Why are photons affected by gravity?

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Discussion Overview

The discussion centers on the question of why photons, which are massless, are affected by gravity. Participants explore this concept through the lens of General Relativity and Newtonian gravity, examining the implications for both photons and other massless particles like neutrinos.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants note that General Relativity couples to energy, momentum, pressure, and twisting forces rather than mass, suggesting that gravity affects photons due to their energy and momentum.
  • Others argue that in the Newtonian framework, massless objects could still be affected by gravity, as gravitational acceleration is independent of mass.
  • One participant points out that while Newtonian gravity predicts the behavior of objects under gravity, it fails to accurately predict the deflection of light, which is actually twice what Newtonian predictions would suggest, according to General Relativity.
  • Another participant mentions that light traveling perpendicular to a gravitational field behaves differently than light traveling along the field, noting that while frequency changes, speed does not, which contradicts Newtonian expectations.

Areas of Agreement / Disagreement

Participants express various viewpoints on the relationship between gravity and massless particles, with some agreeing on the principles of General Relativity while others highlight the limitations of Newtonian gravity. The discussion remains unresolved regarding the implications of these theories for understanding gravity's effect on massless particles.

Contextual Notes

There are limitations in the discussion regarding assumptions about the applicability of Newtonian gravity to massless particles and the conditions under which General Relativity provides a more accurate description. The nuances of gravitational effects on light and other massless particles are not fully resolved.

ayush solanki
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Photons are massless,so why does it get affected by gravity?and are neutrinos affected by gravity too?thank you.
 
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Because General Relativity couples to energy, momentum, pressure, and twisting forces rather than mass. Gravity looks like it only couples to mass in most situations because rest mass energy is so much greater than kinetic energy, momentum, pressure, or twisting forces in most situations.

Photons don't have mass, and can't experience twisting forces, but do have momentum, pressure, and energy.
 
OK thank you a lot.
 
Note that there is a priori no reason not to believe massless objects would not be affected by gravity in the Newtonian setting. The gravitational acceleration in Newtonian gravity is independent of the mass of the object (a = g) also if we take the limit ##m\to 0##.
 
Can you suggest an advanced book to better understand it?
 
Which a 16 year old can understand?
 
ayush solanki said:
Can you suggest an advanced book to better understand it?
If you drop a rock near the Earth, the acceleration imparted to the rock by gravity from the Earth is completely independent of the rock's mass. You can then estimate the impact that Newtonian gravity should have on a light beam by just saying that it's an object that is traveling at speed c.

However, if you try doing this, you get the wrong answer. The actual deflection is twice what you would expect from Newtonian gravity alone. General Relativity predicts this extra factor of two.
 
Chalnoth said:
However, if you try doing this, you get the wrong answer. The actual deflection is twice what you would expect from Newtonian gravity alone. General Relativity predicts this extra factor of two.
It is also worth noting that this is for a light beam traveling perpendicular to the gravitational field. Newtonian gravity alone would also predict the light to accelerate when traveling along the gravitational field - which is also wrong, it changes in frequency but not in speed.
 
Thanks a lot guys.that was very helpful.
 

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