## Saturday, September 26, 2015

### How Does Gravity Work?

How Does Gravity Work?
Multiple Responses:
1.
Every time you jump, you experience gravity. It pulls you back down to the ground. Without gravity, you'd float off into the atmosphere -- along with all of the other matter on Earth.

You see gravity at work any time you drop a book, step on a scale or toss a ball up into the air. It's such a constant presence in our lives, we seldom marvel at the mystery of it -- but even with several well-received theories out there attempting to explain why a book falls to the ground (and at the same rate as a pebble or a couch, at that), they're still just theories. The mystery of gravity's pull is pretty much intact.

So what do we know about gravity? We know that it causes any two objects in the universe to be drawn to one another. We know that gravity assisted in forming the universe, that it keeps the moon in orbit around the Earth, and that it can be harnessed for more mundane applications like gravity-powered motors or gravity-powered lamps.

As for the science behind the action, we know that Isaac Newton defined gravity as a force -- one that attracts all objects to all other objects. We know that Albert Einstein said gravity is a result of the curvature of space-time. These two theories are the most common and widely held (if somewhat incomplete) explanations of gravity.

In this article, we'll look at Newton's theory of gravity, Einstein's theory of gravity and we'll touch on a more recent view of the phenomenon as well.

Although many people had already noted that gravity exists, Newton was the first to develop a cohesive explanation for gravity, so we'll start there.

Newton's Gravity
In the 1600s, an English physicist and mathematician named Isaac Newton was sitting under an apple tree -- or so the legend tells us. Apparently, an apple fell on his head, and he started wondering why the apple was attracted to the ground in the first place.

Newton publicized his Theory of Universal Gravitation in the 1680s. It basically set forth the idea that gravity was a predictable force that acts on all matter in the universe, and is a function of both mass and distance. The theory states that each particle of matter attracts every other particle (for instance, the particles of "Earth" and the particles of "you") with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.

So the farther apart the particles are, and/or the less massive the particles, the less the gravitational force.

The standard formula for the law of gravitation goes [source: UT]:
Gravitational force = (G * m1 * m2) / (d2)
Gravitational force = (G * m1 * m2) / (d2)
where G is the gravitational constant, m1 and m2 are the masses of the two objects for which you are calculating the force, and d is the distance between the centers of gravity of the two masses.

G has the value of 6.67 x 10E-8 dyne * cm2/gm2. So if you put two 1-gram objects 1 centimeter apart from one another, they will attract each other with the force of 6.67 x 10E-8 dyne. A dyne is equal to about 0.001 gram weight, meaning that if you have a dyne of force available, it can lift 0.001 grams in Earth's gravitational field. So 6.67 x 10E-8 dyne is a miniscule force.

When you deal with massive bodies like the Earth, however, which has a mass of 6E+24 kilograms, it adds up to a rather powerful gravitational force. That's why you're not floating around in space right now.

The force of gravity acting on an object is also that object's weight. When you step on a scale, the scale reads how much gravity is acting on your body. The formula to determine weight is [source: Kurtus]:
weight = m * g
where m is an object's mass, and g is the acceleration due to gravity. Acceleration due to gravity on Earth, is 9.8 m/s² -- it never changes, regardless of an object's mass. That's why if you were to drop a pebble, a book and a couch off a roof, they'd hit the ground at the same time.

For hundreds of years, Newton's theory of gravity pretty much stood alone in the scientific community. That changed in the early 1900s.

Einstein's Gravity
Albert Einstein, who won the Nobel Prize in Physics in 1921, contributed an alternate theory of gravity in the early 1900s. It was part of his famous General Theory of Relativity, and it offered a very different explanation from Newton's Law of Universal Gravitation. Einstein didn't believe gravity was a force at all; he said it was a distortion in the shape of space-time, otherwise known as "the fourth dimension" (see How Special Relativity Works to learn about space-time).

Basic physics states that if there are no external forces at work, an object will always travel in the straightest possible line. Accordingly, without an external force, two objects travelling along parallel paths will always remain parallel. They will never meet.

But the fact is, they do meet. Particles that start off on parallel paths sometimes end up colliding. Newton's theory says this can occur because of gravity, a force attracting those objects to one another or to a single, third object. Einstein also says this occurs due to gravity -- but in his theory, gravity is not a force. It's a curve in space-time.

According to Einstein, those objects are still travelling along the straightest possible line, but due to a distortion in space-time, the straightest possible line is now along a spherical path. So two objects that were moving along a flat plane are now moving along a spherical plane. And two straight paths along that sphere end in a single point.

Still more-recent theories of gravity express the phenomenon in terms of particles and waves. One view states that particles called gravitons cause objects to be attracted to one another. Gravitons have never actually been observed, though. And neither have gravitational waves, sometimes called gravitational radiation, which supposedly are generated when an object is accelerated by an external force [source:Scientific American].

Gravitons or no gravitons, we know that what goes up must come down. Perhaps someday, we'll know exactly why. But until then, we can be satisfied just knowing that planet Earth won't go hurdling into the sun anytime soon. Gravity is keeping it safely in orbit.

2.
If all other forces of nature have some particles associated with them why should gravity be an exception?
No, it isn't an exception. Physicists believe that they do appear, it's just they haven't found it yet. Standard Model doesn't have gravity, but extended Standard Model may contain it. Thanks to string theory.

What exactly is the gravitational field and how does it spread over an infinite distance and cause the gravitational force to operate?
It is exactly the space and the time. How do space and time appear? Big Bang. How does gravity operate? A change in space and time give you a gravitation force. Like a change in position gives you velocity, a change in energy gives you a work. A change is very important, it will give you another interesting point. If you have studied differential, you now know how important it is: describing a change.

To real physicists: I am talking with a high school student.

Edit: By referring a change in space and time, I don't mean like a car, travels through cities from morning to afternoon. It's like reforming it.

3.
Because of gravity, if you drop something, it falls down, instead of up. Well, everybody knows that! But, what does this really mean? What is gravity?

Gravity has played a big part in making the universe the way it is. Gravity is what makes pieces of matter clump together into planets, moons, and stars. Gravity is what makes the planets orbit the stars--like Earth orbits our star, the Sun. Gravity is what makes the stars clump together in huge, swirling galaxies.

A great scientist, Albert Einstein, who lived in the 20th century, had a new idea about gravity. He thought that gravity is what happens when space itself is curved or warped around a mass, such as a star or a planet. Thus, a star or planet would cause kind of a dip in space so that any other object that came too near would tend to fall into the dip.

Quite a number of experiments show that Einstein was right about this idea and a lot of others. But there are questions for which even Einstein had no answers.
For example, if gravity is a force that causes all matter to be attracted to all other matter, why are atoms mostly empty space inside? (There is really hardly any actual matter in an atom!) How are the forces that hold atoms together different from gravity? Is it possible that all the forces we see at work in nature are really different sides of the same basic force or structure

Could some of the same laws of nature be at work in the designs of all things in the pictures above?

These are big questions that scientists and ordinary people like us have wondered about for a long time. For a long time we haven't known how to go about finding the answers, other than trying to work things out on paper.

But now NASA has a special program, called…

FUNDAMENTAL PHYSICS

. . . for seeking answers to these and other mysteries of the universe. Fundamental Physics hopes to do two things:
• To discover and explore fundamental physical laws governing matter, space, and time.
• To discover and understand the basic rules nature uses to build the complex and beautiful structures we see around us.

Over the years, scientists and engineers have developed new technologies and instruments that will help us understand nature. Now we can take these new instruments into space and do experiments where the forces of gravity are very, very small (like when the Space Shuttle or the International Space Station are orbiting Earth in "free fall"). This way, scientists can do very delicate experiments to see what single atoms do under special conditions.

NASA hopes these experiments will help us understand our universe and ourselves. NASA also hopes the experiments will help develop technologies that will benefit people in their everyday lives.

4.
Gravity. The average person probably doesn’t think about it on a daily basis, but yet gravity affects our every move. Because of gravity, we fall down (not up), objects crash to the floor, and we don’t go flying off into space when we jump in the air. The old adage, “everything that goes up must come down” makes perfect sense to everyone because from the day we are born, we are seemingly bound to Earth’s surface due to this all-pervasive invisible force.

But physicists think about gravity all the time. To them, gravity is one of the mysteries to be solved in order to get a complete understanding of how the Universe works.

So, what is gravity and where does it come from?
To be honest, we’re not entirely sure.

We know from Isaac Newton and his law of gravitation that any two objects in the Universe exert a force of attraction on each other. This relationship is based on the mass of the two objects and the distance between them. The greater the mass of the two objects and the shorter the distance between them, the stronger the pull of the gravitational forces they exert on each other.

We also know that gravity can work in a complex system with several objects. For example, in our own Solar System, not only does the Sun exert gravity on all the planets, keeping them in their orbits, but each planet exerts a force of gravity on the Sun, as well as all the other planets, too, all to varying degrees based on the mass and distance between the bodies. And it goes beyond just our Solar System, as actually, every object that has mass in the Universe attracts every other object that has mass — again, all to varying degrees based on mass and distance.

With his theory of relativity, Albert Einstein explained how gravity is more than just a force: it is a curvature in the space-time continuum. That sounds like something straight out of science fiction, but simply put, the mass of an object causes the space around it to essentially bend and curve. This is often portrayed as a heavy ball sitting on a rubber sheet, and other smaller balls fall in towards the heavier object because the rubber sheet is warped from the heavy ball’s weight.

In reality, we can’t see curvature of space directly, but we can detect it in the motions of objects. Any object ‘caught’ in another celestial body’s gravity is affected because the space it is moving through is curved toward that object. It is similar to the way a coin would spiral down one of those penny slot cyclone machines you see at tourist shops, or the way bicycles spiral around a velodrome.

We can also see the effects of gravity on light in a phenomenon called gravitational lensing. If an object in space is massive enough – such as a large galaxy or cluster of galaxies — it can cause an otherwise straight beam of light to curve around it, creating a lensing effect.

But these effects – where there are basically curves, hills and valleys in space — occur for reasons we can’t fully really explain. Besides being a characteristic of space, gravity is also a force (but it is the weakest of the four forces), and it might be a particle, too. Some scientists have proposed particles called gravitons cause objects to be attracted to one another. But gravitons have never actually been observed. Another idea is that gravitational waves are generated when an object is accelerated by an external force, but these waves have never been directly detected, either.

Our understanding of gravity breaks down at both the very small and the very big: at the level of atoms and molecules, gravity just stops working. And we can’t describe the insides of black holes and the moment of the Big Bang without the math completely falling apart.

The problem is that our understanding of both particle physics and the geometry of gravity is incomplete.

“Having gone from basically philosophical understandings of why things fall to mathematical descriptions of how things accelerate down inclines from Galileo, to Kepler’s equations describing planetary motion to Newton’s formulation of the Laws of Physics, to Einstein’s formulations of relativity, we’ve been building and building a more comprehensive view of gravity. But we’re still not complete,” said Dr. Pamela Gay. “We know that there still needs to be some way to unite quantum mechanics and gravity and actually be able to write down equations that describe the centers of black holes and the earliest moments of the Universe. But we’re not there yet.”

And so, the mystery remains … for now.