What Would It Be Like To Fall Into A Black Hole?

Let’s say you happened to fall into the nearest black hole? What would you experience and see? And what would the rest of the Universe see as this was happening?

Let’s say you decided to ignore some of my previous advice. You’ve just purchased yourself a space dragon from the Market on the Centauri Ringworld, strapped on your favorite chainmail codpiece and sonic sword and now you’re going ride head first into the nearest black hole.

We know it won’t take you to another world or galaxy, but what would you experience and see on your way to your inevitable demise? And what would the rest of the Universe see as this was happening, and would they point and say “eewwwwww”?

If you were falling toward a black hole, most of the time you would simply feel weightless, just as if you were playing Bowie songs and floating in a most peculiar way in the International Space Station. The gravity of a black hole is just like the gravity of any other large mass, as long as you don’t get too close. But, as we’ve agreed, you’re ignoring my advice and flying dragon first into this physics nightmare. As you get closer, the gravitational forces on various parts of your and your dragon’s body would be different. Technically this is always true, but you wouldn’t notice it… at least at first.

Suppose you were falling feet first toward a black hole. As you got closer, your feet would feel a stronger force than your head, for example. These differences in forces are called tidal forces. Because of the tidal forces it would feel as if you are being stretched head to toe, while your sides would feel like they are being pushed inward. Eventually the tidal forces would become so strong that they would rip you apart. This effect of tidal stretching is sometimes boringly referred to as spaghettification.

I’ve made up some other names for it, such as My Own Private String Cheese Incident, “the soft-serve effect” and “AAAHHHHH AHHHH MY LEGS MY LEGS!!!”.

So, let’s summarize. You wouldn’t survive falling toward a black hole because you wouldn’t listen. Why won’t you ever listen?

A friend watching you fall toward a black hole would never see you reach the black hole. As you fall towards it, gravity would cause any light coming from you to be redshifted. So as you approached the black hole you would appear more and more reddish, and your image would appear dimmer and dimmer. Your friend would see you redden and dim as you approach, but never quite reach, the event horizon of the black hole. If they could still see you past this point, there would be additional red from the inside of you clouding up the view.

Artist's conception of the event horizon of a black hole. Credit: Victor de Schwanberg/Science Photo Library
Artist’s conception of the event horizon of a black hole. Credit: Victor de Schwanberg/Science Photo Library

Hypothetically, if you could survive crossing the event horizon of a black hole, what
would you see then? Contrary to popular belief, you would not see the entire future of the universe flash before you.

What you would see is the darkness of the black hole fill your view and as you approached the event horizon you would see stars and galaxies on the edge of your view being gravitationally lensed by the black hole. The sky would simply appear more and more black until you reach the event horizon.

Many people think that it is at the event horizon where you would be ripped apart, and at the event horizon all sorts of strange things occur. Unfortunately, this goes along with those who suspect black holes are actually some sort of portal. For a solar mass black hole, the tidal forces near the event horizon can be quite large, but for a supermassive black hole they aren’t very large at all.

In fact, the larger the black hole, the weaker the tidal forces near its event horizon. So if you happened to be near a supermassive black hole, you could cross the event horizon without really noticing. Would you still be totally screwed? YOU BETCHA!

What do you think? If you could drop anything into a black hole, what would it be? Tell us in the comments below.

What Time is It in the Universe?

Check your watch, what time is it? But wait, you’ve actually been moving and accelerating, and according to Einstein, everything’s relative. So what time is it really? It all depends…

Flavor Flav knows what time it is. At least he does for Flavor Flav. Even with all his moving and accelerating, with the planet, the solar system, getting on planes, taking elevators, and perhaps even some light jogging. In the immortal words of Kool Moe Dee. Do you know what time it is?

Didn’t Einstein tell us it’s all relative? Does anyone actually know what time it is? I mean, aside from figuratively, or in a political sense, or perhaps as part of rap performance from whence the power is being fought from, requiring the sick skills of a hype man wearing a clock around his neck on a big chain.

So, after all my fancy dancing and longing for a time in rap and hip hop from days gone by, I must present to you “faithful audience member” an answer in the form of your 3 least favorite words I get to deliver.

It all depends…

You have heard that everything is relative, usually we hear it from people who like to talk about “connections on many different levels”, which is just nonsense.

But in physics “everything” is relative in a very particular way. Everything is relative to the speed of light, which is the same in every reference frame. Which is confusing and repeated enough that it can become meaningless.

So I’m going to do my best to explain it. If I shine a flashlight in front of me, I will measure the beam to travel at about 300,000 km/s, which is also known as the speed of light.

And if you are moving at 200,000 km/s faster than me, and shine a flashlight ahead of you, I will see the light from your flashlight moving at the 300,000km/s. It will appear to me, as though the light from your flashlight is moving away from you at 100,000 km/s.

But when you will measure the speed of that light, relative to you, you’d think it’d be moving at 100,000 km/s as well, but instead from your perspective it will ALSO clock in at 300,000 km/s.

Artist's impression - General Relativity.
Artist’s impression – General Relativity.

The speed of light. How is this even possible? It is possible in part because the rate at which you experience time relative to me changes. For you, time will seem normal, but from my perspective your time will seem slower. We agree on how fast light is moving in kilometers per second, but we disagree how long a second is. We also, by the way, disagree on the length of a meter.

This seems strange because we imagine that space and time are absolute things, and light is something that travels through space. This is our experience. Suggesting things like time and space are malleable values at best is unsettling and at worst will make us nanners from thinking too much about.

Hold on to your tinfoil hats, for it is in fact light that is the absolute, and space and time are relative to it. So what time it is depends upon your vantage point, and so there is no single absolute time.

Finally, because of relativity, each point in the Universe experiences time at a slightly different rate. For example, when we observe the cosmic microwave background, we find that we are moving at a speed of about 630 km/s relative to the background. That means we experience time a bit more slowly that something at rest relative to the cosmic background.

It’s just a tiny bit slower, but added over the entire age of the Universe, our cosmic clock is 30,000 years behind the times. Feel free to set your watch. But don’t get too precise about it. Your time could be off by tens of thousands of years.

What about you? What’s your favorite way to explain special relativity to someone. Tell us in the comments below.

Does Light Experience Time?

Have you ever noticed that time flies when you’re having fun? Well, not for light. In fact, photons don’t experience any time at all. Here’s a mind-bending concept that should shatter your brain into pieces.

As you might know, I co-host Astronomy Cast, and get to pick the brain of the brilliant astrophysicist Dr. Pamela Gay every week about whatever crazy thing I think of in the shower. We were talking about photons one week and she dropped a bombshell on my brain. Photons do not experience time. [SNARK: Are you worried they might get bored?]

Just think about that idea. From the perspective of a photon, there is no such thing as time. It’s emitted, and might exist for hundreds of trillions of years, but for the photon, there’s zero time elapsed between when it’s emitted and when it’s absorbed again. It doesn’t experience distance either. [SNARK: Clearly, it didn’t need to borrow my copy of GQ for the trip.]

Since photons can’t think, we don’t have to worry too much about their existential horror of experiencing neither time nor distance, but it tells us so much about how they’re linked together. Through his Theory of Relativity, Einstein helped us understand how time and distance are connected.

Let’s do a quick review. If we want to travel to some distant point in space, and we travel faster and faster, approaching the speed of light our clocks slow down relative to an observer back on Earth. And yet, we reach our destination more quickly than we would expect. Sure, our mass goes up and there are enormous amounts of energy required, but for this example, we’ll just ignore all that.

If you could travel at a constant acceleration of 1 g, you could cross billions of light years in a single human generation. Of course, your friends back home would have experienced billions of years in your absence, but much like the mass increase and energy required, we won’t worry about them.

The closer you get to light speed, the less time you experience and the shorter a distance you experience. You may recall that these numbers begin to approach zero. According to relativity, mass can never move through the Universe at light speed. Mass will increase to infinity, and the amount of energy required to move it any faster will also be infinite. But for light itself, which is already moving at light speed… You guessed it, the photons reach zero distance and zero time.

Photons can take hundreds of thousands of years to travel from the core of the Sun until they reach the surface and fly off into space. And yet, that final journey, that could take it billions of light years across space, was no different from jumping from atom to atom.

There, now these ideas can haunt your thoughts as they do mine. You’re welcome. What do you think? What’s your favorite mind bending relativity side effect? Tell us in the comments below.

The Secret of the Stars

“Say, do you like mystery stories? Well we have one for you. The concept: relativity.

Well look at that, it’s a new video from John D. Boswell — aka melodysheep — which goes into autotuned detail about one of the standard principles of astrophysics, Einstein’s theory of general relativity.

Featuring clips from Michio Kaku, Brian Cox, Neil deGrasse Tyson, Brian Greene and Lisa Randall, I’d say E=mc(awesome).

John has been entertaining science fans with his Symphony of Science mixes since 2009, when his first video in the series — “A Glorious Dawn” featuring Carl Sagan — was released. Now John’s videos are eagerly anticipated by fans (like me) who follow him on YouTube and on Twitter as @musicalscience.

E = mc2… that is the engine that lights up the stars.”

(What does Einstein’s famous mass-energy equivalence equation mean? For a brief and basic explanation, check out the American Museum of Natural History’s page here.)

Do We Really Need Dark Matter?

Hubble mosaic of massive galaxy cluster MACS J0717.5+3745, thought to be connected by a filament of dark matter. Credit: NASA, ESA, Harald Ebeling (University of Hawaii at Manoa) & Jean-Paul Kneib (LAM)

Even though teams of scientists around the world are at this very moment hot on the trail of dark matter — the “other stuff” that the Universe is made of and supposedly accounts for nearly 80% of the mass that we can’t directly observe (yet) —  and trying to quantify exactly how so-called “dark energy” drives its ever-accelerating expansion, perhaps one answer to these ongoing mysteries is maybe they don’t exist at all.

This is precisely what one astronomer is suggesting in a recent paper, submitted Dec. 3 to Astrophysical Journal Letters.

In a paper titled “An expanding universe without dark matter and dark energy” (arXiv:1212.1110) Pierre Magain, a professor at Belgium’s Institut d’Astrophysique et de Géophysique, proposes that the expansion of the Universe could be explained without the need for enigmatic material and energy that, to date, has yet to be directly measured.

In addition, Magain’s proposal puts a higher age to the Universe than what’s currently accepted. With a model that shows a slower expansion rate during the early Universe than today, Magain’s calculations estimate its age to be closer to 15.4 – 16.5 billion years old, adding a couple billion more candles to the cosmic birthday cake.

The benefit to a slightly older Universe, Magain posits, is that it’s not so uncannily close to the apparent age of the most distant galaxies recently found — such as MACS0647-JD, which is 13.3 billion light-years away and thus (based on current estimates, see graphic at right) must have formed when the Universe was a mere 420 million years old.

Read more: Now Even Further: Ancient Galaxy is Latest Candidate for Most Distant

Using accepted physics of how time behaves based on Einstein’s theory of general relativity — namely, how the passage of time is relative to the position and velocity of the viewer (as well as the intensity of the gravitational field the viewer is within) — Magain’s model allows for an observer located within the Universe to potentially be experiencing a different rate of time than a hypothetical viewer located outside the Universe. Not to be so metaphysical as to presume that there are external observers of our Universe but merely to say that an external point would be a fixed one against which one could benchmark a varying passage of time inside the Universe, Magain calls this universal relativity.

A viewer experiencing universal relativity would, Magain claims, always measure the curvature of the Universe to be equal to zero. This is what’s currently observed, a “flatness problem” that Magain insinuates is strangely coincidental.

By attributing an expanding Universe to dark energy and the high velocities of stars along the edges of galaxies (as well as the motions of galaxy clusters themselves) to dark matter, we may be introducing ad hoc elements to the Universe, says Magain. Instead, he proposes his “more economical” model — which uses universal relativity — explains these apparently accelerating, increasingly expanding behaviors… and gives a bigger margin of time between the Big Bang and the formation of the first galactic structures.

Read more: First Images in a New Hunt for Dark Energy

There’s quite a bit of math involved, and since I never claimed to understand physics equations you can check out the original paper here.

While intriguing, the bottom line is that dark energy and dark matter have still managed to elude science, existing just outside the borders of what can be observed (although the gravitational lensing effects of what’s thought to be dark matter filaments have been observed by Hubble) and Magain’s paper is merely putting another idea onto the table — one that, while he recognizes needs further testing and relies upon very specific singular parameters, doesn’t depend upon invisible, unobservable and mysteriously dark “stuff”. Whether it belongs on the table or not will be up to other astrophysicists to decide.

Prof. Magain’s research was supported by ESA and the Belgian Science Policy Office.

At right: Artist’s impression of dark matter (h/t to Steve Nerlich)

Note: this is “just” a submitted paper and has not been selected for publication yet. Any hypotheses proposed are those of the author and are not endorsed by this site. (Personally I like dark matter. It’s fascinating stuff… even if we can’t see it. Want an astrophysicist’s viewpoint on the existence of dark matter? Check out Ethan Siegel’s blog response here.)

A Star’s Dying Scream May Be a Beacon for Physics

When a star suffered an untimely demise at the hands of a hidden black hole, astronomers detected its doleful, ululating wail — in the key of D-sharp, no less — from 3.9 billion light-years away. The resulting ultraluminous X-ray blast revealed the supermassive black hole’s presence at the center of a distant galaxy in March of 2011, and now that information could be used to study the real-life workings of black holes, general relativity, and a concept first proposed by Einstein in 1915.

Within the centers of many spiral galaxies (including our own) lie the undisputed monsters of the Universe: incredibly dense supermassive black holes, containing the equivalent masses of millions of Suns packed into areas smaller than the diameter of Mercury’s orbit. While some supermassive black holes (SMBHs) surround themselves with enormous orbiting disks of superheated material that will eventually spiral inwards to feed their insatiable appetites — all the while emitting ostentatious amounts of high-energy radiation in the process — others lurk in the darkness, perfectly camouflaged against the blackness of space and lacking such brilliant banquet spreads. If any object should find itself too close to one of these so-called “inactive” stellar corpses, it would be ripped to shreds by the intense tidal forces created by the black hole’s gravity, its material becoming an X-ray-bright accretion disk and particle jet for a brief time.

Such an event occurred in March 2011, when scientists using NASA’s Swift telescope detected a sudden flare of X-rays from a source located nearly 4 billion light-years away in the constellation Draco. The flare, called Swift J1644+57, showed the likely location of a supermassive black hole in a distant galaxy, a black hole that had until then remained hidden until a star ventured too close and became an easy meal.

See an animation of the event below:

The resulting particle jet, created by material from the star that got caught up in the black hole’s intense magnetic field lines and was blown out into space in our direction (at 80-90% the speed of light!) is what initially attracted astronomers’ attention. But further research on Swift J1644+57 with other telescopes has revealed new information about the black hole and what happens when a star meets its end.

(Read: The Black Hole that Swallowed a Screaming Star)

In particular, researchers have identified what’s called a quasi-periodic oscillation (QPO) embedded inside the accretion disk of Swift J1644+57. Warbling at 5 mhz, in effect it’s the low-frequency cry of a murdered star. Created by fluctuations in the frequencies of X-ray emissions, such a source near the event horizon of a supermassive black hole can provide clues to what’s happening in that poorly-understood region close to a black hole’s point-of-no-return.

Einstein’s theory of general relativity proposes that space itself around a massive rotating object — like a planet, star, or, in an extreme instance, a supermassive black hole — is dragged along for the ride (the Lense-Thirring effect.) While this is difficult to detect around less massive bodies a rapidly-rotating black hole would create a much more pronounced effect… and with a QPO as a benchmark within the SMBH’s disk the resulting precession of the Lense-Thirring effect could, theoretically, be measured.

If anything, further investigations of Swift J1644+57 could provide insight to the mechanics of general relativity in distant parts of the Universe, as well as billions of years in the past.

See the team’s original paper here, lead authored by R.C. Reis of the University of Michigan.

Thanks to Justin Vasel for his article on Astrobites.

Image: NASA. Video: NASA/GSFC

Data from Black Hole’s Edge Provides New Test of Relativity

From a NASA press release:

Last year, astronomers discovered a quiescent black hole in a distant galaxy that erupted after shredding and consuming a passing star. Now researchers have identified a distinctive X-ray signal observed in the days following the outburst that comes from matter on the verge of falling into the black hole.

This tell-tale signal, called a quasi-periodic oscillation or QPO, is a characteristic feature of the accretion disks that often surround the most compact objects in the universe — white dwarf stars, neutron stars and black holes. QPOs have been seen in many stellar-mass black holes, and there is tantalizing evidence for them in a few black holes that may have middleweight masses between 100 and 100,000 times the sun’s.

Until the new finding, QPOs had been detected around only one supermassive black hole — the type containing millions of solar masses and located at the centers of galaxies. That object is the Seyfert-type galaxy REJ 1034+396, which at a distance of 576 million light-years lies relatively nearby.

“This discovery extends our reach to the innermost edge of a black hole located billions of light-years away, which is really amazing. This gives us an opportunity to explore the nature of black holes and test Einstein’s relativity at a time when the universe was very different than it is today,” said Rubens Reis, an Einstein Postdoctoral Fellow at the University of Michigan in Ann Arbor. Reis led the team that uncovered the QPO signal using data from the orbiting Suzaku and XMM-Newton X-ray telescopes, a finding described in a paper published today in Science Express.

The X-ray source known as Swift J1644+57 — after its astronomical coordinates in the constellation Draco — was discovered on March 28, 2011, by NASA’s Swift satellite. It was originally assumed to be a more common type of outburst called a gamma-ray burst, but its gradual fade-out matched nothing that had been seen before. Astronomers soon converged on the idea that what they were seeing was the aftermath of a truly extraordinary event — the awakening of a distant galaxy’s dormant black hole as it shredded and gobbled up a passing star. The galaxy is so far away that light from the event had to travel 3.9 billion years before reaching Earth.

Video info: On March 28, 2011, NASA’s Swift detected intense X-ray flares thought to be caused by a black hole devouring a star. In one model, illustrated here, a sun-like star on an eccentric orbit plunges too close to its galaxy’s central black hole. About half of the star’s mass feeds an accretion disk around the black hole, which in turn powers a particle jet that beams radiation toward Earth. Credit: NASA’s Goddard Space Flight Center/Conceptual Image Lab

The star experienced intense tides as it reached its closest point to the black hole and was quickly torn apart. Some of its gas fell toward the black hole and formed a disk around it. The innermost part of this disk was rapidly heated to temperatures of millions of degrees, hot enough to emit X-rays. At the same time, through processes still not fully understood, oppositely directed jets perpendicular to the disk formed near the black hole. These jets blasted matter outward at velocities greater than 90 percent the speed of light along the black hole’s spin axis. One of these jets just happened to point straight at Earth.

Nine days after the outburst, Reis, Strohmayer and their colleagues observed Swift J1644+57 using Suzaku, an X-ray satellite operated by the Japan Aerospace Exploration Agency with NASA participation. About ten days later, they then began a longer monitoring campaign using the European Space Agency’s XMM-Newton observatory.

“Because matter in the jet was moving so fast and was angled nearly into our line of sight, the effects of relativity boosted its X-ray signal enough that we could catch the QPO, which otherwise would be difficult to detect at so great a distance,” said Tod Strohmayer, an astrophysicist and co-author of the study at NASA’s Goddard Space Flight Center in Greenbelt, Md.

As hot gas in the innermost disk spirals toward a black hole, it reaches a point astronomers refer to as the innermost stable circular orbit (ISCO). Any closer to the black hole and gas rapidly plunges into the event horizon, the point of no return. The inward spiraling gas tends to pile up around the ISCO, where it becomes tremendously heated and radiates a flood of X-rays. The brightness of these X-rays varies in a pattern that repeats at a nearly regular interval, creating the QPO signal.

The data show that Swift J1644+57’s QPO cycled every 3.5 minutes, which places its source region between 2.2 and 5.8 million miles (4 to 9.3 million km) from the center of the black hole, the exact distance depending on how fast the black hole is rotating. To put this in perspective, the maximum distance is only about 6 times the diameter of our sun. The distance from the QPO region to the event horizon also depends on rotation speed, but for a black hole spinning at the maximum rate theory allows, the horizon is just inside the ISCO.

“QPOs send us information from the very brim of the black hole, which is where the effects of relativity become most extreme,” Reis said. “The ability to gain insight into these processes over such a vast distance is a truly beautiful result and holds great promise.”

Read our previous article on Swift J1644+57

Lead image caption: This illustration highlights the principal features of Swift J1644+57 and summarizes what astronomers have discovered about it. Credit: NASA’s Goddard Space Flight Center

Astronomy Without A Telescope – Light Speed

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The recent news of neutrinos moving faster than light might have got everyone thinking about warp drive and all that, but really there is no need to imagine something that can move faster than 300,000 kilometres a second.

Light speed, or 300,000 kilometres a second, might seem like a speed limit, but this is just an example of 3 + 1 thinking – where we still haven’t got our heads around the concept of four dimensional space-time and hence we think in terms of space having three dimensions and think of time as something different.

For example, while it seems to us that it takes a light beam 4.3 years to go from Earth to the Alpha Centauri system, if you were to hop on a spacecraft going at 99.999 per cent of the speed of light you would get there in a matter of days, hours or even minutes – depending on just how many .99s you add on to that proportion of light speed.

This is because, as you keep pumping the accelerator of your imaginary star drive system, time dilation will become increasingly more pronounced and you will keep getting to your destination that much quicker. With enough .999s you could cross the universe within your lifetime – even though someone you left behind would still only see you moving away at a tiny bit less than 300,000 kilometres a second. So, what might seem like a speed limit at first glance isn’t really a limit at all.

The effect of time dilation is negligible for common speeds we are familiar with on Earth, but it increases dramatically and asymptotically as you approach the speed of light.

To try and comprehend the four dimensional perspective on this, consider that it’s impossible to move across any distance without also moving through time. For example, walking a kilometer may be a duration of thirty minutes – but if you run, it might only take fifteen minutes.

Speed is just a measure of how long it takes you reach a distant point. Relativity physics lets you pick any destination you like in the universe – and with the right technology you can reduce your travel time to that destination to any extent you like – as long as your travel time stays above zero.

That is the only limit the universe really imposes on us – and it’s as much about logic and causality as it is about physics. You can travel through space-time in various ways to reduce your travel time between points A and B – and you can do this up until you almost move between those points instantaneously. But you can’t do it faster than instantaneously because you would arrive at B before you had even left A.

If you could do that, it would create impossible causality problems – for example you might decide not to depart from point A, even though you’d already reached point B. The idea is both illogical and a breach of the laws of thermodynamics, since the universe would suddenly contain two of you.

So, you can’t move faster than light – not because of anything special about light, but because you can’t move faster than instantaneously between distant points. Light essentially does move instantaneously, as does gravity and perhaps other phenomena that we are yet to discover – but we will never discover anything that moves faster than instantaneously, as the idea makes no sense.

We mass-laden beings experience duration when moving between distant points – and so we are able to also measure how long it takes an instantaneous signal to move between distant points, even though we could never hope to attain such a state of motion ourselves.

We are stuck on the idea that 300,000 kilometres a second is a speed limit, because we intuitively believe that time runs at a constant universal rate. However, we have proven in many different experimental tests that time clearly does not run at a constant rate between different frames of reference. So with the right technology, you can sit in your star-drive spacecraft and make a quick cup of tea while eons pass by outside. It’s not about speed, it’s about reducing your personal travel time between two distant points.

As Woody Allen once said: Time is nature’s way of keeping everything from happening at once. Space-time is nature’s way of keeping everything from happening in the same place at once.

What is Space?

First, some simple answers: space is everything in the universe beyond the top of the Earth’s atmosphere – the Moon, where the GPS satellites orbit, Mars, other stars, the Milky Way, black holes, and distant quasars. Space also means what’s between planets, moons, stars, etc – it’s the near-vacuum otherwise known as the interplanetary medium, the interstellar medium, the inter-galactic medium, the intra-cluster medium, etc; in other words, it’s very low density gas or plasma (‘space physics’ is, in fact, just a branch of plasma physics!).

But you really want to know what space is, don’t you? You’re asking about the thing that’s like time, or mass.

And one simple, but profound, answer to the question “What is space?” is “that which you measure with a ruler”. And why is this a profound answer? Because thinking about it lead Einstein to develop first the theory of special relativity, and then the theory of general relativity. And those theories overthrew an idea that was built into physics since before the time of Newton (and built into philosophy too); namely, the idea of absolute space (and time). It turns out that space isn’t something absolute, something you could, in principle, measure with lots of rulers (and lots of time), and which everyone else who did the same thing would agree with you on.

Space, in the best theory of physics on this topic we have today – Einstein’s theory of general relativity (GR) – is a component of space-time, which can be described very well using the math in GR, but which is difficult to envision with our naïve intuitions. In other words, “What is space?” is a question I can’t really answer, in the short space I have in this Guide to Space article.

More reading: What is space? (ESA), What is space? (National Research Council of Canada), Ned Wright’s Cosmology Tutorial, and Sean Carroll’s Cosmology Primer pretty much cover this vast topic, from kids’ to physics undergrad’ level.

It’s hard to know just what Universe Today articles to recommend, because there are so many! Space Elevator? Build it on the Moon First illustrates one meaning of the word ‘space’; for meanings closer to what I’ve covered here, try New Way to Measure Curvature of Space Could Unite Gravity Theory, and Einstein’s General Relativity Tested Again, Much More Stringently.

Astronomy Cast episodes Einstein’s Theory of Special Relativity, Einstein’s Theory of General Relativity, Large Scale Structure of the Universe, and Coordinate Systems, are all good, covering as they do different ways to answer the question “What is space?”

Source: ESA

What is Einstein’s Theory of Relativity?

Albert Einstein's Inventions

There are two, theories of relativity, by Einstein, the special theory of relativity (or just special relativity, SR), and the general theory of relativity (or just general relativity, GR).

The special theory of relativity was published in 1905, in Annalen der Physik (“Zur Elektrodynamik bewegter Körper“, in the original German; “On the Electrodynamics of Moving Bodies” is its English translation), and the general theory of relativity published in 1915, in the Minutes of the Meetings of the Prussian Academy of Sciences (Berlin) (“Die Feldgleichungen der Gravitation” in the original German; “The Field Equations of Gravitation” is its English translation).

In its original form, special relativity is based on just two postulates (or assumptions); namely, that the speed of light (in a vacuum) is constant – no matter who measures it, or when, or where – and that the laws of physics are the same for in all inertial frames of reference (basically, for all observers who are not accelerating) … there are other, logically consistent, ways to construct SR, from different postulates, but they are equivalent to Einstein’s original.

The general principle of relativity at the heart of general relativity is easy to state (something like “the laws of nature are the same, everywhere, everywhen, and to everybody“), but the additional postulate (or postulates) is not. However, the consequence of this postulate is easy to say, in words – gravity is geometry … or in the words of John Wheeler “spacetime tells matter how to move; matter tells spacetime how to curve“.

Special relativity has been tested sixty ways to Sunday, and there are no experimental results which contradict it (or, rather, none which are reproducible). It is incorporated into all modern theories of physics, particularly quantum electrodynamics (which is the most precisely tested scientific theory, period), and general relativity. It is also (!) mathematically consistent (self-consistent, internally consistent), and reduces to Newtonian mechanics in the limit of small speeds.

General relativity has also been extensively tested, though not as thoroughly as special relativity (see The Confrontation between General Relativity and Experiment).

The internet has lots and lots of material on Einstein’s theories of relativity (but do be careful, some sites are overtly anti-science, and some are just crackpot nonsense); two examples of good summaries/intros: Albert Einstein and the Theory of Relativity (from the University of Tennessee), and Relativity Tutorial (Ned Wright, UCLA).

There are lots and lots (and lots and …) of Universe Today articles on Einstein’s theories of relativity; here is a sample: Einstein Still Rules Says Fermi Telescope Team, and New Way to Measure Curvature of Space Could Unite Gravity Theory.

Two Astronomy Cast episodes are worth a special listen, Einstein’s Theory of Special Relativity, and Einstein’s Theory of General Relativity.

Sources:
Stanford University
UT-Knoxville
North Carolina State University