Special Relativity I

Interesting physics explained with many thought experiments and little math.

Special Relativity I

Postby SsDd » Thu Sep 02, 2010 12:12 am

The lecture video is embedded below.

Additionally, slides used in the lecture are embedded below, or can also be downloaded directly from here.

Questions after the lecture? Please feel free to post them in the same thread.


Click to play embedded YouTube video.





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Wikipedia Links:-
Special Relativity
Simultaneity
TIme Dilation
Mass–energy equivalence
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Re: Special Relativity I

Postby keithscientist » Sat Jan 29, 2011 4:15 pm

After enjoying 40 some-odd lectures, some up to 10 times, (I LOVE the whole Idea of Physics lectures on line), I am, for the First time, compelled to make some comments.... Before I watched this Lecture...Below is what I thought about light:

1) The big difference between light and sound is that light behaves more particular-esque, propagating through vacuous media.

2) Why is a supernova bright? It is the light form of a sonic boom.

3) If an object is approaching an observer at super-light speed. The observer can't observe the super-light speed object until after the object arrives.

4) To exceed the speed of light, (heretical and anti-dogmatic as this may be) an object will have to expend a huge amount of energy to exceed the light barrier. I remember when the sound barrier was also thought to be insurmountable for humans.

Now I'm not so sure, after the "C" being the "speed limit of the universe" statement. Is this really for sure true? I'm also a little creeped out by the flash-light and the .9c and .9c example. I thought that red shift, used to measure distance in the visible universe, indicates that not all light travels at the same speed and we can use this to estimate how fast an object is going away???

Puzzled, Keithscientist (Health Physicist - NOT a cosmological physicist)
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Re: Special Relativity I

Postby Chris Peterson » Sat Jan 29, 2011 4:33 pm

keithscientist wrote:After enjoying 40 some-odd lectures, some up to 10 times, (I LOVE the whole Idea of Physics lectures on line), I am, for the First time, compelled to make some comments.... Before I watched this Lecture...Below is what I thought about light:

1) The big difference between light and sound is that light behaves more particular-esque, propagating through vacuous media.

Other than the fact that some similar mathematical tools can be used to describe both, they have nothing physical in common at all. Sound consists of pressure variations traveling in a medium; light is energy carried by photons.

2) Why is a supernova bright? It is the light form of a sonic boom.

A supernova is bright because when a star collapses, gravitational energy is converted to other forms of energy, including light.

3) If an object is approaching an observer at super-light speed. The observer can't observe the super-light speed object until after the object arrives.

An object can't approach an observer at super-light speed.

4) To exceed the speed of light, (heretical and anti-dogmatic as this may be) an object will have to expend a huge amount of energy to exceed the light barrier. I remember when the sound barrier was also thought to be insurmountable for humans.

To exceed the speed of light would require an infinite amount of energy (which I guess you could call "huge"). That's why it can't be done. With respect to the sound barrier, it was recognized well before it was broken that no theoretical constraints were present, only engineering problems. That's very different from the situation with light.

Now I'm not so sure, after the "C" being the "speed limit of the universe" statement. Is this really for sure true? I'm also a little creeped out by the flash-light and the .9c and .9c example. I thought that red shift, used to measure distance in the visible universe, indicates that not all light travels at the same speed and we can use this to estimate how fast an object is going away???

It's really true, although there are cases which sound a bit like exceptions (but aren't, really).

Redshift does not indicate that light travels different speeds. It is always traveling at c; redshift is the increase in wavelength of the light, either because the source is moving away from the observer, or because space is expanding between the two as the light travels.
Chris

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Re: Special Relativity I

Postby neufer » Sat Jan 29, 2011 5:32 pm

Chris Peterson wrote:
keithscientist wrote:
1) The big difference between light and sound is that light behaves more particular-esque, propagating through vacuous media.

Other than the fact that some similar mathematical tools can be used to describe both, they have nothing physical in common at all.
Sound consists of pressure variations traveling in a medium; light is energy carried by photons.

http://en.wikipedia.org/wiki/Phonon wrote:
ImageImage
Igor Tamm
<<A phonon is a quasiparticle representing the quantization of the modes of lattice vibrations of periodic, elastic crystal structures of solids. Phonons play a major role in many of the physical properties of solids, including a material's thermal and electrical conductivities. Thus, the study of phonons is an important part of solid state physics. The name phonon comes from the Greek word φωνή (phonē), which translates as sound, voice, as long-wavelength phonons give rise to sound. The concept of phonons was introduced by Russian physicist Igor Tamm.

A phonon is a quantum mechanical description of a special type of vibrational motion, in which a lattice uniformly oscillates at the same frequency. In classical mechanics these are known as normal modes. These normal modes are important because any arbitrary lattice vibration can be considered as a superposition of these elementary vibrations. While normal modes are wave-like phenomena in classical mechanics, they have particle-like properties in the wave–particle duality description of quantum mechanics.>>
Last edited by neufer on Sat Jan 29, 2011 6:48 pm, edited 2 times in total.
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Re: Special Relativity I

Postby neufer » Sat Jan 29, 2011 6:29 pm

Chris Peterson wrote:
keithscientist wrote:
2) Why is a supernova bright? It is the light form of a sonic boom.

A supernova is bright because when a star collapses, gravitational energy is converted to other forms of energy, including light.

A supernova is bright because months after the star itself explodes
the hot plasma of elements from oxygen to nickel & cobalt that formed the opaque
interior of the star suddenly becomes tenuous enough to be radiantly quasi transparent.
http://annourbis.com/myths-legends/2238 ... piter.html wrote:
<<SEMELE, a beautiful princess, the daughter of Cadmus, king of Phoenicia, was greatly beloved by Zeus. Disguising herself as Beroe, Semele's faithful old nurse, Hera persuaded Semele to insist upon Zeus visiting her, as he appeared to Hera, in all his power and glory, well knowing that this would cause her instant death. The next time Zeus came to Semele, she earnestly entreated him to grant the favour she was about to ask. Zeus swore by the Styx to accede to her request whatsoever it might be. Semele, therefore, secure of gaining her petition, begged of Zeus to appear to her in all the glory of his divine power and majesty. Zeus was compelled to comply with her wish; he therefore revealed himself as the mighty lord of the universe, accompanied by thunder and lightning, and she was instantly consumed in the flames.>>

http://en.wikipedia.org/wiki/Type_Ia_supernova wrote:
Image
Characteristic light curve for a Type Ia supernova.
The peak is primarily due to the decay of Nickel (Ni),
while the later stage is powered by Cobalt (Co).

<<Type Ia supernovae have a characteristic light curve, their graph of luminosity as a function of time after the explosion. Near the time of maximum luminosity, the spectrum contains lines of intermediate-mass elements from oxygen to calcium; these are the main constituents of the outer layers of the star. Months after the explosion, when the outer layers have expanded to the point of transparency, the spectrum is dominated by light emitted by material near the core of the star, heavy elements synthesized during the explosion; most prominently isotopes close to the mass of iron (or iron peak elements). The radioactive decay of nickel-56 through cobalt-56 to iron-56 produces high-energy photons which dominate the energy output of the ejecta at intermediate to late times.>>
Last edited by neufer on Sat Jan 29, 2011 6:50 pm, edited 1 time in total.
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Re: Special Relativity I

Postby neufer » Sat Jan 29, 2011 6:43 pm

Image
keithscientist wrote:
3) If an object is approaching an observer at super-light speed
the observer can't observe the super-light speed object until after the object arrives.

If an object passes an observer at super-light speed in a medium
the observer won't observe the object until after it has passed by.
http://en.wikipedia.org/wiki/Pavel_Alek ... _Cherenkov wrote:
Image
PAC man

<<Pavel Alekseyevich Cherenkov (Павел Алексеевич Черенков, 1904–1990) was a Soviet physicist who shared the Nobel Prize in physics in 1958 with Ilya Frank and Igor Tamm for the discovery of Cherenkov radiation made in 1934. He graduated from the Department of Physics and Mathematics of Voronezh State University in 1928, in 1930 he took a post as a senior researcher in the Lebedev Institute of Physics. That same year he married Maria Putintseva, daughter of A.M. Putintsev, a Professor of Russian Literature.

In 1934, while working under S.I. Vavilov, Cherenkov observed the emission of blue light from a bottle of water subjected to radioactive bombardment. This phenomenon, associated with charged atomic particles moving at velocities greater than the speed of light in the local medium, proved to be of great importance in subsequent experimental work in nuclear physics, and for the study of cosmic rays. Eponymously, it was dubbed the Cherenkov effect, as was the Cherenkov detector, which has become a standard piece of equipment in atomic research for observing the existence and velocity of high-speed particles. The device was installed in Sputnik 3. Pavel Cherenkov also shared in the development and construction of electron accelerators and in the investigations of photo-nuclear and photo-meson reactions.>>

http://en.wikipedia.org/wiki/Cherenkov_radiation wrote:
<<When a high-energy cosmic ray interacts with the Earth's atmosphere, it may produce an electron-positron pair with enormous velocities. The Cherenkov radiation from these charged particles is used to determine the source and intensity of the cosmic ray, which is used for example in the Imaging Atmospheric Cherenkov Technique (IACT), by experiments such as VERITAS, H.E.S.S., and MAGIC. Similar methods are used in very large neutrino detectors, such as the Super-Kamiokande, the Sudbury Neutrino Observatory (SNO) and IceCube. In the Pierre Auger Observatory and other similar projects tanks filled with water observe the Cherenkov radiation caused by muons, electrons and positrons of particle showers which are caused by cosmic rays.

Cherenkov radiation can also be used to determine properties of high-energy astronomical objects that emit gamma rays, such as supernova remnants and blazars. This is done by projects such as STACEE, a gamma ray detector in New Mexico.>>
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Re: Special Relativity I

Postby Chris Peterson » Sat Jan 29, 2011 10:51 pm

neufer wrote:A supernova is bright because months after the star itself explodes
the hot plasma of elements from oxygen to nickel & cobalt that formed the opaque
interior of the star suddenly becomes tenuous enough to be radiantly quasi transparent.

I prefer my answer to that one.
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Re: Special Relativity I

Postby neufer » Sat Jan 29, 2011 11:54 pm

Chris Peterson wrote:
neufer wrote:
A supernova is bright because months after the star itself explodes
the hot plasma of elements from oxygen to nickel & cobalt that formed the opaque
interior of the star suddenly becomes tenuous enough to be radiantly quasi transparent.

I prefer my answer to that one.

I'll grant that you may have a better answer for a Type Ia supernova.

But I'll stick with my answer for a Type II supernova where the gravitation energy will be quickly dispersed by neutrinos.

When trapped photons in a red giant are suddenly released in the a matter of a months rather than millions of years the brightness will increase by ~ 107

http://en.wikipedia.org/wiki/Solar_core wrote:
<<The high-energy photons (gamma rays and x-rays) released in fusion reactions take a long time to reach the Sun's surface, slowed down by the indirect path taken, as well as by constant absorption and reemission at lower energies in the solar mantle. Estimates of the "photon travel time" range from as much as 50 million years to as little as 17,000 years. After a final trip through the convective outer layer to the transparent "surface" of the photosphere, the photons escape as visible light. Each gamma ray in the Sun's core is converted into several million visible light photons before escaping into space.>>
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Re: Special Relativity I

Postby keithscientist » Mon Feb 28, 2011 3:50 am

Thanks so much for your excellent answers to my previous query. I feel that I am at last struggling TOWARD the light.  However, c is STILL freaking me out a bit.

Ok here's another flashlight case. Someone with a  flash light APPROACHES u at .9c. U see the, blue shifted, photons moving at speed = c.

They launch a second flashlight moving at .9c relative to their speed in your direction. 

U see the 2nd light, blue shifted, with photons moving at c? I think u don't see the second light, at all, until it reaches u.  

Am I correct here or fullohuey and what is the experimental evidence one way or the other?

Many thanks for your help.  Keith
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Re: Special Relativity I

Postby FlamencoChuck » Wed May 11, 2011 3:58 am

Hi, Michael,

Thanks much for the lecture; however, I find that after much thought SR is no longer "strange" but entirely consistent with not only QM, but also GR. However, the latter consistency requires a slightly different perspective. However, I do NOT believe it is a new theory, but rather a clarification. In particular, I think the Feyman "Time Dilation" diagram (as such) is misleading; it really is an "E-K" diagram, with v and c expressed normally in the space-time domain .....

In your lecture, both BETA and GAMMA are expressed in the space-time domain which I believe is responsible for much of the confusion regarding SR. From my perspective, BETA (that is v = (v/c)c = BETA*c is expressed in the space-time domain with motion along the space axis, and time only existing to define velocity (with moving particles tracing out world lines in space-time), but GAMMA (T' = T*GAMMA = T/sqr(1 - (V^2/C^2)) is expressed in the E-K domain (momentum/energy) (where CT and VT' are orthogonal, but indicate a relation between rest and kinetic energy)

That is, In space-time, the "metric" is BETA, and in E-K, the "metric" is GAMMA (in GR, the "metric" is the metric tensor).

So (e.g.), there is no twin paradox. In space-time, two points are required to define a velocity. The first point obtains when Mary passes stationary Bob on her journey along an agreed length x, and the second point obtains when Mary passs Bob on the return trip, at which point their clocks are synchronized, and they agree on Mary's velocity. The four points (two each for Bob and Mary) define Bob's velocity (0) and Mary's (v).

The second order equations are irrelevant, since they refer to E-K (Mary may have an increase mass over Bob, but it is irrelevant in space-time. However, different velocities (including that of c) will define different clocks. (i.e., Bob is on the vertical (stationary) time axis and Mary is on an angled world line, but they leave and meet at identical times, no matter what the velocity - which can either be v(BETA) in space-time or v(GAMMA) in E-K.

Does this make any sense to you? There is much more to be said, but this perspective resolves all the paradox "koans" of SR, and provides a basis for understanding both GR and Quantum Field theory. (e.g., the "time" in the Klein-Gordon equation is T and not t, referring to energy and not time).

(I have written a short (.pdf) paper with diagrams and equations that is an easy read, and I do derive the basic equations based on standard hypotheses, which I will send to you if you like; I would be honored if you would read it.....) If not, if you know of a work that uses the above approach I would like to be directed to it (I can't imagine it wasn't thoroughly vetted in 1905, but in all my reading, I haven't come across it (yet), and I am now 70..:-)

Best Regards,
Chuck
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Re: Special Relativity I

Postby FlamencoChuck » Wed May 11, 2011 4:06 am

I should add that in my view, v and c have different physical contexts between space-time and E-K. In space-time, v is defined in terms of orthogonal (x,t) with c being just another velocity. However, in E-K, all v's are independent of c, resulting in restmass being interpreted as m0 = pCT, (a "light stick" with density p = 1) and rest energy E = m0c^2. if a common length for c and v are assumed in space-time, then v and c can be equated with V and C; the result is as quadratic relation between elements of space-time and E-K = 1.

(I can't write this stuff out here in text, but the .pdf is really clear -(I think)....
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