Simultaneity and t=0 - SRT

[ozziemate]

HI guys,

The question that I was pondering upon is:

"If we have two or more observers [ RF's ] at relative v is t=0 for light events simultaneous for both [all] of them?"

I was under the impression that t=0 is relative and not absolute, according to SRT but I have reason to believe I am mistaken.
care to discuss?

 

[ozziemate]

one possible answer that may need to be debated...

t=0 is a completely arbitrary choice for any observer in any situation. It's whenever you want it to be.
Generally, it's convenient to set t=0 to be the start of some time period of interest (eg when timing a running race, you set t=0 to be when the starter's gun fires).

 

[Vorpal]

If
--there are two observers, one of them considered stationary, at the same location (or close to it) with relative velocity v, and
--t = 0 and x = 0 refers to that location and the time that they're both there,
then
--the event (t,x) = (0,0) is trivially in the present of both observers,
but
--events (0,x) with nonzero x are in the present of the stationary observer but not the moving one--unless, of course, v = 0.

The statement "t = 0 is a completely arbitrary choice" is also correct. In the above, "t = 0" refers to the time the observers have the same location; it doesn't really make any difference to call it "t = 42". It's no different than placing mile markers on a highway--what's really important is their differences, but you can start counting from any number you like.

 

[ozziemate]

If
--there are two observers, one of them considered stationary, at the same location (or close to it) with relative velocity v, and
--t = 0 and x = 0 refers to that location and the time that they're both there,
then
--the event (t,x) = (0,0) is trivially in the present of both observers,
but
--events (0,x) with nonzero x are in the present of the stationary observer but not the moving one--unless, of course, v = 0.

The statement "t = 0 is a completely arbitrary choice" is also correct. In the above, "t = 0" refers to the time the observers have the same location; it doesn't really make any difference to call it "t = 42". It's no different than placing mile markers on a highway--what's really important is their differences, but you can start counting from any number you like.

and from the photons persective:

A light event such as a photon particle or wave actually striking an object of mass occurs at t=? ?

meaning that I am suggesting that from a photons perspective it is always t=0.

true or false?

 

[MattusMaximus]

I'm not sure what you're asking. Are the clocks in the two RFs synchronized at t=0 and you're attempting to measure some later event?

 

[ozziemate]

I'm not sure what you're asking. Are the clocks in the two RFs synchronized at t=0 and you're attempting to measure some later event?

Actually the op needs to be rephrased a little to read:

"If we have two or more observers [ RF's ] at relative v is t=0 for light events simultaneous for both or all of them?"
- the correction in bold.

 

[sol invictus]

and from the photons persective:

What's that? How can a photon have a "perspective"?

If you mean, from the perspective of someone moving at the speed of light, the question is meaningless (because such a situation is impossible).

If you mean, from the perspective of someone moving very, very close to the speed of light, there's a clear answer, and it's that no, it's not always "t=0". On the contrary, time passes perfectly normally as observed by that person.

 

[sol invictus]

Actually the op needs to be rephrased a little to read:

"If we have two or more observers [ RF's ] at relative v is t=0 for light events simultaneous for both or all of them?"
- the correction in bold.

What's a "light event"? If you mean the absorption or emission of a photon, then the answer is no - like any other events, simultaneity is relative.

 

[ozziemate]

just to clarify,
scenario [ gedanken ]:

we have a single wave of light heading away from a source.
5 ships hit that same wave all travelling at relative velocity to each other,

question:
Do all ships hit the same wave at the same time [ t=0 ] from all perspectives including that of the photon waves'?
according to SRT?

 

[Vorpal]

No. You can see this most clearly by considering a hypothetical source of the light: the light signal is part of the edge of the source's lightcone. Since neither Lorentz transformations nor spatial translations affect temporal order in the light cone, the ships would observe the light hitting the other ships in the same order as the source would.

 

[ozziemate]

No. You can see this most clearly by considering a hypothetical source of the light: the light signal is part of the edge of the source's lightcone. Since neither Lorentz transformations nor spatial translations affect temporal order in the light cone, the ships would observe the light hitting the other ships in the same order as the source would.

hmmmm....
the problem is not about observers on the source but taking a photon waves perspective as a single wave getting hit by 5 rel. v ships simultaneously.

 

[Vorpal]

I'm not sure what problem you're imagining, but if photons don't have a perspective, that problem no longer exists. It is fortunate that light lacking a perspective is exactly what STR says.

Edit: More precisely, I suppose that one can say that a photon's "perspective" is that everything is simultaneous (this being consistent with it traveling along null trajectories). But that's fine--since all transformations between reference frames preserve lightspeed, it doesn't affect the reference frame of any other object.

 

[Reality Check]

and from the photons persective:

A light event such as a photon particle or wave actually striking an object of mass occurs at t=? ?

meaning that I am suggesting that from a photons perspective it is always t=0.

true or false?

False. An event can happen at any time, i.e. t = any value. t=0 does not have an special significance to a photon.

ETA: You may be thinking of time durations instead. IMHO a hypothetical, very small, massless observer sitting on a photon will see a time of flight from the emission of the photon to its detection of zero. An observer in another reference frame will see a finite flight time. Both viewpoints are valid and correct for that observer. There is no special significance to the zero flight time measured by the observer travelling at the speed of light.

 

[ozziemate]

try this contra:

If 't' can equal say 10 minutes from now when we get to 't' isn't it t=0?

if 't' equals 10 minutes form now then that event is in the future and is yet to occur.
if 't' equals -10 minutes from now then that event is in the past and is no longer observable except as a historical record.

but even so t= -10 must have occurrered at t=0


does that make sense?
and then if you can understand it, is it a correct understanding?

 

[ozziemate]

False. An event can happen at any time, i.e. t = any value. t=0 does not have an special significance to a photon.

ETA: You may be thinking of time durations instead. IMHO a hypothetical, very small, massless observer sitting on a photon will see a time of flight from the emission of the photon to its detection of zero. An observer in another reference frame will see a finite flight time. Both viewpoints are valid and correct for that observer. There is no special significance to the zero flight time measured by the observer travelling at the speed of light.

ok so give an example of t= "anytime you choose" and tell me when that event actually happens what is 't'

 

[Vorpal]

If 't' can equal say 10 minutes from now when we get to 't' isn't it t=0?

If "now" (as you're reading this) has time coordinate t = -10min, then 10 minutes from now it'll be t = 0. If "now" has the time coordinate t = 44min, then 10 minutes from now it'll be t = 54min--not zero, and not anything else but 54min.

but even so t= -10 must have occurrered at t=0

No. You seem to be under the impression that "t = 0" always refers to the present and that time coordinate somehow "self-update" themselves so that it is always so. The the time coordinate for an event doesn't change by itself. If I define a time coordinate so that t = 0 refers to the founding of London, then "now" (as I'm typing it) is t = 1965 years, give or take a year, and t = 0 will never refer to anything other than the time of the founding of London. I am free to change the coordinate system I'm using as I please, but it will be a different system once I change it.

The only reasons people commonly use t = 0 to label the present are psychological factors--it's a natural dividing line to have positives as future and negatives as past. But nature doesn't come with equipped with coordinate systems, and it simply doesn't care what labels we use.

 

[ozziemate]

You may be thinking of time durations instead. IMHO a hypothetical, very small, massless observer sitting on a photon will see a time of flight from the emission of the photon to its detection of zero. An observer in another reference frame will see a finite flight time. Both viewpoints are valid and correct for that observer. There is no special significance to the zero flight time measured by the observer travelling at the speed of light.

interesting.....

 

[ozziemate]

If "now" (as you're reading this) has time coordinate t = -10min, then 10 minutes from now it'll be t = 0. If "now" has the time coordinate t = 44min, then 10 minutes from now it'll be t = 54min--not zero, and not anything else but 54min.


No. You seem to be under the impression that "t = 0" always refers to the present and that time coordinate somehow "self-update" themselves so that it is always so. The the time coordinate for an event doesn't change by itself. If I define a time coordinate so that t = 0 refers to the founding of London, then "now" (as I'm typing it) is t = 1965 years, give or take a year, and t = 0 will never refer to anything other than the time of the founding of London. I am free to change the coordinate system I'm using as I please, but it will be a different system once I change it.


The only reasons people commonly use t = 0 to label the present are psychological factors--it's a natural dividing line to have positives as future and negatives as past. But nature doesn't come with equipped with coordinate systems, and it simply doesn't care what labels we use.

excellent post...well done thanks...

so this is how SRT views t=?

It is interesting that intuitively in the hypothetical year London was declared open it must have occurred for the observers of that event in their moment [ their NOW or t=0 ]

and when I read your Post about 10 minutes ago it occurred in my t=0 at the time....disregarding SRT for a moment does that make sense ?

also another observation I can make from what you have said is that it may be very well to declare the London event t= -1687 but it's has got to be considered that you are using a 't' that must be relative to zero.

after all what is t=-1687 any way. The value -1687 is related to zero yes...

so even if you can give t any value you choose it will still relate back to zero....hmmmmm interesting...

ps I have no idea when London was declared open and teh figure used is only as an example for the purposes of the posts position.

 

[Vorpal]

so this is how SRT views t=?

As far as that much, it's pretty much how all of physics and mathematics views coordinates, including Newtonian physics.

Think of mileposts (or km if you prefer) on a highway: if you see one informing you that it's mile 17, then you can say that the mileposts define a coordinate along the highway, and "here" is x = 17mi. But the coordinate of that place wouldn't change even if you left and your personal "here" changed. The coordinate system remains fixed. It will always be x = 17mi, unless of course someone got rid of all the mileposts and replaced them with new ones, creating an entirely new coordinate system.

It's the same thing here. Define a time coordinate so that, as you're reading it, it's t = 0. Wait, say, two minutes. Your personal "now" would be t = 2min, not zero, because you "left" t = 0 two minutes ago.

and when I read your pst about 10 minutes ago it occurred in my t=0 at the time....disregarding SRT for a moment does that make sense ?

I'm not sure what you mean.

Edit:

also another observation I can make from what you have said is that it may be very well to declare the London event t= -1687

Sure. Pick any number you like and any scale you like (t = -1687 years or t = -1687 millennia). It has no more significance than a highway engineer deciding to start the mileposts with "mile 5" rather than "mile 1" or whatever.

but it's has got to be considered that you are using a 't' that must be relative to zero.

No, not really. If the founding of London is t = -1687 millennia, then there is some time that has t = 0, but it's over a million years in the future, and I might not care about it at all. Now would be t = -1685 millennia, for instance, give or take 0.05 millennia or so.

 

[ozziemate]

As far as that much, it's pretty much how all of physics and mathematics views coordinates, including Newtonian physics.

Think of mileposts (or km if you prefer) on a highway: if you see one informing you that it's mile 17, then you can say that the mileposts define a coordinate along the highway, and "here" is x = 17mi. But the coordinate of that place wouldn't change even if you left and your personal "here" changed. The coordinate system remains fixed. It will always be x = 17mi, unless of course someone got rid of all the mileposts and replaced them with new ones, creating an entirely new coordinate system.

It's the same thing here. Define a time coordinate so that, as you're reading it, it's t = 0. Wait, say, two minutes. Your personal "now" would be t = 2min, not zero, because you "left" t = 0 two minutes ago.


I'm not sure what you mean.

Edit:

Sure. Pick any number you like and any scale you like (t = -1687 years or t = -1687 millennia). It has no more significance than a highway engineer deciding to start the mileposts with "mile 5" rather than "mile 1" or whatever.


No, not really. If the founding of London is t = -1687 millennia, then there is some time that has t = 0, but it's over a million years in the future, and I might not care about it at all. Now would be t = -1685 millennia, for instance, give or take 0.05 millennia or so.

but only if you use a t=0 to derive your other 't's is what I see...

"eg."right now at t=0 the london evnt is t=1678 millenia"