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Allow me to elaborate it further. Let's say ship A and ship B are trying to find each other in space. Ship A uses its passive radar to scan the sky to detect ship B's heat signal. But I'm thinking, because the speed of radio wave is only equal to light speed. So in order for ship A to avoid detection, it must be outside the 300,000 KM range, because at that distance ship A could only see ship B's past location (at least 1 second ago). Is that correct? So if a ship wants to maneuver in space to avoid detection from the enemy's radar, it must be far enough so that the light speed lag could conceal its current position?

I'm asking this because I think, in a realistic sense, laser weapons can fire a beam at light speed. So it's rather pointless to try to maneuver within that 300,000 KM range. Outside that range, the light speed lag will provide both side with a change to avoid detection and getting hit by beam weapons?

closed as off-topic by Valorum, James Sheridan, Ward, Mike Scott, The Fallen Apr 6 '14 at 12:26

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  • 2
    In science fiction, nobody uses radio signals. There are faster than light way of communication. And, there are cloaking mechanisms to evade detection. – Lobo Apr 6 '14 at 5:31
  • @SachinShekhar In some science fiction there are FTL communications and "cloaking mechanisms". Science fiction != Star Wars. – user14111 Apr 6 '14 at 5:56
  • @user14111 Did I say Star Wars? Star Trek also has the same scene. The thing is: Most of popular space based franchises have it. Or, they won't be popular. – Lobo Apr 6 '14 at 6:02
  • Also, there is nothing "boundary-ish" about 300,000 km. Sure, you can see where a spaceship was 1 second ago, but that doesn't stop us seeing it. We can also see the Moon... – Mr Lister Apr 6 '14 at 6:57
  • In sci-fi, scanners often work faster than light-speed. In Trek + Star Wars + Bank's "Culture" novels (for example) scanners are based on hyperspace technology and see things instantly – Valorum Apr 6 '14 at 9:00
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There is nothing special about that 300,000km range - why would there be? If you can't accurately fire at a target whose position you only get with a 1.001s delay, then a 0.999 delay isn't going to be any different. Instead, hitting the target gets continuously harder the greater the delay is.

But the decisive factor is not the delay itself: you can easily hit a target at any delay (be it seconds, minutes or hours), if you know its position, direction and speed with sufficient precision to predict its future position (and can aim with sufficient precision). And direction and speed can be calculated if you know the position at two different times.

Of course, this only holds if the target does not maneuver. If the target knows it's being shot at, it can try to dodge by maneuvering randomly. If it's able to change its actual position from your predicted position within the delay, you can only hit it if you get lucky.

Actually, this is not fundamentally different to what ships at sea did as far back as World War I - because near-instantaneous location of the target does not help if your projectiles have a significant travel time. Even in WWI, ships could (and did) fire at each other at ranges of many kilometers, which meant travel times of sometimes more than a minute for the projectiles. Small ships could not be hit that way because they could dodge, while large battleships were too massive to do that.

  • thank you, I did share the same thought about that. The reason I put 300,000 Km there because I think it – user24716 Apr 6 '14 at 8:59
  • thank you, I did share the same thought about that. The reason I put 300,000 Km because I think it's the minimum distance between 2 ships in order to make any maneuver meaningful. For weapons like laser, trying to maneuver within that range is rather pointless because the beam will hit the ship instantly. But let's say, if the ship is 5 minutes of light speed away, then it will take the same time for the beam to hit the ship. Also, the enemy ship could only see this ship's location 5 minutes ago. So that would be exactly like you said: mostly predicting and guessing the ship's next location. – user24716 Apr 6 '14 at 9:10

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