If Time Doesn't Exist For Photons, How Does Anything Happen To It?

Images showing the expansion of the light echo of V838 Monocerotis. Image credit: NASA, ESA, H.E. Bond (STScI) and The Hubble Heritage Team (STScI/AURA)

Images showing the expansion of the light echo of V838 Monocerotis. Image credit: NASA, ESA, H.E. Bond (STScI) and The Hubble Heritage Team (STScI/AURA)

Originally posted at Forbes!

The concept of photons running with stopped clocks is something that is pulled straight out of relativity; the faster you’re moving, the slower your onboard clocks are moving, and the closer to the speed of light you’re operating, the more sluggish they get. Once you reach the speed of light, your clock runs infinitely slow - for practical purposes, we can say that time doesn't flow for the photon. As with all things relativity, this isn’t an absolute statement- light still has a finite speed, and we can observe light taking fixed amounts of time to traverse large distances.

When light goes zipping around our Universe, it is physically moving through space at a speed of 186,000 miles every second.  But if you could affix a clock to it, an observer that’s not moving at the speed of light would not see the clock moving forwards the way their own clocks do. A hypothetical person moving at the speed of light wouldn’t notice anything weird with their clock, but what they might notice is that the Universe is full of things to smash into.

This artist's impression shows how photons from the early universe are deflected by the gravitational lensing effect of massive cosmic structures as they travel across the universe. Image credit: ESA

This artist's impression shows how photons from the early universe are deflected by the gravitational lensing effect of massive cosmic structures as they travel across the universe. Image credit: ESA

No matter how fast you’re going, if there’s something in front of you, and you can’t dodge it, you will hit it. This is as true for humans as it is for light, and light is even less capable of dodging an oncoming object than we humans are.  Light always travels in locally straight lines - the only way to bend light is to make a curve in the shape of space. A photon will then follow that curve, but there’s no onboard navigation.

Photons are effectively stuck playing the world’s most obnoxious game of bumper cars, continually bouncing from impact to impact. From our non-speedy perspective, the clocks on photons do not tick forward between impacts, so if the photon has the good fortune to get re-emitted by whatever it ran into, it will, from our viewpoint, instantaneously smash directly into something else without its onboard clock ticking onwards at all.

The photon may not get re-emitted by whatever it ran into, (this is one way to get rid of a photon). The energy of whatever it hit will increase, so the energy isn’t lost. However, if it hits something particularly cold, the object won’t be radiating much, and the photon’s energy will be a convenient donation.  More commonly, after some amount of time, a new photon will be produced, at a different energy level, carrying energy away from whatever the photon punched itself into earlier. That new photon has an equally short apparent flight until it smashes into something else.

It’s not the most glamorous of paths through the Universe, but a continual ricocheting from solid matter to solid matter is how photons in our Universe go about it.

The bright cloud is a reflection nebula known as [B77] 63, a cloud of interstellar gas that is reflecting light from the stars embedded within it. There are actually a number of bright stars within [B77] 63. Image credit: ESA

The bright cloud is a reflection nebula known as [B77] 63, a cloud of interstellar gas that is reflecting light from the stars embedded within it. There are actually a number of bright stars within [B77] 63. Image credit: ESA

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Does Light Lose Energy As It Travels The Universe?

As a wave of light travels through the universe, does it lose energy? For example, what is the wavelength of 450 nm (blue) light after traveling a trillion (1,000,000,000,000) km in the universe? Thank you very much.
The anisotropies of the Cosmic microwave background (CMB) as observed by Planck. The CMB is a snapshot of the oldest light in our Universe, imprinted on the sky when the Universe was just 380,000 years old. Image credit: ESA/Planck Collaboration, CC BY SA IGO

The anisotropies of the Cosmic microwave background (CMB) as observed by Planck. The CMB is a snapshot of the oldest light in our Universe, imprinted on the sky when the Universe was just 380,000 years old. Image credit: ESA/Planck Collaboration, CC BY SA IGO

Originally posted at Forbes!

It does! This loss of energy is usually officially termed a cosmological redshift, and it’s an interesting combination of the way that light moves through space, and the nature of our Universe’s expansion.

Light behaves both as a particle and as a wave. Depending on the situation, it can be easier to talk about photons the light particle, or light waves- for travelling the vast distances of space, either works. However, light’s energy is very tightly tied to one of its more wave-like properties - its wavelength. Wavelength measures the distances between peaks, and can be used to measure the distances between ocean waves, or it can be used to very precisely measure the color of light reaching your eyes or a camera. The shorter the wavelength of light, the bluer the color. The bluer the light, the more energy it has, and things like gamma rays and X-rays have more energy still than anything our eyes can detect.

This new false-colored image from NASA's Hubble, Chandra and Spitzer space telescopes shows a giant jet of particles that has been shot out from the vicinity of a type of supermassive black hole called a quasar. Different wavelengths of light are reflected in the different colors. Image credit: NASA/JPL-Caltech/Yale Univ.

This new false-colored image from NASA's Hubble, Chandra and Spitzer space telescopes shows a giant jet of particles that has been shot out from the vicinity of a type of supermassive black hole called a quasar. Different wavelengths of light are reflected in the different colors. Image credit: NASA/JPL-Caltech/Yale Univ.

So let’s imagine that we have a space three feet long, and a flexible spring, almost three feet long. To reach from one edge of our space to the other, we’ll have to stretch the spring a little, but the distance between the coils won’t be very large. If we started to extend the spring, and suddenly found that our space had doubled in size, we’d have to stretch the spring much further, and the distance between coils would be much greater.

This is fundamentally what happens to light, as it travels through an expanding universe. The universe as a whole is expanding, meaning that the space between many galaxies is increasing. As light travels away from a galaxy, the Universe is continually expanding, meaning that the distance the light needs to travel is continually increasing as well. As space stretches out underneath a beam of light, its wavelength increases, and its energy decreases. Measuring this loss of energy is one of the main ways that distance is now measured in the Universe. This metric works well because we have a good sense (from other measurements) of how fast the Universe has been expanding, and what the energy loss should be for light which began its journey at an earlier time.

This artist's concept puts solar system distances in perspective. The scale bar is in astronomical units, with each set distance beyond 1 AU representing 10 times the previous distance. Image credit: NASA/JPL-Caltech

This artist's concept puts solar system distances in perspective. The scale bar is in astronomical units, with each set distance beyond 1 AU representing 10 times the previous distance. Image credit: NASA/JPL-Caltech

However, a trillion kilometers, on an astronomical standard, is still relatively small. A trillion kilometers is roughly a tenth of a light year (about five weeks of light travel time). This distance is sufficiently small that a more useful unit is the astronomical unit (au) which measures the distance between the Earth and the Sun. A trillion kilometers is about 10,000 au. On the scale of our solar system, this would stretch from the Sun to not quite out into the Oort cloud. This distance would put you way past Pluto, which orbits our sun at around 40 au from the Sun, and well past any hypothetical Planet 9, which is supposed to hang out around 200 au from the Sun.

As distant as these measures are, this measures only our very closest cosmic neighborhood, and in this regime, space is not expanding. Everything within our Galaxy is bound gravitationally to each other much more tightly than the expansion of the Universe can pull apart. The expansion is not so rapid that it is able to shear the Galaxy apart. Light’s loss of energy really only comes into play at much larger scales, far beyond the nearest galaxies.

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When You Turn Off A Light, Where Does The Light Go?

In space, light will go on, and on, and on… In a windowless room, where does the light go when we switch it off?

Originally posted on Forbes!

Light is a pretty simple beast. In lieu of any interference, it will go on, and on, and on, as we see it doing in the vast, often empty, realms of interstellar and intergalactic space.

Space is a rather unique case, because in between massive objects, light is traveling through something very close to a pure vacuum. This vacuum environment it's traveling through means that there’s very little chance for the light to run into any kind of interference - it's relatively easy for the light to travel enormous distances without anything changing its path or blocking its way.

So what are the options if there is something in the way? Well, light functionally has two options: reflection or absorption. Reflection we’re quite familiar with, as it’s the physics behind seeing yourself in a mirror. This can happen anytime light hits a surface which is smooth, to its perspective. (The smoothness required depends on the wavelength of the light - optical light needs a smoother surface to reflect cleanly off of than radio waves do, which are much longer in wavelength.)

The other option is absorption. This is the process which makes rocks warm in the sun. The rocks are absorbing sunlight over time, and over that time, the energy collected into the rock will warm the surface. Any light can be absorbed, not just the infrared (heat) portion of sunlight. A terrible mirror could absorb enough light that your reflection is only a faint ghost of an image. Unless you’ve got an old filament bulb left in your lamps, you won’t notice an appreciable warming to any of your possessions, because most light bulbs nowadays are designed very specifically not to produce much heat. We can’t see it, and it’s a waste of energy to produce heat which doesn’t help us see the room.

These one-light-year-tall pillars of cold hydrogen and dust, imaged by the Hubble Space Telescope, are located in the Carina Nebula. This image of dust pillars in the Carina Nebula is a composite of 2005 observations taken of the region in hydrogen light (light emitted by hydrogen atoms) along with 2010 observations taken in oxygen light (light emitted by oxygen atoms), both times with Hubble's Advanced Camera for Surveys. The immense Carina Nebula is an estimated 7,500 light-years away in the southern constellation Carina. NASA, ESA, and the Hubble Heritage Project (STScI/AURA); Acknowledgment: M. Livio (STScI) and N. Smith (University of California, Berkeley)

These one-light-year-tall pillars of cold hydrogen and dust, imaged by the Hubble Space Telescope, are located in the Carina Nebula. This image of dust pillars in the Carina Nebula is a composite of 2005 observations taken of the region in hydrogen light (light emitted by hydrogen atoms) along with 2010 observations taken in oxygen light (light emitted by oxygen atoms), both times with Hubble's Advanced Camera for Surveys. The immense Carina Nebula is an estimated 7,500 light-years away in the southern constellation Carina. NASA, ESA, and the Hubble Heritage Project (STScI/AURA); Acknowledgment: M. Livio (STScI) and N. Smith (University of California, Berkeley)

These two options, absorption and reflection, work in tandem with each other, and most materials will do a little of both. Even your standard bathroom mirror absorbs a little of the light that hits it (typically about 10%), and few naturally occurring materials on Earth are perfect light absorbers. Some ultra-black materials are getting very close, but the only truly perfectly absorbing objects around so far are black body objects; an object heated until it glows. (An old filament light bulb would count.)

So, when considering what happens to the light from your light bulb when you switch it off, let’s consider what’s happening when we have the light on. Light is being continually produced by the bulb, which is streaming outwards through the air, mostly unperturbed by having to go through air instead of a vacuum. It will then hit every surface which faces the bulb, and some fraction of it will reflect in the direction of your eyeballs, which will absorb the light, and tell you how bright the room is, along with some information about the objects within the room.

NGC 1999 is an example of a reflection nebula. Like fog around a street lamp, a reflection nebula shines only because the light from an embedded source illuminates its dust; the nebula does not emit any visible light of its own. NGC 1999 lies close to the famous Orion Nebula, about 1,500 light-years from Earth, in a region of our Milky Way galaxy where new stars are being formed actively.  Image Credit: NASA and The Hubble Heritage Team (STScI)

NGC 1999 is an example of a reflection nebula. Like fog around a street lamp, a reflection nebula shines only because the light from an embedded source illuminates its dust; the nebula does not emit any visible light of its own. NGC 1999 lies close to the famous Orion Nebula, about 1,500 light-years from Earth, in a region of our Milky Way galaxy where new stars are being formed actively.  Image Credit: NASA and The Hubble Heritage Team (STScI)

The difference between this situation and switching the light off is simply that you’re no longer replacing the absorbed photons of light with new ones. The last rays of light that the light bulb produced will behave exactly as the rest of the light did: either absorbing into or reflecting off of the various surfaces in your room. The reflected light will bounce until it’s absorbed, but considering how fast the photon can traverse the room, and how few bounces it takes to absorb light, this loss of light is functionally instantaneous.

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How Far Could You Bounce A Laser Down A Hall Of Mirrors?

Hi! My question is if one has a ray of light to project at an unlimited amount of mirrors, how far could the ray of light be projected by transferring its reflection from each mirror?
Interior view of the Zürichsee-Schiffahrtsgesellschaft (ZSG) paddle steamship Stadt Zürich on Zürichsee (Lake Zürich) in Switzerland. Image credit: wikimedia user Roland zh, CC BY-SA 3.0

Interior view of the Zürichsee-Schiffahrtsgesellschaft (ZSG) paddle steamship Stadt Zürich on Zürichsee (Lake Zürich) in Switzerland. Image credit: wikimedia user Roland zh, CC BY-SA 3.0

Originally posted at Forbes!

All right, let’s tackle this one. To make this a little more straightforward, I’m going to assume you’re bouncing a laser beam down a hall of mirrors. In this case, your question is really how many bounces will the laser beam make before enough light is lost that no more bounces are possible.

Some of the answer will depend on how bright your light is to start with – if you lose 60% of a faint light, you’ll lose sight of that light much faster than 60% of a bright laser light. The other thing we need to be careful with is how much of the light needs to remain for us to count it as still going. We could push down to a single photon, but the human eye definitely does not take a single photon and tell the brain that there was a light. Even the best detectors on some of the biggest optical telescopes don’t have a guarantee of detecting every single photon – at best they capture around 95% of the total light, but that capture fraction drops at the very blue and very red ends to something more like a 50% capture rate.

But we can do the math anyways, making some assumptions along the way. If the mirrored hallway is made of your run-of-the-mill mirror material (basically what you have in your bathrooms), these mirrors reflect around 90% of the visible light that hits them. A second bounce will take 10% of the light that remains, and so on and so on. By the time you’ve bounced off of an 8th mirror, you’ve already lost more than half of the light that you started with. To drop down to 1% of the light you had at the start, you only need 45 mirrors.

Three green lasers are seen emanating from facilities at the Starfire Optical Range on Kirtland Air Force Base, New Mexico. Lasers and deformable optics are used here to eliminate or minimize optical distortions caused by the Earth’s atmosphere. Image Credit: US Air Force

Three green lasers are seen emanating from facilities at the Starfire Optical Range on Kirtland Air Force Base, New Mexico. Lasers and deformable optics are used here to eliminate or minimize optical distortions caused by the Earth’s atmosphere. Image Credit: US Air Force

So, let’s try the math with a laser pointer. According to this site, a 5 milli-Watt green laser pointer (which is the typical strength for a presentation style laser pointer) has a luminance of 5 x 10^12 candela per square meter. If that’s the brightness of the laser before it hits the first mirror, we should expect that by the time we hit the 9th mirror, the laser light is half as bright. This is the case, but half of 10^12 is still a pretty big number.

In fact, the human eye switches from color vision to night vision around a brightness of 0.001 candela per square meter. We’re going to have to bounce this light off of a bunch more mirrors before we drop the laser light down that faint. In fact, you’ll need 344 mirrors before the light is so faint that you’d have to have night-adjusted vision to see it.

To have that laser spot drop below even your night vision? 409 mirrors. After that 409th bounce, enough of the laser light would have been absorbed into each of those mirrors that nothing is left, as far as your eye is concerned, anyways.

Now, obviously, I’ve done some conservative math here – you can do better if you have a very high end mirror, or if you had a high powered laser, or had attached a camera more sensitive than your eye at the end of your hall of mirrors. In any of those cases you’d be able to tell that some light has still made it through after the 409th mirror. But inevitably, with an infinitely long hall of mirrors, at some point you will run out of photons, as some fraction of them will absorb into the mirror with every bounce.

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How Do Solar Sails Work?

I understand that a photon has no mass... so how can it be used to push a solar sail ? How can a “massless” thing transfer momentum?
A 20-meter solar sail and boom system, developed by ATK Space Systems of Goleta, Calif., is fully deployed during testing at NASA Glenn Research Center's Plum Brook facility in Sandusky, Ohio. Blue lights positioned beneath the system help illuminate the four triangular sail quadrants as they lie outstretched in Plum Brook's Space Power Facility -- the world's largest space environment simulation chamber. The sail material is supported by a series of coilable booms, which are extended via remote control from a central stowage container about the size of a suitcase, and is made of an aluminized, plastic-membrane material called CP-1. The material is produced under license by SRS Technologies of Huntsville, Ala. The deployment, part of a series of tests in April, is a critical milestone in the development of solar sail propulsion technology that could lead to more ambitious inner Solar System robotic exploration. Image credit: NASA

A 20-meter solar sail and boom system, developed by ATK Space Systems of Goleta, Calif., is fully deployed during testing at NASA Glenn Research Center's Plum Brook facility in Sandusky, Ohio. Blue lights positioned beneath the system help illuminate the four triangular sail quadrants as they lie outstretched in Plum Brook's Space Power Facility -- the world's largest space environment simulation chamber. The sail material is supported by a series of coilable booms, which are extended via remote control from a central stowage container about the size of a suitcase, and is made of an aluminized, plastic-membrane material called CP-1. The material is produced under license by SRS Technologies of Huntsville, Ala. The deployment, part of a series of tests in April, is a critical milestone in the development of solar sail propulsion technology that could lead to more ambitious inner Solar System robotic exploration. Image credit: NASA

Originally posted at Forbes!

This is not an easy thing to wrap one’s head around, and part of the reason it’s tricky is because light is a little special. Light manages to behave both like a particle and like a wave. In either case, you’re absolutely right that the particle of light has no mass.

To start to get a handle on how this works, let’s think about light as a wave, and ignore the photon-particle behavior. The way we break up the whole electromagnetic spectrum is by the amount of energy carried by that wave. The more energetic the wave is, the higher its frequency, and so dividing by frequency is just another way of slicing by energy levels. When these waves are absorbed by a surface, they will deposit the energy they carry in that surface. This is why we burn in sunlight; the ultraviolet, which we can’t see, carries a lot of energy with it, and that energy is deposited in our skin. Our skin doesn’t handle this energy dose very well, and so we wind up with a burn, as though we’d touched something hot. (Technically, we did! The sun.)

A partial reflective surface, reflecting light but at a weaker intensity than the incoming beam. Image credit: wikimedia user Zátonyi Sándor, CC BY A-SA 3.0

A partial reflective surface, reflecting light but at a weaker intensity than the incoming beam. Image credit: wikimedia user Zátonyi Sándor, CC BY A-SA 3.0

But what about if the surface doesn’t absorb the light, but reflects it, as a mirror does for visible light? In a perfect world, the wave is totally reflected off of the surface, and none of this transfer of energy to the surface happens. In practice, however, most materials are not perfectly reflective, and so the reflected wave has lost some of its energy to the mirror. In most cases, this energy loss is pretty small (otherwise it’s a terrible mirror), but if you’re in the business of trying to make a solar sail, this energy donation pushes the sail along a little bit.

If we go back to thinking about light as a particle, the light-particle must still carry energy. The light particle is a little weird, because it does manage to carry momentum, even without a rest mass. But there is an energy-momentum translation, even without mass, which is in play for photons. Now, if you think about a string of particles bouncing off of a surface, bouncy-ball style, those collisions also transfer a bit of energy into the surface. This energy transfer is giving a little bit of momentum to the surface, so if that surface is floating freely, as a solar sail does, you’ll slowly add speed to the sail.

It’s important to note that both the wave method and the particle method of thinking about light are totally equivalent, but in some cases it is simpler mentally or mathematically to think about light more as a particle or more as a wave. In this case, either method is a decent description of how light can propel a solar sail along.

Model of the Japanese interplanetary unmanned spacecraft IKAROS at the 61st International Astronautical Congress in Prague, Czech Republic. Image credit: Pavel Hrdlička, Wikipedia. CC A-SA 3.0

Model of the Japanese interplanetary unmanned spacecraft IKAROS at the 61st International Astronautical Congress in Prague, Czech Republic. Image credit: Pavel Hrdlička, Wikipedia. CC A-SA 3.0

The technical term for this gradual, tiny momentum-energy transfer is called radiation pressure, and this gradual pressure across the solar sail is what can propel it through the solar system. In any patch of space where there is strong radiation — a.k.a. a huge source of light, of any frequency — you can wind up with radiation pressure shaping the behavior of things around it.

Our sun isn’t even that extreme of an environment for radiation pressure! If it were more extreme, our solar sails might not need to be quite so large, as the pressure would be stronger and we wouldn’t need such a large surface area. The current solar sails have to be tens of meters on a side to be practical – the Japanese IKAROS light sail is a little over 150 feet along one edge, with an even larger one planned now that we know that this sort of technology is feasible!

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