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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Can The Mass Of An Object Ever Change?

What is the difference between “mass” and “rest mass” ? Does this mean that mass is not always the same?
Nuclear particle tracks in the ten-inch bubble chamber mounted inside a superconducting magnet at Argonne show what happened to two negative K mesons that entered the bubble chamber from Argonne's ZGS. c.1966 Image credit: US Department of Energy, public domain

Nuclear particle tracks in the ten-inch bubble chamber mounted inside a superconducting magnet at Argonne show what happened to two negative K mesons that entered the bubble chamber from Argonne's ZGS. c.1966 Image credit: US Department of Energy, public domain

Originally posted on Forbes!

This is a good question, and also a good reminder to be careful with one’s language when writing about physics!

Most of the time, if you’re reading an article, what we mean by mass and rest mass is exactly the same. Rest mass is a slightly more precise term in its phrasing, meaning specifically the mass of that object when it is at rest, relative to the person measuring its mass. This is almost always how we measure mass. If you’re in a lab (or a kitchen), and you measure an object’s weight on a scale, that object is not moving at any speed. If it is moving at a speed, you should probably catch it, because it’s rolling off your scale.

If you’re on Earth, you take the weight — which is the force with which that object is pressing on your scale, within our Earth’s gravitational field – and can then convert it into a mass. This mass you’ve measured is the “rest mass”, since nothing’s moving in this scenario. Generally this number — the rest mass — is the most useful metric of how much material is assembled into whatever you’ve measured, so “rest mass” is often abbreviated into just regular old “mass”.

However, there is a way to make an object sort of behave as if it has a much larger mass than its rest mass, and given my pointing out how nothing’s moving in the definition of rest mass, you will rightly guess that it has something to do with motion. However, it’s not just any motion — an egg rolling off your kitchen scale isn’t any heavier than the egg that managed to stay balanced.

You’d have to accelerate that egg to a significant fraction of the speed of light before the egg started behaving as though it were heavier (and if you can do that without crushing the egg, you’ve won the world’s most difficult Egg Drop Challenge). It’s critical to note that the egg itself is not intrinsically heavier, and so we’re careful to keep “rest mass” separate from this relativistic speed effect. However, because the egg now has so much kinetic energy from moving so fast, and there is an equivalence between mass and energy (hello E=mc²), the energy of the object can masquerade as additional mass by adding to that object’s momentum.

A Newton’s Cradle toy in Darlington-Park in Mülheim an der Ruhr. Image credit: Frank Vincentz, CC BY SA 3.0

A Newton’s Cradle toy in Darlington-Park in Mülheim an der Ruhr. Image credit: Frank Vincentz, CC BY SA 3.0

The faster our relativistic egg is moving, the larger this energy-mass bonus gets, and the egg gains more and more momentum. The relevant feature of momentum here is that it resists changes to its speed. In particular, for our relativistic egg, the faster and faster it goes, the more momentum it has, so the harder it is to speed it up any more – you need to start expending truly ludicrous amounts of energy to speed it up even a little bit. This is one half of the reason it’s physically impossible to accelerate an object all the way to the speed of light — the closer you get, the closer to infinite energy you need to continue speeding it up.

Comparison of Newtonian and relativistic momentum. 1 on the x axis is the speed of light. As you approach the speed of light, in a relativistic framework, momentum rapidly approaches infinity. Image credit: Wikimedia user D.H, CC BY-SA 3.0

Comparison of Newtonian and relativistic momentum. 1 on the x axis is the speed of light. As you approach the speed of light, in a relativistic framework, momentum rapidly approaches infinity. Image credit: Wikimedia user D.H, CC BY-SA 3.0

So you can safely substitute “mass” anytime you see anyone mention a “rest mass,” but there are situations in which an object might behave as though it has more mass than its “rest mass” — these are, however, limited to objects traveling a significant fraction of the speed of light.

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Is There A Zero-Point For Measuring Astronomical Speeds?

Since you know how fast stuff in the universe is moving, where is the point of reference that is not moving at all, to base the speeds at which stuff travels?
A solar storm approaches Mars (artist's concept). The Red Planet is thought to have lost much of its atmosphere to such extreme space weather. Credit: NASA/GSFC

A solar storm approaches Mars (artist's concept). The Red Planet is thought to have lost much of its atmosphere to such extreme space weather.
Credit: NASA/GSFC

Originally posted at Forbes!

This one has a quick answer and a longer explanation. The quick answer is that there is no absolute point of reference that defines motion for all other objects in the Universe. All of our estimates of how fast objects in space are moving are relative to something specific, not absolute.

This lack of an absolutely unmoving object, or point, is one of the principles from the theory of relativity, and simply means that you can’t talk about any motion – at any scale – without also being clear about what that motion is in comparison to. Sometimes these comparisons are implied. When we talk about how fast your car is moving, we assume that we mean relative to the ground, because that’s clearly the relevant metric. But there’s nothing stopping you from measuring the speed of your car relative to the car in the next lane over.

If I sit inside a train car, or a bus, I’m not moving. At least, I’m not moving relative to the bus or train. But the train could be moving relative to the ground. Which speed are you interested in, for describing how I am moving?

You run into the same problem at every stage when you’re describing the Universe. As a point on the surface of the Earth, you are rotating around the Earth’s internal axis, which is revolving around the sun, which is in turn orbiting the center of our galaxy, which is rotating around the center of mass between Andromeda and the Milky Way. The speed of any given piece of this motion is entirely defined relative to some other, arbitrarily defined reference frame, so the speed we’re reporting depends on what piece of this motion we’re interested in describing.

Two galaxies can be crashing into each other with some speed – what speed? Probably the speed of one, relative to the motion of the other. Scientists won’t always spell out the reference frame, because there’s often a standard “relative to…” implied, but it always has to be there.

You can keep playing this game forever – you will always be able to define a speed relative to some defined point – but there’s no special point that will allow you to absolutely calibrate the motions of all things in the universe. There is no universal rest-stop. Any other observer of our universe should find the same thing. They will always be able to measure a velocity, measure a speed, but all of these measurements have to be relative measurements.

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Is time dilation real?

 

Read More

Can we build a wormhole?

Can we build a wormhole? If yes, why is this not priority one?
Exact mathematical plot of a Lorentzian wormhole (Schwarzschild wormhole). Image credit: Wikimedia user AllenMcC, CC A-SA 3.0

Exact mathematical plot of a Lorentzian wormhole (Schwarzschild wormhole). Image credit: Wikimedia user AllenMcC, CC A-SA 3.0

The short answer is that we can’t, and that’s why we’re not doing it.

Wormholes are an interesting thing; like black holes, for many years they were considered to be possible, in principle, but it was unclear whether the actually existed in the universe. Black holes have since moved from the realm of “I guess they’re possible” to “there’s a massive one in the center of every galaxy”, but wormholes have remained only a theoretical possibility.

General relativity gives us a set of rules for how the fabric of space time can behave, and there’s nothing in there that explicitly forbids wormholes from existing. You can even make a geometrically correct wormhole with a sheet of paper, if we assume space and time is a 2 dimensional sheet.

(If you want to do this, get a sheet of paper, and cut two identically sized complete circles out of opposite sides of the paper. Fold the paper over, and tape the inside edges of the circles together.  Presto: you’ve created a wormhole in your paper space-time.)

The problem is, we have no idea how to get this to happen with 3 dimensional space-time. Not only do you have to distort space-time immensely to create a divot deep enough to punch through to some other part of space, you have to bend the other part that you want to reach to meet it. Otherwise, you just have a really big black hole. We’re pretty sure space is intrinsically flat (like your sheet of paper), and trying to bend space that much is effectively impossible. Since we have no idea how nature could go about creating one (there is no mechanism known to physics right now that could make one) it’s pretty hard to go about replicating that process to build our own.

However, if we had any idea how to go about making wormholes, I am 100% sure we would be all over that. Any way to cheat the huge expanses of space is good for exploration!

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