Ask Ethan: How Many Generations Of Stars Formed Before Our Sun?
“[My professor] and I were discussing the status of our Sun. I was leaning towards the Sun being a third generation star and she was thinking the Sun was second. […] Any thoughts? And how might this be resolved with [future] technology?”
How many generations of stars formed before our Sun did? Surprisingly, this is not a questions astronomers know the answer to, nor is it one that we can envision coming up with a definitive answer for. In the modern Universe, we don’t have access to history or pre-history of the Solar System; all we can see is what remains today. The rest of it is up to us, the scientific community, to reconstruct.
But we’re pretty incredible at what we can say based on what we do know. The Sun cannot be a first-generation, pristine star, since none of those have ever been even discovered. It couldn’t be a second-generation star, because those are ubiquitous and have very different properties than our own Sun. But where are we then? And what do we know, and what remains mysterious?
It’s a great question with a remarkable set of facts behind what we know so far, all for your enjoyment on this week’s Ask Ethan!
Ask Ethan: Would An Alien Civilization Classify Earth As An ‘Interesting’ Planet?
“I was thinking about the projection of light through space. My curtain was open and I saw the stars and something from a book popped into my head. It had said that the stars we see are basically reruns. The light is from so long ago, we don’t even know if the star still exists or not.
[…] Whatever signals we send out, or changes in our planet that might be observable to prove intelligent life lives here, would take billions of years to reach anything alive and capable of responding! What do you think?”
The cosmic distances separating the stars and galaxies are absolutely tremendous, and even though the speed of light is the fastest speed there is, it still takes an awfully long time to traverse the astronomical abyss of space. Humanity has only been a technologically advanced civilization for a few hundred years, and we’ve only entered the space age a few decades ago. Yet that doesn’t mean we’re off-limits to advanced aliens who might be looking for us at all. Even if they couldn’t discover our technosignatures, they could still tell, even from billions of light-years away, that Earth was an interesting, inhabited planet, using nothing more than more advanced versions of the technologies we’re using today and in the near-future to look for life on exoplanets in our cosmic backyard.
An alien civilization with more advanced technology could detect cosmically interesting things about Earth from extremely far away. Here’s where your hopes and fears meet with reality: on this week’s Ask Ethan!
Ask Ethan: Can Black Holes And Dark Matter Interact?
“If you do the math, you’ll find that black holes will use both normal matter and dark matter as a food source, but that normal matter will dominate the rate of growth of the black hole, even over long, cosmic timescales. When the Universe is more than a billion times as old as it is today, black holes will still owe more than 99% of their mass to normal matter, and less than 1% to dark matter.
Dark matter is neither a good food source for black holes, nor is it (information-wise) an interesting one. What a black hole gains from eating dark matter is no different than what it gains from shining a flashlight into it. Only the mass/energy content, like you’d get from E = mc2, matters. Black holes and dark matter do interact, but their effects are so small that even ignoring dark matter entirely still gives you a great description of black holes: past, present, and future.”
You might not be able to make a black hole out of dark matter entirely, but once a black hole exists, anything that falls past its event horizon will add to its mass, whether it’s particles, antiparticles, radiation or dark matter. And the longer black holes sit in the galaxy, the more and more dark matter will eventually fall in.
The question isn’t whether dark matter contributes to black holes; it’s how and how much. Let’s give you the answer on this edition of Ask Ethan!
Ask Ethan: How Well Has Cosmic Inflation Been Verified?
“To what margin of error or what level of statistical significance would you say you say inflation has been verified?”
So, you’ve got an alternative theory to our best mainstream scientific ideas? Well, guess what: those are the same shoes that every scientific idea we accept today were wearing at one point in the distant past. The thing that separates them from the ideas that fell by the wayside were three remarkable feats:
1. They reproduced all the earlier successes of the previous prevailing model.
2. They resolved or explained puzzles or problems that the previous model had no sufficient answer for.
3. And, perhaps most importantly, they made new predictions that we could go out and test about the Universe, and those predictions were proven correct by the appropriate experimental or observational test.
Although most people don’t appreciate it, inflation has hurdled all three bars, and has no fewer than four spectacular predictions that have since been confirmed. Come learn how well cosmic inflation has been verified today!
Ask Ethan: Could ‘Cosmic Redshift’ Be Caused By Galactic Motion, Rather Than Expanding Space?
“When we observe a distant galaxy, the light coming from the galaxy is redshifted either due to expansion of space or actually the galaxy is moving away from us. How do we differentiate between the cosmological redshift and Doppler redshift? I have searched the internet for answers but could not get any reasonable answer.”
It’s true: the farther away we look, the greater we find a galaxy’s redshift to be. But why is that? You may have heard the (correct) answer: because space is expanding. But how do we know that? Couldn’t something else be causing this redshift?
The answer is yes, there are actually four other explanations for cosmic redshift that all make sense. But the beauty of science is that there are observational tests we can perform to tell these various scenarios apart! We’ve done those tests, of course, and concluded the Universe is expanding, but wouldn’t you like to know how?
I bet you would! Come and find out how we know that cosmic redshift is caused by the expansion of the Universe, and learn where the alternatives fall apart.
Ask Ethan: What Happens When You Fall Into A Black Hole?
“If you could only know the answer to one question about the Universe, what would you ask? What would you want to know more than anything else? As we get older, most of us lose sight of the things we wondered about as children, which is why I was delighted to get a message from Eric Erb about ten questions that his son, Tristan, brought home from his 2nd grade class. Two of the biggest mysteries of all, gravity and time, dominated his curiosity. After boiling it down, here’s what he wanted to know:
I asked him just now and he wanted two questions to be answered.
1. What happens when you fall into a black hole?
2. Why/how does gravity pull us?
Let’s start at the beginning, and make sure we get to it all.”
If you fell into a black hole, you’d first approach the event horizon from a great distance away, and the light from the Universe around you would start to distort. As you got closer, the event horizon would appear to grow in size faster than you might expect, and the distortions would grow larger in magnitude and color. Time would continue to run at the same, normal speed, and you might not notice crossing the event horizon, but the spaghettifying forces on your body sure would tell you something was up.
Where would you wind up? No one is sure, but the possibilities are both fascinating and terrifying. Find out what happens when you fall into a black hole today.
Ask Ethan: Are Gravitational Waves Themselves Affected By Gravity?
“Are gravitational waves themselves subject to gravity? That is, if a gravitational wave were to pass by a galaxy cluster, would its form get distorted (even though the wave, itself, is a distortion of space-time)? One side of me says gravitational waves are a form of energy so therefore must be affected by gravity. The other side of me says “Nah – that just doesn’t make sense!"”
Think about the fabric of space itself. All the masses and forms of energy in the Universe cause space itself to curve, while the curved space itself alters the path along which any matter or form of energy will travel. Massless particles, like photons, are bent by the fabric of space itself. But what about gravitational waves? Are they also subject to this, or does gravitation lack a self-interaction that it would require for this to be possible?
For a very long time, this was a question that was theoretical only. But over the last three years, we’ve observed a slew of gravitational waves, allowing this idea to be tested for the first time.
What were the results? Gravitational waves are affected by gravity, in at least three different observable ways. Come find out how today!
Ask Ethan: How Do Massless Particles Experience Gravity?
“Given the equation for gravity between two masses, and the fact that photons are massless, how is it possible for a mass (like a star or a black hole) to exert influence on said photon?”
You know the law of universal gravitation: you put in what any two masses are, how far apart they are from each other, and the gravitational constant of the Universe, and you can immediately know what the force is between any two objects. Set one of the masses to zero, and the force goes to zero. So why is it, then, that if you take the ultimate particle with no mass, a photon, and pass it close by a mass, its path does bend? Why do massless particles experience gravity?
To understand why, you should think about what happens if you and I start at the same place near a mass, but I’m stationary and you’re moving. How far away is that mass? What’s the “r” that goes into Newton’s equation? And who’s right: me or you?
The answer is that we both need to be right, and Newton won’t get us there. Come get the real story on gravity, and learn why, in the end, massless particles feel it, too!
Ask Ethan: What Is Energy?
“We talk about energy and we know that there are various forms of energy (PE, KE …) and you can do work with it, and it has to be conserved, and energy and matter are interchangeable, etc. But what is energy?”
Energy is something that touches all aspects of our lives. Yet if you try defining it, you’ll wind up tying yourself in knots. It’s not something we can isolate in a laboratory, but rather is a property inherent to all matter, antimatter, and radiation particles. It can only be defined relative to other particles, rather than absolutely. The definition we use in physics, that it’s the ability to do work, is over 300 years old and is rather circular.
A little over a century ago, the esteemed physicist Henri Poincaré noted the following, “science is built up of facts, as a house is built of stones; but an accumulation of facts is no more a science than a heap of stones is a house.” We speak all the time of what energy can do, how it’s used, where it appears and in what quantities, and how to accomplish a myriad of tasks with it. But a fundamental, universal definition?
For as far as we’ve come, giving an unambiguous, universal definition of energy is still beyond our reach. Come find out why.
Ask Ethan: Does The Measurement Of The Muon’s Magnetic Moment Break The Standard Model?
“[There’s a notable] difference between theory and experiment [for the muon’s magnetic moment]. Is the fact that the [uncertainties are large] more meaningful than the >3 sigma significance calculation? The Mercury precession must have a very small sigma, but is cited as a big proof of relativity. What is a good measure of significance for new physics results?”
Whenever theoretical predictions and experimental results disagree, that’s surely a sign of something interesting. If we’re extremely lucky, it might be a sign of new fundamental physics, which could mean new laws of nature, new particles, new fields, or new interactions. Any of these would be revolutionary, and certainly it’s the great hope of anyone who works on these projects: to peel back the curtain of reality and find the next layer inside. But there are two other possibilities, far more conservative and mundane, that must be ruled out first. One is an error, either on the theoretical or experimental side, that has simply been overlooked. The other is even more subtle, though: an effect from a known physical cause that’s at the heart of this discrepancy, which we haven’t thought we needed to include until now.
The muon’s anomalous magnetic moment might be a harbinger of new physics. But it might also be a subtle effect of gravity that’s appearing for the first time. Come look at the evidence and see for yourself!