How to Think About Relativity’s Concept of Space-Time

Space and time had their own identities in physics. Talking about space-time became unavoidable because of the theory of relativity. It is not true that space and time have different meanings. Space-time is simply a human convention, and slicing it up into space and time is a useful one.

Our intuitions help us to think of space and time as separate things, which is one of the main reasons why relativity is hard to understand. It seems like an objective fact that objects have extent in space. Pre-relativistic physics works because we travel through space at lower speeds than light.

The leap to a space-time perspective is a bit intimidating due to the mismatch between intuition and theory. Presentations of relativity tend to start with our everyday conceptions of space and time and change them in the new context of relativity.

We're going to be a bit different. Our route into special relativity might be thought of as top-down, taking the idea of a unified space-time seriously from the start. The result will be a deeper understanding of the relativistic perspective on our universe.

The theory of electromagnetism was the culmination of a theoretical edifice that had been under construction since the 1860s, thanks to Albert Einstein. A special significance to the speed at which light travels is attached to the theory of light by Maxwell. The idea of a field all by itself wasn't completely intuitive to scientists at the time, and it was natural to wonder what was actually "waving"

The physicists dubbed the medium the luminiferous ether. They had to invent complicated reasons why this substance should not be seen. In 1905, Einstein pointed out that the ether had become pointless, and that we could better understand the laws of physics without it. Accepting a completely new idea of space and time was all we had to do. It turned out to be worth it.

The special theory of relativity is what Einstein's theory was called. He argued for a new way of thinking about length and duration in his paper. He explained that the speed of light is a function of an absolute speed limit in the universe and that everyone would measure it the same regardless of how they were moving. He had to change our notions of time and space.

He didn't advocate joining space and time into a single unified space-time He left that step to his former university professor. Minkowski space-time is an area of special relativity.

When you think of space-time as a four-dimensional continuum, you can ask questions about its shape. Is space-time flat or curved? It's flat, static and infinite.

The force of gravity was studied by Einstein for a decade. He realized that space-time could be dynamic and curved, and that the effects of that curve are whatgravity is. General relativity is the fruit of this inspiration.

Special relativity is the theory of a flat space-time, without gravity, while general relativity is the theory of a dynamic space-time. Even though they replace some of the principles ofNewtonian mechanics, they are still considered Classical Theories. Classical doesn't mean "non-quantum." The principles of classical physics are still in use today.

We should be willing to let go of our pre-relativity fondness for the separateness of time and space. Think more carefully about what we mean by time. To back, once again, to how we think about space is the best way to go.

Take into account your home and your favorite restaurant. The distance between them is not known.

You think that depends on the situation. If we could take a straight line path between the two points, there would be a distance. On a real-world journey, you might not be able to take public streets and sidewalks, avoiding buildings and other obstacles along the way. Since a straight line is the shortest distance between two points, the route you take is always longer than the distance from one point to another.

Consider the events in space time. A single point in the universe is specified by locations in both space and time. One event could be at home at 6 p.m. or at the restaurant at 7 p.m. If you want to reach the event in space, you have to wait until 7 pm.

We can ask ourselves how much time elapsed between these two events, just as we did for the distance between home and restaurant.

This is a trick question, right? There is one hour between the two events.

Einstein doesn't think it's so fast. It's in an antiquated conception of the world. If the time between two events is less than an hour, there is nothing to say.

A different story is told by the company. There are two different ideas of what time means. Time is a coordinate on space time. Space-time is a four-dimensional continuum, and if we want to specify locations within it, it's convenient to attach a number called "the time" to every point within it. When we think of 6 p.m. and 7 p.m., we usually mean that. A coordinate on space-time is a label that helps locate events. Everyone is supposed to understand what we're saying when we say we're going to meet at the restaurant.

The duration of time that you experience between two points in space will not be the same as the universal coordinate time. You have a clock with you on the journey that you can use to measure the time. This is the right time. The duration measured by a clock is similar to the distance traveled on your car's odometer.

It means that time is relative. A coordinate on space-time and a personal time that we experience along our path are both possibilities. Time and space are not related. It is not a coincidence that Einstein worked in a Swiss patent office at a time when rapid rail travel was forcing Europeans to think about what time it was in other cities across the continent.

There must be some way in which time isn't like space, otherwise we'd just talk about space. We are in a world with few moving parts where we don't have to worry about the arrow of time.

Straight lines describe the shortest distance between two points. Straight path gives the longest elapsed time between two events. Time is different from space due to the flip from shortest distance to longest time.

A straight path in space-time means both a straight line in space and a constant speed of travel. A trajectory with no acceleration is called an insturment. Two locations in space and corresponding moments in time need to be fixed. A traveler can either travel between them in a straight line at constant speed or they can travel in a non-inertial path. The back-and-forth route will always involve more spatial distance than the straight route.

It's like that. Because physicists say so. That's the way the universe is. It is one of the bedrock assumptions upon which we build physics and it is not a conclusion we derive from deeper principles. Straight lines in space are shorter than straight paths in space-time.

It seems odd that paths of greater distance take less time. It is ok. It wouldn't have taken Einstein to come up with the idea if it were intuitive.

Adapted from The Biggest ideas in the Universe by Sean Carroll with permission from the publisher of the book. SeanCarroll's work is Copyright 2022.