# A square looks at light squared

#### tbco

Non physicist asks question-why speed of light squared?
Its the derivitive that makes the equation work.
The speed of light is some kind of ultimate measuring device.
You are too stupid to understand.

I think the questioners are incorrectly phrasing what they actually want to know
We see the realtionship between Mass and Energy almost intuitively. What we dont see is
why light has anthing to do with it at all.( and dont answer-it doesnt,its the SPEED of light because saying light and the speed of light are not connected makes no sense) Some say it doesnt matter because the equation is so cool and valuable-who cares? Well, could it be that the next entire level of understanding of Physics could pivot on figuring out why these realtionships exist?

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#### ZapperZ

Staff Emeritus
2018 Award
Non physicist asks question-why speed of light squared?
Its the derivitive that makes the equation work.
The speed of light is some kind of ultimate measuring device.
You are too stupid to understand.

I think the questioners are incorrectly phrasing what they actually want to know
We see the realtionship between Mass and Energy almost intuitively. What we dont see is
why light has anthing to do with it at all.( and dont answer-it doesnt,its the SPEED of light because saying light and the speed of light are not connected makes no sense) Some say it doesnt matter because the equation is so cool and valuable-who cares? Well, could it be that the next entire level of understanding of Physics could pivot on figuring out why these realtionships exist?
This is utterly puzzling.

Please look at this FAQ entry:

You'll notice that the FULL relativistic equation has another term to it! In fact, for light itself, its energy is only the other term, ie. E= pc.

So where's the "square" there? How come people are not "obsessing" over the fact that there is no square of the speed of light here?

Sometime, I just don't understand where and how people form their ideas.

Zz.

Last edited by a moderator:

#### phinds

Gold Member
Sometime, I just don't understand where and how people form their ideas.

Zz.
+1 on that

#### Dale

Mentor
Non physicist asks question-why speed of light squared?
What else could it possibly be? It has to have units of speed^2. It has to be composed entirely of universal constants. That makes it c^2.

#### Drakkith

Staff Emeritus
2018 Award
Non physicist asks question-why speed of light squared?
It's just the way the math works out. You might as well ask why the Lorentz transformation is set up as it is. Both are the result of how the rules of the universe interact with each other. Nothing more, nothing less.

What we dont see is why light has anthing to do with it at all.( and dont answer-it doesnt,its the SPEED of light because saying light and the speed of light are not connected makes no sense)
I'm sorry to say, but light has nothing to do with it. Light happens to travel at the maximum possible velocity in the universe. That's it. The fact that there is a maximum speed limit, along with other ways physics works, results in mass and energy being related by c2.

#### sophiecentaur

Gold Member
It's just the way the math works out. You might as well ask why the Lorentz transformation is set up as it is. Both are the result of how the rules of the universe interact with each other. Nothing more, nothing less.

I'm sorry to say, but light has nothing to do with it. Light happens to travel at the maximum possible velocity in the universe. That's it. The fact that there is a maximum speed limit, along with other ways physics works, results in mass and energy being related by c2.
OR everything - depending on how you look at it?

#### Drakkith

Staff Emeritus
2018 Award
OR everything - depending on how you look at it?
You must be looking at it differently than I am.

#### sophiecentaur

Gold Member
You must be looking at it differently than I am.
I was looking at it from the point of SR. There is a strong link between velocity and mass and it involves the speed of light and light is an EM wave. You seem to be arguing against a very obvious association between those quantities. Or am I reading you wrong?

#### Drakkith

Staff Emeritus
2018 Award
I was looking at it from the point of SR. There is a strong link between velocity and mass and it involves the speed of light and light is an EM wave. You seem to be arguing against a very obvious association between those quantities. Or am I reading you wrong?
I'm saying that there is a maximum speed limit in the universe, c, and that quantity is used in a variety of important ways in SR and other theories. It was my understanding that light itself simply happens to move at this velocity. Is that clearer?

#### phinds

Gold Member
I was looking at it from the point of SR. There is a strong link between velocity and mass and it involves the speed of light and light is an EM wave. You seem to be arguing against a very obvious association between those quantities. Or am I reading you wrong?
I think Drakkith's point, which I agree with, is that there is a universal speed limit that exists REGARDLESS of whether photons happen to be massless. The fact that they ARE massless means that they travel at that speed limit. We use the speed of light in the models really as a stand-in for the universal speed limit.

Uh ... I think I just said the same thing Drakkith just said.

#### sophiecentaur

Gold Member
I don't feel particularly happy with using photons along with a theory like SR. That always leads to circular arguments about impossible characteristics for photons. SR, being non quantum, is more to do with fields and the rate at which 'disturbances' propagate. No surprise that this involves a speed limit of c. (No surprise, once Albert introduced the idea at least). The fact that 'light' is and was used in the arguments which lead to SR is a mere chance, based on our visible wavelength range. If we worked at LF radio frequencies, SR would still have been the conclusion but it is less likely that we would be as obsessed with trying to include photons in all our discussions.
Saying that light just happens to behave that way seems to lose the intimate connection between all the quantities. To my mind it is not just peripheral. The maths just yields the answers that the input physical ideas force it to.

Staff Emeritus
The OP hasn't been back.

#### sophiecentaur

Gold Member
The OP hasn't been back.
Yep. It's like dads with their son's radio controlled car. They take over. I'm sure the OP never expected the overkill response.

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