Will we run out of light elements?

Chris - This one says, does the equation E=MC²hold for dark matter and dark energy as well?

Katie - Well, for sure it does for dark matter because that's a kind of matter. And we use that equation as a way of counting up the energy density of stuff. And we don't know what dark matter is. So we don't know if it's a particle or, or whatever, but when we talk about what dark matter could be, we do talk about it in terms of its mass, in terms of its energy. And so that conversion between matter and energy as a way of accounting for how much stuff there is, is just everywhere in physics. As for dark energy, dark energy is a little bit of a more complicated thing because it might just be a kind of property of the universe and, whether or not you can reasonably talk about that in terms of mass is a whole other thing. But for dark matter, we use that conversion factor all the time.

Chris - Here's one relevant to The End of Everything. This person says "since light elements are transforming into heavier elements in stars. Is it true to predict that one day there will be no new star being born?"

凯蒂-是的。不一定,我们going to run out of light elements. I think that's the question that's being asked. What's really going to happen is that there will be fewer ways for the light elements to come together in enough density to form new stars. So the stars in the Milky Way will die. And there won't be processes that compress the gas enough to form new stars over time. And in the rest of the universe, everything will be getting farther and farther apart. And so there will be fewer stars forming just in general, out in the cosmos. And so it's more a matter of the fact that in the dense places, the stars will form and die and scatter elements, but the places where we normally have star formation in little clumps of gas in galaxies, or in violent events of mergers of galaxies, those kinds of events, those kinds of environments will become less common.

Chris - And lastly, before we let you take a well-earned break, this person says, "do you think aliens would use lasers rather than radio telescopes to attempt to contact with us here on Earth?"

Katie - It really depends on what they're trying to do. And we have used all sorts of technologies to try to search for alien life, you know, aliens communicating with us or communicating with each other or anything like that. And the nice thing about a radio telescope, depending on how it's built, you might be able to search a large part of the sky. If you're trying to transmit something then you might do better with a laser, cause you can have a very, very strongly collimated beam and just aim at one thing in particular. But if you're, if you're aiming at one thing in particular, you have to know that there's something they’re receiving. So if there's an alien civilization out there, if they know we're here, then using laser to try to communicate with us would probably be a pretty good idea. Using a radio transmitter of some kind might be better if they're trying to sweep a broad region.