<< Chapter < Page Chapter >> Page >
  • Define Yukawa particle.
  • State the Heisenberg uncertainty principle.
  • Describe pion.
  • Estimate the mass of a pion.
  • Explain meson.

Particle physics as we know it today began with the ideas of Hideki Yukawa in 1935. Physicists had long been concerned with how forces are transmitted, finding the concept of fields, such as electric and magnetic fields to be very useful. A field surrounds an object and carries the force exerted by the object through space. Yukawa was interested in the strong nuclear force in particular and found an ingenious way to explain its short range. His idea is a blend of particles, forces, relativity, and quantum mechanics that is applicable to all forces. Yukawa proposed that force is transmitted by the exchange of particles (called carrier particles). The field consists of these carrier particles.

The image shows the creation a pion from a proton and its exchange to a neutron. After the exchange, the proton has become a neutron and the neutron has become a proton.
The strong nuclear force is transmitted between a proton and neutron by the creation and exchange of a pion. The pion is created through a temporary violation of conservation of mass-energy and travels from the proton to the neutron and is recaptured. It is not directly observable and is called a virtual particle. Note that the proton and neutron change identity in the process. The range of the force is limited by the fact that the pion can only exist for the short time allowed by the Heisenberg uncertainty principle. Yukawa used the finite range of the strong nuclear force to estimate the mass of the pion; the shorter the range, the larger the mass of the carrier particle.

Specifically for the strong nuclear force, Yukawa proposed that a previously unknown particle, now called a pion    , is exchanged between nucleons, transmitting the force between them. [link] illustrates how a pion would carry a force between a proton and a neutron. The pion has mass and can only be created by violating the conservation of mass-energy. This is allowed by the Heisenberg uncertainty principle if it occurs for a sufficiently short period of time. As discussed in Probability: The Heisenberg Uncertainty Principle the Heisenberg uncertainty principle relates the uncertainties Δ E size 12{ΔE} {} in energy and Δ t size 12{Δt} {} in time by

Δ E Δ t h 4 π size 12{ΔEΔt>= { {h} over {4π} } } {} ,

where h size 12{h} {} is Planck’s constant. Therefore, conservation of mass-energy can be violated by an amount Δ E size 12{ΔE} {} for a time Δ t h 4πΔ E size 12{Δt approx { {h} over {4πΔE} } } {} in which time no process can detect the violation. This allows the temporary creation of a particle of mass m size 12{m} {} , where Δ E = mc 2 size 12{ΔE= ital "mc" rSup { size 8{2} } } {} . The larger the mass and the greater the Δ E size 12{ΔE} {} , the shorter is the time it can exist. This means the range of the force is limited, because the particle can only travel a limited distance in a finite amount of time. In fact, the maximum distance is d c Δ t size 12{d approx cΔt} {} , where c is the speed of light. The pion must then be captured and, thus, cannot be directly observed because that would amount to a permanent violation of mass-energy conservation. Such particles (like the pion above) are called virtual particles    , because they cannot be directly observed but their effects can be directly observed. Realizing all this, Yukawa used the information on the range of the strong nuclear force to estimate the mass of the pion, the particle that carries it. The steps of his reasoning are approximately retraced in the following worked example:

Practice Key Terms 3

Get Jobilize Job Search Mobile App in your pocket Now!

Get it on Google Play Download on the App Store Now




Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'College physics' conversation and receive update notifications?

Ask