Theoretical physics: The origins of space and time

Many researchers believe that physics will not be complete until it can explain not just the behaviour of space and time, but where these entities come from.

Finding that one huge theory is a daunting challenge. Nature explores some promising lines of attack — as well as some of the emerging ideas about how to test these concepts (see 'The fabric of reality').

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Credit: Zeeya Merali


What happens to a liquid in a cold vacuum? Does it boil or freeze? These animations of liquid nitrogen (LN2) in a vacuum chamber demonstrate the answer: first one, then the other! The top image shows an overview of the process. At standard conditions, liquid nitrogen has a boiling point of 77 Kelvin, about 200 degrees C below room temperature; as a result, LN2 boils at room temperature. As pressure is lowered in the vacuum chamber, LN2’s boiling point also decreases. In response, the boiling becomes more vigorous, as seen in the second row of images. This increased boiling hastens the evaporation of the nitrogen, causing the temperature of the remaining LN2 to drop, the same way sweat evaporating cools our bodies. When the temperature drops low enough, the nitrogen freezes, as seen in the third row of images. This freezing happens so quickly that the nitrogen molecules do not form a crystalline lattice. Instead they are an amorphous solid, like glass. As the residual heat of the metal surface warms the solid nitrogen, the molecules realign into a crystalline lattice, causing the snow-like flakes and transition seen in the last image. Water can also form an amorphous ice if frozen quickly enough. In fact, scientists suspect this to be the most common form of water ice in the interstellar medium. (GIF credit: scientificvisuals; original source: Chef Steps, video; h/t to freshphotons)

You want a physicist to speak at your funeral. You want the physicist to talk to your grieving family about the conservation of energy, so they will understand that your energy has not died. You want the physicist to remind your sobbing mother about the first law of thermodynamics; that no energy gets created in the universe, and none is destroyed. You want your mother to know that all your energy, every vibration, every Btu of heat, every wave of every particle that was her beloved child remains with her in this world. You want the physicist to tell your weeping father that amid energies of the cosmos, you gave as good as you got.

And at one point you’d hope that the physicist would step down from the pulpit and walk to your brokenhearted spouse there in the pew and tell him that all the photons that ever bounced off your face, all the particles whose paths were interrupted by your smile, by the touch of your hair, hundreds of trillions of particles, have raced off like children, their ways forever changed by you. And as your widow rocks in the arms of a loving family, may the physicist let her know that all the photons that bounced from you were gathered in the particle detectors that are her eyes, that those photons created within her constellations of electromagnetically charged neurons whose energy will go on forever.

And the physicist will remind the congregation of how much of all our energy is given off as heat. There may be a few fanning themselves with their programs as he says it. And he will tell them that the warmth that flowed through you in life is still here, still part of all that we are, even as we who mourn continue the heat of our own lives.

And you’ll want the physicist to explain to those who loved you that they need not have faith; indeed, they should not have faith. Let them know that they can measure, that scientists have measured precisely the conservation of energy and found it accurate, verifiable and consistent across space and time. You can hope your family will examine the evidence and satisfy themselves that the science is sound and that they’ll be comforted to know your energy’s still around. According to the law of the conservation of energy, not a bit of you is gone; you’re just less orderly. Amen.

—  Aaron Freeman

You guys ask great questions—I’ve missed replying to them! But I love talking about entropy so prepare yourself.

The Second Law of Thermodynamic is part of a set of three fundamental, beautifully simple physical laws of a thermodynamic system.

  • First Law: Energy is not created or destroyed.
  • Second Law: In an isolated system, entropy must always increase.
  • Third Law: Absolute zero cannot be achieved.

Most people understand the Second Law to mean that “In an isolated system, disorder must always increase”, as in things always tend from order to disorder, but entropy does not automatically equate to disorder. It’s just a measure of disorder.

For those who are unfamiliar with entropy, the basic concept is pretty simple because we see it all around us everyday—objects break, relationships disintegrate, people age. To understand the terms of “high” and “low” entropy means, think of it like this: if a library is highly-organised and has an efficient indexing system, it could be called a low entropy library, whereas if a library has jumbled shelving, a disorganised indexing system and books all over the place, it could be called a high entropy library.

By referring to an ‘isolated system’, the Second Law is referring to a system where no energy is being added to it or taken from it, and entropy is a gauge of the energy in such a system that can’t be used anymore. This unavailable energy hasn’t left the system—it’s just become irretrievably disordered—but even though this will increase, it doesn’t mean that elsewhere in the system other energy can’t become more ordered.

Sure, a system (for example, the universe) contains unavailable energy, but the rest of the universe’s energy needs to be accounted for. It’s still out there, still doing stuff, and can still behave in a bunch of different ways depending on a variety of forces. It’s completely possible for a closed system to create even complex and elaborate order, just as long as there’s a balance and there’s an increase of disorder elsewhere in the system—importantly, you can only create order by increasing disorder too, because creating order involves expending energy, which is inevitably inefficient and so it adds to the unavailable energy.

True to the Second Law of Thermodynamics, the total amount of order in the universe is always decreasing, but that doesn’t mean parts of the universe can’t continue to become ordered too.

It’s like a struggle against the tide on a cosmic-scale. Structures, stars and organisms are created as low-entropy systems, but it’s fruitless because disorder is a byproduct of order and will triumph in the end—and yet, the universe keeps striving for order all the same.

Watch on fuckyeahfluiddynamics.tumblr.com

If you find yourself some place really cold this holiday season, may I suggest stepping outside and having some fun freezing soap bubbles? The crystal growth is quite lovely, as seen in this photograph. If you live in warmer climes, fear not, you can always experiment in your freezer. It would be particularly fun, I think, to see how a half-bubble sitting on a cold plate freezes in comparison to a droplet like this one. (Video credit: Mount Washington Observatory)

Watch on fuckyeahfluiddynamics.tumblr.com

Reader kylewpppd asks:

Have you seen the post of a man in Siberia throwing boiling water off of his balcony? Can you provide a better explanation of what’s going on?

As you can see in the video (and in many similar examples on YouTube), tossing near boiling water into extremely cold air results in an instant snowstorm. Several effects are going on here. The first thing to understand is how heat is transferred between objects or fluids of differing temperatures. The rate at which heat is transferred depends on the temperature difference between the air and the water; the larger that temperature difference is the faster heat is transferred. However, as that temperature difference decreases, so does the rate of heat transfer. So even though hot water will initially lose heat very quickly to its surroundings, water that is initially cold will still reach equilibrium with the cold air faster. Therefore, all things being equal, hot water does not freeze faster than cold water, as one might suspect from the video.

The key to the hot water’s fast-freeze here is not just the large temperature difference, though. It’s the fact that the water is being tossed. When the water leaves the pot, it tends to break up into droplets, which quickly increases the surface area exposed to the cold air, and the rate of heat transfer depends on surface area as well! A smaller droplet will also freeze much more quickly than a larger droplet.

What would happen if room temperature water were used instead of boiling water? In all likelihood, a big cold bunch of water would hit the ground. Why? It turns out that both the viscosity and the surface tension of water decrease with increasing temperature. This means that a pot of hot water will tend to break into smaller droplets when tossed than the cold water would. Smaller droplets means less mass to freeze per droplet and a larger surface area (adding up all the surface area of all the droplets) exposed. Hence, faster freezing!

Researchers at the University of Warwick and Oxford University have developed a form of crystal that can deliver highly accurate temperature readings, down to individual milli-kelvins, over a very broad range of temperatures: -120 to +680 degrees centigrade.

The researchers used a “birefringent” crystal which splits light passing through it into two separate rays. Research has already shown that the size of the effect will increase or decrease in proportion to the temperature of the crystal. Therefore, in theory, you could calibrate such crystals to be highly accurate temperature gauges.

However, the use of birefringence in this way has significant problems in practice. This temperature measuring ability of highly birefringent crystals is badly compromised by changes in the thickness and orientation of the crystal. This adds expense to the manufacture and calibration of such crystals and makes them almost unusable in situations where, for example, vibration could alter the orientation of the crystal.

However the Warwick and Oxford researchers have developed a reproducible and low-cost method of modifying the properties of crystalline lithium tantalate so that its birefringence is virtually independent of the crystal’s thickness and position making it resistant to vibration and cheaper to manufacture. In fact, they have made the birefringence almost zero in magnitude in all directions (the material is close to being optically isotropic just like ordinary glass). However, the slightest temperature change induces a rapid increase in birefringence in these materials, making this a reliable, robust and very sensitive method for measuring temperature. The inventors have named their device a Zero-Birefringence Optical Temperature Sensor (Z-BotS) and are currently seeking follow-on funding to develop the device from the bench-top proof-of-concept to a miniaturized commercially-viable package.

(via Research gives crystal clear temperature readings from toughest environments)

It has come to my attention that the entropy tag is full of things which are not actually related to entropy. Here is my attempt to fix that.

Entropy is a measure of the number of specific ways in which a system may be arranged, often taken to be a measure of disorder, or a measure of progressing towards thermodynamic equilibrium. The entropy of an isolated system never decreases, because isolated systems spontaneously evolve towards thermodynamic equilibrium, which is the state of maximum entropy. (x)

What people don’t seem to understand is that entropy is always happening. It’s all around us all the time!

It’s not some sort of decay into madness, it’s not a loss of touch with reality, it’s not even a set of random events happening to you.

Actually, it has a lot to do with heat.

The third law of thermodynamics states:

Any pure crystalline substance at a temperature of absolute zero (0.0 K) has an entropy of zero (S = 0.0 J/K mol).

Because of this law, we are able to determine some general rules about entropy.

  1. Entropy increases as one goes from a solid to a liquid, or a liquid to a gas.

  2. Entropy increases if a solid or liquid is dissolved in a solvent. 

  3. Entropy increases as the number of particles (molecules) in a system increases.

  4. The Entropy of any material increases with increasing temperature

  5. Entropy increases as the mass of a molecule increases

  6. Entropy is higher for weakly bonded compounds than for compounds with very strong covalent bonds
  7. Entropy increases as the complexity (# of atoms, # of heavier atoms, etc.) of a molecule increases

Basically, all of this means that the final state of a system is more energetically favorable if energy can be dispersed over a greater number and variety of molecules, and if the particles of the system can be more dispersed (more disordered).

The greater the dispersal of energy or matter in a system, the higher is its entropy. The greater the disorder (dispersal of energy and matter, both in space and in variety) the higher the entropy. Adding heat to a material increases the disorder. (x)

Thermal imaging of emperor penguins in Antarctica shows that, in still conditions, large portions of their bodies remain colder than ambient temperatures. In the image above, the heads, beaks, eyes, and flippers of this pair of penguin are the warmest while much of their feathered surface remains several degrees colder than the temperature around them. Not only does this indicate that the penguins’ skin and feathers are extremely effective insulators—the core temperature of each penguin is roughly the same as a human’s—but it means that the penguins are losing heat via radiative cooling toward the sky, the same way your car does when frost forms. The measurements in the study are for penguins at least one body length away from any other penguins; of course penguins typically huddle together to generate additional warmth. The mathematics of this behavior are under active research. (Photo credit: D. McCafferty et al.; via Wired)