Exotic Objects Test Einstein’s Theory

Astronomers are using the most extreme objects they can find to put Einstein's theory of general relativity to the test.

An artist's illustration of a pulsar — the mass of the Sun packed into a sphere the size of Manhattan.
General relativity, Einstein’s breakthrough theory of the nature of gravity, has withstood every experimental challenge since he published it in 1916. No test has yet found it to be even the tiniest trace off course. But since general relativity doesn’t mesh with quantum mechanics — the other great foundation of modern physics, which governs the other three of the universe’s four fundamental forces — astronomers haven’t stopped trying to push general relativity beyond its limits in hopes of finding hints of something new.

At the 7th Harvard-Smithsonian Conference on Theoretical Astrophysics held May 14-17 in Cambridge, Massachusetts, astronomers described test after test of general relativity using extreme-gravity objects from pulsars to supermassive black holes. Here’s a small sampling.

Pulsars

If you want to push general relativity to its limits, you’ll need the strong gravity near compact, massive objects. Neutron stars fit the bill; they form when a star roughly the mass of our Sun collapses into a sphere with the density of an atomic nucleus and the size of Manhattan. Even better is when a spinning neutron star’s magnetic field whirls jets of radiation that sweep past Earth. The jets look like lighthouse beacons, flashing us as the pulsar rotates. Some pulsars spin ridiculously quickly — with periods down to a millisecond or two — and with fantastic regularity that matches or beats the best artificial atomic clocks.

Binary pulsar
An artist illustrates a binary pulsar system like J0737-3039.
John Rowe
The theorist’s real treasure, though, is to find two pulsars orbiting each other. One example of this holy grail goes by the unglamorous name PSR J0737-3039. Some 1,800 light-years away in Puppis, a pulsar rotating every 23 milliseconds (pulsar A) orbits another rotating every 2.8 seconds (pulsar B). In a scale model, they would be two marbles 750 feet apart. When pulsar A’s radio blips travel through the strong gravitational field around pulsar B on their way to us, the timing of this otherwise perfect clock is distorted in several ways predicted by general relativity. So far, general relativity has passed five different tests using this one binary, all with flying colors (S&T: August 2010). Read more about the binary pulsar.

Another test requires a more unevenly balanced system, such as the pulsar (PSR J1738+0333) that orbits a white dwarf only one-fifth the mass of the Sun. Any two massive objects orbiting closely will gradually spiral together, speeding up as the system emits energy in the form of gravitational waves. These gravitational waves have a certain pattern to them, one that is magnified in an asymmetric system like J1738+0333. Most alternative theories of gravity predict that the pattern should be dipolar, but the orbit of J1738+0333 is slowing in agreement with Einstein’s theory instead, which predicts quadrupolar gravitational waves. Read more about the pulsar – white dwarf binary.

If the mass asymmetry of J1738+0333 is a boon for theorists, imagine if astronomers found a pulsar orbiting the Milky Way’s central black hole, weighing in at 4 million times the mass of the Sun. Star-formation theories say that anywhere from 10 to 100 pulsars should be doing so. Scott Ransom (National Radio Astronomy Observatory) has been looking for them, but to no avail. His “last, best search” for these perfect clocks in relativistic orbits around the giant black hole will happen later this year. If he doesn’t find any, then not only do astronomers miss out on some fantastic tests of relativity, but they’ll also have to acknowledge that something is wrong with their understanding of how stars form in the center of the Milky Way.

Imaging Supermassive Black Holes

A very direct test of Einstein’s theory would be to actually see a black hole. It’s hard to believe that that once-distant goal is scheduled to become a reality by 2015. Shep Doeleman (Haystack Observatory) presented a status update on the Event Horizon Telescope, an array of millimeter-wave radio antennae that should image the silhouette of the Milky Way’s central black hole (S&T: February issue, page 20).

There are four phases to putting this array together. The Large Millimeter Telescope in Mexico is already operating, but the real excitement will start when the sensitive and powerful Atacama Large Millimeter/submillimeter Array (ALMA) in Chile is incorporated in 2015. The team would also like to add two more arrays, one at the South Pole (South Pole Telescope) and another in Europe (IRAM), to create longer baselines.

“This is not a ‘first-light’ instrument” where the whole telescope goes online at once and starts taking images, Doeleman said at the conference. “We have planned technical improvements that should result in big jumps in performance along the way.”

black hole silhouette
Strong gravity bends light around the Milky Way's black hole in this simulation, leaving a silhouette against a glowing background.
A. Loeb & A. Broderick (CfA)
The black hole’s silhouette should actually be the size and shape of the “last photon orbit.” The hole, which is a bit smaller, will swallow any photon that orbits inside that radius. Once the Event Horizon Telescope is able to resolve the size and shape of the dark silhouette, the proof will be in the pictures — black holes exist. Then the real fun begins.

One test of general relativity is to look for the black hole’s “hair.” General relativity says black holes “have no hair,” which is physicists’ delightful way of saying that a black hole has only three properties that define it completely: its mass, spin, and electrical charge. In other words, no matter what you toss into a black hole — stellar matter, popcorn, the kitchen sink — no outside observer would be able to tell what you added, other than by the effect on the black hole’s mass, spin, and charge. It has no other characteristics whatsoever.

But some alternative theories of gravity suggest that black holes do have a bit more to show, if only a vanishingly small amount. If the Milky Way’s black hole is somehow hairy, that will affect the shape of the silhouette. So in 2015, we may see whether our local supermassive black hole is as bald as predicted.

The Future of Gravity

Personally, I don’t think they’ll soon find proof that general relativity is wrong — Einstein’s theory has passed every test for almost a century already. Proponents of alternative theories, by contrast, have had to modify their theories over and over to cope with observations. (For example, some alternative theories of gravity that were initially formulated to do away with the need for dark matter later had to incorporate a modified form of dark matter in their equations after all.)

But it’s clear that general relativity is not complete. Until gravity can be united with the electromagnetic, weak, and strong forces, astronomers will keep testing general relativity to its limit, hoping for clues to a Theory of Everything.

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Monica Young

About Monica Young

Monica Young, a professional astronomer by training, is web editor of Sky & Telescope, where she creates, manages, and maintains website content, and contributes to the magazine.

15 thoughts on “Exotic Objects Test Einstein’s Theory

  1. Peter WilsonPeter

    "…a black hole has only three properties that define it completely: its mass, spin, and electrical charge." What about the magnetic field? I cannot imagine a black hole just sitting there in space without a magnetic field.

  2. Monica YoungMonica Young

    Hi Peter, you ask a great question. A black hole actually can’t have a magnetic field. What *can* happen is that a magnetic field is generated around a black hole. For example, plasma will swirl into what’s called an accretion disk as it comes close to the black hole. Magnetic fields can certainly thread the accretion disk and come very close to the black hole itself. However, magnetic field lines cannot thread the black hole itself.

  3. Edward Schaefer

    Peter –

    You are correct that there can be a magnetic field surrounding a black hole. However, the strength and geometry of that field is entirely a function of the mass, charge, and spin of the black hole. So once again, those three properties completely define a black hole.

  4. Edward Schaefer

    The tests of GR have never gotten down to the second order in Post_Newtonian approximations. To be more specific, effects that are of the order of m/r (where m is 1/2 of the Schwarzschild radius for an object’s mass and r is the distance from the center of the mass at which GR is being studied) have been confirmed. However, at the level of m^2/r^2 (or m/r squared), no study has had either an intense enough gravitational field or the accuracy needed see deviations there. Alternative theories that diverge from GR at that level remain viable. So future studies may yet reveal some surprises. Even so, note that I put "wrong" in quotes: Much of the fundamentals of GR have proven to be robust, and must carry forward into any new theory, which will be more an improvement on GR than a replacement.

  5. Mike W. Herberich

    To Monica, Ed and the rest: with respect to the last paragraph: "… it’s clear that general relativity is not complete. Until gravity can be united with the electromagnetic, weak, and strong forces, …". Just a low-brow, humble question/ doubt from an ardent enthusiast: if the two don’t match, does it absolutely HAVE to be GR that is incomplete? How about Quantum Mechanics? As I have heard QM is based on point-shaped particles. Although extremely successful in itself, this (very convenient/ necessary) base assumption/ simplification is per se not very trustworthy as for reality, especially when it comes to uniting it with GR! Couldn’t it be as well that QM has to be extended to allow for accomodating GR? After all, strings deal with this in a non point particle way, don’t they?

  6. Edward Schaefer

    To Mike: QM has been confirmed to a much higher level of accuracy than GR and has less wiggle room for improvement. Even so, your point is well taken: It is possible that QM may be incomplete instead of GR or even that both are incomplete. I for one really don’t expect that, but then again I also have ideas of my own of how GR may be changed to remove black holes from theory (which for me are another red flag the something is wrong with GR) if not create consistency between gravitational theory and QM. So my biased opinion is that GR is what will give, but we will see what we will see. If these exotic objects start confirming GR to the second post-Newtonian order, then I for one will need to rethink my views.

  7. Mike W. Herberich

    Hi Ed, thank you so much for reacting to my post. Absolutely fascinating what you’re insinuating about your own GR ideas: could you let us (me, most of all!) in on it? Just a little bit, please! "Completeness": of course, enormous chances are that both theories are not "complete", independently of what one prefers to comprehend by "complete". (Somehow all of "reality" comes down to what we mean by language, how we use it in accordance with whom and how many, etc.! Per se, our minds and thus language and all that springs from both are incomplete (Gödel!)). Yet I understand and support all you say about it. Particularly interesting is the terms-of-second-order thought of yours. I’ve studied special relativity enough to understand that -developing, say, the Lorentz-Factor, into a series- by dropping all terms higher than those of 2nd order, one arrives at Newton. But, is the current state of affairs really how you portray it? No second-order measurements done yet, at all? Just fascinating! Opens up all kinds of speculations … within me, at least!

  8. Mike W. Herberich

    But: how to drop black holes from GR? … and WHY? My gut feeling (if that is of ANY value AT ALL ;-}!) is that black hole singularities sort of "fall through" to the "other side", into (or within) (an-)other/ further dimension(s), thereby crossing the border from minus- to plus-infinity (possibly by changing matter to anti-matter and vice versa). This sort of brings Big Bang back into play: on the "other" side, this all would appear to be a Big Bang, yet this "other" side still is/ becomes "our"/ "this" side (by means of the above other/ further dimensions). So, what we think/ see as "our" Big Bang is/ was just ONE cycle of such a pass-through system, completely closed in itself. Although this is all mere "babble", it still makes a certain sense to me. In a certain way as much as Einstein’s(/ Perlmutter’s?!) Lambda and subsequent "dark" matter/ energy … with the tiny flaw of course that it relies on NO measurements whatsoever … I guess I have to work on that further ;-] ! Jest aside: the fact that math/ physics break down at singularities could be viewed as a mirror rather than a wall. Just brainstorming!

  9. Mike W. Herberich

    But: how to drop black holes from GR? … and WHY? My gut feeling (if that is of ANY value AT ALL ;-}!): black hole singularities "fall through" to the "other side", into (within?) (an-)other/ further dimension(s), thereby crossing the border from – to + infinity (e. g. by changing matter to anti-matter & v. v.). This brings Big Bang back into play: on the other side this all would appear to be a Big Bang, yet this other side still is/ becomes our/ this side (by means of above other/ further dimensions). So, what we see as "our" Big Bang is/ was just ONE cycle of such a pass-through system, completely closed in itself. Although this is all mere "babble" it still makes a certain sense to me. In a way as much as Einstein’s(/ Perlmutter’s?!) Lambda and subsequent dark matter/ energy … with the tiny flaw that it relies on NO measurements whatsoever … I have to work on that further ;-] ! Jest aside: the fact that math/ physics break down at singularities could be viewed as a mirror rather than a wall. Just brainstorming!

  10. Peter WilsonPeter Wilson

    In other words, calculations done with quantum-mechanics always agree with experiment, whereas in GR there are black holes, dark matter and dark energy. GR is probably complete, but remains suspect by association with those Big Three unsolved mysteries. As for uniting gravity with the electromagnetic, weak, and strong forces, consider the 3-body problem: There is no solution, using Newtonian gravity, GR, electromagnetism or quantum-mechanics for the 3-body problem. Three equations can be found that predict the rate-of-change for each of the 3 bodies, but these equations cannot be solved to yield the total change after an arbitrary amount of time. A solution can only be approximated, by computer modeling and other techniques, such as the second-order that Edward refers to. So none of the four force fields have exact solutions to the 3-body problem, yet theorists are supposed to come up with a united, single equation for the 4 known force fields? If that is the criterion, it could be a long time before GR is considered complete!

  11. Mike W. Herberich

    Hi Peter! So nice to hear from you! Would it be fair to say that QM is so "good"/ successful AT THE COST of applying statistics to POINT-shaped particles, both pretty much simplifying and smudging the idea of what we commonly are able to comprehend as "reality"? From Heisenberg we can never "really" "know" what the single particle is doing. The behavior of the extremely small leaves us with no other option (except strings and the like). GR, conversely, i. e., the macroscopic, does and most likely may not rest on such premises: rather, it aspires for and claims "absolute" exactness, as you illustrated in the 3-body-problem! This fundamental dichotomy (if true at all!) seems to me at the base of the incompatibility of the two worlds. So, as long as GR does not lean towards statistics/ point-particles or/ and QM does not lean towards microscopic exactness OR BOTH do not lean towards a third, yet unknown world, there is, I feel, no land in sight in unifying the two.

  12. Mike W. Herberich

    Hi Peter! So nice to hear from you! Would it be fair to say QM is so successful AT THE COST of applying statistics to POINT-shaped particles, both pretty much simplifying/ smudging the idea of what we commonly comprehend as reality? By Heisenberg we can never "really know" what a single particle is doing. The behavior of the extremely small leaves us with no other option (except strings and the like). GR, conversely, the macroscopic, does and most likely may not rest on such premises: rather, it aspires for/ claims "absolute" exactness, as you illustrated in the 3-body-problem! This fundamental dichotomy -if true at all- seems basic to the incompatibility of the 2 worlds. So, as long as GR doesn’t lean towards statistics/ point-particles or/ and QM doesn’t lean towards microscopic exactness OR BOTH don’t lean towards a 3rd, yet unknown world, there is, I feel, no land in sight in unifying the 2.

  13. Mike W. Herberich

    Hi Peter! So nice to hear from you! Would it be fair to say QM is so successful AT THE COST of applying statistics to POINT-shaped particles, both pretty much simplifying/ smudging the idea of what we commonly comprehend as reality? By Heisenberg we can never "really know" what a single particle is doing. The behavior of the extremely small leaves us with no other option (except strings and the like). GR, conversely, the macroscopic, does and most likely may not rest on such premises: rather, it aspires for/ claims "absolute" exactness, as you illustrated in the 3-body-problem! This fundamental dichotomy -if true at all- seems basic to the incompatibility of the 2 worlds. So, as long as GR doesn’t lean towards statistics/ point-particles or/ and QM doesn’t lean towards microscopic exactness OR BOTH don’t lean towards a 3rd, yet unknown world, there is, I feel, no land in sight in unifying the 2.

  14. Mike W. Herberich

    I just seem to unable to post. Please excuse potential double posting …
    Hi Peter! So nice to hear from you! Would it be fair to say QM is so successful AT THE COST of applying statistics to POINT-shaped particles, both pretty much simplifying/ smudging the idea of what we commonly comprehend as reality? By Heisenberg we can never "really know" what a single particle is doing. The behavior of the extremely small leaves us with no other option (except strings and the like). GR, conversely, the macroscopic, does and most likely may not rest on such premises: rather, it aspires for/ claims "absolute" exactness, as you illustrated in the 3-body-problem! This fundamental dichotomy -if true at all- seems basic to the incompatibility of the 2 worlds. So, as long as GR doesn’t lean towards statistics/ point-particles or/ and QM doesn’t lean towards microscopic exactness OR BOTH don’t lean towards a 3rd, yet unknown world, there is, I feel, no land in sight in unifying the 2.

  15. Bruce Mayfield

    A few more sincere questions from another interested reader. If GR and QM work perfectly well as descriptive and predictive tools in the macro and micro worlds respectively, why do they need to be unified at all? Also why the push to unify gravity with the other forces? Couldn’t gravity in fact be a completely separate and distinct force?

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