Light, gravity, and red shift.

In summary, light responds to gravitation and has momentum, meaning that it can be affected by massive bodies and has its own gravitational field. As it travels through the universe, its course is constantly altered by gravitational fields, potentially causing a loss of energy and red shift. However, through calculations, it is determined that photons behave like test particles and do not emit gravitational waves, making it unlikely that red shift in light from distant stars is due to losing energy in this form.
  • #1
mrspeedybob
869
65
Light responds to gravitation. Light has momentum.

Given these two postulates it follows that if a photon passes a massive body both the photon and the body will be effected, though admittedly the effect on the massive body will be extremely slight.

Put another way, a photon has its own gravitational field.

Now a photon traveling through the universe is not going to travel in a straight line, it's course will be constantly altered by the gravitational fields it goes through. Since it is constantly changing directions it seems that some of its energy would be radiated away as gravitational waves. Since its velocity is fixed this would mean that it would shift to a lower energy frequency. The longer the photon traveled through the universe the more energy it would lose and the more red shifted it would become. How do we know that the red shift in light from distant stars is shifted due to the star moving away and not due to losing energy in the form of gravity waves?
 
Physics news on Phys.org
  • #2
mrspeedybob said:
Now a photon traveling through the universe is not going to travel in a straight line, it's course will be constantly altered by the gravitational fields it goes through.

The world-line of a test particle, such as a photon, is essentially the definition of a "straight" line in general relativity. A good introduction to this kind of thing is Relativity Simply Explained, by Martin Gardner.
 
  • #3
How do we know that the red shift in light from distant stars is shifted due to the star moving away and not due to losing energy in the form of gravity waves?
By doing calculations.
The lighter a particle is, the more does it resemble a massless "test particle", i.e. a hypothetical particle which doesn't affect spacetime at all. bcrowell is talking about such a test particle.

Photons are already very light. The worst thing - in terms of acceleration - that can happen to a photon is to orbit around a small (~sun-sized) black hole. It's at the very least some 20 orders of magnitude worse than what happens to a photon in interstellar space.
Even then, if the usual Quadrupole approximations still hold for a photon, they lose a significant amount of energy at a timescale of 10^60 s (for optical photons), which is ridiculously longer than the age of the universe.
IOW: they behave like test particles and don't radiate gravitational waves.
 
  • #4
Ich said:
By doing calculations.
The lighter a particle is, the more does it resemble a massless "test particle", i.e. a hypothetical particle which doesn't affect spacetime at all. bcrowell is talking about such a test particle.

Photons are already very light. The worst thing - in terms of acceleration - that can happen to a photon is to orbit around a small (~sun-sized) black hole. It's at the very least some 20 orders of magnitude worse than what happens to a photon in interstellar space.
Even then, if the usual Quadrupole approximations still hold for a photon, they lose a significant amount of energy at a timescale of 10^60 s (for optical photons), which is ridiculously longer than the age of the universe.
IOW: they behave like test particles and don't radiate gravitational waves.

Thank you for the reply. That makes sense.
 

1. What is light?

Light is a form of energy that is visible to the human eye. It is part of the electromagnetic spectrum and travels in waves. Light can also be described as particles called photons.

2. How does gravity affect light?

Gravity is a force that attracts objects towards each other. It affects light by bending its path, causing it to follow a curved trajectory. This effect is known as gravitational lensing and has been observed in space.

3. What is red shift?

Red shift is a phenomenon where light from objects in space appear to be shifted towards the red end of the electromagnetic spectrum. This is caused by the Doppler effect, which is the change in frequency of waves as the source and observer move relative to each other.

4. How does red shift provide evidence for the expansion of the universe?

The red shift of light from distant galaxies is evidence that the universe is expanding. This is because the further away a galaxy is, the greater the red shift in its light. This suggests that the galaxies are moving away from each other and the space between them is expanding.

5. Can gravity affect the red shift of light?

Yes, gravity can affect the red shift of light. If light is passing through a region with a strong gravitational field, such as near a black hole, it can experience a significant red shift. This is known as gravitational red shift and is a key concept in Einstein's theory of general relativity.

Similar threads

  • Special and General Relativity
Replies
26
Views
1K
  • Special and General Relativity
2
Replies
46
Views
3K
Replies
19
Views
1K
  • Special and General Relativity
Replies
30
Views
2K
  • Special and General Relativity
4
Replies
132
Views
6K
  • Special and General Relativity
Replies
6
Views
1K
Replies
1
Views
123
  • Special and General Relativity
Replies
9
Views
1K
  • Special and General Relativity
Replies
10
Views
1K
  • Astronomy and Astrophysics
Replies
3
Views
823
Back
Top