1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

I want to understand relativity

  1. Mar 14, 2004 #1
    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.
  2. jcsd
  3. Mar 14, 2004 #2
    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.

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?