I Spacetime and Events in (x,y,z,t)

RossBlenkinsop

Summary
does space describe where and event happened , but not when and event happened

The dude in this video appears to say that "space" (ie a coordinate system that does not involve time ) describes where an event happened but not when. To describe when and where an event happened you need both space and time

so if I dont care "when" an event took place, only "where", "space" (x,y,z) can precisely describe "where", is that correct ?

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RossBlenkinsop

so to clarify "space" describes "where" an event took place but not when?

"spacetime" describes both where and when?

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jbriggs444

Homework Helper
so if I dont care "when" an event took place, only "where", "space" (x,y,z) can precisely describe "where", is that correct ?
A three dimensional coordinate can uniquely identify a position in a three dimensional slice called "now" carved from four dimensional space time. But only in the context of a defined coordinate system covering that three dimensional space.

None of that helps you determine whether an event that took place five minutes ago did or did not occur at the same place as the one whose (x,y,z) coordinates you have in hand.

PeterDonis

Mentor
if I dont care "when" an event took place, only "where", "space" (x,y,z) can precisely describe "where", is that correct ?
No, because of relativity, which tells us that "space" and "time" are not independent things. They are combined into "spacetime". How spacetime is split up into space and time depends on your choice of reference frame, so there is no such thing as "space" independent of a choice of reference frame. (And that's leaving out the further complications that arise when you include gravity and thus have to go to General Relativity and allow spacetime to be curved.)

Your failure to grasp the above is what has caused your threads on this topic to go on and on with no resolution. You need to read the above again and again until it sinks in.

we have a single strobe that emits a single flash of light and there multiple people in multiple frame of reference
People (and anything else, for that matter) are not "in" particular frames of reference and not others; all frames of reference describe all objects and events, so all objects and events are "in" all frames of reference. The correct way of saying what you appear to want to say here is that you have multiple people in different states of motion.

1 they will all agree on "where" the flash took place and that will be the same place according to all of them(remember they dont care when, just where)
2 they will all disagree "where" it took place and each will think it took place at a different point(remember they dont care when, just where)
3 they cant agree where and it is impossible for any of them to determine where(I think this is the only remaining possibility)
All of these are wrong.

The observers will agree on the point in spacetime at which the flash was emitted. But they will not, in general, agree on the coordinates x, y, z, t to assign to that point. They will also, in general, observe that the "point in space" at which the flash was emitted is moving, which means that in each of their coordinate systems, there will in general not be a single set of spatial coordinates x, y, z that describes "where the flash was emitted". (The fact that you appear completely oblivious to this possibility is part of why you continue to get wrong answers.) At most there can be one of them who can assign a single set of spatial coordinates x, y, z to "where the flash was emitted" that will be valid for all coordinate times t in his frame; but even this is not guaranteed.

Ibix

@RossBlenkinsop - here's a quick problem for you. Look at this diagram:

I'm thinking of a point specified by (X,Y) in the black coordinate system. I've marked the X position on the relevant axis, but the Y position is not marked and could be anything. Can you mark the X-position of the point on the red axis?

Why am I asking this? This is a Euclidean analogy to a 2d spacetime. Different frames pick different directions in spacetime to call "space", just as in the example above two frames have picked different directions to call "x". Specifying just the x, y, and z coordinates of an event in spacetime is analogous to specifying just the x coordinate of a point on a plane. So if, as you claim, specifying (x,y,z) in one frame uniquely specifies a fixed point in space for all frames, then specifying X on one set of axes uniquely specifies the x value on the other.

pervect

Staff Emeritus
so to clarify "space" describes "where" an event took place but not when?

"spacetime" describes both where and when?
Basically, yes. Of course, the two interact. For instance, if you have a moving object, "where" the moving object is depends on "when" you observe it. There are other more subtle and less obvious interactions in special relativity, as well.

FactChecker

Gold Member
2018 Award
When two coordinate systems are moving with respect to each other, the location in one coordinate system does not tell you where it is in the other coordinate system unless you also specify a time. So the OP is only correct when no other moving coordinate systems are involved.

RossBlenkinsop

"........ They will also, in general, observe that the "point in space" at which the flash was emitted is moving, which means ....... "

if the speed of light is indeed invariant the only logical result of any experiment is that the point that lies at the centre of the photon wavefront must not move over time ie ds/dt where s is distance will be zero for that point for all time

if the result of any experiment is that that point is moving over time then that must mean the speed of light is variant

If anyone perceives that point as moving then they have a paradox
1 either the speed of light is variant or
2 there is something wrong with their experiment or
3 both or
4 the point is stationary and they are moving relative to that point

which one is a logical conclusion that fits with the current science ?

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PeterDonis

Mentor
if the speed of light is indeed invariant the only logical result of any experiment is that the point that lies at the centre of the photon wavefront must not move over time
This does not follow at all.

if the result of any experiment is that that point is moving over time then that must mean the speed of light is variant
Incorrect.

which one is a logical conclusion that fits with the current science ?
None of them. Your premise is wrong. See above.

FactChecker

Gold Member
2018 Award
if the speed of light is indeed invariant the only logical result of any experiment is that the point that lies at the centre of the photon wavefront must not move over time
This is at the heart of where Special Relativity gets tricky. To define a point as the "centre" of the wavefront, one must measure the distance between the point and one part of the wavefront and simultaniously measure the distance to the wavefront in the opposite direction. But even the definition of simultanious depends on the motion of the reference frame.

"You did not take relativity of simultaneity into account." - The answer to 99% of all paradox threads in the relativity forum
-- @Orodruin

RossBlenkinsop

choose what ever coordinate system u want, place the source of the light and the photon wave front in that coord system. If you get a result that the source of the light is moving in some direction ie one of the coordinate is changing over time then you have the above paradox

PeterDonis

Mentor
choose what ever coordinate system u want, place the source of the light and the photon wave front in that coord system. If you get a result that the source of the light is moving in some direction ie one of the coordinate is changing over time then you have the above paradox
If all you are going to do is continue to repeat your erroneous statements, there is no point in discussion.

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