The concept of time feels so natural that we often forget just how peculiar it is. Time governs all our everyday lives. We keep track of it, waste it and wish we had more of it. It seems to go fast when we’re having fun, and drag when we’re not. But figuring out what exactly ‘time’ is remains one of the most difficult problems in physics.

One of the biggest misconceptions is the notion of ‘now’. What we perceive as happening in the present is actually based on things that have happened in the past. This is because light does not travel instantaneously but takes time to reach us.

The Sun is about 93 million miles away from Earth (that’s about 150 thousand million metres meaning that even though light travels at just under 300 million metres per second through space it still takes light from the Sun just over 8 minutes to reach us. That means we actually see the Sun how it was 8 minutes ago. Looking into the night sky we see the stars how they were millions or billions of years ago, as they are so far away the light has only just reached us. Even reading these words from your computer screen there is a delay of about 3 nanoseconds – so small you never notice it, but the fact is this intake of information is not instantaneous.


Everyday life is defined by hours, minutes and seconds. Scientific experiments regularly use much smaller intervals like nanoseconds, with some precise time measurements now on the attosecond (10-18) scale. This is still a relatively large division of time – the smallest unit of  time in current theoretical physics is thePlanck time of 10-43 seconds. It’s almost impossible   to imagine how small the Planck length is (although try this). To put in in some kind of context, imagine a ‘grain’ of time being blown up to be the size of a grain of sand. If we also blew up real grain of sand in the same proportion they would  end up being 10000 trillion times the size of the Milky Way.

Einstein fundamentally altered the way we think about time. His theory was that time is a dimension just like the three spatial ones, and they are intertwined into a four-dimensional ‘spacetime’ that forms the fabric of the universe. Moving through  the time dimension is what gives us the feeling of time passing.

One of the well known laws of physics is that you can’t travel at, or faster than, the speed of light. More accurately, you can’t travel through space at the speed of light. This restriction doesn’t apply to that of time. Because space and time are together as 4Dspacetime, the faster you travel through space, the slower you travel through time. This means that if you’re stationary in space, you’re travelling through time at the speed of light.

The nature of time is even stranger. It’s not just about how fast you’re moving, but also what you’re close to that influences it. This is because massive objects such as stars and planets – in fact anything with a gravitational field – warps the spacetime around it.


Because the Earth’s gravity warps spacetime, and this effect diminishes further from the planet’s centre of mass, each day our heads age around 10-11 seconds more than our feet. Live until 100 and that adds up to around 365 nanoseconds. This effect has been most notable in space missions – the Russian cosmonaut Sergei Krikalev has spent 803 days on space stations, meaning he’s around 21 milliseconds younger than he would have been had he stayed on Earth.

In the 1960s Irwin Shapiro  conducted experiments that showed the Sun’s warping of spacetime causes a signal sent between Earth and Mercury to  travel at a different time to  that expected without this effect This validation of Einstein’s theory is called the Shapiro Time Delay. On earth the effect is miniscule, but out in the universe the vast mass of the stars and galaxies warp spacetime so much that time ticks all over the place.

There are even things that can warp spacetime so much they appear to not just slow down time, but stop it altogether. Black holes are phenomena that warpspacetime so much that they appear to stop time. Observing someone’s watch as they moved towards a black hole it would be seen to tick slower and slower, until when they reach its event horizon time would appear to stop completely.

Our intuition about time is that we live in the present. The past has happened and no longer exists, and the future is yet to happen. But if time is simply a dimension like those of space, as Einstein theorised, then we can come up with a completely counter-intuitive idea: that time and all events have always existed and always will. The past, the present, and the future all exist simultaneously.

Think of this as like your journey from home to work. As you travel away from your home, it still exists in space, and similarly past events still exist in time. Your work place already exists and you are simply travelling through space to it. Similarly future events already exist and you’re travelling through time to when they occur. Travelling through time is no different to travelling through space.

This idea that the future is as real as the past is poignant but doesn’t seem quite right. That’s because it’s a classical viewpoint and doesn’t take into account quantum mechanics. In the QM world nothing is certain and the future least of all. Teaming Einstein’s ideas with QM should give us a better grasp on the true nature of time.

An alternative theory is that time is not the smooth structure proposed by Einstein, but rather that it is granular. Intervals of time are like grains of sand, with the passing of time able to be thought of as like sand flowing through an hour glass.

If spacetime is grainy then it could grow grain by grain, event by event. This is not possible in Einstein’s vision, where spacetime is a continuum with all events already and always in existence. This theory is a bit strange, but does take into account the uncertainty of quantum mechanics which we know defines how the universe behaves at its smallest scale.

Time is a fundamental concept, governing not only our everyday lives but the entire universe. It’s remarkable that we still know so little about it. Will we ever understand the true nature of time? That’s uncertain. Or perhaps it isn’t.

What is nothing? It’s a very difficult question to answer, because wherever we look around us, there always seems to be something there. Even just trying to imagine absolute nothingness is tricky.

Let’s think of a box:


Now let’s remove everything we possibly can from inside of it – all the air, molecules, particles, every single atom – until there are no things left in it. Now what still exists inside the box? It is really nothing?

This is an important question because such emptiness makes up most of the universe; the atoms that make up everything, including us, are mostly empty space. Studying this nothingness has revealed nature’s deepest secrets and helped explain why we exist. This is because our best theories say that the entire universe appeared nearly 14 billion years ago out of nothing.

For over a thousand years, our understanding of nothingness was though the ideas of the Greek philosopher Aristotle who believed that nature would always oppose the existence of true nothingness. This all changed in the 17th Century due to work by Torricelli who created the first sustained vacuum and Pascal who furthered this work. These experiments revealed a profound truth – that nothing is everywhere. Our Earth is just a tiny point floating through in an enormous void.


Newton’s balls. Action and reaction. Cause and effect. This is how our everyday classical world behaves – sensible, predictable, and understandable. But the microscopic quantum world is very different. It’s strange and based on uncertainty. You can never be sure of what’s going to happen, not because the experiments and measurements aren’t good enough, but simply because of the fundamental uncertainty.

Heisenberg’s uncertainty principle (HUP) states that there is a fundamental limit to the precision with which two physical properties of a particle can be simultaneously known. This means the more precisely we know where a particle is, the less we know about its movement. Unfortunately there’s no way around this, it’s an inescapable feature of reality at this scale.

So what has this quantum weirdness got to do with the concept of nothingness? Well, the HUP can take a different form, in terms of energy and time. Let’s think again about that ‘empty’ box:


If we examined a small volume of the space inside the box, we could in principle know very precisely how much energy it contains. But, if we could slow down time and look carefully at a very short interval, things start to get strange. According to HUP, because we’re looking at a small interval of time we’ve lost the precise information about the exact energy in that part of the box.

If we could examine an even smaller volume of space inside the box over an even smaller interval of time, then HUP suggests something weird can happen. How much energy there is in that part of the box will be so uncertain that there is a chance it could contain enough energy to create particles literally out of nowhere.

The uncertainty principle suggests that in extremely tiny amounts of space and time, something could come from nothing. But how?

The void, contrary to what we would intuitively expect, is teeming with what physicists call quantum fluctuations, little packets of energy that appear and disappear very quickly. This is perfectly allowed by the laws of physics, with the HUP telling us it is possible to borrow energy from nothing as long as it’s paid back quickly enough. This concept, strange though it seems, is fundamental to our universe. This theory of quantum mechanics explains physical phenomena at the microscopic scale, and is the most accurate and powerful description we have of our universe.

But there is a much more dramatic way idea that we can see the effects of these quantum fluctuations than envisaging an empty box. The ‘Big Bang,’ our best theory of how the Universe began, says it appeared from nothingness and expanded very rapidly. So the rules of the quantum world contributed to the large scale structure we see today. Tiny quantum fluctuations suddenly expanded, and continued to grow into stars, galaxies, and everything else in the universe. The strange truth about reality is the profound connection between our infinite universe and the nothingness from which it originated – nothing really has led to everything.

“Take the blue pill and the story ends, you wake up and believe whatever you want to believe. Or take the red pill, stay in wonderland, and see how deep the rabbit hole goes” [Morpheus, The Matrix (1999)].

In the film The Matrix humanity has become confined to an existence within a highly detailed artificial reality. Over a decade later, the idea that we could be living in a computer simulation continues to intrigue us. Several years ago Oxford University professor Nick Bostrom postulated the Simulation Argument – now physicists at the University of Bonn in Germany say it may be possible for us to actually test its validity.

The simulation argument, or hypothesis, put simply is the idea that our reality is a simulation and that we are unaware of this. This idea frequently features in some way in science fiction, most notably in Star Trek and in films such as The Truman Show and Inception. The concept can be thought of as similar to the holodeck in Star Trek, where the crew on board the spaceship can simulate an artificial reality that they can then play out scenarios within.


According to the physicists, if the universe is a simulation then it will have certain constraints, and we can try find out if they are there. The team’s work ‘Constraints on the Universe as a Numerical Simulation’, essentially involves studying a computer simulation of our universe, investigating the fundamental properties of its artificial construction and how that knowledge can be used towards the further understanding of our own reality.

The reasoning is that if the universe is just a mathematical simulation then there should be clues around us, glitches in the system, which could reveal its true nature. The main problem with any simulation is that ultimately it is not smooth, not totally perfect. In order to model physical phenomena, including the laws of physics, the world has to be represented by numerous separate points in space and time. Even though the separation distance can be incredibly small, a three-dimensional grid structure always exists. So in a computer-simulated world there would be limits on the energy particles can have, because nothing can exist that is smaller than the spacing between these lattice points of the ‘reality’ framework.

A limit like this does actually exist in our universe – high energy particles are subject to the well studied Greisen-Zatsepin-Kuzmin (GZK) cut-off condition. This limits the energy that cosmic rays can have, and results from particles losing energy due to interactions with the cosmic microwave background as they travel long distances across the universe.

The researchers created a detailed computer simulation of our universe, which models quantum chromodynamics (QCD). This theory describes how the strong nuclear force binds together quarks and gluons into protons and neutrons, which then form nuclei that interact. It describes the universe at a fundamental level, so it is believed that the QCD simulation is equivalent to simulating accurately the workings of our universe. In the simulation spacetime, the fabric of the universe, is replicated by tiny cubic lattices in a process known as lattice gauge theory. At the moment the simulation only models nuclei and their interactions, but the researchers believe it could eventually be extended to include modelling of larger things like molecules and potentially even humans. The most interesting outcome is that the simulation is effectively indistinguishable from the real thing – at least on our level of understanding.

These QCD processes are complex, and so take a lot of computing power to model. Even using the world’s most powerful supercomputers, physicists have still only managed to simulate tiny regions of space (on the femtometre scale, that is, just a few quadrillionths of a metre).  It is likely that eventually, with greater computing power, physicists will be able to simulate larger regions of space. Even if they managed just a few micrometers, that would allow for the modelling of biological cells.

There’s also something else to look for. These cosmic rays would have a tendency to travel along the gridlines of the lattice, so they wouldn’t show up equally in all directions. This is something that can be checked using current technology, and the researchers point out that seeing this effect would be the same as seeing the lattice on which our universe is simulated.

However there are some limitations, and not finding evidence wouldn’t necessary rule out the theory of a computer-simulated world. The experimental research can only identify a certain type of simulation, which may well be different to the one we are searching for. The computer simulation, if it does exist, could be based on technology far superior to our understanding.  The cosmic rays’ theoretically preferred direction of travel would only be apparent if the lattice spacing is the same as the GZK cut-off. This occurs when the lattice spacing is about 10-12 femtometres – if it is smaller than that, there is no way of us finding it. Even if these conditions were found, it would be difficult to definitively say this is due to the universe being a simulation. It would be much more likely that the result arose from inaccuracies in the model. We may be able to create a highly accurate model of our own universe, including simulating ourselves, but still be unable to determine if we are likewise living in a simulated world.

In 2009 it was reported that the GEO600 gravitational wave detector had potentially given us a real insight into the theory of a simulated reality. Unexpected noise affecting the experiment was attributed to the fundamental limit of spacetime – the point where spacetime stops behaving as a smooth continuum and instead exists in a granular state. Theoretical physicists have long believed that the fabric of spacetime is grainy, similarly to pixels making up an image, and would have waviness due to energy fluctuations described by Heisenberg’s quantum-mechanical uncertainty principle.

The size of this granulation is billionths of billionths of the size of protons, defined by a distance known as the Planck length, which is 10-35 metres. This scale is far beyond the reach of any experiment, so finding evidence of it was unexpected. However, new investigative work on the noise has shown that it actually needs to be much smaller than that found by GEO600 to be confidently attributed to an artificial construct. Ongoing and future research work on detecting gravitational waves has the potential to also find evidence of an artificial reality, a far more fundamental property of nature.

Digital physics, a collection of theoretical perspectives postulating that that the world can be explained and therefore modelled as a computer program, offers an alternative approach to finding out the nature of reality. One aspect of this is a concept by John Wheeler called “it for bit” that states “everything in the universe derives its existence entirely from the outcome of yes/no questions, binary choices, digital bits”. This idea that the entire universe is effectively just a digital computer was actually first proposed by computer scientist Konrad Zuse in his 1969 Calculating Space. This concept has gained some following in physics, with recent work showing that the mathematical equations we use to describe the laws of physics may contain elements analogous to computer code.

Whether our universe is a computer simulation or not, the reality is that we are not aware of the majority of the world around us. We can only see a small proportion of light, called the visible spectrum, and are mostly unaware of both the higher and lower frequency electromagnetic radiation that’s all around us. Similarly, we can only hear a small range of audio frequencies. We still have to explore the depths of the oceans, and are yet to venture far into space. Perhaps most poignant, we live on a tiny planet in an incomprehensively vast universe of which we only understand 4% despite all our scientific and technological advances. It seems we need to first find the rabbit hole before we can jump in and see how deep it goes.

The idea of invisibility cloaking – being able to hide things from view – has been a point of interest to humans for thousands of years. Plato wrote in The Republic about The Ring of Gyges which allowed the wearer to become invisible. Within nature there are many different species of creatures that have evolved survival skills focused on hiding themselves from both their predators and prey. Although man-made camouflage techniques and stealth technology have been around for many years now, mainly developed and used by military forces worldwide, neither of these concepts provides true invisibility in the scientific sense of the term.

The concept of an invisibility cloak has come to prominence from its use throughout science fiction, most notably in the Star Trek TV series and more recently in the books and films of the highly popular Harry Potter franchise, but only in the last few years has scientific understanding and technological developments allowed us to finally realise this idea.

In the simplest sense, we see things around us because light reflects off them into our eyes, with our brains then using these signals to form images. This means it is easy to hide objects, for example consider the ‘cup and balls’ trick or the ‘smoke and mirrors’ analogy. However it is also important to note that all objects give off electromagnetic radiation, and whilst they may be hidden from our sight, they can still be ‘seen’ by using devices such as infrared detectors. Therefore from a scientific perspective the concept of an optical cloak is a device that reflects no light and absorbs no energy, giving the impression that the region of space it occupies is empty.

The properties and behaviour of electromagnetic waves are described by Maxwell’s equations. The theory of invisibility cloaking lies in utilising this set of equations within a field of physics called transformation optics. Metamaterials, artificially structured materials engineered to have particular properties, are used to control the propagation of electromagnetic waves.

Several different types of optical cloaking devices have been theorised and designed; the most well-known is the cylindrical/spherical design.


Through some relatively simple mathematics is it possible to formulate a coordinate transformation; basically creating a two-part region of space that can be used to conceal an object by acting as a single uniform region. This approach is important because it means any arbitrary object can be hidden without having to study the material properties.

This type of optical cloaking device works by bending the incoming electromagnetic waves around a central region that contains the object to be hidden. The idea is that the waves enter and exit the device on the same trajectory, meaning a device would be unable to detect whether the waves had been bent around an object or just travelled through a region of empty space – thereby creating an invisibility cloak that has hidden the object.

Physicists have now managed to successfully demonstrate object cloaking using a cylindrical device, albeit under very specific conditions and parameters. We can see how usually light hitting an object (in this case a cylinder) is scattered away in various directions and the object is easily detectable.


The cloaking device bends the light around the object so it then moves away in the same direction as before the interaction. This means a detector would perceive that the light propagation was unaffected so must have moved through a region of free space, i.e. the presence and effects of the object have been cloaked from detection.


Invisibility cloaks have now been demonstrated, finally turning ideas into reality. However, whilst the theory has been validated and experimentally shown, problems still persist mainly with perfecting the art of bending the light rays and making devices that can operate over a wide range of wavelengths although these issues continue to be worked on by numerous research groups. Until an invisibility cloaking device is created that operates for visible light, which can hide an object from right in front of our eyes, this new science remains quite a niche area of physics. There is continued rapidly growing interest and in the last few years hundreds of scientific papers have been published detailing the theoretical and experimental research of physicists in this field worldwide; so it’s surely only a matter of time before a true invisibility cloak, once residing only in science fiction, finally becomes part of everyday reality.

Scientists at the University of St Andrews have developed a tractor beam, a device which uses light to attract microscopic particles. Their work was published in Nature Photonics. Tractor beams feature prominently in science fiction, notably in the Star Trek and Star Wars franchises, where they are used to move large objects such as spacecraft.


Now this science fiction idea has become reality – albeit on a much smaller scale.

When a beam of light hits microscopic objects, they are usually forced along in the direction of the beam by the momentum of photons, the little packets of energy that make up light. This new technique reverses that force, meaning a light beam hitting microscopic objects a few hundred nanometres in size such as individual molecules can attract them towards it. The tractor beam can be seen in action here.

Whilst the technique is new, it has great future potential. The tractor beam is selective in the properties of the particles it acts upon, so could be used to pick up and move specific particles in a mixture for example those of a particular size. NASA has studied how the technique might help with manipulating samples whilst on space explorations.

The downside of the technique is that there is a significant transfer of energy. On a microscopic scale that doesn’t matter so much, but on an everyday scale this would cause huge problems. Increasing the power of the laser, as would be required to provide enough energy to move larger objects, would make it  so powerful it would just destroy the object. Because of this, making a real-life tractor beam that can manoeuvre a spacecraft wouldn’t be possible.

Despite this, it’s still a huge scientific step forward in using light in technological applications. The technique could be used for intricate engineering or in medical testing, such as to analyse blood samples.

Time Cloak Edits History

Posted: February 1, 2013 in Science
Tags: , , ,

Time cloaks not only sound cool, but offer exciting new technological possibilities.

At just under 300 million meters per second, the speed of light in a vacuum is one of the most important numerical constants of physics. In everyday life however, the speed of light is actually less than this and depends on what it’s travelling through, whether that’s air, water, glass, or something else. The speed of light is therefore defined by a thing called the refractive index, a unitless number that describes how electromagnetic radiation (light) propagates through a medium. Different media have different refractive indices, for example air is n=1 and water n=1.33. A larger refractive index means the light moves slower and experiences more refraction (a change in direction) – this is why if you put an object such as a spoon or straw in a cup of water it will appear to bend at the boundary between the air and the water, and why a glass prism splits white light into a rainbow.

Different colours of light travel at different speeds depending on their wavelength/frequency, a phenomenon of optical physics called dispersion. Dispersive media, materials in which this occurs, can therefore be used to control the movement of light.

Physicists have now extended the theory of spatial cloaks and started researching temporal cloaks, devices that control electromagnetic waves not just in space but also in time. Researchers at Cornell University recently demonstrated a working device dubbed a ‘time lens’ that was able to hide an event from detection, building on theoretical work by a team led by Professor McCall at Imperial College London.

Consider a simple scientific experiment, where a laser beam is projected in a straight line to a detector. If anything is placed in the path, thereby breaking the continuous beam, the detector will easily notice this has happened. This process is called an ‘event’.

The researchers developed a device that splits a beam of light into a spectrum of wavelengths and then speeds up and slows down different parts. The first optical fibre increases the frequency of the laser beam, towards the blue end of the spectrum, and then decreases it to the opposite red end. The higher frequency bluer part travels faster than the lower frequency redder part, creating a gap in the beam. The beam then moves, with the gap maintained, out and along to the next fibre. The second fibre slows down the first part of the beam, allowing the second part to catch up and close the gap. The laser beam reaches the detector exactly as it left the emitter, with the manipulation of the beam undetectable.

This setup allows for an object to move through the laser beam, without the detector registering that has happened. So the temporal devices have been able to cloak an event from detection. Unfortunately the device was only able to achieve a gap of 50 picoseconds. That’s 50 trillionths of a second (0.000000000050 seconds). It should be possible to increase that time slightly, perhaps up to a few nanoseconds (billionths of a second), but scattering and dispersion effects will make this a big challenge. Considering the current setup, to hide 1 second of time from detection would require a device with emitter and detector about the same distance apart as the Earth from the Sun. Also, the device needs to be developed into 3D to allow cloaking from all directions; trickier to do than the current 2D experiment.

The practical demonstration of concealing events from detection has profound implications for future science and technology. Using time lenses, data can be manipulated and then restored, so the applications of temporal cloaking go beyond simply hiding single events; the process could be used to allow the insertion of something into a continuous beam without causing disruption. This phenomenon could be incorporated into optical devices, with multiple interlaced – but non-interacting – beams allowing much faster data processing and the development of quantum computing. Whilst time cloaks are still very much in their infancy, they promise to have significant involvement in future and allow us to have a better understanding of the spacetime around us.

Want a hoverboard like Marty in Back To The Future? Well soon you might be able to. Recently a group of physicists at Université Paris Diderot showed off the “Mag Surf” – their experimental demonstration of superconductor magnetic levitation.


Supercold liquid nitrogen enables a superconductor on the bottom of the board to repel the magnetic field of a metal rail underneath it, fixing the board at a small distance above and creating an invisible guide for it to move along.

Pretty cool physics… but what’s going on?

Electricity is defined as the flow of electrons. As these electrons move through a material, they encounter resistance due to collisions with the atoms in the material. At higher temperatures these atoms have more energy so vibrate around more, causing a greater resistance due to an increased amount of electron-atom interactions. In a cooler material, the atoms vibrate around much less, so the electrons can travel through more freely.

This electrical resistance is the reason we can’t achieve perpetual motion; why the energy that comes out of a system is always less than the energy put in. The energy lost by electrons due to their impeded travel causes electrical devices to heat up, reducing their efficiency.

So what happens  to a material at absolute zero, that is, at -273°C. Here, the atoms in the material are stationary, so the electrical resistance is zero. This is known as superconductivity.

Superconductivity is the quantum-mechanical phenomenon of zero electrical resistance.  As materials are cooled down and become superconducting they emit a magnetic field, a process called the Meissner effect.

Superconducting wires transmit electrical current without interference and can hold electrical current indefinitely. There’s just one catch – they only work at very low temperatures. Research work is ongoing and has increased their working temperature, but we’re still a long way off being able to use them easily in everyday life.

If we can develop room-temperature superconductors, they’ll revolutionise our world. Using superconductors in power cables and electrical devices would mean the energy in would equal energy out, giving near-perfect efficiency.

The Meissner effect exhibited by superconductors gives rise to Quantum Locking – a phenomenon where a superconducting material is ‘locked’ in 3D space above a magnet. This levitation effect results from the magnetic field lines of the magnet penetrating the superconductor. The levitating material has the ability to support a substantially heavier objects, keeping it at a fixed distance above the magnet.

It is this concept that was demonstrated by the French physicists, and could have many future implementations, mainly through incorporation in mechanical devices to get frictionless moving parts.

That’s the sensible stuff… now here’s literally the coolest toy train set in the world: