Special Relativity & Radiative Processes

Transcription

1 Special Relativity & Radiative Processes

2 Special Relativity Special Relativity is a theory describing the motion of particles and fields at any speed. It is based on two principles: 1. All inertial frames are equivalent for all experiments i.e. no experiment can measure absolute velocity. 2. Maxwell's equations and the speed of light must be the same for all observers.

3 Galilean Relativity Applies Appliesto toall allinertial inertialand andnon-inertial non-inertial frames framesat atlow lowspeeds. speeds. The laws of motion are the same in all inertial frames. Special Relativity Applies Appliesto toall allinertial inertialand andnon-inertial non-inertial frames. frames. The laws of motion are the same in all inertial frames. General Relativity Applies Appliesto toall allinertial inertialand andnon-inertial non-inertial frames frames++gravitational gravitationalfields. fields.

4 Lorentz Transformations Lorentz LorentzTransformations: Transformations:Both Bothspace spaceand andtime time are subject to transformation. The description of are subject to transformation. The description ofevents events occurring at a certain location in space and time depends occurring at a certain location in space and time depends on onthe theparticular particularreference referenceframe frameof ofchoice. choice.

5 Light Cones

6 Relativity of Simultaneity

7 The Andromeda Paradox Formulated first by R. Penrose to illustrate the apparent paradox of relativity of simultaneity

8 Lorentz-Fitzgerald Contraction Note: the two observers in K and K' would measure the same effect with respect to each other. How is that possible? Solution: Lorentz transformation of time is NOT Lorentz invariant since it depends also on space. Therefore temporal simultaneity is NOT Lorentz invariant. Therefore each observer does not see the other carrying the measurement of the two ends of the stick at the same time.

9 Time Dilation Time in the lab frame flows faster than in the moving frame. Same story here: both observers will see the each other's clock slowing down. Each would object that the clocks used by the other to measure the time interval were not synchronized.

10 Observability of Lorentz contraction Question for you: Lorentz contraction and time dilation assume that you are carrying your measurements with rods, i.e., you can carry the measurement in place. But what happens when you use photons? This is the situation we encounter in astronomy, basically all information is carried by photons and we make measurements by collecting photons on a detector.

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12 Diameter: light years

13 This picture does not represent an instant of the Andromeda Galaxy. Indeed the photons you're recording were emitted with up to 200,000 years difference. Diameter: light years What you're seeing are photons arriving at the same time, but NOT emitted at the same time.

14 Question: Given that the speed of light has a finite propagation speed, can we ever observe the Lorentz contraction and time dilation when we measure photons to carry our measurements? (As is the case in astronomy) J. Teller: Invisibility of the Lorentz Contraction R. Penrose: The Apparent Shape of a Relativistically Moving Sphere A. Lampa: "Wie erscheint nach der Relativitätstheorie ein bewegter Stab einem ruhenden Beobachter?

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16 Here the square represents an object of finite size and extension. In other words any real object we can observe.

17 Measured with a ruler Measured with photons

18 A sphere would be contracted along the direction of motion (Lorentz contraction) However, due to the finite speed of propagation of light, photons arriving simultaneously at the observer will still produce a spherical object with no visible Lorentz contraction.

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20 Time Dilation or Contraction?

21 Relativistic Doppler Boost Let's start from this expression we derived before: Δ t =γ (1 β cos θ)δ t ' We know that frequency is the inverse of time so we can write: ' ' ν ν= =ν δ γ(1 β cos θ) This is the relativistic Doppler effect, based on the time dilation AND the finite time for light propagation.

22 Aberration of Light

23 Lorentz Transformations of Velocities There are the velocity transformations when the velocity is on the x-axis direction. What about a more general form?

24 Take v along an arbitrary direction. Take the parallel and perpendicular components of u to v. Aberration formula Aberration of light (u = c)

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28 Radius R ASSUMPTIONS 1. Jet is moving with bulk Lorentz factor Gamma 2. BL photons are produced in a sphere of radius R 3. The radiation is monochromatic

29 Radius R Blueshifted 90 deg. 1/Gamma by a factor Γ

30 Radius R Intensity boost: 4 I ' =δ ' I Blueshifted 90 deg. 1/Gamma by a factor Γ

31 Radius R Intensity boost: 4 I ' =δ ' I Monochromatic flux spreads across frequencies. Blueshifted 90 deg. 1/Gamma by a factor Γ

32 Radius R Intensity boost: 4 I ' =δ ' I Monochromatic flux spreads across frequencies. Blueshifted 90 deg. 1/Gamma by a factor Γ

33 Intensity, opacity and emissivity Transformation Iν = Lorentz Invariant 3 ν Sν = Lorentz Invariant 3 ν

34 Intensity, opacity and emissivity Transformation Since exp(-tau) gives the fraction of photons passing through the material, the optical depth must be a Lorentz Invariant (i.e., simple counting does not change the outcome in any reference frame). l αν τ= = Lorentz Invariant sin θ Similar arguments can be used to show that also the emissivity divided by the frequency squared is a Lorentz Invariant: jν = Lorentz Invariant 2 ν

35 Superluminal Motion Special Relativity states that the speed of light cannot be crossed. So how do you explain the following image?

36 To answer this question look at the exercise 4.7 of the R&L Δ te Time to move from 1 to 2 (in reference frame K) 2 is closer to observer than 1, therefore: Δ t a=δ t 'e γ(1 β cos θ)=δ t e (1 β cos θ) 3 The displacement 3 2 is v sin θ Δ t e Therefore the apparent velocity must be: Observer v sin θ Δ t e v sin θ v app = = Δ ta 1 β cos θ

37 To answer this question look at the exercise 4.7 of the R&L How can we now find the maximum of this apparent velocity? v sin θ Δ t e v sin θ v app = = Δ ta 1 β cos θ 3 Observer

38 To answer this question look at the exercise 4.7 of the R&L How can we now find the maximum of this apparent velocity? v sin θ Δ t e v sin θ v app = = Δ ta 1 β cos θ Differentiate (wrt the angle theta) and set the expression to zero: 3 v Observer max app v 1 β = =γ v 2 1 β 2 So if the velocity v is large and gamma is >>1 you can easily go to apparent velocities >> c

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42 Covariance of Electromagnetic Phenomena Consider a capacitor, with plate separation d and charge density σ. We know that the field within the plates is E and it does not depend on the plate separation d.

43 Covariance of Electromagnetic Phenomena E per =γ E per ' B per = γ β E per y E v B field created by surface current density: ' ' μ = σ v x ' par = E par (Remember that the E field of a capacitor does not depend on the plate separation d)