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!
Ask Ethan: How Will Our Universe End?
“When will our universe reach the point of maximum entropy? And what other possibilities exist for our universe in the far future?”
It’s nearly 14 billion years since the hot Big Bang gave rise to our observable Universe, which now consists of some 2 trillion galaxies spread out across a sphere over 46 billion light years in radius. But despite how plentiful the matter in our Universe is, it won’t last forever. The stars will all burn out, and even the new stars that form will eventually run out of gas to form from. Dark energy will drive the unbound galaxies away, while gravitation will pull the bound ones into a single structure. Over time, ejections and mergers occur, littering the Universe with isolated masses and setting up enormous black holes embedded in dark matter halos as the last remnants of galaxies. After enough time passes, the final black holes decay, leaving only low-energy, ultra-high-entropy radiation behind.
It will take a long time, but this is the ultimate fate of everything in the far future of the Universe!
Ask Ethan: How Fast Could Life Have Arisen In The Universe?
“How soon after the Big Bang would there have been enough heavy elements to form planets and possibly life?”
Making anything in this Universe takes time. After the Big Bang, there are a whole slew the Universe needed to take before rocky planets and life were possible. This includes the formation of atomic nuclei, neutral atoms, dense enough gas clouds to make stars, multiple generations of stars living-and-dying, and only then will the Universe be filled with the right ingredients to create rocky worlds and, potentially, life. But Earth didn’t come into existence until more than 9 billion years after the Big Bang, and these ingredients were around long before that. The heavy elements from the first supernovae could have made rocky, Earth-like planets very early on, but interestingly enough, it takes longer to form enough carbon to make life a reasonable possibility.
Let’s run through the Universe and find when life could have first evolved. The answer might be sooner than you think!
Ask Ethan: What Happens When Stars Pass Through Our Solar System?
“How bad would it be if a star passed near the Sun? How close/large would it have to be to pose serious danger? How likely would such an event be?”
Space is a pretty empty place; it’s more than four light years to the nearest star. But despite this, we’re moving through the galaxy at around 220 km/s, passing and being passed by other stars at about 10% of that speed. Over long periods of time, stars occasionally make close passes by our own, meaning that they could pertub the Oort cloud, the Kuiper belt, or (if they got close enough) even the orbits of the planets themselves. Which of these is a realistic concern, and how often do these events actually occur? Moreover, when they do occur, what are the implications for what we’ll experience here on Earth? Will there be a pretty light show? A series of cometary impacts? Or a complete disruption of our orbit?
There’s only one way to find out, and that’s to calculate it so we know! Let’s walk you through exactly that, and what it means, for this week’s Ask Ethan!