I want to understand relativity

[insane_pete]

HI, i don't understand a lot of things about relativity so I will write the things I am confused about and maybe someone can helop me

1. How can light be forced to go slower than a certain speed if space is empty? To me it looks like light rays are pieces of luggage falling on one of those moving conveyor belts or something. doesn't matter how fast you throw them on it they all end up going the same speed But if a star is moving at 1/2 the speed of light then you could also look at it like the star is not moving and the universe is moving past at 1/2 the speed of light, but if space is empty and doesn't effect light-speed it would still travel away from the star at light spee, so to the rest of us it would travel at 1 1/2 times the speed of light. but it doesn't so there is something that is slowing the light from the star and the force is exactly the same as the amount it's speed goes over the speed limit. a really big coincidence! how does it work?

2. Is it true that gravity can change the wavelength of light but not it's speed?

3. DOesn't light slow down in water thenhow do we know that space is really completely empty and that there are some things that slow it down even just a little?

4. e=mc2 i know kind of what this means but i don't know how it works. mass times the speed of light squared, so it's kind of like the same force of the speed light working against itself times something's mass gives you it's energy. but why does it work? why does the speed of light have anything to do with mass or energy?


5. what are the measurements? if something weighs 10 kilograms then you times it by 300 000 kilometres? and end up with watts or something?
Well, those are my questions. I hope someone knows the answers here.

 

[cookiemonster]

1. I'm going to cheat a little and sidestep the real question, but this might help you a bit anyway.

You're familiar with the distance formula for 3-d space, right? x^2 + y^2 + z^2 = s^2. Well, in order to keep light going constant speed, we're going to have to expand space a little bit and add time into there. So we're no longer talking about only spatial distance in special relativity, we're talking about spatial distance and time "distance". That's why the word "spacetime" is thrown around a bunch. When we throw time in there, we change the distance formula from x^2 + y^2 + z^2 = s^2 to x^2 + y^2 + z^2 - (ct)^2 = s^2, and we say that light can only travel so far in spacetime in a given amount of time. You can see that in order to keep s^2 constant in the new distance equation, if one of the spatial lengths is large (x or y or z or a combination of them), then the time (ct) term becomes small, i.e. time slows down! Can you kind of see how time dilation is going to come into play?

2. Nothing except a medium can change light's speed.

3. I think the best we're going to get is that there is nothing in outer space that light interacts with. So there's no apparent medium in space that could slow light down.

4. The most famous equation in history and most people have no idea what it means! Don't feel bad if you don't yet, that makes you normal!

E = mc^2 relates energy to mass (obviously). It is the amount of energy that if the mass were annihilated (perhaps by encountered some anti-matter) that would be released (in the form of gamma rays, in that case). I guess you could say that it's the mass's energy "value." I'll let someone else try for a more thorough explanation.

5. E = mc^2. E has units (in SI) kg*m^2/s^2 (Joules). m has units kg. c has units m/s, so m^2 has units m^2/s^2. So therefore mc^2 has units kg*m^2/s^2. It works out!

Hope this helps a bit. I know I avoided the whole "why" issue, so sorry about that. If you're lucky, some of the smarter guys on here will throw in their two cents. And if I'm lucky, they won't tell me I came in and said something horribly stupid.

cookiemonster

 

[M98Ranger]

1. The concept of relativity states that the laws of physics are the same for all observers, regardless of their relative motion. This means that the speed of light is constant, and it does not matter if it is observed from a stationary or moving object. However, this does not mean that the speed of light cannot be affected by other factors. In fact, the speed of light can be slowed down by certain mediums, such as water or glass. This is due to the fact that these mediums have a different refractive index, which causes light to travel at a slower speed. So, while space may be empty, it is not completely void of any substance that can affect the speed of light.

2. Yes, it is true that gravity can change the wavelength of light but not its speed. This is because gravity is a force that can bend the path of light, but it does not directly affect the speed of light. This phenomenon is known as gravitational lensing and has been observed in space.

3. Light does indeed slow down in water, but this does not necessarily mean that space is not empty. As mentioned earlier, different mediums can affect the speed of light. In the case of water, it is due to the molecules in the water that can interact with light and cause it to slow down. However, in the vast expanse of space, there is very little matter present, and therefore, the speed of light remains constant.

4. The famous equation, E=mc², is a fundamental principle of relativity. It shows the relationship between energy, mass, and the speed of light. Essentially, it means that mass and energy are interchangeable, and a small amount of mass can produce a large amount of energy. This equation has been proven through various experiments and is a crucial concept in understanding the universe.

5. The units used in the equation E=mc² are kilograms, meters, and seconds. The speed of light, which is represented by 'c', is measured in meters per second. So, when you multiply a mass (measured in kilograms) by the speed of light squared (meters per second), you get energy (measured in Joules). This equation is used to calculate the amount of energy released in nuclear reactions and is essential in understanding the behavior of matter and energy in the universe.

I hope these answers help in your understanding of relativity. It is a complex and fascinating theory, and there is still much to be