## Tuesday, May 3, 2011

### Poll 74 - Twins, Photons, and Mass

Poll 74 - "One twin stays here, the other travels by near speed of light rocket, returns to earth in 60 years looking not much older. This thought experiment shows us that a photon (traveling at the speed of light) has no experience of time: it has zero duration." Poll ended November 14 2010. 83.8% agreed, while the remaining 16.2% did not.

The Twins Paradox, as this is often called, comes from Einstein's Special Theory of Relativity. I remember as a child watching a Wonderful World of Disney episode which included an animation about this concept and thinking it was impossible. How could it be that one twin gets in a rocket ship, the other stays home, and yet when the rocket twin returns he still looks the same age while the twin who stayed home now looks old? I really had a hard time with how strange the whole idea seemed to be.

And yet if you take the logic of this paradox even further, you arrive at the amazing conclusion we first looked at in Light Has No Speed. The faster you go, the larger the disparity between your experience of time and those who you leave behind: but the rocket ship twin will never be able to travel right at the speed of light, because it would take an infinite amount of energy to move them. Even traveling at near the speed of light, as we describe in this poll question, would require unimaginably large amounts of energy to push the rocket ship twin along.

So how does a photon travel at the speed of light if it takes an infinite amount of energy to reach that speed? It's because photons have no mass.  Here's a quote from the NASA website about this:

"Photons do not have mass, but they do have momentum. The proper, general equation to use is E^2 = m^2c^4 + p^2c^2 So in the case of a photon, m=0 so E = pc or p = E/c. On the other hand, for a particle with mass m at rest (i.e., p = 0), you get﻿ back the famous E = mc^2."
Here's one more interesting idea about all this - photons have no mass, but they do carry energy. E=mc^2 is often used to show that there's an equivalence between mass and energy. The exact number varies according to the assumptions used, but as you'll see at this U.S. Department of Energy web page, if you were to take the energy planet Earth is receiving from the sun and convert it to mass, it works out to over 2 kilograms per second! That's a bit of scientific trivia I hadn't come across before.

So, to get back to the poll question: isn't it quite a leap to go from the twins paradox to the idea that light has zero duration? I'm thrilled that almost 84% of the visitors to this blog were willing to come along for the ride to make that leap with me. And I'm hoping it's because many of you saw and approved of the ideas presented in this video for my blog entry published around the same time as this poll question, Light Has No Speed.

Next up is the new video for Is Spacetime Flat or Curved. Enjoy the journey!

Rob Bryanton

Next - Is Spacetime Flat or Curved?