Short Answer
Gravitational redshift is a phenomenon in which light coming from a strong gravitational field appears to shift toward the red side of the spectrum. This happens because gravity affects the energy and frequency of light. When light moves away from a massive object, its frequency decreases and its wavelength increases, making it look redder.
This effect is predicted by Einstein’s general theory of relativity. Gravitational redshift has been observed in light from stars, galaxies, and even in experiments on Earth. It shows that gravity not only affects matter but also influences light.
Detailed Explanation :
Gravitational redshift
Gravitational redshift is an important concept in Einstein’s general theory of relativity. It refers to the change in the wavelength and frequency of light as it moves through a gravitational field. According to relativity, gravity can stretch space and time. Because of this, light traveling away from a massive object loses energy. When light loses energy, its wavelength becomes longer, and the color shifts toward red. This phenomenon is called gravitational redshift.
The word “redshift” means that the light has moved toward the red end of the spectrum, where wavelengths are longer. It is called “gravitational” because this shift happens due to gravity, not due to motion or expansion of the universe.
How gravitational redshift works
To understand gravitational redshift, we first need to understand how light behaves near gravity. Even though light has no mass, it still interacts with gravity because gravity affects space-time. When light travels away from a massive object, such as a star, black hole, or planet, it must climb out of the gravitational “well.” In doing so, it loses energy.
Energy of light is given by:
Where:
- = energy
- = Planck’s constant
- = frequency
If light loses energy, its frequency decreases. Since wavelength and frequency are related by:
A decrease in frequency means an increase in wavelength. Red color has the longest wavelength in the visible spectrum, so the shift is called “redshift.”
Gravitational redshift and general relativity
Einstein’s general relativity explains gravitational redshift as a result of time dilation. In a strong gravitational field, time runs slower. Clocks near massive objects tick more slowly than clocks far away. Because the light’s frequency depends on the ticking of these “clocks,” the frequency of light emitted from a strong gravitational field is lower when observed from a weaker gravitational field.
This effect is important evidence that gravity affects time, space, and energy.
Mathematical expression
For a small gravitational field like Earth’s, gravitational redshift can be approximated using:
Where:
- = change in wavelength
- = original wavelength
- = gravitational acceleration
- = height difference
- = speed of light
This formula shows that the shift is extremely small near Earth but becomes significant near very massive bodies.
Real-world examples of gravitational redshift
- Light from stars
Light from stars and galaxies is shifted toward red when the light climbs out of their gravitational field. The effect is strongest in extremely dense stars such as white dwarfs and neutron stars.
- Black holes
Near a black hole, gravity is extremely strong. Any light that escapes from near the black hole’s event horizon experiences enormous redshift. In fact, light can be redshifted so much that it becomes undetectable.
- Solar redshift
Light emitted from the surface of the Sun shows a tiny gravitational redshift. This was one of the early tests that confirmed general relativity.
- GPS satellites
Gravitational redshift is important in the functioning of GPS systems. GPS satellites orbit far from Earth’s surface, where gravity is weaker. Their onboard clocks run faster compared to clocks on Earth. Engineers must correct this effect to ensure accurate positioning.
- Laboratory tests
The Pound–Rebka experiment in 1959 confirmed gravitational redshift on Earth using gamma rays. This experiment provided strong evidence for general relativity.
Difference between gravitational redshift and Doppler redshift
Although both effects shift light toward the red side, they occur for different reasons:
- Gravitational redshift happens due to gravity.
- Doppler redshift happens due to relative motion between the source and observer.
Both effects can occur together in astronomical observations, but scientists can separate them based on conditions.
Importance of gravitational redshift
Gravitational redshift helps scientists:
- Understand the structure of stars.
- Study black holes and neutron stars.
- Test general relativity with high precision.
- Measure the strength of gravity in different regions of space.
- Improve technologies like GPS that depend on accurate time measurement.
Without understanding gravitational redshift, many observations in astronomy would remain unexplained.
Gravitational redshift and energy conservation
Even though the frequency decreases as light escapes gravity, energy is not lost from the universe. Instead, the energy is transformed as the light moves through curved space-time. General relativity ensures that energy conservation still holds when understood correctly in curved space.
Why gravitational redshift proves relativity
Gravitational redshift is one of the key tests that confirmed Einstein’s predictions. Classical physics could not explain why light from the Sun or white dwarfs had a shift in wavelength. Only relativity provided a correct explanation. This experiment helped establish relativity as a fundamental theory of physics.
Conclusion
Gravitational redshift is the increase in the wavelength of light as it moves away from a strong gravitational field. It occurs because light loses energy while escaping gravity, leading to a lower frequency and a shift toward red. This effect is a powerful prediction of general relativity and has been confirmed through many experiments and astronomical observations. Gravitational redshift helps us understand stars, black holes, time dilation, and the nature of space-time.