Something can move faster than the speed of light. How can you explain this?

Question

Imagine you have a bird, flying directly north. The sun is shining directly east (from the west). There is a wall to the east of the bird which runs at an angle so that as the bird flies forward it's perpendicular distance from the wall increases (e.g. running south-west to north east). So, as the bird flies north, it's shadow moves up the wall, going north at exactly the same speed. But because the wall is at an angle, the shadow is actually moving a greater distance than the bird in a set amount of time. So, the shadow is moving faster than the bird by a certain ratio.
Now imagine that (somehow) this bird could travel at nine tenths the speed of light (this is theoretically not impossible according to our current knowledge of physics,). The shadow would therefore be travelling faster than the speed of light.

I know that the explanation for this is that a shadow is not a 'thing', but using this method to send a signal, information could be transferred faster than the speed of light.

I see a lot of you are saying that information couldn't be transferred this way.

Let's say the information you want to transfer is that somebody said "hello". You position the person who will say "hello" and the bird right next to the wall, and you can stand, say, two light seconds away, on the wall. As soon as the man says "hello", the bird will set off, and LESS THAN two seconds later, the birds shadow will pass over your face. If you have good reactions, you will notice and know that the man has said hello, and if the signal was sent to you using light, it would have taken two seconds, whereas this way is quicker. Thus, the information that somebody said "hello" reached you faster than light could have got it to you.

I know it is impractical (and a bit pointless) but it is only theoretical, and although there is probably a simple explanation it does seem to challenge the idea that information cannot be transferred faster than the speed of light.

PhysicsDude, that logic is terrible.
The expression for V' is not dx/dt + dy/dt
Where dx/dt is the same as it is for V.

Maybe it's confusing you to think of it in terms of speeds. In a certain amount of time (say 1 second), the bird will have moved 3x10^8 m north. So, because the sun shines east, the shadow will also be 3x10^8m north of where it started. But the position of the WALL at that Latitude (for want of a better word) is also 3x10^8m East of the starting point, so the total distance along the wall it's travelled is 3xsqrt2 x10^8 m, in one second, so the speed is roughly 4x10^8m/s, faster than c.

Also, if you're bothered about the length of the bird think of it as a particle of infantesimally small length.

Answer 1

The shadow will move faster than the speed of light but try to think of a way you could use this to send information........

The answer is you can't. If you're at the point where the shadow starts, you can't do anything to influence the shadow, just observe it so no information can be sent and causality remains intact.

Answer 2

if i have understood the question properly, this would be a paradox it would only appear to move a greater distance in the same time to observers on the ground the shaddow to the persective of the bird would appear normal,

but if the shaddow was traveling at a faster speed than the bird then it would arrive at the end of the wall before the bird had got there - which would be inconsistant. In each perspective nothing has changed and as the shadow's passing is merely the blocking of the light then its observed "speed of travel" is affected only by the angle at which the shadow is oberved from, and the angle of insidence to the ground, the physical "collum" shaddow is traveling with the bird. I hope this makes sence.
as to the second part- could information be sent this way im not sure as it would be very inpracticle to set up but maby this could be a good experiement for exmple you could set up the experement as you say in a room and a wall that is filmed at a fast fillm rated as a speeding bullet passes between it and a light source to observe the phenomon more closely, then this might give posible idears as to the application.

Answer 3

There is a flaw in your thinking and it has to do with the fact that there is a time delay between when the bird is moving north and when the shadow starts moving, and that is dependent on how far the bird is from the wall, which increases as the bird keeps moving north. In your example, you're assuming that the bird and the shadow move at the same time. That's how you can get V(shadow) always = sqrt(2) V(bird), but this equation assumes c = infinite.

So if we now calculate V(s) but set the speed of light to C:

V(b) = x/t, where x is the distance the bird moves northward and t is the amount of time elapsed.

V(s) = sqrt(2)x/(t+t'), where t' is the time it takes light to travel from the bird to the wall. Note that the total time used to calculate V(s) is (t+t'), because we need to wait for the light from the bird to reach the wall.

t' = x/C, since the wall is at a 45 degree angle thus the light will travel the same distance x as the bird.

Hence, V(s) = sqrt(2)x / [(x/V(b)+(x/C)], factor and cancel out x

V(s) = sqrt(2)V(b) / [1+(V(b)/C)]

This is the correct equation for V(shadow) as a function of V(bird), and you can see that in the limit that C => infinity, V(s) = sqrt(2)V(b), but if C is finite and V(b) = C, we do NOT get V(s)=sqrt(2)C, faster than light! Instead, we get something much more sensible:

V(s) = C/sqrt(2), less than the speed of light!!! Q.E.D.

Now you can eat my logic, dude...

Answer 4

its far to late to be thinking about this sort of thing really but ill have a go. lets forget about shadows for a second and lets say we are standing on your wall looking at the bird. lets say the bird is 2 light seconds away. you will see the bird as it is 2 seconds ago not as it is now. its shadow will also be 2 seconds behind the bird for the same reason (so it is a dark spot where you see the bird) but for the 2 seconds in between the bird passing and its image reaching you, light will still be arriving because the bird has not been past to block the photons (remember it is like looking back in time). as the bird gets further away all that will happen is the shadow and where the bird is in reality will just get further apart the birds shadow will not increase its speed to keep up with the bird. i think i have disproved it but as im falling asleep here i probably haven't explained myself very well

Answer 5

the thing is that as the bird goes north the distance between the bird and the wall is increasing so the light also takes more time to reach the wall in order to imprint the shadow of bird. so the light source, the bird and the shadow location at a given moment will not be in line. if you do the calculation you will find that Einstein was wright when saying that the speed of light can not be overcome.

Answer 6

really good question. think i've figured it out, but i may be talking absolute rubbish...
As a point further along the wall is further west from your bird, it will take longer for the light to reach it. The time for the shadow to move between points A and B on the wall is longer than the time the bird takes to move between the corresponding A and B points. As the shadow hits the wall's point B after the bird hits the point B on its trajectory, the shadow must move slower along the wall in a northerly direction, so staying within the speed of light.

ish.

Answer 7

Exactly the shadow is not a thing, in the same way a "beam" of light can do the same, but it is not something traveling faster than the speed of light but lots of identical things arriving in different places so as to look like a moving thing.

As for sending messages this way I had never thought of that...

I'm doubting the need or practicality at this moment in time but maybe a long time in the future!

Answer 8

good question! its got me preoccupied when i should be working. I think you're right that a shadow is not a 'thing'. Light speed is the limit for a travelling object because at that speed its mass becomes infinity - but a shadow has no mass and therefore has a speed limit of infinity. Not sure but i think its something along those lines

Answer 9

Wht ur trying to say in first & third para is totally impossible.
In second para it might be possible tht shadow move faster than light.This is something very serious i need to think more on this
as we know the basic lines in physics
1)shadow cannot exist without light
2)shadow is just the impression of object created by light itself.
3)from ur theory if it is possible to send info or signal it actually cannot reach its destination as it is just impression of the data reaching the destination.tht means light also have to reach at same time the shadow is reaching because if shadow in the form of signal or info reaches its destination it will be unrecognisable or it will be never felt tht signal have reached unless the light also reaches in same time it is recognisable.

Another topic which is storming my brain is tht
Consider our Universe 99% of Universe is made up of Dark matters. Think again dark matters which can also be said as black matters.Another thing is light because of which we all are in existence we call it as white matters as when it falls on any thing it makes them bright.this co-relation between bright & dark matter lies the premieval of whole Universe.
Wht iam trying to say is WHT WE SEE IN OUR UNVERSE AS DARK MATTER(BLACK) IS THIS IS A SHADOW OF ANOTHER UNIVERSE ON US.
ok i can be wrong but its my view any way
coming to the point ur theory is impossible.

Answer 10

This is a well know effect called beaming. In astrophysics it appears that some jets from active galaxies are exceeeding the speed of light!

Bt=(B Sin A)/ (1-B Cos A)

Where Bt is the Bt is the beaming factor, B is the v/c and A is the angle between the direction of movement and the line of sight.

Sorry but it is just an optical effect.