Smartphone

Your smartphone knows physics: The science inside mobile devices

Ryan Smith

Ryan Smith

Ryan is an Associate Professor of Physics at California State University - East Bay, in the San Francisco Bay Area. Ryan’s lab group researches nanoscale-sized semiconductor materials using short pulses (less than a billionth of a second in duration!) of laser light. Such materials may become building blocks for future computing and renewable energy applications. Ryan is also a musician, outdoors enthusiast, rock climber, and beekeeper.

Physics is the reason computers shrunk from building-size to ones that fit in your pocket. We explore how the smartphone celebrates a variety of scientific and technological advancements: semiconductor nanotechnology, sensors, fiber optics, satellites, and atomic clocks are some of the puzzle pieces that have conspired to make such a useful (and distracting!) device. Upon exploring some of the technologies used by your phone, you may not be able to look at your phone in the same way and may even think that your phone is sort of smart.
 
While it may not feel like an inspiring moment each time you pull out your smartphone to check your email, there is a lot of fascinating physics going on inside your device. Smartphones are a rich showcase of many of the triumphs of modern physics. This article is intended to give some framework for understanding smartphone technology, and hopefully leave the reader with a sense of wonder, and maybe even some curiosity about future technologies that may soon make ‘smartphones’ antiquated technology.
 

The physics of using an app

Let’s take a look at some of the physics involved in an example of everyday use of a rideshare app on your phone. Turning on your phone screen, you are presented with approximately 3 million individually controlled microscopic light emitters that convince your eye that it is seeing images made up of reds, greens, and blues. Your finger applies a small amount of pressure on a glass screen, which electrically senses the pressure and converts this into meaningful information: you want to open an app. After collecting time-encoded radio signals from several orbiting satellites that allow the pocket computer to determine its position within a couple meters, it then relays its position information through a variety of electromagnetic waves (Wi-Fi and cellular signals, usually then encoded into fiber optic signals – all three of these examples are electromagnetic waves at different frequencies) to another computer that then collects position information from various drivers in your area. After some negotiation, another packet of waves is sent to a driver, who at some point will also have a payment be directed to their bank account after providing the service. A combination of continuous measurement of radio waves from satellites and nearby cell towers helps the driver, the company, and you, to know the constantly changing locations of both client and driver. The fact that this all works consistently and with very few glitches may feel like a miracle or something futuristic from this perspective, but this is the reality of the capability of the little devices we each have in our pockets.
 

Smaller means faster

To give a little perspective on how the technology has evolved, let’s look at a typical smartphone today and compare it with the IBM System/360 Model 75 mainframe computer (1) that was used to help send NASA astronauts to the moon. A smartphone with 4 GB of RAM (memory) has 500 times the memory, a cost ($800) that is 30,000 times cheaper (adjusted for inflation) and is 3,000 times faster than the IBM mainframe. Not to mention, compared with taking up a large room’s worth of space, just a bit smaller as well…

A smartphone is a miniature computer.  So how did we move from expensive, inefficient, building-sized computers to more powerful ones that fit in a pocket and have access to a global network of information? A big part of this story is the innovation of smaller transistors.

The driving force behind smaller devices has been shrinking the transistor, the basic building block of computation. A transistor, a controllable valve for electrons, can be assembled with other transistors to make a logic operation based on inputs. As an example of such an operation: if my two friends choose coffee, I will too; otherwise, I’ll choose tea. Put together billions of such logic operations and we are talking about the level of complexity of running an application or calculating the trajectory of a spacecraft. Remarkably, every two years for the last five decades, researchers have been able to nearly double the number of transistors that fit on a computer chip (mostly by shrinking the size of the transistor) (2). This strange phenomenon, predicted by Intel co-founder Gordon Moore in 1965, is known as Moore’s Law.

Why should we be motivated to shrink these building blocks to be smaller and smaller? For one, the speed of a computer fundamentally depends on its size. Nothing can travel faster than light and since it takes light a nanosecond to travel 30 cm (1 foot), a larger computer can be limited by the time it takes for a signal to travel through it. So, computer research has been motivated to miniaturize computers. Another important benefit of a smaller computer is that it produces less heat in making a calculation, which means lower consumption of energy and less need for fancy cooling solutions. Lastly, smaller features in computers mean making more efficient use of limited silicon ‘real estate,’ resulting in lower overall cost of making a computer chip while more logic operations can be made. Thus, smaller transistor size is simultaneously a necessary condition for having a miniature, portable, and low-cost device, and it helps to make a faster computer.
 

From sand to semiconductors

Most transistors are made from semiconductors, the materials at the backbone of the computer industry. The “semi-”, meaning “half” in Latin, implies that such a material can be made to sometimes behave as a conductor of electricity (like a metal) and other times be made to behave as an electrical insulator (such as plastic). Silicon has been the material of choice for decades now because it is abundant (28% of the earth’s crust) and cheap to produce – it is literally extracted from sand, which is a silicon atom bonded with two oxygen atoms – i.e., SiO2. Additionally, there is a convenient process for making parts of the material conducting or insulating by adding impurity atoms in a process called doping, thus allowing precise control of the flow of electrons in a circuit. The process for transferring an image of a circuit onto a silicon crystal wafer is called photolithography – for more on how this works, check out the semiconductor physics chapter in the book referenced below (2). Breakthroughs in photolithography have allowed nanometer-scale circuit features such as transistors to be routinely constructed in silicon.
 

Quantum world effects at small scales

Can transistors be made tinier without any limit? Gordon Moore also predicted, “One day transistors will be so small that they become affected by the bizarre reality of the quantum world and will thus have reached their smallest usable size.” Transistors have shrunk near this limit to several nanometers in size (that’s less than a thousandth the thickness of a human hair, and just a few atoms thick!), yet industry keeps finding creative ways to fit more transistors on a chip, including building three-dimensionally instead of only on a flat surface.

What are the ‘quantum world effects’ that become tricky for small sizes? When electrons are confined to small spaces around a few nanometers in size, they begin to exhibit a wave nature clearly. This behavior includes electrons oscillating at specific frequencies and interfering with other electrons. Both effects present difficulties to control of electron flow, a task at the heart of a computer.

While quantum effects can present challenges, some quantum effects can be useful for constructing computing components. One example is quantum tunneling – particles like electrons have the possibility to penetrate thin walls even when they don’t have the energy required to break through. This effect is used in transistors and flash memory (such as in a USB thumb drive) (2). Another use of the quantum effects is the development of a quantum computer, which could in principle perform calculations in hours that would take today’s best computers thousands of years. Viable quantum computers are a topic of active research.
 

On-board sensors

Besides receiving information through radio waves, a phone has many on-board sensors that continuously update the computer with information. These sensors include accelerometers and gyroscopes (e.g., to detect if you are making a turn when in navigation or the device has been dropped), magnetic sensors (sensing the Earth’s magnetic field and thus acting as a compass), and temperature sensor (tells phone to turn off if it gets too hot, keeping sensitive components from melting), and more recently pressure sensors (detecting your altitude and weather conditions).

As an example of how a sensor works, let’s look at how the accelerometer sensor works in a smartphone. This sensor looks like a microscopic version of two interleaved forks (acting as a capacitor), shown below. One of the forks is fixed in place, and the other can move a bit. A tiny current flows back and forth between the two forks. When you suddenly take a turn with your car or are in a suddenly rising elevator, one fork moves, changing the tiny spacing between the fork prongs. The amount of current that flows is sensitive to this spacing, and the phone then measures the current as a proxy for the acceleration that happens. In the phone there would be three sets of forks, one pair for each primary spatial direction. The data from all three sensors is sent continuously to the computer (up to hundreds of times per second), and this information is used to refine the computer’s tracking of the phone’s movement through space.
 

Interleaved forks as an analogy for how an accelerometer works

The sensors in smartphones are like a pocket science lab, and indeed smartphones are being integrated into science education. Many educators are making use of the on-board sensors to allow students to explore physics ideas such as motion and magnetic fields by using the devices already in their pockets (3). This turns out to be an exciting way for students to perform experiments, learning physics concepts and cultivating curiosity around what is happening inside their phones.
 

From phone to fiber

 Fiber optics offer long-distance, high-speed, heavy-data transmission of information by carrying infrared light. Creative Commons NC-SA 3.0 Unported License (photo by Ben Felten)

When we bring up a webpage on a smartphone, we are making use of a communication network which can involve cell phone towers, ‘telephone wire’ lines, and optical fibers. An optical fiber is a hair-thin tube of glass, encased in a thin plastic sheath, that guides light, as in the image above. When you send some communication, packet of information, a long series of 1’s and 0’s, encoded into long or short Morse-code-like pulses, is initially sent from your phone via radio waves. These waves cannot travel more than a few kilometers and so are converted at network nodes either into voltages to travel in wires, or a series of short bursts of infrared light through an optical fiber. Optical fiber is becoming the most common way to send information, as it can send enormous amounts of information over long distances very rapidly. Compared with AM radio, a single optical fiber can carry millions of times more data per second (2)! Light in an optical fiber can travel hundreds of kilometers with minimal losses. Many fibers are usually bundled together and buried as cables underground or ocean. On the other side of a long fiber, a network node converts this signal into electrical signals, and then sends this new signal into a radio wave or electrical signal, relaying your communication to a receiver.
Another interesting sort of physics that plays an important role in the way smartphones function is related to Einstein’s theory of relativity.
 

Connecting time and space with satellites

 

A Global Positioning System (GPS) satellite with solar panels transmits a radio wave packet to your phone that signals when the packet was sent. Creative Commons: NASA / Public domain.

Our initial example of using a rideshare app involved frequent use of determining locations of users. Smartphones don’t typically use satellites to communicate (e.g. internet, phone calls, text messages) but do receive radio signals from satellites as part of the global positioning system (GPS). The key to accurately knowing your physical position turns out to be connected to timekeeping.

To know your position, you must also know the time very well. Your phone needs to know a distance from several GPS satellites to determine its position. The distance to each satellite is figured by the time it takes for a radio signal to arrive at your device. GPS satellites orbit 20,000 km above the surface of the earth, meaning it takes about 70 milliseconds for each radio signal to reach you. The delay time for each satellite signal reaching you tells you how far away it is since the speed of light is fixed (about 1 billion kilometers per hour). The better you know this time delay exactly, the better you know your distance from each satellite. The time is kept by an average of several laboratories around the world that use lasers to create rapid oscillations of electron energy levels in atoms of elements such as Cesium. These ‘atomic clocks’ are so accurate that they would neither gain nor lose a second in 100 million years! This accurate timekeeping is radio broadcast to satellites and to you in order to determine the distance that each satellite is from you. By accurately knowing your distance from three satellites (there are 24 in total which are in orbit and available for public use), you can determine your position on earth. If you know your distance from four or more satellites, you can also know your altitude as well as your location even more accurately.

It turns out that clocks moving fast or experiencing a different gravitational field (both conditions are true for GPS satellites) will experience time differently. These differences, predicted by Einstein’s theory of relativity, have been precisely measured and are routinely incorporated into GPS protocol.  His theory predicts that clocks moving fast run slower compared with a stationary frame of reference. He also determined that a clock in a weaker gravitational field will run faster compared with a clock in a stronger gravitational field.  This relativistic effect is very strong near a black hole and may not seem to have any consequence for life near earth, which has a comparatively weak gravitational field.  However, GPS satellites are experiencing 17x weaker gravitational force than we are on earth, and they are also traveling at 14,000 km/hour around the earth. This means that in a year, without including the effects of relativity, my clock would disagree with the clock on a satellite by 20 minutes. Corrections from Einstein’s theory of relativity must be included into the system to reduce errors in determining your position. Without these relativistic corrections, our determination of position on earth would have errors between 10-20 meters (4).
 

Smartphones: a symbol of our changing civilization

In this short exploration of the physics behind smartphones, we traced through a few examples of the sorts of physics going on behind the gadget. We saw myriad physics effects in the use of a single app, the value of miniaturizing transistors, sensors that work with the phone, the role of fiber optics, and how atomic clocks and relativity play roles in GPS. The technology is evolving quickly, and new paradigms are being tested out.

How will the technology transform in the years to come? How will the ways that we communicate with each other continue to morph? Some of the trends we have seen will likely continue – e.g., more transistors fitting into smaller spaces, and reliance on satellites and atomic clocks for determining location. Will ‘phones’ continue to be items that we carry in our hands? Visions of sci-fi cyborgs may scare some from becoming physically connected to an electronic system, but ongoing experiments with ‘wearable’ technologies such as watches, and glasses offer new ways to interact with devices. On the software side, developments in natural language processing (NLP), eye tracking technology, and recommendation engines make interacting with a screen often unnecessary for carrying out tasks such as navigation or sending an email.

Some technologies will be enabled by developments in fundamental research. Studies of various ‘2D materials’ such as graphene (a sheet of single carbon atoms) permit the stacking of atomically thick layers (analogous to Lego blocks) to construct precise circuits (5). Devices made from such circuits could be smaller than a pinhead and consume very low amounts of energy, conceivably being charged either by body heat using thermoelectrics or through motions such as walking using piezoelectric energy harvesting.

The story of the smartphone is a human story of where we have come from and what we have discovered along the way. The smartphone is an interesting symbol of modern civilization, representing individuality alongside global interconnectedness, the capacity to gain knowledge about the world around us, and the level of convenience and efficiency we seem to value as a society.  It will be interesting to see developments in the coming years as the science – and society’s communications needs – continue to evolve.

 

Ryan Smith

 

References:

  1. Cortada, J.W., “IBM: The Rise and Fall and Reinvention of a Global Icon”, The MIT Press, 2019.
  2. Raymer, M.G., “The Silicon Web: Physics for the Internet Age”, Taylor & Francis, 2009.
  3. Wright, K., “Smartphone Physics on the Rise”, Physics, 2020.
  4. Faraoni, V., “Special Relativity (illustrated ed.)”, Springer Science & Business Media, 2013.
  5. Geim, A., Grigorieva, I. “Van der Waals heterostructures.”, Nature, 2013.
Received: 10.09.20, Ready: 02.11.20, Editors: Laura Mariotti, Omaina H. Aziz.

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