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## Expanding spheres of light

In this blogpost here, I derived the Lorentz transformations from first principles. The derivation uses a simple thought experiment; two reference frames $S \text{ and } S^{\prime}$ are moving relative to each other with a speed $v$. The origins of $S \text{ and } S^{\prime}$ coincide spatially at time $t=t^{\prime}=0$. At this moment, a flash of light is created at their origins, and expands as a sphere.

The radius of this sphere of light will be $r \text{ and } r^{\prime}$ in $S \text{ and } S^{\prime}$ respectively. So, we can write

$x^{2} + y^{2} + z^{2} = r^{2} \text{ in } S \text{ (1) }$
and
$x^{\prime}{^2} + y^{\prime}{^2} +z^{\prime}{^2} = r^{\prime}{^2} \text{ in } S^{\prime} \text{ (2) }$

But, in reference frame $S$, the distance $r$ that light travels in time $t$ is related to the speed of light $c$, as $c = r/t$, so for $S$ we can write that

$r = ct$

We can do the same thing for reference frame $S^{\prime}$. The distance $r^{\prime}$ that light travels in time $t^{\prime}$ in $S^{\prime}$ is related to the speed of light $c^{\prime}$ in $S^{\prime}$, $c^{\prime} = r^{\prime} / t^{\prime}$, so for $S^{\prime}$ we can write

$r^{\prime} = c^{\prime} t^{\prime}$.

However, crucially, Einstein said that the speed of light $c$ is the same for all inertial observers. This is the most important principle which underpins special relativity. It means that $c^{\prime} = c$, and so we can write

$r^{\prime} = c t^{\prime}$

which allows us to write Equations (1) and (2) as

$x^{2} + y^{2} + z^{2} -c^{2}t^{2} = 0 \text{ in } S \text{ (3) }$

and

$x^{\prime}{^2} + y^{\prime}{^2} + z^{\prime}{^2} - c^{2}t^{\prime}{^2} = 0 \text{ in } S^{\prime} \text{ (4) }$

Using the Galilean Transformations, which can be written as

we can substitute $for$latex x^{\prime} (=x – vt)$, $y^{\prime}=y$, $z^{\prime}=z$ and $t^{\prime} =t$ in Equ. (4) to give us $(x-vt)^{2} + y^{2} + z^{2} - c^{2}t^{2} = x^{2} - 2vxt +v^{2}t^{2} + y^{2} + z^{2} - c^{2}t^{2}$ which is the same as Equ. (3), except for the two extra terms $-2vxt \text{ and } v^{2}t^{2}$. This shows that Equations (3) and (4) are not equal under a Galilean transformation, even though the two equations should be equal (as both are equal to zero). Let us now show that Equations (3) and (4) are equal using the Lorentz transformations. These can be written as where $\gamma$ is the Lorentz factor, and is defined as $\gamma = \frac{ 1 }{ \sqrt { \left( 1 - v^{2}/c^{2} \right) } }$ We will substitute these expressions for $x^{\prime},y^{\prime},z^{\prime}$ and $t^{\prime}$ into Equ. (4). When we do this, we have $\left( \gamma (x-vt) \right)^{2} + y^{2} + z^{2} - c^{2} \left( \gamma (t - vx/c^{2}) \right)^{2}$ Multiplying this out, and dropping the $y^{2}$ and $z^{2}$ terms, we get $\gamma^{2}(x^{2} -2vxt +v^{2}t^{2}) -c^{2}\gamma^{2}\left(t^{2} - \frac{2vxt}{c^{2}} + \frac{v^{2}x^{2}}{c^{4}}\right)$ $\gamma^{2}x^{2} -2\gamma^{2}vxt + \gamma^{2}v^{2}t^{2} - c^{2}\gamma^{2}t^{2} +2\gamma^{2}vxt - (\gamma^{2}v^{2}x^{2})/c^{2}$ The two terms $2\gamma^{2} vxt$ disappear and, gathering terms in $x^{2}$ and $t^{2}$ together, we can write $\gamma^{2}x^{2} - \left( \frac{ (\gamma^{2}v^{2}) }{ c^{2} } \right) x^{2} + \gamma^{2}v^{2}t^{2} - \gamma^{2}c^{2}t^{2}$ $\gamma^{2}x^{2} (1 - v^{2}/c^{2}) + \gamma^{2}t^{2}(v^{2}-c^{2})$ Remembering that $\gamma^{2} = \frac{ 1 }{ ( 1 - v^{2}/c^{2} ) }$, and changing the sign of the $t^{2}$ term, we can write $\frac{ 1 }{ (1 - v^{2}/c^{2} ) } \cdot (1 - v^{2}/c^{2}) \cdot x^{2} - \frac{ 1 }{ (1 - v^{2}/c^{2} ) } \cdot (c^{2} - v^{2}) \cdot t^{2}$ which is $x^{2} - \frac{ 1 }{ (1 - v^{2}/c^{2} ) } \cdot c^{2}(1 - v^{2}/c^{2}) \cdot t^{2}$ which is $x^{2} - c^{2}t^{2}$, exactly as in Equ. (3). We have therefore shown that Equations (3) and (4) are equal if we use the Lorentz transformations. Advertisements ## Antimatter and Dirac’s Equation Yesterday I introduced Paul Dirac, number 10 in “The Guardian’s” list of the 10 best physicists. I mentioned that his main contributions to physics were (i) predicting antimatter, which he did in 1928, and (ii) producing an equation (now called the Dirac equation) which describes the behaviour of a sub-atomic particle such as an electron travelling at close to the speed of light (a so-called relativistic theory). This equation was also published in 1928. ## The Dirac Equation In 1928 Dirac wrote a paper in which he published what we now call the Dirac Equation. This is a relativistic form of Schrödinger’s wave equation for an electron. The wave equation was published by Erwin Schrödinger two years earlier in 1926, and describes how the quantum state of a physical system changes with time. ### The Schrödinger eqation The various terms in this equation need some explaining. Starting with the terms to the left of the equality, and going from left to right, we have $i$ is the imaginary number, remember $i = \sqrt{-1}$. The next term $\hbar$ is just Planck’s constant divided by two times pi, i.e. $\hbar = h/2\pi$. The next term $\partial/\partial t \text{ } \psi(\vec{r},t)$ is the partial derivative with respect to time of the wave function $\psi(\vec{r},t)$. Now, moving to the right hand side of the equality, we have $m$ which is the mass of the particle, $V$ is its potential energy, $\nabla^{2}$ is the Laplacian. The Laplacian, $\nabla^{2} \psi(\vec{r},t)$ is simply the divergence of the gradient of the wave function, $\nabla \cdot \nabla \psi(\vec{r},t)$. In plain language, what the Schrödinger equation means “total energy equals kinetic energy plus potential energy”, but the terms take unfamiliar forms for reasons explained below. ## The Beatles rooftop concert (January 30th, 1969) Today (January 30th) marks the 50th anniversary of the last time The Beatles played live together, in the infamous “rooftop” concert in 1969. Although they would go on to make one more studio album, Abbey Road in the summer of 1969; due to contractual and legal wranglings the rooftop concert, which was meant to be the conclusion to the movie they were shooting, would not come out until 1970 in the movie Let it Be. It is also true to say that some of the songs on Abbey Road were performed “live” in the studio with very little overdubbing (as opposed to separate instrument parts being recorded separately as was done on e.g. Sgt. Pepper). But, the rooftop concert was the last time the greatest band in history were seen playing together, and has gone down in infamy. It has been copied by many, including the Irish band U2 who did a similar thing to record the video for their single “Where the Streets Have no Name” in 1987 in Los Angeles. The Beatles were trying to think of a way to finish the movie that they had been shooting throughout January of 1969. They had discussed doing a live performance in all kinds of places; including on a boat, in the Roundhouse in London, and even in an amphitheatre in Greece. Finally, a few days before January 30th 1969, the idea of playing on the roof of their central-London offices was discussed. Whilst Paul and Ringo were in favour of this idea, and John was neutral, George was against it. The decision to go ahead with playing on the roof was not made until the actual day. They took their equipment up onto the roof of their London offices at 3, Saville Row, and just start playing. No announcement was made, only The Beatles and their inner circle knew about the impromptu concert. The concert consisted of the following songs : 1. “Get Back” (take one) 2. “Get Back” (take two) 3. “Don’t Let Me Down” (take one) 4. “I’ve Got a Feeling” (take one) 5. “One After 909” 6. “Dig a Pony” 7. “I’ve Got a Feeling” (take two) 8. “Don’t Let Me Down” (take two) 9. “Get Back” (take three) People in the streets below initially had no idea what the music (“noise”) coming from the top of the building was, but of course younger people knew the building was the Beatles’ offices. However, they would not have recognised any of the songs, as these were not to come out for many more months. After the third song “Don’t Let Me Down”, the Police were called and came to shut the concert down. The band managed nine songs (five different songs, with three takes of “Get Back”, two takes of “Don’t Let Me Down”, and two takes of “I’ve Got a Feeling”) before the Police stopped them. Ringo Starr later said that he wanted to be dragged away from his drums by the Police, but no such dramatic ending happened. At the end of the set John said I’d like to thank you on behalf of the group and ourselves, and I hope we’ve passed the audition. You can read more about the rooftop concert here. Here is a YouTube video of “Get Back” (which may get taken down at any moment) and here is a video on the Daily Motion website of the whole rooftop concert (again, it may get taken down at any moment). Enjoy watching the greatest band ever perform live for the very last time! ## First ever asteroid from another solar system detected In late October 2017, astronomers announced the first ever discovery of an asteroid (or comet?) coming into our Solar System from another stellar system. The object was first spotted on 19 October by the University of Hawaii’s Pan-STARRS telescope, during its nightly search for near-earth objects. Based on its extreme orbit and its rapid speed, it was soon determined that the object has come into our Solar System from somewhere else, and this makes it the first ever asteroid/comet with an extra-solar origin to have been discovered. Originally given the designation A/2017 U1, the International Astronomical Union (IAU) have now renamed it 1I/2017 U1, with the I standing for “interstellar”. The object, given the designation A/2017 U1, was deemed to be extra-solar in origin from an analysis of its motion. In addition to its strange trajectory, observations suggest that the object also has quite an unusual shape. It is very elongated, being ten times longer than it is wide. It is thought to be at least 400 metres long but only about 40 metres wide. This was determined by the rapid and dramatic changes in its brightness, which can only be explained by an elongated object tumbling rapidly. The object has also been given the name Oumuamua (pronounced oh MOO-uh MOO-uh), although this is not its official name (yet). This means “a messenger from afar arriving first” in Hawaiian. In other respects, it seems to be very much like asteroids found in our own Solar System, and is the confirmation of what astronomers have long suspected, that small objects which formed around other stars can end up wandering through space, not attached to any particular stellar system. To read more about this fascinating object, follow this link. ## The Only Living Boy in New York – Simon & Garfunkel (song) A couple of weeks ago I was in New York City. Although I have been to NYC many times since I first visited it in October 1985, this was the first time I had spent the night in Manhattan. On the evening of Saturday 17th November I booked a ticket to go up the Empire State Building to take night-time photos. The cost of a ticket to the 80th floor is US$32, and I’d say that it’s good value. But, what is not is the extra \$20 to go up to the 102nd floor. I had forgotten from my last time that this is not worth the extra money. Hopefully I’ll remember next time.

I was on the observation deck of the Empire State (86th floor) for about 2 hours taking photos and videos. I’ll post some of them over the next few weeks. Then, at about 11:30pm I walked to Time Square. I stopped to get a coffee and warm up a bit so got to Time Square at about 12:30am. The place was heaving, hundreds of people were milling around and many of the shops were open.

At about 1:30am I caught the subway to Brooklyn as I wanted to get a photo of Manhattan with the Brooklyn Bridge in the foreground. I took my photos from the Brooklyn Bridge Park, then walked back to Manhattan over the Brooklyn Bridge.

By this time it was lashing it down with rain and I was soaked. The rain had seeped through my winter coat and my sweater and trousers were pretty much wet through. But, as I walked over the bridge I caught sight of the Statue of Liberty illuminated (it was about 3am by this time). So I decided to walk to Battery Park to take photos of her at night.

When I got to Battery Park and set up my camera I discovered that my camera battery was dead from all the long exposure photos I’d been taking. So I got my spare battery out of my backpack, only to find that it too was dead. So, I didn’t get any photos of Lady Liberty at night. It was now 4am.

I then walked back to my hotel which was in the Little Italy part of Manhattan. The rain was still lashing it down, and by now my phone was getting damp leading to Google maps misbehaving. The app kept on going haywire every minute or two, so I couldn’t use it to guide me back from Battery Park to Little Italy. Instead I just tried to figure it out, and it took me two hours!

I collapsed into my bed at 6am, having spent nearly 12 hours wandering around nighttime Manhattan taking photos.

When I was walking across the Brooklyn Bridge at about 3am this great Simon & Garfunkel song kept playing in my head.

## The Only Living Boy in New York

This song appears on Simon & Garfunkel’s last album Bridge Over Troubled Water. Written, of course, by Paul Simon, the “Tom” in the lyrics refers to Art Garfunkel. When they were teenagers in Queens they released a single and briefly called themselves”Tom & Jerry”.

By 1970 Simon and Garfunkel were arguing and about to go their separate ways. Garfunkel decided to have a go at acting, he appears in the movie Catch 22. Simon is wishing him the best for his part in the movie Garfunkel is filming in Mexico.

As is usual with Paul Simon, the song’s lyrics are exquisite.

Tom, get your plane right on time
I know your part’ll go fine
Fly down to Mexico
Da-n-da-da-n-da-n-da-da a
And here I am
The only living boy in New York

I get the news I need on the weather report
Oh, I can gather all the news I need on the weather report
Hey, I’ve got nothing to do today but smile
Da-n-do-da-n-do-da-n-do
Here I am
The only living boy in New York

Half of the time we’re gone but we don’t know where
And we don’t know where
Here I am
Half of the time we’re gone, but we don’t know where
And we don’t know where

Tom, get your plane right on time
I know that you’ve been eager to fly now
Hey, let your honesty shine, shine, shine
Like it shines on me
The only living boy in New York
The only living boy in New York

Here is a video of this beautiful song. Enjoy!

## The origin of the elements

A couple of weeks ago this fascinating version of the periodic table of the elements was the NASA Astronomy Picture of the Day (APOD). Most people have seen the periodic table of the elements, it is shown on the wall of most high school chemistry classrooms. But, what is totally fascinating to me about this version is it shows the origin of each element.

It has been a long process of several decades to understand the origin of the elements. In fact, we have not totally finished understanding the processes yet. But, we do know the story for most elements. All the hydrogen in the Universe was formed in the big bang. This is true for nearly all the helium too. A small amount of the 25% or so of helium in the Universe has been created within stars through the conversion of hydrogen into helium. But, not much has been created this way because most of that helium is further converted to carbon.

The only other element to be formed in the big bang is lithium. About 20% of the lithium in the Universe was formed in the big bang, the rest has been formed since,

Together, hydrogen and helium comprise 99% of the elements in the Universe by number (not by mass).

Where Your Elements Came From – from the NASA Astronomy Picture Of the Day (APOD) 24 October 2017.

I have decided to use this fascinating table as the basis for a series of blogs over the next few weeks to explain each of the 6 processes in these six boxes

## Forever Young – Bob Dylan (song)

The Bob Dylan song about which I am going to blog about today is his 1973 song “Forever Young”, another one of his songs included on his Vevo channel. It appears on his 1974 album Planet Waves in two different versions, a slow and a fast version. It is the slow version which is in the video included here. As an aside, Planet Waves is the only studio album which Dylan released through Asylum Records. Apart from the live album Before The Flood which is his next album after Planet Waves, all his other albums have been with Columbia Records.

“Forever Young” was recorded by Dylan in November 1973. The slow version runs for 4m57s and is the 6th track on Planet Waves, the last track on the first side of the record. The fast version (which is a shorter track at 2m49s) is the 7th track on the album, the first track on the second side of the record. Dylan first performed “Forever Young” live in January 1974 and his most recent live performance of it was in November 2011. He has performed it live a remarkable 493 times as of my writing this.

“Forever Young” was recorded by Bob Dylan in November 1973 and appears on his 1974 album Planet Waves. There are two version of the song on the album, a slow version and a fast version.

You may be familiar with a 1988 Rod Stewart song by the same name. Confusingly, it is not a cover version in the traditional sense, but bears such a remarkable similarity to Dylan’s song in both melody and some of the lyrics that Stewart agreed to share his royalties with Dylan (presumably to avoid a lawsuit).

The inspiration for the song was Dylan’s eldest son Jesse who was born in 1966. Dylan wrote “Forever Young” as a lullaby to his young son, and over the years it has been covered by many artists.

May God bless and keep you always
May your wishes all come true
May you always do for others
And let others do for you
May you build a ladder to the stars
And climb on every rung
May you stay forever young
Forever young, forever young
May you stay forever young

May you grow up to be righteous
May you grow up to be true
May you always know the truth
And see the lights surrounding you
May you always be courageous
Stand upright and be strong
May you stay forever young
Forever young, forever young
May you stay forever young

May your hands always be busy
May your feet always be swift
May you have a strong foundation
When the winds of changes shift
May your heart always be joyful
May your song always be sung
May you stay forever young
Forever young, forever young
May you stay forever young

Here is the official Vevo video of this great song. Enjoy!