Science Sundays with John Duffield: Black Holes

Like I was saying last week, once you understand one thing in physics it’s easy to understand the next. Once you understand time, you understand what Einstein was on about with his variable speed of light. After that you understand gravity: light curves because the speed of light varies with position. And after that, you can understand black holes.

Straight up. Yes, straight up. Because imagine you’re standing on a planet shining a laser beam straight up into space. The light goes straight up. It doesn’t curve. Now imagine it’s a denser more massive planet. The light still goes straight up. It still doesn’t curve. Let’s make it really massive. That light still goes straight up. It still doesn’t curve. But when we make it so massive that it’s a black hole, all of a sudden light can’t escape. Why not?

Some will tell you that the light curves back down into the black hole. When you challenge that by saying the light didn’t start curving on the massive planet, they’ll change tack. They’ll say it’s because spacetime is curved. Then when you challenge that by saying spacetime is an abstract mathematical space, they’ll change tack again and tell you about the waterfall analogy. That’s where space is falling inwards so the light beam doesn’t make any progress. That’s crap. In no sense is space falling inwards in a gravitational field. A gravitational field alters the motion of light through space, but it doesn’t suck space in. Because it’s a region of inhomogeneous space, like Einstein said. Like this:



Optical clocks go slower when they’re lower because the space down there is different to the space up here. They call it gravitational time dilation, but it’s just the light going slower when it’s lower, so the clocks go slower too. How much slower? Have a google on infinite gravitational time dilation. What pops up time and time again? Yes, black holes. Remember those parallel-mirror light-clocks at different elevations? Gravitational time dilation is said to go infinite at the black hole event horizon. So an optical clock at the event horizon doesn’t tick at all. And if it doesn’t tick, it can only be because the light isn’t moving. Because the speed of light at the event horizon is zero. That’s why your laser beam doesn’t get out of the black hole. Not because of some mystic curvature, or because the sky’s falling in, but because at that location the speed of light is zero, zip, zilch. The light isn’t moving, so it  doesn’t go up and it doesn’t get out because it is effectively “frozen”.

Did you know that black holes were originally called frozen stars? If you google on frozen star Oppenheimer you can find references to this. However if you google on frozen star and follow the links, what comes up is black hole along with a point singularity. It’s like history has been rewritten. It’s like the original “frozen star” has been airbrushed away, and replaced with something else. Something stupid. See the picture below?



My mate Jesse put it up on the internet. It’s a screenshot from Misner/Thorne/Wheeler, the “bible” of gravitation. It depicts Schwarzschild coordinates for a body falling into a black hole. See the dashed line up the middle? That’s the event horizon. See how to the right of it  the curve goes up? Do you know where that’s headed? It’s headed to the end of time. Only it’s cut off vertically, and then it comes back down. Yes, according to MTW if you fall into a black hole, you go to the end of time and back in no time flat. That’s why you read about the elephant and the event horizon, where the elephant is in two places at once. Sadly there’s even an echo of this in Kevin Brown’s the formation and growth of black holes. He refers to the frozen star, but doesn’t favour it, and talks about “future infinity” instead. Fall into a black hole and that’s where you go. And back again.

It’s crap. The truth is that when you fall into a black hole, everything goes slower and slower  until it stops. Your clocks go slower and slower, and so do you. Some people think you can cancel out a stopped clock with a stopped observer, who somehow sees the clock ticking normally. But he doesn’t.  He’s stopped. He doesn’t see anything. Ever. At the black hole event horizon the speed of light is zero, so he can’t see. His light is stopped, and because of the wave nature of matter, he’s stopped too. And because light can’t go slower than stopped, light isn’t going to curve any more, and he isn’t going to fall any further. So the frozen-star black hole grows like a hailstone. Which is apt.

Only there’s something I forgot to mention. If you fall towards a black hole you fall faster and faster. And all the while the speed of light is getting slower and slower. So, is there some crossover point where you end up going faster than the local speed of light? Relativity says no, the wave nature of matter says no, and The Man from Del Monte says no. So it stands to reason that something bad is going to happen. Like you get ionised and annihilated. Hence gamma-ray bursters. Hence Friedwardt Winterberg’s firewall, which is “very different to Hawking radiation”.  

There’s other things different too. For example you’ll have heard of gravitational blue shift. But remember your conservation of energy. When you drop a 511kev photon into a black hole, the black hole mass increases by 511keV. So whilst people talk about blue-shifted photons, those photons don’t actually gain any energy or increase in frequency as they descend. All that happens is that clocks go slower when they’re lower, so you measure a higher frequency when you’re lower. That’s relativity for you. You understand time, then the speed of light, then gravity, then black holes, and then you realise just how much crap is out there.  Because you realise just how simple it all is.

And if you don’t, keep plugging away, because believe you me, it is.

9 comments on “Science Sundays with John Duffield: Black Holes

  1. Ed
    October 20, 2013 at 9:41 am #

    Nope. Still don’t get any of it. Wish I did but life’s a bit short for idiots like me to devote all the necessary (variable speed) time it would take to get me there. I’ll leave it to brainiacs while I supply the potatoes, which is as noble a contribution as any.

  2. shorelark
    October 20, 2013 at 11:41 am #

    Hi DJ. Starting from the premise that the tidal forces around any black-hole would shred any material body pretty soon, I wouldn’t have thought people would have much to say on the subject. Thought I would agree that most of what they have to say is wrong.

    There is an important point here, which everyone side-steps. The photon falling into the well experiences an increase in kinetic energy – hence the blue shift – while its potential energy is reduced by a similar amount. Or I might say the photon’s momentum increases, and hence its wave-length is reduced.


  3. duffieldjohn
    October 20, 2013 at 12:49 pm #

    Ed: go back a couple of weeks and start with the article about time, then read last week’s article about gravity. It’s simpler than you might think.

    Shorelark: tidal forces are there because the force of gravity at your feet is stronger than at your head. And the force of gravity is due to the “local slope” of gravitational potential. That equates to the gradient in the (coordinate) speed of light. If your feet are at the event horizon where the speed of light is zero, it can’t go any lower. So your feet aren’t being pulled down, but your head still is. Ouch!

    The falling electron increases its kinetic energy at the expense of its potential energy. If you catch it and throw away the kinetic energy, its rest mass is reduced. See mass in general relativity on wiki*. It’s different for the photon. It doesn’t have any rest mass, or potential energy, it’s *just* kinetic energy. This doesn’t actually increase as it descends. It doesn’t gain any energy. Conservation of energy applies Throw a 511keV photon into a black hole, and the black hole mass increases by 511keV, no more. The photon only appears to gain energy because when you measure it at the lower elevation, you and your clocks are going slower there. So you measure a higher frequency. The photon energy-momentum is higher relative to you, but it hasn’t changed one jot. Instead, you have.


    • shorelark
      October 21, 2013 at 8:57 am #

      DJ: “The falling electron increases its kinetic energy at the expense of its potential energy. If you catch it and throw away the kinetic energy, its rest mass is reduced.” I am afraid most physicists would blink at this claim. Throwing away the particle’s kinetic energy means bringing it rest; then its mass equals its rest mass.

      ” It doesn’t have any rest mass, or potential energy, it’s *just* kinetic energy.” True, the photon is a zero-rest mass particle, but gravity doesn’t discriminate; all mass is grist to its mill. Here the mass equals the energy divided by C-squared. The idea that photons don’t have potential energy is one of the fictions of general relativity.

      My guess would be that it got started with the short-lived idea that antiparticles have negative mass. So neither the electron-positron pair or the gamma rays resulting resulting from annihilation would have any mass.

  4. duffieldjohn
    October 21, 2013 at 12:21 pm #

    They might blink, shorelark, but that’s general relativity for you. When you raise a brick (or an electron) you do work on it. You add energy. It’s then at rest on some shelf, and its mass has increased. When you drop it, some of its mass-energy is converted into kinetic energy. Catch the brick and get rid of the kinetic energy, and the mass is reduced again. Check out mass deficit.

    The potential energy of a body is essentially hidden kinetic energy. It isn’t hidden for a photon, which isn’t a body in the E=mc² sense. I’m not kidding you about this. Check out Einstein’s original paper* along with Light is Heavy**. The body loses mass when it emits a photon. The photon isn’t at rest, so rest mass doesn’t apply. It has “relativistic mass”, which is a measure of energy, and it has “inertial mass” because it conveys inertia from the emitting body to the absorbing body. That’s a measure of energy too, and is “active gravitational mass”.


  5. jdseanjd
    October 22, 2013 at 6:48 am #

    I’ve read Stephen Hawking’s A Brief History Of Time, oh 3 or 4 times, using up & wasting a fair amount of my precious & increasingly limited time thereby.

    It’s all Greek to me John, & I only studied Latin. 😦

    An attempt to reconcile Newtonian Physics & Quantum Mechanics? The big stuff & the invisible stuff? OK, right.
    But when it comes to strings & black holes, my mind shuts down, time & time again. ;-).

    Your stuff flies over my head in exactly the same way.
    Whoosh, there it went same as last time.

    I salvage my self respect by consoling myself that I have an artistic, rather than a scientific type of mind.

    Keep up the good work, John.
    Perhaps I’ll catch on next time. 🙂

  6. duffieldjohn
    October 22, 2013 at 12:43 pm #

    Take it one step at a time, jdseanjd. Get a handle on time, then the speed of light, then gravity:

    Then read about black holes. If there’s anything that’s not clear ask me about it. I’m always trying to make this stuff as straightforward as I can.


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