Science In Real Life

Altering the Past, part 1 September 11, 2011

Discussions of time travel among physicists never fail to revolve around a rather jargon-y term: “closed timelike curves”. The exact definition of this term is not important right now. What is important is that it fixes the discussion to the question of whether or not it is possible for a chunk of ordinary matter to revisit a previous point in its own personal timeline. This is a very interesting, very speculative, and oftentimes very mathematical question. Of course, it’s not the only discussion of time travel – consider your average time-travel science fiction story. With a few notable exceptions, the same thing always happens: our heroes are flung back into the past and they must find a way to return to their own time without changing history. Sometimes they succeed, sometimes they find they were meant all along to have done something in the past, sometimes they have to undo an accidental change they already made. Save history or else!, is always the message. Frankly, I think we could all do with a little less history these days, but that’s a digression for another time.

There is a question lying at the intersection of these two realms of time travel that I found myself recently pondering. What does it mean, in the context of a physical understanding of time and space, to have altered the past? How much of a disruption is small enough to go unnoticed, and how much is enough to make sure that the planet has been conquered by giant ants when you get back to your present day?

They campaigned on a platform of free sugar cubes. What were we going to do, vote for the spiders?

To get started on this, I will have to drop some physics on you. What you’re about to see is called a Minkowski diagram, and it is the best method ever devised by man for representing the fundamental, ethereal concept of causality in a drawing simple enough to be a doodle on someone’s coffee napkin.

Pictured: Cosmic wisdom.

There’s a lot going on here. Let’s unpack it. First, we have the horizontal axis, in light years. This represents all of space. On the vertical axis is time, in years. That means that a light ray, traveling one light year per year, is a 45° line. The tricky part is that these light rays represent the limits of temporal influence. Let’s say I, sitting at Here & Now, at coordinates (0,0), want to influence something three light years away, at x = 3. Let’s call this place Deep Space Three. The fastest way to do it is to send a signal of light. That light ray will travel for three years and then arrive at Deep Space Three, at coordinates (3,3). I absolutely cannot send any information to Deep Space Three, three light years away, any sooner than that. Any events that happen at Deep Space Three before that are utterly beyond my power to alter to even the smallest extent. I cannot so much as budge a single electron. Why? Because the light ray hasn’t gotten there yet. And the light ray carries the electromagnetic force. From currently known physical laws, all influence, all information transmission, happens through interactions of one or more of the four fundamental forces. Gravity, strong, weak, and electromagnetic. That’s it. For an object to be influenced in any way is for it to interact with another object via one of those forces.

Love is not a force, no matter what Randall Munroe tells you.

But now let us turn to the past. The diagram also shows all the points, events, and information that can possibly influence me, at Here & Now. Anything that can send me a light ray exists on one of the diagonal yellow borders of the triangle labeled Past. What about the rest of the triangle? It’s largely irrelevant. With one large exception, every single interaction you have ever had and will ever have with anything else in your entire life happens by the exchange of photons, carriers of the electromagnetic force. What you think of as physical touch is the exchange of photons between the EM fields of your skin and another object. Sound waves, pressure waves in air, are transmitted when one group of air molecules pushes another. That push happens by the same EM field photon exchange. Think about everything that’s ever happened to you. Every event, every point on your version of that graph, is an electromagnetic interaction. You have lived a life built out of light rays.

Oh man, remember this day? That was hilarious.

I’m afraid we’re out of time, dear readers. Next time I will, with this background fresh in your minds, discuss the meat of the question. And, I’m going to give you some homework. Just two questions, to roll around in your minds.

1) I mentioned a large exception in the final paragraph. What is it?

2) Try to think of minimal ways to change events. What’s the smallest amount of energy required to get someone to change their mind on any kind of decision, or alter their actions in some culturally recognizable way?

Happy sciencing!

The Physics of Cracking Skulls March 2, 2011

Gruesome, no? But these are the important questions of our times. And thankfully a team of intrepid forensic pathologists (and one physicist!) have stepped forward to bring us this study:  Are full or empty beer bottles sturdier and does their fracture-threshold suffice to break the human skull?

Goodness, even the title is a little hard to wrap one’s head around. Throw in the first sentence of the introduction: “The examination of living or deceased victims of bar fights is not uncommon in routine forensic practice.” – and we can see we’re off to a rollicking good start. You’ll need institutional access or some other subscription to see the article, but fear not, I intend to explain the basics of the research here.

First, the results are described in terms of “impact energy”, at 30 Joules and 40 Joules for full and empty beer bottles respectively. A Joule is a unit of energy. For comparison, 35 Joules is about enough energy to lift a newborn baby up to a height of one meter. Let’s say we have an exactly 8 lb baby. If you have 30 Joules, you can lift that baby 0.84 meters. If you have 40 Joules, you can lift that baby 1.12 meters. So we’re dealing with a small, but not insignificant energy difference here, at least as far as our arms and lifting things are concerned. Now let’s put little Julianna down and pick up some beer bottles instead.

In the experiment, the researchers measured the energy difference needed to break a bottle by dropping a 1 kg steel ball on the bottles from various heights. They then already knew how much energy would be imparted to each bottle during the impact. How? What arcane powers of divination provided them with this knowledge? The answer is gravitational potential energy. Just by lifting the steel ball to a position further away from the center of the earth, the ball acquires potential energy – energy that manifests as the ball’s potential to fall down. The higher up the ball is when it falls, the harder it hits.

Now, to the results – you might think it would be easier to break an empty bottle over someone’s head compared to a full one, but you’d be wrong. In one of the most memorable phrases I’ve ever seen in a paper, the authors explain: “beer is an almost incompressible fluid.” That means that the shock of the impact in a full bottle gets transferred straight through to the opposite glass wall with almost no dispersion or degradation. Boom! Cracked bottle. By contrast, in an empty bottle, the stress and strain of the collision is transmitted throughout the bottle’s structure. This makes a full bottle more dangerous in a fight – when it cracks, it can cause severe cuts and lacerations. Of course, as the authors note, either one is sufficient to cause cranial trauma and is just generally a bad idea.

That about does it for beer bottles. I suggest you all go drink one instead of swinging one.

Neutrinos and the Human Body June 28, 2010

So, some of the other grad students and I were discussing something or other about neutrinos. I don’t remember exactly what prompted it, but I proposed the question: How likely is it for a human body to register the presence of a neutrino during an average 70 year lifespan? Before I get into telling you guys about the answer, I should explain why the question is of any interest whatsoever.

Ever since the idea of neutrinos, tiny weakly-interacting electrically neutral particles, was proposed by a guy named Wolfgang Pauli on Dec 4, 1930, neutrino detection has been a matter of volume. The largest detector in the biz, Super-Kamiokande, contains 50000 tons of pure water, and registers about 5000 neutrino detections per year. That should stagger you. Consider that uncounted millions of neutrinos pass through every gallon of empty space every single second of every single day. And yet it takes the biggest structures mankind can muster to record just a few of the interactions. This is because interactions, on the level of fundamental particles, between neutrinos and ordinary atoms are exceedingly rare. Thus the question of human bodies as neutrino detectors acquires at least the status of a curiosity worth spending a few hundred words analyzing.

Now, down to business. According to Wolfram Alpha, 50000 tons of pure water has a volume of about 750,000 human bodies. Human bodies are, let’s be honest here, mostly water. So it’s not a stretch to say that it takes 750,000 of us to interact with 5000 neutrinos per year. A quick division gets us 150 humans interacting with one neutrino per year. Each and every one of us has a 1/150 chance of physically interacting with one of the 2×10^20 neutrinos that pass through our bodies every year. Postulating a 70 year lifespan, we all of us have slightly less than even odds on becoming a living neutrino detector for just one neutrino sometime within our lives.

So what happens if you’re one of the lucky ones? Not a whole lot. At this point, things start to depend on how much energy the neutrino has. But we can still draw some general conclusions. Without getting into the details of the various possible reactions, one thing you would absolutely get is an energetic electron or five. Depending on what the neutrino hits, you might also get a sudden episode of alchemy – a carbon atom changing to a nitrogen isotope, for example. More electrons would result when it decays back to carbon. And all of these things would be throwing out light rays. What does this mean for you, the consumer? Very little. Depending on where this happens, you might end up with a single broken molecule. Such detritus is routinely cleaned out by cellular processes. But, if you’re really unlucky, that broken molecule could cause a mutation in the DNA of one of your cells. If you’re really really unlucky, the mutation could be in an activated gene, which might cause that cell to die. Maybe you should look into some cellular insurance. I’m sure they have a neutrino clause in the policy.

Science and Death June 23, 2010

Written June 21, 2010.

Today is Monday, and today I have to go to a funeral. To a scientist, in some ways, death is both more and less permanent. There is no empirical justification as yet for any kind of afterlife, and so we lose the comforting notion that the mind, soul, or other consciousness of a loved one continues to be a part of our world or of any other. Ashes to ashes, dust to dust, in the truest and most literal sense of the expression. A human being, according to my own amalgamation of the ideas of several scientific disciplines, is a highly energetic mass of complex molecules undergoing a continual and continually changing set of intricate regulatory processes enabling it to absorb oxygen, nutrients, and information and interact with other human beings to survive, reproduce, and thrive. To those of who object “Is that all?” I suggest that they underestimate the complexity of those molecules and their regulatory processes.

So what happens when a human being dies? These days death is defined by Western medicine as irreversible loss of electrical activity in the brain. Aside from a few difficulties with taking the relevant measurements, this is a perfectly serviceable definition. But it’s only the beginning of the process. What really gets us happens on the cellular level. Without the larger-scale processes like breathing, individual cells (those marvelous factories for proteins and energy) simply shut down. The bakery cannot make any cakes if it does not get any flour. The bakers lose their jobs, the building falls into disrepair and eventually falls apart. Thus it is with the cell. Of course, there the analogy fails, because bakeries are not continually fending off an invading army of decomposer bacteria. Without the active defense of the immune system (in all its multifarious glory), everything can eat us. This is why corpses stink. It’s not all your pent-up farts finally escaping, it’s the farts of the bacteria as they chow down on your femurs. Death is a disgusting business. Dying with dignity, or glory, is just not possible. You break down, you decay, you rot. You are eaten, digested, and absorbed. Nothing of what you were remains.

Or does it?

Those brutally hungry bacteria may efficiently disassemble your large scale structures, but your molecules mostly remain intact. Your atoms almost certainly so. Are you destroyed, or are you merely disseminated? There is some weight, and some lingering comfort, in this viewpoint. In a deep sense of the word, what you were remains. Now no longer part of a single human body, what you were disperses, joining other molecules from other lost loved ones to feed a bird, or roll along in an ocean wave. This is the conservation of matter and energy at its most personal, at its most touching. Nothing ever really dies, nothing is ever truly lost. Everything is reduced, recycled, reused, and reincarnated. Even those of us who are still alive are made of the remains of those past. The same molecules that made Caesar, Cleopatra, Aristotle, and Joan of Arc are us today. And it goes further than that – before that, the atoms that make those molecules were part of incomprehensibly massive stars that exploded with astronomical force to produce the substances that formed Earth and the rest of our solar system. When our system ends, out of the remaining gas clouds may form new stars, and the cycle will begin once more.

Ashes to ashes, dust to dust.

Pardon Our Construction April 2, 2010

So, some of you loyal readers may have been wondering where Science In Real Life has been lately. To you I offer my apologies – the formats in which I have attempted to write this blog, while fun and educational, have ultimately proven unsustainable for reasons peculiar to my own foibles. However, all hope is not lost. I have a new idea for a more natural way to communicate the scientific thinking that goes on in my head, one that is in some ways truer to the original stated mission of this blog. Over the next few weeks I will be testing this idea, and if all goes well you will start seeing new entries, in a looser and more fluid format. Until then, happy sciencing.

SiRL: Quantum Weirdness, pt 2 December 10, 2009

Welcome, welcome, one and all to the second helping of quantum weirdness, in case you didn’t get enough the first time around. We’ll be continuing along the same basic theme; see if you can guess what it is before I reveal it at the end, and tie this whole thing up in a pretty little bow with flowers on it.

Next up, quantum tunneling, which is in all honesty one of the coolest things ever. This is not to be confused with quantum Chunneling, which is when you suddenly find yourself transported from England to France. If you have a wavefunction approaching a barrier, which is to say a region where the particle is not allowed to be, there is a small probability that it will simply appear on the other side, like so:

This graph, again, is a wavefunction, a probability wave, so don’t think of this reduced amplitude as a less energetic particle. The size of the wave indicates only the relative likelihood of finding a particle when you do a measurement. This process has given us the Scanning Tunneling Microscope, and subatomic particles do this on a daily basis; it has effects in scenarios cosmological and biological.

So why can’t we walk through walls? What gives? Clearly not the walls. As you can see here, the thickness of the barrier is a very important factor. To us, a wall is sturdy, but only about as thick as we are. To one of your protons … you might as well try to launch an iPod unscathed through the thronging crowds of Macworld. It’s not going to end well. And for a macroscopic object like us to do it, all of our particles have to do this simultaneously and in perfect unison, otherwise you end up as a fine paste smeared across the floor. So things on our scale stay where they are, mostly.

There is one last bit of weirdness I want to discuss, and this is the weirdest of all. Just like the position of a quantum particle is fundamentally indeterminate, so too is the amount of energy of the entire Universe. It simply has no definite value, and we are sadly lacking in interactions with other Universes to fix it. What that means is, at any moment, there is a chance that a pair of virtual particles will pop into existence and then immediately annihilate again. For small particles, it’s a rather good chance. In fact, this is happening all the time, everywhere. Yes, even there. Please nobody be alarmed! I promise it’s harmless. Well, mostly harmless.

You see, this process is governed by the energy-time uncertainty principle, younger and more mysterious brother to the position-momentum version. Essentially, the shorter something is around, the bigger the variation in its energy. So on very very short timescales, you can get things bigger than particles. Like pogo sticks and ponies and Christopher Walken.

These are fun to think about, but I cannot stress enough how unlikely it would be for even a single atom to begin existence in this fashion. And so it is that the realm of zero point energy production, like so much else from quantum mechanics, remains hidden from our normal view.

The underlying concept behind all of these is something called superposition of states, and I don’t mean some mutant abomination hybrid of California and Texas. First, in the language of quantum mechanics the state of the system is whatever we need to know about it to describe it as distinct from other states. In classical physics (think Baroque) everything is in one state, and we can predict what state it will go to next with absolute certainty. This turns out to be very wrong. Objects can be in more than one state, this is that superposition business. It is only an interaction that forces it into one state or another, as dictated by the probabilistic laws that govern its behavior.

Into the very general framework I have just sketched you can color in just about any situation you want, from Schrodinger’s cat to the electrons in an atom to quantum tunneling and indeed the energy of the entire Universe. One need only change what is the possible selection of states to which one refers. So where does this weirdness go? We don’t see the delicate selection process. Our brains, accustomed to having evolved from a need to react quickly, need to make assumptions about where things are. Our brains are expert spatiotemporal librarians, quickly filing everything into a sequence of locations and making snap judgements about how to react. Any errors in this are small enough to not kill us, so we don’t notice them.

And so it is that quantum effects remain shielded from our view. But I hope that you will leave here tonight having made a quantum leap in perspective, the better to appreciate the magnitude of weirdness permeating our existence.