# What's the symbol behind Einstein's head? [closed]

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!

• 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 – anna v Jan 6 '15 at 5:34
• Cross-posted from math.stackexchange.com/q/1092619 (closed) – David Z Jan 6 '15 at 5:55
• This question appears to be off-topic because it is about mathematical symbol identification. – Brandon Enright Jan 6 '15 at 6:39
• 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. – John Rennie Jan 6 '15 at 8:24
• @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. – Gaurav Jan 6 '15 at 8:56

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

• This seems more suited for a comment (to Goodies' post?) than an answer. – Kyle Kanos Jan 6 '15 at 13:57
• 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. – Old Name Jan 6 '15 at 16:05
• 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. – David Z Jan 6 '15 at 18:03
• 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. – LouisB Jan 6 '15 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).

• It's an E in cursive script. – LDC3 Jan 6 '15 at 5:33
• Yes. Ignore the initial left swoop, and you'll see it. – BMS Jan 6 '15 at 5:43
• 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. – Hypnosifl Jan 6 '15 at 6:10
• 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 – Luboš Motl Jan 6 '15 at 7:14