What units of measure are in the theory of relativity?

[Roger Feeley]

If my time with this forum has taught me anything, it’s that knowledge is both wide and deep.

We all know the deceptively simple equation. But it seems incomplete to me. C is the speed of light but what number does it represent? Cubits per fortnight?

[Nicholas Lawrence]

The internet tells me “cleritas” is the Latin word for “swift” and at some point the abbreviation c began to be used by physicists to represent the speed of light.

[Rod Sheridan]

In the SI system,

Mass = Kg

E= energy in joules

C = 300,000,000 metres per second (3 X 10 to the 8th exponent)

C squared would be 9 X 10 to the sixteenth exponent or 9 followed by 16 zeros

So 1 Kg of mass would convert to 90,000,000,000,000,000 joules of energy

Regards,, Rod

[Roger Feeley]

Buuut… those are earthbound units. The meter is based on the distance from the equator to the North Pole. The kilogram is related to the meter in that it represents the weight of water in cubic centimeters.

what’s SI?

[roger wiegand]

what’s SI?

Système International d'Unités, aka the metric system

[Dave Fitzgerald]

The choice of units is arbitrary. For example, you can measure length / distance in meters (or any "units of 10" fraction of meters - centimeters, millimters, micrometers, nanometers), angstroms, miles, feet, inches, cubits, parsecs, light years, ... you get the idea. Two things are the same distance apart whether you measure an inch or 2.54 cm - just gotta know which units you're using.

The relationship (E=mc^2) holds no matter what units you're working with. Science (usually) uses SI units because they're standardized, and they make most of the math easy.

[Patty Hann]

Buuut… those are earthbound units. The meter is based on the distance from the equator to the North Pole. The kilogram is related to the meter in that it represents the weight of water in cubic centimeters.

what’s SI?

The meter and KG standards you cite have been superceded:

The meter was redefined by international agreement in 1983 as the length of the path traveled by light in a vacuum in 1/299,792,458 of a second. This definition also locked the speed of light at 299,792,458 meters per second in a vacuum. Length was now no longer an independent standard but rather was derived from the extremely accurate standard of time and a newly defined value for the speed of light made possible by the technology developed at NIST.

The kilogram is defined by taking the fixed numerical value of the Planck constant, ℎ, to be 6.626 070 15 × 10-34 when expressed in the unit J s, which is equal to kg m2 s−1, where the metre and the second are defined in terms of the speed of light, and the hyperfine transition frequency of the caesium-133 atom, ∆ν, respectively. (This was a new definition in May 2019.)

More info on measurement standards can be found on the NIST site https://www.nist.gov (https://www.nist.gov/)

Full disclosure: I worked in a measurement lab (Aerospace calibration) for 26 years

[Jim Koepke]

Full disclosure: I worked in a measurement lab (Aerospace calibration) for 26 years

I've carried a tape measure in my pocket for about 35 years. :D

jtk

[Bill Dufour]

In the English system, as used in the USA. The measurements are joules equal to slugs and feet per second squared.
Bill d

[Patty Hann]

I've carried a tape measure in my pocket for about 35 years. :D

jtk

We calibrated tape measures too :D

[Tom M King]

Good answers. Einstein, like most of us men, was helped a Lot by his first Wife who was better at math and physics than he was. He was a remarkable person understanding the big picture of many things, and not simply a kook like he has been presented many times. I have a good friend who has family pictures of him visiting them at their home in the back yard.

[Alan Lightstone]

I always thought that 1 kg was $50,000 USD - the monetary unit of measure of what cocaine from Columbia cost in the 80's. My bad.

[Jim Becker]

In the English system, as used in the USA. The measurements are joules equal to slugs and feet per second squared.
Bill d
Science and industry in the US uses the international measurements... ;) So the slugs can have a nice vacation and enjoy their sticky trails.


I always thought that 1 kg was $50,000 USD - the monetary unit of measure of what cocaine from Columbia cost in the 80's. My bad.

ROFLOL!!!

[Stan Calow]

Like most physics formulas, the terms are not dependent on the units until you are actually making a calculation. And then they are chosen by the context. Just like in the formula, area equals length times width (A = L x W) can be done in metric or imperial or square furlongs depending on what you're going to do with it.

[Tom M King]

Units of measure are of no importance in this. It's merely a formula for the amount of energy in mass. Think about a BB that probably most of us shot through our BB guns when we were little. The formula tells us that the amount of energy in the atoms of that BB, if released, equals the BB going in all directions at once at the speed of light.

[Patty Hann]

Units of measure are of no importance in this. It's merely a formula for the amount of energy in mass. Think about a BB that probably most of us shot through our BB guns when we were little. The formula tells us that the amount of energy in the atoms of that BB, if released, equals the BB going in all directions at once at the speed of light.

Do you know how much energy that would result if 100% of the mass of ONE BB were totally converted to energy? 30600000 MJ.
It takes 36000MJ to power the average home for a year. You could power 850 homes for a year on one BB converted to energy.
(We need to work on getting that to be feasible.. :D.)

By way of comparison: in nuclear fission reaction (as in a nuclear reactor) 0.1% mass is converted to energy.
The sun is a little better...0.7% of hydrogen gets converted to energy (altho' this is a fusion reaction, not a a fission reaction).
So the Sun is only 7 times more efficient than our nuclear reactors. Except the quantity is huge...off the scale.

[Tom Bender]

So we could define a new energy unit the clumber such that 1 clumber = 4.31 stone furlongs squared per fortnight squared. So how many watt seconds is that? I'll leave the math to my wife.