What's the symbol called that's behind and to the right of Einstein's head in this picture? Bonus if you can tell me what the whole formula is!

image of Einstein and formulas

  • $\begingroup$ you are used to the E=mc**2 form of the formula, where m is the relativistic mass, and do not recognize it when represented by using the behavior of the rest mass. hyperphysics.phy-astr.gsu.edu/hbase/relativ/conrel.html $\endgroup$
    – anna v
    Jan 6, 2015 at 5:34
  • 1
    $\begingroup$ Cross-posted from math.stackexchange.com/q/1092619 (closed) $\endgroup$
    – David Z
    Jan 6, 2015 at 5:55
  • 8
    $\begingroup$ This question appears to be off-topic because it is about mathematical symbol identification. $\endgroup$ Jan 6, 2015 at 6:39
  • 1
    $\begingroup$ It's not something I feel strongly about, but I think the close votes are a bit unfair. The question is basically asking what the equation is, and that seems a fair question. $\endgroup$ Jan 6, 2015 at 8:24
  • $\begingroup$ @JohnRennie Doesn't it not ask a specific physics question ? Just because it's related to physics doesn't mean it's fit for physics.se. Such questions are better asked on chat. $\endgroup$
    – Gaurav
    Jan 6, 2015 at 8:56

2 Answers 2


Here is a link to the Einstein archives online. You will see the same cursive letter E used in his signature.

  • $\begingroup$ This seems more suited for a comment (to Goodies' post?) than an answer. $\endgroup$
    – Kyle Kanos
    Jan 6, 2015 at 13:57
  • $\begingroup$ Thanks @Louis. I appreciate everyone's responses and feedback but I found the link you provided to be the most useful and pertinent to the root of my question. Thanks again to everyone for all the great feedback. $\endgroup$
    – Old Name
    Jan 6, 2015 at 16:05
  • 1
    $\begingroup$ The answer does identify the symbol as a cursive $E$, so as far as I'm concerned it meets the criterion to be considered an answer. $\endgroup$
    – David Z
    Jan 6, 2015 at 18:03
  • $\begingroup$ I only intended to confirm what had already been posted in other comments by citing a proper reference. I will accept the reputation points, with humility, since they now enable the comment privilege. Thank you all for your patience, support and constructive criticism. $\endgroup$
    – LouisB
    Jan 6, 2015 at 20:21

The equation is that of the relativistic energy of a 'particle' with a nonzero mass. $$ E=\frac{mc^2}{\sqrt{1-\frac{v^2}{c^2}}} $$

The symbol itself I've never seen before. However, I think it's important to mention that $1/\sqrt{1-\frac{v^2}{c^2}}$ is represented by $\gamma$ ($\text{gamma}$).

The factor $\gamma$ is often used in special relativity in Lorentz Length Contraction and Time Dilation (space and time contract with relativistic speed).

  • 3
    $\begingroup$ It's an E in cursive script. $\endgroup$
    – LDC3
    Jan 6, 2015 at 5:33
  • $\begingroup$ Yes. Ignore the initial left swoop, and you'll see it. $\endgroup$
    – BMS
    Jan 6, 2015 at 5:43
  • $\begingroup$ Also worth noting that with a little algebra, you can show this formula is equivalent to $E^2 = m^2 c^4 + p^2 c^2$, where $p = mv / \sqrt{1 - v^2/c^2}$ is the relativistic momentum. So the formula is telling you the total energy in a given frame is a combination of the rest mass energy and energy due to motion in that frame. $\endgroup$
    – Hypnosifl
    Jan 6, 2015 at 6:10
  • 2
    $\begingroup$ By the way, on the blackboard, the symbol for the speed is really $q$, not $v$, see comments about Geschwindigkeit $q'$ here: einsteinpapers.press.princeton.edu/vol4-doc/71 $\endgroup$ Jan 6, 2015 at 7:14

Not the answer you're looking for? Browse other questions tagged or ask your own question.