- binding energy
- the energy equivalent of the difference between the mass of a nucleus and the masses of its nucleons

- ether
- scientists once believed there was a medium that carried light waves; eventually, experiments proved that ether does not exist

- frame of reference
- the point or collection of points arbitrarily chosen, which motion is measured in relation to

- general relativity
- the theory proposed to explain gravity and acceleration

- inertial reference frame
- a frame of reference where all objects follow Newtonâ€™s first law of motion

- length contraction
- the shortening of an object as seen by an observer who is moving relative to the frame of reference of the object

- mass defect
- the difference between the mass of a nucleus and the masses of its nucleons

- postulate
- a statement that is assumed to be true for the purposes of reasoning in a scientific or mathematic argument

- proper length
- the length of an object within its own frame of reference, as opposed to the length observed by an observer moving relative to that frame of reference

- relativistic
- having to do with modern relativity, such as the effects that become significant only when an object is moving close enough to the speed of light for $\text{\xce\xb3}$ to be significantly greater than 1

- relativistic energy
- the total energy of a moving object or particle $E=\mathrm{\xce\xb3}m{c}^{2}\text{,}$ which includes both its rest energy
*mc*^{2}and its kinetic energy

- relativistic factor
- $\mathrm{\xce\xb3}=\frac{1}{\sqrt{1\xe2\u02c6\u2019\frac{{u}^{2}}{{c}^{2}}}}$, where
**u**is the velocity of a moving object and*c*is the speed of light

- relativistic momentum
**p =***Î³m***u**, where $\text{\xce\xb3}$ is the relativistic factor,*m*is rest mass of an object, and**u**is the velocity relative to an observer

- relativity
- the explanation of how objects move relative to one another

- rest mass
- the mass of an object that is motionless with respect to its frame of reference

- simultaneity
- the property of events that occur at the same time

- special relativity
- the theory proposed to explain the consequences of requiring the speed of light and the laws of physics to be the same in all inertial frames

- time dilation
- the contraction of time as seen by an observer in a frame of reference that is moving relative to the observer