Starts With A Bang

Comments of the Week #158: from event horizons to time travel

The star in the great Andromeda Nebula that changed our view of the Universe forever, as imaged first by Edwin Hubble in 1923 and then by the Hubble Space Telescope nearly 90 years later. Note, also, that the galaxy has not rotated at all in that time. Image credit: NASA, ESA and Z. Levay (STScI) (for the illustration); NASA, ESA and the Hubble Heritage Team (STScI/AURA) (for the image).

“Time travel used to be thought of as just science fiction, but Einstein’s general theory of relativity allows for the possibility that we could warp space-time so much that you could go off in a rocket and return before you set out.” -Stephen Hawking

As always, there’s been a new fantastic week of articles here at Starts With A Bang, punctuated by our new podcast this month, on the physics of time travel!

Have a listen (or download it and take it with you) and thank our Patreon supporters for making it possible! Now, what was this past week all about? Come enjoy some fabulous stories if you missed anything, including:

There’s always so much more I could have talked about or so many more details I could have gone into but didn’t, and that’s what makes doing a follow-up so informative and educational. So let’s jump into what you wanted to learn or discuss more about here, and enjoy our comments of the week!

Image credit: Mark Garlick / SPL.

From Michael Mooney on his ‘key issues’ with relativity: “I am sorry that you refuse to address the core of the issue I bring to your forum.
Is there a “real world” independent of observation or not? (Yes, there is.)
Does the Earth physically change dimensions when (if ever) viewed from near light speed or not? (Ans: Not)
Does Earth’s atmosphere physically change in depth/height “for” every incoming muon or not? (Ans: Not.)”

When I “refuse to address” your issue it’s because you are asking a question that doesn’t have a scientific answer. I’m not in the business of giving you uninformed opinions and asserting them as facts. So if you want these three questions answered with scientific accuracy, let’s go.

1.) We cannot and have not ever measured the world independent of observation, and will never be able to do so. That is a fundamental limit of being a physical being in a physical Universe. Some things we can not observe for a very long time, then look at them and they will be exactly the same as a deterministic equation predicted. Others, if we observe them continuously, will give different results than if we didn’t look. So is there a “real world” independent of observation? You can’t answer, and I also assert that your quoted phrase “real world” is ambiguously defined.

Moving close to the speed of light results in times and distances transforming, with lengths — including the length of your starship — becoming shorter in the direction of motion. Image credit: David Taylor of Northwestern, via

2.) Does Earth physically change dimensions when viewed near the speed of light? The answer is yes, dependent on which observer you ask. If I ask you, on Earth, whether Earth’s physical dimensions change, the answer is no. If I ask the traveler whether they do, the answer is yes. There are some things that are invariant under relativistic transformations, but physical extents in space are not one of them.

Cosmic rays shower particles by striking protons and atoms in the atmosphere, but they also emit light due to Cherenkov radiation. Image credit: Simon Swordy (U. Chicago), NASA.

3.) Does the Earth’s atmosphere change for every incoming relativistic particle? It’s not a “change,” as you incorrectly use the word, but it achieves a unique size/extent with respect to that particle, defined by that particle’s motion. The size of the atmosphere is entirely dependent on that particle’s frame-of-reference, and there is no definition of size/length/distance/spatial extent independent of reference frame in relativity.

Now, I have stated previously to you that there is an apparent length contraction that is not visually real, and I want you to understand why. If an object is moving close to the speed of light and it is emitting/reflecting photons, the way the object appears will often be uncontracted because of the superimposed effects of length contraction atop the fact that the speed of light is finite and not much greater than the moving object. You can also get visual effects that appear to move faster-than-light, known as photonic booms. But the thing you want to be invariant — the physical size of a physical object — simply isn’t, nor is it well-defined in the absence of an observer.

Image credit: CERN/Maximlien Brice, of the CMS detector, the small detector at the LHC.

From Elle H.C. on the future of science: “‘Real’ physical testing isn’t going to open any new doors (physics). The future will come from fiddling and experimenting with computer simulations.”

The day we stop doing experimental or observational tests of our theories — the day we totally divorce science from testability — is the day we stop doing physics. Do you know where the word physics comes from? It’s from the ancient greek work, φύσις, which is best translated as “nature.” The study of physics is a study of the natural, physical world. Simulations are useful, particularly about the past, but are only really informative when you can compare the results of a simulation with something physically observable.

If you don’t deem it necessary to compare the existing Universe with what you predict or simulate, you are engaging in exercise for your imagination and for possibilities, but you are not doing physics. If you think there are no more doors to be opened, that’s fine; I don’t care if you give up. Just don’t expect many followers, particularly among physicists.

Artist’s impression of a black hole. What goes on outside the black hole is well understood, but inside, we run up against the limits of fundamental physics… and potentially, the laws governing the Universe itself. Image credit: XMM-Newton, ESA, NASA.

From Andrew on event horizons in classical vs. quantum realities: “Remarkable, that event horizon, as it is described by GRT is far from what quantum theory says. When an observer falls to the black hole and crosses aforementioned event horizon, he notice nothing extraordinary (well, if the black hole is large enough). But, in accordance with quantum theory, there will be a firewall…”

So just to refresh your memory, because it’s been about four years since I wrote about it, here’s the deal with black holes, event horizons and firewalls:

Given that particles fall into black holes and quantum entanglement is real, are firewalls real, and do you get fried falling in as a result? Probably not.

Image credit: Sabrina Herbst of Penn State.

A 2013 paper showed that entanglement across all event horizons is maximized, which pushed the time of black hole firewall formation out to… infinity. Since black holes decay in a finite time, these firewalls shouldn’t exist in our Universe. It’s not that quantum physics is in conflict with classical physics, though; it’s that this is an effect that shows up only when you add quantum effects in to your background of classical spacetime.

We still have not figured out a quantum theory of spacetime, or of gravity, which some argue will be necessary to fully understand the firewall paradox, and other paradoxes about black holes.

The eight planets of our Solar System and our Sun, to scale in size but not in terms of orbital distances. Mercury is the most difficult naked-eye planet to see. Image credit: Wikimedia Commons user WP.

From eric on an amusing fact about the 8 planets in our Solar System: “Amusing factoid: every planet in our solar system is more massive than all the planets smaller than it, combined. Thus, Venus has more mass than Mercury+Mars, Earth has more mass than Mercury+Mars+Venus, Uranus has more mass than Mercury+Mars+Venus+Earth, and so on.”

What’s also interesting is not just that this is true, but that it’s barely true! Venus (82% of Earth) plus Mars (11% of Earth) plus Mercury (5.5% of Earth) is almost equal to the mass of Earth, but not quite. Uranus (at 14.54 times the mass of Earth) plus all the rocky planets (1.98 Earth masses) comes out to 16.52 Earth masses, just shy of Neptune’s 17.15 Earth masses.

TRAPPIST-1 system compared to the solar system; all seven planets of TRAPPIST-1 could fit inside the orbit of Mercury. Note that at least the inner six worlds of TRAPPIST-1 are all locked to the star. Image credit: NASA / JPL-Caltech.

If you were to count Earth’s Moon and the four moons of Jupiter as “planets,” by the way — or go whole hog and buy into the “planetary science” definition of a planet — this fun fact would cease to be true. We know it is not a universal law, by the way, as many other solar systems, such as the TRAPPIST-1 system, violate this egregiously. In fact, if we consider that our Solar System likely once had a fifth rocky world similar in size to Mars that once collided with a proto-Earth, and that simulations indicate we once had a 5th gaseous world, it wasn’t always true for us, either.

Satellite photo of the Galapagos islands overlayed with the Spanish names of the visible main islands. The islands themselves are, at most, only a few million years old. Image credit: Jacques Descloitres, MODIS Rapid Response Project at NASA/GSFC.

From eric on Planet Earth II: “Watching Planet Earth II, I was struck by just how inhospitable Zavadovski Island is. Evidently the volcano is still active and has covered much of the island’s surface in fresh volcanic rock as recently as the 1800s. So basically, there’s little or no growing things on it. And yet, it’s home to a million penguins. So it’s not just mosses and algae that start the process of turning rocky wastelands into green pastures.”

Can I tell you how much I adore all of the BBC/Attenborough documentaries about nature and naturalism here on Earth? If you asked me to design my dream job — what I’d really like to do with my life if given the opportunity — it would be to do my own version of that for the Universe, astronomy and astrophysics. The Universe is out there, waiting for us to discover it, and the cosmic story is one we all share. I’d love to tell that story, as accurately and excitingly as possible, to anyone willing to listen and learn.

With all the active volcanic activity on the island, it’s possible — as the names suggest — that Zavodovski island is also the smelliest place in the world. Image credit: UK Antarctic Place-names Committee / British Antarctic Survey / NERC.

Zavodovski Island, by the way, is not in the Galapagos, but is rather a 5 km by 5 km island located at the northern edge of the South Sandwich Islands in the southern Atlantic Ocean. It’s maybe 1,000 kilometers north of Antarctica, as the crow flies. And it was discovered two centuries ago, but two recent eruptions, in 2012 and 2016, have put the penguin colonies located there at risk. Remember, penguins go there for safety from predators, to breed, and to warm themselves in the Sun, not for food or shelter; they don’t exactly create green pastures on their own, but they are an important part of the ecosystem nonetheless.

Niels Bohr and Albert Einstein together in 1925, engaging in their famous conversations/debates about quantum mechanics. Public domain image.

From Denier on winning a scientific debate: “At the end of the day, more and better science will win. If you want to win a scientific debate then forget the debate and focus on doing more and better science.”

So what do you do when a scientific debate is over and won, and yet the policies and politics of the world refuses to accept the results? What would you do, hypothetically, in that case as it applies to a flat Earth, to relativity, to evolution, to the Big Bang, etc.? I bring up the Big Bang in contrast to the other three, because unlike a flat Earth (with implications for travel and commerce), relativity (technologies like GPS), evolution (health and medical breakthroughs), there is no tangible, physical, capitalistic benefit or consequence to “believing in” the Big Bang.

How would doing more and better science help the world win?

Image credit: Japan Meteorological Association (JMA), of the monthly average temperatures in February, going back as far as temperature records do. Via the Sydney Morning Herald at

From Ragtag media exemplifying the difference between scientific and non-scientific debate: “OK my evidence is that a Steve Koonin was a Professor Of Theoretical Physics at Caltech, Was an Under Secretary of Science at the dept of Energy, and is the Director at NYU center for science and urban progress.
Says press releases put out by the govt was about climate data and climate analysis was “MISLEADING and sometimes just WRONG”

How does one person’s credentials, employment history, and claims made by them without showing the supporting scientific evidence count as a valid part of a scientific debate? What is the data? What do you think the world’s temperature is doing? What do you think atmospheric gases are doing? What do you think is the connection between temperature and these gases, and what do you think is responsible for the changes in gas concentration?

Do you see the difference between scientific questions and politicized accusations, and does it matter to you? Or is your goal to find and amplify the voices of the people who agree with you to sow doubt and outrage among those doing more and better science to further verify and solidify an already robust conclusion?

Mongo McMichael vs. Jeff Jarrett, 1997, WCW.

From lloyd on commenter policies: “Regarding denier, CFT and ragtag media: can we have Wow back now please?”

Wow is welcome back, as he was after his temporary 1 week ban. If denier, CFT, Ragtag media or anyone else engage in outrageous personal attacks or affronts against other individuals here they will be banned as well. Affronts against reason, logic, or scientific facts are not given bans in the same way; unless someone else is promoting their own pet theories/websites/spam sources, I don’t ban for that.

The Atmospheric Infrared Sounder (AIRS) instrument aboard NASA’s Aqua satellite senses temperature using infrared wavelengths. This image shows temperature of the Earth’s surface or clouds covering it for the month of April 2003. The scale ranges from -81 degrees Celsius (-114° Fahrenheit) in black/blue to 47° C (116° F) in red. Higher latitudes are increasingly obscured by clouds, though some features like the Great Lakes are apparent. Northernmost Europe and Eurasia are completely obscured by clouds, while Antarctica stands out cold and clear at the bottom of the image. Image credit: NASA AIRS.

From John on what science should be: “I think the purpose of Science is to describe – ideally to explain – the physical world. The purpose of Science is not to win debates. Science “wins” debates by providing a correct description or explanation of an event or process. Leave oratorical sleights of hand to others.”

I think that first sentence is definitely true; the second is arguably true; the third is demonstrably untrue as respects human beings on this planet. So… then what? Where are the “others” who will sway public opinion into agreement with the scientific conclusions? Will the real slim shady please stand up?

Cosmic rays produced by high-energy astrophysics sources can reach Earth’s surface. By detecting these fast-moving particles correctly, we can put Einstein’s relativity to the test. Image credit: ASPERA collaboration / AStroParticle ERAnet.

From D.C. Sessions on proving relativity for yourself: “$100? Spendthrift. Why not just run a current through a pair of wires and watch as they attract (or repel) each other. Special relativity at work, as even the slow drift velocity is enough to unbalance the effective balance between the forces exerted by moving electrons and stationary protons.”

And you are measuring the drift velocity of electrons how? And this is special relativity rather than electric current how? I am not saying that you are wrong; I am saying that this is not exactly a clear and obvious demonstration of relativity the way that receiving and observing a slew of unstable particles created very large distances away — that shouldn’t exist without relativity — are.

Image credit: Einstein deriving special relativity, 1934, via

From Frank on proving Einstein wrong: “I had read somewhere that one time a reporter said to Einstein “There are hundred professors who say you are wrong.”
Einstein answered “If I was really wrong just one professor would be enough.””

This is very, very relevant. I will try and keep this in mind, and see if I can source that and verify its validity.

A quantum eraser experiment setup, where two entangled particles are separated and measured. No alterations of one particle at its destination affect the outcome of the other. Image credit: Wikimedia Commons user Patrick Edwin Moran, under c.c.a.-s.a.-3.0.

From Sinisa Lazarek quoting the Stanford Encyclopedia of Philosophy: ““11. Conclusion
Grammatical variants of the term ‘observation’ have been applied to impressively different perceptual and non-perceptual process and to records of the results they produce. Their diversity is a reason to doubt whether general philosophical accounts of observation, observables, and observational data can tell epistemologists as much as local accounts grounded in close studies of specific kinds of cases. Furthermore, scientists continue to find ways to produce data that can’t be called observational without stretching the term to the point of vagueness.
It’s plausible that philosophers who value the kind of rigor, precision, and generality to which l logical empiricists and other exact philosophers aspired could do better by examining and developing techniques and results from logic, probability theory, statistics, machine learning, and computer modeling, etc. than by trying to construct highly general theories of observation and its role in science. Logic and the rest seem unable to deliver satisfactory, universally applicable accounts of scientific reasoning. But they have illuminating local applications, some of which can be of use to scientists as well as philosophers.””

If you’re more concerned with an ideology or the “purity” of a definition than what’s actually scientifically robust about observations, prediction, measurement and reproducibility, that’s your prerogative, but please stay out of the way of those doing the science and adding to our actual, empirical understanding of the world. You are free to advocate for your preferred logical systems or syllogisms or definitions all you like, but don’t try and place unnecessary restrictions on physics; it won’t obey them anyway.

Image credit: John D. Norton, via

From Kasim Muflahi on misinterpreting relativity: “>>if you observe someone in motion relative to you, their clock will appear to run slow.
That’s because, as the clock moves further and further away from you, the images of the clock take longer and longer times to get to you giving the illusion that the clock is running slow.”

So I always hesitate to tell someone that they are wrong because I worry that they will not listen if they hear that… so let’s try a different approach.

Sure! When an object moves away from you, all the waves coming from it — including light waves — will take longer to reach you from crest-to-crest. So if your idea is correct, time should be sped-up as an object moves towards you, and it should remain constant when an object moves transverse to you.

Unfortunately, we can do the experiment, and not only is the clock really running slow (it’s not an illusion), but it does so independent of the redshift/blueshift of the waves. So we can put your idea to the test and show, scientifically, that it is not the way things actually work.

Thanks for a good week, everyone, and looking forward to another great week of science ahead!