Charged particles will be forced to accelerate and improve energy in the electric field, which is the principle adopted by particle accelerators so far, and neutral particles cannot be accelerated under this principle. Therefore, a particle accelerator should be defined as a device that uses electromagnetic fields to accelerate charged particles. Particle accelerators can accelerate charged particles such as electrons, protons and ions, making the speed of particles reach several thousand kilometers per second, tens of thousands of kilometers per second, and even close to the speed of light (the propagation speed of light in vacuum is 300,000 kilometers per second). According to the principle of relativity, the accelerator can increase the speed of charged particles to infinitely close to the speed of light, but it can't reach the speed of light anyway.

　　Modern accelerators should be called "energizers". For example, an electron accelerator injects 20GeV(1GeV=109 eV, that is, 1 billion electron volts), and the corresponding electron speed is 0.999999999979 times the speed of light. After the electron is accelerated by the accelerator, the energy can reach 100GeV, and the electron speed can reach 0.999999987 times the speed of light. This shows that the speed of electrons in this accelerator has hardly increased, but the energy has increased by four times.

　　In fact, the accelerator is not far from people's lives. The picture tube used in TV and computer monitors, which has been popularized in modern life, is a small electron accelerator.