Faster-Than-Light Lasers Could “Illuminate” the Universe

It’s a cornerstone of modern physics that nothing in the Universe is faster than the speed of light (c). However, Einstein’s theory of special relativity does allow for instances where certain influences appear to travel faster than light without violating causality. These are what is known as “photonic booms,” a concept similar to a sonic boom, where spots of light are made to move faster than c.

And according to a new study by Robert Nemiroff, a physics professor at Michigan Technological University (and co-creator of Astronomy Picture of the Day), this phenomena may help shine a light (no pun!) on the cosmos, helping us to map it with greater efficiency.

Consider the following scenario: if a laser is swept across a distant object – in this case, the Moon – the spot of laser light will move across the object at a speed greater than c. Basically, the collection of photons are accelerated past the speed of light as the spot traverses both the surface and depth of the object.

The resulting “photonic boom” occurs in the form of a flash, which is seen by the observer when the speed of the light drops from superluminal to below the speed of light. It is made possible by the fact that the spots contain no mass, thereby not violating the fundamental laws of Special Relativity.

An image of NGC 2261 (aka. Hubble's Variable Nebula) by the Hubble space telescope. Credit: HST/NASA/JPL.
An image of NGC 2261 (aka. Hubble’s Variable Nebula) by the Hubble space telescope. Image Credit: HST/NASA/JPL.

Another example occurs regularly in nature, where beams of light from a pulsar sweep across clouds of space-borne dust, creating a spherical shell of light and radiation that expands faster than c when it intersects a surface. Much the same is true of fast-moving shadows, where the speed can be much faster and not restricted to the speed of light if the surface is angular.

At a meeting of the American Astronomical Society in Seattle, Washington earlier this month, Nemiroff shared how these effects could be used to study the universe.

“Photonic booms happen around us quite frequently,” said Nemiroff in a press release, “but they are always too brief to notice. Out in the cosmos they last long enough to notice — but nobody has thought to look for them!”

Superluminal sweeps, he claims, could be used to reveal information on the 3-dimensional geometry and distance of stellar bodies like nearby planets, passing asteroids, and distant objects illuminated by pulsars. The key is finding ways to generate them or observe them accurately.

For the purposes of his study, Nemiroff considered two example scenarios. The first involved a beam being swept across a scattering spherical object – i.e. spots of light moving across the Moon and pulsar companions. In the second, the beam is swept across a “scattering planar wall or linear filament” – in this case, Hubble’s Variable Nebula.

Artist view of an asteroid (with companion) passing near Earth. Credit: P. Carril / ESA
Photonic booms caused by laser sweeps could offer a new imaging technique for mapping out passing asteroids. Credit: P. Carril / ESA

In the former case, asteroids could be mapped out in detail using a laser beam and a telescope equipped with a high-speed camera. The laser could be swept across the surface thousands of times a second and the flashes recorded. In the latter, shadows are observed passing between the bright star R Monocerotis and reflecting dust, at speeds so great that they create photonic booms that are visible for days or weeks.

This sort of imaging technique is fundamentally different from direct observations (which relies on lens photography), radar, and conventional lidar. It is also distinct from Cherenkov radiation – electromagnetic radiation emitted when charged particles pass through a medium at a speed greater than the speed of light in that medium. A case in point is the blue glow emitted by an underwater nuclear reactor.

Combined with the other approaches, it could allow scientists to gain a more complete picture of objects in our Solar System, and even distant cosmological bodies.

Nemiroff’s study accepted for publication by the Publications of the Astronomical Society of Australia, with a preliminary version available online at arXiv Astrophysics

Further reading:
Michigan Tech press release
Robert Nemiroff/Michigan Tech

How We Know Gravity is Not (Just) a Force

When  we think of gravity, we typically think of it as a force between masses.  When you step on a scale, for example, the number on the scale represents the pull of the Earth’s gravity on your mass, giving you weight.  It is easy to imagine the gravitational force of the Sun holding the planets in their orbits, or the gravitational pull of a black hole.  Forces are easy to understand as pushes and pulls.

But we now understand that gravity as a force is only part of a more complex phenomenon described the theory of general relativity.  While general relativity is an elegant theory, it’s a radical departure from the idea of gravity as a force.  As Carl Sagan once said, “Extraordinary claims require extraordinary evidence,” and Einstein’s theory is a very extraordinary claim.  But it turns out there are several extraordinary experiments that confirm the curvature of space and time.

The key to general relativity lies in the fact that everything in a gravitational field falls at the same rate.  Stand on the Moon and drop a hammer and a feather, and they will hit the surface at the same time.  The same is true for any object regardless of its mass or physical makeup, and this is known as the equivalence principle.

Since everything falls in the same way regardless of its mass, it means that without some external point of reference, a free-floating observer far from gravitational sources and a free-falling observer in the gravitational field of a massive body each have the same experience. For example, astronauts in the space station look as if they are floating without gravity.  Actually, the gravitational pull of the Earth on the space station is nearly as strong as it is at the surface.  The difference is that the space station (and everything in it) is falling.  The space station is in orbit, which means it is literally falling around the Earth.

The International Space Station orbiting Earth. Credit: NASA
The International Space Station orbiting Earth. Credit: NASA

This equivalence between floating and falling is what Einstein used to develop his theory.  In general relativity, gravity is not a force between masses.  Instead gravity is an effect of the warping of space and time in the presence of mass.  Without a force acting upon it, an object will move in a straight line.  If you draw a line on a sheet of paper, and then twist or bend the paper, the line will no longer appear straight.  In the same way, the straight path of an object is bent when space and time is bent.  This explains why all objects fall at the same rate.  The gravity warps spacetime in a particular way, so the straight paths of all objects are bent in the same way near the Earth.

So what kind of experiment could possibly prove that gravity is warped spacetime?  One stems from the fact that light can be deflected by a nearby mass.  It is often argued that since light has no mass, it shouldn’t be deflected by the gravitational force of a body.  This isn’t quite correct. Since light has energy, and by special relativity mass and energy are equivalent, Newton’s gravitational theory predicts that light would be deflected slightly by a nearby mass.  The difference is that general relativity predicts it will be deflected twice as much.

Description of Eddington's experiment from the Illustrated London News (1919).
Description of Eddington’s experiment from the Illustrated London News (1919).

The effect was first observed by Arthur Eddington in 1919.  Eddington traveled to the island of Principe off the coast of West Africa to photograph a total eclipse. He had taken photos of the same region of the sky sometime earlier. By comparing the eclipse photos and the earlier photos of the same sky, Eddington was able to show the apparent position of stars shifted when the Sun was near.  The amount of deflection agreed with Einstein, and not Newton.  Since then we’ve seen a similar effect where the light of distant quasars and galaxies are deflected by closer masses.  It is often referred to as gravitational lensing, and it has been used to measure the masses of galaxies, and even see the effects of dark matter.

Another piece of evidence is known as the time-delay experiment.  The mass of the Sun warps space near it, therefore light passing near the Sun is doesn’t travel in a perfectly straight line.  Instead it travels along a slightly curved path that is a bit longer.  This means light from a planet on the other side of the solar system from Earth reaches us a tiny bit later than we would otherwise expect.  The first measurement of this time delay was in the late 1960s by Irwin Shapiro.  Radio signals were bounced off Venus from Earth when the two planets were almost on opposite sides of the sun. The measured delay of the signals’ round trip was about 200 microseconds, just as predicted by general relativity.  This effect is now known as the Shapiro time delay, and it means the average speed of light (as determined by the travel time) is slightly slower than the (always constant) instantaneous speed of light.

A third effect is gravitational waves.  If stars warp space around them, then the motion of stars in a binary system should create ripples in spacetime, similar to the way swirling your finger in water can create ripples on the water’s surface.  As the gravity waves radiate away from the stars, they take away some of the energy from the binary system. This means that the two stars gradually move closer together, an effect known as inspiralling. As the two stars inspiral, their orbital period gets shorter because their orbits are getting smaller.

Decay of pulsar period compared to prediction (dashed curve).  Data from Hulse and Taylor, Plotted by the author.
Decay of pulsar period compared to prediction (dashed curve). Data from Hulse and Taylor, Plotted by the author.

For regular binary stars this effect is so small that we can’t observe it. However in 1974 two astronomers (Hulse and Taylor) discovered an interesting pulsar. Pulsars are rapidly rotating neutron stars that happen to radiate radio pulses in our direction. The pulse rate of pulsars are typically very, very regular. Hulse and Taylor noticed that this particular pulsar’s rate would speed up slightly then slow down slightly at a regular rate. They showed that this variation was due to the motion of the pulsar as it orbited a star. They were able to determine the orbital motion of the pulsar very precisely, calculating its orbital period to within a fraction of a second. As they observed their pulsar over the years, they noticed its orbital period was gradually getting shorter. The pulsar is inspiralling due to the radiation of gravity waves, just as predicted.

Illustration of Gravity Probe B.  Credit: Gravity Probe B Team, Stanford, NASA
Illustration of Gravity Probe B. Credit: Gravity Probe B Team, Stanford, NASA

Finally there is an effect known as frame dragging.  We have seen this effect near Earth itself.  Because the Earth is rotating, it not only curves spacetime by its mass, it twists spacetime around it due to its rotation.  This twisting of spacetime is known as frame dragging.  The effect is not very big near the Earth, but it can be measured through the Lense-Thirring effect.  Basically you put a spherical gyroscope in orbit, and see if its axis of rotation changes.  If there is no frame dragging, then the orientation of the gyroscope shouldn’t change.  If there is frame dragging, then the spiral twist of space and time will cause the gyroscope to precess, and its orientation will slowly change over time.

results_graph-lg
Gravity Probe B results. Credit: Gravity Probe B team, NASA.

We’ve actually done this experiment with a satellite known as Gravity Probe B, and you can see the results in the figure here.  As you can see, they agree very well.

Each of these experiments show that gravity is not simply a force between masses.  Gravity is instead an effect of space and time.  Gravity is built into the very shape of the universe.

Think on that the next time you step onto a scale.

Here’s What A Spacecraft Looks Like Burning Up (Plus Correction of Past Article)

Flame and fireworks. That’s what the Automated Transfer Vehicle Albert Einstein appeared to astronauts to be like as it made a planned dive into Earth’s atmosphere Nov. 2. The European Space Agency ship spent five months in space, boosting the International Space Station’s altitude several times and bringing a record haul of stuff for the astronauts on board the station to use.

According to the European Space Agency, this is the first view of an ATV re-entry that astronauts have seen since Jules Verne, the first, was burned up in 2008. Controllers moved the spacecraft into view of the Expedition 37 crew to analyze the physics of breakup.

Also, yesterday you may have seen an article concerning a picture a photographer snapped of the ATV burning up on Earth. After publishing it, we then realized we were in error with that information. But it turns out the photographer actually DID capture the ATV-4 ina subsequent image. We’ve now updated the article a second time. Senior Editor Nancy Atkinson writes:

Here’s a story that we’ve updated a couple of times, and now it ultimately has a happy ending. We originally posted a picture from Oliver Broadie who thought he captured an image of the ATV-4 Albert Einstein right before it burned up in the atmosphere. That image, see below, was ultimately determined to be of the International Space Station and not the ATV-4, so yesterday we pulled the image and explained why. But now, thanks to a great discussion between the photographer and satellite tracker Marco Langbroek (see it in the comment section), they have determined that Oliver actually did capture the ATV-4 in a subsequent image taken about 4 minutes later. Thanks to both Ollie and Marco for analyzing the timing and images. Also, we were in error for saying that the image showed the ATV-4 burning up in the atmosphere. That was my mistake (Nancy).

More orbital pictures of the ATV burning up are available in this ESA Flickr set.

Automated Transfer Vehicle Albert Einstein burning up in the atmosphere at 12:04 GMT on Nov. 2, 2013. Picture snapped from the International Space Station. Credit: ESA/NASA
Automated Transfer Vehicle Albert Einstein burning up in the atmosphere at 12:04 GMT on Nov. 2, 2013. Picture snapped from the International Space Station. Credit: ESA/NASA

Photographer Catches ATV-4’s Fiery Plunge Through the Atmosphere

UPDATE: Editor’s note: Here’s a story that we’ve updated a couple of times, and now it ultimately has a happy ending. We originally posted a picture from Oliver Broadie who thought he captured an image of the ATV-4 Albert Einstein right before it burned up in the atmosphere. That image, see below, was ultimately determined to be of the International Space Station and not the ATV-4, so yesterday we pulled the image and explained why. But now, thanks to a great discussion between the photographer and satellite tracker Marco Langbroek (see it in the comment section), they have determined that Oliver actually did capture the ATV-4 in a subsequent image taken about 4 minutes later. Thanks to both Ollie and Marco for analyzing the timing and images. Also, we were in error for saying that the image showed the ATV-4 burning up in the atmosphere. That was my mistake (Nancy).

And you can now actually see images of ATV-4’s fiery plunge taken by the ISS astronauts here — Nancy Atkinson, Senior Editor.

Universe Today reader Oliver Broadie captured this shot of the International Space Station, shot from Sukhothai, Thailand. Just a few minutes later, the ATV-4 flew by at a lower altitude. Credit: Oliver Broadie
Universe Today reader Oliver Broadie captured this shot of the International Space Station, shot from Sukhothai, Thailand. Just a few minutes later, the ATV-4 flew by at a lower altitude. Credit: Oliver Broadie

Each Automated Transfer Vehicle series ferries cargo to the International Space Station, stays attached for a few months to do routine boosts to the station’s altitude, then leaves with a haul of trash to burn up in Earth’s atmosphere.

ATV-4 Albert Einstein backs away from the space station after five months in space. It burned up in the Earth's atmosphere Nov. 2, 2013. Credit: ESA/NASA
ATV-4 Albert Einstein backs away from the space station after five months in space. It burned up in the Earth’s atmosphere Nov. 2, 2013. Credit: ESA/NASA

Albert Einstein carried a record 5,467 pounds (2,480 kg) of cargo for its type of vehicle and also brought away the most garbage of the series of vehicles. It did six reboosts of the ISS’ altitude and among its precious cargo was a GPS antenna for Japan’s Kibo laboratory as well as a water pump for Europe’s Columbus laboratory, according to the European Space Agency.

The cargo ship undocked from the space station on Oct. 28 after five months in space. It burned up Nov. 2 at 12:04 GMT within sight of the astronauts. The next of the series, Georges Lemaitre, is in French Guiana for a launch aboard an Ariane 5 rocket that will take place in June 2014.

The ATVs are just one of many space trucks that visit the International Space Station. Check out this recent article on cargo ships past and present to see other ones that ferry stuff into space.

First-Ever Video of an ATV Vehicle Into Orbit!

Yesterday, June 5, the European Space Agency launched their ATV-4 Albert Einstein cargo vessel from their spaceport in French Guiana. Liftoff occurred at 5:52 p.m. EDT (2152 GMT), and in addition to over 7 tons of supplies for the ISS a special payload was also included: the DLR-developed STEREX experiment, which has four cameras attached to the Ariane 5ES rocket providing a continuous 3D view of the mission, from liftoff to separation to orbit and, eventually, docking to the Station on June 15.

The dramatic video above is the first-ever of an ATV vehicle going into free-flight orbit — check it out!

“The highlight of the STEREX deployment will be observing the settling of ATV-4 in orbit. STEREX for this event will include three-dimensional video sequences to study the dynamic behavior of the spacecraft during the separation phase. This opens up for the ATV project engineers an entirely new way to monitor the success of their work and also to gain important new experiences for the future.”DLR blog (translated)

If you look along the horizon at around 5:20, you can make out the plume from the launch.

At 20,190 kg (44, 511 lbs) ATV Albert Einstein is the heaviest spacecraft ever launched by Ariane. Read more here.

(HT to Daniel Scuka at ESA.)

Book Review: Time Reborn

Time Reborn: From the Crisis of Physics to the Future of the Universe is one of those books intended to provoke discussion. Right from the first pages, author Lee Smolin — a Canadian theoretical physicist who also teaches philosophy — puts forward a position: time is real, and not an illusion of the human experience (as other physicists try to argue).

Smolin, in fact, uses that concept of time as a basis for human free will. If time is real, he writes, this is the result: “Novelty is real. We can create, with our imagination, outcomes not computable from knowledge of the present.”

Physics as philosophy. A powerful statement to make in the opening parts of the book.  The only challenge is understanding the rest of it.

Smolin advertises his book as open to the general reader who has no background in physics or mathematics, promising that there aren’t even equations to worry about. He also breaks up the involved explanations with wry observations of fatherhood, or by bringing up anecdotes from his past.

Artist concept of Gravity Probe B orbiting the Earth to measure space-time, a four-dimensional description of the universe including height, width, length, and time.  Image credit: NASA
Artist concept of Gravity Probe B orbiting the Earth to measure space-time, a four-dimensional description of the universe including height, width, length, and time. Image credit: NASA

It works, but you need to be patient. Theoretical physics is so far outside of the everyday that at times it took me (with education focusing on journalism and space policy, admittedly) two or three readings of the same passage to understand what was going on.

But as I took my time, a whole world opened up to me.

I found myself understanding more about Einstein’s special and general relativity than I did in readings during high school and university. The book also made me think differently about cosmology (the nature of the universe), especially in relation to biological laws.

While the book is enjoyable, it is probably best not to read it in isolation as it is a positional one — a book that gathers information scientifically and analytically, to be sure, but one that does not have a neutral point of view to the conclusions.

We’d recommend picking up other books such as the classic A Brief History of Time (by physicist Stephen Hawking) to learn more about the universe, and how other scientists see time work.

How Fast Does Light Travel

How Does Light Travel?

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One of the most interesting constants and challenges in physics is the speed of light. The speed of light has a lot of important implications for physics from General Relativity to the search for a unified theory. Physicists and aeronautics engineers designing future space craft see it as the last great barrier to practical interstellar travel. So how fast does light travel?

We know that light has a finite speed and it travels at the speed of 300,000 kilometers per second. This a great distance to travel. On earth this speed is almost instantaneous. However we now know that its limits can be determined on the larger scale of space. For example it takes about 8.3 minutes for light from the Sun to reach the Earth. To reach the nearest star to the Solar System it takes about 3 to 4 years. This limitation of light is what we call the light speed barrier.

In the early days of science the argument of whether the speed of light was instantaneous or not was a major source of debate. As early as the Greeks, there were proponents that argued for both a finite and infinite speed for light. There were also writings during the 11th century by Arab philosophers that proposed that the speed of light depended on the medium it traveled through. It would not be until the 20th century that physicists such as Planck and Einstein would discover the actual speed of light and light’s properties.

As mentioned earlier the speed of light does change. It is actually only 300,000 km in a vacuum. The speed varies slightly in air and other mediums depending on transparency and refractive quality. The speed of light however tends to still be considerably faster than that of others waves such as sound waves. It was also discovered that the speed of light applies to all forms of electromagnetic radiation not just visible light. Physicists are also proposing that the speed of light also applies to gravity waves.

Understanding of the speed of light has led to some interesting theories in physics. Many of them can be found in Einstein’s theories of General Relativity and Special relativity. First off, only massless particles such as photons can naturally reach the speed of light otherwise it would take essentially infinite energy to reach this speed. However objects with mass can theoretically achieve a significant percentage of light speed. It is also proposed that even if light speed could be reach it would produce certain side effects. One is time dilation where while traveling at light speed a Rip Van Winkle effect occurs where years would pass by for observers while a person traveling at light speed would only experience moments of time in the same perceived period. It has also been theorized exceed light speed would lead to time travel.

We have written many articles about light for Universe Today. Here’s an article about gravity moving at the speed of light, and here’s an article about galaxies moving faster than the speed of light.

If you’d like more info on the speed of light, check out The Speed of Light According to Einstein, and here’s a link to The Speed of Light on a Rocket by NASA.

We’ve also recorded a Question Show about the Speed of Light. Check it out!

Sources:
Wikipedia: Speed of Light
Wikipedia: Time Travel
Newton Ask a Scientist!
University of Illinois