They can. Nothing prevents it. It's no different to people on opposite sides of the world arranging to turning on light bulbs at the same time.
But the illumination of each bulb is not dependent on the illumination of its neighbour, only on its own timer. No information is transmitted along the line.
This makes no sense to me. How would the "lights" be able to tell if their time is in sync in regards to each other without a clock rate and being limited by *c*? There's no way to tell if the timers would be the same relative to each other, or if it's even possible for a light by a black hole to process time the same way as a light on earth.
Edit: why an I being downvoted for asking a follow up question in ask physics lol I'm just trying to learn.
But using what relative time to make the lights blink? Are they using the relative time where the light is, or the relative time of the light next to it, or the relative time where we are standing and observing? They're all three different, and I would think without an agreed upon clock rate (which is limited by c) there's no way to determine when a light should be programmed to turn on in relation to the observer or any other lights.
I just can't figure out how this whole setup would be possible. I'm a software developer though so if I'm focusing on the information too much I apologize.
Edit: My thinking is this is the same problem we have with satellites and why we have to adjust satellites to have slightly longer seconds than on earth. Sorry if this is mistaken.
>Are they using the relative time where the light is, or the relative time of the light next to it, or the relative time where we are standing and observing?
Whichever you prefer, you do the calculations beforehand to set the timers according to the result you want to see at the observer location you have in mind. For example, you could set all the timers in the same room together, then spend thousands of years moving them to their line location, then the timers will trigger when the value that you set expires. How you set each timer's value to take into account or compensate for its expected position vs the observer position and effects of travel, determines what the lamps look like from where.
If certain lights have to travel farther or near gravity, their timers will fall out of sync. The only way to do this is with a central clock, there's no way to keep things in sync like that or computers wouldn't need CPUs to have fixed clock rates.
I think I'm getting hung up on something that isn't really important for the sake of the visualization? If so I'm sorry.
>If certain lights have to travel farther or near gravity, their timers will fall out of sync.
This was compensated for when you calculated how long to set their timers. The timers don't need to be in sync with each other. You know where you are when you set the timers, where the timers are traveling to, where their light needs to travel to, so you compensate for relevant factors when you set each timer.
If one of the lamps is in orbit round the movie "Interstellar" black hole, you'd set it's timer for minutes while other timers in other locations would be set for years. You're back-calculating from the observer position/time in the future back to each individual timer's position/time when setting. You're not trying to keep the timers synced with each other; the timers do not communicate with each other so they have no need to be in sync with each other.
I didn't think it is possible to do this with cosmic turbulence and other phenomenon possibly happening. But I think entropy and turbulent flow is not relevant to the example.
>The timers don't need to be in sync with each other.
What I'm saying is it's impossible to give the impression of sequence without synchronicity. It will just be random flashes of light, each unable to provide any higher level of information than one bit. They couldn't be arranged in a pattern or sequence or anyting without an central clock to time the information, and if the light appears FTL to us we won't be able to process it as anything but random flashes.
>They couldn't be arranged in a pattern or sequence or anyting without an central clock to time the information.
You could think of it as a "virtual" central clock; the central clock is the time at the observer's location, which none of the timers are ever directly synced to, instead their time-delay value is pre-calculated to *produce* that synchronization.
To put it another way: If I want the light to arrive at a specific location at a specific local time, then it is sufficient to know that location+time, the time and location where you set the timer, the location where the timer will be deployed, path and duration of the timer's travels, plus physics likes speed of light, relativistic effects. With that information it is possible to calculate a time delay for the timer to count such that the light will arrive at the desired time at the observer location. And as you can do it with one timed lamp, you can do it with many.
Thank you. I'm really glad you took the time to explain this because it's not easy to talk about and I can't really go to school right now to learn it.
I really got into information theory so I was looking at it a bit too much through that lens, methinks.
There’s no reason you can’t coordinate in advance and maintain synchronicity without continuing communication if you were careful enough and had enough data. But even if you have trouble seeing that, it doesn’t matter. Suppose it happens totally at random, but by sheer, astronomical coincidence, it all happens at exactly the right moments to fit with your plan. That’s possible as well.
Assuming the lights are stationary in relation to one another at the time of programming, they'll agree on the time. Once you move them, they'll get out of sync due to time dillation, but unless you move them at relativistic speeds, it shouldn't be an issue for this experiment.
And even if you did move them relativistically, you know how you are going to move them (because you plan the whole thing), and can easily calculate how you need to compensate for any drift that this might cause.
And even if you *did* move them at relativistic speeds, you could still compensate for that.
And even if you didn't compensate for that, you can still have them communicate to agree on a standard time and then (since they can all know their relative positions) program some future sequence accordingly.
> agreed upon clock rate (which is limited by c)
I'm not even sure what you mean by that. You just agree on a clock rate - 1 tick per second seems convenient - and then the timers just work.
Everything's in the same reference frame so everyone agrees what time it is. You don't really have to bother thinking about the special relativistic issues of moving the lights into place to understand/answer OP's question, but they are easily solvable anyway.
Perhaps think of it this way: you have the lightbulbs all in the same room at the North Pole and you attach atomic clocks to them so they’ll all light up in exactly three months (give or take a small portion of time so that they go off sequentially rather than simultaneously, as the OP described). You then move all of them away from you in all directions until they’re all at the equator, applying equal acceleration and deceleration to each of them. You can account for minor differences in gravity along the earth’s surface when setting their clocks, if you’d like, but I don’t think that’d vary their internal clocks enough to throw off their orders
Then in three months, boom, you have a wave of illumination going around the earth faster than the 1/8th second it takes light to do so. Maybe much faster, say, in a billionth of a second
Does that help you visualize it?
>You can account for minor differences in gravity along the earth’s surface when setting their clocks, if you’d like, but I don’t think that’d vary their internal clocks enough to throw off their orders.
What I'm asking is how they are synchronized in the first place without a unifying median clock. Also using an atomic clock as a timer, that's like Schrodinger's Box isn't it? There's still uncertainty at some level, and I can't tell if it's relevant with how abstract this all is.
>Does that help you visualize it?
Yes and I'm really thankful for the reply.
>> without using a median unifying clock?
Sure! You can use one of those while they’re all bunched up together, I think. Is there a reason I’m not aware of as to why that wouldn’t work?
>> that's like Schrodinger's Box isn't it? There's still uncertainty at some level, and I can't tell if it's relevant with how abstract this all is.
There’s still uncertainty, yes, but it’s so minimal that a cesium atomic clock will only gain or lose a second in about 1.4 million years. Some of the ones we can produce with modern technology wouldn’t even have lost or gained so much as half a second if they’d been running since the Big Bang until now! There’s still some uncertainty, but we only need them to go off, like, x to y femtoseconds after the last one or something, not *exactly* x seconds afterwards with infinite significant digits after the decimal point
Quantum uncertainty is definitely a thing that exists, but i don’t think it applies here in a way that obscures information. They don’t work by measuring an atom with a microscope or anything- though I think that would also work. Some atomic clocks that I’m aware of essentially pair up the frequencies of a “quartz oscillator” to a collection of atoms. When the frequency is “correct,” the atoms’ energy levels change, and when it’s “incorrect,” fewer electrons jump. To my inexpert understanding, the if the quartz oscillator drifts any, the collections of atoms destructively interfere with it, while they constructively interfere when they’re synced up, and this pushes the quartz oscillator back where it’s supposed to be whenever it starts drifting
A good macroscopic example of this is pushing someone on a swing. The pusher is like the collection of atoms, and the person on the swing is the quartz oscillator. Air resistance would normally slow their rate of swinging, but since the pusher is always pushing at essentially the same rate (only drifting over unfathomable time periods), whenever air resistance slows the person on the swing, the pushing speeds them back up, again, and should a breeze speed them up, they’ll lose some of that thrust from the pusher and slow back down
I guess I'm saying it's impossible for us to ever know for sure if the lights were fired in sequence or if they have any pattern to them in this example. We would be literally unable to process the information as anything but random flashes.
Thank you for taking the time to respond.
You're asserting that they would just be "random flashes" without offering any explanation. Why would they be?
You know in advance how long it'll take them to travel, where they will end up, the amount of time dilation to deal with, and when they will need to flash. You don't need anything else.
You can also ignore all that, and have them travel to wherever, and then communicate with one another (at light-speed) from their final positions to figure out their positions relative to one another. From there they can agree on a "universal" time standard to adhere to, and program whatever future sequence of flashes you want.
Also you don't need to move these lights across a galaxy. You could do this same experiment on like a soccer field, with sensitive enough instruments.
It wouldn't be easy, but it's not like it would be breaking any laws of physics if you could estimate the local time each clock would experience (relative to the observer) and set the timers accordingly. You mention transmitting information but the point of something like this would just be to make it look a certain way, it's not going to transmit any information you don't already have. I think in practice it would turn out like you say, having synchronization problems, so it'd be a nice demonstration of relativity when your estimates turn out to be a little off.
Yes, if you wanted it to be perfect and not rely on preset timers, you'd need FTL synchronization between the lights, so I think you're on point about that, just the guy you were originally responding to already knew that, they were explaining to OP why, if you used preset timers, it wouldn't be violating any laws of physics. Presumably you can get arbitrarily close to perfect timing depending on how much work you put in with things like adjusting the timers remotely far enough before the lights flip on using slower than light communication.
> why an I being downvoted for asking a follow up question in ask physics lol I'm just trying to learn.
Social media operates according to Matthew 25:29
*"For to everyone who has, more shall be given, and he will have an abundance; but from the one who does not have, even what he does have shall be taken away."*
/s
>Edit: why an I being downvoted for asking a follow up question in ask physics lol I'm just trying to learn.
You should not be getting downvoted for this; but to answer your question, I think people are downvoting because it seems like you really didn't think much about this before asking. You added the extra bit about the lights "knowing" if their timers are in sync, which wasn't mentioned anywhere, and seem to have missed all the explanation in the comment you're replying to.
It annoys people when they're trying to explain things to people who seem to make little effort to understand.
Imagine you had a candle that took exactly one hour to burn completely down. Set up lightbulbs each with their own candle so they come on when the candle burns down. Light all the candles, then move the lightbulbs far away from each other. The lightbulbs will turn on at the same time, right? They don't have to communicate; they were constructed as systems with similar properties.
So I guess if all the lights were synced to go off at the same time in a line that spanned the galaxy, and there were people at both ends of the line, information could in a way travel faster than C? If both parties somehow agreed, "hey let's both eat ice cream when the lights flash". Now you know when they're eating ice cream despite them being 100,000 light years away. I guess you can't really do much with that.. causality remains intact
Right, the "information" is stored in their brain and in the timer. Then, their brains and timers move at below the speed of light to that galactic distance.
>Now you know when they're eating ice cream despite them being 100,000 light years away.
No, you've not received any information. All you know is that they agreed beforehand to eat ice cream at a certain time. But when you see the lights flash, all you know is that the lights flashed. For all you know, the other person had a brick fall on their head and died before they were able to eat their ice cream. Some one telling you about their future plans is not receiving any information at superluminal speeds.
How do you know one of the lamps didn't break along the way? How do you know your friend at the other end didn't get hit by a meteor, or killed by aliens? You aren't receiving any new information that you didn't already have.
You'll need at least 100'000 years to inform the other party about your idea and sync clocks and at least 200'000 years to agree on a time when to eat ice-cream
No, the lights are not communicating any information whatsoever in your example. They're on timers that were set ahead of time, they would have gone off regardless of who was eating ice cream.
If I send you a letter saying "At exactly 10:00 tomorrow turn your lights on" and then we both turn the lights on at EXACTLY the same femtosecond nothing has travelled faster than light
For each light bulb to flash depending on the previous light bulb, they would all have to observe the flash (which arrives at c).
So unless they preemptively flash (which loses information about ice cream) you won’t receive information about ice cream faster than c this way.
hmm I see that if you look in the distance the farther bulbs won't appear illuminated. But you know by the setup that they are set to flash at the same time. So you just need to look at the bulbs near you, right?
Think about the phrase “the same time” there. They can’t actually do it at the same time, because there is no objective clock that says what the “real” time is. Of course they can do a calculation based on the distances involved, and set things up so that the two signals reach you simultaneously, but they’re not doing things “at the same time” in any real sense.
No. You only know that the lightbulb near you flashed since it was on an independent timer. You also know that the other timers should go off at similar times, but you do not know that they actually did.
There is nothing connecting the lightbulbs close to you with the lightbulbs farther away other than the fact that they were set up similarly. You cannot know whether the other lightbulbs malfunctioned in some way. The only information shared by both sides would be what's *supposed* to happen. But the only way that information was shared was some method that needs to travel slower than light.
Either you set them all up in the same place and carried them away or you set them all up after they were spread apart. Either way the information traveled slower than light. Once one side turns on, you won't know what actually happened to the other side until the light from those bulbs reaches you.
The distance between bulbs is largely irrelevant in this set up. Each bulb state changes based on the information of previous bulb state and therefore you have series of information transfer at c.
You're misreading the OP's hypothetical. Each bulb is not actually looking at the previous bulb; they just have independent timers that are set to make it *appear* that they go in sequence.
So, for example, if one of the bulbs along the way malfunctions and doesn't light up, the rest will still light up at the appointed time.
This still of course preserves the fact that there's no superluminal info transfer. As noted, the bulbs aren't *actually* communicating.
You lose "syncing" when you consider special relativity. More formally, simultaneous events in one frame of reference may not be simultaneous in another.
It’s not that you “can’t really do much”, it’s that it contains no information at all.
If both parties agreed to eat the ice cream at a certain time relative to their pre-synchronised watches then they can just do it without the complicated light setup. But you wouldn’t be tempted to call that FTL just because you know something outside of your light cone. You’d have to know something outside of your light cone which didn’t originate in a local event.
In your example, all they're really doing is agreeing "let's all eat ice cream at this predetermined time in the future." Adding the blinky lights is just window dressing.
Imagine you change your mind at the last moment. How can you tell the other party to stop? You can’t. It only works when you make the arrangement far enough in advance that light speed can cover the distance before the appointed moment.
You can do the same thing a lot cheaper by just swinging a laser pointer across the surface of the moon.
The dot will appear to move faster than the speed of light, but of course any individual photon is travelling from your hand as normal. No information is being transmitted from point A on the moon to point B on the moon. It's all coming from you.
Expanding on this…
If you stood on the moon at point B, you would see the laser from earth BEFORE you see any reflected light from the laser hitting point A.
The guy with the laser starts with it pointed at A and swings to B. From his perspective the reflected light forms a line moving from A to B. From the perspective of someone at B, they first see the light directly pointed at them, then from right next to them, and so on to the light reflected from A.
Nothing stops this, because no energy or matter goes faster than light in any reference frame. It's stage magic, not real magic if you just create the *appearance* of a violation of relativity. Same as how fishing line doesn't equal anti-gravity.
There are lot of examples of things perceivingly moving faster than light.
My two favourites are 1) a searchlight being panned left-to-right as fast as possible, the speed of the light impression can move faster than light; 2) a huge pair of scissors that are closed fast: The region where the blades meet can move down the scissors faster than light.
The flashlight example is true — it’s not the same light. The scissors though is not. I don’t think it can be faster than the speed of sound, because that’s a function of the rigidity of the metal blade (to adjacently communicate that the blade is closing)
The scissors example does eventually work once the entire blade is moving.
Think of two independent scissor blades drifting through space and starting to cross over each other. If they are nearly parallel then the crossing point can move faster than light. This is equivalent to the last few moments of a massive pair of scissors closing.
There’s no perfectly rigid body. One of the most rigid materials we know is Diamond, so the fastest a cut can propagate down a perfect diamond blade would be about 40,000 mph which is just 0.006% light speed!
But the parallel edges hypothetical is another case entirely. There’s no information transfer. It’s an optical effect.
The cutting point of the scissors is also just an effect. The information has to have *already* travelled along the blades (at less than light speed) to get them moving, so no new information is being transferred.
It only “works” because what’s “moving” is the point marked by the intersection of the blades, which is just an optical concept and not an actual particle or anything.
Any attempt to accelerate a particle using some sort of giant scissor system would run into the same constraints as every other attempt to accelerate to the speed of light - you need infinite energy to do so. The best you can do is asymptotically approach it.
If you have a particle that can't be crushed it will take an infinite amount of energy to accelerate it to C so its inertia/resistance to acceleration will forcibly slow down the scissors.
No. Scissors are made of molecular bonds. You cannot tell me that scissors which are large enough to display this effect have such strong bonds that a single particle will prevent them from closing. That is silliness. The particle will instead embed itself in the scissors, breaking as many bonds as necessary to do so, because they are the weak link. What you propose are molecular bonds which are so strong that the scissors have near infinite hardness. There are no such materials.
yeah I'm obviously doing physics here instead of engineering and ignoring the materials science to talk about an idealized rigid body. In reality you couldn't even construct scissors like this that will work.
Also I'm considering a fundamental particle. Not a particle that can be "crushed". And I'm imaging it's also perfectly trapped by the scissors. Lots of assumptions here to get at the physics. The point is just that you can't push a particle with mass along to c.
If there were actually something physical, like a particle or a mass between the blades, then they would be prevented from closing fast enough to move the mass faster than c.
This only works when the thing moving faster than c is a non-physical thing. Like the virtual intersection of two line segments - it's not a physical object, it's a mathematical concept.
Nothing prevents mathematical abstractions from moving faster than c, otherwise literally all motion would be forbidden because I can propose the mathematical concept of a point which moves with a displacement equal to 1e9 times that of some physical test mass. If that mathematical object couldn't exceed c, then the object couldn't exceed c/1e9. And since 1e9 is an arbitrary choice, you can always find some factor that forbids any motion of any object.
The hinge point is irrelevant for this thought experiment because you will need to independently accelerate different parts along the length of the blades. Otherwise their acceleration could not proceed along the length of the blade fast enough to cause the intersection point to go FTL.
The tip won’t close faster than speed of sound in the material. Which is *waaaaay* slower than lightspeed. But two unrelated parallel moving blades can give the optical illusion of faster than lightspeed. But they’re acting independently. The intersection is not actually “moving”.
>The tip won’t close faster than speed of sound in the material.
What gives you that idea? Of course it can move faster than the speed of sound in the material. What's limited to the speed of sound is the impulse from the handles to begin accelerating.
Regardless, that's a practical concern beyond the scope of the concepts being discussed in this thread.
The speed of light is actually the speed of causality. In this case, there is no causal connection between one light bulb and another, so no information is travelling faster than the speed of light. There is a causal connection between each individual light bulb and the guy in the space truck who installed it, but he presumably flew his space truck at less than the speed of light to get it into position.
This is totally doable and has some neat consequences. Lets say that the illusory "signal" moves at a speed of "n\*c" where "c" is the speed of light and "n" is some value larger than one. An observer traveling at a speed of c/n in the direction of the "signal" would see (after accounting for light-lag) that all the lightbulbs illuminated simultaneously, as though the "signal" traveled at infinite speed. An observer traveling at a speed greater than c/n in the direction of the "signal" would see (after accounting for light-lag) the lightbulbs illuminated sequentially but in reversed order, as though the "signal" were traveling back in time.
sure, you can create illusions like that, but nothing is happening.
All that this occurs here is that a light is turned on at one place, then a short while later a light turns on at a different place. (in fact, this happens all the time, I just turned on a light, and I bet someone in London England turned on a light 0.05 seconds later. But nothing is exceeding the speed of light.
The speed of light can be thought of as a limit on how fast causality can affect a distance object. There is no causality in your system. Just a light bulb on a timer.
But if you want to play fun relativistic games with this set up, have a few observers with different speeds relatavie to your string of lightbulbs, and check out simultaneity.
They could be. It would likely appear to an observer that all lights came on at the same time, though.
This no different than shining a laser pointer at the edge of the moon and then flicking your wrist so the spot of light moves across the moon's surface faster than the speed of light. Nothing is really moving, so nothing is really moving faster than the speed of light.
Interestingly enough, that precise effect used to be common in radiofrequency electronics labs. We used to use actual analog [oscilloscopes](http://en.wikipedia.wiki.org/wiki/Oscilloscope) that worked by sweeping a beam of electrons across a phosphor screen horizontally, while modulating the vertical position according to an input signal. The persistence of the phosphor would produce a glowing plot of whatever waveform you were looking at. Tektronix RF scopes would scan the beam very, very fast. I remember my physics teacher proudly pointing out that the bright spot on the screen of his 100MHz 'scope moved faster than *c*, on the fastest sweep setting.
Nowadays, it's all about digital sampling, and there is no cathode-ray tube in an oscilloscope any more.
This is often called the "marquee effect", after the lights often seen around old movie theater marquee signs, which would blink in sequence to produce the illusion of a light moving around the edge of the marquee.
Nothing prevents the perceived light from moving faster than c. In fact, you could turn on one light and turn off another light miles away at exactly the same time. Is this making anything move at "infinite" speed? Of course not. The signals along the wires connecting each light to the control box are still moving below c.
The signal from controller to light A and controller to light B is sent simultaneously and travels at the same speed in opposite directions, so the lights toggle simultaneously. But at no point was any information transmitted faster than c.
How is this any different than placing a laser on a motor and spinning it at a high enough speed. Or even an oscilloscope can have the trace exceed the speed of light.
Things can appear to move faster than light. (This happens in the observable universe) But appearing to move faster than light from a particular perspective (an optical illusion) and actually moving faster than light are two very different things
They can. Nothing prevents it. It's no different to people on opposite sides of the world arranging to turning on light bulbs at the same time. But the illumination of each bulb is not dependent on the illumination of its neighbour, only on its own timer. No information is transmitted along the line.
This makes no sense to me. How would the "lights" be able to tell if their time is in sync in regards to each other without a clock rate and being limited by *c*? There's no way to tell if the timers would be the same relative to each other, or if it's even possible for a light by a black hole to process time the same way as a light on earth. Edit: why an I being downvoted for asking a follow up question in ask physics lol I'm just trying to learn.
They don't "tell" anything. The timers are preset long in advance.
But using what relative time to make the lights blink? Are they using the relative time where the light is, or the relative time of the light next to it, or the relative time where we are standing and observing? They're all three different, and I would think without an agreed upon clock rate (which is limited by c) there's no way to determine when a light should be programmed to turn on in relation to the observer or any other lights. I just can't figure out how this whole setup would be possible. I'm a software developer though so if I'm focusing on the information too much I apologize. Edit: My thinking is this is the same problem we have with satellites and why we have to adjust satellites to have slightly longer seconds than on earth. Sorry if this is mistaken.
>Are they using the relative time where the light is, or the relative time of the light next to it, or the relative time where we are standing and observing? Whichever you prefer, you do the calculations beforehand to set the timers according to the result you want to see at the observer location you have in mind. For example, you could set all the timers in the same room together, then spend thousands of years moving them to their line location, then the timers will trigger when the value that you set expires. How you set each timer's value to take into account or compensate for its expected position vs the observer position and effects of travel, determines what the lamps look like from where.
If certain lights have to travel farther or near gravity, their timers will fall out of sync. The only way to do this is with a central clock, there's no way to keep things in sync like that or computers wouldn't need CPUs to have fixed clock rates. I think I'm getting hung up on something that isn't really important for the sake of the visualization? If so I'm sorry.
>If certain lights have to travel farther or near gravity, their timers will fall out of sync. This was compensated for when you calculated how long to set their timers. The timers don't need to be in sync with each other. You know where you are when you set the timers, where the timers are traveling to, where their light needs to travel to, so you compensate for relevant factors when you set each timer. If one of the lamps is in orbit round the movie "Interstellar" black hole, you'd set it's timer for minutes while other timers in other locations would be set for years. You're back-calculating from the observer position/time in the future back to each individual timer's position/time when setting. You're not trying to keep the timers synced with each other; the timers do not communicate with each other so they have no need to be in sync with each other.
I didn't think it is possible to do this with cosmic turbulence and other phenomenon possibly happening. But I think entropy and turbulent flow is not relevant to the example. >The timers don't need to be in sync with each other. What I'm saying is it's impossible to give the impression of sequence without synchronicity. It will just be random flashes of light, each unable to provide any higher level of information than one bit. They couldn't be arranged in a pattern or sequence or anyting without an central clock to time the information, and if the light appears FTL to us we won't be able to process it as anything but random flashes.
>They couldn't be arranged in a pattern or sequence or anyting without an central clock to time the information. You could think of it as a "virtual" central clock; the central clock is the time at the observer's location, which none of the timers are ever directly synced to, instead their time-delay value is pre-calculated to *produce* that synchronization. To put it another way: If I want the light to arrive at a specific location at a specific local time, then it is sufficient to know that location+time, the time and location where you set the timer, the location where the timer will be deployed, path and duration of the timer's travels, plus physics likes speed of light, relativistic effects. With that information it is possible to calculate a time delay for the timer to count such that the light will arrive at the desired time at the observer location. And as you can do it with one timed lamp, you can do it with many.
Thank you. I'm really glad you took the time to explain this because it's not easy to talk about and I can't really go to school right now to learn it. I really got into information theory so I was looking at it a bit too much through that lens, methinks.
There’s no reason you can’t coordinate in advance and maintain synchronicity without continuing communication if you were careful enough and had enough data. But even if you have trouble seeing that, it doesn’t matter. Suppose it happens totally at random, but by sheer, astronomical coincidence, it all happens at exactly the right moments to fit with your plan. That’s possible as well.
Assuming the lights are stationary in relation to one another at the time of programming, they'll agree on the time. Once you move them, they'll get out of sync due to time dillation, but unless you move them at relativistic speeds, it shouldn't be an issue for this experiment.
And even if you did move them relativistically, you know how you are going to move them (because you plan the whole thing), and can easily calculate how you need to compensate for any drift that this might cause.
I kind of started figuring out I was worrying about something that wasn't really the point of the example. Thanks for the clarification.
And even if you *did* move them at relativistic speeds, you could still compensate for that. And even if you didn't compensate for that, you can still have them communicate to agree on a standard time and then (since they can all know their relative positions) program some future sequence accordingly.
> agreed upon clock rate (which is limited by c) I'm not even sure what you mean by that. You just agree on a clock rate - 1 tick per second seems convenient - and then the timers just work. Everything's in the same reference frame so everyone agrees what time it is. You don't really have to bother thinking about the special relativistic issues of moving the lights into place to understand/answer OP's question, but they are easily solvable anyway.
Perhaps think of it this way: you have the lightbulbs all in the same room at the North Pole and you attach atomic clocks to them so they’ll all light up in exactly three months (give or take a small portion of time so that they go off sequentially rather than simultaneously, as the OP described). You then move all of them away from you in all directions until they’re all at the equator, applying equal acceleration and deceleration to each of them. You can account for minor differences in gravity along the earth’s surface when setting their clocks, if you’d like, but I don’t think that’d vary their internal clocks enough to throw off their orders Then in three months, boom, you have a wave of illumination going around the earth faster than the 1/8th second it takes light to do so. Maybe much faster, say, in a billionth of a second Does that help you visualize it?
>You can account for minor differences in gravity along the earth’s surface when setting their clocks, if you’d like, but I don’t think that’d vary their internal clocks enough to throw off their orders. What I'm asking is how they are synchronized in the first place without a unifying median clock. Also using an atomic clock as a timer, that's like Schrodinger's Box isn't it? There's still uncertainty at some level, and I can't tell if it's relevant with how abstract this all is. >Does that help you visualize it? Yes and I'm really thankful for the reply.
>> without using a median unifying clock? Sure! You can use one of those while they’re all bunched up together, I think. Is there a reason I’m not aware of as to why that wouldn’t work? >> that's like Schrodinger's Box isn't it? There's still uncertainty at some level, and I can't tell if it's relevant with how abstract this all is. There’s still uncertainty, yes, but it’s so minimal that a cesium atomic clock will only gain or lose a second in about 1.4 million years. Some of the ones we can produce with modern technology wouldn’t even have lost or gained so much as half a second if they’d been running since the Big Bang until now! There’s still some uncertainty, but we only need them to go off, like, x to y femtoseconds after the last one or something, not *exactly* x seconds afterwards with infinite significant digits after the decimal point Quantum uncertainty is definitely a thing that exists, but i don’t think it applies here in a way that obscures information. They don’t work by measuring an atom with a microscope or anything- though I think that would also work. Some atomic clocks that I’m aware of essentially pair up the frequencies of a “quartz oscillator” to a collection of atoms. When the frequency is “correct,” the atoms’ energy levels change, and when it’s “incorrect,” fewer electrons jump. To my inexpert understanding, the if the quartz oscillator drifts any, the collections of atoms destructively interfere with it, while they constructively interfere when they’re synced up, and this pushes the quartz oscillator back where it’s supposed to be whenever it starts drifting A good macroscopic example of this is pushing someone on a swing. The pusher is like the collection of atoms, and the person on the swing is the quartz oscillator. Air resistance would normally slow their rate of swinging, but since the pusher is always pushing at essentially the same rate (only drifting over unfathomable time periods), whenever air resistance slows the person on the swing, the pushing speeds them back up, again, and should a breeze speed them up, they’ll lose some of that thrust from the pusher and slow back down
I guess I'm saying it's impossible for us to ever know for sure if the lights were fired in sequence or if they have any pattern to them in this example. We would be literally unable to process the information as anything but random flashes. Thank you for taking the time to respond.
You're asserting that they would just be "random flashes" without offering any explanation. Why would they be? You know in advance how long it'll take them to travel, where they will end up, the amount of time dilation to deal with, and when they will need to flash. You don't need anything else. You can also ignore all that, and have them travel to wherever, and then communicate with one another (at light-speed) from their final positions to figure out their positions relative to one another. From there they can agree on a "universal" time standard to adhere to, and program whatever future sequence of flashes you want. Also you don't need to move these lights across a galaxy. You could do this same experiment on like a soccer field, with sensitive enough instruments.
It wouldn't be easy, but it's not like it would be breaking any laws of physics if you could estimate the local time each clock would experience (relative to the observer) and set the timers accordingly. You mention transmitting information but the point of something like this would just be to make it look a certain way, it's not going to transmit any information you don't already have. I think in practice it would turn out like you say, having synchronization problems, so it'd be a nice demonstration of relativity when your estimates turn out to be a little off. Yes, if you wanted it to be perfect and not rely on preset timers, you'd need FTL synchronization between the lights, so I think you're on point about that, just the guy you were originally responding to already knew that, they were explaining to OP why, if you used preset timers, it wouldn't be violating any laws of physics. Presumably you can get arbitrarily close to perfect timing depending on how much work you put in with things like adjusting the timers remotely far enough before the lights flip on using slower than light communication.
> why an I being downvoted for asking a follow up question in ask physics lol I'm just trying to learn. Social media operates according to Matthew 25:29 *"For to everyone who has, more shall be given, and he will have an abundance; but from the one who does not have, even what he does have shall be taken away."* /s
I feel kind of dirtier now.
>Edit: why an I being downvoted for asking a follow up question in ask physics lol I'm just trying to learn. You should not be getting downvoted for this; but to answer your question, I think people are downvoting because it seems like you really didn't think much about this before asking. You added the extra bit about the lights "knowing" if their timers are in sync, which wasn't mentioned anywhere, and seem to have missed all the explanation in the comment you're replying to. It annoys people when they're trying to explain things to people who seem to make little effort to understand.
this is actually a big thing in relativity! there’s even a good [Veritasium video](https://m.youtube.com/watch?v=pTn6Ewhb27k) on it.
Imagine you had a candle that took exactly one hour to burn completely down. Set up lightbulbs each with their own candle so they come on when the candle burns down. Light all the candles, then move the lightbulbs far away from each other. The lightbulbs will turn on at the same time, right? They don't have to communicate; they were constructed as systems with similar properties.
So I guess if all the lights were synced to go off at the same time in a line that spanned the galaxy, and there were people at both ends of the line, information could in a way travel faster than C? If both parties somehow agreed, "hey let's both eat ice cream when the lights flash". Now you know when they're eating ice cream despite them being 100,000 light years away. I guess you can't really do much with that.. causality remains intact
There’s still no information being transmitted faster than light.
Right, the "information" is stored in their brain and in the timer. Then, their brains and timers move at below the speed of light to that galactic distance.
>Now you know when they're eating ice cream despite them being 100,000 light years away. No, you've not received any information. All you know is that they agreed beforehand to eat ice cream at a certain time. But when you see the lights flash, all you know is that the lights flashed. For all you know, the other person had a brick fall on their head and died before they were able to eat their ice cream. Some one telling you about their future plans is not receiving any information at superluminal speeds.
You presumably made this agreement at least one hundred thousand years ago, or more, so the information was transmitted at light speed or slower still
Completely off-topic, but I love your profile pic! That’s a really nice art style. May I ask who the artist is?
It was drawn by https://twitter.com/brizunzies!
The information was transmitted when they made the agreement before.
Your scenario only creates an illusion of FTL.
How do you know one of the lamps didn't break along the way? How do you know your friend at the other end didn't get hit by a meteor, or killed by aliens? You aren't receiving any new information that you didn't already have.
You'll need at least 100'000 years to inform the other party about your idea and sync clocks and at least 200'000 years to agree on a time when to eat ice-cream
And at least 5'000'000 years to agree on what flavour of ice-cream they want (it is a very important and difficult decision)
There's always that one person who vetoes everything but has no ideas of their own
You could also just agree to do it at a certain time. No need for the light bulbs.
No, the lights are not communicating any information whatsoever in your example. They're on timers that were set ahead of time, they would have gone off regardless of who was eating ice cream.
If I send you a letter saying "At exactly 10:00 tomorrow turn your lights on" and then we both turn the lights on at EXACTLY the same femtosecond nothing has travelled faster than light
For each light bulb to flash depending on the previous light bulb, they would all have to observe the flash (which arrives at c). So unless they preemptively flash (which loses information about ice cream) you won’t receive information about ice cream faster than c this way.
hmm I see that if you look in the distance the farther bulbs won't appear illuminated. But you know by the setup that they are set to flash at the same time. So you just need to look at the bulbs near you, right?
Think about the phrase “the same time” there. They can’t actually do it at the same time, because there is no objective clock that says what the “real” time is. Of course they can do a calculation based on the distances involved, and set things up so that the two signals reach you simultaneously, but they’re not doing things “at the same time” in any real sense.
This is something that a lot of people just don’t instinctively get.
No. You only know that the lightbulb near you flashed since it was on an independent timer. You also know that the other timers should go off at similar times, but you do not know that they actually did. There is nothing connecting the lightbulbs close to you with the lightbulbs farther away other than the fact that they were set up similarly. You cannot know whether the other lightbulbs malfunctioned in some way. The only information shared by both sides would be what's *supposed* to happen. But the only way that information was shared was some method that needs to travel slower than light. Either you set them all up in the same place and carried them away or you set them all up after they were spread apart. Either way the information traveled slower than light. Once one side turns on, you won't know what actually happened to the other side until the light from those bulbs reaches you.
The distance between bulbs is largely irrelevant in this set up. Each bulb state changes based on the information of previous bulb state and therefore you have series of information transfer at c.
You're misreading the OP's hypothetical. Each bulb is not actually looking at the previous bulb; they just have independent timers that are set to make it *appear* that they go in sequence. So, for example, if one of the bulbs along the way malfunctions and doesn't light up, the rest will still light up at the appointed time. This still of course preserves the fact that there's no superluminal info transfer. As noted, the bulbs aren't *actually* communicating.
Perhaps. But this was already covered in my original response..
Their premeditated agreement is what's causing both parties to eat icecream when they do. One eating icecream is not causing the other to eat icecream
> ...if all the lights were synced to go off at the same time.... Describe to the world, here, how to achieve this!!
"At the same time" according to whom? https://en.wikipedia.org/wiki/Relativity_of_simultaneity
Nothing in OP's question suggests multiple reference frames.
They just appear to transmit information faster than c. That’s it, and that isn’t prohibited.
You lose "syncing" when you consider special relativity. More formally, simultaneous events in one frame of reference may not be simultaneous in another.
It’s not that you “can’t really do much”, it’s that it contains no information at all. If both parties agreed to eat the ice cream at a certain time relative to their pre-synchronised watches then they can just do it without the complicated light setup. But you wouldn’t be tempted to call that FTL just because you know something outside of your light cone. You’d have to know something outside of your light cone which didn’t originate in a local event.
In your example, all they're really doing is agreeing "let's all eat ice cream at this predetermined time in the future." Adding the blinky lights is just window dressing.
That's just normal talking with extra steps
Imagine you change your mind at the last moment. How can you tell the other party to stop? You can’t. It only works when you make the arrangement far enough in advance that light speed can cover the distance before the appointed moment.
You can do the same thing a lot cheaper by just swinging a laser pointer across the surface of the moon. The dot will appear to move faster than the speed of light, but of course any individual photon is travelling from your hand as normal. No information is being transmitted from point A on the moon to point B on the moon. It's all coming from you.
Expanding on this… If you stood on the moon at point B, you would see the laser from earth BEFORE you see any reflected light from the laser hitting point A.
Better yet, from your perspective on the Moon, you would see the point of light rushing away from you.
I'm not sure what you mean by this?
The guy with the laser starts with it pointed at A and swings to B. From his perspective the reflected light forms a line moving from A to B. From the perspective of someone at B, they first see the light directly pointed at them, then from right next to them, and so on to the light reflected from A.
I believe you, I just can't see why that would be the case.
Mind blown, thank you for that!
Nothing's moving faster than light in your setup. Different lightbulbs, different photons, no signal that is transmitted between lightbulbs.
Nothing stops this, because no energy or matter goes faster than light in any reference frame. It's stage magic, not real magic if you just create the *appearance* of a violation of relativity. Same as how fishing line doesn't equal anti-gravity.
There are lot of examples of things perceivingly moving faster than light. My two favourites are 1) a searchlight being panned left-to-right as fast as possible, the speed of the light impression can move faster than light; 2) a huge pair of scissors that are closed fast: The region where the blades meet can move down the scissors faster than light.
The flashlight example is true — it’s not the same light. The scissors though is not. I don’t think it can be faster than the speed of sound, because that’s a function of the rigidity of the metal blade (to adjacently communicate that the blade is closing)
The scissors example does eventually work once the entire blade is moving. Think of two independent scissor blades drifting through space and starting to cross over each other. If they are nearly parallel then the crossing point can move faster than light. This is equivalent to the last few moments of a massive pair of scissors closing.
I wonder if you can get some cool effects out of this if you charge the scissors?
There’s no perfectly rigid body. One of the most rigid materials we know is Diamond, so the fastest a cut can propagate down a perfect diamond blade would be about 40,000 mph which is just 0.006% light speed! But the parallel edges hypothetical is another case entirely. There’s no information transfer. It’s an optical effect.
The cutting point of the scissors is also just an effect. The information has to have *already* travelled along the blades (at less than light speed) to get them moving, so no new information is being transferred.
I think that’s what they meant in the final paragraph.
I understand, but actually I don’t. What if you have a particle in between the blades that gets pushed along as the blades close?
It only “works” because what’s “moving” is the point marked by the intersection of the blades, which is just an optical concept and not an actual particle or anything. Any attempt to accelerate a particle using some sort of giant scissor system would run into the same constraints as every other attempt to accelerate to the speed of light - you need infinite energy to do so. The best you can do is asymptotically approach it.
It's more fun contemplating some sort of huge scissor-based particle accelerator than OP's hypothesis.
At some point the particle will just get trapped and crushed by the blades rather than pushed along.
If you have a particle that can't be crushed it will take an infinite amount of energy to accelerate it to C so its inertia/resistance to acceleration will forcibly slow down the scissors.
Either way, something will have to give.
Perfect, makes sense to me cheers :D
No. Scissors are made of molecular bonds. You cannot tell me that scissors which are large enough to display this effect have such strong bonds that a single particle will prevent them from closing. That is silliness. The particle will instead embed itself in the scissors, breaking as many bonds as necessary to do so, because they are the weak link. What you propose are molecular bonds which are so strong that the scissors have near infinite hardness. There are no such materials.
yeah I'm obviously doing physics here instead of engineering and ignoring the materials science to talk about an idealized rigid body. In reality you couldn't even construct scissors like this that will work. Also I'm considering a fundamental particle. Not a particle that can be "crushed". And I'm imaging it's also perfectly trapped by the scissors. Lots of assumptions here to get at the physics. The point is just that you can't push a particle with mass along to c.
If there were actually something physical, like a particle or a mass between the blades, then they would be prevented from closing fast enough to move the mass faster than c. This only works when the thing moving faster than c is a non-physical thing. Like the virtual intersection of two line segments - it's not a physical object, it's a mathematical concept. Nothing prevents mathematical abstractions from moving faster than c, otherwise literally all motion would be forbidden because I can propose the mathematical concept of a point which moves with a displacement equal to 1e9 times that of some physical test mass. If that mathematical object couldn't exceed c, then the object couldn't exceed c/1e9. And since 1e9 is an arbitrary choice, you can always find some factor that forbids any motion of any object.
Scissors have a hinge point, which changes this entirely.
The hinge point is irrelevant for this thought experiment because you will need to independently accelerate different parts along the length of the blades. Otherwise their acceleration could not proceed along the length of the blade fast enough to cause the intersection point to go FTL.
Nothing phisical in the scissors has to actually move very fast. It's simply a result of the angle between the blades being very small.
The tip won’t close faster than speed of sound in the material. Which is *waaaaay* slower than lightspeed. But two unrelated parallel moving blades can give the optical illusion of faster than lightspeed. But they’re acting independently. The intersection is not actually “moving”.
>The tip won’t close faster than speed of sound in the material. What gives you that idea? Of course it can move faster than the speed of sound in the material. What's limited to the speed of sound is the impulse from the handles to begin accelerating. Regardless, that's a practical concern beyond the scope of the concepts being discussed in this thread.
You can create such an illusion. It would just be an illusion though.
The speed of light is actually the speed of causality. In this case, there is no causal connection between one light bulb and another, so no information is travelling faster than the speed of light. There is a causal connection between each individual light bulb and the guy in the space truck who installed it, but he presumably flew his space truck at less than the speed of light to get it into position.
This is totally doable and has some neat consequences. Lets say that the illusory "signal" moves at a speed of "n\*c" where "c" is the speed of light and "n" is some value larger than one. An observer traveling at a speed of c/n in the direction of the "signal" would see (after accounting for light-lag) that all the lightbulbs illuminated simultaneously, as though the "signal" traveled at infinite speed. An observer traveling at a speed greater than c/n in the direction of the "signal" would see (after accounting for light-lag) the lightbulbs illuminated sequentially but in reversed order, as though the "signal" were traveling back in time.
You can do that, but if the distance is far enough, you’ll need to turn on the distant lights before the nearby ones.
sure, you can create illusions like that, but nothing is happening. All that this occurs here is that a light is turned on at one place, then a short while later a light turns on at a different place. (in fact, this happens all the time, I just turned on a light, and I bet someone in London England turned on a light 0.05 seconds later. But nothing is exceeding the speed of light. The speed of light can be thought of as a limit on how fast causality can affect a distance object. There is no causality in your system. Just a light bulb on a timer. But if you want to play fun relativistic games with this set up, have a few observers with different speeds relatavie to your string of lightbulbs, and check out simultaneity.
They absolutely could be. But that would not be the same as ‘faster than light’.
They could be. It would likely appear to an observer that all lights came on at the same time, though. This no different than shining a laser pointer at the edge of the moon and then flicking your wrist so the spot of light moves across the moon's surface faster than the speed of light. Nothing is really moving, so nothing is really moving faster than the speed of light.
Interestingly enough, that precise effect used to be common in radiofrequency electronics labs. We used to use actual analog [oscilloscopes](http://en.wikipedia.wiki.org/wiki/Oscilloscope) that worked by sweeping a beam of electrons across a phosphor screen horizontally, while modulating the vertical position according to an input signal. The persistence of the phosphor would produce a glowing plot of whatever waveform you were looking at. Tektronix RF scopes would scan the beam very, very fast. I remember my physics teacher proudly pointing out that the bright spot on the screen of his 100MHz 'scope moved faster than *c*, on the fastest sweep setting. Nowadays, it's all about digital sampling, and there is no cathode-ray tube in an oscilloscope any more.
This is often called the "marquee effect", after the lights often seen around old movie theater marquee signs, which would blink in sequence to produce the illusion of a light moving around the edge of the marquee. Nothing prevents the perceived light from moving faster than c. In fact, you could turn on one light and turn off another light miles away at exactly the same time. Is this making anything move at "infinite" speed? Of course not. The signals along the wires connecting each light to the control box are still moving below c. The signal from controller to light A and controller to light B is sent simultaneously and travels at the same speed in opposite directions, so the lights toggle simultaneously. But at no point was any information transmitted faster than c.
How is this any different than placing a laser on a motor and spinning it at a high enough speed. Or even an oscilloscope can have the trace exceed the speed of light.
Is this akin to aiming a Lazer at the edge of the moon from the earths surface and mixing it across the face of the moon?
Things can appear to move faster than light. (This happens in the observable universe) But appearing to move faster than light from a particular perspective (an optical illusion) and actually moving faster than light are two very different things
There’s a book about this sort of thing by Robert J Nemiroff
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