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Chapter 18: Problem 16

An astronomer sees a redshift in the spectrum of an object. Without any other information, can she determine whether this is an extremely dense object (exhibiting gravitational redshift) or one that is receding from her (exhibiting Doppler redshift)? Explain your answer.

Short Answer

Expert verified
No, without additional information, the astronomer cannot determine if the redshift is gravitational or Doppler.

Step by step solution

01

Define Redshift

Redshift refers to the phenomenon where the wavelength of light or other electromagnetic radiation from an object is increased, leading to the object appearing redder.
02

Types of Redshift

There are primarily two types of redshift relevant to this problem: gravitational redshift and Doppler redshift. Gravitational redshift occurs when light escapes from a very dense object, while Doppler redshift occurs when an object is moving away from the observer.
03

Gravitational Redshift

Gravitational redshift is caused by the effect of gravity on light. When light travels away from a massive object, it loses energy, increasing its wavelength and causing the redshift. This is typically observed near extremely dense objects like black holes or neutron stars.
04

Doppler Redshift

Doppler redshift, on the other hand, happens when an object is moving away from the observer. The speed of the object causes the light waves to stretch, resulting in a shift towards the red end of the spectrum.
05

Comparison and Limitations

Without additional information, such as the context of the object's environment or spectrum details, it's not possible to conclusively determine whether the redshift is due to a gravitational effect or the Doppler effect. Both phenomena result in similar redshift effects.
06

Conclusion

Therefore, the astronomer cannot definitively determine the cause of the redshift—whether it's due to an extremely dense object exhibiting gravitational redshift or an object receding from her exhibiting Doppler redshift—without further data.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Gravitational Redshift
Gravitational redshift is a fascinating phenomenon that happens because of gravity's influence on light. Imagine light trying to escape from an immensely dense object like a black hole or a neutron star. As it moves away from the gravitational pull, it loses energy. This loss of energy stretches the light's wavelength, shifting it towards the red end of the spectrum. This is why it's called 'redshift'. This kind of redshift is typically observed in extremely dense environments where the gravitational forces are very strong. Key points to remember include:
  • It's caused by gravity.
  • Occurs near extremely dense objects.
  • Light loses energy as it escapes gravity, leading to an increased wavelength.
Understanding gravitational redshift helps astronomers study the nature and environment of dense astronomical objects.
Doppler Redshift
Doppler redshift is another important type of redshift. It happens when an object is moving away from an observer. Think of a car moving away from you; the sound gets quieter. Similarly, when a star or galaxy moves away from Earth, its light waves stretch. This stretching of waves leads to a shift towards the red end of the spectrum—hence the redshift. This is often observed in galaxies, showing that the universe is expanding. Important points include:
  • Caused by the motion of objects moving away.
  • Light waves stretch, increasing wavelength and shifting towards red.
  • Often linked with the expansion of the universe.
By studying Doppler redshift, astronomers can understand the movement and speed of celestial objects.
Light Wavelength
Light wavelength is a fundamental concept in understanding redshift. Wavelength is the distance between two peaks of a wave of light. When light's wavelength increases, we see a shift towards the red part of the spectrum (redshift). Conversely, a decrease in wavelength shifts light towards the blue part of the spectrum (blueshift). Key elements include:
  • Wavelength is the distance between peaks of light waves.
  • Increased wavelength means redshift, decreased means blueshift.
Light wavelength plays a crucial role in identifying and analyzing various cosmic phenomena. Knowing how to measure and interpret these changes helps astronomers make important discoveries about the universe.

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Most popular questions from this chapter

Astronauts in the International Space Station a. have no mass. b. have no energy. c. are outside of Earth's gravitational field. d. are in free fall.

Go to the LIGO website (http://ligo.org/science.php) and read about gravitational waves. Click on "Sources of Gravitational Waves" and listen to the example. What are the differences among the four listed sources of gravitational waves? Click on "Advanced LIGO." What's new with the project?

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If a spaceship approaches you at \(0.5 c\), and a light on the spaceship is turned on pointing in your direction, how fast will the light be traveling when it reaches you? a. \(1.5 c\) b. between \(1.0 \mathrm{c}\) and \(1.5 \mathrm{c}\) c. exactly \(c\) d. between \(0.5 c\) and \(1.0 c\)

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