The Passage of Time in Space

In almost every interstellar movie there is out there, there is always talk of a “space-time continuum,” how it can be disrupted, and how it affects humans… however, what exactly is the space-time continuum, and what does it have to do with the way time works in outer space?

To understand space-time and its role in the fabrication of the universe, there are a couple of ground theories that we need to layout. Mainly, the theories of both special and general relativity.

The Theory of Relativity

The theory of relativity is the most accurate description of the universe that we have today. However, it is one that is extremely difficult to fully wrap your head around.

Let us start with something most people have heard of … E = mc²

This equation, devised by Albert Einstein, demonstrates the theory of relativity. The equation itself states that the relative mass of an object (m) multiplied by the speed of light squared (c²) is equal to the kinetic energy of that object (E). Which is to say that Einstein theorized that mass and energy were equivalents, or rather, two manifestations of the same thing. This observation of mass-energy leads to the theory of special relativity, and derived from the theory of special relativity, general relativity … another one of Einstein’s theories, and arguably his greatest achievement.

Special Relativity

Special relativity essentially theorizes that time, space, and distance are all relative to the observer. In addition to this, it also states that the mass of an object will increase the faster it moves, which puts a cap on how fast an object can move. Uncoincidentally, this cap is the speed of light. However, for right now let’s focus only on the first statement.

Space, time, and distance are all relative to the observer

This means that someone moving close to the speed of light will experience time and distance different than someone standing stationary. To explain this concept, Einstien used the train example. Imagine you are standing directly between two lightning strikes, you perceive the lightning strikes to be hitting the ground simultaneously. Now, your friend is on a train moving at the speed of light, while they pass, they happen to see both lightning strikes, but the lightning strikes at different times.

General relativity

The theory of general relativity states that gravity is a product of space-time; in other words, objects with mass can warp space-time. This warp in space-time is what we call gravity. So, matter shapes space-time, but space-time tells matter how to move. Think of general relativity as a bowling ball on a trampoline, the bowling ball acts as a planet, and the trampoline is the fabric of space-time. If a bowling ball is placed on that trampoline, the trampoline will curve to accommodate the bowling ball.

What does that have to do with time in space?

There are two types of time dilation, that is to say, two ways that we know of right now to warp time; gravitational time dilation and relative velocity time dilation. Both these “methods” of dilating time affect astronauts in space and people here on Earth.

Gravitational Time Dilation

Gravitational time dilation deals with the way gravity warps and bends space-time. As previously stated, gravity is a product of the way large objects warp space-time, so scientists can conclude that an object’s gravitation pull affects time as well. How?

Time passes slower near objects with a large amount of gravity.

That would mean that the more gravity (, the more space-time is bent) an object experiences, the slower time passes for said object. Take a clock for example: Eventually, after a great deal of time, a clock in a basement will fall behind another clock at the top of a building.

So what does that mean for astronauts in space?

Since much of space is full of dark matter, there are not a lot of large objects, in comparison to empty space, around to slow time down for the astronauts. Once the astronaut comes into the gravitational field of an object, time will slow for them. Theoretically, if the astronaut does not come into contact with a gravitational field, they will age faster than someone on Earth. However, there is another factor involved in time dilation.

Relative Velocity Time Dilation

The second type of time dilation is called relative velocity time dilation, which is a fancy way of saying that time slows as an object moves faster. So what is the relationship between gravitational and relative velocity time dilation?

Velocity time dilation has a more significant effect on objects than gravitational time dilation.

Consider the astronauts on the space station. Since they are very high above the surface of the Earth, they should be experiencing time moving faster on the ISS than on Earth. However, they are also moving about 5 miles a second. The speed at which they are traveling is also affecting the way they experience time, so without the consideration of gravity, they should be aging slower. When added together, we see time internally struggling with itself, however, since relative velocity time dilation has a more significant effect on the object around it, the astronauts in space will actually be aging slower than their human counterparts on Earth.

What does that mean for Space Travel?

These changes in time throughout the universe could potentially spell out big problems for astronauts and the scientists working with them. As astronauts spend more time in space, they would age slower than the rest of us here on Earth. This would mean that there is a chance that when the astronaut came back to Earth, those that they were working with could be too old for work, or dead. Since time passes differently in space, anyone in deep space would have no idea how much time passed on Earth while they were out exploring. Even if they did, if they or Earth had an emergency, it could be ages before help arrived. Therefore, if we really want to make interstellar travel a reality, humanity as a whole would have to travel together to avoid this time difference problem.

Key Takeaways

• The theories of relativity help us understand how time passage is affected by large objects in the universe
• Special relativity theorizes that although it may look like time, distance and space are all static, time, distance, and space are all relative to the observer
• General relativity states that gravity is a production of a warp in space-time. Objects can warp space-time, and space-time tells objects how to move
• There are two types of time dilation derived from the theories of relativity: gravitational time dilation and relative velocity time dilation
• gravitational time dilation means that time slows down the more gravity is present
• general velocity time dilation means that the faster an object’s velocity is, the slower time moves for them
• This means that if humans ever want to travel into deep space, we would most likely have to all abandon Earth in favor of interstellar travel

Sources:

• https://www.businessinsider.com/do-astronauts-age-slower-than-people-on-earth-2015-8
• https://www.britannica.com/science/time-dilation
• https://www.britannica.com/science/relativity/Relativistic-mass#ref252888
• Science ABC — time dilation — “Einstein’s Theory Of Relativity Explained!” (youtube)
• https://einstein.stanford.edu/SPACETIME/spacetime2.html
• https://www.forbes.com/sites/startswithabang/2018/01/23/the-three-meanings-of-emc2-einsteins-most-famous-equation/#43ce6a5271c0

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