**Metric expansion of space and The Big Bang Theory**

General Relativity brought about the discovery that objects in our universe are actually moving away from each other. While this effect is predicted by the equations of general relativity themselves, it was Hubble who first observed that this effect does indeed occur. The development of Hubble’s Law states that objects observed in deep space are moving away from Earth at relative velocity. Through his observations, Hubble determined the rate of expansion of our universe. The realization that our universe is expanding serves as evidence for one of the biggest theories of universal origin.

The Big Band Theory came about as a direct result of the realization that the Universe was expanding due to the fact that objects were moving at a certain velocity away from each other. The Big Bang Theory is currently the prevailing cosmological model of how our universe came about. Thinking about the metric expansion of the universe, we notice that if objects are distant because they are moving away from each other, then at some point these objects must have been close to each other. This theory suggests that, originally, the universe was a lot denser and hotter in the very distant past; eventually, it grew so hot and dense that a massive explosion occurred, causing objects to shoot out in all directions.

Watch the following videos that introduce the Big Bang Theory:

https://www.youtube.com/watch?v=q8lNooOiK1g

https://www.youtube.com/watch?v=hVApTLE7Csc

https://www.youtube.com/watch?v=uabNtlLfYyU

https://www.youtube.com/watch?v=-Ip5BAEfZuA

**Time Travel**

As has been discussed in Physics Learning Lesson 5, we know that time dilation is a result of relativity. While currently there is no way to prove that time travelling to the past is possible, Einstein’s Theory of Relativity easily allows for time travel into the future. Using atomic clocks, physicists have shown that time moves faster at higher elevations. The same thing occurs when you move at speeds close to the speed of light. So why is this happening? It is because space and time, are actually one and the same. The concept of space-time is a very important result of the Theory of Relativity. The faster you move, the slow time will move for you. In regards to space, consider the fact that there is less gravity acting on you the further away from the Earth you are, as such, when you are at really high altitudes, gravity’s effect on you is lower. This in turn, leads the passage of time to be faster, since there is less of a force from gravity acting on your body. Time travel into the future is possible mainly due to the constant speed behavior of light. One of the theorized means of time travel involves wormholes. Wormholes are objects that are connected to two different points in space-time, thus allowing you to time travel between those two points.

Watch the following videos that discuss Time travel:

https://www.youtube.com/watch?v=Y3ZLg0GPCG8

https://www.youtube.com/watch?v=FflcA85zcOM

https://www.youtube.com/watch?v=X02WMNoHSm8

https://www.youtube.com/watch?v=n_3d7O619tA

**Black Holes**

In Physics Learning Lesson 5, we mentioned that an important differentiator between Special and General relativity was the fact that gravity and acceleration is the same thing. Large enough masses bend space-time, and this effect is what we refer to as gravity. So what happens when a mass is so enormous that it just collapses onto itself? This is what is called a Black Hole, an object so massive that causes gravity to be so high that not even light, which is the fastest object according to relativity, can escape it. The concept is similar to how rockets need propulsion to escape the Earth’s gravity; if the Earth was bigger, it would bend space-time more, resulting in more gravity and more propulsion being needed for a rocket to escape. One of the effects that black holes have on our universe is the fact that they show that mass can be hidden; after all, since light cannot escape black holes, it is currently impossible to see what resides within it.

Watch the following videos that discuss Black holes:

https://www.youtube.com/watch?v=_VStBw95K34

https://www.youtube.com/watch?v=VC2XYNqZVEo

https://www.youtube.com/watch?v=PbvvRATFGB0

https://www.youtube.com/watch?v=mWm8fmkTsUo

https://www.youtube.com/watch?v=ZvAsvLjEdIo

**Gravitational Waves**

Similar to Black Holes, gravitational waves are an effect that comes as direct result to general relativity’s space-time concept. A gravitational wave is what we refer to as the after-effect of the “bending” of space-time brought about by the object’s mass. Gravitational waves can be compared to the ripple effect that occurs when you throw a rock into the water; the bending of the space-time is the splash, whereas the gravitational waves are the ripples that follow the splash. Indeed, just like water ripples, gravitational waves become weaker the further they travel. The importance of gravitational waves lies in the fact that they are produced through the interaction of two or more masses; as such, they can be used as a means to detect the creation of black holes and the merger of galaxies and other celestial bodies.

Watch the following videos that discuss gravitational waves:

https://www.youtube.com/watch?v=rJ4U2VdKlW0

https://www.youtube.com/watch?v=v1tkM_f5B9s

https://www.youtube.com/watch?v=y7-_Bi8aSxY

**Atomic Energy**

Einstein’s (and perhaps in all physics) most famous equation, E = mc^{2}, describes the mass-energy equivalence. If you pay attention to the equation, you’ll notice that c^{2} refers to square of the speed of light, which is an enormous number equaling 449,726,663,091,323,641 miles per hour. This equation suggests that very small masses can release an absurdly enormous amount of energy. This equation gave birth to the concept of atomic energy and, along with it, the development of nuclear energy and weapons. The wide array of applications that atomic energy has lent itself to ranges from X-rays to nanotechnology. The concept of nuclear weapons involves concept of nuclear fission/fusion. In nuclear fission, the nucleus of an atom splits into smaller parts. Nuclear fusion is a nuclear reaction in which two or more atomic nuclei collide at a very high speed and join to form a new type of atomic nucleus. Nuclear fusion is process that the Sun uses to generate its energy. The energy released by these processes, as per the mass-energy equivalence, is enormous.

Watch the following videos that explain atomic energy:

https://www.youtube.com/watch?v=hW7DW9NIO9M

https://www.youtube.com/watch?v=4HgvPBAOea8

https://www.youtube.com/watch?v=rK67Dj1jyc8

https://www.youtube.com/watch?v=fIbACkLU-38

**Quantum Physics**

Quantum Mechanics was a necessity born out of the fact that Classical/Newtonian Mechanics failed to explain the physical phenomena at very small scales (often referred to in QM as Planck Scale). While Albert Einstein did not believe in Quantum Physics/Mechanics, his Theory of General Relativity cemented the pavement for its development. Einstein proved that Planck’s hypothesis, which said that light was not a wave but a stream of quantified particles, was correct. Einstein proved this by showing that light energy striking a metal surface releases a flurry of electrons. Unfortunately, as it stands, Quantum Mechanics and General Relativity are not compatible theories and are often in conflict with one another on various points due to gravitation not fitting in with any of the other fundamental forces. Quantum mechanics provides a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. Quantum mechanics provides a substantially useful framework for many features of the modern periodic table of elements including the behavior of atoms during chemical bonding and has played a significant role in the development of many modern technologies. Quantum physics theory has provided us with the insight on developing technology such as microchips, USB drives and computers.

Watch the following videos that introduce some concepts of Quantum Mechanics

https://www.youtube.com/watch?v=45KGS1Ro-sc

https://www.youtube.com/watch?v=9up_NkrhL9E

https://www.youtube.com/watch?v=t2mjoI7bgXM

**Comparison between Newtonian Physics and Relativity**

*I. When are Newtonian Mechanics good enough?*

We can use Newtonian Mechanics without concern when we are dealing with the motion of macroscopic objects, from projectiles to parts of machinery, as well as astronomical objects, such as spacecraft, planets, stars, and galaxies. This also provides extremely accurate results as long as the domain of study is restricted to large objects and the speeds involved do not approach the speed of light. Newtonian Mechanics follow the concept of Galilean transformations, and can only be applied when the physics involved concern only inertial frames of reference, that is, frames of reference that are not under the effects of acceleration.

*II. In what situations do we use Relativity?*

In contrast to Newtonian Mechanics, we make use of relativity when the objects in question are going at speeds close to the speed of light. In addition, as mentioned above, we can deal with non-inertial frames of reference thanks to the switch from Galilean transformations to Lorentz transformations. Due to the fact that space and time are treated as a single space-time, a lot of the things that are related to gravity can be calculated through relativity.

*III. Limitations of the Theory of Relativity*

Currently General Relativity might have some limitations that are far too complex to discuss it with you now. As mentioned in the blurb of quantum mechanics, these two theories conflict under very specific conditions. The Theory of Unification, which attempts to join all fundamental forces, is currently stuck on how to add gravitation into harmony with the fundamental forces. In quantum mechanics, gravitation would not work the way it is predicted to in General Relativity. Unfortunately, it is currently incompatible.