Albert Einstein during a lecture in Vienna in 1921.
Credit: National Library of Austria/F Schmutzer/Public Domain
In the history of science and physics, several scholars, theories, and equations have become household names. In terms of scientists, notable examples include Pythagoras, Aristotle, Galileo, Newton, Planck, and Hawking. In terms of theories, there’s Archimede’s “Eureka,” Newton’s Apple (Universal Gravitation), and Schrodinger’s Cat (quantum mechanics). But the most famous and renowned is arguably Albert Einstein, Relativity, and the famous equation, E=mc2. In fact, Relativity may be the best-known scientific concept that few people truly understand.
For example, Einstein’s Theory of Relativity comes in two parts: the Special Theory of Relativity (SR and the General Theory of Relativity (GR). And the term “Relativity” itself goes back to Galileo Galilee and his explanation for why motion and velocity are relative to the observer. As you can probably tell, explaining how Einstein’s groundbreaking theory works require a deep dive into the history of physics, some advanced concepts, and how it all came together for one of the greatest minds of all time!
Special relativity is one of the most strongly validated theories humanity has ever devised. It is central to everything from space travel and GPS to our electrical power grid. Central to relativity is the fact that the speed of light in a vacuum is an absolute constant. The problem is, that fact has never been proven.
A time machine connects to the past. Credit: David Revoy / Blender Foundation (CC BY 3.0)
Time travel is a staple of science fiction, and not without reason. Who wouldn’t want to go back in time to explore history, or save the world from catastrophe. Time travel has also been deeply studied within the context of theoretical physics because it tests the limits of our scientific theories. If time travel is possible, it has implications for everything from the origin of the universe to the existence of free will. One of the central problems of time travel theory is that it gives rise to logical paradoxes. But a couple of researchers think they have solved the pesky paradox problem.
During the 1930s, venerable theoretical physicist Albert Einstein returned to the field of quantum mechanics, which his theories of relativity helped to create. Hoping to develop a more complete theory of how particles behave, Einstein was instead horrified by the prospect of quantum entanglement – something he described as “spooky action at a distance”.
Despite Einstein’s misgivings, quantum entanglement has gone on to become an accepted part of quantum mechanics. And now, for the first time ever, a team of physicists from the University of Glasgow took an image of a form of quantum entanglement (aka. Bell entanglement) at work. In so doing, they managed to capture the first piece of visual evidence of a phenomenon that baffled even Einstein himself.
Relativity is used in more day to day situations than you may realize. In this episode, we will count (some of) the ways. This episode is brought to you live from the All-Stars Star Party in Indian Wells, California. Continue reading “Ep. 536: Everyday Relativity”
The sign of a truly great scientific theory is by the outcomes it predicts when you run experiments or perform observations. And one of the greatest theories ever proposed was the concept of Relativity, described by Albert Einstein in the beginning of the 20th century.
In addition to helping us understand that light is the ultimate speed limit of the Universe, Einstein described gravity itself as a warping of spacetime.
He did more than just provide a bunch of elaborate new explanations for the Universe, he proposed a series of tests that could be done to find out if his theories were correct.
One test, for example, completely explained why Mercury’s orbit didn’t match the predictions made by Newton. Other predictions could be tested with the scientific instruments of the day, like measuring time dilation with fast moving clocks.
Since gravity is actually a distortion of spacetime, Einstein predicted that massive objects moving through spacetime should generate ripples, like waves moving through the ocean.
The more massive the object, the more it distorts spacetime. Credit: LIGO/T. Pyle
Just by walking around, you leave a wake of gravitational waves that compress and expand space around you. However, these waves are incredibly tiny. Only the most energetic events in the entire Universe can produce waves we can detect.
It took over 100 years to finally be proven true, the direct detection of gravitational waves. In February, 2016, physicists with the Laser Interferometer Gravitational Wave Observatory, or LIGO announced the collision of two massive black holes more than a billion light-years away.
Any size of black hole can collide. Plain old stellar mass black holes or supermassive black holes. Same process, just on a completely different scale.
Colliding black holes. Credit: LIGO/A. Simonnet
Let’s start with the stellar mass black holes. These, of course, form when a star with many times the mass of our Sun dies in a supernova. Just like regular stars, these massive stars can be in binary systems.
Imagine a stellar nebula where a pair of binary stars form. But unlike the Sun, each of these are monsters with many times the mass of the Sun, putting out thousands of times as much energy. The two stars will orbit one another for just a few million years, and then one will detonate as a supernova. Now you’ll have a massive star orbiting a black hole. And then the second star explodes, and now you have two black holes orbiting around each other.
As the black holes zip around one another, they radiate gravitational waves which causes their orbit to decay. This is kind of mind-bending, actually. The black holes convert their momentum into gravitational waves.
As their angular momentum decreases, they spiral inward until they actually collide. What should be one of the most energetic explosions in the known Universe is completely dark and silent, because nothing can escape a black hole. No radiation, no light, no particles, no screams, nothing. And if you mash two black holes together, you just get a more massive black hole.
The gravitational waves ripple out from this momentous collision like waves through the ocean, and it’s detectable across more than a billion light-years.
Arial view of LIGO Livingston. Credit: The LIGO Scientific Collaboration
This is exactly what happened earlier this year with the announcement from LIGO. This sensitive instrument detected the gravitational waves generated when two black holes with 30 solar masses collided about 1.3 billion light-years away.
This wasn’t a one-time event either, they detected another collision with two other stellar mass black holes.
Regular stellar mass black holes aren’t the only ones that can collide. Supermassive black holes can collide too.
From what we can tell, there’s a supermassive black hole at the heart of pretty much every galaxy in the Universe. The one in the Milky Way is more than 4.1 million times the mass of the Sun, and the one at the heart of Andromeda is thought to be 110 to 230 million times the mass of the Sun.
In a few billion years, the Milky Way and Andromeda are going to collide, and begin the process of merging together. Unless the Milky Way’s black hole gets kicked off into deep space, the two black holes are going to end up orbiting one another.
Just with the stellar mass black holes, they’re going to radiate away angular momentum in the form of gravitational waves, and spiral closer and closer together. Some point, in the distant future, the two black holes will merge into an even more supermassive black hole.
View of Milkdromeda from Earth “shortly” after the merger, around 3.85-3.9 billion years from now. Credit: NASA, ESA, Z. Levay and R. van der Marel (STScI), T. Hallas, and A. Mellinger
The Milky Way and Andromeda will merge into Milkdromeda, and over the future billions of years, will continue to gather up new galaxies, extract their black holes and mashing them into the collective.
Black holes can absolutely collide. Einstein predicted the gravitational waves this would generate, and now LIGO has observed them for the first time. As better tools are developed, we should learn more and more about these extreme events.
In a previous article, I talked about how you can generate artificial gravity by accelerating at 9.8 meters per second squared. Do that and you pretty much hit the speed of light, then you decelerate at 1G and you’ve completed an epic journey while enjoying comfortable gravity on board at the same time. It’s a total win win.
What I didn’t mention how this acceleration messes up time for you and people who aren’t traveling with you. Here’s the good news. If you accelerate at that pace for years, you can travel across billions of light years within a human lifetime.
Here’s the bad news, while you might experience a few decades of travel, the rest of the Universe will experience billions of years. The Sun you left will have died out billions of years ago when you arrive at your destination.
Welcome to the mind bending implications of constantly accelerating relativistic spaceflight.
With many things in physics, we owe our understanding of relativistic travel to Einstein. Say it with me, “thanks Einstein.”
The effect of time dilation is negligible for common speeds, such as that of a car or even a jet plane, but it increases dramatically when one gets close to the speed of light.
It works like this. The speed of light is always constant, no matter how fast you’re going. If I’m standing still and shine a flashlight, I see light speed away from me at 300,000 km/s. And if you’re traveling at 99% the speed of light and shine a flashlight, you’ll see light moving away at 300,000 km/s.
But from my perspective, standing still, you look as if you’re moving incredibly slowly. And from your nearly light-speed perspective, I also appear to be moving incredibly slowly – it’s all relative. Whatever it takes to make sure that light is always moving at, well, the speed of light.
This is time dilation, and you’re actually experiencing it all the time, when you drive in cars or fly in an airplane. The amount of time that elapses for you is different for other people depending on your velocity. That amount is so minute that you’ll never notice it, but if you’re traveling at close to the speed of light, the differences add up pretty quickly.
But it gets even more interesting than this. If you could somehow build a rocket capable of accelerating at 9.8 meters/second squared, and just went faster and faster, you’d hit the speed of light in about a year or so, but from your perspective, you could just keep on accelerating. And the longer you accelerate, the further you get, and the more time that the rest of the Universe experiences.
The really strange consequence, though, is that from your perspective, thanks to relativity, flight times are compressed.
I’m using the relativistic star ship calculator at convertalot.com. You should give it a try too.
Proxima Centauri. Credit: ESA/Hubble & NASA
For starters, let’s fly to the nearest star, 4.3 light-years away. I accelerate halfway at a nice comfortable 1G, then turn around and decelerate at 1G. It only felt like 3.5 years for me, but back on Earth, everyone experienced almost 6 years. At the fastest point, I was going about 95% the speed of light.
Let’s scale this up and travel to the center of the Milky Way, located about 28,000 light-years away. From my perspective, only 20 years have passed by. But back on Earth, 28,000 years have gone by. At the fastest point, I was going 99.9999998 the speed of light.
Let’s go further, how about to the Andromeda Galaxy, located 2.5 million light-years away. The trip only takes me 33 years to accelerate and decelerate, while Earth experienced 2.5 million years. See how this works?
The Andromeda Galaxy. Credit: NASA/JPL-Caltech/WISE Team
I promised I’d blow your mind, and here it is. If you wanted to travel at a constant 1G acceleration and then deceleration to the very edge of the observable Universe. That’s a distance of 13.8 billion light-years away; you would only experience a total of 45 years. Of course, once you got there, you’d have a very different observable Universe, and billions of years of expansion and dark energy would have pushed the galaxies much further away from you.
Some galaxies will have fallen over the cosmic horizon, where no amount of time would ever let you reach them.
If you wanted to travel 100 trillion light years away, you could make the journey in 62 years. By the time you arrived, the Universe would be vastly different. Most of the stars would have died a long time ago, the Universe would be out of usable hydrogen. You would have have left a living thriving Universe trillions of years in the past. And you could never get back.
Our good friends over at Kurzgesagt covered a very similar topic, discussing the limits of humanity’s exploration of the Universe. It’s wonderful and you should watch it right now.
Of course, creating a spacecraft capable of constant 1G acceleration requires energies we can’t even imagine, and will probably never acquire. And even if you did it, the Universe you enjoy would be a distant memory. So don’t get too excited about fast forwarding yourself trillions of years into the future.
At the turn of the 20th Century, Einstein’s theory of relativity stunned the physics world, but the experimental evidence needed to be found. And so, in 1919, another respected astronomer, Arthur Eddington, observed the deflection of stars by the gravity of the Sun during a solar eclipse. Here’s the story of that famous experiment. Continue reading “Astronomy Cast Ep. 371: The Eddington Eclipse Experiment”
One of the most amazing implications of Einstein’s relativity is the fact that the inertial mass of an object depends on its velocity. That sounds like a difficult thing to test, but that’s exactly what happened through a series of experiments performed by Kaufmann, Bucherer, Neumann and others. Continue reading “Astronomy Cast Ep. 370: The Kaufmann–Bucherer–Neumann Experiments”
Cosmologists are intellectual time travelers. Looking back over billions of years, these scientists are able to trace the evolution of our Universe in astonishing detail. 13.8 billion years ago, the Big Bang occurred. Fractions of a second later, the fledgling Universe expanded exponentially during an incredibly brief period of time called inflation. Over the ensuing eons, our cosmos has grown to such an enormous size that we can no longer see the other side of it.
But how can this be? If light’s velocity marks a cosmic speed limit, how can there possibly be regions of spacetime whose photons are forever out of our reach? And even if there are, how do we know that they exist at all?
The Expanding Universe
Like everything else in physics, our Universe strives to exist in the lowest possible energy state possible. But around 10-36 seconds after the Big Bang, inflationary cosmologists believe that the cosmos found itself resting instead at a “false vacuum energy” – a low-point that wasn’t really a low-point. Seeking the true nadir of vacuum energy, over a minute fraction of a moment, the Universe is thought to have ballooned by a factor of 1050.
Since that time, our Universe has continued to expand, but at a much slower pace. We see evidence of this expansion in the light from distant objects. As photons emitted by a star or galaxy propagate across the Universe, the stretching of space causes them to lose energy. Once the photons reach us, their wavelengths have been redshifted in accordance with the distance they have traveled.
Two sources of redshift: Doppler and cosmological expansion; modeled after Koupelis & Kuhn. Bottom: Detectors catch the light that is emitted by a central star. This light is stretched, or redshifted, as space expands in between. Credit: Brews Ohare.
This is why cosmologists speak of redshift as a function of distance in both space and time. The light from these distant objects has been traveling for so long that, when we finally see it, we are seeing the objects as they were billions of years ago.
The Hubble Volume
Redshifted light allows us to see objects like galaxies as they existed in the distant past; but we cannot see all events that occurred in our Universe during its history. Because our cosmos is expanding, the light from some objects is simply too far away for us ever to see.
The physics of that boundary rely, in part, on a chunk of surrounding spacetime called the Hubble volume. Here on Earth, we define the Hubble volume by measuring something called the Hubble parameter (H0), a value that relates the apparent recession speed of distant objects to their redshift. It was first calculated in 1929, when Edwin Hubble discovered that faraway galaxies appeared to be moving away from us at a rate that was proportional to the redshift of their light.
Fit of redshift velocities to Hubble’s law. Credit: Brews Ohare
Dividing the speed of light by H0, we get the Hubble volume. This spherical bubble encloses a region where all objects move away from a central observer at speeds less than the speed of light. Correspondingly, all objects outside of the Hubble volume move away from the center faster than the speed of light.
Yes, “faster than the speed of light.” How is this possible?
The Magic of Relativity
The answer has to do with the difference between special relativity and general relativity. Special relativity requires what is called an “inertial reference frame” – more simply, a backdrop. According to this theory, the speed of light is the same when compared in all inertial reference frames. Whether an observer is sitting still on a park bench on planet Earth or zooming past Neptune in a futuristic high-velocity rocketship, the speed of light is always the same. A photon always travels away from the observer at 300,000,000 meters per second, and he or she will never catch up.
General relativity, however, describes the fabric of spacetime itself. In this theory, there is no inertial reference frame. Spacetime is not expanding with respect to anything outside of itself, so the the speed of light as a limit on its velocity doesn’t apply. Yes, galaxies outside of our Hubble sphere are receding from us faster than the speed of light. But the galaxies themselves aren’t breaking any cosmic speed limits. To an observer within one of those galaxies, nothing violates special relativity at all. It is the space in between us and those galaxies that is rapidly proliferating and stretching exponentially.
The Observable Universe
Now for the next bombshell: The Hubble volume is not the same thing as the observable Universe.
To understand this, consider that as the Universe gets older, distant light has more time to reach our detectors here on Earth. We can see objects that have accelerated beyond our current Hubble volume because the light we see today was emitted when they were within it.
Strictly speaking, our observable Universe coincides with something called the particle horizon. The particle horizon marks the distance to the farthest light that we can possibly see at this moment in time – photons that have had enough time to either remain within, or catch up to, our gently expanding Hubble sphere.
And just what is this distance? A little more than 46 billion light years in every direction – giving our observable Universe a diameter of approximately 93 billion light years, or more than 500 billion trillion miles.
The observable universe, more technically known as the particle horizon.
(A quick note: the particle horizon is not the same thing as the cosmological event horizon. The particle horizon encompasses all the events in the past that we can currently see. The cosmological event horizon, on the other hand, defines a distance within which a future observer will be able to see the then-ancient light our little corner of spacetime is emitting today.
In other words, the particle horizon deals with the distance to past objects whose ancient light that we can see today; the cosmological event horizon deals with the distance that our present-day light that will be able to travel as faraway regions of the Universe accelerate away from us.)
Dark Energy
Thanks to the expansion of the Universe, there are regions of the cosmos that we will never see, even if we could wait an infinite amount of time for their light to reach us. But what about those areas just beyond the reaches of our present-day Hubble volume? If that sphere is also expanding, will we ever be able to see those boundary objects?
This depends on which region is expanding faster – the Hubble volume or the parts of the Universe just outside of it. And the answer to that question depends on two things: 1) whether H0 is increasing or decreasing, and 2) whether the Universe is accelerating or decelerating. These two rates are intimately related, but they are not the same.
In fact, cosmologists believe that we are actually living at a time when H0 is decreasing; but because of dark energy, the velocity of the Universe’s expansion is increasing.
That may sound counterintuitive, but as long as H0 decreases at a slower rate than that at which the Universe’s expansion velocity is increasing, the overall movement of galaxies away from us still occurs at an accelerated pace. And at this moment in time, cosmologists believe that the Universe’s expansion will outpace the more modest growth of the Hubble volume.
So even though our Hubble volume is expanding, the influence of dark energy appears to provide a hard limit to the ever-increasing observable Universe.
Our Earthly Limitations
Cosmologists seem to have a good handle on deep questions like what our observable Universe will someday look like and how the expansion of the cosmos will change. But ultimately, scientists can only theorize the answers to questions about the future based on their present-day understanding of the Universe. Cosmological timescales are so unimaginably long that it is impossible to say much of anything concrete about how the Universe will behave in the future. Today’s models fit the current data remarkably well, but the truth is that none of us will live long enough to see whether the predictions truly match all of the outcomes.
Disappointing? Sure. But totally worth the effort to help our puny brains consider such mind-bloggling science – a reality that, as usual, is just plain stranger than fiction.
