物理学第5回

Summary

This lecture delves into the concept of cosmic expansion, focusing on essential understandings such as light spectrum, Doppler effect, and Hubble's Law. It explains how light's wavelength varies, resulting in a spectrum when passed through a prism, and introduces the phenomenon of absorption lines in stellar spectra. The Doppler effect is discussed in relation to light, illustrating how light wavelengths shift towards blue or red when a source moves towards or away from an observer. Hubble's Law, correlating the velocity at which galaxies recede with their distance from us, is introduced, underlining the ongoing expansion of the universe.

Takeaways

• 🌌 The lecture discusses the expansion of the universe over two sessions, starting with fundamental concepts necessary for understanding cosmic expansion.
• 📊 The script explains the importance of light spectrum, Doppler effect, and Hubble's Law in understanding the universe's expansion.
• 🔍 Light from the sun, when passed through a prism, splits into various colors, which are differentiated by their wavelengths, measured in nanometers (NM).
• 🌈 The script introduces the concept of a spectrum, which is the separation of light into its constituent wavelengths, and mentions that light is a type of electromagnetic wave.
• 🌠 The presence of emission and absorption lines in the spectrum of light from celestial bodies is highlighted, which are indicative of specific wavelengths being more intense or less intense than others.
• 🚨 The Doppler effect is described, noting that the wavelength of light changes depending on whether the light source is moving towards or away from the observer.
• 🔵 The script uses the example of a siren to explain the Doppler effect in sound, which shifts in pitch as the source moves closer or further away.
• 🌠 The Doppler shift is observed in light from celestial bodies, with blue shift indicating movement towards the observer and red shift indicating movement away.
• 📚 The script provides a formula for the Doppler effect in light, relating the observed wavelength to the speed of the light source and the speed of light.
• 🌌 Hubble's Law is introduced, stating that the speed at which galaxies are moving away from us is proportional to their distance from us, with the proportionality constant known as the Hubble constant.
• 📉 The lecture concludes with a historical note on the naming of Hubble's Law, acknowledging the contributions of both Edwin Hubble and Georges Lemaître, and discusses the implications of the law for the understanding of the universe's expansion.

Q & A

What is the primary focus of the lecture on the expansion of the universe?

The primary focus of the lecture is on understanding the expansion of the universe, covering topics such as the spectrum of light, the Doppler effect, and Hubble's law.

How does a prism affect light from the sun, and what does this demonstrate?

A prism separates sunlight into various colors, demonstrating that these colors correspond to different wavelengths. This separation is known as a spectrum.

What is the significance of wavelength in understanding light?

Wavelength is crucial in understanding light as it determines the color observed. Light is a type of electromagnetic wave, and different wavelengths correspond to different colors, with red light having longer wavelengths than blue light.

What are absorption lines, and how do they appear in the spectrum of light from celestial bodies?

Absorption lines are dark lines that appear in the spectrum when certain wavelengths of light are absorbed more than others. They indicate the presence of specific elements in the atmosphere of celestial bodies.

Can you explain the Doppler effect as it relates to light from celestial bodies?

The Doppler effect refers to the change in wavelength of light depending on whether the source is moving towards or away from the observer. If a celestial body moves towards Earth, its light shifts to shorter wavelengths (blue shift); if it moves away, the light shifts to longer wavelengths (red shift).

What is the difference between blue shift and red shift in the context of the Doppler effect?

Blue shift occurs when a celestial body moves towards the observer, resulting in shorter wavelengths. Red shift occurs when the celestial body moves away, resulting in longer wavelengths.

How does Hubble's Law relate to the expansion of the universe?

Hubble's Law states that the speed at which a galaxy moves away from us is proportional to its distance. This suggests that the universe is expanding, with galaxies moving away from each other.

What is the historical significance of Hubble's and Lemaître's contributions to the understanding of the expanding universe?

Edwin Hubble is credited with discovering the relationship between galaxy distance and velocity in 1929, now known as Hubble's Law. However, it was later recognized that Georges Lemaître had published similar findings in 1927, leading to the renaming of the law to the Hubble-Lemaître Law in 2018.

What method is used to measure the speed at which galaxies move away from Earth?

The speed at which galaxies move away from Earth is measured using the Doppler effect, where the shift in the wavelengths of light from the galaxies is observed and calculated.

What does the Hubble-Lemaître Law imply about the relationship between galaxy distance and velocity?

The Hubble-Lemaître Law implies a direct proportionality between the distance of a galaxy from Earth and its velocity of recession, meaning that more distant galaxies are moving away faster, supporting the concept of an expanding universe.