How Do We Track Photons Through Space?

When a particle moves through spacetime, how do we know it is the same particle and not some excitation that is passed from place to place?
Wispy tendrils of hot dust and gas glow brightly in this ultraviolet image of the Cygnus Loop Nebula, taken by NASA’s Galaxy Evolution Explorer. The nebula lies about 1,500 light-years away, and is a supernova remnant, left over from a massive stellar explosion that occurred 5,000-8,000 years ago. Image credit: NASA/JPL-Caltech

Wispy tendrils of hot dust and gas glow brightly in this ultraviolet image of the Cygnus Loop Nebula, taken by NASA’s Galaxy Evolution Explorer. The nebula lies about 1,500 light-years away, and is a supernova remnant, left over from a massive stellar explosion that occurred 5,000-8,000 years ago. Image credit: NASA/JPL-Caltech

Originally posted on Forbes!

We don’t! This is a really interesting feature of our universe, and it comes from the observation that all subatomic particles are described by a few key properties, but are otherwise completely and utterly identical. Electrons appear to be identical to all other electrons. All photons (if they carry the same energy within them) are identical to all other photons of that energy. Protons are identical to other protons, and neutrons are identical to other neutrons.  All of these particles are distinguished from each other by their mass, electric charge, and a property called their spin. However, each and every single electron in the Universe has the exact same mass, electric charge, and spin. There are no other measurements we can do to distinguish a given electron from another.

We could think of it along these lines: let’s say I give you a ping pong ball, and tell you that this one is special because it’s yours. But then we throw that ping pong ball into a bag full of other balls which look just like yours and mix them up. It’d be quite difficult to tell if the one I pull out of the bag next is the one I initially gave you or another one. If that newly drawn ball is identical in all measurable ways to the original one I told you was yours, there’s really no way to tell if it’s the one I originally handed to you or not.

Photons have one extra parameter that can distinguish them from each other and it’s the amount of energy they’re carrying. This energy corresponds to the color of the light - the more energy, the further to the blue the light appears, and the less energy, the harder to the red it falls. I can distinguish a blue photon from a red one as it hits my camera because of this difference in energy, but the mass, electric charge, and spin of those two photons are the same.

If the photons have the same energy when they arrive, then I’ve run out of ways to distinguish them.

Flaring, active regions of our sun are highlighted in this new image combining observations from several telescopes. High-energy X-rays from NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) are shown in blue; low-energy X-rays from Japan's Hinode spacecraft are green; and extreme ultraviolet light from NASA's Solar Dynamics Observatory (SDO) is yellow and red. Image credit: NASA/JPL-Caltech/GSFC/JAXA

Flaring, active regions of our sun are highlighted in this new image combining observations from several telescopes. High-energy X-rays from NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) are shown in blue; low-energy X-rays from Japan's Hinode spacecraft are green; and extreme ultraviolet light from NASA's Solar Dynamics Observatory (SDO) is yellow and red. Image credit: NASA/JPL-Caltech/GSFC/JAXA

So if I dump a bunch of photons into my metaphorical bag, and they all come out again, there’s no way for me to tell if my favorite photon came out first or last. The closest astrophysical approximation to this simple setup is light which strikes the surface of the Sun and is then absorbed. That photon is now mixing with a huge number of other photons created within the depths of the Sun, and I have no way of flagging that particular photon to distinguish it from the flood of other, identical photons which are streaming outwards away from the Sun.

The energy that a photon carries isn’t a fundamental property of the photon in the way that its electric charge (which is neutral) and its spin are fundamental properties. Fundamental properties cannot be changed, no matter what happens to these photons in the course of bouncing around the Universe. So the energy of a photon, not being a fundamental property, canbe changed. And this energy often is changed, making our attempt to keep track of individual photons even more difficult. The photons that stream from the Sun and onto the surface of the Earth deposit some of their energy into the matter of the Earth, heating up the ground. That heating process depletes the energy remaining in the photon, and so the photon which reflects away has changed the amount of energy that it carries with it. So, if I see photons streaming into a region of space where they must interact with other objects, the identities of individual photons are even more scrambled than they would have been while they were streaming freely through space.

Your idea of energy excitations passing from place to place is precisely the right one for fundamental particles - nothing we can measure will tell me which electron is my favorite.

 

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Can A Star Ever Turn Its Spin Backwards?

Can a star reverse its rotational direction during some time in their life, and if so, how would it affect any planets around it?
This artist’s impression of the water snowline around the young star V883 Orionis, as detected with ALMA. Image credit: A. Angelich (NRAO/AUI/NSF)/ALMA (ESO/NAOJ/NRAO)

This artist’s impression of the water snowline around the young star V883 Orionis, as detected with ALMA. Image credit: A. Angelich (NRAO/AUI/NSF)/ALMA (ESO/NAOJ/NRAO)

Originally posted on Forbes! 

The stars in the night sky all have their preferred direction of rotation, which depends directly on the exact way that the cloud of gas and dust that the star formed out of collapsed. If there was slightly more random motion in a clockwise or a counterclockwise direction, as the cloud of gas collapsed, the star would have magnified that hint of rotation, spinning up the same way that a figure-skater does, pulling in their arms and legs.

Once spun up this way, another piece of fundamental physics comes into play - inertia. Inertia tells us that objects in motion tend to stay in motion unless there’s something else that’s causing that object to slow down. On Earth, that something else can come in many forms - we have the mass of the Earth, whose gravity will pull objects down towards the surface, an atmosphere to move through, which will slow objects moving through it, or quite simply mountains and buildings which objects can bounce off of and away from.

If you’re in space, these sorts of Earthly obstacles which can serve to stop a moving object aren’t around. There are many fewer things which could serve to stop an object’s motion - without a thick atmosphere to move through, the planets and our spacecraft continue at their current speeds without any impediments. The most common method of slowing down (or speeding up) an object in space is by traveling near another large object (ones that are of a similar mass to yourself are the most effective) and letting the force of gravity alter your path.

These requirements for an external force hold both for motion as we normally think about it (a forward or sideways motion) and for spin. So if we think about spinning a bicycle tire, that wheel will continue to spin until the forces of friction in the axle (primarily) will slow it down, or until you clamp down on the brakes. An object which is spinning in space has no friction-containing axle around which to spin, and so if it’s isolated, without any external objects which can act as a braking force, that object should continue to spin as it is, ad infinitum. This is basically the situation that stars find themselves in. Stars do not reverse their spins as a standard part of their lifetimes.

Planet formation begins with a brilliant young star at the center of what’s called a protoplanetary disk. Collisions within the disk form rocks that act as planetary building blocks. They settle into orbit around the star, creating gaps in the disk. Image credit: NASA's Goddard Space Flight Center Video and images courtesy of NASA/JPL-Caltech

Planet formation begins with a brilliant young star at the center of what’s called a protoplanetary disk. Collisions within the disk form rocks that act as planetary building blocks. They settle into orbit around the star, creating gaps in the disk. Image credit: NASA's Goddard Space Flight Center Video and images courtesy of NASA/JPL-Caltech

A reversal in the rotation of a star is extremely difficult to accomplish without something external to the star punching it backwards in the other direction, to slow down each rotating particle that makes up the star. If you could, using magic, reverse the rotation of our own star, without changing anything else, the planets surrounding our Sun wouldn’t be influenced at all - the orbits of the planets are determined by the gravitational pull of the Sun, which hasn’t changed if we haven’t changed its mass, plus the planets’ own velocities.

What sort of objects could act as a brake in space? The is easiest is always a good old large-scale collision. This is how we think the Earth formed the Moon, how Uranus got tipped onto its side, and Venus got tipped completely upside down. So if we wanted to reverse the spin of the Sun, we’d have to hit it with something pretty catastrophic in order to both stop the rotation of the gas & plasma which currently makes up the Sun, and reverse the direction of the spin. Any impact on that scale would definitely impact the planets. If it was another star that hit our own, we’d have gravitational and temperature-related chaos even before the impact, setting aside whatever cataclysm of energy would be unleashed during the collision.

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Why Must Cassini Crash Into Saturn?

Why does Cassini have to crash into Saturn?
In this screenshot from the short animated film Cassini's Grand Finale, the spacecraft is shown breaking apart after entering Saturn's atmosphere. The planned end of Cassini will occur on Sept. 15, 2017. Image credit: NASA/JPL-Caltech

In this screenshot from the short animated film Cassini's Grand Finale, the spacecraft is shown breaking apart after entering Saturn's atmosphere. The planned end of Cassini will occur on Sept. 15, 2017. Image credit: NASA/JPL-Caltech

Originally posted on Forbes!

Cassini has served us well over the past twenty years, giving us incredible images of Saturn since 2004; thirteen years of imagery from its vantage in the outer solar system. With any lengthy mission, it is sad to see a spacecraft reach its end, and Cassini has been exceptional. Cassini has truly changed the way we see Saturn and its moons. With such an illustrious explorer, it’s natural to wonder if we really had to say goodbye.

We must - there’s no escaping the finite nature of all of our robotic explorers of the solar system. 

Cassini, like all orbiting spacecraft far from Earth, has limited fuel reserves, on both of its two fuel sources. The first is the power generator, which replaces the usefulness of solar panels in the outer solar system, where sunlight is too weak. Onboard Cassini, this is a piece of plutonium, which generates heat, and is converted into electrical power. This is the source of the power to operate Cassini’s instruments, and has gradually decreased in power over time, but not enough to incapacitate Cassini. But then there’s the fuel for the thrusters. The thrusters are responsible for changes in Cassini’s orbit, both to direct the craft towards a moon for a gravitational assist, or to correct its path afterwards, so that the craft continues onwards in exactly the right position. These thrusters have been 90 percent empty since 2009, and Cassini has been extremely economical with the remaining propellant for its thrusters for a long time.  Unlike the International Space Station, which can be refueled by the shuttling crafts between the Earth and low orbit, we can’t refuel Cassini.

This graphic shows Cassini's final plunge toward Saturn, with tick marks representing time intervals of 2 minutes, leading to the spacecraft's entry into the atmosphere. Image credit: NASA/JPL-Caltech

This graphic shows Cassini's final plunge toward Saturn, with tick marks representing time intervals of 2 minutes, leading to the spacecraft's entry into the atmosphere. Image credit: NASA/JPL-Caltech

We can’t afford to run Cassini dry, either. It’s true that right now the spacecraft still has fuel, and we could have opted to let Cassini continue to orbit, which might have let us squeeze a few more images out of the spacecraft, but by doing that, we consign Cassini to an end-of-mission which leaves it wandering derelict around Saturn, completely uncontrolled, once its fuel reserves are exhausted.

An uncontrolled Cassini is an unacceptable end. It’s unacceptable because we cannot control at what point it might crash into one of Saturn’s many moons, and Saturn has some extremely special places that we cannot risk even the slightest chance of contaminating.  Enceladus, a moon of Saturn, was discovered (by Cassini) to have a global ocean of warm, salty water underneath its icy crust.

Places in our solar system with liquid water fall under the strongest planetary protection rules.  These are rare and delicate places within our solar system, and we have agreed as an international community that unless our robotic explorers have met the absolute highest standards for cleanliness, sterilization, and testing of that sterilization, we should stay away from the watery places. The goal is simply this: don’t contaminate worlds that might have their own unique chemistry (and, perhaps, simple life) with the ultimate invasive species from Earth.

Saturn's icy moon Enceladus hovers above Saturn's exquisite rings in this color view from Cassini. The rings, made of nearly pure water ice, have also become somewhat contaminated by meteoritic dust during their history, which may span several hundred million years. Enceladus shares the rings' nearly pure water ice composition. Image credit: NASA/JPL

Saturn's icy moon Enceladus hovers above Saturn's exquisite rings in this color view from Cassini. The rings, made of nearly pure water ice, have also become somewhat contaminated by meteoritic dust during their history, which may span several hundred million years. Enceladus shares the rings' nearly pure water ice composition. Image creditNASA/JPL

Cassini was sterilized before launch, but not to the specifications required for watery worlds. It is an orbiter, and it’s not designed for interactions with the atmospheres or surfaces of planets or their moons. Because landing on a watery moon wasn’t part of its mission, Cassini never needed to go through the gauntlet of extra sterilization. And while Cassini has been in the vacuum of space for 20 years, bacteria from our home planet have been shown to survive for years in outer space.

As we can’t say with confidence that Cassini has no microbial hitchhikers, we need to place Cassini carefully, while we still have enough propellant to direct it accurately. For Saturn's moons, the safest place for Cassini is Saturn itself. Saturn is not hospitable to life, and its atmosphere will destroy Cassini the way that Earth’s atmosphere destroys a meteorite. Much of Cassini will vaporize, becoming one with the clouds of Saturn, and anything that survives will melt in the heat of the deeper planetary layers, sinking gradually towards the center of Saturn.

It is through the skill and experience of Cassini’s flight crew and science team that we have been able to undertake the Grand Finale set of orbits before the spacecraft has its final encounter with the atmosphere of Saturn. It is a gift to the scientists who have studied and will study Saturn in past and future years - a wealth of new data has flooded through Cassini’s transmissions home. And for those of us who do not research Saturn, it has been a gift of tremendous imagery of the sixth planet from the Sun.

NASA's Cassini spacecraft gazed toward the northern hemisphere of Saturn to spy subtle, multi-hued bands in the clouds there. Image credit: NASA/JPL

NASA's Cassini spacecraft gazed toward the northern hemisphere of Saturn to spy subtle, multi-hued bands in the clouds there. Image credit: NASA/JPL

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Why Does The Moon Look Upside Down From Australia?

Why does the Moon look upside down from Australia?
The Moon seen from the southern hemisphere, taken on the 25th of November 2012, from Montevideo, Uruguay. Image credit: Fernando da Rosa, CC BY A-SA 3.0

The Moon seen from the southern hemisphere, taken on the 25th of November 2012, from Montevideo, Uruguay. Image credit: Fernando da Rosa, CC BY A-SA 3.0

Originally posted on Forbes!

Those of us who live in the Northern Hemisphere of our planet are used to a very specific view of the Moon, and, if you never travel outside of the Northern Hemisphere, journeying only to Europe, North America, Asia or the Arctic expanses, that view of the Moon would never change by very much.

However, once you move to the Southern Hemisphere, visiting South America, Africa, Australia or New Zealand, something will indeed seem off about the Moon. It’s upside down in the sky, relative to what you’d be used to in the Northern Hemisphere. Likewise, if you’re used to a Southern Hemisphere sky, moving to the Northern Hemisphere will turn the Moon upside down relative to what you’re used to.

Many of the portraits of the Moon are oriented in the way you’d see them from the Northern Hemisphere. There’s nothing fundamental about this orientation relative to the Southern Hemisphere orientation, but we’ve designated North as “up” for long enough that that convention has expanded outwards to the whole solar system. With that convention, it makes sense to display the Moon “right-side up,” with the view from the Northern half of the planet.

Full Moon photograph taken 10-22-2010 from Madison, Alabama, USA. Photographed with a Celestron 9.25 Schmidt-Cassegrain telescope. Image credit: Gregory H. Revera, CC BY A-SA 3.0

Full Moon photograph taken 10-22-2010 from Madison, Alabama, USA. Photographed with a Celestron 9.25 Schmidt-Cassegrain telescope. Image credit: Gregory H. Revera, CC BY A-SA 3.0

Why does the Moon look upside down from Australia? It’s because we’re on a spherical planet. If I stand at the North Pole, with my head “up,” and have a friend stand on the South Pole, with their head “up,” relative to the ground, our two heads are pointed in exactly opposite directions. If we both look at the Moon, then I see a Moon with dark Mare stretching along the “top” of the Moon, and a bright region at the bottom. At the South Pole, to a person whose head is pointed in the other direction, the Mare go along the bottom edge of the Moon, with the brighter region stretching across the top. If I were to move between the North and South poles, I would watch the Moon appear to rotate in the sky, as my perspective of “up” changes with the curvature of the Earth.  If I, on the North Pole, wanted to replicate my South Pole friend’s view onto the sky, I should do a perfect handstand, mimicking manually what the curve of the Earth has done more naturally. Obviously, this method of replicating the South Pole’s view isn’t perfect, because things that are directly overhead on the South Pole are blocked from my view at the North Pole by the bulk of the Earth.

In a less extreme case, someone living at 45 degrees North of the Equator (exactly halfway between the North Pole and the Equator) and someone living at 45 degrees south of the Equator, (halfway between the South Pole and the Equator) both standing on the ground, have their heads both pointed “up” but at 90 degrees relative to each other. Since their North/South separation is still an up/down change, then if the two moongazers could swap places, they’d say the Moon had rotated by about 90 degrees. It’s exactly the same kind of perspective shift on the Moon that my friend and I, at the North and South poles, have when looking outward.

The great hunter Orion hangs above ESO’s Very Large Telescope (VLT), in this stunning, previously unseen, image. As the VLT is in the Southern Hemisphere, Orion is seen here head down, as if plunging towards the Chilean Atacama Desert. Image credit: ESO/Y. Beletsky 

The great hunter Orion hangs above ESO’s Very Large Telescope (VLT), in this stunning, previously unseen, image. As the VLT is in the Southern Hemisphere, Orion is seen here head down, as if plunging towards the Chilean Atacama Desert. Image credit: ESO/Y. Beletsky 

The Moon is probably the most dramatic example of this in the night sky, simply because we know it so well, but it’s not the only object that may appear odd in the Southern sky if you’re used to the Northern view. Constellations do the exact same thing. Some Northern constellations are not visible in the Southern skies, but Orion, one of the brightest and easiest-to-spot constellations in the Northern winter sky, is visible from both hemispheres. And just like the Moon’s change, Orion appears upside down, his head towards the ground instead of the rest of the stars overhead.

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