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physics.subha.peperonity.net

TWENTIETH-CENTURY PHYSICS

by subhankar karmakar

Physics is the branch of science that tries to explain the physical phenomena occurring all around us. As a matter of fact Physics is the knowledge by which we try to explain our Universe too in a great way. The main impetus of the modern day physics had been accelerated due to the path breaking theories of motion proposed by Issac Newton.

As Newtonian Mechanics were more than able to predict about the dynamics of a rigid body and essentially a macrobody, treating mass as a continuous physical quantity, in all practical purposes theory was successful enough. As this theory was mainly depended upon the classical concept of mass as a continuous quantity it is named as Classical Physics.

The three great strands of twentieth-century physics were special relativity, general relativity, and quantum physics.

Albert Einstein was the central developer of both special and general relativity, and he also developed important aspects of quantum physics. Time magazine named Albert Einstein the man of the century.

Twentieth-century physics was important to the twentieth century.

THEORY OF SPECIAL RELATIVITY:

Lonnie is the stay-at-home type, but her twin sister Bonnie is an astronaut. At age 20, Lonnie starts to raise a family while Bonnie goes to visit the planets in a nearby star system.

It’s a long trip for Bonnie and her crewmates, but this trip is taking place with advanced technology allowing travel at very high speeds. Great discoveries are made and, after the long voyage home, Bonnie returns just at her 30th birthday.

Lonnie is 70 years old and greets the returning astronauts at Cape Canaveral with her grandchildren.

Huh?

Lonnie is 70 and Bonnie is 30.

Moving clocks run slow.

This is a fact. A physical reality.

Whether Bonnie is 30 or 31 or 69 or 69.99999 years old when she returns—this is a detail that depends on Bonnie’s exact speed on her voyage.

But Bonnie will be younger than Lonnie when she returns. All contemporary scientists agree with this reality. It’s how the universe works.

Why we don’t notice in our everyday lives that this is how the universe works is because we on earth travel very slowly.

Bonnie would have to travel at over 650 million miles per hour in order for the age difference to be 40 years. She would have to travel over 90 million miles per hour for there to be even a 1% difference in aging.

But just the concept—the idea that Bonnie’s speed has anything to do with how fast time travels for her—sounds absurd.

Perhaps more absurd-sounding is the idea that time is not universal. Time does not flow like a smooth river, second by second, the same everywhere. Time is a local phenomenon whose passage varies with how fast we travel.

There is nothing about this that matches anything from how we experience our everyday lives, but nevertheless it’s true. It’s completely imperceptible to us because we don’t experience speeds of 650 million or even 90 million miles per hour, so we have no intuition for this. We may safely be ignorant of this fact, yet function perfectly normally for our entire lives.

Part of Einstein’s brilliance is evidenced by his being able to draw these conclusions about special relativity simply by thinking about the implications of the speed of light being the same for all travelers. When I’m moving away from an object, the light waves emitted from this object will inevitably be more spread out for me than for someone not moving away from the object, due to my motion away from the object. But because the speed of light is invariable, it is time itself that must spread out.

There’s more to special relativity, of course, and you don’t get college credit for reading these few pages. For example, in addition to time passing more slowly for a fast-moving traveler, length contracts and mass increases. But for our brief overview, we’ll just accept that, if our normal lives took place at 650 million miles per hour, or if we routinely dealt with distances of intergalactic magnitude, we would never have created for ourselves the conceptualizations that we have of distance, time, space, and mass. Our everyday concepts don’t work for scientists who work with the fast-moving particles of the subatomic world, and they don’t work for scientists dealing with the vast distances of the universe. For these scientists, the adjustments developed through special relativity must be made, because special relativity is reality.

THEORY OF GENERAL RELATIVITY:

It gets weirder. If you think of planets as giant masses revolving around the mass of the sun, held in place by gravitational force, you’ve got it wrong.

What’s actually happening is that the sun reshapes space. Gravitational waves emanate from the sun and change the shape of space around it. For millions of miles around the sun, space is not shaped with a north-south dimension at ninety degrees to an east-west dimension at ninety degrees to a vertical dimension. Space is curved. And the planets float effortlessly, taking the path of least resistance through this curved space.

Now this may actually sound to you like a distinction without a difference. After all, what’s the practical difference between a universe in which planets are held in place by gravitational force, and a universe in which the force of gravity is propelled at light speed via gravitational waves which reshape space?

Not much, it turns out. But there are differences, and they were first observed by precise measurements of perturbations in the orbit of the planet Mercury about the sun, and by similarly precise measurements involving phenomena that can be observed only during a solar eclipse, when measurements matched general relativity’s predictions for the bending of light. Only after many decades of additional observation has near-unanimity been reached on the existence of gravitational waves.

Again, no college credit for this summary description. But the important point for this book’s perspective is that—to understand the universe—we have more strange and counter-intuitive concepts to absorb, for example a concept that the force of gravity is propelled in waves throughout the universe, reshaping space and time, and setting up the motions and interactions among the planets, the galaxies, all matter.

This understanding will not help you in your everyday life. In fact, your day will go just fine if you don’t make any adjustments at all for either general relativity or special relativity. This is because we obtain only a very refined degree of additional accuracy by introducing relativity’s corrections: Isaac Newton’s seventeenth-century classical physics is good enough to point you in the right direction to get to work or to the grocery store, and you will not have to be concerned with the very small discrepancy that has been created between your wristwatch and those of the people you encounter. But relativity’s adjustments do make our measurements of time and space just a bit more accurate, and perhaps more importantly they give us a truer understanding of how the universe works.

UNIFYING THE FUNDAMENTAL FORCES:

Throughout the twentieth century, physicists have been obsessed with the creation of a unified force theory, and it is gravity that has proven the most troublesome force to unify with the other forces.

Four forces exist in the universe—the electromagnetic force, the weak and strong forces that operate within the structure of atoms, and the force of gravity.

Back in the nineteenth century, physicists had an even earlier success at this, unifying the force of electricity and the force of magnetism, by showing that these two forces are actually the "same" force, the electromagnetic force.

Now this is odd, because common sense—our common basis for understanding the universe—tells us that electricity and magnetism are not the same thing. They’re different. One goes through wires, and the other involves magnets and iron filings.

But the point is that we’re just not understanding the universe correctly if we continue to think that electricity and magnetism are different forces.

The physicist’s perspective is that electricity and magnetism are best understood as dual aspects of a single phenomenon, modeled mathematically as interacting fields, moving in tandem, electricity producing magnetism, and magnetism producing electricity.

With electricity and magnetism unified, physicists focused on unifying electromagnetism and gravity. But then two additional forces were identified within the physics of atomic particles—the strong force and the weak force—increasing from two to four the number of basic interactions of the universe that physicists are now challenged to unify.

So why the obsession? What’s so important about unifying the forces, creating a physical framework in which there is only one force?

BIG BANG: THE GREAT EXPLOSION

If the universe began as a minuscule point that has been expanding for fourteen billion years—well, what was in that minuscule point? There couldn’t have been room in there for four forces (so the thinking goes, loosely expressed). There must have been just one force, whose manifestations varied (especially as experienced through our simple senses) as the universe expanded (and cooled). So let’s figure out how it can be that these four forces can all be understood as one. Let’s move backward in time to the big bang.

Now you may not be convinced that the only conclusion we can draw from the fact of the big bang is that there exists only one, unified force. You may not even be convinced of the fact of the big bang. But the fact is that, as the twentieth century ended, physicists had come very close to achieving a consensus view—within the standard model of particle physics—on how it is that all the forces except gravity are unified. This consensus view is called the Grand Unified Theory, and we’ll be discussing in more depth the mainstream view of the extent of Grand Unification. It’s certainly been a great achievement of physics and mathematics to have gotten so far during the past century.

You may be thinking: aren’t physicists getting a bit carried away here, in calling Grand Unification something that seems more properly called “Three-Quarters Unification”? After all, “Grand Unification” is unifying only three of the four known forces.

Maybe you’re right if this is what you’re thinking. But if so, physicists have paid a price for this overstatement: what will they call it when gravity too is brought into the unification? Their answer to that is the “Theory of Everything.” That’s what physicists are striving for as they work to unify all of the forces, ...
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