Why does it matter if communications happen instantly or faster than the speed of light.
If we send a message to the Mars lander in 1 second and get a reply in another second then so what?.Some say this violates causality,I say not.
It is violated only if the reply arrives before the send.
That is backwards time travel, not for this topic.

Am I missing something?.

Did Einstein say that FTL communication was impossible because at that time it was?.

FTL travel of anything physical is impossible. Typically, communications to space probes are sent in radio waves, which can't be faster than the speed of light.
Quantum communication is possible but it's not technically FTL because it does not travel but relies on twin particles.

And yes, it does matter. For example, the communication delay from light speed even to mars means no real-time remote steering is possible - the feedback time takes too long. And think of how much longer it will be once we get out of our solar system...

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the fact that events x, y and z can happen in the time it takes light/information to go from A to B means that we CAN have time, if that weren't possible then there would be no time.

At least, that is how I see it.

I only think I understand what 'Frog march' has just said... and he's right.
When watching television news feeds from halfway around the Earth, I see the time gap as the distance consumes the seconds and we wait............ for the response.
Obviously from Mars, some seconds will be lost in light speed communications.
We know of no way to communicate faster., and may never.
Phoning home will be a time consuming exercise.

I agree. If someone did discover a way of communicating information faster than light can travel, I don't see how it would violate causality. (In science-fiction they have sublight communication.)

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Yes he did, and that is the general assumption today.

But you are right about how “instant” communication would not violate causality. With hypothetical “instant” communication, all you’d have is the same as when you are standing in front of someone talking to them (assuming the shortest possible distance for sound and light travel). So, someone is not going to answer your question before you ask it. They can only answer it after you ask it, and that would be the same with instant communication at great distances.

I read a popular FTL book a few years ago, and the author speculated that if someone could invent a computer that had electronic signals that could travel through its circuits faster than light, the computer could answer a question before the question was asked, but that’s not true. The computer could only answer the question “instantly”, but not before it is asked.

That is indeed a strange phenomenon. One time I had to talk to a technician in India who was giving me instructions about how to program an electronic device here in the US. There were so many up and down satellite links used by the phone companies, there was at least a two second delay in our conversation. Since the geostationary satellites are about 22,240 miles up, one up and down link to and from a single satellite would require about 44,480 miles of signal travel distance, which is about 1/4th of a second of light travel time (radio signal travel time). So, just 4 up and down links would delay the signal about 1 second, since the total signal travel distance would be about 177,920 miles, and the signal travels at about 186,000 miles per second. Since there is cross-communication going on in telephone calls to India and TV broadcasts from England or Iraq to the US, it would take about 2 seconds to complete the cross-communication link if 4 satellites are used.

A reporter in the US asks a question, there is a 1 second delay while the signal travels to England or Iraq or India, the reporter on the other end answers the question, and there is another 1 second delay for the signal containing the answer to return to the US. That’s why the remote field reporters seem to be standing there not doing or saying anything after the question is asked in the US. The reporter actually answers the question 1 second earlier than we see him answer it.

On some occasions, such as half-way around the world communications, more than 4 satellites might be used for the up and down links, and that would delay the signals even longer.

Interestingly, if we could cause the satellites to move at high speed out and away from the earth while this communication was going on, the signals would be received as being redshifted and the voices would be heard in slow motion. Of course we would have to re-tune our electronic receivers to be able to pick up the new frequency of the redshifted signals.

This is more ATM than anything...

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Here's a good explanation why a causality violation is implied by FTL:
http://www.sheol.org/throopw/tachyon-pistols.html

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No it's not. No one here is saying that there is or could ever be any such thing as FTL communication. I think there never will be any FTL communication. I think such a thing is impossible. So there is no ATM discussion going on here at all.

Faster than light communication, plus the well-tested special or general theory of relativity (take your pick), means one can send signals to their own past, violating causality. To say otherwise IS most definitely ATM.

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cjl

^If you warp out, go to Tau Ceti, then with normal reaction engines accelerate away from earth, warp out again to go back to earth, you will indeed get back before you left. (Presuming that the real-space delta-v before the warp/hyperdrive/tachyon-watziz trips was "large enough"... there are formulas for such things in the textbooks). ^

What nonsense.
It would take a certain time to get there (call it T)and no time to get back(call it zero).Total travel time is T+zero.Return time is T ,AFTER leaving.

I didn't even notice that part, and it's irrelevant to the argument. Did you even read the other part?

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cjl, in the first place, there are no such things as "tachyon pistols", so all you've posted is a sci-fi fiction story.

That was the question in the OP: What if there was faster than light communication? Or, put another way, "tachyon pistols." It's not science fiction to consider the implications of FTL under theory.

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The tachyon pistols are arbitrary devices that can transport information faster than the speed of light.

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The way I understood the OP was that it had two objects that were basically stationary with each other, us on earth and the Mars lander. The question was about what if we could send a FTL or “instant” signal to the lander. That question didn’t involve any objects moving at high speeds while sending FTL or “instant” signals to each other.

Yes, but the Sci-Fi example you linked to had people moving at high speeds relative to each other. But the OP did not.

Doesn't matter. If you have FTL communication, there will be causality violation in some frames.

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Impossible. Instantaneous signals going from earth to Mars and back would be the same as a light signal in your room going from a light bulb to your eyes. The travel distance is so short in your room, that’s virtually the same as instantaneous light signal travel time. You can’t receive the light until the bulb starts to glow, so you can’t receive the light signal before the light is turned on, and the light can’t activate a photocell receiving device before the light is turned on, even if the light signal travel time from the light to the photocell is instantaneous, i.e. with the light bulb pressed up against the photocell. So the photocell can't activate before the bulb is turned on, even if the travel time from the bulb to the photocell is instantaneous.

You can make such statements, but you can give no examples where what you say actually happens, when the signaling device is stationary with the receiving device as stated in the OP and the signal travel time is instantaneous.

Going back to the OP, I think the simplest way is to think this: information has to be carried by something. It could be light signals or radio signals. But it has to be something. And according to Einstein, nothing, not even light, can travel faster than the speed of light.