Okay, so then is the second “u” silent?

Wow, time, what a concept,,,

GAry 7

But what I really don’t understand is that there does not appear to be a word to explain the concept of the actual happening of two, or more, even millions of events, similtaneously. We *know* they happen – it is a dynamic universe – so they must all have an effect on each other, however slight or massive.

I sometimes wonder that there may be an effect on us at that actual moment, possibly insignificant, but there nevertheless. That the visual display of it, and/or the gravitational and/or ionic effect, or even the blast wavefront as it overwhelms our little Third Rock, or whatever, may not reach us for 8 minutes/years/millenium/lightyears… is the tangible evidence of that event. I think I can safely say that at this moment somewhere in our universe a mighty event is happening, but it won’t annoy us until we see it, and then it will be too late. It is a bit like the disclaimer on the ads for the lottery promotion – “the Prize may have already have been won before your ticket is bought”. (Or the thought that with FOI provisions, what our politicians are doing now, will be in the papers in 30 years, showing them in a different light.)

It is generally agreed that nothing travels faster than Light, not even Gravity, and I know this is 101 stuff, but what about in-tangibles? Thought, Perception and even Knowledge are in that grab-bag. (Sorry for the capitalisation of those words – it’s a literary thing, nothing to do with the supernatural!). Or is there something else?

Ivan.

That assumes constant speed and all that, but that’s the principle here.

That’s good enough for me, for the moment. Thanks, BA.

That assumes constant speed and all that, but that’s the principle here.

We cannot say anything about that event until the light reaches us, and in a real sense that event has not happened until the light reaches us. Time flows like light, I sometimes say, meaning that the event itself happens when the light reaches us. So it is acceptable to say that the explosion actually happened in 1680.

By that same logic, couldn’t we say that the Cas A supernova occurred at the same place as the Earth? A light cone is a null surface. That means that the distance of any path along the light cone is zero. If you think about it, this makes sense.: a photon traveling along the light cone will experience time dilation and length contraction. The time “experienced” by an observer riding the photon (its proper time) will be zero because of the time dilation. Similarly, the distance “experienced” by that observer (its proper distance) will be zero, because of the length contraction. In that sense, the supernova and the light reaching the Earth occur at the same place. Most people wouldn’t actually put it that way, though.

Note that the zero distance along the light cone is not relative. In relativity, space and time are melded into spacetime. The geometry of spacetime is absolute. The distance between two points in spacetime is given by that geometry. It’s invariant under coordinate transformation — that is, whatever strange way we label points in spacetime, the distance between those points doesn’t change [1]. (In the sort of the same way, the distance between places on Earth does not depend on whether we’re using a Meractor projection or a cylindrical projection map of the Earth.) If the distance between two points in spacetime is zero, we say that they are lightlike separated [2]. If the distance between them is positive, we say that they are spacelike separated, and they have no way of knowing of each other; they are outside of each others’ light cones. If the distance between them is negative, then they are timelike separated, and they are within each others light cones [3].

To label points uniquely in spacetime, we need four coordinates. At a given point, one of these will serve as a temporal coordinate, and three as a spatial coordinate (this is another property of spacetime — but which coordinate acts as time can change within your coordinate system depending on where you are [4]). No coordinate system is privileged – they all work well. That’s why time and space are relative. But spacetime, considered as a whole, is absolute [5].

Consider, you say that the supernova occurred about 300 years ago. But how can you say how long ago the light reached Earth from now? How are you measuring the distance from Earth right now to the point where light reached Earth from Cas A? You could use, say, the proper time experienced by Earth in the intervening time. This would always give you a consistent answer, since that proper time is absolute. But why not use the proper time experienced by a hypothetical rocket traveling at 50% of the speed of light away from Earth to the Galactic Center? If you use the proper time experienced by the astronauts, you will still get a consistent answer. Basically, you need some way to name your light cones uniquely. You chose to name them with the proper time experienced by Earth, but that’s not the only way you could do it.

Similarly, while the proper distance between the Cas A supernova and Earth in 1680 is zero, that does not mean that they actually were at the same point. Otherwise, Earth would’ve been might toasty. To have a useful coordinate system, you need to have a way of saying where on Earth’s light cone the supernova was. You say that the Cas A remnant is (and was) 10,000 light years away. Which coordinate system are you using? If you use the hypothetical astronaut’s coordinate system, you’ll get something different, and the astronaut’s coordinate system is just as good as yours. The point is, you need three more numbers to uniquely name a point on your light cone, for a grand total of four numbers. But again, you could’ve labeled it differently.

Now, your coordinate system, as far as I can see it, is perfectly valid. When someone complains that Cas A was 10,000 years ago, not 300 years ago, that’s because they’re saying they’re using a different coordinate system than you are. When they think of a specific “moment” in time, think of a specific spacelike surface in spacetime. You carve up spacetime differently – your “moment” is a lightlike surface. You are perfectly in your right to do this. In fact, that sort of thing can happen naturally in general relativity [4]. But you can be no more right than they are — your coordinate system is just as relative as theirs. When you say the Cas A supernova happened in 1680 and 10,000 light years away, that’s just your label for a specific point in spacetime. You could’ve labeled it differently, even if you do use light cones as moments of time, by naming those light cones and the points on those light cones differently. You can’t claim to be more correct just because the coordinates are relative, since it’ll apply to you as well.

So, it all comes down to simply a matter of convenience. Just because a coordinate system is valid, doesn’t mean it’s useful. For everyday life, both results will give pretty much the same results. There are some advantages, in that you don’t have to say things like “Supernova 1987A happened 1987 minus 180,000 years”, or “The Mars probe has now deployed its parachutes, and we’re getting the telemetry for that.” It can be a bit confusing, though — because other people carve up spacetime into different moments in a different way than you do. And someone could just as easily use the light cones for one of the spatial coordinates instead of one of the temporal coordinates. Usually, people just say which coordinate system they are using. Timelines for space probe events, for example, often account for light delay, and just give the time when we should recieve the probe’s radio signals from an event.

Um, closer to topic, are we sure the Cas A supernova was caused by a massive star? I ask that because I vaguely remember hearing it was suspected to be a Type Ia supernova, in which case it would’ve been a white dwarf (or two?) exploding. The basic point would still be the same — type Ia supernovae produce a lot of iron, especially since the entire white dwarf undergoes fusion, and all of it is blown away. Or have they pretty much ruled that out?

[1] That’s basically the solution to the Twin Paradox, if someone’s wondering. In general relativity, the astronaut’s coordinate system is just as valid as Earth’s. But even so, when you integrate the distance traveled by the rocket, it will always come out as shorter than the Earth’s, even in the astronaut’s coordinate system.

[2] This may be a bit confusing: in relativity, just because two points are separated by a proper distance of zero, does not mean they are the same point. If readers are interested, in flat spacetime, the proper distance between two points is found by:

ds^2 = -(c dt)^2 + (dx)^2 + (dy)^2 + (dz)^2

(If someone doesn’t know what the “d”s are, just ignore them for now. They’re sort of like deltas.) This is basically the Pythagorean Theorem for spacetime. It’s because of the minus sign that you can get the zero distances, if (c dt) is equal to the distance in space. It’s also the reason we get all sorts of weird special relativistic effects.

[3] Yes, that’s even true when you have things like wormholes or Alcubierre warp drives. They provide a shortcut between points in spacetime that would look far away in flat spacetime. When you fall through a wormhole, for example, you are still within your past self’s light cone — it’s just that the light cone as a whole has a weird shape. Of course, wormholes and warp drives have other problems.

[4] The event horizon of a black hole is lightlike. Think about it. If you’re falling into a black hole, and you emit a photon upwards as you cross the event horizon, that photon can never escape. But it won’t fall in, either. It’ll stay trapped at the event horizon forever.

In radial coordinate systems, where the event horizon is defined as being at radius r = R, then r = R is a lightlike surface. For the Schwarzchild coordinates, surfaces of constant r are spacelike inside the event horizon. That is, the distance r from the singularity acts like a spatial coordinate outside of the horizon. Inside the horizon, it acts like a time coordinate.

That’s also why you can’t escape from a black hole once you pass the horizon. The singularity, at r = 0, acts more like (what most people would call) a moment in time than a point in space. Once you are in the horizon, the singularity covers your entire future light cone. Assuming general relativity holds all the way in.

(By the BA’s way of defining a moment in time, the event horizon would be a specific time instant, not the singularity, though.)

[5] That’s also why you don’t really need to talk about time “flowing” in relativity. Time is on the same footing as space, because they are both integral part of spacetime. So, one could just as easily argue that space “flows”. Similarly, how would you say which moment it is right now? The very way you define “now” depends on which way you carve up spacetime.

Because the state is not knowable until the light arrives, it seems fair to say it hasn’t happened, and when the state is known, we retcon it.

facetious (also uses each vowel in order)

behaviour, sequoia

uncomplimentary (uses each vowel once, including Y)

There are many others

And yes, I grok the other arguments. They’re unecessry obfuscation when dealing with objects at non-relativistic velocities.

It happened 10,000 years ago. Period. We can say that because we know when the light reached us, where it came from, and how far away the light source is. There’s some corrections to be made for relative motion of Earth and Cas A (another known or knowable figure), by 10 KYears is probably about right.

The oldest “light” we can see if the microwave background radiation, and that dates, if I recall, to roughly 1/2 million years after the Big Bang. Yet we happily talk about what happened before that, all the way back to 10^-23 seconds after the Bang.

Huh? I understand that perhaps we cannot interpret new “data” until we receive it. But, you want to say it hasn’t happened until we can perceive it? I can understand the event didn’t happen “to us” until we received the light. But, to say it didn’t happen earlier doesn’t make sense…unless I am not taking into account some theory of physics.

Now, it would be interesting to have a new word (such as k-light) to indicate when light from an event significantly far away reached us.

PS. I believe that simultaneously is the only word in the English language that uses all vowels (exactly once).