Comments of the Week #74: from the Universe's age to the love of science

“To me there has never been a higher source of earthly honor or distinction than that connected with advances in science.” -Isaac Newton

And the advances continue, not just here at Starts With A Bang but everywhere humans are engaged in the practice of gathering knowledge about the world and Universe itself. This past week, we covered:

And for those of you who want to catch up on nuclear fusion, check my latest over at Forbes:

Later this weekend (probably on Sunday), the first advance copies of Chapter 2: A Relatively Different Story: How Einstein’s Relativity Revolutionized Space, Time, And The Universe will go out to my Patreon supporters, so make sure you don't miss out for a spectacular preview of my upcoming book, Beyond The Galaxy. Now, dive with me into our Comments of the Week!

Image credit: NASA, ESA, and A. Feild (STScI), via Image credit: NASA, ESA, and A. Feild (STScI), via

From Sinisa Lazarek on the puzzling problem of special relativity: "what I find fascinating and bizzare at the same time is if you view it from a POV of a photon being created i.e at CMB time and traveling through spacetime.. is that photon will colide with things that don’t yet exist in his timeline.. i.e. with a gold atom created in star cores which is 2 billion years in the future from a photon’s clock.. in photon’s reference that star hasn’t even been born.. yet in some other reference frame you could observe that photon hiting that gold atom somewhere in space. If photon could think, what would he say? what the hell did I just hit? where did this atom come from??"

Imagine not that you were a photon, since the laws of physics return instantaneous answers at the speed of light (the hazards of being a null vector), but rather that you were the very first observer ever to be created out of matter, say, 2 billion years after the Big Bang. Out in the great distance, there's a CMB that's some four times as hot as ours is at present, the largest galaxy clusters contain only hundreds (instead of thousands) of galaxies, the visible Universe is only 14 billion light years in diameter instead of the present 93, and the galaxies that do exist are bluer, smaller and far less evolved than the ones we know today.

Image credit: NASA, ESA, P. van Dokkum (Yale University), S. Patel (Leiden University), and the 3D-HST Team. Image credit: NASA, ESA, P. van Dokkum (Yale University), S. Patel (Leiden University), and the 3D-HST Team.

Yet if you got into a spaceship, accelerated at 9.8 m/s^2 for about 30 years in your frame of reference, and then just coasted, you'd be able -- assuming you didn't run into something and fry yourself -- to travel through billions of years of cosmic evolution. When you looked out your front windshield, the Universe would appear to evolve incredibly rapidly, as the galaxies you'd encounter would change tremendously in short order. The ones behind you, contrariwise would still appear just as ancient as when you left, since the photons would struggle to overtake you.

But the big surprise would come if you screeched to a halt, and slipped back into the CMB's rest frame.

Image credit: NASA; ESA; G. Illingworth, D. Magee, and P. Oesch, University of California, Santa Cruz; R. Bouwens, Leiden University; and the HUDF09 Team. Image credit: NASA; ESA; G. Illingworth, D. Magee, and P. Oesch, University of California, Santa Cruz; R. Bouwens, Leiden University; and the HUDF09 Team.

At that moment, the Universe would look like whatever it looks like in its rest frame at its present age, your relativistic journey notwithstanding. You would essentially suffer the craziest episode of time dilation of all, where everyone you knew had been dead for billions of years, where your own star had died and very likely, your parent galaxy had long since merged with another. The Universe would come to be dominated by dark energy, meaning that your original location would now be unreachable, and that you literally -- unless you invent a wormhole -- you can never go back.

Physics sure is strange, isn't it? Thanks, Einstein.

Image credit: NASA, ESA, Jean-Paul Kneib (Laboratoire d’Astrophysique de Marseille) et al. Image credit: NASA, ESA, Jean-Paul Kneib (Laboratoire d’Astrophysique de Marseille) et al.

From PB on whether this could every practically happen: "While technically correct, such an accelerated galaxy is extremely unlikely."

Typically, galaxies reach peculiar velocities of a few hundred to the low thousands of km/s. In extreme cases, we've seen speeds in the tens of thousands of km/s. To reach near-light speed? Yes, it's exceedingly unlikely. But this is the whole purpose of a thought experiment: to test the scenarios unachievable in the physical Universe itself! While galaxies might not get there, individual star systems can come far closer, and of course a spacefaring civilization... well, call me in 1,000 years and let's see where we are.

Image credit: Stuart Palley, from his instagram feed at Image credit: Stuart Palley, from his instagram feed at

From PJ on California's (and other) wildfires: "From a firies point of view, I can say there is no beauty in a fire situation. There is no time to enjoy such things – rather, observe the extent, calculate the risks, endeavor to extinguish; ensure the safety of your men & others, stock, then property.
I entirely agree with the photographer that education is an absolute necessity for those living in the bush. Goodness knows how many campaigns we would run each year in the off-season to try & make property owners aware of their surrounds."

It's a very hard task to think of beauty when you're someone who either works firsthand with or is close to the destruction. I've lived up in the Pacific Northwest (Oregon/Washington states) for the past seven years, and this year, all three coastal states (OR, WA, CA) are experiencing drought and rampant wildfires. The air quality where I am the past few weeks has been awful, the sunsets have been reddish for hours, and just last weekend, a 10,000 gallon propane tank just a few miles from my house almost went up in flames. Lit cigarettes or unattended campfires have been the cause of nearly half of the wildfires around here.

Image credit: Stuart Palley, from his instagram feed at Image credit: Stuart Palley, from his instagram feed at

And yet, there is a beauty to it all. It's kind of amazing that Stuart was able to capture that aspect amid all the chaos. Somewhat unexpectedly, he reached out to me this past week, having seen my article:

Stuart Palley here, photographer out in Los Angeles working on the Terra Flamma project. Thanks for your article detailing my work and for your insight on the wildfire and drought issue. I enjoyed your treatment of the work and words as well.
I like the wildfire song too.
Thank you for your support!
That's good stuff, and I hope you liked it -- and that you never underestimate its destructive power -- too.
Image credit: The pre-launch Planck Sky Model: a model of sky emission at submillimetre to centimetre wavelengths — Delabrouille, J. et al.Astron.Astrophys. 553 (2013) A96 arXiv:1207.3675 [astro-ph.CO]. Image credit: The pre-launch Planck Sky Model: a model of sky emission at submillimetre to centimetre wavelengths — Delabrouille, J. et al.Astron.Astrophys. 553 (2013) A96 arXiv:1207.3675 [astro-ph.CO].

From Michael Kelsey on how fast we're moving through space: "Ethan, I must be missing something really obvious. Toward the end of your piece, you wrote, “[T]he Solar System moves relative to the CMB at 368 ± 2 km/s, and that when you throw in the motion of the local group, you get that all of it — the Sun, the Milky Way, Andromeda and all the others — are moving at 627 ± 22 km/s relative to the CMB.”The first half of that is just converting the +/-3.354 mK dipole into a velocity (z = 1.23e-3, so v = zc = 368 km/s). But how do you get the 627 km/s? Is that the motion of the center of mass of the Local Group relative to the CMB? Do you get that by summing the apparent (peculiar) motions of the other members of the LG, along with the Sun’s galactic orbital motion?"

So there are two pieces to this, as you identify: our total motion relative to the CMB, which we get simply from the CMB dipole: 368 km/s. The uncertainty there, by the way? That isn't a measurement uncertainty! That extra ± 2 km/s comes from our ignorance of how intrinsically large (or small) the actual primordial cosmic dipole is. We know it exists (or ought to exist), but we have no measurable way to disentangle the CMB's dipole from ours. If it's the same magnitude as the other multipole moments, it ought to be ± 1-or-2 km/s, and so that's where our uncertainty comes from.

Image credit: Cosmography of the Local Universe — Courtois, Helene M. et al. Astron.J. 146 (2013) 69 arXiv:1306.0091 [astro-ph.CO]. Image credit: Cosmography of the Local Universe — Courtois, Helene M. et al. Astron.J. 146 (2013) 69 arXiv:1306.0091 [astro-ph.CO].

But the second piece is that we can measure the Earth's motion around the Sun, and the Sun's motion around the galaxy (which has an uncertainty of around ± 20 km/s in magnitude, mind you), and the Milky Way's motion towards Andromeda and the other local group objects, and we'll get a number that's close to (but just under) 300 km/s total for our motion through this part of the cosmos. But it's nearly opposite to the direction of the CMB dipole, and so therefore the entire local group must be moving (using vector addition) at some 627 km/s relative to the CMB, with the uncertainty mostly coming from the Sun's motion around the galaxy.

Hope this helps!

Image credit: ESA and the Planck Collaboration. Image credit: ESA and the Planck Collaboration.

From Doug Henderson on the CMB: "I was puzzled by this statement: “Prior to that time, some 380,000 years ago, it was too hot to form them, as photon collisions would immediately blast them apart, ionizing their components.”"

I was puzzled by your puzzlement, initially, because my mind just read it and went, "Yeah, CMB! 380,000 years from the Big Bang, and then we come to the present." Because that's what I write. That's what I always write. And so that I accidentally wrote "years ago" instead, I didn't even catch it at all.

So there's your proof: I'm not a robot. I'm a human, an error-prone human. I've fixed it in the text without much fanfare, but I wanted to acknowledge my mistake. Thanks for the catch.

Image credit: NASA / JPL-Caltech. Image credit: NASA / JPL-Caltech.

From Denier on the evaporation rate of black holes: "While it should have been obvious, I didn’t realize until reading this article that there is not a single moderate or large black hole that has lost even a single atom’s worth of mass to Hawking Radiation in the history of our universe to this point. Even if a black hole was completely cut off from being able to absorb any matter for the past 13 billion years, it would still absorb more energy from the CMB than it could lose in Hawking Radiation."

This is a lot of fun, actually, and lucky for you, something I worked through in detail last year in one of my Ask Ethans. Here's an excerpt of the relevant bits:

In fact — although you actually have to do the quantum field theory calculations in curved spacetime to find this out — Hawking radiation predicts that you’ll get a blackbody spectrum of photons with a temperature given by:

which is a temperature of less than one microKelvin for a black hole the mass of our Sun, less than one picoKelvin for the black hole at the center of our galaxy, and just a few tens of attoKelvins for the largest known black hole. These decay rates that this radiation corresponds to are so small that it means that black holes will continue to grow so long as they continue to absorb even one proton’s worth of material per present-age-of-the-Universe, which is estimated to occur for the next 10^20-some-odd years.

After that, black holes the mass of the Sun will finally start to lose more energy due to Hawking radiation (on average) than they’ll absorb, completely evaporating after ~10^67 years, and with the largest black holes in the Universe disappearing after around ~10^100 years. That may be far longer than the age of the Universe, but it’s still not forever. And the way it will decay is through the mechanism of photon emission via Hawking radiation.

You might look at numbers like 10^10 years (the present age of the Universe) and think it's not such a big leap to 10^20 years, but you've got to remember how orders of magnitude work. It takes ten times the present age of the Universe to get to 10^11 years; ten times that to get to 10^12 years, etc. You absorb one proton per 10^20 years, and you'll grow faster than you shrink due to Hawking radiation. No one's decaying anytime soon.

Image credit: XMM-Newton, ESA, NASA. Image credit: XMM-Newton, ESA, NASA.

From Michael Kelsey (again!) on the black hole conference wrapping up today: "@Ethan and/or @Sabine: Do either of you know if the talks at the HR Conference are going to be posted to the program ("

Hawking's talk is available in an embedded video here (it's short), and it looks like there was a live feed of the other talks, as there's photographic evidence on the Nordita facebook page of Malcolm Perry's talk:

Image credit: Nordita live feed, via Image credit: Nordita live feed, via

But as far as I know, there aren't any permanent ways to view the lectures. Too bad, because much like you, Michael, I'd like to see them too! Guess we'll have to wait until the Hawking/Perry/Strominger paper comes out, which I'm betting isn't going to live up to the hype.

Image credit: Nicolas George. Image credit: Nicolas George.

And finally, from MandoZink on the follow-up to the above picture of liquid nitrogen: "A photo from this same event, also credited to Nicolas George, is on Wikipedia’s “Liquid nitrogen” page. Hands possibly acquiring brown spots. Must be seconds later in sequence. Probably solid and sitting on (or stuck to) a shelf a few moment after that."

Because the internet is amazing, I was able to find that picture and ID those brown spots right away. Have a look for yourself (emphasis mine).

Image credit: Nicholas George. Image credit: Nicolas George.

I have done this to myself, by the way: given myself a cold burn with liquid nitrogen. I do not recommend it, and I do instead strongly recommend wearing the proper safety/protective equipment when you do any sort of work.

You only get one body, and having worked for an experimental physicist with fewer than ten fingers (who lost what he lost while taking a risk during his day job), I cannot emphasize the importance of basic safety awareness. You get one life; don't let your impatience cost you the highest quality one you can have.

Thanks for the great week, looking forward to the next one, and if you're my Patron on Patreon, can't wait to deliver your rewards to you!

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