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Eka Eka 4 4 gold badges 13 13 silver badges 28 28 bronze badges. Timtam Timtam 1 1 gold badge 4 4 silver badges 18 18 bronze badges. So I think the answer by anna v is more accurate. Is there a point at which the velocity of the photon is half way between the velocity it travels through the two media? If not, the jump in velocity is discontinuous, is not differentiable, and so, cannot be described as having an acceleration.
Do Photons Have Mass? | Sciencing
Nick 1, 10 10 silver badges 24 24 bronze badges. The photon is still following a null geodesic, so in the frame of the light, there is no acceleration.
There might only appear to be some acceleration to another observer. The problem is that you can reduce your answer to "Due to gravitation, the notion of 'motion in an unchanging direction' is obsolete.
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Photons, or plane electromagnetic waves, or whatever model you want to use for light, move or propagate or however you want to call it along null geodesics, which result not only in curved light paths from a distant observer, but also in time propagation delays. This is bread and butter for extragalactic astronomers and cosmologists It is there and it is measured and used every day, and cannot be denied or called innacurate. Light loses energy with the expansion though, the frequency changes.
The speed is still c. Schroeder Schroeder 1 1 silver badge 8 8 bronze badges. Sorry, but that is just the way General relativity works. Kilo Kilo Featured on Meta. Unicorn Meta Zoo 8: What does leadership look like in our communities? Linked 2. Related 8. Hot Network Questions. Question feed.
Electron and photon beams interacting with plasma.
Moreover, external guiding recently allowed reaching energies of up to 8 GeV from a cm-scale accelerator [ 12 ]. These continuously improving characteristics have excited significant interest in the use of these electron beams to generate secondary sources, such as photon and positron beams. In the present manuscript we present a brief overview of recent developments in this area. Even though other mechanisms for photon and positron generation from a laser-driven electron beam have been proposed and are actively studied, we will focus our attention here only on Inverse Compton Scattering and quantum cascades in solid targets, respectively.
The remainder of the article is structured as follows: the current state-of-the-art of inverse-Compton-scattered photon sources from LWFA electron beams is discussed in section 2, whilst experimental results on the generation of positron beams from LWFA electrons interacting with a solid target are discussed in section 3. Finally, concluding remarks are given in section 4. These extend from radiotherapy and oncology based on photon irradiation [ 16 , 17 ] to active interrogation of nuclear material and shipping containers [ 18 ] for security purposes. Moreover, high frequency radiation has been found to lead to nuclear resonance fluorescence [ 19 ] as well as photo-induced fission [ 20 ].
In addition to these applications of interest in industry, the value of these sources from the scientific point of view is also extremely attractive. Besides the potential interest for laboratory astrophysics [ 21 ], inverse-Compton-scattering ICS from an ultra-relativistic electron beam is an ideally suited mechanism to probe strong field quantum electrodynamics [ 22 ].
High-field quantum electrodynamic phenomena such as Breit-Wheeler pair production [ 23 ], elastic photon-photon scattering [ 24 ], quantum radiation reaction [ 25 — 27 ], and vacuum breakdown [ 28 ] can then be triggered and precisely studied, with preliminary experimental results already obtained using this setup [ 29 , 30 ]. High energy photons can be mainly obtained via two mechanisms: radioactive sources or accelerating charges.
In the latter case, the properties of the generated radiation are mostly dictated by the specific acceleration of the charge, an electron for definiteness.
Three Failures of Classical Physics
Arguably, the simplest mechanism to reproduce in the laboratory is bremsstrahlung, whereby the radiation is generated during the scattering of an ultra-relativistic electron beam as it propagates through a high-Z solid target [ 31 — 33 ]. This mechanism produces broad-band photon spectra with their maximum energy coincident with that of the primary electron beam.
Moreover, the scattering processes inside the material significantly broaden the divergence and source size of the photon beam, resulting in a relatively low peak brilliance. Gas harmonics [ 34 , 35 ], betatron [ 36 — 39 ], laser-thin foil irradiation [ 40 , 41 ], synchrotron radiation [ 42 ], and free electron lasers [ 43 , 44 ] also constitute viable sources of high energy photons.
However, the photon maximum energy at which these methods operate is typically in the sub-MeV range. Similarly, betatron radiation, i. However, they usually present broad-band spectra and have typical energies per photon in the range of 10s of keV. These properties, together with the intrinsic tuneability in energy of these sources, make them appealing for practical applications especially in nuclear physics and several experimental results have been recently reported in this area [ 30 , 34 , 46 — 54 ].
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A summary of the typical energy and peak brilliance achievable by different photon sources is depicted in Figure 1 , showing how ICS is uniquely suited to provide, simultaneously, multi-MeV energy per photon and a high peak brilliance. Due to these appealing properties, we will focus here only on this photon generation mechanism and refer the reader to other reviews available in the literature for a more in-depth discussion of alternative mechanisms see for instance [ 60 ] and [ 10 ].
Figure 1. Existing Compton scattering facilities have conventional accelerators at the core of their operation. While they constitute reliable, tuneable and efficient means of seeding Compton facilities, conventional accelerators are not only large scale, but also expensive machines to shield and operate. The relative long duration of the photon beams generated also limits the peak brilliance achievable and the temporal resolution of probing schemes based on this technology. The all-optical alternative to the generation of Compton scattered photons poses the advantage that the Compton pulse duration is mainly dictated by the pulse length of the particle beam [ 53 ], due to the relativistic compression of the laser pulse in the rest reference frame of the particle.
In LWFA, the typical bunch duration of an electron beam can be down to a few tens of fs [ 9 ] Moreover, practical advantages of all-optical ICS sources include the compact nature of laser laboratories, the relatively low running cost, and the wide degree of tuneability that they guarantee. Moreover, the capabilities of all-optical Compton sources will be extended by upcoming facilities in the multi-petawatt regime, including the Extreme Light Infrastructure ELI [ 69 , 70 ] and Vulcan 20PW [ 71 ].
It must be noted, however, that aside from peak scattering laser intensity, the seed electron beam quality is of prime importance. In the following section, a brief description of the main properties of all-optical ICS photon sources is presented. Thomson scattering can be described as the elastic scattering of a photon by a free charged particle an electron, for definiteness , with Compton loosely denoting the inelastic counterpart. Traditionally, both Compton and Thomson scattering refer to processes in which an electron gains energy from the photon, whereas Inverse Compton and Inverse Thomson scattering are the terms employed for those interactions in which the photon gains energy from a high-energy particle.
The physics governing the dynamics of electrons scattering in a high intensity electromagnetic field and the subsequent radiation emitted closely resembles the nature of conventional undulators. The theoretical study of the multi-photon variant of Compton Scattering has been active since the mid-twentieth century see Di Piazza et al. Multiple experimental campaigns have observed these effects [ 54 ], and noticed the effects on both induced radiation [ 46 ] and spectrum of the primary electron beam [ 29 , 30 ].
These may be expressed as [ 60 ]. This latter parameter is defined as the ratio of the peak electric field in the electron reference frame and the Schwinger field [ 22 ], and it indicates the importance of quantum effects in the interaction in question.
Photons May Emit Faster-Than-Light Particles, Physicists Suggest
As given by Equations 1, 2, the energy and flux of the X-ray beam can be enhanced by entering the non-linear regime; i. However, despite these more favorable scalings, the bandwidth of the emitted radiation also increases with a 0 [ 72 ]:. An increase in the bandwidth of the generated radiation contributes to the reduction of the quality of the photon beam for applications.
In an all-optical scenario, several ICS experimental campaigns have been carried out in the past few years. Experiments in the non-linear regime, however, have pushed the boundaries of peak energy while maintaining similar brilliance performance. Notably, some of these experiments have also observed the onset of quantum effects in the interaction [ 29 , 30 ]. Multi-photon effects play a vital role in these experiments, as first observed experimentally by Bula et al.
Measuring Photon Momentum
However, multiple photon interactions contribute not only to the energy of the outgoing radiation, but also to the divergence of the beam. However, the finite divergence of the electron beam changes the distribution, as shown in Figure 2 for an electron beam with a 10 mrad divergence [ 46 ]. This effect serves anyway as a useful tool to estimate the peak intensity of a laser beam in focus.
Related Theory of Photon Acceleration
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