# Waveform produced by a collapsing magnetic field

• I
Gold Member
A thought experiment:

A electron is moving in a straight line at velocity v. It instantly stops dead. It doesnt move another femtometer.

Obviously its magnetic field collapses and produces light. What is the waveform of the light produced?

Is it something like this: https://en.wikipedia.org/wiki/Dirac_delta_function

But then again it will take time for the magnetic field to collapse. Hmm.

Dale
Mentor
2020 Award
Just plug it into the Lienard Wiechert field equations.

Delta2
Homework Helper
Gold Member
Your setup implies infinite acceleration (actually deceleration) of the electron. All accelerated charged particles emit EM waves and such is the case with this electron. However because the acceleration in this setup is infinite, the frequency and amplitude of the produced EM wave will be infinite as well.

em3ry
vanhees71
Gold Member
The acceleration is some ##\delta## distribution in time. I'm not sure, whether this extreme simplification of "stopping" will produce a finite result for the bremsstrahlung em. field. As @Dale said, just try it out plugging it into the Lienart-Wiechert (retarded) potentials.

Ibix
2020 Award
Do we need to do anything so complex as the LW potentials for this? Surely the infinite acceleration makes this easy: write down the EM field of a charge moving at constant velocity and write down the EM field of a stationary charge. The former applies everywhere outside the future lightcone of the speed change event. The latter applies inside. The field on the future lightcone isn't well defined because of the discontinuous velocity. Or am I oversimplifying?

Dale and Nugatory
vanhees71
Gold Member
I think so, because there must be a radiation field on top of the (boosted) Coulomb field because of the acceleration, but I'm not sure about the em. response to a ##\delta##-distribution singularity in the acceleration of the charge.

Ibix
2020 Award
I think so, because there must be a radiation field on top of the (boosted) Coulomb field because of the acceleration, but I'm not sure about the em. response to a ##\delta##-distribution singularity in the acceleration of the charge.
Wikipedia says that the covariant form of the Lienard-Wiechert potentials is$$A^\mu(X^\mu)= -\frac{\mu_0qc}{4\pi}\left(\frac{U^\mu}{R_\mu U^\mu}\right)_{t_r}$$where ##A^\mu## is the potential at an event ##X##, ##U^\mu## is the four velocity of the charge, and ##R^\mu## is a vector from the source to ##X##, and the source position and velocity is evaluated at the retarded time ##t_r##.

I assume that the two ##\mu## in the denominator should be ##\nu## (or something) and the left hand side shouldn't really have an index on the ##X##.

It's easiest to work in Einstein coordinates with their origin at the deceleration event. Then ##R^\nu=X^\nu-(R_s)^\nu##, where ##R_s## is the position of the source at the retarded time. Since the charge is stationary at the spatial origin for all ##t>0## we find that for all events with positive ##t_r## (i.e. all events inside the future lightcone), ##R_\nu U^\nu=R_t U^t=(t-t_r)=r##, where ##r## is the distance of event ##X## from the spatial origin. So everywhere inside the lightcone you have ##\phi\propto 1/r## and ##\vec A=0##.

You can make the same argument outside the lightcone by working in the initial rest frame of the source, or simply observe that causality requires that the field outside the lightcone be the Lorentz boosted Coulomb field.

So I don't think I am oversimplifying - unless there's more wrong with the Wiki article than dodgy indexes. I think the radiation field, as far as one can define it in this case, lies on the future lightcone where the behaviour isn't well defined. I would imagine that to consider this formally, one should consider a finite period of proper acceleration ##\alpha## linking the two periods of inertial motion and take the limit as ##\alpha\rightarrow\infty## while keeping the velocity change constant.

Dale, em3ry, etotheipi and 1 other person
vanhees71
Gold Member
The formula in the Wiki is correct, and I also thin your arguments.

But where is the radiation field, which must be there, right? There must be some singular contribution along the light cone. Usually such apparently "simple idealizations" turn out to be much more complicated. A famous example is the radiation from a charge in (eternal) hyperbolic motion ;-)).

em3ry and Ibix
tech99
Gold Member
Do we need to do anything so complex as the LW potentials for this? Surely the infinite acceleration makes this easy: write down the EM field of a charge moving at constant velocity and write down the EM field of a stationary charge. The former applies everywhere outside the future lightcone of the speed change event. The latter applies inside. The field on the future lightcone isn't well defined because of the discontinuous velocity. Or am I oversimplifying?
It looks to me that during the period of deceleration we are doing work on the electron, as we are decelerating it against its own field. This energy appears as radiation. The electric field before and after deceleration is radial; only during deceleration does it have a tangential component, so constituting radiation. I think the radiated energy will have a spectrum corresponding to the Fourier analysis of the deceleration curve. Clearly we cannot supply the necessary energy in zero time as it would correspond to infinite power.

em3ry
Staff Emeritus
I'm kind of puzzled by this. If I have infinities in the problem description - and the electron has an infinite force applied to it for zero time - why is it surprising that there infinities in the solution?

vanhees71 and em3ry
Dale
Mentor
2020 Award
The field on the future lightcone isn't well defined because of the discontinuous velocity.
The LW fields will give that also, plus they are good to know since they answer all similar questions too.

But I agree that your simple method works everywhere else

vanhees71 and em3ry
Gold Member
But where is the radiation field, which must be there, right? There must be some singular contribution along the light cone. Usually such apparently "simple idealizations" turn out to be much more complicated. A famous example is the radiation from a charge in (eternal) hyperbolic motion ;-)).
Below shows an image of the electric field with a kink in it as would be expected after a sudden deceleration. Clearly the curl at A and B is nonzero but the curl at C is zero. The curl of A is of the opposite sign of the curl at B.

According to the Feynman link I just posted above, this kink which is due to the acceleration of the charge, falls off as 1/x (think conservation of energy) while the electric field itself falls off as 1/x^2. When the electric field has fallen off so much as to be negligible this kink will still be there but will consist of electric (and magnetic) fields going around in circles (curl > 0). In other words it will be light.

Edit: This is actually the electric field of a stationary particle that suddenly moves and then stops again immediately but its enough to illustrate my point about curl.

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tech99
Gold Member
I notice this diagram portrays radiation field components pointing only to the right. Of course, we cannot have EM radiation with a DC component. So other accelerations must have occurred, or will occur, so that the radiated waveform is balanced. The timing of these events (the initial switch-on etc), seems to dictate the lowest frequency component.

Gold Member
You dont seem to understand. The curl is the radiation. The curl is there but doesnt look like curl (doesnt look like field lines going around in circles).

The circular field lines only become visible after the electric field due to the charge becomes negligible

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Ibix
2020 Award
Below shows an image of the electric field with a kink in it as would be expected after a sudden deceleration.
That isn't a sudden deceleration in the sense you were using it earlier. Rather, it's the result of a short period of deceleration. Furthermore, it's a snapshot of a small part of the field very far from the source (which is why the field lines are nearly parallel).

I think that answers @tech99's objection - the overall picture is one of radial field lines with a kink at radius ##c\Delta t##, where ##\Delta t## is the time since the deceleration and ##c\Delta t## is much greater than the width of the diagram. The above picture is an exceedingly zoomed-in diagram of a small part of the field.

em3ry
Gold Member
OK so I think I've got my answer now. Thanks everyone.

To sum up: A charge experiencing a very sudden acceleration would emit very high frequency radiation.

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hutchphd
Homework Helper
elow shows an image of the electric field with a kink in it as would be expected after a sudden deceleration.
Here's an interesting image for the E field which seems correct to me (acceleration instead of decceleration) Then there will also be the B field. Food for thought.

em3ry, vanhees71 and Ibix
tech99
Gold Member
Here's an interesting image for the E field which seems correct to me (acceleration instead of decceleration) Then there will also be the B field. Food for thought.
I think the B radiation field is something a stationary observer sees as the electric wave passes. I am not sure about generation of the B radiation field by the kinking of field lines.

hutchphd
Homework Helper
Not sure I understand your point. The "electric wave" occurs as the kinks in the E field pass and marks the edge of the B field. This is then an outgoing radiation pulse representing the frequency content of the acceleration pulse.

vanhees71
vanhees71
Gold Member
I started to calculate the field.

Of course the above solution given by @Ibix above, i.e., in the open region excluding the light cone at the origin of the Minkowski space (with the particle moving such that it arrives at ##\vec{x}=0## at time ##t=0## and is then suddenly stopped). So it's a boosted Coulomb field for a charge at rest in the entire region ##t<0## and outside the forward lightcone, i.e., for ##|x|< c t## a "simple Coulomb field of a charge at rest in the origin" in the interior of the forward lightcone, i.e., for ##|x|>c t##.

The trouble is to get what happens precisely at the light cone, where the field is singular, and this singularity represents somehow the radiation field, i.e., seen in position space it's a singular spherical wave pulse exactly on the sphere ##|\vec{x}|=c t## (##t>0##). Of course, when calculating the fields the derivatives of the involved Heaviside unitstep functions from the LW four-potential give the appropriate Dirac ##\delta## distribution "peaked" at the light cone. I don't think that it's worth the effort to really calculate it, because as I guessed before, this complication is the result of overidealizing a real "braking process" of a charged particle, which always takes a finite time over a finite distance and is smooth. So if it's done rapidly enough, i.e., when the overidealization is not too bad, you get some spherical wave packet with a smooth distribution peaked around the sphere ##|\vec{x}|=c t## (of course superimposed with the Coulomb field and boosted Coulomb field as described above).

hutchphd
Ibix
2020 Award
Even with a finite braking time the field between the lightcones appears to be... complicated to describe.

vanhees71
vanhees71
Gold Member
Well, while the "sudden stop" idealization is at least calculable in closed form, I guess it's hard to find a more realistic motion of a point charge of this kind such that you can solve for the LW potential analytically. The LW potential only looks simple. The problem is to calculate the retarded time (or the "retarded proper" time when using the relativistic covariant formulation of particle motion).

What's easier is the Fourier decomposition and/or multipole expansion (if the particle motion is restricted to a finite region in space). See Landau Lifshitz vol. 2.

em3ry and hutchphd
Ibix