Posts Tagged Einstein

The arrow of time Marcus Wilson Mar 02


Benjamin is now two-and-two-thirds, or near enough. As ever, his grasp of physics continues to improve.  In the last few weeks, he has been picking up the idea of time. 

We have a large (more accurately, LARGE) analogue clock on the wall of our lounge. He's watched me take it off the wall, change the battery and move the hands to a new position when it started to run slow. It's clear to Benjamin that what the hands do on the clock is related to the time of day, although just how I think is some way off.  Over the weekend he wanted me to read him a book, but didn't want me to get it for him. He shoved me away, and rather matter-of-factly said "Daddy stay there. Poppet will get a story." Then he turned to get the book, but quickly came back to me and added "I'll be back at half-past-four. See you soon." And off he went to get his favourite book (which, as ever, is about excavators and other large machines). 

That was just one of those amusing moments you get with a young child. But another 'time' incident was rather more interesting. Karen was out, and Benjamin was somewhat upset over her absence. I was trying to reassure him that she would be back. "She'll come home at about half-past-eight". Benjamin looked at the clock longingly and said "Daddy change the clock so it's time for Mummy to come home". 

Now there's a thought! Wouldn't it be great if we could just manipulate time by turning the hands of the clock? So, somehow, if we changed what the clock says, then the time of day would actually change. Does Benjamin think that this is how it actually is? Maybe. It would be useful if it were true – extended weekends, short work days; one could cut hours of a plane trip to Europe by taking the clock with you. 

While it might work for science fiction writers, unfortunately that's not how time works for us. While the clock and time are intimately linked, it is time that controls the clock, not the other way around. We are stuck with progressing through time at the rate of one second every second. 

That makes time a rather strange concept from a physics perspective. Unlike space, where we are free, more or less, to move to any point in it, we don't have that option with time. We can only move forward in it, and only move forward at the same rate – one second every second. The past is forever behind us; and the future is always unknown. Physicists call this the 'Arrow of Time'. It points one way: forward. 

Special Relativity makes it more interesting still. The way time works for you may not be the same as it works for me. If I were to get on a Really Fast Spaceship and travel close to the speed of light for a while, then return to earth, I would be noticeably younger than my identical twin brother. (I actually DO have an identical twin, by the way.)  Not only would the clock in my spaceship be telling me less time had past, I would have actually aged less. From my point of view, I might have been gone for two weeks; from yours, I might have been gone ten years. But even so, each of us will still have perceived time as travelling at one second every second. Forward. 

What about real time-travel – going back in time. Just maybe physics permits this to happen. That's in the realms of General Relativity and Quantum Gravity and involves some really big masses indeed. Matt Visser's work at Wellington might give us some pointers here. But the summary of it is: Don't expect a such a time-machine to be built in Benjamin's lifetime, even if he does prolong it indefinitely take the battery out of our lounge clock.






Undiscovery in physics Marcus Wilson Nov 28


With the recent undiscovery of Sandy Island I’ve begun wondering what other things might be ripe for undiscovery. Wasps, for example. Wouldn’t it be great if we realized that there wasn’t actually any evidence for the existence of wasps after all. Their discovery had been just a mistake made by an entomologist back in the depths of history. We can all tell our children not to worry about them – they don’t exist. Our chickens would love to see the neighbour’s cat undiscovered (as would we – at least from our garden).  I’m sure a variety of places might feature strongly too. Hamilton is bound to be on the list of some people; but, I can assure you, the last time I looked it was still there.

I don’t think in physics there has been a great deal of undiscovery in the last few centuries. I struggle to think of any real undiscoveries.  Sure, there have been changes to our thinking. For example, relativity superseded Newtonian physics, but it would be wrong to say that Einstein undiscovered Newton’s Laws of motion. The latter are still a cornerstone of physics – but their applicability has been reduced to the realm where things aren’t travelling close to the speed of light. That would be more like discovering the coastline of Sandy Island is a bit different to what the maps have it. One might say that the Michelson-Morley experiment undiscovered the aether, but in reality the aether had never been discovered – it was just a well-accepted hypothesis. Likewise Joule’s experiments with heat put pay to the idea that heat was a fluid, but since no-one had claimed (supported by real evidence)  to have observed this fluid, it wasn’t really an undiscovery either.

Underlying modern science (by which I mean Galileo and beyond) is experimental evidence. No change in understanding of science, in any discipline, is going to happen without well collected and well analyzed data. This makes undiscovery of something (by which I mean overturning of some knowledge, theory or principle that has been believed based upon evidence, as opposed to mere hypothesis) unlikely. There have been a few instances of reports of new things that have been made prematurely, with unreliable evidence, such as cold fusion and faster-than-light neutrinos, and these have been embarrassing for the groups concerned and undiscovered very rapidly.  But undiscovery here has happened because they were never properly discovered in the first place.

That said, neither was Sandy Island properly discovered. My spell-checker’s underlining of the word undiscovery may be for good reason.

I’d love to hear readers’ thoughts on this one. Is there any piece of modern science that has been genuinely undiscovered?





Apparent weight? Apparently not Marcus Wilson Jul 08

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Here’s a thorny problem that no doubt every physics teacher has grappled with. In a space-station, orbiting the earth, are you weightless?  

There are at least two ways of answering this:

1. Yes, you are. Let’s face it, you float around inside the space-station, water forms large blobs, some plants don’t know which way up is, pendulums don’t swing, and you need frightening machines to help you go to the toilet.

2. No, you still have weight, because you still feel the gravitational force of the earth. If you didn’t, there would be nothing keeping you in orbit.  You ‘feel’ weightless, because you and the space-station are accelerating at the same rate towards the centre of the earth (centripetal acceleration).

There is confusion here because we are often very loose with what we mean when we say ‘weight’. We often think of weight as being the force gravity exerts on us, and so we run into problems because when we are in the space-station, the earth’s gravitation field is still there, and so we are forced to conclude that we must still have weight. To alleviate this conundrum, often we talk about ‘apparent’ weight. Since the space-station is accelerating towards the centre of the earth at the same rate as its occupants, the occupants feel as if they are weightless – so they have no ‘apparent’ weight.

 In the article  "Apparent Weight: A Concept that Is Confusing and Unnecessary" in "The Physics Teacher", Albert Bartlett argues that us physicists should get our act together when it comes to talking about weight. If we abandon the concept of apparent weight and the equally confusing "acceleration due to gravity", and stick to a decent definition of weight and use "free-fall acceleration", the confusion should be alleviated.  I’m inclined to agree with him, and also stick my hand up as being guitly of  sometimes doing just what he wants stopped.

If you’re physics-educated, have a read and see what you think (the article is downloadable for free).

If we stick with the definition that an object’s weight (a force) is what a spring-balance would read when the object is placed on it, we should have no problems. In this case the weight might result from gravity, but it could also result from being in a rotating frame (e.g. a rotating spacestation in deep space), or a combination of the two (e.g. being at the equator on the earth), or, bringing in general relativity, in an accelerating lift.  Take your bathroom scales into work and go up and down in the lifts, and note that your weight really does change when the lift accelerates and decelerates.

Thanks to Steve Chrystall for sending this article to me.

Bartlett, A. A. "Apparent Weight": a concept that is confusing and unnecessary. (2010). The Physics Teacher, 48(8), 552.

Physicists on film Marcus Wilson May 17


Last night we went to the movies. We had free passes that needed using by the end of the week, so we turned up at Chartwell early evening without knowing what was showing. Not wishing to see a film about adultery, we decided against ‘Water for Elephants’ (or whatever it’s called) and picked Thor. 

If you haven’t seen this film, I’ve got just one word of advice. Don’t. We paid nothing for it, and it was  worth nearly every cent. It takes a ridiculous scenario, embellished with wooden acting, mixed with overcooked special effects, and held together with a plot so transparent you could stick it in a frame and call it a window. I can only think that Antony Hopkins was paid a huge amount for his appearance in a film that clearly didn’t deserve his presence.  

And then there is the gripe with the physicist characters. Three of the characters are physicists. They are portrayed as sad losers, with no friends, a single-minded focus on astrophysics, driving a van packed with electronics with lots of flashing LEDs and ‘beep’ and ‘ping’ sounds. Seldom do we see on film physicist characters who bear much relation to real physicists. True, it’s a (bad) work of fiction, and their relativity obsession was rather essential to what passed for the plot, but how much does that portrayal influence people’s thinking about what physicists are like.  I wonder. All I can say is real physicists aren’t like that.

But given that rather few people are likely to go and see this film, I guess the impact is hardly going to be significant.

What’s cooking at the LHC? Marcus Wilson Nov 01

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I’ve just been perusing CERN’s Twitter site for some of their latest news.

While the Higgs is still hiding inside some time-travelling baguette, there’s still some really nice physics arising. This example is one that caught my attention – it’s the detection of a bound state made up of a beauty (or bottom) quark and anti-beauty quark.

There’s some similarity here to the hydrogen atom – this has a single, positively charged proton, and a single, negatively charged electron bound together by the electrostatic force. In this new ‘atom’, a beauty quark is bound with its anti-particle opposite – the anti beauty quark. Both particles are charged (the beauty has 1/3 the charge of an electron and is negative; the anti-beauty has the same charge but positive); however, it is the strong nuclear force, rather than the electrostatic force, that holds the two together.

In a hydrogen atom, the electron can exist in several energy states. It can move between these states by absorbing or emitting a photon of the appropriate frequency – this is what spectroscopy is about.  Likewise, this ‘beautiful’ atom can have excited states. This is what the diagram on the right of the report is showing. Along the bottom there are labels S and L, which refer to the spin and angular momentum states of the beautiful atom, and on the upward axis we see the ‘energy’ of the states (quoted as a mass, through Einstein’s E = mc^2). Transitions between the states are indicated.

Plenty for a particle physicist to chew over for a while.

Happy Birthday Marcus Wilson Nov 06

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PhysicsStop is one today!

That means I’m a year older than I was when I wrote the first entry, give or take a few nanoseconds as a result of special and general relativistic effects while on aircraft journeys. Eeek.


Anti-gravity Marcus Wilson Oct 28

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There are some lovely physics demonstrations that get repeatedly wheeled-out for things like Open Day and visits from school groups. Things like holding a spinning bike wheel on a rotating chair (flip it over and you start rotating – conservation of angular momentum) and levitating a piece of superconductor above a magnet at liquid nitrogen temperatures.  But one thing that I’ve yet to get my hands on to demonstrate is Cavorite.

I think it’s an excellent suggestion, but with two minor drawbacks. One, it comes with significant health and safety concerns, and two, it is fictional.

The material Cavorite makes an appearance in H.G. Wells’ novel "The First Men in the Moon". Its basic characteristic is that it is impervious to gravity – that is, if you put a sheet of Cavorite on the floor, anything you put on top of the Cavorite will be weightless.  Now, you can imagine that would be great fun, but it plays havoc with air pressure which is why one needs an appropriate risk management strategy before children can be allowed near it.

In the novel, Dr Cavor uses it to power a space ship to the moon – first of all he uses it to control air pressure to blast his ship off from the earth, then by opening and closing shutters of Cavorite he allows the moon’s gravitational attraction to pull  the ship towards its destination, but shuts off that of the earth.

A fun concept for a novel, but more seriously we can ask whether it is at all plausible that Cavorite could exist. For example, it is possible to shield the electrostatic force - simply putting a sheet of electrically conductive material between two charges will mean that neither can ‘see’ each other (there are issues though – both charges will be attracted to the sheet)  – so why not gravity? Now, in some ways electrostatics is similar to gravity – both obey the inverse square law for example, but in other ways it is not. There are positive and negative charges, but there is not positive and negative mass. Gravity is always attractive. From what we understand from General Relativity, a mass distorts space-time, which will be felt by another mass. To create Cavorite, we have somehow to put a rip through space-time. Can it be done?

It is perhaps just possible that the Higgs Boson can shed some light on this. I don’t think anyone seriously expects its discovery  (if it happens) and subsequent analysis will help us create Cavorite (would we want to?)  but it will probably help us to understand just what mass really is. And that is of great interest to physics. Roll on Large Hadron Collider switch-on.

Gravitational Waves Marcus Wilson Oct 20

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One of my undergraduate students has been researching gravitational waves this year. Last Friday, he gave a nice presentation on the subject.

Gravitational waves are one of the many examples of waves in physics. We are perhaps more used to waves on the surface of water, or waves along a guitar string, or electromagnetic waves (such as radio waves and light), and, in many ways, gravitational waves aren’t much different.

But they are a little strange. Whereas a radio wave travels through space and time, a gravitational wave (caused for example by a supernova)  travels ‘on’ space and time, rather like a water wave (caused by throwing a stone into a pond,) travels on the surface of water. This means that space-time distorts as the wave goes past. When a gravitational wave hits us front on, we will alternately squash in height and expand widthways, before squashing widthways and growing in height (the preferred option for most of us).

These changes in lengths are not some mathematical construction, they are real. At least, they are predicted to be real, but, to date, no-one has actually detected a gravitational wave. The problem is, that unless you are standing next to a supernova, the changes in length due to gravitational waves are very small indeed.  Imagine a rod the same length as the distance from the earth and the sun.  Now imagine it growing in length by about the width of an atom. That is the sort of distortion we are talking about. Not surprising that no-one has built a detector sensitive enough yet.

But that doesn’t stop people trying. Detectors on earth are limited by, for example, seismic vibrations, and the constraints on how large an object you can build. But space doesn’t have those problems. And so there is the LISA concept; three satellites in a large triangle 5 million kilometres apart, following the earth in its orbit around the sun, linked with laser beams. And you thought the Large Hadron Collider was ambitious.

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