- #1

artie

**Speed of Light. What is c? Why use the letter "c'?**

In the equation E=mc2

**E**is energy

**is mass**

m

m

What is

**c**? I know that

**c**is the speed of light, but why use the letter

**c**? What does

**c**stand for?

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- Thread starter artie
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- #1

artie

In the equation E=mc2

m

What is

- #2

nicksauce

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- #3

artie

That's a little strange in this case, because

- #4

Doc Al

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I believe that

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- #5

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- #6

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I believe thatwas chosen forcceleritas, Latin for "speed". (Butfor "constant" works too.) Read: http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/c.html" [Broken]c

^^^^^^^^^^^^

or that haha

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- #7

Mapes

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- #8

artie

I believe thatwas chosen forcceleritas, Latin for "speed". (Butfor "constant" works too.) Read: http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/c.html" [Broken]c

Excellent!

Since Einstein was German, I had tried to find a German word for "light" that begins with

Thanks

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- #9

malawi_glenn

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^{2}and crossed it out, then wrote E=mb^{2}and crossed that out...

that is just a joke.

- #10

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^{2}and crossed it out, then wrote E=mb^{2}and crossed that out...

wow, i thought i saw that one too, but it had different powers for

i guess it's a way to do physics; guess (and see if experiment supports your guess). about

- #11

malawi_glenn

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Hmm we have derived E = mc^2 at school, so I dont think Einstein just "guessed" that solution.. ;-)

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and as i think c^2 is the greatest constant in the physics

i dont know exactly how Eisntein found it...

- #13

malawi_glenn

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Just google "derivation of E=mc^2" or search in textbooks about special relativity

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Just google "derivation of E=mc^2" or search in textbooks about special relativity

i said i dont know how Einstein found this equation....

when E=mc^2 apllicated on the particles...it was right

so the scientists has no reason to change c^2 to c^5345345345

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Just google "derivation of E=mc^2" or search in textbooks about special relativity

my own explanation why E=mc^2

E means Energy, measured by Joule

we all now that F=ma and E=Fr =mar(r is the distance)

so Joule=Newton*Meter=Kg*Meter*sec^-2

for E=mc^2 --> Joule=Kg*(Meter*sec^-2)^2

anybody agree with me??

- #16

malawi_glenn

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my own explanation why E=mc^2

E means Energy, measured by Joule

we all now that F=ma and E=Fr =mar(r is the distance)

so Joule=Newton*Meter=Kg*Meter*sec^-2

for E=mc^2 --> Joule=Kg*(Meter*sec^-2)^2

anybody agree with me??

That is just an argument from units, why couldn't it be: E = 8*mc^2 ?

You must do the full derivation.

And WHY is c^2 the biggest constant in physics?

i) There are formulas which have c^6 .. aren't that a bigger constant?

ii) c is the constant, c^2 = c*c, i.e the constant c is multiplied with another constant c ...

iii) c is not a real constant, it also have a unit: Lenght/time, so c^2 has units lenght^2/time^2

iv) How can we compare e.g G with c? They have different nummerical values, but they have also different units. Also, you can easy come up with a unit system where G has a bigger nummerical value than c. It is like comparing colour with sound.

/Glenn - 1 term from Masters degree in physics.

- #17

HallsofIvy

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That was not original with Einstein. Maxwell had already used "c" for the speed of light in his derivation of the wave equation from his equations for the Electric and Magnetic fields and I suspect it had been used that way before. My understanding is that it is from "celerity" which is Latin for "speed".

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I'm still unclear as to what the question is! You say you know that "E" is "energy" and "m" is "mass" and understand that "c" represents the speed of light. Are you really only askingwhythe letter "c" is used for "speed of light?

That was not original with Einstein. Maxwell had already used "c" for the speed of light in his derivation of the wave equation from his equations for the Electric and Magnetic fields and I suspect it had been used that way before. My understanding is that it is from "celeritas" which is Latin for "speed".

yes that's right

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That is just an argument from units, why couldn't it be: E = 8*mc^2 ?

You must do the full derivation.

And WHY is c^2 the biggest constant in physics?

i) There are formulas which have c^6 .. aren't that a bigger constant?

ii) c is the constant, c^2 = c*c, i.e the constant c is multiplied with another constant c ...

iii) c is not a real constant, it also have a unit: Lenght/time, so c^2 has units lenght^2/time^2

iv) How can we compare e.g G with c? They have different nummerical values, but they have also different units. Also, you can easy come up with a unit system where G has a bigger nummerical value than c. It is like comparing colour with sound.

/Glenn - 1 term from Masters degree in physics.

i said first....im not sure of it...but maybe c^2 is the

biggest physical constant

and tell me about these formulas which have c^6... i hav no idea abt them...

about c is not a real constant?? i didnt understand

...it is a unit that`s right.... so where`s the problem???

c is specified unit (at least in caccum)

- #20

malawi_glenn

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i said first....im not sure of it...but maybe c^2 is the

biggest physical constant

and tell me about these formulas which have c^6... i hav no idea abt them...

about c is not a real constant?? i didnt understand

...it is a unit that`s right.... so where`s the problem???

c is specified unit (at least in caccum)

Can you please spell better?

The thing is that we can only compare things that have equal units. For example 3Joules - 1 Joule = 2 Joule. But what is 3Joules - 5m/s ? And what is biggest between 300 000 000m/s and 1.626*10^-34 J*s ?

Using your argument, why is not h^-1 bigger than c^2? If we only look at the figures, using SI-units, h^-1 = 1.51*10^33 ...

It is only meaningful to compare quantities with the same units. c.f. pi vs. natural number e, or 2000m/s with 10m/s.

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Can you please spell better?

The thing is that we can only compare things that have equal units. For example 3Joules - 1 Joule = 2 Joule. But what is 3Joules - 5m/s ? And what is biggest between 300 000 000m/s and 1.626*10^-34 J*s ?

Using your argument, why is not h^-1 bigger than c^2? If we only look at the figures, using SI-units, h^-1 = 1.51*10^33 ...

It is only meaningful to compare quantities with the same units. c.f. pi vs. natural number e, or 2000m/s with 10m/s.

excuse me.....i learned myself the english

if u know arabic....maybe we can communicate better :D

coz im from Syria

- #22

malawi_glenn

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I hope you understand what I wrote.

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W= T2-T1= integral F.dr

W Work

T kenitic energy

F force

dr deplacement element

this formula is generalisation of the case in classical mechanics

the force is rate of change of momentum in time

the component // to r is

F//=G(v)m(dv/dt)

m rest mass

G Gamma factor = 1/sqrt(1-v²/c²)

then F.dr=F//.dr

and dr=vdt

W= integrale m G(v) v.dv

the bound of velocity 0"rest" and v

W= T2-T1=G(v)mc²-mc²

this is for free particule " no interaction "

like we know in this case E=T=G(v)mc²

rest energy =mc²

in presence of interaction the effect must be take into account in enrgy's formula

- #24

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Can you please spell better?

The thing is that we can only compare things that have equal units. For example 3Joules - 1 Joule = 2 Joule. But what is 3Joules - 5m/s ? And what is biggest between 300 000 000m/s and 1.626*10^-34 J*s ?

Using your argument, why is not h^-1 bigger than c^2? If we only look at the figures, using SI-units, h^-1 = 1.51*10^33 ...

It is only meaningful to compare quantities with the same units. c.f. pi vs. natural number e, or 2000m/s with 10m/s.

I think that i understand your viewpoint

in classiccal mechanics the kenitic energy is T=(1/2)mv²

but if we take the expression of energy in relativity for free particule

E=mc²/sqrt[1-(v²/c²)]

and make a taylor development for v very smaller than c we find

E=mc²+ (1/2)mv²+... =T

u may ask about the origin of mc²

this is not a physical problem because the principle of consevation energy is not broken if we add a constant , because the interising is the amount of change make by the interaction of physical systems in this case representing by the work W= T2-T1 =(1/2)mv2²-(1/2)mv1²

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let's say it's non-relativistic to start with:

work done: [tex] T = \int_0^{x_0} F \ dx [/tex]

the force

in the classical case. we can pull the mass

so this work integral comes out as:

[tex] T = \int_0^{x_0} F \ dx = \int_0^{x_0} \frac{d(m_0 v)}{dt} \ dx [/tex]

with two substitutions of variable (that we learn in calculus):

[tex] T = \int_0^{m_0 v_0} v d(m_0 v) = m_0 \int_0^{v_0} v dv [/tex]

where

[tex] T = m_0 \int_0^{v_0} v dv = \frac{1}{2} m_0 v_0^2 [/tex]

the familiar kinetic energy formula for classic mechanics. we had to do

hey guys, i just realized i gotta get going before i finish this. i'll come back to it tonight. you can kinda see how it will go. in the relativistic case, the mass is not constant, the integral will come out different and the kinetic energy will come out to be

[tex] T = m c^2 - m_0 c^2 [/tex]

where [tex] m = m_0 \frac{1}{\sqrt{1 - \frac{v_0^2}{c^2}}} [/tex]

the relativistic mass. the interpretation of the kinetic energy in the above equation is

[tex] T = m c^2 - m_0 c^2 = E - E_0 [/tex]

where

i'll get back to this later.

- #26

artie

This is the question:

In the equation E=mc2

Eis energy

is mass

m

What isc? I know thatcis the speed of light, but why use the letterc? What doescstand for?

And this is the answer:

I believe thatwas chosen forcceleritas, Latin for "speed". (Butfor "constant" works too.) Read: http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/c.html" [Broken]c

Einstein began by using an upper-case V for the speed of light [9]. At that time he was also writing papers about the thermodynamics of radiation, and in those he used up upper-case L

Light(n.) - "brightness," O.E. leht, earlier leoht, from W.Gmc. *leukhtam (cf. O.Fris. liacht, M.Du. lucht, Ger. Licht), from PIE *leuk- "light, brightness" (cf. Skt. rocate "shines;" Arm. lois "light," lusin "moon;" Gk. leukos "bright, shining, white;" L. lucere "to shine," lux "light," lucidus "clear;" O.C.S. luci "light;" Lith. laukas "pale;" Welsh llug "gleam, glimmer;" O.Ir. loche "lightning," luchair "brightness;" Hittite lukezi "is bright"). The -gh- was an Anglo-Fr. scribal attempt to render the O.E. hard -h- sound, which has since disappeared.

It would seem that since, according to the Theory, the speed of light is absolute speed, that he would change it to a word for "speed" -

Thanks for every one who posted here. I still must review all of your posts and since I have no understanding of physics jargon, that may take me some months.

In the meantime, I have another question that I offer, and I hope that it is not a stoopid one, but here goes:

If the speed of light is absolute, then how can the speed of light be squared? How can E=mc2 be possible?

Thanks

artie

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- #27

Doc Al

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What does the speed of light being "absolute" (a better term would be invariant) have to do with it being squared in that expression?In the meantime, I have another question that I offer, and I hope that it is not a stoopid one, but here goes:

If the speed of light is absolute, then how can the speed of light be squared? How can E=mc2 be possible?

- #28

artie

What does the speed of light being "absolute" (a better term would be invariant) have to do with it being squared in that expression?

What I think I understand is that the speed of light can not be increased. This must be true if it is invariant. Therefore, I am led to ask, "How can it be squared?" If the speed of light is squared, then the speed of light varies and is not absolute speed.

- #29

Doc Al

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I don't understand the logic of that sentence. Just because a quantity is squared in some expression, does not mean that the quantity varies.If the speed of light is squared, then the speed of light varies and is not absolute speed.

Trivial example: A car of mass "m" moves at constant speed "v". Its (non-relativistic) kinetic energy is 1/2mv^2--the speed is squared in that expression--yet its speed remains constant.

I suspect you are getting messed up with the apparent implications of English grammar. Just because the speed of light appears squared in some expression, does not mean that we did something to physically change the speed of light.

- #30

HallsofIvy

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Can you cite a specific formula that involves the speed of light to the 6That is just an argument from units, why couldn't it be: E = 8*mc^2 ?

You must do the full derivation.

And WHY is c^2 the biggest constant in physics?

i) There are formulas which have c^6 .. aren't that a bigger constant?

ii) c is the constant, c^2 = c*c, i.e the constant c is multiplied with another constant c ...

iii) c is not a real constant, it also have a unit: Lenght/time, so c^2 has units lenght^2/time^2

That I agree with completely.iv) How can we compare e.g G with c? They have different nummerical values, but they have also different units. Also, you can easy come up with a unit system where G has a bigger nummerical value than c. It is like comparing colour with sound.

/Glenn - 1 term from Masters degree in physics.

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