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There exists an alternate dimension where its velocity of light equals our velocity of sound and its velocity of sound equals our velocity of light. The velocities are reversed but absolutely anything this would realistically affect stays the exact same and is ignored simply because that's how the dimension exists.

On this dimension's Earth — it being the exact same historically — all known wars have been fought, but the technologies and war tactics used during them were drastically different.

What would be some useful technologies and feasible war tactics based on these altered physics? Long ago, it was a best-practice to detect approaching enemies by listening to the ground. I was thinking something like the reverse of this could be possible in this world. Also, technologies such as laser guns wouldn't be nearly as effective or useful.

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    $\begingroup$ Handwavium aside, this causes a lot of consistency problems that your question needs to address directly. Do the atomic forces still react at traditional light speed? If not, i'm fairly certain the planets never coalesced. If so, I'm absolutely positive hardwire communication still approaches traditional c. $\endgroup$
    – Nohbdy
    Oct 27, 2015 at 4:35
  • $\begingroup$ Welcome to the site Adam. This feels way way to broad, particularly without defining how the physics work first. $\endgroup$
    – James
    Oct 27, 2015 at 5:36
  • $\begingroup$ Would you mean speed of light as in c or just as in the magnitude of its velocity in the medium of the fictional planet's atmosphere? $\endgroup$
    – timuzhti
    Oct 27, 2015 at 5:43
  • $\begingroup$ @Alpha3031 The magnitude of its velocity. I'm trying to keeping the physics knowledge here as simple as possible. $\endgroup$
    – Adam
    Oct 27, 2015 at 5:58
  • $\begingroup$ @James I included that the speeds are reversed and absolutely anything this would realistically affect is ignored for this case (besides the fact that technology and war tactics would change, of course). $\endgroup$
    – Adam
    Oct 27, 2015 at 5:59

4 Answers 4

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Talking realistically, such a world is not possible. Speed of light is supposed to be the fastest possible velocity in the universe. If you theoretically cross the speed of light, you would find yourself in another universe which would have some amazing mind-bending properties. Another thing is that you will see an extremely weird phenomena of stars in the night sky. Even with the current speed of light, the stars are light-years away from us. When the speed of light is just 0.0001% of its original value ...

Anyhow, forgoing that, let us focus on your question. If the speed of sound is equal to the speed of light in your world, it would mean that ... well distance communication would be really really slow. Sound fades quickly over distance and even though it has an extremely fast speed, that would not affect its diffusion rate in the air (unless you specifically make it so).

Sound cannons would be a potentially lethal weapon in your world. Transmitting high volume, concentrated waves of sound, the weapon would immediately kill or permanently disable its target(s). Also since sound is travelling at extremely fast velocity, you should be able to detect the march of an army the moment the horses start trampling the ground.

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  • $\begingroup$ Actually c is an upper bound to the speed of light (it's the speed of light in vacuum). Light can go slower, all the way down to about 40 miles an hour. $\endgroup$
    – user11599
    Oct 27, 2015 at 3:59
  • $\begingroup$ Yes, but nothing can go faster than the speed of light in vacuum. Plus, most of the light we see around go around at 40 mph. $\endgroup$
    – Youstay Igo
    Oct 27, 2015 at 4:16
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It's hard to think of all of the ramifications, but satellite based communications would not exist, because sound does not travel through space as there is a lack of air.

You would be able to hear an enemy plane coming, but not see it, so a different form of targeting would be needed. Perhaps a series of microphones to function as a traditional antenna array.

Missiles would be invisible so stealth technology would be based on making things very quiet.

Strangely music and conversations wouldn't change that much since they sound instantaneous to us anyway. Megaphones might replace military walkie talkies, but the enemy would be able to hear it too.

A major problem you'd have to define is what happens to EM fields that are not visible. Radio waves travel at the speed of light, so I assume they wouldn't be as useful, but electricity shouldn't be affected, so running a lot of wire for communications would be more common.

Sorry. These aren't real answers, but maybe it will help just the same.

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    $\begingroup$ Satellites would still be possible, just unbelievably slow (in comparison to what we are used to). It might take a day to get a message to the other side of the world, but this might still be the best way. Electrical signals would almost certainly be slowed since EM radiation is slowed, and electrons influence each other through propagation of EM radiation. $\endgroup$
    – Alex S
    Oct 27, 2015 at 4:04
  • $\begingroup$ @AlexS: I agree with your first point, but the second one is harder. See oposite argument in the comment to the question by Sean Boddy. What about nervous signals? On the other hand, I do not know why slow interactions inside atoms would cause planets not to form. $\endgroup$
    – BartekChom
    Oct 27, 2015 at 5:36
  • $\begingroup$ @BartekChom I'm sticking by my guns on this one. Light, visible or not, is just the propagation of electromagnetic radiation. Saying light is slower is just saying that electromagnetic radiation propagates more slowly. An electric current is created by one electron moving closer to another and increasing the electric field nearby, causing the other electron to move, and so begins a domino effect. A slower propagation speed for electromagnetic radiation leads to this whole electron dance happening much more slowly. $\endgroup$
    – Alex S
    Oct 27, 2015 at 14:30
  • $\begingroup$ But I concede that Sean Boddy is probably right that this would prevent gravity (since gravity also works at the speed of light) from pulling together masses or prevent the weak nuclear force from doing its job. But that makes for a really boring story, so I've got to suspend my disbelief somewhere. $\endgroup$
    – Alex S
    Oct 27, 2015 at 14:34
  • $\begingroup$ I didn't say satellites weren't possible, I said satellite communication wouldn't be possible or at least not feasible. $\endgroup$
    – ozone
    Oct 28, 2015 at 1:53
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All wars up until the 1850's would be much the same. The telegraph was developed during this time. This was the first time communication speed was comparable to the speed of light. Even so, it was still very slow for a while. The first transatlantic telegraph message took seventeen hours to transmit (in 1858). Things quickly picked up. By 1866, transatlantic telegraph messages could be exchanged at eight words per minute. By contrast, even at peak speeds, a message traveling at the speed of sound would take four and a half hours to go from New York to London.

It would be impossible to replace light speed communication with sound because sound dissipates very fast compared to electromagnetic radiation, so it would take an unbelievable amount of power to produce messages in sound which can be heard more than just few dozen miles away. Not to mention the damage done to the ears of those near the source.

In the real universe, rapid messaging is essential between military units within a theater. Spread across hundreds of miles, communication at the speed of sound would be on the scale of hours. I do not think this would affect tactics too much, except each unit would be in the habit of communicating large amounts of information all at once so their comrades have as much information as possible, without the need for back and forth communication. Those sending messages would be better at making messages detailed and clear. Plans would be laid out in great detail with many contingencies to limit the need for real-time communication.

Drones and the like would be pretty much impossible. You couldn't react fast enough to make it useful. Even computers would be noticeably slower. That said, modern computer technology is very fast, so weapons with onboard computers may have small, modern-looking computers which are as slow as computers in the seventies.

There is one thing that would work better: sonar. Sound under water has a greater range than sound in air. It's not great, but active sonar between submarines works on the range of tens of nautical miles. if this communication was at what we would call the speed of light, then fast communication is possible through sonar. For instance, a system of buoys twenty nautical miles apart could be arranged across the Atlantic ocean. If each buoy receives a sonar message and then re transmit it. If each transmission takes a minute, then you could send a message from New York to London in less than three hours, a little faster.

If this situation were reality, technologies would be developed to take advantage of the fast speed of sounds. We would develop better microphones, louder speakers, and longer under water communication. I disagree that sound could be used as a weapon more effectively in this universe than here. The speed of sound is the speed at which molecules transfer energy. I don't think that over the range of several hundred feet we would really notice a difference. Even at large distances, you would have to pay close attention to notice a difference.

There are other implications to faster than light communications. There is potential to violate causality: you could observe an effect before a cause. This is not good, and very complicated. Unfortunately, I am not a good enough theorist to come up with any specific examples.

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That does not hold together logically. Sound has a speed due to the stiffness of material. How could the speed of sound change without changing all the essential properties of the materials?

You didn't say "in what" as sound travels differently in air, water, steel, etc.

Assuming you mean spefically air, you are talking about air being as stiff and dense as neutron star matter, which is very different from air as we have it.

You can't change sound and claim that nothing of interest is different!

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  • $\begingroup$ I disagree. The premise of my question relies on the fact that this is an alternate dimension. It's perfectly reasonable to assume these changes will break the universe, but in this dimension, it simply works because that's how the dimension exists. $\endgroup$
    – Adam
    Oct 27, 2015 at 7:28
  • $\begingroup$ "in every way exactly like our own except..." $\endgroup$
    – JDługosz
    Oct 27, 2015 at 7:44
  • $\begingroup$ I wrote that knowing what I meant in my head without considering anything else. I'll clarify what I mean by it. $\endgroup$
    – Adam
    Oct 27, 2015 at 7:52

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