E:mc2

[Paigntonian]

This came up today: Understand the energy bit. Understand the mass bit. Get the square of the speed of light. But, clearly, you can measure the speed of light as per second or per hour. They are artificial units. So don't quite get how you should measure the speed of light.

[THECORBYLOON]

This explains the dimensions of the equation.

https://youtu.be/s49jC0w3EOc

I always used to wonder how various equations were arrived at (the period of a pendulum for example) until dimensions were covered during physics and then it made sense.

I've forgotten it all now, mind...

[bhg481]

This is exactly why scientists have conventions for units. Most scientists nowadays use the KGS system, kilograms, metres, seconds. When I first learnt science it was the cgs system, centimetres, grams, seconds. The unit of energy quoted in your example will have a name which reflects the system being used.

[jim360]

Doesn't matter what units you use for the speed of light, as long as they are consistent with the units for energy and mass. Physicists typically use units in which energy and mass are both measured in terms of the so-called "electronvolt", and the speed of light has a value of exactly 1, partly to avoid all that faff.

[Buenchico]

>>> Most scientists nowadays use the KGS system, kilograms, metres, seconds

. . . which should be the MKS system (metre, kilogram, second) and which, in turn, has been incorporated into SI (Système international d'unités) since 1960: https://en.wikipedia.org/wiki/International_System_of_Units

[Paigntonian]

Thanks guys

[bhg481]

Chris - yes, I realised the abbreviation mistake after I'd gone to bed. Interesting that jim was referring to using EV for measuring energy; in my line of work we used Kev for temperature measurements, Celcius not being very appropriate for the temperatures we dealt with despite being the standard SI unit. Also interesting to note that the computer codes we used were first written in the 1950s, so input units (density etc) were in the cgs system because that it what was in use at that time.

[THECORBYLOON]

Is the kelvin not the SI unit for temperature?

[bhg481]

TCL - yes, you're correct. Same as the Celcius but a different starting point.

[Buenchico]

>>> Is the kelvin not the SI unit for temperature?

Yup!