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!
Ask Ethan: Can We Build A Sun Screen To Combat Global Climate Change?
“[W]hy don’t we evaluate building a “sun screen” in space to alter the amount of light (energy) earth receives? Everybody who did feel a total eclipse knows temperature goes down and light dims. So the idea is to build something that would stay between us and sun all year long…”
The Earth’s temperature is rising: that’s a fact. The overwhelming cause of this warming is human-caused emissions of greenhouse gases, like CO2 and methane. The CO2 concentration has now passed 410 parts per million, an increase of over 50% over pre-industrial revolution levels. If we cannot or will not reverse what’s driving the temperature change, we could instead try to counteract it. One proposal is to put a large device in space, between the Sun and the Earth, to block some of the incoming sunlight. As it turns out, blocking 2% would be enough to completely counteract the effects of human-caused global warming. Modifying our atmosphere is risky, but placing a sun screen in space to do the job has only one real drawback: its expense.
But how expensive is it, truly, when you consider the positive effects it would have on the planet? Let’s consider what it would take to combat global climate change by putting a shade in space!
Ask Ethan: Will Future Civilizations Miss The Big Bang?
“If intelligent life re-emerges in our solar system in a few billion years, only a few points of light will still be visible in the sky. What kind of theory of the universe will those beings concoct? It is almost certain to be wrong. Why do we think that what we can view now can lead us to a “correct” theory when a few billion years before us, things might have looked completely different?”
Incredibly, the Universe we know and love today won’t be the way it is forever. If we were born in the far future, perhaps a hundred billion years from now, we wouldn’t have another galaxy to look at for a billion light years: hundreds of times more distant than the closest galaxies today. Our local group will merge into a single, giant elliptical galaxy, and there will be no sign at all of young stars, of star-forming regions, of other galaxies, or even of the Big Bang’s leftover glow. If we were born in the far future, we might miss the Big Bang as the correct origin of our Universe. It makes one wonder, when you think about it in those terms, if we’re missing something essential about our Universe today? In the 13.8 billion years that have passed, could we already have lost some essential information about the history of our Universe?
And in the far future, might we see something that, as of right now, hasn’t yet grown to prominence? Let’s explore this and see what you think!
Ask Ethan: How Big Will The Universe Get?
“The current estimate for the diameter of the universe is 93 billion light years. With the current acceleration of the universe measured by redshift, and the future exponential acceleration, how long until “we” hit a diameter of 100 billion light years?”
Our Universe is made up of a number of different types of energy, including dark energy, dark matter, normal matter, neutrinos, and radiation. When you combine those different forms of energy with our observed expansion rate, you arrive at a Universe prediction for how the Universe expanded in the past and how it will continue to expand into the future. As distant galaxies accelerate away from us, we can make predictions for how large our observable Universe will get as time goes on. At present, our visible Universe is 92 billion light years in diameter, with an age of 13.8 billion years. When will we hit 100 billion light years? Or a trillion? Or a quadrillion?
The answer is straightforward, fun, and profound. Come find out how big the Universe will get, and how fast it will get there, on this week’s Ask Ethan!
Ask Ethan: Could The Universe’s Missing Antimatter Be Found Inside Black Holes?
“It is a mystery why we see matter without corresponding antimatter. Some remote and old super massive black holes evolved much faster than current theory is able to predict. Could the missing antimatter be hiding inside those primordial black holes? Does the total mass of super massive black holes come even close to the amount of missing anti matter?”
When we look out at the Universe today, we see that everything is made of matter and not antimatter. This is a puzzle, because the laws of physics appear to be symmetric between matter and antimatter: you can’t create or destroy either one without creating or destroying an equal amount of the other. Is it possible that we actually created equal amounts of both, and that the antimatter collapsed into black holes, which might be responsible for either supermassive black holes or primordial black holes as dark matter? While, on the other hand, the normal matter didn’t collapse, and became the stars, gas, galaxies, and more that we observe today?
It’s a fascinating alternative to the standard picture that our Universe is fundamentally asymmetric, but does it hold up? Find out on this week’s Ask Ethan!