[geordief (OP)]

I understand that an "event" is used to mean a point on a spacetime coordinate system  or  it can be one element  of  the set of aĺl events (which doesn't help).

But if we take  an event as a physical oçcurence , what can we say about it?

If a particle is moving through the vacuum does an event only occur when it interacts with another object and are there no events between the start of its life and this interaction?

It can travel all the way across the universe without encountering any events?

Or do particles interact with themselves in any way and would that be an "event" ?

[Colin2B]

If a particle is moving through the vacuum does an event only occur when it interacts with another object and are there no events between the start of its life and this interaction?

We define what an event is. Usually it is when something happens and we can define the point at which it happens as a spacetime coordinate. The happening can be anything, an object passing a point, changing speed, hitting something, emitting a photon, etc. You name it 

[alancalverd]

I would use the word to indicate the boundary in spacetime where a change occurs between "before" and "after", as distinct from a spatial boundary between "here" and "there", a material boundary between "this" and "that", or a temporal boundary between "then" and "now".

[Bill S]

Alan, I’m trying to clarify the distinction between “spacetime where a change occurs between "before" and "after",” and “a temporal boundary between "then" and "now".”

In the case of Geordie’s particle travelling through the vacuum: at t1 it is at position 1; at t2 it is at position 2; between these is a temporal boundary, which you do not define as an event. (?) 

What additional factor must be present to make this a “boundary in spacetime where a change occurs between "before" and "after",”?

[Bill S]

We define what an event is.

Would it be that all observers would agree on the occurrence of a spacetime event, but would not all agree on its timing, as that would depend on their individual reference frames?

[geordief (OP)]

We define what an event is. Usually it is when something happens and we can define the point at which it happens as a spacetime coordinate. The happening can be anything, an object passing a point, changing speed, hitting something, emitting a photon, etc. You name it

So no restrictions? Can events fit inside one another like Russian dolls?

For example the fall of Rome  could be an event  that seems one dimensional from  a distance  and reveals "sub events" as one approaches.

Is that process  relevant in physics?

Also is the location of a "fundamental" event viewed mathematically as the intersection of  two or more surfaces of simultaneity ?

[alancalverd]

Alan, I’m trying to clarify the distinction between “spacetime where a change occurs between "before" and "after",” and “a temporal boundary between "then" and "now".”

Midnight on 31 December 1999 was a temporal boundary between then and now, but to everyone's amazement, no planes fell out of the sky and Beelzebub did not claim his earthly inheritance, so it was a non-event. Even the London fireworks were disappointing, and the Dome was complete crap.

In the case of Geordie’s particle travelling through the vacuum: at t1 it is at position 1; at t2 it is at position 2; between these is a temporal boundary, which you do not define as an event. (?)

If you observed it at either point, that would have been an event because there would be a bit more information in the universe, so a distinction between before and after.

What additional factor must be present to make this a “boundary in spacetime where a change occurs between "before" and "after",”?

"after" must be distinguishable from "before": the new universe must not map exactly onto the old one.

[Halc]

So no restrictions? Can events fit inside one another like Russian dolls?

Points don't fit inside each other, but a particle traces a path of points along its way, so those are all events even if no interaction takes place at most of them.

For example the fall of Rome  could be an event  that seems one dimensional from  a distance  and reveals "sub events" as one approaches.

This is using a different definition of 'event'.  Rome did not fall at a point in spacetime.  Rome is larger than a point, and the process took (a lot of) time.

Also is the location of a "fundamental" event viewed mathematically as the intersection of  two or more surfaces of simultaneity ?

Two 3D hyper-surfaces of simultaneity will form a 2D plane of events which are simultaneous in both frames.  All a third hyper-surface and that plane it reduced to a line.  A 4th one reduces it to one event, meaning with four different inertial frames, no two events are simultaneous in all of them.

[geordief (OP)]

Am I right to suppose that an event in quantum theory  is a different  to an event in Relativity?

Is an event in Quantum Theory perhaps   the same as an observation?

[Halc]

Am I right to suppose that an event in quantum theory  is a different  to an event in Relativity?

The 'point in spacetime' seems universal across physics, but QM may additionally leverage that word to indicate an occurrence of some state change like the decay of a nucleus, which for most practical purposes takes place at a spacetime point.  I'm no expert on QM terminology.

Is an event in Quantum Theory perhaps   the same as an observation?

It depends on the context I suppose.  Usually 'observation' is an epistemological change, typically of a human learning the result of a measurement.  A measurement is one system acquiring information about another system, hence changing the state (wave function) of the measured system.  The exact wording is very interpretation dependent, and hence are metaphysical differences, not scientific ones.

[geordief (OP)]

suppose.  Usually 'observation' is an epistemological change, typically of a human learning the result of a measurement.  A measurement is one system acquiring information about another system, hence changing the state (wave function) of the measured system.  The exact wording is very interpretation dependent, and hence are metaphysical differences, not scientific ones.

I think I am more familiar with "observation" and " measurement" being synonymous ** -so that no actual acquisition of knowledge is implied  simply ,perhaps  a change of state  in one system  being directly connected to a change of state in another system.

But my understanding of Quantum Theories is flimsy.

**and "measurement" perhaps being synonymous with "interaction".

[Bill S]

Usually it is when something happens and we can define the point at which it happens as a spacetime coordinate.

A point is zero dimensional; so, if an event is something that happens at a point, the notion of an event is an idealisation, because the point at which it happens has no physical extent, but the actual event must have a finite extent. (?)

[geordief (OP)]

The  mathematical "events" could be approximately centred wrt the physical events (however they are described)

[Halc]

A point is zero dimensional; so, if an event is something that happens at a point, the notion of an event is an idealisation, because the point at which it happens has no physical extent, but the actual event must have a finite extent. (?)

An event is a mathematical point which has no extent.  Sure, this is an idealization.  One can talk about the event of departure of a twin in the twins experiment, and for the purposes of the discussion, that event is treated as a point in spacetime.

[evan_au]

Simple particles like a photon or an electron don't interact with themselves to change state.

More complex particles like a neutron, or a plutonium nucleus or a hand grenade (with no pin) could spontaneously decay, and this could be considered an "event".

You could consider a photon passing through a glass window as an event, even if that particular photon did not interact with the glass in any dramatic way (more of a non-event).

And you could consider a photon (or a cat) passing through the event horizon of a supermassive black hole as an event (from the viewpoint of a distant observer), even though nothing special happens to the photon/cat (from it's viewpoint).

[jeffreyH]

But is the cat dead or alive?

[Bill S]

An event is a mathematical point which has no extent.  Sure, this is an idealization.  One can talk about the event of departure of a twin in the twins experiment, and for the purposes of the discussion, that event is treated as a point in spacetime.

An event may be well described as “a mathematical point which has no extent”, but as you say, “this is an idealization”.  The idealisation has no physical extent, but the physical event it describes does.  The “departure of a twin in the twins experiment” happens in time.

[Bill S]

I think you just echoed what I just said, yes.

What is it they say about imitation?

Actually it was:

  that event is treated as a point in spacetime.

I thought you were equating an event with a point.

 

[Halc]

Actually it was:

  that event is treated as a point in spacetime.

I thought you were equating an event with a point.

Lots of things that are not points are treated as such. Earth is X many km from the sun.  Never seen a page that listed it as a range (except when speaking of aphelion and perihelion, which are still point references).

[evan_au]

An event is a mathematical point which has no extent.  Sure, this is an idealization.

For many purposes an approximation is fine. eg the Earth is an extended object, thousands of km across, which you could approximate as the center of the Earth; knowing this location within 1km is fine (unless you are a GPS satellite).

Similarly, for a cat entering a supermassive black hole, measuring the position of a cat within 100m may be enough. If you want more accuracy, you could measure from the center of mass of the cat, or the center of the pupil of its left eye.

But is the cat dead or alive?

For extended objects like a cat, there is not only an uncertainty in space, there is an uncertainty in time as well.

Quantum theory says that there will always be some uncertainty in space and/or time...