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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Why Are We Limited To Only Seeing The Past?

We are moving from A to B. Yet everywhere we look, we are going backwards! I mean the direction we are moving along [trajectory] must [?] have something in front of earth-solar system-galaxy. Yet it is all the past. No future! Now could this be because we are at the event horizon [so to speak] the very edge of the beginning? However one ‘material’ that has moved faster-than-light is space. Light is still catching up. Why can’t we see even this?
Our solar journey through space is carrying us through a cluster of very low density interstellar clouds. Right now the Sun is inside of a cloud (Local cloud) that is so tenuous that the interstellar gas detected by IBEX is as sparse as a handful of air stretched over a column that is hundreds of light years long. These clouds are identified by their motions, indicated in this graphic with blue arrows. Credit: NASA/Goddard/Adler/U. Chicago/Wesleyan

Our solar journey through space is carrying us through a cluster of very low density interstellar clouds. Right now the Sun is inside of a cloud (Local cloud) that is so tenuous that the interstellar gas detected by IBEX is as sparse as a handful of air stretched over a column that is hundreds of light years long. These clouds are identified by their motions, indicated in this graphic with blue arrows. Credit: NASA/Goddard/Adler/U. Chicago/Wesleyan

Originally posted on Forbes!

There’s a couple things blurring together here, but the fundamental thing here is the distinction between the observable universe and the universe which exists, independent of our ability to observe it.

You’ve got a good handle on the observable universe. This is the universe that we see, where everything away from our own planet has some kind of time lag to it. We see our Sun as it was eight minutes ago, in the past. We see Jupiter as it was, about 30 minutes ago in the past. We see the stars as they were, a few years to a few thousand years ago. We see the Andromeda galaxy as it was 2.5 million years ago. As we look farther out into the universe, and strain our technology to the limits, we push further and further back into time, capturing light which has traveled longer and longer stretches of time to reach us.

But because we can only see the universe as it was, some varying degree of delay later, doesn’t mean the universe is actually delayed the farther away from us we go. Mars is a few minutes away from us, but that doesn’t mean that Mars’s “now” is actually a few minutes behind our “now” – that few minutes’ delay is just the quickest any Mars-related information can get to us.

So space hasn’t really traveled faster than light to get “ahead” of our ability to see the Universe. Space describes what and where the universe is, and is not particularly concerned with how well we can observe it.

Think of it this way. We’re sort of walking down a road, but backwards. We can see all the things we’ve passed by, all of the pieces that we know about. Now, if the road is straight, and we know where the road is supposed to go, and where we are, we can pretty safely assume that walking backwards in a straight line will keep us walking along the roadside. We might be able to predict how long it will take us to get there, walking backwards. So it is with the universe. We can see where the universe has been, and we can roughly figure out what the rules are which govern its changes. So we can predict where we will go, and check our predictions.

So maybe the road has a bend in it. You might notice that near you, the shape of the curb is different from what it has been, and that will clue you that maybe, if you want to stay near the road, you should bend that direction as well. We can change our ideas of how this particular road goes, and similarly we can constantly check how well our models of the universe’s evolution predict what we should see, versus what we actually see.

This image shows New Horizons’ current position along its full planned trajectory. The green segment of the line shows where New Horizons has traveled since launch; the red indicates the spacecraft’s future path. Positions of stars with magnitude 12 or brighter are shown from this perspective, which is slightly above the orbital plane of the planets. Image credit: NASA/JHU

This image shows New Horizons’ current position along its full planned trajectory. The green segment of the line shows where New Horizons has traveled since launch; the red indicates the spacecraft’s future path. Positions of stars with magnitude 12 or brighter are shown from this perspective, which is slightly above the orbital plane of the planets. Image credit: NASA/JHU

We do this kind of prediction in all kinds of ways — we assume that the physics of the world around us are stable, so that when I step forward onto concrete, the concrete will still be there when I put weight on it, even though I can’t see into the future to check that it will be. Any kind of rover delivered to the surface of another world, or a spacecraft very carefully planned to swing past a planet, requires these predictions of what the space outside our range of sight will look like. And they work — New Horizons, arriving at Pluto, took only one minute less to travel there than was predicted in 2006, at the start of a ten year journey.

So while we’re not able to see into the future – the speed of light simply will never allow this – we’re not going totally blind into the future. From looking into the past, we have learned the direction our future is unspooling.

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Is Cause & Effect Limited By The Speed Of Light?

Imagine lots of dominoes all lined up and ready to go over the length of a light-year. Am I right to assume that the speed of light is also the speed of causality (how fast things lead to one another in the universe) and that no matter what I do, the last domino will fall at best one year after the first?
A narrow wedge showing the evolution of the clustering across cosmic time. Image credit: Gus Evrard and Andrzej Kudlicki, Galaxy clusters in Hubble Volume Simulations, Evrard et al., 2002, ApJ, 573, 7

A narrow wedge showing the evolution of the clustering across cosmic time. Image credit: Gus Evrard and Andrzej Kudlicki, Galaxy clusters in Hubble Volume Simulations, Evrard et al., 2002, ApJ, 573, 7

Originally posted on Forbes!

The speed of light is usually thought of as a speed limit for how fast an object can travel, but you’re right to also think of it as a speed limit on the transfer of information.  If you want an object to influence another object, you first have to transfer some information around.  In the case of light, this influence can come in the form of light arriving on a detector, or perhaps a burst of radio waves, and this light unsurprisingly traverses the cosmos at precisely the speed of light.

The definition of causality from a physics perspective goes beyond a simple cause & effect link.  It’s more than just tying an event to the thing that caused it, though this is a critical component.  If I knock a glass over onto the floor, we can reasonably blame me for being the cause of that glass tipping over – that’s the cause and effect part.

However, say I was in the other room, and I just heard the glass fall, and don’t know what caused it. The physical principle of causality imposes limits on the number of things which could have caused the glass to fall.  The first rule of causality is that the order of time must be kept. Nothing you can do now will influence events that have already happened, earlier in time.  The second rule is that to influence anything in the universe later in time, the first event or object must transfer information across space and time.  Bu we already know that we have a maximal speed of information transfer – a maximum speed that can link two things causally; the speed of light.

The speed of light seems quick, and on human frames of reference, it is. But a lot of information travels much slower than the speed of light.  Sound, for instance, travels significantly slower than the speed of light. You’ll catch the flash of light from a lightning flash, but it’s the rolling thunder that will rattle the windows. The causal link between lightning strike and rattled windows travels slower than the speed of light, and so you have to wait for the sound wave to arrive for it to influence your windows.

Example of a light cone. Image credit: Wikimedia user Stib, CC 3.0 A-SA.

Example of a light cone. Image credit: Wikimedia user Stib, CC 3.0 A-SA.

You can draw out the regions of space which can possibly affect anything around you, and the regions of space which you can affect in the future, and it looks a bit like an hourglass.  This hourglass is called a light cone, with the point at the very centre as the present. In your domino example, you start the dominos tumbling at “now”, at this central point.  Your cone of influence extends out through time, as information about your push of the first domino extends outwards in space and time.  Someone watching you pushing over the domino sits along the edge of the light cone, as the information they need to see (light) travels at the speed of light. Of course, as time progresses, the “now” point progresses, and the light cones travel with it. In your example, the domino hits the next domino at some point in the future, which in turn hits the domino after that, all of which must be contained within this cone-space.

So yes – your dominos are bound to fall one after another within this light cone, which limits us to communicating at the fastest, at the speed of light.  So assuming that the dominos fell exactly instantaneously (unlikely to impossible), and no time was taken in transferring the energy from domino to domino (also unlikely to impossible), the fastest the furthest one could fall, while having the first domino be the cause, is one year after the first.  By the same token, if you see the last dominos fall before a year has gone by, it guarantees that something other than the domino stack is responsible for its fall.

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How Do We Know We Have The Speed Of Light Correct?

Compared with intergalactic space, our galaxy is gravitationally dense. Compared with interstellar space, our solar system is gravitationally dense. We know that light waves bend with gravitational compression of space. How do we know that our “speed of light in a vacuum” isn’t slower in solar space because of all the nearby objects creating drag than it would be in intergalactic space which is gravitationally sparse by comparison?
This photograph shows the Laser Ranging Facility at the Geophysical and Astronomical Observatory at NASA's Goddard Space Flight Center in Greenbelt, Md. The observatory helps NASA keep track of orbiting satellites. In this image, the lower of the two green beams is from the Lunar Reconnaissance Orbiter's dedicated tracker. The other laser originates from another ground system at the facility. Both beams are pointed at the moon - specifically at LRO in orbit around the moon. Image Credit: NASA

This photograph shows the Laser Ranging Facility at the Geophysical and Astronomical Observatory at NASA's Goddard Space Flight Center in Greenbelt, Md. The observatory helps NASA keep track of orbiting satellites. In this image, the lower of the two green beams is from the Lunar Reconnaissance Orbiter's dedicated tracker. The other laser originates from another ground system at the facility. Both beams are pointed at the moon - specifically at LRO in orbit around the moon. Image Credit: NASA

Originally posted at Forbes!

You’re right that our galaxy represents a much denser population of stuff than intergalactic space, and that our solar system is similarly a more dense collection of stuff than the space between stars within our galaxy. However, it sounds as though you’re thinking of gravitational field strength (which is certainly correlated with the density of matter) like an atmosphere of material that light must make it through.

Now, if light is indeed travelling through a dense material (for instance, air or water), light does slow down. This slowdown is related to the index of refraction of the material, which is the technical term applied to how much light bends when it enters that material. So, light going from air to water has a certain bend to it, which we can measure, and that bend tells us how much slower light will move through water. You can do the same experiment with air. Light in air is 1.0003 times slower than light in a vacuum, which slows it all the way down from 299,792,458 meters per second to 299,702,547 meters per second. That’s a slowdown of 89,911 meters per second, which looks like a lot but is only three ten-thousandths of the speed of light. Light in water goes even slower – water’s refractive index is 1.33, so the speed of light in water is slowed by 74,384,595 meters per second. If you have a sufficiently dense material, light can slow down really considerably.

A ray of light being refracted in a plastic block. Image credit: public domain, via wikimedia user ajizai

A ray of light being refracted in a plastic block. Image credit: public domain, via wikimedia user ajizai

But if you’re in a vacuum, the index of refraction is precisely 1; there is no change to the speed of light in a vacuum. There’s no material in a vacuum, quite by definition, for the light to encounter. The solar system is dense, but it’s dense with material in very specific locations – to extend your metaphor a bit, compared to interplanetary space, the planet is very dense. But it’s dense with matter, the physical pieces of you and me and rocks and the atmosphere. Outside of our atmosphere’s region of influence, you are very rapidly in a vacuum.

That’s not to say that the presence of an object with a strong gravitational field won’t affect light – it certainly does, but the way that a strong gravitational field influences light is a bit different from the slowing down you get from going through a thick substance. Gravity changes the shape of the space surrounding an object, and since light always travels in locally straight lines, light is affected by this warping. The more gravitationally weighty objects between the light source and your detector, the longer a path your light must travel. However, if you know the masses and locations of the gravitationally weighty objects, you can calculate the exact shape of space that light will have to pass through, and therefore how long it should take to travel between any two points.

But how do we know that the speed is right? There’s an early experiment that tackled this question from the comfort of our own planet. You can shoot a laser down a long tube, which you know the distance of quite precisely (this one you can physically measure). To measure the speed of light, you first bounce the light off of a rotating, 8 sided mirror, and then send it down your tube, with mirrors on each end. When the light bounces back out of your tube and back onto the rotating mirror, the mirror will have rotated a bit, which means that the light bounces out at a slightly different angle than it went in. The slower the speed of light, the more time the mirror has to rotate, so the bigger the difference in angle. This experiment was run by Michelson, Pease and Pearson in the 1930s, and successfully determined the speed of light to within 11,000 meters per second! Pretty good.

Apparatus used in physicist Albert A. Michelson, Fred Pease and astronomer Francis Pearson’s 1930-35 determination of the speed of light. It consists of a mile long 3 ft diameter vacuum chamber in a Southern California valley containing an optical system with two large concave mirrors at either end. Inside the vacuum chamber a beam of light from an arc lamp is reflected from an eight-sided mirror spinning at 512 revolutions per second, then makes ten passes through the tube, after which it returns and reflects again from the same face of the mirror. During the light beam’s ten-mile journey the mirror rotates through a small angle, so the reflected beam has a small angle to the outgoing beam. The apparatus measures this angle, which is proportional to the time of flight of the beam. The tube is evacuated to a pressure of about 10 Torr. E. C. Nichols designed the optics. Michelson died in 1931 with only 36 of the 233 measurement series completed, but Pease and Pearson carried on. The experiment’s accuracy was limited by geological instability and condensation problems, but in 1935 a result of 299,774 ± 11 km/s was obtained, the most accurate measurement of the speed of light to that date. Image use: public domain.

Apparatus used in physicist Albert A. Michelson, Fred Pease and astronomer Francis Pearson’s 1930-35 determination of the speed of light. It consists of a mile long 3 ft diameter vacuum chamber in a Southern California valley containing an optical system with two large concave mirrors at either end. Inside the vacuum chamber a beam of light from an arc lamp is reflected from an eight-sided mirror spinning at 512 revolutions per second, then makes ten passes through the tube, after which it returns and reflects again from the same face of the mirror. During the light beam’s ten-mile journey the mirror rotates through a small angle, so the reflected beam has a small angle to the outgoing beam. The apparatus measures this angle, which is proportional to the time of flight of the beam. The tube is evacuated to a pressure of about 10 Torr. E. C. Nichols designed the optics. Michelson died in 1931 with only 36 of the 233 measurement series completed, but Pease and Pearson carried on. The experiment’s accuracy was limited by geological instability and condensation problems, but in 1935 a result of 299,774 ± 11 km/s was obtained, the most accurate measurement of the speed of light to that date. Image use: public domain.

There’s another way to test the speed of light, a bit further from home. We can do tests of the strength of gravity on Earth, and given Newton’s equations, we can calculate the mass of the Earth. With the mass of the earth, and the length of a month, we can figure out the mass of the moon, so we can tell the exact shape of the space between the earth and the moon. With all those pieces in place, we should be able to predict the length of time it takes for a beam of light to make it from the earth outwards (and probably back again, if we’re testing the speed of light). If light makes it out and back again in the length of time you’d expect given the shape of space, then light is behaving the way we think it should. And it does behave the way we think it should.

Every test we can think of has given us very consistent results for the speed of light in a vacuum, from experiments on earth, to the length of time it takes to communicate with our satellites out in the distant solar system, and if the speed of light depended on anything beyond the geometry of the space it’s travelling through, we’d have seen some sign of it – experiments would run fast or slow in a certain time of year, or would have changed over time. There’s been no sign of that, so the speed of light in a vacuum seems to be one of the fundamental constants to our universe.

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Are We Missing Intelligent Life Because We're Looking Into The Past?

Scientists around the world say that they have found new planets thousands, millions or billions of light years away from Earth. Doesn’t that mean that the images those scientist receive are thousands or millions or billions of light years old? If intelligent life formed in one of those planets, and if they are also searching for new planets, then doesn’t it mean that those life forms will see only a rocky planet not habitable to life?
This artist’s impression shows the super-Earth 55 Cancri e in front of its parent star. Using observations made with the NASA/ESA Hubble Space Telescope and new analytic software scientists were able to analyse the composition of its atmosphere. It was the first time this was possible for a super-Earth. 55 Cancri e is about 40 light-years away and orbits a star slightly smaller, cooler and less bright than our Sun. As the planet is so close to its parent star, one year lasts only 18 hours and temperatures on the surface are thought to reach around 2000 degrees Celsius. Image credit: ESA/Hubble, M. Kornmesser

This artist’s impression shows the super-Earth 55 Cancri e in front of its parent star. Using observations made with the NASA/ESA Hubble Space Telescope and new analytic software scientists were able to analyse the composition of its atmosphere. It was the first time this was possible for a super-Earth. 55 Cancri e is about 40 light-years away and orbits a star slightly smaller, cooler and less bright than our Sun. As the planet is so close to its parent star, one year lasts only 18 hours and temperatures on the surface are thought to reach around 2000 degrees Celsius. Image credit: ESA/Hubble, M. Kornmesser

Originally posted at Forbes!

The search for planets outside our solar system has been expanding pretty rapidly recently with the data coming back from the Kepler mission, but nobody has managed to detect a planet quite as far away as a billion light years from Earth. Kepler can only detect Earth-like planets that are less distant than 3000 light years away from our solar system, in a very narrow region of our galaxy.

Our galaxy is about 50,000 light years from center to edge (so about 100,000 light years across), and the next nearest large galaxy is Andromeda, sitting about two and a half million light years away from us. While our current observations of the Milky Way lead us to believe that there’s a planet around pretty much every star in our galaxy, we haven’t been able to survey that much of our own galaxy, let alone the stars in Andromeda, which would be exponentially more difficult to observe. The furthest solid detection of an exoplanet is still only about 21,000 light years away.

But you’re absolutely correct - our images of exoplanets are just as out of date as they are distant from us, and we won’t ever be able to get around that limitation unless we can go visiting them so that the light-delay isn't so severe. A planet that we see at 10,000 light years distant from us will be an image that has traveled for 10,000 years.

In this rare image taken on July 19, 2013, the wide-angle camera on NASA's Cassini spacecraft has captured Saturn's rings and our planet Earth and its moon in the same frame. It is only one footprint in a mosaic of 33 footprints covering the entire Saturn ring system (including Saturn itself). At each footprint, images were taken in different spectral filters for a total of 323 images: some were taken for scientific purposes and some to produce a natural color mosaic. This is the only wide-angle footprint that has the Earth-moon system in it. Image credit: NASA/JPL-Caltech/Space Science Institute

In this rare image taken on July 19, 2013, the wide-angle camera on NASA's Cassini spacecraft has captured Saturn's rings and our planet Earth and its moon in the same frame. It is only one footprint in a mosaic of 33 footprints covering the entire Saturn ring system (including Saturn itself). At each footprint, images were taken in different spectral filters for a total of 323 images: some were taken for scientific purposes and some to produce a natural color mosaic. This is the only wide-angle footprint that has the Earth-moon system in it. Image credit: NASA/JPL-Caltech/Space Science Institute

On a geological timescale, 10,000 years is just a blip of time - the Earth was pretty much in the same shape as it is now, though we humans had made fewer changes to its surface. On a human timescale, 10,000 years has made a big difference. 10,000 years puts us back into the Neolithic era - the end of the Stone Age, around the time when pottery was developing, and we were beginning to cultivate plants for agriculture. So an intelligent civilization, 10,000 light years distant, that is just now looking for other life in the Universe would spy our Earth as a rocky planet with an atmosphere, far enough away from our sun that water could exist in our atmosphere, and if they managed to examine our atmosphere, they would notice that it is mostly nitrogen, with some oxygen and carbon dioxide in it as well, and that it contains water vapor. They would not be able to tell that there are creatures on that planet that are 10,000 years away from developing the internet, neurosurgery, and machines able to detect tiny distortions in space itself.

This kind of time delay is one of the reasons that scientists get extra excited when they find a nearby rocky planet that might be able to have liquid water on its surface - if the planet is close to us, then the time delay isn’t as bad as a more distant planet. (It is also much easier to observe these nearby planets in any degree of detail - the farther away you are from the Earth, the harder these measurements get.) We only managed to detect the contents of the atmosphere of a slightly-bigger-than-Earth planet for the first time a few days ago — unfortunately that planet is totally devoid of water, having an atmosphere of mostly hydrogen and helium, with some hydrogen cyanide thrown in for extra poisonous flavor. This planet is only 60 light years away, so our image of it is only out of date as far as 1976— this particular planet won’t have evolved into a friendlier, life-hosting planet in such a short time.

But let’s say a super-intelligent civilization out there has built an impossibly large telescope, and has the power (and the time) to detect planets orbiting stars in a distant galaxy, and they pointed it at our Earth. If they happened to be 2.5 billion light years distant, our planet’s atmosphere would be in the middle of a dramatic change. 2.5 billion years ago, our planet was in the middle of the Oxygen Catastrophe - the earliest photosynthetic bacteria were dumping oxygen into the atmosphere faster than it could be absorbed, and oxygen was slowly building up. As oxygen was a toxic byproduct to the single-celled life which had been living in a delightfully oxygen-free environment, they would have to adapt or die off. Observations of our planet from that distance would be able only to tell the observer that our planet existed, it has water in its atmosphere, and how rapidly we travel around our star, but not so much as a hint to our space-exploring future.

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