In Layman's Terms
In Layman's Terms
Imagine a object five units long, four wide, and two high—easy enough, right? However, what if it had no persistence? Without continuity, it could not really exist. If we could roll back the days of this object’s existence, we’d see a long worm-like line stretching all the way back to the time it was first formed. If we could look into the future, we might see it as a ghostly trail of potential showing all the possible future paths of its existence.0 That is why there isn’t separate space and time. They are both important parts of the same thing. It is called a continuum because space and time are intimately intertwined, and only together can you describe the Universe in a scientifically useful way.
In its simplest form you could say that the space-time continuum is all of the four basic dimensions. Space is comprised of length, width, and height, while time is measured in units of duration. Humans tend to take duration for granted because the everyday things we experience in our daily lives typically don’t vanish spontaneously.Technically we could describe a person as “John, born 1990, two meters tall, a half meter wide, a quarter metre thick, who will die in 2070…” but that is highly imprecise and we actually don’t know all these things about John with any great degree of accuracy. Most humans exist for about 50-100 years, so we just say “John” and accept that the general things about duration will also be true for “John”.
Of course, you might have a hard time convincing some adults about an object’s continuity when their phone or their car keys are missing. Explaining to a child where the balloon “went” when it pops is equally difficult. So there are some circumstances that people are familiar with which show how duration, or time, is actually essential.
Yes, time and space change together, because they are aspects of the same thing. For example, if you took out your phone to use its GPS (Global Positioning System) to find a local restaurant, if it weren’t corrected for space-time changes and perturbations caused by Earth’s gravity, that restaurant would be shown six miles (10 kilometers) further away from its actual position every day. Not because it moved, of course, but because the GPS uses satellites in orbit to describe a position on the planet’s surface. They send radio pings back and forth to the surface to know their own location relative to the Earth’s surface and the other GPS satellites.
Those satellites are 12,500 miles (20,000 km) above us, so they are in a weaker gravity field than on Earth’s surface. Earth’s gravity field bends space, and at that altitude space is less curved than on the surface, so time flows faster. The difference is very small, of course, but it is enough to render uncorrected GPS completely useless.We solved this problem using Albert Einstein’s General Relativity equation. After applying his formulas describing space-time curvature, we knew exactly how much to adjust the reading to keep GPS accurate—and it worked perfectly. It’s also how we finally figured out why the planet Mercury had such a weird orbit.
The Sun’s gravity field is immensely strong because it is so massive compared to everything else in the solar system. It represents 99.85% of all the Solar System’s mass. Here is Earth compared to the Sun—and Mercury is even smaller than Earth! Since Mercury is so close to the Sun, it experiences time at a slightly slower rate than we do (we’re much farther away). Newtonian Physics (the Laws of Physics as described by Isaac Newton) let astronomers calculate how all the planets would affect each other, and made it possible to predict exactly where they would be at any given time—but it was slightly wrong for the planet Mercury. The space-bending effect hadn’t been described by Einstein yet.
The Sun’s massive gravity curves the space around it and the closer you are the more curved it is. Mercury’s orbit is much more curved than Venus or Earth’s, to the point that it orbits 7% faster than we thought it should back in the 1900s for its size and distance from the Sun.At first we thought there was another planet closer to the Sun that we hadn’t found, and we called it Vulcan (from Roman mythology’s god of fire, since it would be so hot). It didn’t exist, so we speculated that Mercury had some sort of asteroid field that gave it more mass to account for its speed, but that didn’t work out either.
Finally, it was Albert Einstein that pointed out that space-time is curved by the mass within it, and his General Relativity equations finally explained Mercury’s orbit exactly. So even if we can’t see it, space bends around all mass.
We use that effect today to study objects behind other massive objects such as the above image of the famous Einstein’s Cross. The four surrounding pools of light are actually that same object being bent around a galaxy that is between Earth and the object.The distant (four surrounding images) object is a quasar (the highly active nucleus of another galaxy), hidden behind the nearer one in the center. The gravity of the nearer galaxy bends the light around each side of itself. It’s unusual to have four distinct images like this as it typically shows as a ring of light surrounding the nearer object.
This is also how we just detected the most distant star ever seen, named Earendel (Dawn star), which is 12.9 billion light years away, because it formed soon after the Big Bang, near the dawn of time. We would ordinarily have no possibility of seeing a single star at these distances. It was only because of this bending of space-time, creating a gravitational lens, that we could see it at all.
When we move, time slows down around us. If you fly in a jet at 500 mph/800kph, you age slightly slower than someone on the ground. However, because jets fly at great heights, you would also be in a weaker gravity field, so you would age more quickly! Once you do all the calculations, if you flew for about two years, you would end up being about one second further into the future than a non-flying person—or you could say that you would be one second younger.
Now on a fast spaceship, travelling at a good percentage of the speed of light through space, your aging would really slow down because time would be compressed for you. More importantly, distance would shrink the faster you travelled. The faster you go, the shorter the distance becomes.
To understand this you need to know that the dimensions of space change based on speed and time. At 65% of c (the speed of light), distance is reduced to 75% of the distance measured when not moving. For example, to get to the Alpha Centauri star system (4.3 light years away), at that speed you would actually only travel 3.26 light years. Weird, right?
If you were moving at 85% of c, the distance would shrink to 2.2 light years. At 95% of c, the distance would be just 1.4 light years. At 99.9% of c, the distance would be 0.19 light years.Since time and space are locked together, the time you experienced on the space ship would be smaller, too. At 99.9% of c, the trip would only take 70 days, but far away, back on Earth, time and space stayed “Earth normal” and they experienced the full 4.3 years of time where you experienced only 70 days!
On this theoretical trip, you just effectively time travelled into the future by over 4 years. If you turned around and came straight back, you would be almost nine years into the future, that you experienced as only 140 days! Now you’re a time traveller!
Space and Time are aspects of the same thing. Even though we often regard them separately, they are related and affect each other. The effects are measurable with appropriate instruments, even on our daily scale, but we largely ignore them because they are so minuscule.Can space-time be broken? In a way, yes, it can. Black holes seem to break space-time because they take away our ability to measure what is going on inside a black hole. They seriously distort space-time in their vicinity, and actually fold space in on themselves at some point, severing the connection with the rest of the universe.
That isn’t permanent though, because once the Universe cools down enough, there will be less energy outside of a black hole than inside, and the process will reverse, letting matter leak back into the cold, dead universe. That’s a little too late for us, of course, but maybe some future beings will be able to make use of it and start up the Universe again.