Relativity is two related theories. Special relativity explains the relationship between space, time, mass, and energy. General relativity describes how gravity fits into the mix. Albert Einstein proposed these theories starting in 1905. By the 1920s, they were widely accepted by physicists.
Special relativity involves two key ideas.
The first idea is that the speed of light in a vacuum (a space that has no matter in it) is the same for any observer. This is true regardless of the observer’s location or motion, or the location or motion of the light source.
The second idea has to do with reference frames. A reference frame is an environment in which an observer is not moving relative to the immediate surroundings. If you are a passenger in a car, the car is your reference frame. You are not moving or changing position with respect to your car and everything in it. Another example is an astronaut on a spaceship. The astronaut is not moving relative to the spaceship. However, if the astronaut is bouncing a ball on the spaceship, the ball is moving within the reference frame of the spaceship.
It gets more complicated once a reference frame is moving relative to another reference frame, like two spaceships. Those two frames each have a different perspective on time and space. (The combination of the three dimensions of space [length, width, height] and the one dimension of time makes up spacetime). The second key idea of special relativity says that the laws of physics are the same for all reference frames even if they are moving at different speeds – as long as the reference frames are not speeding up or slowing down relative to each other.
Einstein’s most famous equation builds on these ideas. It describes the relationship between energy, mass, and the speed of light. It says energy (E) equals mass (m) times the speed of light (c) squared (2), or E=mc2. Mass is essentially the amount of material an object contains. (This is different from weight, which is the force of gravity on an object.) Each object has a different mass. In contrast, the speed of light is a constant—it is the same everywhere in the universe. The equation describes how mass and energy are related and can transform from one to the other. This transformation occurs in nuclear fusion, which happens inside the sun and other stars. In nuclear fusion, two nuclei merge to form a heavier nucleus. However, the heavier nucleus still has less mass than the two lighter nuclei put together. The leftover mass transforms into energy. That’s why fusion can produce so much energy.
Another result of the theory of special relativity is that as an object moves faster, its observed mass increases. This increase is negligible at everyday speeds. But this increase is significant at much higher speeds. As an object approaches the speed of light, its observed mass becomes infinitely large. As a result, an infinite amount of energy is required to make an object move at the speed of light. For this reason, it is impossible for any matter to travel faster than light speed.
Special relativity describes how the universe works for objects that are not accelerating. However, it doesn’t incorporate gravity. That’s part of the theory of general relativity.
Before Einstein, the traditional view was that gravity was an invisible force pulling things together. Instead, general relativity states that gravity is how mass warps space and time. The bigger the mass, the more it warps things. Imagine that the universe is a rubber sheet covered with objects of different weights, each sitting in a curved depression formed by that object’s weight. More massive objects will bend the sheet more. General relativity is why stars, which are incredibly massive, bend the path of light. Black holes, with huge amounts of mass in a small space, bend space so much they actually trap light.
Special and general relativity come together to show how time is measured differently in different frames of reference. This effect happens because different frames of reference perceive time and space differently.
The classic example is a pair of twin astronauts. One twin stays on Earth, while the other climbs on a rocket going close to the speed of light. Both have their own frames of reference that are moving at different speeds. The Earth is moving around the sun, while the rocket is accelerating away from the Earth. The twin on the rocket will experience time differently than the one on Earth. In fact, she will experience much less time overall than the one on Earth, called time dilation. When she returns to Earth, she will be significantly younger than her twin.
Subatomic particles called muons are a more practical example. Cosmic rays hitting Earth’s atmosphere create muons. Muons travel at nearly the speed of light and decay incredibly fast. In fact, they decay so fast that it seems like they shouldn’t reach the Earth’s surface. Nonetheless, many do! This bizarre result is because of time dilation. Imagine being an astrophysicist standing on the Earth’s surface. To you, a muon should travel only 0.4 miles over the course of its 2.2 microsecond life. But muons travel very close to the speed of light. As a result, their time within their own reference frame time is about 40 times slower than experienced by an observer on Earth. As 2.2 times 40 equals 90 microseconds, this gives the muon far more time to get to Earth. Over this period of time, the muon can travel 16 miles, a much further distance than 0.4 miles.
DOE Office of Science: Contributions to Special and General Relativity
As fundamental theories of physics, special and general relativity underpin all the work supported by the Department of Energy’s (DOE) Office of Science.
Relativity is particularly important to the research of the DOE Office of Science Nuclear Physics and High Energy Physics programs. Research on muons, nuclei, and other subatomic particles relies on understanding the effects of relativity on these particles. In addition, large-scale studies of the universe to understand dark matter and cosmic acceleration rely on studying how relativity explains the phenomena that have shaped our universe.
Understanding relativity is also essential to many of the scientific facilities the DOE Office of Science supports. For example, DOE’s particle accelerator User Facilities, speed subatomic particles to nearly the speed of light. Scientists at these facilities must take relativity into consideration when they gather data.
Relativity Fast Facts
- As particle accelerators speed subatomic particles up to close to the speed of light, those particles become incredibly massive.
- Global positioning system (GPS) satellites fly in different orbits around the Earth. These orbits are different frames of reference, so GPS has to take special relativity into consideration to help us navigate.
Resources
- DOE SC Nuclear Physics program
- DOE SC High Energy Physics program
- Fermilab video: What’s relativity all about?
- DOE Office of Science Nuclear Physics user facilities
- DOE Office of Science High Energy Physics user facilities
DOE Explains offers straightforward explanations of key words and concepts in fundamental science. It also describes how these concepts apply to the work that the Department of Energy’s Office of Science conducts as it helps the United States excel in research across the scientific spectrum.