Archive for special relativity

Lorentz-Fitzgerald or Fitzgerald-Lorentz?

Posted in Beards, History, The Universe and Stuff with tags , , , , on December 9, 2020 by telescoper

I’ve recently moved on to the part about Special Relativity in my module on Mechanics and Special Relativity and this afternoon I’m going to talk about the Lorentz-Fitzgerald contraction or, as it’s properly called here in Ireland, the Fitzgerald-Lorentz contraction.

The first thing to point out is that the physicists George Francis Fitzgerald and Hendrik Lorentz, though of different nationality (the former Irish, the latter Dutch), both had fine beards:

George Francis Fitzgerald (1851-1901)

Hendrik Lorentz (1853-1928)

One of the interesting things you find if you read about the history of physics just before Albert Einstein introduced his theory of special relativity in 1905 was how many people seemed to be on the verge of getting the idea around about the same time. Fitzgerald and Lorentz were two who were almost there; Poincaré was another. It was as if special relativity was `in the air’ at the time. It did, however, take a special genius like Einstein to crystallize all that thinking into a definite theory.

Special relativity is fun to teach, not least because it throws up interesting yet informative paradoxes (i.e. apparent logical contradictions) arising from  that you can use to start a discussion. They’re not really logical contradictions, of course. They just challenge `common sense’ notions, which is a good thing to do to get people thinking.

Anyway, I thought I’d mention one of my favorite such paradoxes arising from a simple Gedankenerfahrung (thought experiment) here.

Imagine you are in a railway carriage moving along a track at constant speed relative to the track. The carriage is dark, but at the centre of the carriage is a flash bulb. At one end (say the front) of the carriage is a portrait of Lorentz and at the other (say the back) a portrait of Fitzgerald; the pictures are equidistant from the bulb and next to each portrait is a clock.The two clocks are synchronized in the rest frame of the carriage.

At a particular time the flash bulb goes off, illuminating both portraits and both clocks for an instant.

It is an essential postulate of special relativity that the speed of light is the same to observers in any inertial frame, so that an observer at rest in the centre of the carriage sees both portraits illuminated simultaneously as indicated by the adjacent clocks. This is because the symmetry of the situation means that light has to travel the same distance to each portrait and back.

Now suppose we view the action from the point of view of a different inertial observer, at rest by the trackside rather than on the train, who is positioned right next to the centre of the carriage as the flash goes off. The flight flash travels with the same speed in the second observer’s frame, but this observer sees* the back of the carriage moving towards the light signal and the front moving away. The result is therefore that this observer sees the two portraits light up at different times. In this case the portrait of Fitzgerald is lit up before the portrait of Lorentz.

Had the train been going in the opposite direction, Lorentz would have appeared before Fitzgerald. That just shows that whether its Lorentz-Fitzgerald contraction or Fitzgerald-Lorentz contraction is just a matter of your frame of reference…

But that’s not the paradoxical thing. The paradox is although the two portraits appear at different times to the trackside observer, the clocks still appear show the same time….

*You have to use your imagination a bit here, as the train has to be travelling at a decent fraction of the speed of light. It’s certainly not an Irish train.

3 Comments »

The Funeral of Lorentz

Posted in History, The Universe and Stuff with tags , , on December 6, 2019 by telescoper

In a post a couple of days ago I mentioned the Dutch physicist Hendrik Lorentz, whose work helped establish the foundations of the theory of special relativity.

Hendrik Lorentz (1853-1928)

Doing a quick google about Lorentz I came across this remarkable silent footage of his funeral in 1928 in the town of Haarlem in the Netherlands.

from the Wikipedia page of Lorentz:

The funeral took place at Haarlem at noon on Friday, February 10. At the stroke of twelve the State telegraph and telephone services of Holland were suspended for three minutes as a revered tribute to the greatest man the Netherlands has produced in our time. It was attended by many colleagues and distinguished physicists from foreign countries. The President, Sir Ernest Rutherford, represented the Royal Society and made an appreciative oration by the graveside.

The footage of the funeral procession shows a lead carriage followed by ten mourners, followed by a carriage with the coffin, followed in turn by at least four more carriages, passing by a crowd at the Grote Markt, Haarlem from the Zijlstraat to the Smedestraat, and then back again through the Grote Houtstraat towards the Barteljorisstraat, on the way to the “Algemene Begraafplaats” at the Kleverlaan (northern Haarlem cemetery).
Einstein later gave a eulogy at a memorial service at Leiden University.

It was clearly a very grand affair which demonstrates high regard in which Lorentz was held not only by physicists but by the wider public.

 

1 Comment »

The Relativity of Beards

Posted in Beards, History, The Universe and Stuff with tags , , , , on December 4, 2019 by telescoper

In my first-year module on Mechanics and Special Relativity, I’ve just moved on to the part about Special Relativity and this afternoon I’m going to talk about the Lorentz-Fitzgerald contraction or, as it’s properly called here in Ireland, the Fitzgerald-Lorentz contraction.

The first thing to point out is that the physicists George Francis Fitzgerald and Hendrik Lorentz, though of different nationality (the former Irish, the latter Dutch), both had fine beards:

George Francis Fitzgerald (1851-1901)

Hendrik Lorentz (1853-1928)

One of the interesting things you find if you read about the history of physics just before Albert Einstein introduced his theory of special relativity in 1905 was how many people seemed to be on the verge of getting the idea around about the same time. Fitzgerald and Lorentz were two were almost there; Poincaré was another. It was like special relativity was `in the air’ at the time. It did, however, take a special genius like Einstein to crystallize all that thinking into a definite theory.

Special relativity is fun to teach, not least because it throws up interesting yet informative paradoxes (i.e. apparent logical contradictions) arising from  that you can use to start a discussion. They’re not actually paradoxes really logical contradictions, of course. They just challenge `common sense’ notions, which is a good thing to do to get people thinking.

Anyway, I thought I’d mention one of my favorite such paradoxes arising from a simple Gedankenerfahrung (thought experiment) here.

Imagine you are in a railway carriage moving along a track at constant speed relative to the track. The carriage is dark, but at the centre of the carriage is a flash bulb. At one end (say the front) of the carriage is a portrait of Lorentz and at the other (say the back) a portrait of Fitzgerald; the pictures are equidistant from the bulb and next to each portrait is a clock.The two clocks are synchronized in the rest frame of the carriage.

At a particular time the flash bulb goes off, illuminating both portraits and both clocks for an instant.

It is an essential postulate of special relativity that the speed of light is the same to observers in any inertial frame, so that an observer at rest in the centre of the carriage sees both portraits illuminated simultaneously as indicated by the adjacent clocks. This is because the symmetry of the situation means that light has to travel the same distance to each portrait and back.

Now suppose we view the action from the point of view of a different inertial observer, at rest by the trackside rather than on the train, who is positioned right next to the centre of the carriage as the flash goes off. The flight flash travels with the same speed in the second observer’s frame, but this observer sees* the back of the carriage moving towards the light signal and the front moving away. The result is therefore that this observer sees the two portraits light up at different times. In this case the portrait of Fitzgerald is lit up before the portrait of Lorentz.

Had the train been going in the opposite direction, Lorentz would have appeared before Fitzgerald. That just shows that whether its Lorentz-Fitzgerald contraction or Fitzgerald-Lorentz contraction is just a matter of your frame of reference…

But that’s not the paradoxical thing. The paradox is although the two portraits appear at different times to the trackside observer, the clocks still appear show the same time….

 

*You have to use your imagination a bit here, as the train has to be travelling at a decent fraction of the speed of light. It’s certainly not an Irish train.

 

 

 

9 Comments »

R. I. P. Wolfgang Rindler (1924-2019)

Posted in Books, Talks and Reviews, Education, The Universe and Stuff with tags , , , , on March 5, 2019 by telescoper

A recent comment on this blog drew my attention to the sad news of the death, at the age of 94, of Wolfgang Rindler. He passed away almost a month ago, in fact, but I have only just heard. My condolences to his family, friends and colleagues.

Wolfgang Rindler was a physicist who specialized in relativity theory and especially its implications for cosmology. Among other things he is attributed with the first use of the phrase `Event Horizon‘ as well as elucidating the nature of horizons in general relativity, both in the context of black holes and in cosmology. I never met him personally but to me, and I think to many other people, Wolfgang Rindler will be familiar through his textbooks on relativity theory. I have two in my collection:

I bought the one on the right on recommendation when I was an undergraduate over thirty years ago and the other (shorter) one I acquired second-hand some years later. Both are still very widely used in undergraduate courses.
I found the first one then (as I do now) rather idiosyncratic in approach and notation but full of deep insights and extremely effective from a pedagogical point of view. I still recommend it to students, to balance more conventional modern texts which tend to be far more conventional. It’s no easy thing to write textbooks and Wolfgang Rindler deserves high praise for having devoted so much of his time, and considerable talent, into writing ones whose impact has been so widespread and lasted so long.

Rest in peace, Wolfgang Rindler (18th May 1924 – 8th February 2019).

6 Comments »

The Joy of Natural Units

Posted in The Universe and Stuff with tags , , , on March 5, 2010 by telescoper

I’m glad it’s the end of the week. It’s been ridiculously busy. It didn’t help that I was already exhausted before it started, after a hectic three days in Geneva. Part of the reason for being so heavily occupied is that my teaching duties have just doubled. I teach the second half of a module called Nuclear and Particle Physics, and I’ve just taken over  for the second half of the semester to cover the part about particle physics. I started my set of 11 lectures with one about natural units, which is a lot of fun because it usually divides the class into two opposing camps.

About half the students think natural units are crazy, and the other half think they’re great. I’m in the second camp. The motivation is straightforward: particle physics combines quantum theory, which involves Planck’s constant

\hbar \simeq 1.05 \times 10^{-34}\,\,\,{\rm Js}

with special relativity, which involves the speed of light

c\simeq 3 \times 10^{8}\,\,\,{\rm m s}^{-1} .

Using everyday SI units (metres, seconds and kilograms) to deal with quantities that are either ridiculously small or ridiculously large doesn’t make any sense but, more importantly, the SI units don’t really reflect the physics very clearly.

In natural units we take these two constants to be equal to unity, so they don’t appear in any formulae:

\hbar = c =1

For example, the energy invariant in special relativity is usually written

E^2=p^2c^2 + m^2c^4

This is where the most famous equation in physics

E=mc^2

comes from. However, the equivalence between mass and energy (and also momentum) is much more clearly expressed in the natural units system:

E^2=p^2 + m^2

None of those tiresome factors of c^2 to remember! Mass, energy and momentum are all expressed in terms of the same natural unit of energy (usually, in particle physics, the GeV).  You can keep track of which is which by the simple expedient of using different names.

Velocities are, of course, always expressed as a fraction of c in this system so have no units.

In quantum theory we find energy E=\hbar \omega becomes E=\omega so energy is expressed in the same units as frequency. Energy is thus a measure of inverse time.  Momentum p =\hbar k becomes just p= k so momentum is an inverse length.  This is in accord with the various forms of Heisenberg’s Uncertainty Principle too:  \Delta p \Delta x \sim \hbar is \Delta p \Delta x \sim 1 and \Delta E \Delta t \sim \hbar becomes \Delta E \Delta t \sim 1. A particle with a finite lifetime thus has a finite energy width which is inversely proportional to the lifetime. It makes sense to use energy units for both of these things.

As an extra bonus we can dispense with the clumsy way that electromagnetism is handled in the SI system by noting that

\frac{e^2}{4\pi \epsilon_0 \hbar c} \equiv \alpha\simeq \frac{1}{137}

is dimensionless. In the SI system the coulomb force between two electrons is \frac{e^2}{4\pi \epsilon_0 r^2} whereas in natural units it is just \frac{\alpha}{r^2}, which is much nicer. Incidentally, the strange quantity \epsilon_0 that appears in the SI version is called the permittivity of free space. Nice name, but I wonder what it means?

The dimensionless quantity \alpha on the other hand, has a very clear  physical meaning: it is the fine structure constant,  a coupling constant that measures the strength of the electromagnetic interaction.

Some people – including emeritus professors of observational astronomy – object to natural units because they hide the units that things are expressed in. They don’t actually. What they do is express things in units that are better geared to the physics. In any case, if you want to convert back to SI units you can always do so straightforwardly with a little bit of dimensional analysis. This is necessary if you have to talk to engineers and the like, perhaps so they can build you a particle accelerator, but in the more elevated company of particle physicists you should definitely follow proper etiquette and keep your units natural.

22 Comments »