View Full Version : Speed of Light Question


Fifty
Feb 12, 2008, 12:27 PM
Can an event have causal contact with another event faster than it would take for light to traverse the distance between the place where those events happened???

brennan
Feb 12, 2008, 01:02 PM
As far as we know: no.

It's usually expressed on the lines of "information cannot travel faster than the speed of light."

xienwolf
Feb 12, 2008, 01:19 PM
There is a hypothesis that for paired particles in superposition there could be FTL communication. But to keep a paired set in superposition long enough for equipment to measure the speed of the communication is not currently possible.

Fifty
Feb 12, 2008, 03:48 PM
Ok so here's something weird, and I'm curious as to whether this is actually true, or if I'm just muddling things up or what:

So suppose Mr. X lives on planet A, and his wife Mrs. X lives on planet B, which happens to be one light year away. They live this far apart because Mr. X is an intergalactic space-miner out on a mission for alien ore! Suppose that last Tuesday at midnight, Mr. X was killed in a terrible space-mining accident! Now consider this further event, the event of "Mrs. X's becoming a widow". Did Mr. X's dying cause Mrs. X to become a widow? I mean it seems like it does, but it also seems odd to suggest that she has to wait 1yr to be properly referred to as a "widow". And I'm not talking about whether or not she knows she is a widow, I'm talking about whether she actually is one! It seems like, in that case, either two events have causal contact instantaneously despite the rules governing FTL, or Mrs. X isn't actually a widow until a year later, which seems absurd! Can we get out of this without supposing that there are such things as "non-physical events"????

So, um, what the crap is going on here!?!?? I don't doubt that I'm wrong about something here, I'm just not sure what...?

Mise
Feb 12, 2008, 04:00 PM
Ok so here's something weird, and I'm curious as to whether this is actually true, or if I'm just muddling things up or what:

So suppose Mr. X lives on planet A, and his wife Mrs. X lives on planet B, which happens to be one light year away. They live this far apart because Mr. X is an intergalactic space-miner out on a mission for alien ore! Suppose that last Tuesday at midnight, Mr. X was killed in a terrible space-mining accident! Now consider this further event, the event of "Mrs. X's becoming a widow". Did Mr. X's dying cause Mrs. X to become a widow? I mean it seems like it does, but it also seems odd to suggest that she has to wait 1yr to be properly referred to as a "widow". And I'm not talking about whether or not she knows she is a widow,
It would seem rather odd to call someone a widow when they are completely unaware that they are one. But anyway...

I'm talking about whether she actually is one! It seems like, in that case, either two events have causal contact instantaneously despite the rules governing FTL, or Mrs. X isn't actually a widow until a year later, which seems absurd! Can we get out of this without supposing that there are such things as "non-physical events"????
The event was the death of the husband. Mrs X becoming a widow is the same event.

Erik Mesoy
Feb 12, 2008, 04:01 PM
Fifty: Your theory was proposed by Terry Pratchett in Mort. Monarchy is instantly hereditary and is probably propagated by FTL particle called kingons or queenons, and one could send hypothetically send information by having a heir in one place and torturing a king in another place. However, before this could be put into practice, the bar where it was being suggested closed. :p

What's going on here, I think, is that simultaneity doesn't exist as we know it across relativistic distances. "Last Tuesday at midnight" for Mr X may be a completely different time for Mrs X depending on frame of reference, so whether she "waits" a year or no time or up to two years is observer-dependent.

Gogf
Feb 12, 2008, 04:05 PM
Ok so here's something weird, and I'm curious as to whether this is actually true, or if I'm just muddling things up or what:

So suppose Mr. X lives on planet A, and his wife Mrs. X lives on planet B, which happens to be one light year away. They live this far apart because Mr. X is an intergalactic space-miner out on a mission for alien ore! Suppose that last Tuesday at midnight, Mr. X was killed in a terrible space-mining accident! Now consider this further event, the event of "Mrs. X's becoming a widow". Did Mr. X's dying cause Mrs. X to become a widow? I mean it seems like it does, but it also seems odd to suggest that she has to wait 1yr to be properly referred to as a "widow". And I'm not talking about whether or not she knows she is a widow, I'm talking about whether she actually is one! It seems like, in that case, either two events have causal contact instantaneously despite the rules governing FTL, or Mrs. X isn't actually a widow until a year later, which seems absurd! Can we get out of this without supposing that there are such things as "non-physical events"????

So, um, what the crap is going on here!?!?? I don't doubt that I'm wrong about something here, I'm just not sure what...?

I would argue that "Mr. X dying" and "Mrs. X becoming a widow" are two names for the same event, so there is no causation happening in the first place.

EDIT: Grah, I said it on chat before you, damnit, Mise :p!

Fifty
Feb 12, 2008, 04:05 PM
The event was the death of the husband. Mrs X becoming a widow is the same event.

Ahh! So you're sayin' that they are numerically identical, and that the utterance "Mr. X's death caused Mrs. Y to become a widow" is meaningless??? I like it!

Heretic_Cata
Feb 12, 2008, 04:08 PM
@post4

I guess it depends who asks your question. From a god-mode perspective (or computergamer perspective :D) you will realise that in the instant the man dies she becomes a widow even tho she doesn't know it.

I'd like to thank my supreme lack of knowledge about quantum(?) physics for coming up with this.

Bill3000
Feb 12, 2008, 04:10 PM
Ok so here's something weird, and I'm curious as to whether this is actually true, or if I'm just muddling things up or what:

So suppose Mr. X lives on planet A, and his wife Mrs. X lives on planet B, which happens to be one light year away. They live this far apart because Mr. X is an intergalactic space-miner out on a mission for alien ore! Suppose that last Tuesday at midnight, Mr. X was killed in a terrible space-mining accident! Now consider this further event, the event of "Mrs. X's becoming a widow". Did Mr. X's dying cause Mrs. X to become a widow? I mean it seems like it does, but it also seems odd to suggest that she has to wait 1yr to be properly referred to as a "widow". And I'm not talking about whether or not she knows she is a widow, I'm talking about whether she actually is one! It seems like, in that case, either two events have causal contact instantaneously despite the rules governing FTL, or Mrs. X isn't actually a widow until a year later, which seems absurd! Can we get out of this without supposing that there are such things as "non-physical events"????

So, um, what the crap is going on here!?!?? I don't doubt that I'm wrong about something here, I'm just not sure what...?

In this specific instance, both the events "Mr. X dies" and "Mrs. W is a widow" are simultaneous because they occur at the same time and place. Simultaneity will be broken if the two events occur at different distances, and if there is a relative velocity between the two planets (which there obviously will). This is a result of the Lorentz transformations in special relativity. It doesn't require them to be the same event.

lovett
Feb 12, 2008, 07:22 PM
So suppose Mr. X lives on planet A, and his wife Mrs. X lives on planet B, which happens to be one light year away. They live this far apart because Mr. X is an intergalactic space-miner out on a mission for alien ore! Suppose that last Tuesday at midnight, Mr. X was killed in a terrible space-mining accident! Now consider this further event, the event of "Mrs. X's becoming a widow". Did Mr. X's dying cause Mrs. X to become a widow? I mean it seems like it does, but it also seems odd to suggest that she has to wait 1yr to be properly referred to as a "widow". And I'm not talking about whether or not she knows she is a widow, I'm talking about whether she actually is one! It seems like, in that case, either two events have causal contact instantaneously despite the rules governing FTL, or Mrs. X isn't actually a widow until a year later, which seems absurd! Can we get out of this without supposing that there are such things as "non-physical events"????


Well, I'd suggest that 'widowship' is not an absolute truth. Rather, as a concept based on human constructs (marriage, society, ectcetera) it would rely on society becoming aware of it before it could be argued as an objective fact.


On a similiar note to the original question, does anyone know how fast gravity is meant to travel?

Fifty
Feb 12, 2008, 09:06 PM
Well, I'd suggest that 'widowship' is not an absolute truth. Rather, as a concept based on human constructs (marriage, society, ectcetera) it would rely on society becoming aware of it before it could be argued as an objective fact.

huh? that doesn't make any sense at all...

Souron
Feb 12, 2008, 09:51 PM
Ahh! So you're sayin' that they are numerically identical, and that the utterance "Mr. X's death caused Mrs. Y to become a widow" is meaningless??? I like it!The word "caused" has so many different meanings. On gut instinct I would say that it is ok to say "Mr. X's death caused Mrs. Y to become a widow". Since "caused" is not a technical term, and is not being used formally, gut instinct should be sufficient.

But if you try to find a unambiguous synonym, you realize all that is being said is: Mr. X is dead, therefore Mrs. Y is a widow. That implication completely and unambiguously states the previous claim. So apparently caused is simply being used as a synonym for implication. Implication does not have anything to do with time, so the speed of light is irrelevant.

Speedo
Feb 12, 2008, 11:11 PM
As far as we know: no.

It's usually expressed on the lines of "information cannot travel faster than the speed of light."
What about tachyons?

Souron
Feb 12, 2008, 11:26 PM
Actually, there is another issue here that can arise that does relate to simultaneity and the speed of light.

We can agree that in order to be a widow, a woman needs to outlive her husband.

Let us define a coordinate system such that Mr X lived above Mrs Y.

Like all people, they both die. Assume that there exists a reference frame in which they both die at the same time. Alice happens to live in this reference frame. Bob is in a different reference frame. He was moving up when he observed -- saw -- Mr X and Mrs Y die. Claire is also observed both deaths. She was moving down. Both Claire and Bob were moving at relativistic speeds relative to Alice.

Assume that Alice, Bob, and Claire were all the same distance from both Mr X and Mrs Y when Alice saw both deaths. Then Bob, who is moving up toward Mr X, will observe Mr X die first, then Mrs Y die. Because he is equidistant form Mr X and Mrs Y, and the light traveling from both is at the same speed, he concludes that Mr X does indeed die first. Claire's perspective is Symmetric to Bob's except she is moving down toward Mrs Y. So she observes Mrs Y die first.

Furthermore, there is nothing special about Alice's frame of reference either. If Bob claims that Mr X died t seconds before Mrs Y, then it is possible to deduce what fraction of the speed of light he was traveling at. This function is 1 to 1, so it's inverse is also a function. For any time t we can calculate Bob's speed.* Now Bob moving relative to Alice is the same as Alice moving relative to Bob. Consequently, for any period t between the two deaths, there is a perspective such that the two death's are simultaneous.

Note that Mr X's and Mrs Y's speed were never mention in this rant.

Therefore, whether Mr X widows Mrs Y or Mrs Y widow Mr X depends the velocity and the position of the person making the claim.

PS. Sorry, I got carried away.

EDIT:
*Note however that Bob's speed must be between 0 and the speed of light. Which is a bounded region. I don't know the equation in question, but I think that it would yield bounds for the time that Bob observes between events. The obvious bound it that bob will observe Mr X die sooner than Mrs Y. This bound is approached as Bob's speed approaches 0. I'm not sure what value the time difference would be as Bob approaches the speed of light.

EDIT 3:
This should be correct now.

brennan
Feb 13, 2008, 03:20 AM
What about tachyons?
Tachyons are purely hypothetical.

Sure Mrs X is a widow, but she won't know it until the information of her husband's death gets to her. On the other hand Mr X's partner, who was present at the death will know and can propogate that information at a speed up to c.

Souron: "For simplicity, assume that Alice, Bob, and Claire were all equidistant from Mr X and Mrs Y when Alice saw both deaths. Then Bob, who is moving up toward Mr X, will observe Mr X die first, then Mrs Y die. Because he is equidistant form Mr X and Mrs Y"

If he's still equidistant then he isn't moving towards X and still thinks they happened at the same time. :p

Mise
Feb 13, 2008, 07:02 AM
@lovett: No-one knows how fast gravity travels. People assume it travels at the speed of light, but I don't think it's ever been proven. We haven't even confirmed the existence of gravitons, the gravitational analog of photons, yet. It's fairly safe to say, however, that it propagates no faster than light.

Souron
Feb 13, 2008, 12:19 PM
Souron: "For simplicity, assume that Alice, Bob, and Claire were all equidistant from Mr X and Mrs Y when Alice saw both deaths. Then Bob, who is moving up toward Mr X, will observe Mr X die first, then Mrs Y die. Because he is equidistant form Mr X and Mrs Y"

If he's still equidistant then he isn't moving towards X and still thinks they happened at the same time. :p
Nope. Since He's moving towards Mr X the light from Mr X reaches him first. Since He's equidistant and since the speed of light is constant, he must conclude that Mr X does in fact die first.

Mise
Feb 13, 2008, 12:27 PM
Souron, can you draw a diagram? I saw it the same way Brennan did.

Souron
Feb 13, 2008, 01:31 PM
Souron, can you draw a diagram? I saw it the same way Brennan did.

After some googleing:
Some university site (http://www.phys.unsw.edu.au/einsteinlight/jw/module4_time_dilation.htm#simultaneity)
Youtube Video (http://www.youtube.com/watch?v=wteiuxyqtoM&feature=related)

Mise
Feb 13, 2008, 02:06 PM
After some googleing:
Some university site (http://www.phys.unsw.edu.au/einsteinlight/jw/module4_time_dilation.htm#simultaneity)
Youtube Video (http://www.youtube.com/watch?v=wteiuxyqtoM&feature=related)
The video was cool :D Makes sense now.

lovett
Feb 13, 2008, 02:40 PM
huh? that doesn't make any sense at all...

Well, the concept of being a widow is human. I.e, widowship was orignally created and then sustained by human society. Thus, surely it relies on being known by society (or a segment thereof) before being 'fact'.

In contrast, physical constants are completely independent of humanity.

xienwolf
Feb 13, 2008, 03:37 PM
I've been tossing this one through my head all day since reading it this morning. Decided not to post then because I was VERY tired and figured I might be wrong, so wanted to draw up the World Lines for it all.

The problem with this particular case is the wording of the example. The video you linked from You-Tube is how it is typically set up. They are at the equidistant point when the event occurs.

But it was phrased here that they were at the equidistant point when the information for the simultenaity arrived. In that particular case, they will agree that it was simultaneous, because they are both getting the information at the same time, because in that location, at that time, both sets of data exist together.

Now of course you could argue that they are not technically perfectly centered on the 2 events because then they would have to be sitting in the exact same point in space as each other, which is quite catastrophic at best. But in that case you wind up having it so that the first event viewed is the one you are traveling AWAY from, because you were closer to that event and it was traveling faster than you are, so it catches up to you.


So, the proper wording for the statement to have the results intended is that all 3 are at the same point when the deaths occur, and then 2 of them move away at relativistic speeds in the direction of each death (opposite directions)

warpus
Feb 13, 2008, 08:12 PM
I think that the woman in question is a widow as soon as her husband dies - she just doesn't know it yet.

Say that I'm married.. and my wife is somewhere in Africa doing missionary work. Am I a widower as soon as she is hacked to pieces by a group of crazed monkeys?.. or do I become a widower as soon as I find out? I move that I am a widower the entire time, but just do not know it yet.

Now, since there is no such thing as a universal present, the question of WHEN I become a widower is a bit more complicated, even assuming we all go with my definition above.

It really depends on the velocity through space-time of the observer. So, to one observer I become a widow March 21st, 2008, 5:48pm, to another one, same day at 5:47pm, to yet another one Feb 20th, 2874. I suppose my own reference point is the most important here, so for most intents and purposes we should just go with that.. but yeah, I'd say she's a widow without knowing it.. until she finds out.

Souron
Feb 13, 2008, 08:55 PM
Words in dispute:
For simplicity, assume that Alice, Bob, and Claire were all equidistant from Mr X and Mrs Y when Alice saw both deaths.

I've been tossing this one through my head all day since reading it this morning. Decided not to post then because I was VERY tired and figured I might be wrong, so wanted to draw up the World Lines for it all.

The problem with this particular case is the wording of the example. The video you linked from You-Tube is how it is typically set up. They are at the equidistant point when the event occurs.

So, the proper wording for the statement to have the results intended is that all 3 are at the same point when the deaths occur, and then 2 of them move away at relativistic speeds in the direction of each death (opposite directions)That's what I said. They are equidistant when Alice sees both events. "When the events occur" is confusing since the events do not occur at the same time in all reference frames. Heck, same time at all reference frames is not even a meaningful description.

Now of course you could argue that they are not technically perfectly centered on the 2 events because then they would have to be sitting in the exact same point in space as each other, which is quite catastrophic at best. Alice, Bob, and Claire do not have to be at the same point. They just have to be equidistant from both events. In fact, not only does their position not matter, but any sideways motion would not matter either. Only motion along the up-down axis matters.

Erik Mesoy
Feb 14, 2008, 02:55 AM
After some consideration, I move that Mrs. X is a widow starting when the light cone of Mr. X's death intersects Mrs. X's world line.

Any objections?

brennan
Feb 14, 2008, 03:15 AM
Points equidistant from Mr and Mrs X occupy a Plane Xienwolf. There are plenty of equidistant points for different observers.

Really all I was doing was pointing out that Souron's wording was wrong.

Erik, I would say that Mrs X is a widow as soon as her husband dies. She just isn't aware of the fact.

Imagine the legal wrangling that will occur the first time this becomes a real concern.

Erik Mesoy
Feb 14, 2008, 07:12 AM
Erik, I would say that Mrs X is a widow as soon as her husband dies. She just isn't aware of the fact.I don't even understand what "as soon as" means in this context. Please be so kind as to spell it out for me, because I'm under the impression that special relativity explicitly denies absolute simultaneity.

My approximate mental state in this thread:
:confused: :crazyeye: :twitch: :dunno: :shake: :ack: :huh: :rolleyes: :smoke: :dubious: :hmm:

brennan
Feb 14, 2008, 07:42 AM
Well If Mr and Mrs X are on planets, then they have a roughly similar reference frame to the rest of the galaxy they are in. That makes it fairly easy to establish an arbitrary standard reference frame for all such measurements.

Basically, unless the universe you occupy is empty except for people zipping about at relativistic velocities in all directions, then it's easy to set a standard reference. :)

Erik Mesoy
Feb 14, 2008, 09:12 AM
Earth orbits Sol at 30 km/s which gives a Lorentz factor around 1.00005, which adds about 26min to a year (light or otherwise). That's enough to shift widowhood before or after midnight with a good margin of error, so I think it counts as a relativistic velocity when it can mess up bureacracy.

Edit: Hmm, this was my 8888th post. :)

xienwolf
Feb 14, 2008, 09:43 AM
After some consideration, I move that Mrs. X is a widow starting when the light cone of Mr. X's death intersects Mrs. X's world line.

Any objections?

This is precisely how the problem must be approached for a correct answer.

Points equidistant from Mr and Mrs X occupy a Plane Xienwolf. There are plenty of equidistant points for different observers.

Really all I was doing was pointing out that Souron's wording was wrong.

Erik, I would say that Mrs X is a widow as soon as her husband dies. She just isn't aware of the fact.

Imagine the legal wrangling that will occur the first time this becomes a real concern.


Yes, points that are equidistant from them both do occupy a plane. However, the light will expand from each event in a sphere, and the spheres will intersect each other in an expanding Circle (which is in that plane). So I was incorrect to state that they would have to be in the exactly same individual point because that is only true to see the events as simultaneous at the soonest possibile moment. To see the events as simultaneous at any other point, they may occupy any position in the expanding circle of intersecting light cones.

Words in dispute:
For simplicity, assume that Alice, Bob, and Claire were all equidistant from Mr X and Mrs Y when Alice saw both deaths.

That's what I said. They are equidistant when Alice sees both events. "When the events occur" is confusing since the events do not occur at the same time in all reference frames. Heck, same time at all reference frames is not even a meaningful description.

"When the events occur" is defined by tracing the light back in the reference frame of your stationary observer. Since you have defined them as your stationary frame, you have defined that as your overall reference frame, and thus you are allowed to state that both events occured in that frame at the same time just to set the initial conditions. It will not matter which person's reference frame you actually define as your stationary observer, you will get the same results for every drawing of the world lines.

Alice, Bob, and Claire do not have to be at the same point. They just have to be equidistant from both events. In fact, not only does their position not matter, but any sideways motion would not matter either. Only motion along the up-down axis matters.

Sideways motion does matter. It is TANGENTIAL motion that doesn't matter. And it is not possible to move tangential to both spheres at once. That is part of what led me to stating incorrectly that they would have to be at the exact same point as each other because I had them traveling along the axis connecting the two events to avoid tangential elements.

Souron
Feb 14, 2008, 01:05 PM
Sideways motion does matter. It is TANGENTIAL motion that doesn't matter. And it is not possible to move tangential to both spheres at once. That is part of what led me to stating incorrectly that they would have to be at the exact same point as each other because I had them traveling along the axis connecting the two events to avoid tangential elements.
:hammer2: That makes so much sense! Thanks for correcting me. :hammer2:

Tank_Guy#3
Feb 14, 2008, 04:12 PM
Here's another speed of light question I've pondered:

It's stated that nothing with mass can't accelerate to the speed of light. However, would it be possible (if not concievable) to have an instantaneous jump from motionless to light speed?

Let's for a moment completely disregard the stresses applied to the object that is doing the "jumping".

xienwolf
Feb 14, 2008, 05:46 PM
Well, any change in velocity is called an acceleration. An non-continuous shift would simply be an infinte acceleration (definably with a Dirac Delta for different levels of jumping Velocity), not a lack of it.

Souron
Feb 15, 2008, 07:19 PM
"When the events occur" is defined by tracing the light back in the reference frame of your stationary observer. Since you have defined them as your stationary frame, you have defined that as your overall reference frame, and thus you are allowed to state that both events occured in that frame at the same time just to set the initial conditions. It will not matter which person's reference frame you actually define as your stationary observer, you will get the same results for every drawing of the world lines.But see, I did not define a stationary observer. Fifty defined Mr and Mrs X as being stationary, but that has no impact on the scenario.

xienwolf
Feb 16, 2008, 12:20 PM
You said Alice was in that reference frame, so while not neccessarily completely stationary, she would be at the least non-relativistically mobile in relation to the event. Which is good enough to define that as the stationary reference frame, no?

Souron
Feb 16, 2008, 05:09 PM
I did not say Alice was in that reference frame. I only said that Alice saw both events happen at the same time, and that she was equidistant from the two events. Neither of those imply that she is in a non non-relativistically mobile plane relative to Mr X and Mrs Y. It is entirely possible that Claire or Bob is in that reference frame.

Souron
Feb 16, 2008, 05:23 PM
After some consideration, I move that Mrs. X is a widow starting when the light cone of Mr. X's death intersects Mrs. X's world line.

Any objections?While this will yield consistent results (I think), that fact is if Mr X's death's light code intersects Mrs's Y's light cone, Mr X must already have died at a time proportional to the distance between the intersection, and the point of death (and the speed of light).

xienwolf
Feb 16, 2008, 07:55 PM
Ok, looked back and read closer, you did indeed never set a reference frame for a stationary observer as the one which includes the 2 people's own reference frames during their lives.

Assume that there exists a reference frame in which they both die at the same time. Alice happens to live in this reference frame. Bob is in a different reference frame.

But that does state a reference frame, and the very nature of establishing a reference frame is to set your "stationary observer" because that is just another way to state that it is your reference frame. I do not remember precisely why we are debating the reference frame anyway at this point, and am disinclined to check :) Needless to say, you are correct in what you stated ;)

While this will yield consistent results (I think), that fact is if Mr X's death's light code intersects Mrs's Y's light cone, Mr X must already have died at a time proportional to the distance between the intersection, and the point of death (and the speed of light).

Correct, and the same can of course be stated about Mrs. Y (that she has to have already died at a time proportional to the distance between the intersection and the point of death).

And the light cone/World-line approach will indeed yield cinsistent results, it was designed for just these kind of thought experiments.


If you want a really fun one, you look at a train entering into a tunnel that is the same length as the train itself in the Earth Frame. Set the train moving at relativistic speeds and compare the sequence of events (front of train enters tunnel, back of train enters tunnel, front of train leaves tunnel, back of train leaves tunnel), as seen from an observer on the train and an observer standing outside the tunnel (equidistant from the 2 ends).

This is the standard thought experiment to demonstrate mass contraction, which is far more enjoyable than Time dilation anyday.