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Author Topic: How does E=MC2 relate to general relativity?  (Read 709 times)

Offline thedoc

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How does E=MC2 relate to general relativity?
« on: 18/01/2016 18:50:02 »
Leonard Patrick Knight asked the Naked Scientists:
   I found this explanation of the two theories of relativity easy to comprehend (I'm an elderly pensioner who has never properly understood them before). However, I am still at a loss to understand the related formula E=MC2. I have mentioned this to a number of intelligent people including a cell biologist and the best they can offer is that the speed of light is a constant (which I knew anyway if you are speaking of a vacuum of course). My problem is: How can this formula be used in a concrete way to calculate the energy in a given mass of matter? The speed of light may be a constant but to use that speed in a formula one has to use units of some kind and miles per hour gives a very different figure to say kilometres per second. Who says what the units are in any real calculation and has anyone ever proved it by experiment?
What do you think?
« Last Edit: 18/01/2016 18:50:02 by _system »


 

Offline evan_au

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Re: How does E=MC2 relate to general relativity?
« Reply #1 on: 18/01/2016 20:16:04 »
Quote from: Leonard Patrick Knight
miles per hour gives a very different figure to say kilometres per second
This is true in all equations - you must use coherent units, or the numerical answer will be nonsense.

The set of coherent units most used by scientists uses the meter (length), second (time), Kilogram (mass) and other related units such as the Joule (energy) and Volt (voltage). 
See: http://en.wikipedia.org/wiki/International_System_of_Units

Quote
How can this formula be used in a concrete way to calculate the energy in a given mass of matter?
Actually, the iconic E=mc2 is only part of the equation that calculates the energy in a piece of matter.

The full equation tells us that the energy consists of energy due to the movement of the object, plus a residual energy, even when the object is not moving.

The full equation is this:
E2 = (pc)2 + (m0c2)2
Where:
  • E is the energy in the lump of matter (as measured by you)
  • c is the speed of light in a vacuum
  • m0 is the mass of the object when it is stationary (not moving, compared to you)
  • p is the momentum of the lump of matter (as measured by you) 

As we were taught in science class, the kinetic energy of an object increases as it moves faster (compared to us). Surprisingly, Einstein showed that there is an amount of energy (a huge amount of energy) that is present even when the object is stationary (compared to you). This is the mass/energy equivalent.

If you set the velocity=0, then momentum=0, and you are left with the almost familiar E=m0c2.

Einstein showed a variety of other surprising things which are also implicit in this equation:
  • As the speed of an object changes, it's mass changes.
  • There is a maximum speed that a massive object can travel (c)
  • The energy of an object when calculated in this way is the same when measured by different observers, traveling at different speeds. (The kinetic energy we calculated at school does not have this useful  property.)
  • There are things (like photons of light) that have no rest mass, but do have momentum and energy, and they also obey this equation
  • If you measure the mass of a moving object (m), and use that in this equation, you do end up with the familiar E=mc2.
See: http://en.wikipedia.org/wiki/Energy%E2%80%93momentum_relation

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has anyone ever proved it by experiment?
Yes. Every day in summer, the Large Hadron Collider accelerates protons up to an energy of 7 Terra-electron Volts (atomic physicists and astronomers use some non-coherent units to express very small or very large quantities).

If you look up an information sheet on a proton, you will see that it's rest mass m0 is shown as 938 MeV/c2 = 1.672 x 10-27kg. This means that as the LHC accelerates protons up to almost the speed of light, their mass and energy grows by a factor of about 7,000.
See: http://en.wikipedia.org/wiki/Proton   
« Last Edit: 18/01/2016 21:03:10 by evan_au »
 

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Re: How does E=MC2 relate to general relativity?
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