# Spacetime Interval & Metric: Equivalent?

• jmatt
In summary, the statements "The spacetime interval is invariant" and "The spacetime metric is a tensor" are not exactly equivalent, but are closely related. The spacetime interval is a scalar, represented by ##ds^2##, which is obtained by contracting the metric tensor ##g_{ab}## with infinitesimal vectors ##dx^{a}##. This scalar remains invariant under Lorentz transformations. It is not a four-vector, as it is just a single number and not made up of four components.
jmatt
This may seem an odd question but it will clear something up for me. Are "The spacetime interval is invariant." and the "The spacetime metric is a tensor." exactly equivalent statements? Does one imply more or less information than the other?

Thanks!

They aren't exactly equivalent, but they are closely related. The space time interval is ##ds^2=g_{ab}dx^a dx^b##. Because the metric ##g_{ab}## is a tensor, contracting it with (infinitesimal) vectors ##dx^{a}## will yield an invariant scalar ##ds^2##.

Matterwave said:
They aren't exactly equivalent, but they are closely related. The space time interval is ##ds^2=g_{ab}dx^a dx^b##. Because the metric ##g_{ab}## is a tensor, contracting it with (infinitesimal) vectors ##dx^{a}## will yield an invariant scalar ##ds^2##.
Thanks! A little confused why you referred to ##ds^2## as a scalar. Isn't it a four-vector?

jmatt said:
Thanks! A little confused why you referred to ##ds^2## as a scalar. Isn't it a four-vector?
Nope. It's the scalar product of (differential) four-vectors.

jmatt said:
Thanks! A little confused why you referred to ##ds^2## as a scalar. Isn't it a four-vector?

Why would it be a 4 vector? What are the 4 components that you are thinking of? It is a scalar because it's just 1 single number, which does not change under an arbitrary Lorentz transformation.

Got it now. Thanks very much.

## 1. What is the concept of spacetime interval?

The spacetime interval is a measure of the distance between two events in four-dimensional spacetime. It takes into account both the spatial and temporal components of the events.

## 2. How is the spacetime interval calculated?

The spacetime interval is calculated using the Pythagorean theorem in four dimensions. It involves taking the square root of the sum of the squares of the spatial and temporal components of the events.

## 3. What is the significance of the spacetime interval in Einstein's theory of relativity?

In Einstein's theory of relativity, the spacetime interval is a fundamental concept that is used to define the geometry of spacetime. It is invariant, meaning that it remains the same for all observers regardless of their relative motion.

## 4. What is the difference between spacetime interval and proper time?

The spacetime interval is a measure of the distance between two events in four-dimensional spacetime, while proper time is a measure of time experienced by a single object or observer. The spacetime interval is used to calculate proper time, but they are not interchangeable.

## 5. How is the spacetime interval related to the metric tensor?

The metric tensor is a mathematical tool used to calculate the spacetime interval. It represents the geometric properties of spacetime, including the curvature caused by the presence of mass and energy. The components of the metric tensor are used to calculate the spacetime interval between events.

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