# Why are massless photons affected by gravity?

• jtbell
In summary, general relativity explains gravitation as the manifestation of spacetime curvature, affecting the motion of all objects regardless of mass. Light follows straight paths in flat spacetime and curved paths in curved spacetime. This is based on the concept of invariant mass, which is zero for photons. However, if you prefer to think in terms of relativistic mass, all photons still follow the same paths regardless of their energy. This has been verified through experiments, such as comparing the deflection of visible light and radio waves near the sun.
jtbell
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In general relativity, gravitation is a manifestation of the curvature of spacetime. The motion of all objects is affected by this curvature, regardless of whether they have mass or not. Light follows geodesic paths in spacetime, which are straight lines in flat spacetime, and curved paths in curved spacetime.

Note that by "mass" above I mean "invariant mass" as discussed in the following FAQ:

because it is the invariant mass that is zero for a photon. If you prefer to think in terms of "relativistic mass" (which is related to energy via $E = m_{rel} c^2$, note that all photons (as far as we know) follow the same geodesics, regardless of their energy. This has been verified, for example, by comparing the deflection of visible light as it passes close to the sun, with the deflection of radio waves from distant sources.

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jtbell

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Manpriyam
jtbell said:
In general relativity, gravitation is a manifestation of the curvature of spacetime. The motion of all objects is affected by this curvature, regardless of whether they have mass or not.
So, in GR there are no attractive forces between two masses?

Mass produces spacetime curvature (as do energy and momentum), so two masses affect each other gravitationally. Whether to call this "force" is a matter of semantics. If you are falling freely under only the influence of gravity, you do not "feel" it, unless the curvature is so strong as to produce tidal stresses in your body. Therefore many people do say that "gravity is not a force."

afcsimoes

## 1. Why are photons, which have no mass, affected by gravity?

According to Einstein's theory of general relativity, gravity is not a force between masses, but rather a curvature of space and time caused by the presence of mass and energy. Photons, despite having no mass, still carry energy and can therefore interact with the curvature of spacetime created by massive objects like planets and stars.

## 2. How does gravity affect the path of a photon?

Gravity bends the path of a photon, causing it to follow a curved trajectory. This is because the spacetime curvature caused by massive objects changes the straight line path of light. The more massive an object is, the stronger its gravity and the more it will bend the path of a photon.

## 3. Why do photons travel at the speed of light despite being affected by gravity?

The speed of light is a fundamental constant in the universe, and it is not affected by external forces like gravity. This means that even though gravity may bend the path of a photon, it will continue to travel at the speed of light.

## 4. Can photons escape the gravitational pull of a black hole?

No, photons cannot escape the gravitational pull of a black hole. This is because black holes have such strong gravity that they create a region of spacetime from which nothing, including light, can escape. This region is called the event horizon.

## 5. How does the warping of spacetime by gravity affect the wavelength of photons?

When a photon travels through a region with a strong gravitational field, its wavelength is stretched or compressed due to the warping of spacetime. This phenomenon is known as gravitational redshift or blueshift, depending on the direction of the wavelength shift. It has been observed in the light emitted by stars near massive objects like black holes.

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