π Introduction to Redshift
Redshift is a phenomenon where electromagnetic radiation (like light) from an object undergoes an increase in wavelength. This essentially means the light shifts towards the red end of the spectrum. There are two main types: Redshift (caused by the Doppler effect) and Gravitational Redshift.
π Redshift (Doppler Effect)
This type of redshift occurs when a light source is moving away from an observer. Think of it like the sound of a siren changing as an ambulance passes you. The faster the object moves away, the greater the redshift.
- π‘ Cause: The relative motion between the source and the observer.
- π Formula: $z = \frac{v}{c}$ (for small velocities), where $z$ is the redshift, $v$ is the velocity of the source, and $c$ is the speed of light. For larger velocities, the relativistic Doppler effect must be considered: $1 + z = \sqrt{\frac{1 + \frac{v}{c}}{1 - \frac{v}{c}}}$
- π Example: Observing distant galaxies. The redshift of their light indicates they are moving away from us, supporting the expansion of the universe.
π Gravitational Redshift
This type of redshift occurs when light escapes from a gravitational field. As light climbs out of a gravitational well, it loses energy, causing its wavelength to increase (shift towards the red end of the spectrum).
- π Cause: The effect of gravity on light as it escapes a massive object.
- π Formula: $z = \frac{GM}{Rc^2}$, where $G$ is the gravitational constant, $M$ is the mass of the object, $R$ is the radius of the object, and $c$ is the speed of light.
- β« Example: Light escaping from a black hole or a neutron star will experience a significant gravitational redshift. This redshift can be used to study these extreme objects.
π Key Differences Summarized
Here's a table summarizing the key differences:
| Feature |
Redshift (Doppler) |
Gravitational Redshift |
| Cause |
Relative motion |
Gravity |
| Effect |
Wavelength increases due to source moving away |
Wavelength increases due to escaping gravity |
| Dependence |
Velocity |
Mass and Radius of the object |
| Typical Scenarios |
Moving galaxies, objects in space |
Light escaping massive objects |
π§ͺ Practice Quiz
- β Light from a distant galaxy is observed to be redshifted. What does this indicate about the galaxy's motion relative to Earth?
- β A photon escapes from the surface of a neutron star. Will it experience redshift or blueshift? Why?
- β What is the main difference in the cause of redshift and gravitational redshift?
- β How does the mass of an object affect the gravitational redshift of light escaping from it?
- β If a star is moving towards us, would we observe redshift or blueshift? What is this effect called?