Tag Archives: #speed

Scientists Find Upper Limit For The Speed Of Sound (Physics)

A research collaboration between Queen Mary University of London, the University of Cambridge and the Institute for High Pressure Physics in Troitsk has discovered the fastest possible speed of sound.

The result- about 36 km per second—is around twice as fast as the speed of sound in diamond, the hardest known material in the world.

Waves, such as sound or light waves, are disturbances that move energy from one place to another. Sound waves can travel through different mediums, such as air or water, and move at different speeds depending on what they’re travelling through. For example, they move through solids much faster than they would through liquids or gases, which is why you’re able to hear an approaching train much faster if you listen to the sound propagating in the rail track rather than through the air.

Einstein’s theory of special relativity sets the absolute speed limit at which a wave can travel which is the speed of light, and is equal to about 300,000 km per second. However until now it was not known whether sound waves also have an upper speed limit when travelling through solids or liquids.

The study, published in the journal Science Advances, shows that predicting the upper limit of the speed of sound is dependent on two dimensionless fundamental constants: the fine structure constant and the proton-to-electron mass ratio.

These two numbers are already known to play an important role in understanding our Universe. Their finely-tuned values govern nuclear reactions such as proton decay and nuclear synthesis in stars and the balance between the two numbers provides a narrow ‘habitable zone’ where stars and planets can form and life-supporting molecular structures can emerge. However, the new findings suggest that these two fundamental constants can also influence other scientific fields, such as materials science and condensed matter physics, by setting limits to specific material properties such as the speed of sound.

The scientists tested their theoretical prediction on a wide range of materials and addressed one specific prediction of their theory that the speed of sound should decrease with the mass of the atom. This prediction implies that the sound is the fastest in solid atomic hydrogen. However, hydrogen is an atomic solid at very high pressure above 1 million atmospheres only, pressure comparable to those in the core of gas giants like Jupiter. At those pressures, hydrogen becomes a fascinating metallic solid conducting electricity just like copper and is predicted to be a room temperature superconductor. Therefore, researchers performed state-of-the-art quantum mechanical calculations to test this prediction and found that the speed of sound in solid atomic hydrogen is close to the theoretical fundamental limit.

Professor Chris Pickard, Professor of Materials Science at the University of Cambridge, said: “Soundwaves in solids are already hugely important across many scientific fields. For example, seismologists use sound waves initiated by earthquakes deep in the Earth interior to understand the nature of seismic events and the properties of Earth composition. They’re also of interest to materials scientists because sound waves are related to important elastic properties including the ability to resist stress.”

References: K. Trachenko, B. Monserrat, “Speed of sound from fundamental physical constants”, Science Advances 09 Oct 2020, Vol. 6, no. 41, eabc8662 DOI: 10.1126/sciadv.abc8662 link: https://advances.sciencemag.org/content/6/41/eabc8662

Provided by Queen Mary, University of London

The Universe Is Expanding, But How Fast Is Up For Debate (Physics)

In the 1920s, astronomer Edwin Hubble brought about the modern age of cosmology when he discovered that the universe is expanding at a predictable rate, which has since been called the Hubble constant. Nearly 100 years later, more precise measurements have sharpened his accuracy — but may also put our current understanding of physics in limbo.

At the turn of the 20th century, the hot topic in cosmological circles was nebulae. We could see them with telescopes, but we didn’t know how far away they were — were they interstellar clouds within the Milky Way, or were they far-off galaxies of their own? If you’ve ever wondered whether a light in the night sky is a satellite or a star, you know how difficult it is to tell the distance of things in space. In the early 1900s, Henrietta Swan Leavitt discovered what became known as Cepheid variables, a type of star whose brightness varied at a rate that could be used to calculate their absolute luminosity, or intrinsic brightness. That’s important because astronomers can use a star’s luminosity to measure its distance from us. A decade or so later, Edwin Hubble realized that many nebulae contained these variable stars, and could, therefore, make the important discovery that nebulae weren’t located in our own galaxy, but far beyond it, existing as galaxies in their own right.

Oh, but that’s not all. Next, Hubble compared these distance measurements with each galaxy’s velocity and found that the further away the galaxy was, the faster it was moving away from us. That led to the bombshell of the century: the universe was expanding. (To understand why more distant galaxies are moving faster, imagine a loaf of raisin-bread dough. When you put it in the oven, all of the raisins are evenly distributed, but as it rises in the oven, the raisins near the edges move outward faster than those in the center.) Hubble’s formula to determine the speed of a galaxy, called Hubble’s law, is v = H0d. H0 is, you guessed it, the Hubble constant, which astronomers have used ever since to judge the rate at which the universe is expanding.

When Hubble made this discovery, technology was not what it is today, to put it mildly. As a result, Hubble’s estimate for the value of the Hubble constant was pretty imprecise. One big reason for launching the Hubble Space Telescope in the 1980s was to get a more precise estimate for a number that at that time was somewhere between 50 and 100 km/sec/Mpc (kilometers per second per Megaparsec) — they wanted to whittle the accuracy down to at least 10 percent, which is still a wildly imprecise margin for science. Fast forward 30 years, and even more precise instruments such as the Wilkinson Microwave Anisotropy Probe (WMAP) honed the number to 69.3 km/sec/Mpc. Then in 2013, the expansion rate of the universe put on the brakes when the Planck satellite used background radiation from the Big Bang to find that the Hubble constant was closer to 67 km/sec/Mpc.

But in December 2016, a group called H0 Lenses in COSMOGRAIL’s Wellspring, or H0liCOW (get it?), used Einstein’s theory of general relativity to determine that the Hubble constant was a much faster 72 km/sec/Mpc. Despite its name, improved technology means that the Hubble constant keeps changing. What does that mean? A lot. It could mean that there are yet discovered elementary particles at play. It could mean that dark energy, which was previously blamed for shifts in the expansion rate, isn’t there at all, and is instead a theoretical form called phantom energy. This all would mean new physics and a drastic change in our understanding of the universe. But for now? It’s too soon to tell.

What Would Happen If You Traveled Faster Than The Speed Of Light (Universe / Science And Technology)

When we were kids, we were amazed that Superman could travel faster than a speeding bullet. We could even picture him, chasing down a projectile fired from a weapon, his right arm outstretched, his cape rippling behind him. If he traveled at half the bullet’s speed, the rate at which the bullet moved away from him would halve. If he did indeed travel faster than the bullet, he would overtake it and lead the way. Go, Superman! In other words, Superman’s aerial antics obeyed Newton’s views of space and time: that the positions and motions of objects in space should all be measurable relative to an absolute, nonmoving frame of reference.

In the early 1900s, scientists held firm to the Newtonian view of the world. Then a German-born mathematician and physicist by the name of Albert Einstein came along and changed everything. In 1905, Einstein published his theory of special relativity, which put forth a startling idea: There is no preferred frame of reference. Everything, even time, is relative.

Two important principles underpinned his theory. The first stated that the same laws of physics apply equally in all constantly moving frames of reference. The second said that the speed of light — about 186,000 miles per second (300,000 kilometers per second) — is constant and independent of the observer’s motion or the source of light. According to Einstein, if Superman were to chase a light beam at half the speed of light, the beam would continue to move away from him at exactly the same speed.

These concepts seem deceptively simple, but they have some mind-bending implications. One of the biggest is represented by Einstein’s famous equation, E = mc², where E is energy, m is mass and c is the speed of light. According to this equation, mass and energy are the same physical entity and can be changed into each other. Because of this equivalence, the energy an object has due to its motion will increase its mass. In other words, the faster an object moves, the greater its mass. This only becomes noticeable when an object moves really quickly. If it moves at 10 percent the speed of light, for example, its mass will only be 0.5 percent more than normal. But if it moves at 90 percent the speed of light, its mass will double.

As an object approaches the speed of light, its mass rises precipitously. If an object tries to travel 186,000 miles per second (299,792 kilometers per second), its mass becomes infinite, and so does the energy required to move it. For this reason, no normal object can travel as fast or faster than the speed of light.

That answers our question, but let’s have a little fun and modify the question slightly.

We covered the original question, but what if we tweaked it to say, “What if you traveled almost as fast as the speed of light?” In that case, you would experience some interesting effects. One famous result is something physicists call time dilation, which describes how time runs more slowly for objects moving very rapidly. If you flew on a rocket traveling at 90 percent of light-speed, the passage of time for you would be halved. Your watch would advance only 10 minutes, while more than 20 minutes would pass for an earthbound observer.

You would also experience some strange visual consequences. One such consequence is called aberration, and it refers to how your whole field of view would shrink down to a tiny, tunnel-shaped “window” out in front of your spacecraft. This happens because photons (those exceedingly tiny packets of light) — even photons behind you — appear to come in from the forward direction.

In addition, you would notice an extreme Doppler effect, which would cause light waves from stars in front of you to crowd together, making the objects appear blue. Light waves from stars behind you would spread apart and appear red. The faster you go, the more extreme this phenomenon becomes until all visible light from stars in front of the spacecraft and stars to the rear become completely shifted out of the known visible spectrum (the colors humans can see). When these stars move out of your perceptible wavelength, they simply appear to fade to black or vanish against the background.

Of course, if you want to travel faster than a speeding photon, you’ll need more than the same rocket technology we’ve been using for decades. Perhaps pulling on blue tights and a red cape isn’t such a far-fetched idea after all.

References: (1) https://science.howstuffworks.com/dictionary/famous-scientists/physicists/isaac-newton.htm (2) https://www.howstuffworks.com/videos (3) https://science.howstuffworks.com/science-vs-myth/everyday-myths/time-dilation.htm (4) https://science.howstuffworks.com/science-vs-myth/everyday-myths/doppler-effect.htm