Is there a concept of up and down in the universe?
On the International Space Station (ISS), where you can experience zero gravity, you can do things that are impossible on Earth, such as eating dinner while floating in the air and imitating a flying superhero. Scientific media Live Science explains whether there is up and down in outer space where gravity is almost non-existent.
Is there an 'up' and a 'down' in space?
On the earth, the gravitational force works toward the center of the earth, so the top and bottom are determined according to the direction of the gravitational force. In addition, it is possible to determine the north, south, east, and west directions based on the earth's magnetism, and to express any position on the earth as coordinates using latitude and longitude.
On the other hand, it is often thought that ``outer space is a zero-gravity space, so there is no top and bottom,'' but according to Sanjana Curtis, a nuclear astrophysicist at the University of Chicago, astronauts staying on the ISS also experience gravity. He said he would. The ISS, which exists at an altitude of about 400 km, orbits around the earth, but because the gravity of the earth and the centrifugal force generated by the orbit of the ISS are balanced, it is able to orbit in a stable orbit. In other words, even if the ISS is seemingly weightless, the gravity of the earth exists. Therefore, at least within the ISS, it can be said that the concept of top and bottom exists.
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“The fact that gravity exists on the ISS is one of the most exciting things about physics,” Curtis said. ' may be a vague term, but in physics you can always come up with a definition that works.'
In the first place, about what gravity is, Albert Einstein, known for the theory of relativity, expresses it as 'a distortion of the cloth called space-time.' For example, imagine taut bed sheets. If you put a bowling ball on top of this sheet, the weight of it will cause the sheet to dent. And when you put a marble on this sheet, the marble rolls towards the dent in the sheet. Being 'pulled by gravity' is exactly like a marble rolling into the dent of a bowling ball.
Any object with mass distorts the space-time continuum. And the greater the mass of the object, the deeper the distortion of the space-time continuum and the greater the gravitational force. Mr. Curtis explained, ``On the ISS, we can define up and down based on the gravity of the earth,'' but in outer space other than the ISS, the concept of up and down defined by gravity does not apply.
For example, the ISS is being pulled toward the Earth, but at the same time the Earth itself is being pulled towards the center of the Sun. Also, the solar system is part of the Milky Way, which is attracted to a massive black hole at its center. And the galaxy itself is strongly tugged on by other galaxies. Even if you say 'universe' in a nutshell, the gravity source that forms the upper and lower standards differs depending on what level you look at.
Gravity itself is explained by Einstein's general theory of relativity, but there are some points that are incompatible with modern elementary particle physics theory, and there are many things that have not been clarified yet. So while defining top and bottom by gravity can help us understand the space around us, it can also hinder our understanding of fundamental physics.
Fermilab particle physicist Jessica Eskivel said, 'One of the most difficult things in my job is trying to think outside the dichotomy of particle physics and general relativity. It's about imagining a space that has no up or down, no front or back, no past or present.It's really hard, but it's also the most fun part of the job.'
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