I don’t think that the question about the sonic boom from light was answered by this post. I think the reader is actually asking about why light doesn’t cause a SONIC boom. Here is the answer: airplanes and other objects push air out of the way when they travel through it. It is this push that causes the wave. When the object is traveling vaster than the speed of sound, the waves get piled up, causing the shock wave or boom. Since air is transparent, light travels though it without pushing it out of the way. Therefore it produces no sound waves and no sonic boom. The mass of the object doesn’t matter. All that matters is if it pushes the air.

John made a very interesting point about how objects can make Cerenkov radiation (an optic flash). Charged particles make this flash because they distort the electromagnetic field as the travel. Photons do not distort the electromagnetic field, so they cannot cause the flash.

Gavin

I bet if you sent a strong pulse of light through a medium that interacted strongly with light you could get something akin to a sonic boom. You just need some way for the light to transfer its energy to vibrational modes of the medium.

Any experimentalists out there?

I am heartened to see the answers to questions from a non-scientist. One question that continues to bother me, and I have yet to see addressed, is why the big bang was not a black hole? All the mass of the universe concentrated at a point? Doesn’t that sound very like a black-hole situation? Why — how — could the universe escape black-hole destiny? Any hint greatly appreciated.

“And why do we presume an object 13.7 billion light years away to be only 13.7 billion years old?”

Maybe I’m not following what you mean. Your question seems meaningless to me because it starts with the false presumption that we have seen any objects which are 13.7 billion light years away. To my knowledge we have not and can not. (Maybe in a few more billion years we will be able to.)

why the big bang was not a black hole? All the mass of the universe concentrated at a point? Doesn’t that sound very like a black-hole situation?

IANAC (I am not a Cosmologist), but I thought it would be fun to give this question a crack. A black hole is characterized by the presence of an event horizon, which divides space time into two distinct regions (inside & outside). The Big Bang singularity is all of spacetime itself at a single point, not a point of infinite density within spacetime, so there’s no where for an event horizon (or anything else) to be.

That’s a somewhat lame way to point out a qualitative distinction between a black hole singularity and the Big Bang singularity. You might also ask, shortly after the big bang, wouldn’t the mass/energy density within space have been large enough to form event horizons everywhere? This requires a more technical explanation.

The Schwartzchild metric describes spacetime outside of a spherical mass. The event horizon is possible because the signs of the spacelike/timelike coordinates can switch (so a black hole singularity is more properly thought of as a time, not a place).

The Friedmann (et. al.) metric describes spacetime with the mass/energy evenly spread out everywhere. Note there’s no way to change the signs, thus no event horizon possible at any mass density.

So, event horizons are only possible at local concentrations of mass. In the early universe, density fluctuations would have been very tiny, so any primordial black holes that formed would have been very tiny.

I’d like to have someone explain why a witch weighs the same as a duck.

You write of my “first” Nobel Prize. Please tell me where to pick up my check for the second, which is news to me. Thanks! -Al

Because the universe is expanding, there’s no unique definition of “distance.” In fact, in the concordance cosmology where the universe is now accelerating, very distant objects are something like 40 billion light years away, even though the light from them left about 13.7 billion years ago (in some coordinate system…). Just because the distance between there and here has been getting bigger.

Hi Sean,

Probably once we understand quantum gravity, we’ll realize that there never was a singularity, but that’s another topic.

question: I see that QG is a good shot to solve the problem, but I don’t see that a priori both – quantizing gravity and singularity avoidance – have to be related to each other. I.e. consider there’s no such thing as a quantized gravity, but the apparent disagreements between QFT and gravity are resolved in some other way. Couldn’t it as well be that there’s a classical modification of GR that solves the singularity problem (should probably circumvent energy conditions).

Best,

B.

Thanks for the clarifications – I guess.

I am still not following how we see light from objects 13.7 billion light years away. I thought we are seeing light which started much closer to us and has caught up as both we and it moved farther and farther from our starting point at the Big Bang. Otherwise, it would not be in our “light cone”. Maybe I’m thinking in the wrong coordinate system.

B– Sure, that’s possible. I doubt it, just based on the limited success so far. And even if classical modifications of GR resolved singularities, the requisite modifications would likely kick in near the Planck scale, where quantum gravity is going to be important anyway.

?blackhole..
Novice alert..
In theory if our universe was “falling into” a black hole, would it not hold to relativity that the event horizon would never actually be reached due to acceleration and relative observation?
Would it not hold true that as our universe is being accelerated into the “black hole” at a certain point due to acceleration and relative speed, from our vantage point it would actually appear as if the universe was expanding as we would be whitnessing it from a relative point of view, and light from our vantage (being sucked in) is traveling faster than light further form the event, and so we would whitness events in reverse cronology?
Giving rise to the question: If we were being sucked in and we were within the gravitational boundary of the event, how would we know we were being drawn in if the relative view makes it appear that we are being expanded away from the event?

……O_o…..*reads the very intellectual comments left by people who know somethin and print out to show and tell for current event*……….ducks quack dont echo?…………

Keith: It’s not a typo. The speed of light in some media (glass for example) is actually slower than the speed of light in a vacuum. What’s actually going on is that the light is absorbed, re-emitted, and scattered by the atoms in the material — any given photon still travels at the speed of light between bumps. The speed of light in a medium is closely related to the medium’s index of refraction, which you may have heard about in relation to lenses and light-bending.

At any rate, it is possible for something to travel faster than the speed of light in a medium, as long as it’s slower than the speed of light in a vacuum. When it does so, you get Cerenkov radiation.

Why doesn’t light make a sonic boom when it travels?

Very Simple.
It did when it passed the speed of sound 14. billion The years prior (big bang). Backgroud Radiation…

You guys should think less and observe more.
Dar

I just want some chicken nuggets.

David, I think your instructor was on the wrong track. A typical black-hole collapse won’t be perfectly isotropic, but it can be very close, and that’s not really the point. The spacetime geometry very close to the Big Bang is quite similar to what you get when a star collapses to a black hole — except that one is the time-reverse of the other. That’s really the point.

Thanks Sean,

What you said makes alot more sense.

@ Sean #24

I doubt it, just based on the limited success so far. And even if classical modifications of GR resolved singularities, the requisite modifications would likely kick in near the Planck scale, where quantum gravity is going to be important anyway.

Sorry, I almost forgot I left that comment here. Thanks for the answer. What do you mean with ‘quantum gravity’ in the above context? I see it this way: gravity works perfectly fine also at the Planck scale. The only thing that goes wrong in GR are the singularities. What we don’t know is what to do with quantum field theory when curvature gets close to the Planckian regime, and we don’t know how to couple QFT to GR (okay, and even if we knew, backreaction effects would make the equations enormously ugly, but lets put that aside for a moment). If I look at it this way, it seems to me it’s rather background indep. quantization that we don’t know how to deal with instead of gravity. So, what good reason do we have to believe that the metric has to be subject to quantization as well?

Bee, to play the devil’s advocate for a moment, even though I agree with Sean that gravity must be quantized: suppose it were possible to couple a classical theory of gravity to QFT. How do you know which classical theory of gravity? There are infinitely many background-independent classical theories of gravity, the Einstein-Hilbert action is just the one that keeps the dominant term at low energies. So it appears you still have a problem at the Planck scale.

(The quantum version of this question is the problem of nonrenormalizability of gravitational theories, which as far I can tell from zillions of blog threads on the topic, LQG ignores completely.)

… suppose it were possible to couple a classical theory of gravity to QFT. How do you know which classical theory of gravity? There are infinitely many background-independent classical theories of gravity, the Einstein-Hilbert action is just the one that keeps the dominant term at low energies. So it appears you still have a problem at the Planck scale.

(The quantum version of this question is the problem of nonrenormalizability of gravitational theories, which as far I can tell from zillions of blog threads on the topic, LQG ignores completely.) – anon.

To answer the first point. You choose the classical theory of gravity which, when coupled to QFT, is based on verified facts and makes accurate predictions.

Regards the second point. Renormalization in gravitation will be a change in the effective value of gravitational charge, i.e., mass. Mass is supposed to be given by the Higgs mechanism, which must be gravitational charge because Einstein’s equivalence principle says gravitational mass is the same as inertial mass.

Renormalization of electric charge in QED is explained by the polarization of the vacuum around the bare core charge, which cancels part of the latter as observed from a distance. You can’t apparently polarize the vacuum to shield gravitational force as you can for electric force.

Polarization in an electric field works because virtual positive charges get attracted closer to the bare core negative charge of a particle than virtual positive charges, so the virtual charges give a net radial electric field which opposes and cancels part of the core charge.

Clearly this can’t occur in a gravitational field because all mass moves the same way; there are no opposite poles for mass and gravity, so no polarization or shielding occurs, at least directly. This makes it hard to see how any quantum theory of gravity can physically include renormalization of gravitational charge (mass).

However, the equivalence principle between gravitational and inertial mass in the context of quantum gravity has been attacked by Rabinowitz in http://arxiv.org/abs/physics/0601218 where it is argued:

“… a theory of quantum gravity may not be possible unless it is not based upon the equivalence principle, or if quantum mechanics can change its mass dependence. …”

In QED, both electric charge and electron mass are renormalized parameters and are scaled by similar factors. This seems to suggest that maybe the source of the electron’s mass is the mass-giving (‘Higgs’) vacuum field outside the polarization region, if mass is associated with the electron by a coupling depending on the electric field of the electron. Thus, the polarization-shielded electric charge (not the core or bare electron charge) would be responsible for coupling external mass-giving Higgs bosons to the electron. So renormalization of the electric field automatically causes renormalization of the gravitationam charge (mass), because the shielded electron charge is responsible for the vacuum field effects which give mass to an electron.

In the Standard Model, all masses are given to particles by field which is separate to electric charge. The only way such a mass-viving field can couple to an electron core without mass is obviously by some kind of coupling to the electron’s electric field. So renormalization effects in quantum gravity are likely to be indirect, i.e., the effect of electric field renormalization (which does have a very simple, empirically confirmed physical mechanism; vacuum charge radial polarization).

Joe said
“When lightning strikes it makes some what of a sonic boom which is the clap of thunder we hear eminate after the strike. Just as Gavin said, “airplanes and other objects push air out of the way when they travel through it.” Lightning does the same thing, except it has to find a path all the way from the cloud to the ground that moves inbetween the air molecules and when full connection is made it pushes the air molecules out of the way so fast that it creates thunder, or mini sonic boom. Some of this maybe incorrect or could be explained in a more efficient mannor, but that is the gist of it.”

That’s entirely different. Thunder is caused when all that electrical force is channeled through air (which is actually a pretty good insulator), super heating it. What you hear, in effect, is the air expanding at a supersonic speed.