# What does pressure mean in regards to GR?

• BWV
In summary, pressure in the context of General Relativity (GR) refers to the force per unit area exerted by a substance's internal energy and particle motion. It contributes to the energy-momentum tensor, which affects the curvature of spacetime and thus plays a role in shaping the gravitational field. Pressure can change the trajectory of an object by influencing the curvature of spacetime. In the principle of equivalence, pressure contributes to the indistinguishability of gravity and acceleration. Negative pressure, or tension, can exist in GR and can have implications for the behavior of the universe.
BWV
I cannot get past thinking of it like air pressure

BWV said:
I cannot get past thinking of it like air pressure

It is!

Pressure (inside a star, or wherever) is just pressure!

In the context of GR, pressure refers to the force exerted by a fluid or gas on a given area. This can be seen in the Einstein field equations, where the energy-momentum tensor includes terms for both energy density and pressure. In this sense, pressure plays a crucial role in the curvature of spacetime and the overall dynamics of the universe.

While it may have some similarities to air pressure, pressure in GR is a much more complex and nuanced concept. It is not just a measure of the force exerted by a gas, but rather a fundamental property of matter and energy that has a significant impact on the behavior of spacetime. In addition, the concept of pressure in GR extends beyond just gases and can also apply to other forms of matter and energy, such as radiation or dark energy.

Overall, understanding pressure in the context of GR is essential for comprehending the intricate relationships between matter, energy, and spacetime in our universe. It is a key component in the study of gravity and the behavior of massive objects, and continues to be a topic of ongoing research and exploration in the field of physics.

## 1. What is pressure in the context of General Relativity (GR)?

In GR, pressure refers to the force per unit area that a substance exerts on its surroundings. This can be a result of the substance's internal energy and motion of its particles.

## 2. How does pressure affect the curvature of spacetime in GR?

In GR, pressure contributes to the energy-momentum tensor, which is a mathematical representation of the distribution of matter and energy in spacetime. This tensor affects the curvature of spacetime, and thus, pressure plays a role in shaping the gravitational field.

## 3. Can pressure change the trajectory of an object in GR?

Yes, pressure can change the trajectory of an object in GR. This is because pressure affects the energy-momentum tensor, which in turn affects the curvature of spacetime. As objects move through curved spacetime, their trajectories are influenced by the surrounding matter and energy, including pressure.

## 4. How does pressure relate to the principle of equivalence in GR?

The principle of equivalence in GR states that the effects of gravity are indistinguishable from the effects of acceleration. Pressure plays a role in this principle as it contributes to the energy-momentum tensor, which is used to describe the curvature of spacetime. This tensor is what determines the gravitational field, and thus, pressure is a factor in the equivalence of gravity and acceleration.

## 5. Can pressure be negative in GR?

Yes, pressure can be negative in GR. This is known as negative pressure or tension. In some cases, negative pressure can counterbalance the effects of positive pressure, resulting in a net pressure of zero. This can have implications for the curvature of spacetime and the overall behavior of the universe.

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