Page 1 of 2 12 LastLast
Results 1 to 15 of 17

Thread: What units of measure are in the theory of relativity?

  1. #1
    Join Date
    Oct 2007
    Location
    Falls Church, VA
    Posts
    2,344
    Blog Entries
    1

    What units of measure are in the theory of relativity?

    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?

  2. #2
    Join Date
    Mar 2015
    Location
    Virginia
    Posts
    1,211
    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.

  3. #3
    Join Date
    Dec 2006
    Location
    Toronto Ontario
    Posts
    11,272
    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

  4. #4
    Join Date
    Oct 2007
    Location
    Falls Church, VA
    Posts
    2,344
    Blog Entries
    1
    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?

  5. #5
    Join Date
    Sep 2013
    Location
    Wayland, MA
    Posts
    3,665
    Quote Originally Posted by Roger Feeley View Post

    what’s SI?
    Système International d'Unités, aka the metric system

  6. #6
    Join Date
    Jan 2016
    Location
    Northern Virginia
    Posts
    40
    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.

  7. #7
    Join Date
    Apr 2017
    Location
    Southwest US
    Posts
    1,041
    Quote Originally Posted by Roger Feeley View Post
    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

    Full disclosure: I worked in a measurement lab (Aerospace calibration) for 26 years
    "What you see and what you hear depends a great deal on where you are standing.
    It also depends on what sort of person you are.”

  8. #8
    Join Date
    Sep 2007
    Location
    Longview WA
    Posts
    27,430
    Blog Entries
    1
    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.

    jtk
    "A pessimist sees the difficulty in every opportunity; an optimist sees the opportunity in every difficulty."
    - Sir Winston Churchill (1874-1965)

  9. #9
    Join Date
    Sep 2016
    Location
    Modesto, CA, USA
    Posts
    9,975
    In the English system, as used in the USA. The measurements are joules equal to slugs and feet per second squared.
    Bill d

  10. #10
    Join Date
    Apr 2017
    Location
    Southwest US
    Posts
    1,041
    Quote Originally Posted by Jim Koepke View Post
    I've carried a tape measure in my pocket for about 35 years.

    jtk
    We calibrated tape measures too
    "What you see and what you hear depends a great deal on where you are standing.
    It also depends on what sort of person you are.”

  11. #11
    Join Date
    Feb 2014
    Location
    Lake Gaston, Henrico, NC
    Posts
    9,023
    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.
    Last edited by Tom M King; 02-25-2024 at 8:14 AM.

  12. #12
    Join Date
    Jan 2010
    Location
    Tampa Bay, FL
    Posts
    3,925
    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.
    - After I ask a stranger if I can pet their dog and they say yes, I like to respond, "I'll keep that in mind" and walk off
    - It's above my pay grade. Mongo only pawn in game of life.

  13. #13
    Join Date
    Mar 2003
    Location
    SE PA - Central Bucks County
    Posts
    65,842
    Quote Originally Posted by Bill Dufour View Post
    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.

    Quote Originally Posted by Alan Lightstone View Post
    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!!!
    --

    The most expensive tool is the one you buy "cheaply" and often...

  14. #14
    Join Date
    Apr 2013
    Location
    Kansas City
    Posts
    2,665
    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.
    < insert spurious quote here >

  15. #15
    Join Date
    Feb 2014
    Location
    Lake Gaston, Henrico, NC
    Posts
    9,023
    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.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •