# Question concernig E=MC^2

#### whatzzupboy

What do the variables M and C reffer to, in the equastion E=MC^2?

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#### Concorde

C stands for the speed of light I think and I know M stands for mass.

#### Pengwuino

Gold Member
m = relativistic mass of the particle in question
c = the speed of light in a vacuum, ~300,000km/s

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#### whatzzupboy

i thought the speed of light was 299,292.6Km/S

#### irrehaare

i thought that it depend on the environment where is the light, so the velocity of light in air is different than in water or glass . see more about diffraction of light

#### whatzzupboy

ok well im basing the equastion out side the universe so... what then is C?

#### Pengwuino

Gold Member
whatzzupboy said:
ok well im basing the equastion out side the universe so... what then is C?
C is the speed of light in a vacuum

#### Pengwuino

Gold Member
irrehaare said:
i thought that it depend on the environment where is the light, so the velocity of light in air is different than in water or glass . see more about diffraction of light
Well the speed of light can change, but as far as the formula goes, you are able to determine the energy based off of its maximum speed i think. Since you can make it so that there's no actual light flying around in an isolated nuclear reaction, theres no speed to base a change in c off of.

#### whozum

whatzzupboy said:
ok well im basing the equastion out side the universe so... what then is C?
Outside the universe? Wheres that? Why would there be light there?

#### Pengwuino

Gold Member
whozum said:
Outside the universe? Wheres that? Why would there be light there?
And what matter would there be "outside the universe"???

#### HallsofIvy

Homework Helper
whatzzupboy said:
ok well im basing the equastion out side the universe so... what then is C?
You've been told what c is several times now. (And that "out side the universe" makes no sense.)

Pengwino said:
c = the speed of light in a vacuum, ~300,000km/s
Did you mean "outside the atmosphere" (i.e. in vacuum) rather than "out side the universe"?

whatzzupboy said:
i thought the speed of light was 299,292.6Km/S
Do you know what "~" means?

#### xFlankerx

HallsofIvy said:
You've been told what c is several times now. (And that "out side the universe" makes no sense.)

Did you mean "outside the atmosphere" (i.e. in vacuum) rather than "out side the universe"?

Do you know what "~" means?
Owned. But I thought the "C" was the speed of light in meters which would make it ~300,000,000 m/s. What difference does it make? In multipling, you would be multipling by 300,000,000 instead of 300,000.

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#### quasar987

Homework Helper
Gold Member
I suggest you keep you "owned" and "pwned" for yourself. Please.

Btw, I'm not sure I understand your post. When you say "What difference does it make?", are you saking yourself a question and then answering it, or are you really asking that question? Cuz my answer would be: "In so far as every unit system is as good as any other, it makes no difference wheter you take 'c' to be 300,000 km/s, 300,000,000 m/s or 1 M/s, where I have define the lengh 1M to be equivalent to 300,000,000 m (I heard this is what particle physicists use as the unit of lenght in their calculations)

#### Pengwuino

Gold Member
xFlankerx said:
Owned. But I thought the "C" was the speed of light in meters which would make it ~300,000,000 m/s. What difference does it make? In multipling, you would be multipling by 300,000,000 instead of 300,000.
C can be done in any length. Heck it can be done in feet or centimeters or earths!

#### Dr.Brain

$E=mc^2$ is basically an idea that when 'm' mass is annihilated , a radiation carrying energy 'E' will be radiated , signifying the idea that mass is a sort of condensed form of energy . In this equation 'c' is the light speed in vacuum , though light speed varies with the medium apparently , the photons that make up the light always move at this constant 'c'.

BJ

#### HallsofIvy

Homework Helper
xFlankerx said:
Owned. But I thought the "C" was the speed of light in meters which would make it ~300,000,000 m/s. What difference does it make? In multipling, you would be multipling by 300,000,000 instead of 300,000.
Oh, dear! I completely missed the "missing" 000!!

#### xFlankerx

quasar987 said:
I suggest you keep you "owned" and "pwned" for yourself. Please.

Btw, I'm not sure I understand your post. When you say "What difference does it make?", are you saking yourself a question and then answering it, or are you really asking that question? Cuz my answer would be: "In so far as every unit system is as good as any other, it makes no difference wheter you take 'c' to be 300,000 km/s, 300,000,000 m/s or 1 M/s, where I have define the lengh 1M to be equivalent to 300,000,000 m (I heard this is what particle physicists use as the unit of lenght in their calculations)
Sorry, its just a habit. I'll try to keep it to myself from now on. And I was asking the question in anticipation of someone else asking it, so I answered it as I thought fit in the next sentence.

HallsofIvy said:
Oh, dear! I completely missed the "missing" 000!!
Sarcasm or not? I can't tell.

#### Pengwuino

Gold Member
How can people think that 300,000km/s is different then 300,000,000 m/s?

#### h_k331

whatzzupboy said:
i thought the speed of light was 299,292.6Km/S
To be exact, the speed of light in a vacuum is actually 299,792.458 km/s.

hk

#### pmb_phy

irrehaare said:
i thought that it depend on the environment where is the light, so the velocity of light in air is different than in water or glass . see more about diffraction of light
If the object in question is a particle then the E in the relation $E = mc^2$ is the inertial energy of the particle. c is the speed of like in a vacuum and has the value c = 299,792,456.2 m/s. m is the mass of the particle (and yelling will ensue if I don't point out that I'm speaking of relativistic mass). If the object is not a particle but a closed system of free particles or it is an object of finite size which is isolated in space then the same thing applies.

Pete

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