# Speed of Light Question

Discussion in 'Science & Technology' started by Fifty, Feb 12, 2008.

1. ### Fifty!!!!!!!!!!!!!!!!!!!!!!!!!

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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???

2. ### brennanArgumentative Brit

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As far as we know: no.

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

3. ### xienwolfChieftain

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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.

4. ### Fifty!!!!!!!!!!!!!!!!!!!!!!!!!

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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...?

5. ### Miseisle of lucy

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It would seem rather odd to call someone a widow when they are completely unaware that they are one. But anyway...

The event was the death of the husband. Mrs X becoming a widow is the same event.

6. ### Erik MesoyCore Tester / Intern

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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.

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.

7. ### GogfIndescribable

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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 !

8. ### Fifty!!!!!!!!!!!!!!!!!!!!!!!!!

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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!

9. ### Heretic_CataWe're gonna live forever

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@post4

I guess it depends who asks your question. From a god-mode perspective (or computergamer perspective ) 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.

10. ### Bill3000OOOH NOOOOOOO!Supporter

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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.

11. ### lovettChieftain

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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?

12. ### Fifty!!!!!!!!!!!!!!!!!!!!!!!!!

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

13. ### SouronThe Dark Lord

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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.

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15. ### SouronThe Dark Lord

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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.

16. ### brennanArgumentative Brit

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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.

17. ### Miseisle of lucy

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@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.

18. ### SouronThe Dark Lord

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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.

19. ### Miseisle of lucy

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Souron, can you draw a diagram? I saw it the same way Brennan did.

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