How Does Light Travel?

Magnetic Fields in Inter-cluster Space: Measured at Last

Article Updated: 24 Dec , 2015


The strength of the magnetic fields here on Earth, on the Sun, in inter-planetary space, on stars in our galaxy (the Milky Way; some of them anyway), in the interstellar medium (ISM) in our galaxy, and in the ISM of other spiral galaxies (some of them anyway) have been measured. But there have been no measurements of the strength of magnetic fields in the space between galaxies (and between clusters of galaxies; the IGM and ICM).

Up till now.

But who cares? What scientific importance does the strength of the IGM and ICM magnetic fields have?

The Large Area Telescope (LAT) on Fermi detects gamma-rays through matter (electrons) and antimatter (positrons) they produce after striking layers of tungsten. Credit: NASA/Goddard Space Flight Center Conceptual Image Lab

Estimates of these fields may provide “a clue that there was some fundamental process in the intergalactic medium that made magnetic fields,” says Ellen Zweibel, a theoretical astrophysicist at the University of Wisconsin, Madison. One “top-down” idea is that all of space was somehow left with a slight magnetic field soon after the Big Bang – around the end of inflation, Big Bang Nucleosynthesis, or decoupling of baryonic matter and radiation – and this field grew in strength as stars and galaxies amassed and amplified its intensity. Another, “bottom-up” possibility is that magnetic fields formed initially by the motion of plasma in small objects in the primordial universe, such as stars, and then propagated outward into space.

So how do you estimate the strength of a magnetic field, tens or hundreds of millions of light-years away, in regions of space a looong way from any galaxies (much less clusters of galaxies)? And how do you do this when you expect these fields to be much less than a nanoGauss (nG), perhaps as small as a femtoGauss (fG, which is a millionth of a nanoGauss)? What trick can you use??

A very neat one, one that relies on physics not directly tested in any laboratory, here on Earth, and unlikely to be so tested during the lifetime of anyone reading this today – the production of positron-electron pairs when a high energy gamma ray photon collides with an infrared or microwave one (this can’t be tested in any laboratory, today, because we can’t make gamma rays of sufficiently high energy, and even if we could, they’d collide so rarely with infrared light or microwaves we’d have to wait centuries to see such a pair produced). But blazars produce copious quantities of TeV gamma rays, and in intergalactic space microwave photons are plentiful (that’s what the cosmic microwave background – CMB – is!), and so too are far infrared ones.

MAGIC telescope (Credit: Robert Wagner)

Having been produced, the positron and electron will interact with the CMB, local magnetic fields, other electrons and positrons, etc (the details are rather messy, but were basically worked out some time ago), with the net result that observations of distant, bright sources of TeV gamma rays can set lower limits on the strength of the IGM and ICM through which they travel. Several recent papers report results of such observations, using the Fermi Gamma-Ray Space Telescope, and the MAGIC telescope.

So how strong are these magnetic fields? The various papers give different numbers, from greater than a few tenths of a femtoGauss to greater than a few femtoGauss.

“The fact that theyโ€™ve put a lower bound on magnetic fields far out in intergalactic space, not associated with any galaxy or clusters, suggests that there really was some process that acted on very wide scales throughout the universe,” Zweibel says. And that process would have occurred in the early universe, not long after the Big Bang. “These magnetic fields could not have formed recently and would have to have formed in the primordial universe,” says Ruth Durrer, a theoretical physicist at the University of Geneva.

So, perhaps we have yet one more window into the physics of the early universe; hooray!

Sources: Science News, arXiv:1004.1093, arXiv:1003.3884

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24 Responses

  1. Astrofiend says:

    Very interesting study. Magnetic fields is relative isolation are notoriously difficult to study in the universe – there are relatively few techniques available. But the field has come a long way recently, and this result is another good (if small) step forward.

  2. Astrofiend says:

    >”Magnetic fields in relative isolation… “

  3. Lawrence B. Crowell says:

    Quick! Before the EU guys rubbish up this page!

    A quick point about the gamma on IR/MW photon. The interaction occurs at the TeV range because there you get the electroweak interaction (g = gamma)

    g + g —> Z^0 —> pions, muons etc —> e-e^+ pairs

    This interaction occurs because of electroweak unification. At lower energy photons don’t interact with each other.

    This is clever deduction from a measurement.


  4. IVAN3MAN_AT_LARGE says:

    @ Lawrence B. Crowell,

    I, too, was going to say the same thing about ‘them’! ๐Ÿ˜‰

  5. Jean Tate says:

    Lawrence B. Crowell, IVAN3MAN_AT_LARGE,

    Not so long ago Nancy wrote:

    if you don’t want the EU folks to comment, why did you basically open the door and lay down the red carpet for them to start commenting on this post? Certainly I don’t support what they say, but lord, don’t feed the trolls! Your opening comment pretty much said, “Here you go EUer’s โ€” here’s one you should post a comment on!”

    Consider what counts as a success for a troll; now think about how gleeful a troll would feel if the very first words you write are about them, even when they are nowhere to be seen.


    The effects of magnetic fields on photons is exactly what we use to estimate their presence, strengths, and directions; in fact, other than cosmic rays, photons are the only thing we have to ‘see’ the universe with when it comes to magnetic fields.

    Those effects are rather few – rotation measure, Zeeman effect, the very high energy photon-photon collisions my article relies one, maybe one or two others – and it’s not always easy to disentangle the effects of magnetic fields from others (all we have to work with are the photons we detect, right here).

    If you haven’t already done so, I recommend you read the last of my sources (arXiv:1003.3884), it’s a recent review paper titled “Cosmic Magnetic Fields: from Stars and Galaxies to the Primordial Universe”

  6. Silenus says:

    Hey, will you guys pipe down with your annoying, boring and pointless remarks about the EU guys?

    People are trying to read here!

    (No, I’m not one of them, it’s just very annoying having to read this same remark over en over and over again)

  7. SteveZodiac says:

    Silenus is right, this isn”t SDC. At this risk of being branded an EU follower I suspect that magnetism plays a larger role in the cosmos than it currently gets credit for. After all, most matter in the universe is actually in the form of plasma.. If magnetic fields pervade the interstellar and intergalactic medium then they must have some effect on the EM radiation passing through them – the same EM radiation which we detect and use as the basis of our cosomological theories.

  8. Duncan Ivry says:

    I can only support what Silenus said. We have been told again and again, that certain stupid and annoying persons will be here soon. Well, *you* are already here! My thanks go to Jean Tate for dealing with the comment policy, if you know what I mean.

  9. DrFlimmer says:

    @ SteveZodiac

    If magnetic fields pervade the interstellar and intergalactic medium then they must have some effect on the EM radiation passing through them […]

    Nope. Photons are not influences by magnetic fields. As is pointed out above, only secondary particles that are created in photon collisions can be used to trace magnetic fields. But radiation itself does not interact with magnetic fields!

  10. DrFlimmer says:

    (next to the comment policy we need an edit facility!)

    Correction to the first sentence:

    …are not influenced by….

  11. Aodhhan says:

    The EU guys are just about as annoying as those who make remarks about them.
    Especially if they cut and paste some non-intellectual babble which doesn’t add anything to the article.
    Wait… isn’t this why they bash the EU guys?

  12. IVAN3MAN_AT_LARGE says:

    @ Aodhhan,

    May I inquire: what’s your beef with Lawrence B. Crowell? Which, I must say, is also as annoying as ‘them’.

    Now, I don’t pretend to understand everything L.B.C. says and the supporting equations that he posts, but he does prompt me to look-up the subject matter on Wikipedia, Scholarpedia, and what-have-you, whenever I don’t know what the bloody hell he’s talking about — subsequently, I have learned something new and then I do know what the bloody hell he’s talking about! ๐Ÿ™‚

  13. Hon. Salacious B. Crumb says:

    @ Jean

    Thanks for the excellent link to Elisabete M. de Gouveia Dal Pino paper “Cosmic Magnetic Fields: from Stars and Galaxies to the Primordial Universe”. I agree this is both well balanced and explains to role of magnetic fields on various astronomical scales.
    I especially thought what piqued my interesting was quoting van der Hulst’s statement; “Astrophysical community with regard to cosmic magnetic fields: โ€…Magnetic Fields are to Astrophysics as sex is to Psychologyโ€ฆโ€; which is now more poignant to what we have learnt in recent years is it has evolved to the proper study of “the soul” or “the mind.” In my view, this is the watershed where the extreme views fall down – as the evidence before us literally kills it for good!
    Another rather important point that is worth mentioning here, is that magnetic fields are far more prevalent in the extreme end of phenomena – from stars to neutron stars and black holes, where the size of the fields can be easily modelled in familiar territory.
    However, as a perspective on the relative strengths of fields in the spaces between stars (interstellar medium) and galaxies (extragalactic medium) these fields are on the other hand rather small.
    IMO, the liberal use (and deliberate abuse) of “plasma cosmology” has clearly the wrong perspective – destroying for good the suggestion that comic fields are on par with the extreme phenomena. This dream is now mostly shattered in a million pieces. Clearly our evidence to date shows there is a huge range of magnitude / strengths of magnetic / electric fields and the progenitors generating the fields – via the ‘active’ plasmas that are create by it. (This is why, of course, why we say these fields are NOT scaleable.) It predominates some and barely rates a mention in others.

    So this is why it is quite true magnetic field play a significant part in formation and activity in celestial objects, it is equally true that the effects can have significant or minor influences.

    In my view, astrophysical plasmas and the improved observations via more accurate polarimetry measures is the future of comic plasmas and our knowledge of magnetic fields. Laboratory experiments here on Earth can only yield so much regarding any improved astrophysical models – failing because the size of the fields are as tenuous as the vacuum of space. This is of course quite separate from the applied plasma physics (i.e. IEEE) and its application to technology. (and what our ‘friends’ fail to see!)

    I must thank you for this excellent and informative article. The open perspective here, foe a change, is realistic and in the proper way of things. I hope in the future our enthusiastic ‘combatants’ might see that their is indeed a place for them in the astronomical sciences – if only they are prepared to let go of the past speculations and move with the flow regarding our current advancements as seen as such in these discoveries in astrophysical plasmas.
    You efforts, especially here, is very greatly appreciated.

  14. Hon. Salacious B. Crumb says:

    Aodhhan said;

    “The EU guys are just about as annoying as those who make remarks about them.”

    Now I know I’ve absolutely been doing the right thing, here. Thanks.

    There is some truth in the adage; “You teach what you most need to learn.”

    You could actually learn something about yourself from that!

    (All my text above was written independent of either Goggle or Wikipedia, without any “cutting or pasting” at all – in case you wanted to know.)

  15. IVAN3MAN_AT_LARGE says:


    But radiation itself does not interact with magnetic fields!

    Au contraire, mon ami! Have you forgotten the Faraday effect?

  16. DrFlimmer says:

    @ Ivan3man

    Indeed, I did, as I already noticed (but didn’t mention) after Jean Tate’s comment above. My apologies.

  17. Lawrence B. Crowell says:

    @ DrFlimmer: You are right that magnetic field do not influence photons, at least classically and for low to moderate magnetic fields. I spent a bit of time just now thinking and am wondering if this is the case for absolutely extreme magnetic fields. 10^{12}Gauss and higher B fields polarize the QED vacuum. Extreme polarizations are essentially squeezed states. This makes me ponder whether or not this could lead to nonlinear effects with radiation passing through such extreme magnetic fields. This is a quantum electrodynamics question, but classically EM radiation does not interact with magnetic fields.

    Sorr for the EU crack, but I just couldn’t resist ๐Ÿ™‚

    @ IVAN3MAN_AT_LARGE: The faraday effect is the interaction of a charged particle with a magnetic field. That is a different process than any interaction between photons and magnetic fields.


  18. Jean Tate says:

    Let’s see if I can undo some of the confusion, concerning magnetic fields and photons.

    The only case I can think of in which a magnetic field affects photons, directly, is vacuum birefringence, as Lawrence B. Crowell notes, which is observable only in magnetars (in principle), because we can’t create magnetic fields strong enough here on Earth (and in the universe only magnetars have fields strong enough).

    All the other effects are effects of magnetic fields on matter, mostly atoms/ions/molecules, which subsequently produce effects on the photons (in the sense that without the magnetic field the photons would be different, in some systematic way).

    For example, the Zeeman effect: in a magnetic field, some of the energy states of an atom (or ion) are different (than the same states in the absence of such a field). When an electron jumps between two such states, the photon emitted will have a different frequency (or wavelength).

    In the case of Faraday rotation, it’s the plane of polarization which is changed, as photons pass through a medium pervaded by a magnetic field (no medium, no Faraday rotation, even if there is a magnetic field).

    The photon-collision/pair-production mechanism that my article relies on has nothing to do with a magnetic field; it’s what subsequently happens to the electrons and positrons that (eventually) leaves its mark on the photons we see … and the key is that they behave differently if there’s a magnetic field around (compared with when there’s not).

  19. IVAN3MAN_AT_LARGE says:

    @ Lawrence B. Crowell,

    On rechecking the Wikipedia reference to the Faraday effect, that I gave above, it states in the first paragraph:

    In physics, the Faraday effect or Faraday rotation is a magneto-optical phenomenon, or an interaction between light and magnetic field in a medium. The rotation of the plane of polarization is proportional to the intensity of the component of the applied magnetic field in the direction of the beam of light.

    However, in the second paragraph, it states:

    This effect occurs in most optically transparent dielectric materials (including liquids) when they are subject to strong magnetic fields.

    Furthermore, in the “Faraday rotation in the interstellar medium” section, it states:

    The effect is imposed on light over the course of its propagation from its origin to the Earth, through the interstellar medium. Here, the effect is caused by free electrons and can be characterized as a difference in the refractive index seen by the two circularly polarized propagation modes.

    It then lists a series of equations involving the charge on an electron, the mass of the electron, and the speed of light in a vacuum; that section concludes:

    Faraday rotation is an important tool in astronomy for the measurement of magnetic fields, which can be estimated from rotation measures given a knowledge of the electron number density. […] In particular, Faraday rotation measurements of polarized radio signals from extragalactic radio sources occulted by the solar corona can be used to estimate both the electron density distribution and the direction and strength of the magnetic field in the coronal plasma.

    Therefore, the “Faraday effect” is, in fact, caused by electromagnetic radiation interacting with electrons in “transparent dielectric materials” (e.g., the “interstellar medium”), which means that the effect on EM waves is indirect and not direct as I had at first thought.

    So, Lawrence B. Crowell, you’ve prompted me, yet again, to check “the subject matter” (as I had stated above) more thoroughly, and it appears that you’re right!

    DrFlimmer, my apologies to you for my earlier presumptuousness! ๐Ÿ™‚

  20. IVAN3MAN_AT_LARGE says:

    P.S. Hmm… I now see that Jean Tate has also posted a clarification on magnetic fields and photons.

    Anyway, this is the difference between rational, scientifically minded people, and pseudoscience cranks (mentioning no names!): the former are always self-correcting themselves with new information; the latter never admit that they were in error — no matter how trivial!

  21. Torbjorn Larsson OM says:

    Darn, too late for the fun stuff! Indeed, magnetic fields are felt by local sissies (materials) not classical devil-may-care far traveling photons as they rip through the vacuum with utter abandon.

    The easiest way to see that is (again, classically) to realize that if they don’t completely separate from effects of the near field that creates them, such as E, M and coupled EM fields, they would have a durn difficult time to freely go to infinity (as r^-2) in the far field. (And, as LBC points out, this is low energy behavior in space. No guarantees for, say, black holes!)

    But I appreciate all the funny effects I haven’t been aware of before. Thanks!

  22. Torbjorn Larsson OM says:

    Chucks, this is before my first coffee. I’m not making myself clear.

    Of course it is the other way around, as it is the coupling EM near field that doesn’t go to infinity. Which means that photons, that does, are separated from such coupling behavior and its components E and M. (Except in the sense that photons themselves are already fully coupled.)

    I’m fairly certain a better analysis would be to look at energy, as suggested by the different radius behavior – but I lack it (energy) right now. ๐Ÿ˜ฎ

  23. DrFlimmer says:


    So, I was somehow not entirely wrong in the first place.

    I don’t know, if my following description is correct. If not, please correct it anyone!
    I just thought of a “semi-classical” approach to make myself clear, why “weak” magnetic fields do not interact with photons. IIRC, the transmitter of the “electromagnetic force” (or the fields) is the photon. So, an interaction between a magnetic field and a photon would actually be an interaction between two photons. And we’ve learned from this post that photons do not interact with each other at low energies (again, assuming not too strong magnetic field strength). Therefore, there should be no interaction between a magnetic field and a photon (neglecting media…).

    As I said before, I don’t know if this simple picture is correct, because it mixes quantum mechanical and classical views (“semi-classical”). But I think it gives an idea.

    CONFUSION mode off!

  24. Lawrence B. Crowell says:

    My comment with potential interactions between photons and huge magnetic fields is a “I wonder if” sort of thing. In a quantum setting this might induce nonlinear QED effects, maybe similar to parametric amplification. As for other presumed effects presented here by others, these do involve photons in media subjected to magnetic fields. Ultimately there are electrons in these media which act as an intemediate actors.

    I should check out whether there is anything on nonlinear QED with hyper-magnetic fields.


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