It can look dumb, but I always had this question as a kid, what physical principles would prevent this?
For anyone looking for other cool ideas or videos about speed of light etc
What Is The Speed of Dark? - Vsauce (13m:31s)
- Cool older vsauce video going over shadows and light speed etc
The Faster-Than-Light Guillotine - Because Science (w/ Kyle Hill) (14m:19s)
- Basically goes over the “FTL Scissor action” that a lot of people have covered but he does a good segment covering it.
There’s a thought experiment about this in most intro classes on relativity, talking about “length compression”. To a stationary observer a fast-moving object appears shorter in its direction of travel. For example, at about 87% of the speed of light, length compression is about 50%. If you are interested in the formula look up Relativistic Length Compression. Anyway, if you are carrying a pole 20 meters long and you run past someone at that speed, to them the pole will only look 10 meters long.
In the thought experiment you run with this pole into a barn that’s only 10 meters long. What happens?
The observer, seeing you bringing a 10-meter pole into a 10-meter barn, shuts the door behind you, closing it exactly at the point where you’re entirely in the barn. What happens when you stop, and how does a 20-meter pole fit in a 10-meter barn in the first place?
First, when the pole gets in the barn and the door closes, the pole is no longer moving, so now to the observer it looks 20 meters long. As its speed drops to zero the pole appears to get longer, becoming 20 meters again. It either punches holes in the barn and sticks out, or it shatters if the barn is stronger.
Looking at the situation from the runner’s point of view, since motion is relative you could say you’re stationary and the barn is moving toward you at 87% of the speed of light. So to you the 10-meter barn only looks 5 meters long. So how does a 20-meter pole fit in?
The answer to both questions is compression - or saying it another way, information doesn’t travel instantly. When the front end of the pole hits the inside of the barn and stops, it takes some time for that information to travel through the pole to the other end. Meanwhile, the rest of the pole keeps moving. By the time the back end knows it’s supposed to stop, from the runner’s point of view the 20-ft pole has been compressed down to 5 meters. From the runner’s point of view the barn then stops moving, so it’s length returns to 10 meters, but since the pole still won’t fit it either punches holes in the barn or shatters.
One of my physics profs had double-majored in theatre, and loved to perform this demo with a telescoping pole and a cardboard barn.
The compression on the end of the stick wouldn’t travel faster than the speed of sound in the stick making it MUCH slower than light.
But… But… The stick is unfoldable!
You’re forgetting the speed at which the shockwave from the compression travels through the stick. I guess it’s around the speed of sound in that material, which might be ~2 km/s
What about the speed of the earth’s rotation though, could that fuck up the stick holding?
It’ll knock the moon and earth out of orbit!
You’re pushing the atoms on your end, which in turn push the next atoms, which push the next ones and so on up to the atoms at the end of the rod which push the hand of your friend on the moon.
As it so happens the way the atoms push each other is electromagnetism, in other words sending photons (same thing light is made of) to each other but these photons are not at visible wavelengths so you don’t see them as light.
So pushing the rod is just sending a wave down the rod of atoms pushing each other with the gaps between atoms being bridged using photons, so it will never be faster than the speed at which photons can travel in vacuum (it’s actually slower because part of the movement of that wave is not the lightspeed-travelling photons bridging the gaps between atoms but the actual atoms moving and atoms have mass so they cannot travel as fast as the speed of light).
In normal day to day life the rods are far too short for us to notice the delay between the pushing the rod on one end and the rod pushing something on the other end.
So folks have already explained the stick, but you’re actually somewhat close to one of the ways you can sort of bend the rules of FTL, at least when it comes to a group of photons.
Instead of a stick, imagine a laser on earth pointed at one edge of the moon. Now suddenly shift the laser to the other side of the moon. What happens to the laser point on the moon’s surface?
Well, it still takes light speed (1.3 seconds to the moon) for the movement to take effect, but once it starts, the “point” will “travel” to the other side faster than light. It’s not the same photons; and if you could trace the path of the laser, you’d find that the photons space out so much that there are gaps like a dotted line; but if you had a set of sensors on each side of the moon set up to detect the laser, they would find that the time between the first and second sensor detecting the beam would be faster than what light speed would typically allow.
It’s not exactly practical, and it’s such an edge case that I doubt we can find a good way to use it, but yeah; FTL through arc lengths can kind of be a thing. At least if you tilt your head and squint funny at it.
With your example, nothing is “moving”.
Imagine a giant wave in the ocean that is almost lined up perfectly parallel to the shore. Imagine the angle that the wave is off by is astronomically small (0.0000000001 degrees off from parallel). Also imagine the shore line is astronomically long (millions of kilometers).
One end of the wave will crash the shore slightly before the other end of the wave at the opposite end of the shore. The difference in time between the two sides of the shore is also astronomically small (so small that not even light could reach the other end in time)
Now let me ask you: did the wave “crash” travel faster than the speed of light? Of course not. I think that is a similar analogy to the laser movement concept you described.
Edit: Fun thought experiment. Depending on where you are on the shore (which end you are closer to), you may see one end crash before the other end (one event happening before the other event). Have two people at different locations on the shore, once they meet up with each other, they might disagree on which end crashed first! And they would BOTH be correct! Relativity is fucking crazy
You’d still be limited by light speed to transmit the information between the two locations to compare times or indicate they received a signal.
this isn’t at all what this example depicts, here there is actual information transfer.
this depiction is actually just false, the light would send information faster than the stick, because in the stick information only travels as fast as speed of sound in the stick, which is why completely rigid objects don’t exist
Sure, the time between detections is faster than the time it takes light to travel from one detector to the other. Nothing is actually traveling faster than light and no physical laws are broken.
I’m not sure. The beam of light would bend as it travels to the moon, delaying the projected dot on the moons surface.
Just like it happens with a stream of water coming out of a hose. You point the hose in a new direction, but it won’t get wet before the the time it takes the water to travel from the hose to the pointed location.
When you push something you push the atoms in the thing. This in turn pushes the adjacent atoms, when push the adjacent atoms all the way down the line. Very much like pushing water in the bathtub, it ripples down the line. The speed at which atoms propogate this ripple is the speed of sound. In air this is roughly 700mph, but as the substance gets harder* it gets faster. For example, aluminum and steel it is about 11,000mph. That’s why there’s a movie trope about putting your ear to the railroad line to hear the train.
If you are talking about something magically hard then I suppose the speed of sound in that material could approach the speed of light, but still not surpass it. Nothing with mass may travel the speed of light, not even an electron, let alone nuclei.
*generalizing
Best answer
Putting it on the moon is just a distraction. It doesn’t matter if the rod is 1m long or 100,000km.
There’s no such thing as a perfectly rigid object.
There was, but now I’m getting older and more tired
Have you spoken to your healthcare provider about Viagratm? It may be able to help with your issue. (Please seek immediate medical help with an erection lasting more than 4 hours).
Even if it were perfectly rigid, supernaturally so, your push would still only transmit through the stick at the speed of light. The speed of light is the speed of time.
The push would travel at the speed of sound in the stick, much slower than the speed of light
In a “perfectly rigid” stick (a fictional invention), the speed of sound is the speed of light.
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Sound is air vibration
Sound is not exclusive to air, it can be generalized to vibrations in any media. Whale song and dolphin echolocation are certainly sounds, and we’re almost always talking about them propagating in water rather than air.
which has to travel from one place to the next
No, that isn’t how sound works. In air this would be a description of wind, not sound.
just transfer kinetic energy to the adjacenct atom
This is actually a good description of how sound waves propagate.
Think of it like this. If our universe is a simulation, then the speed of light is the maximum speed at which information can propagate through reality. We know that for anything to move through space, it must move from one adjoining position to another, then another, then another, incrementally. Each one of those increments takes, at minimum, one ‘tick’ of the universe. That’s one tick to increment each bit of information, that is, the position of something moving at light speed from position x,y,z to x+1,y,z. Light moves as fast as the universe allows; if there was a faster speed, light would be doing it, but it turns out that our universe’s clock speed only supports speeds of up to 299,792,458 meters per second.
What you have here is sound. Motion propagates through material at the speed of sound in that material. That’s part of the reason why moving large scale objects quickly gets weird.
Edit: to be clear, I am not making the case that we’re in a simulation. I’m only trying to use computers to make it relatable.
Matter is made of atoms. Things are only truly rigid in the small scales we deal with usually.
