What is relativity? What does E=MC2 actually mean?

Question

It's a very famous concept that was drawn up nigh on 100 years ago and I have no idea what it means or why I should.
Seems the original theory of Special relativity said that if two people are standing looking at each other from across the gulf of space then they cannot say if the other person is moving or not.
Seems reasonable, the gulf of space is a long way to look for somebody.
Then Einstien said that planets etc cause dents in spacetime relative to their weight or gravity or something, and that all things are subject to the same laws of physics no matter what speed they are moving at, and something about the speed of light in a vacuum.
I still don't understand any of this.

Answer 1

I recommend you take a look at some of the sources I link below. In particular, see the "introduction" articles. There are some good pictures and demonstrations that may help you.

Special relativity is a direct consequence of the observation that light's speed is constant in any reference frame. Classical mechanics suggests that light should appear faster moving toward a light source and slower moving away. However, observations that were constructed to measure that effect showed that light's speed was always constant regardless of how fast the source or observer was traveling. It could not be made slower or faster due to movement. However, something must change when things are moving, so Einstein showed that moving objects are perceived with shorter lengths and slower times.

In other words, if the light cannot "slow down" when an object is traveling away from you, information about the object will get to you "quicker" and so you will perceive its distance away from you as being shorter and the time as moving slower.

These observations have an effect on momentum, and the mathematical effect on momentum leads to the expression that E=mc^2, which really means that the rest mass of any object has energy. It shows that matter is just another form of energy.

Now, what's really significant about all of this is that time is shown to be flexible, just like the other dimensions. In other words, we don't live in a three dimensional world of (x,y,z). We live in a four dimensional world of (x,y,z,t). We're always moving in the "time" direction through this 4 dimensional universe. If we start moving in one of the other directions, we trade some of the speed we're moving in the time direction. In other words, time ticks slower for moving objects because their motion borrows speed from the clock. Eventually, objects traveling at the speed of light tick off no time because all speed has been diverted to the spatial velocities.

General relativity extends this idea. It says that if we live in a 4 dimensional universe, matter deforms that universe and forces us to travel in curved directions. Remember that we're always moving in the "time" direction through spacetime. Without gravity, we'd be traveling in a straight line. However, gravity curves space and forces us to travel in a curved line even when we're standing still. However, if the universe was not dented and we were moving in curved directions, we would feel an acceleration. Accelerations are represented by curved trajectories through spacetime. This is why any force can be replaced with an equivalent gravitational field and vice-versa -- they both have the same impact on our trajectory through spacetime.

None of this is immediately intuitive. It's natural for it to take a while. Take a look at some of the sources I link below and take your time. These are very novel ideas. It's a completely different way of approaching things. We're taught science through most of our education from a Newtonian point of view. Relativity corrects that view. Try not to think about it in terms of the Newtonian physics you've been taught. Try to think in a completely new and different way.

Answer 2

Relativity is the idea that observations depend on the frame of reference. Some examples:
- The classical relativity principle (Gallilei, Newton): The speed of an object depends on the speed of the observer. If you go by 100 kilometers per hour while bypassing a car that goes by 90 km/h, you see the other car as going 10 km/h backwards.
- The special relativity principle (Einstein, Lorentz): Also the mass of an object, the time between events etc. depends on the object's speed relative to the observer. Also, the classical relativity principle is slightly inaccurate (only important for very high speeds).
- The speed of light is NOT relative (Einstein, Michelson, Morris): No matter who observes a photon in vaccum, they all agree that it moves by 3*10^8 meter per second.

E=mc^2 is not the most interesting part of the theory of relativity but probably got famous because it is so simple. It says that mass and energy is "the same". The constant of proportionality is c^2 or 9*10^16 joule per kilogram.

Answer 3

The equation E=MC2 is not the defining one for the General Theory of Relativity, or Specific Relativity.

Relativity defines the relationship between time, mass and the speed of an objects travel. Roughly translated , light is mass less. Light is going as fast as anything can go, because nothing can have a mass less than 0. As mass is constant (a Kg of sugar always has a mass of 1 kg. This is not to say it always weighs 1 kg) if something is traveling close to light speed, something else has to change.
Speed is in distance per time unit. Therefore as distance traveled per time unit changes and mass does not, Einstien surmised that the time unit changes. This is also tied in with gravity, time space and a lot of other confusing stuff. Read Steven Hawkings a brief history of time for a better explanation.

E=MC2

E = the energy released in a total mass conversion.
M = the mass of the object
C = the speed of light (36with loads of 0's miles per hour)
2 is a function of the C (C2 means CXC).

It means that the amount of energy in an object is its mass x the speed of light x the speed of light (Quite a big number!).

Answer 4

E=MC2 was the famous formula given by a renowned Scientis Albert Einstein.

The formula gives the amount of energy released by the loss of weight of an atom during fission.

For eg : Uranium weight of 235 is broken down into lower weight compounds during fission

E = Energy released
M = Mass of atom lost
C + Speed of light ( i.e. 3lakh km/second )

Answer 5

But it does sound incredibly cool at parties... as does the voice box that Mr Hawkings uses (probably why he keeps attracting women).

I have also never understood it, but enjoyed Disney's The Black Hole. Which I'm sure was based on Einstein's thinking.

If you think I've got too much time on my hands, check out the US Navy's view of this equation...

Answer 6

"A man sits with a pretty girl for an hour and it seems shorter than a minute. But tell that same man to sit on a hot stove for a minute, it is longer than any hour. That's relativity."

Albert Einstein

Answer 7

Check out some of Steven Hawking's work.

Answer 8

I have a headache!
Your question is easy to find on the Web, my pal!