Recent Trends in Physics

1. Introduction

This chapter surveys a few modern and applied areas of physics that connect classroom concepts to present-day science and to Nepal's own geology. It has two parts: seismology (the physics of earthquakes and the elastic waves they send through the Earth), and a set of frontier ideas, gravitational waves, nanotechnology, and the Higgs boson. The treatment is largely descriptive, but the wave ideas rest directly on the mechanical-wave physics developed earlier (longitudinal versus transverse waves, wave speed, amplitude, and the medium dependence of propagation).

2. Seismology

Seismology is the scientific study of earthquakes and of the propagation of elastic (seismic) waves through the Earth.

2.1 Origin of earthquakes

• The Earth's outer shell (the crust plus the upper mantle) is broken into large tectonic plates that move slowly and continuously.
• Nepal lies on the boundary where the Indian Plate is pushing northward into the Eurasian Plate at roughly $5\ \text{cm}$ per year.
• Because the plates lock at their boundary, elastic strain energy builds up over years. When the accumulated stress exceeds the strength of the rock, the rock suddenly slips and releases this stored energy as seismic waves. This sudden slip is an earthquake.
• The point underground where the rupture begins is the focus (or hypocentre). The point on the Earth's surface directly above the focus is the epicentre. A shallow focus generally produces more surface damage than a deep one.

2.2 Types of seismic waves

Seismic waves are divided into body waves, which travel through the Earth's interior, and surface waves, which travel along the surface.

a) Body waves

P-waves (Primary or longitudinal waves):

• Particle motion is parallel to the direction of propagation, so they travel as a series of compressions and rarefactions (just like sound in air).
• They are the fastest seismic waves, about $6\ \text{km s}^{-1}$ in the crust and up to $13\ \text{km s}^{-1}$ in the mantle, so they are the first to arrive at a seismograph.
• They can travel through solids, liquids, and gases.
• They cause a forward-and-backward (push-pull) ground motion.

S-waves (Secondary or transverse waves):

• Particle motion is perpendicular to the direction of propagation (a shearing motion).
• They are slower than P-waves, about $3.5\ \text{km s}^{-1}$ in the crust, and arrive after the P-waves.
• They travel only through solids, not through liquids, because a fluid cannot sustain a shear stress. The fact that S-waves fail to pass through the Earth's outer core is the key evidence that the outer core is liquid.
• They cause a side-to-side or up-and-down shearing motion of the ground.

The time gap between the arrival of the P-wave and the S-wave at a station increases with distance from the epicentre, which lets seismologists locate the source.