Physics Discussion Forum

Time Explained

Hold your hands up. See that gap between them? That's a space, and you can see it’s there, even though you can't see the space itself. Now waggle those hands. That's motion, and you can see that’s there too. But can you see time? No. Can you see time flowing? No. Can you see any travelling through time? No. These phrases are just figures of speech. There’s no scientific evidence for time flowing or time travel. Time travel is science fiction, and it’s going to stay that way forever because travelling back in time is impossible. It’s impossible because we don’t even travel forward in time, not really.

To understand this, think about a stasis box. That’s science fiction too. It’s the ultimate refrigerator. No motion occurs inside the box, so when I put you inside one, electromagnetic phenomena don’t propagate. So you can’t see, you can’t hear, and you can’t even think. Hence when I open the door 5 years later, to you it’s like I opened the door just as soon as I closed it. And get this: you “travelled” to the future by not moving at all. Instead everything else did. And that motion wasn’t through time, and it wasn’t through spacetime, it was through space. Yes, the stasis box is fighting fire with fire, but don’t forget, we can freeze embryos now, so “in the future” maybe we’ll be able to freeze an adult. Then you could “travel” to the future by stepping into a freezer. But you aren’t really travelling. You aren’t moving. Everything else is.

That’s the size of it. Time is very simple, once you get it. But “getting it” is so very difficult. That’s because for most people, the current concept of time is so deeply ingrained, from childhood in a catch ‘em young way. They’re locked into a conviction that days pass, that time flows, and that a journey takes a length of time. It takes some steely rationality to break out of this, but it can be done.

Motion is King

First of all we need to look at the senses and the things we experience. Let’s start with sight. Look at the picture below:



Now, squares A and B are the same colour. They’re the same shade of grey. Oh no they’re not, I hear you say. Oh yes they are I insist. Oh no they’re not you answer back. We could do this all day, but they really are the same colour. Squares A and B are the same shade of grey. The apparent difference in colour is an illusion. Just look at the screen from a narrow angle to break the illusion. See Ed Adelson’s website for details. Check it out for yourself. Satisfy yourself. Be empirical, test your belief, then you realise that A and B really are the same colour. What this tells you is that colour is subjective. It isn’t a real property of things in the world. It’s perception, a "quale", and it’s in your head. A photon doesn’t actually have a colour. It has a wavelength, an oscillation, a frequency. What’s it’s got is a motion.

Let’s move on to sound. Imagine a super-evolved alien bat with a large number of ears, like a fly’s eye. This bat would “see” using sound, and if it was sufficiently advanced it might even see in colour. But we know that sound is pressure waves, and when we look beyond this at the air molecules, we know that sound relies on motion.



Pressure is related to sound, and to touch. You feel it in your ears on a plane, or on your chest if you dive. This pressure of air or water is not some property of the sub-atomic world. It’s a derived effect, and the Kinetic Theory of Gases tells us that pressure is derived from motion.

You can also feel kinetic energy. If a cannonball in space travelling at 1000m/s impacted your chest you would feel it for sure. But apologies, my mistake. It isn't the cannonball doing 1000m/s. It's you. So where's the kinetic energy now? Can you feel it coursing through your veins? No. Because what’s really there is mass, and relative motion.

You can also feel heat. Touch that stove and you feel that heat. We talk about heat exchangers and heat flow as if there’s some magical mysterious fluid in there. And yet we know there isn’t. We know that heat is another derived effect of motion.



The point of all this is there’s a lot of motion out there, and most of your senses are motion detectors. But it probably never occurred to you because you’re accustomed to thinking about the world in terms of how you experience it, rather than the scientific, empirical, fundamental, ontological things that are there.

And nowhere is this more so than with time.

The definition of the second

To understand time, the best place to start is with the NIST caesium fountain clock, also looking at and wikipedia for the definition of the second:

"Since 1967, the second has been defined to be the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom. This definition refers to a caesium atom at rest at a temperature of 0 K (absolute zero), and with appropriate corrections for gravitational time dilation."

In essence lasers and a microwave cavity are employed to cause hyperfine transitions, which are electron “spin flips” within caesium atoms. The hyperfine transition emits microwaves, which is light in the wider sense. There’s a peak frequency in the emitted light, which is found and measured by the detector. But note that frequency is measured in Hertz, which is defined as cycles per second, and the second isn't defined yet. What the detectors essentially do, is count incoming microwave peaks. When they get to 9,192,631,770, that's a second. Hence the frequency is 9,192,631,770 Hertz by definition.



Note the mention of gravitational time dilation in the wiki article. If you were to take this clock and place it in a region of low gravitational potential, it would be like pressing a slow-motion button. All electromagnetic and other processes would then occur at a reduced rate, including the hyperfine transition and the motion of the resultant light towards the detector. However regardless of this, when the detectors get to 9,192,631,770, that's a second.

It's important to realise here that in this situation, the light is moving slower and this is why the second is bigger. We then use this second... to measure the speed of light. That's why we always measure the local speed of light in vacuo to be 299,792,458 m/s.

Time is in the events

Now let’s look at that frequency some more. What’s the definition in English?

Frequency is the measurement of the number of times that a repeated event occurs per unit of time.

Our unit of time is the second. Frequency is the number of events per second. A second is nine billion periods of electromagnetic radiation. A period of radiation is an electromagnetic event, caused by an electromagnetic event happening inside an atom. For an event to happen, something has to move. Some component of the caesium atom has to travel some distance. A hyperfine transition is to do with magnetic dipole movement, a flip-flop interaction between the nucleus and an electron. It’s magnetic, so it’s electromagnetic in nature. Like the electron is electromagnetic in nature. Like the photon is electromagnetic in nature. So in some simple respect, we can consider some vital component of the atom to be electromagnetic just like light.



The answer is clear enough once you know about pair production and spin angular momentum, and magnetic dipole moment and the Einstein de-Haas effect which “demonstrates that spin angular momentum is indeed of the same nature as the angular momentum of rotating bodies as conceived in classical mechanics.” It’s a form of “light” moving inside the atom, and it causes more light, radiation with a frequency, waves with peaks, We sit there counting them as they go by, and when we get to nine billion, we say that’s a second. Then we use this second to measure the speed of light. We measure the speed of light in terms of the speed of light. In caesium atoms, in hydrogen atoms, in our own atoms, in the atoms of everything. No wonder it never changes.

And so the penny drops: the interval between events is measured in terms of other events. And the interval between those events is measured in terms of other events. Until there are no events left, only intervals. And intervals are frozen timeless moments. But you need events, not frozen timeless intervals to mark out the time. So the events aren’t in the time, the time is in the events. Because time is merely the measure of events, of change, measured against some other change. And for things to change, there has to be motion. You don’t need time to have motion. You need motion to have time. It’s blindingly obvious when you look at a clock. A mechanical clock is full of gears and sprockets, and it’s clocking up motion, not the flow of time. A quartz watch is clocking up the oscillatory motion of a crystal, a pendulum clock is clocking up the swings of the pendulum, a sundial is clocking up the motion of the earth. A year is calibrated on the motion of the earth too, and atomic clocks clock up electromagnetic motion. It always comes back to motion.

You don’t need regular atomic motion to mark out time. Any regular motion will do. Yes, we counted nine billion oscillations and called it second. One, two, three… nine billion. But you don’t have to count microwave wavepeaks emitted by a caesium atom. You could count beans in a bucket. Ping, ping, ping, chuck them in, regular as clockwork.

The Arrow of Time

You’re sitting there counting beans into the bucket, ping, ping, ping, regular as clockwork. Now, what is the direction of time? The only direction that is actually there, is the direction of the beans you’re throwing, and that direction is the direction of motion through space. A fuller bucket is not the direction of time. More beans is not the direction of time. The direction of time is the direction of your counting, and I could have asked you to count the beans out of the bucket. There is no real direction. It’s as imaginary as the direction you take when you count along the set of integers.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 →

It’s imaginary, so you cannot actually point in this direction. Nor can an arrow. Yes, entropy is a measure of disorder, but for that to occur motion has to occur. There’s motion in the universe, but collectively you can’t assign a direction to it and fire an arrow in that direction. And since there’s no real direction, there’s no direction you can possibly “travel” in. And since you can’t travel, you can’t travel a length, and a length can’t pass you by. It’s all abstraction, a false concept rooted in the language we use to think. Yet we never ever think about what the words actually mean. Instead we say the clock is running slow as if a clock is an athlete. We say the day went quickly but it didn’t go anywhere. We say years pass, but they don’t go by like buses.



The only directions that are there, are the directions of the spatial motions that make the events that we use to measure the intervals between the other events. What’s there is the motion of light, the motion of atoms, and the motion of clocks, buses, and rivers. What’s there is the motion of the earth, and the sun, moon, and stars. And these motions are being counted, incremented, added up. We count regular atomic motion to use as a ratio against some other motion, be it of light, clocks, or buses. All of these things have motion, both internal motion and travelling motion. And all those motions are real, with real directions in space. But the time direction isn't real. It's as imaginary as a trip to nine billion.

That's why the past is only in your head, in your memory, in your records. It isn’t a place you can travel to. It’s just the places where things were. All those places that are still here in the universe. And while the past is the sum of all nows, now lasts for no time at all. Because there’s no time like the present, and time needs events, and when you take away the events, you take away the time. A second isn’t some slice of spacetime. It’s just nine billion motions of light from a caesium atom. Accelerate to half the speed of light and a second is still nine billion motions of light from a caesium atom. But there's only half the local motion there used to be, because the other half is already doing the travelling motion through space. That’s why time dilates.

Time Dilation

It’s easy to understand time dilation. Imagine yourself as a metronome. Each tick is a thought in your head, a beat in your heart, a second of your time. If you’re motionless with respect to me I see you ticking like this: |||. If you flash by in a spaceship, I see you ticking like this: /\/\/\. If you could reach c and we know why you can’t, you wouldn’t tick at all. Your time would flatline like this ______ because any transverse motion would cause c to be exceeded. You wouldn’t tick for me, you wouldn’t tick for you, and you wouldn’t tick for anybody else in the universe.

That’s the thing we’re interested in. The universe. That’s the thing that’s out there, the thing we’re a part of, the thing we’re trying to understand. It’s full of motion, and this is what it’s like:



What can you see? What can you measure? You can measure the height. You can measure the width. And if it wasn't just a picture you could measure the depth. That's three Dimensions, with a capital D because we have freedom of movement in those dimensions. What else can you see? What else can you measure? You might imagine a fourth dimension, a time dimension. But the picture comes from the wikipedia temperature page. It’s a gif, a moving image, and in that image, those red and blue dots are moving. The thing you can measure is temperature.

Temperature is an aspect of heat, an emergent property, a derived effect of atomic and molecular motion. When you measure the temperature, you are measuring an aggregate motion. If you were one of those dots, you would not talk of climbing to a “higher temperature”. There is no real height. You can’t literally climb to a higher temperature. Hence we don’t call temperature a dimension. But people did. Temperature used to be called a dimension, but the word has gradually changed from its original meaning of “measure”, and is now assumed to be something that offers a degree of freedom, something you can move through.

We are immersed in time like the dots are immersed in temperature. It’s a different measure, but just as we cannot travel in temperature because there is no real height, we cannot travel in time because there is no real length. Because time is a dimension with a small d. There is no degree of freedom. I can hop backwards a metre but not backwards a second. Because time is a measure of change rather than a measure of place, and it has no absolute units, because you can only measure one change of place against another. It’s a relative measure of motion. The units are relative, and that’s what Special Relativity was telling us all along.

Special Relativity tells us that your relative velocity alters your measurement of space and time compared to everybody else. You increase your relative velocity and space appears to contract while time dilates by a factor of 1/√(1-v²/c²). It’s just Pythagoras’s theorem really. See wikipedia. The hypotenuse is the light path, and we’re using natural units where c=1. The base of the triangle is your speed as a fraction of c, and the height is the Lorentz factor. You need a reciprocal because time “dilation” is the opposite sense to length “contraction”. So if you travel at .99c, space appears to contract to one seventh of its former size. So your trip to a star seven light years away only takes you a year. But physics is about the universe, and in that universe it took you seven years. The star didn’t become a disc because you flashed by. The space in the universe didn’t really contract because you travelled through it. But your time did.



If you travel through the universe your local motion is reduced by virtue of your macroscopic motion. Hence your clocks run slower. Yes motion is relative, and there’s a symmetry because we can’t always say whether it’s you moving or me. But if you travel out and back we both agree that it was you moving, and we compare grey hairs and agree that you were time dilated, because your local motion occurred at a reduced rate. Then we apply the Principle of Equivalence, and know that the same applies if you’d been orbiting a black hole. When you "spend time" at a lower gravitational potential, local motion occurs at a reduced rate, hence you’re gravitationally time dilated. It’s like plunging a clock into an oil bath. The oil is viscous, and makes the clock run slower. Only you’re like a clockwork man. When you jump in after it, you run slower too. That’s why we have the GPS clock adjustment.

Time exists like heat exists

I'm what you'd call an "Einstein fan", but I have to say Einstein didn’t seem to fully understand time until later in life. He started off by saying there is no absolute time, using a postulate that says the speed of light is always the same. When he did general relativity, he said the speed of light varies with position, and there are some hints when he talked about an operational definition of time. But he never achieved his goal of combining gravity and electromagnetism, so I think it wasn’t until he was with Godel in Princeton that he really got it, in 1949:

"It is a widely known but insufficiently appreciated fact that Albert Einstein and Kurt Godel were best friends for the last decade and a half of Einstein's life. They walked home together from Princeton's Institute for Advanced Study every day; they shared ideas about physics, philosophy, politics, and the lost world of German-Austrian science in which they had grown up. What is not widely known is that in 1949 Godel made a remarkable discovery: there exist possible worlds described by the theory of relativity in which time, as we ordinarily understand it, does not exist. He added a philosophical argument that demonstrates, by Godel's lights, that as a consequence, time does not exist in our world either. If Godel is right, Einstein has not just explained time; he has explained it away... (Palle Yourgrau, A World Without Time)".



And what he got was this: time as we know it doesn’t exist. It still exists however, like heat exists. It’s real because it does things to us. But just like heat it’s an emergent property, a derived effect of motion. This means time isn’t a dimension like the dimensions of space. We don’t see the 4 dimensions of spacetime, just as we can’t point up to the sky at a frame of reference. We see space and motion through it. Yes, the mathematics of Minkowski spacetime works perfectly, but the time dimension is derived from motion through space, that’s why it’s 3+1 dimensions not 4, because we have no freedom of motion in the time dimension. The thing we call c is a conversion factor, between the measure we call distance and the measure we call time, and both are derived from the motion of light. When we move fast, we talk of [url=Lorentz Transformation]Lorentz_transformation[/url] and how our measures rotate. But it’s the motion that’s king, whether it’s the motion of light, or us. The universe is not a block universe, it’s a world in motion. The worldlines are only in mathematical space, and in your head, and the map is not the territory. There’s no place that’s the future, and no place that’s the past. There’s only this place, and the time is always now whatever our various clocks say. We don’t travel in time at one second per second. We don't travel in time at all, and thus we don’t travel through spacetime either. To travel backwards in time we'd need to unevent events, we’d need negative motion. But motion is motion whichever way it goes. You can’t have negative motion, just as you can’t have negative distance. So you can’t travel in time. There are no time travel paradoxes, and there’s no need for a Chronology protection conjecture. Because there is no time travel, because time is just a relative measure of motion. And motion is travel. You can’t travel through travel.

So those celebrity physicists who talk earnestly about time travel are misguided. They’re not even wrong. And those good folk who puzzle about the beginning of time are actually chasing the wrong horse. There never was any beginning of time. Time didn’t start thirteen point seven billion years ago. Because time didn’t start in the first place. It was motion that started in the first place. It was a place, not a time. And it’s this place, the place we call the universe, marked out by every light path we can track through timeless space. The thing we call time is only there because it's a world of motion, and without motion there is no time. Because:

Time exists like heat exists, being an emergent property of motion. It's a cumulative measure of motion used in the relative measure of motion compared to the motion of light, and the only motion is through space. So time doesn’t really flow and we don’t really travel through it.

Very informative set of posts.

We could think of a cause for the time dilation we notice when objects move. Since time is the repetition rate of patterns in the caesium atom, pattern components would need move a greater distance to complete patterns when in motion.

Maybe?

Certainly. See the Simple inference of time dilation on wikipedia, where light between parallel mirrors moves a greater distance. Then relate this to matter and simplify things by thinking of a hydrogen atom rather than a caesium atom. The hyperfine transition is an electron spin flip, and we can annihilate the electron with a positron, and annihilate the proton with an antiproton. The result is typically photons. So we can assert that the electron is a "photon configuration" involving spin ½ rather than lateral motion at c. We can then infer that the proton is of similar ilk, and that the hydrogen atom is a combined configuration of the two. Hence in simple terms we can say it's all just light moving in patterns. Like I was saying on the other thread, we don't classify neutrinos as light, so maybe "action" is a better word. Either way, time dilation seems pretty trivial.

Well said Farsight! Time dilation is trivial, easily understood, and totally necessary in a universe that has light as its final irreducible constituent.

The neutrino thing bothers me though. There seems to be evidence that they exist. But in the universe I like to imagine, they can't exist.