

The arena featured fog, strobe lights, many twists, turns, dead ends, sniper locations, and an 80's techno soundtrack. The music ranged from crappy to not half bad.

Shooting the enemy's base 3 times: 200 points
Shooting an enemy: 10 points
Shooting a teammate: Priceless, ahem, -30 points
Getting shot: -10 points

Every time you were shot, you had to go to a recharge station, to power your equipment back up. The equipment itself was large, and when I last played it (around age 8 or so), probably weighed at least a quarter of my total body weight. It consisted of a Helmet, Armor, a battery pack, and the gun itself. Each suit had LED lights according to the team's colors, so while the arena itself was fairly dark, you could still see your teammates and opponents well enough to shoot them.

Most Photon locations were equipped with a fairly large arcade, as well as a balcony where people, for the price of $.25, could shoot at the players. I don't recall if the players were deactivated if shot by the spectators, but I believe the spectators had a score readout near their gun. This made the game more interesting, as when I couldn't cough up the money to actually play the game, I could go shoot those who could. Of course, this paled in comparison to actually playing the game, as the fun of being shot at yourself, and trying for the opponents target was lost.

There was also a show on television based upon Photon, but I cannot recall of it's details, other than it had Photon gun wielding monsters of some type, and all the bad guys were on the green team. If I find anything further, I will update this node.

Despite my young age, I was placing within the top 4 or 5 each game. The team sizes were roughly 11-12 per team. I vaguely recall getting some compliments for doing so well while being so young.

Alas, my favorite Photon location, in Ocean City, Maryland, has seen some drastic changes. For 8 or 9 years after the photon franchise folded, it was home to a wax museum. This was really awkward, seeing wax dolls occupy the space where I used to shoot people. Seeing the remnants of the targets (they were an octagonal shape, if I recall correctly) was also difficult to adjust to. Finally, in the past four years, the Wax Museum closed, and a 90's clone of Lazer-Tag/Photon (Lazersport, I believe), opened up in the same building.

Interesting fact: According to the Theory of Relativity, photon particles have not aged a second since the Big Bang.

Source: The Elegant Universe, by Brian Greene

That blows my mind. I can (after a bit of contemplation) understand how they can travel through space--time has stopped from the perspective of the photon, but from any other perspective, it continues on regardless. But...well...what happens from the perspective of the photon?

No, scratch that I don't understand how the damned thing can travel through space. How can it travel through space without it's perspective travelling through the intervening time?

Also, when scientists manage to slow a photon down, does it begin to age?

A quantum of electromagnetic energy. The photon is the gauge boson responsible for carrying the electromagnetic force.

Making Photons

A photon is emitted when an atom dumps some of its energy. An excited atom, one in a high energy state, has an electron in a higher energy orbit than normal. Nothing likes having more energy than it needs, so the electron will drop to a lower energy state. The difference in energy between the two levels is emitted as electromagnetic radiation, a photon.

How the atom gets in the excited state in the first place can be through many mechanisms. A commonly exploited example is electronic excitation in the cathode ray tube. Electrons accelerated in the tube strike the phosphor on the front of the screen, exciting the electrons in the phosphor atoms to high energy states. They then decay back to lower energy states, emitting a photon on the way. We see these photons.

The same thing can happen in the nucleus of the atom. If one of the nucleons is in a high energy state it can drop to a more desirable low energy state by emitting the excess energy as a photon. The energies involved in a nucleus are orders of magnitude greater than those for the electrons around the nucleus, so the photon has much more energy and we label it gamma radiation.

Wave or Particle?

Well, both. The wave-particle duality, as it's known. All matter, not just light is both a wave and a particle. Most of the time light is a wave, an electromagnetic wave. It is only when it interacts with something that is assumes a particle-like nature. For example, when the light travels towards your eye it is as a wave. Then when the light falls on your retina it can only deposit energy in distinct lumps, or quanta - so it appears particulate. This is true for all matter, just it is more noticeable for light.

You can neatly explain the double-slit experiment 'paradox' by thinking of light in this way. The light remains as a wave during its passage through both the slits and a standing wave pattern is set up on the other side. This standing wave is the quantum mechanical wave function, which directly relates to the probability of observing a photon at a given position. It is only when the light interacts with the screen that it appears as a particle. A full quanta of energy must be deposited.

Photon Mass

Mass is a confusingly misused term. We all know Einstein's rather famous mass-energy equivalence, E=mc², and it's fairly obvious that photons have energy, so they must have mass, right? Well, no. The correct way to interpret E=mc² is to use it define the energy of an object when it is not moving, or rest energy, E0, in terms of a fixed quantity, it's mass. This is sometimes emphasised by calling this value the rest mass, but this isn't helpful, a particle only has one mass, it may have variable energy depending on how fast it is travelling, but mass is constant.

When a particle is moving the total energy is given correctly by, E² = m²c4 + p²c², where p is momentum. You can see if the object is at rest then p²c²=0, and the equation reduces back to E0=mc². In the case of a massless, but moving, particle then it reduces to E=pc. This means that a particle can have energy without mass. You can't stop photons so they always have momentum.

Theoretically, if photons did have mass we would see deviations from the Coulomb inverse square law. It is photons that transfer the electromagnetic force, they are gauge bosons. If they are massless then they can have infinite range and the 1/r² law holds true, if they have mass, they become limited in their range so the 1/r² rule will not hold anymore. Experimental tests for photon mass concentrate on finding such deviations. The upper limit for photon mass so far stands at 3x10-27 eV, which is about 10-46 kg.

Other stuff

The photon is a boson. This means that many photons can exist with the same energy states at the same time in the same place. Not all particles are like this, the Fermions exclude each other from being in the same state - electrons for example, otherwise atoms would collapse. Because the photons can have the same state it is possible to superimpose many of them with the same energy in the same place, this is basically what a laser is.

A photon is a spin-1 Boson, usually considered a pointlike particle. A photon moved at a fixed velocity called the 'speed of light.' This specific velocity is denoted by the letter 'c' which originally comes from the latin word, 'Celeritas,' which means ''swiftness.'' In SI units, the speed of light in a vacuum is 299,792,458 meters per second (1,079,252,848.8 km/h).

Why does the speed of light move at lightspeed?

The reason has to do with the permittivity and permeability of spacetime which can help define the speed of light. We may write this as

(√με)^(-1) = c

That is, 1 divided by the square root of the product of permittivity ε and permeability μ (which are inherent properties of the spacetime continuum) yields the speed of light again, given by c.

Moving on, a photon has momentum given by the equation

E = pc

The energy-momentum relationship is given as

E² = (Mc²)² + (pc)²

Since the rest mass of a photon is zero, this reduces to

E = pc

(We can ignore square root signs since there is no such thing as a negative momentum)

It is interesting to note, that at the limit of a particle at rest we have

E = Mc²

However, particles are never truly at rest in nature. The reason has to do with the Uncertainty Principle. If a particle did come to complete rest, this would violate the Uncertainty Principle which briefly states that the momentum and the position of a particle may never be completely known simultaneously - there is always an element of uncertainty in your system. If a particle was at complete rest, the particle would have a tremendously accurate position making it's momentum highly uncertain. Therefore, a particle always retains a little bit of momentum, so effectively E = Mc² is a good approximation and nothing more. I say nothing more, but in reality there is deep significance to relating energy to mass with the coefficient of the speed of light squared, but that is irrelevant for this post. Keep in mind, a photon is never at rest because or become at rest, as we have explained it has no rest mass.

The photon is the 'quantum of light' - that is, the sunshine you see on a fresh spring morning is made up of these tiny particles we call the photon. Photons are very frequent in nature - cosmologically-speaking, they make up the background radiation of spacetime. The background radiation is believed to be, the radiation remnant of the Big Bang. The Big Bang marks the beginning of space, matter and energy and even time itself according to theory.

A photon does not decay spontaneously in spacetime. A photon decays when interacting with another particle. One possible decay mode is that a pair of photons can decay into an electron and a positron. A positron is the antiparticle of the electron. An interesting phase transition occurs when an electron and a positron come together, they reduce back into gamma energy - those photons which first created it. The reason why this happens is because of charge conservation. An interesting thing to consider is that there are no anti-photons in nature - this is not reserved for photons alone, but is an interesting facet for a number of particles in this universe. The reason why the photon does not have an antiparticle is because the photon is it's own antiparticle.

Because a photon has momentum, it can also be related to the wave vector k

p = ħk

Where ħ is the angular momentum or also known as the quantum action which has dimensions of momentum multiplied by distance or energy multiplied by time.

As has been discussed, the photon does not have a rest mass. If it did have a mass, it would have a mass on the scale of around 10^(-51)kg which is tremendously small. The fact a photon has zero mass is in fact a wonderful prediction, both of experimental and mathematical gauge theory. Interestingly however, a photon does behave like it has a mass in a superconductor - the reason why is because the electromagnetic force inside a superconductor becomes short range and so behaves like it has a rest mass.

Particles, like an electron emit photons when being accelerated. This is predicted by the bizarre-looking Larmor formula

P = ⅔ (e²/c³)a²

Where P denotes power (units of energy over time), a is the acceleration and e is the electric charge. Power is the rate in which energy is transferable. This process for an electron can be thought of as a type of electromagnetic inertia.

It is also said that a photon does not experience time because time is stretched to infinity for the reference frame of the photon. Essentially, a photon which does not move in time would not move in space as well if one was to take relativity seriously... however there is a way out of this problem. The solution is by saying the photon does not have a frame of reference.

To finish up, the photon today remains as one of the most understood particles in the standard model. It has taken years of understanding no less, since the day Max Planck predicted their existence from his work on black body radiation whereas our modern understanding of the photon may be fully attributed to Albert Einstein.

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