if a fly is flying in the opposite direction to a train speeding at 100 mph..the fly must change direction from oving forwards to moving backwards with the train...therefore there must be a point between the fly changing direction in which the fly has stopped moving. If the fly has stopped then so has the train.

More bits O' wisdom by Billy.

if a fly is flying in the opposite direction to a train speeding at 100 mph..the fly must change direction from oving forwards to moving backwards with the train...therefore there must be a point between the fly changing direction in which the fly has stopped moving. If the fly has stopped then so has the train.

Firstly, the fly only ever stops moving if it would go from forward motion straight to backward motion (reverse). Flies don't do this, they just turn real quick.
Secondly, IF the fly stopped moving why would the train stop moving as well ? There is no real connection between the train and the fly, in other words.. its pure nonsense.

if a fly is flying in the opposite direction to a train speeding at 100 mph..the fly must change direction from oving forwards to moving backwards with the train...therefore there must be a point between the fly changing direction in which the fly has stopped moving. If the fly has stopped then so has the train.
Two things here.

First, for this puzzle to work you would have to assume that the train was a perfectly rigid body. In real life, the very front of the train (probably no more than the outer surface of the paint) would deform slightly as the fly impacted. It would be rather like gluing a pillow to the front of a car and whacking it with a bat as it drove by. The car never stops moving forward, but the surface of the pillow itself does stop (or even move backward slightly) because it's a compressible material. Similarly, when the fly hit the train there would be a microscopically small outer-layer of the train that compressed itself as the fly impacted, because the outer-layer would have been stopped by the fly even though the train was still moving.

Second, even if you had some hypothetical train that was perfectly rigid and the fly collided with it perfectly elastically, there would be no need for the train to stop just because the fly has stopped, because in your scenario the fly is only stopped for an infinitely small period of time - the exact, infinitely small moment in which the collision occurs. Since we would also expect the distance traveled by the train to be zero over an infinitely small period of time regardless of its velocity, there is no paradox

Firstly, the fly only ever stops moving if it would go from forward motion straight to backward motion (reverse). Flies don't do this, they just turn real quick.
Secondly, IF the fly stopped moving why would the train stop moving as well ? There is no real connection between the train and the fly, in other words.. its pure nonsense.

A bit like your answer, Enmos.

:D

Hm ok, I understand know that you let the fly hit the train. I didn't figure that from the OP.
I believe Nasor answered your question perfectly already, no need to repeat :)

A bit like your answer, Enmos.

:D

Apologies, i didn't understand the question the first time. :o

Two things here.

First, for this puzzle to work you would have to assume that the train was a perfectly rigid body. In real life, the very front of the train (probably no more than the outer surface of the paint) would deform slightly as the fly impacted. It would be rather like gluing a pillow to the front of a car and whacking it with a bat as it drove by. The car never stops moving forward, but the surface of the pillow itself does stop (or even move backward slightly) because it's a compressible material. Similarly, when the fly hit the train there would be a microscopically small outer-layer of the train that compressed itself as the fly impacted, because the outer-layer would have been stopped by the fly even though the train was still moving.

Second, even if you had some hypothetical train that was perfectly rigid and the fly collided with it perfectly elastically, there would be no need for the train to stop just because the fly has stopped, because in your scenario the fly is only stopped for an infinitely small period of time - the exact, infinitely small moment in which the collision occurs. Since we would also expect the distance traveled by the train to be zero over an infinitely small period of time regardless of its velocity, there is no paradox

:D

I'm not sure that you understand the hypothetical question any more than, Enmos...if the fly deviates from going forward to going backwards then between that nanosecond the fly is inbetween fwds/bwds.

Got that.

Now if the fly in the nanosecond the fly is not going fwds or back therefore it must by definition be inbetween..like a Car if a car crashes into another it may be going backwards but it has to crash to go backwards.

It does not matter that one car is going faster because at the point of impact both are - for a moment - stopped in motion not just the fly (or the slow car) but the train (the fast car).

Hm ok, I understand know that you let the fly hit the train. I didn't figure that from the OP.
I believe Nasor answered your question perfectly already, no need to repeat :)

Not really, Enmos..but one at least admires your consistency in getting everything wrong by any means possible.

Not really, Enmos..but one at least admires your consistency in getting everything wrong by any means possible.

Do you have to be an ass ALL of the time ?

I'm not sure that you understand the hypothetical question any more than, Enmos...if the fly deviates from going forward to going backwards then between that nanosecond the fly is inbetween fwds/bwds.

Got that.

Now if the fly in the nanosecond the fly is not going fwds or back therefore it must by definition be inbetween..
No offense, but I don't think you understand how collisions between perfectly rigid bodies work. There is only one specific, infinitely short instance of time in which the fly is "changing direction". You seem to be imagining that the fly is stopping for a very small but non-zero amount of time when it hits the train, as shown by your description of the time that the fly is stopped as being a "nanosecond." That's not the case. Even a nanosecond is a non-zero duration of time, and if the fly was actually stopped for a nanosecond then there would indeed be a logic problem here. But that's not the case.

The fly cannot suddenly go from a forwards motion to a backwards motion without anything inbetween. Its a very simple point.

No offense, but I don't think you understand how collisions between perfectly rigid bodies work. There is only one specific, infinitely short instance of time in which the fly is "changing direction". You seem to be imagining that the fly is stopping for a very small but non-zero amount of time when it hits the train, as shown by your description of the time that the fly is stopped as being a "nanosecond." That's not the case. Even a nanosecond is a non-zero duration of time, and if the fly was actually stopped for a nanosecond then there would indeed be a logic problem here. But that's not the case.

I think we can put it this way: the fly stops for an infinitely small duration of time.

If it would stop my pet toad would eat it! :D

Anyway, you would have to specify in relation to what the fly stops.

reported to PETA for the unethical treatment and discrimination of flies.

My toad has a eye problem and can't see flys fly!

The classic case as given to me 30 years ago was a ball-bearing rather than a fly.
It does stop, while in contact with the train.
The train also, therefore, stops.
The flaw in the thinking is the use of the word "train".
PART of the train stops - the part in contact with the fly/ ball bearing - that's why there's a dent in the front of the train afterwards (although admittedly in the case of the fly a very small dent :)).
The impact causes deformation in the material of the train (the part which actually stops), this deformation allows the rebound of the ball bearing as it passes through zero velocity and reverses direction.
"The train" is not a monolithic structure... easy.

The fly cannot suddenly go from a forwards motion to a backwards motion without anything inbetween. Its a very simple point.
Sure it can. Have you ever had a physics class?

Suppose the fly and the train are 2 meters apart, and are each moving forward toward each other at 1 m/sec. Before time=1 second, they will each be moving toward each other. At exactly time=1 second, they collide. Immediately after t=1 sec, the fly and the train are both moving in the same direction. The only time the fly is stopped is at exactly t=1 sec. If you are any time before or after t=1, the fly is not stopped; it either moving forward or backward. The instant of t=1 is a point in time, not a duration of time.

if a fly is flying in the opposite direction to a train speeding at 100 mph..the fly must change direction from oving forwards to moving backwards with the train...therefore there must be a point between the fly changing direction in which the fly has stopped moving. If the fly has stopped then so has the train.

Pieces of the fly (like a squishy accordian) will achive zero velocity for a few billions of a billionth of a second; however, those same pieces will have 100 mph acceleration.

Sure it can. Have you ever had a physics class?

Suppose the fly and the train are 2 meters apart, and are each moving forward toward each other at 1 m/sec. Before time=1 second, they will each be moving toward each other. At exactly time=1 second, they collide. Immediately after t=1 sec, the fly and the train are both moving in the same direction. The only time the fly is stopped is at exactly t=1 sec. If you are any time before or after t=1, the fly is not stopped; it either moving forward or backward. The instant of t=1 is a point in time, not a duration of time.

And if you plot the velocity vectors the fly comes to zero speed relative to the ground - while in contact with the train.
So, logically, if they are in contact and the fly has zero velocity so should the train - hence the "conundrum".

Pieces of the fly (like a squishy accordian) will achive zero velocity for a few billions of a billionth of a second; however, those same pieces will have 100 mph acceleration.
Again, the pieces of the fly are theoretically only actually at zero velocity for an infinitely short duration – a single instance of time. If you plot the velocity of the fly (or a single atom of the fly) vs. time on a graph, the fly’s speed will stay constant, then begin to slow down as it begins to hit the train, then pass through zero and continue to go negative as the fly begins to accelerate in the opposite direction. But the plot will only pass through zero at a single value, not a range of values. That single value is the one, infinitely small duration of time at which the fly’s speed is actually zero. It’s not a billionth of a billionth of a second, because that would imply that the fly’s velocity passed through zero over some range of values (ie, x to x+10^-18), rather than at a single value.

It might seem nitpicky, but it’s the key to understanding why there’s no problem here, even with hypothetical perfectly rigid flies and trains.

edit:
So, logically, if they are in contact and the fly has zero velocity so should the train - hence the "conundrum".
No. The fly's velocity is only zero for an infinitely small duration of time. Since the train would not be expected to move at all over an infinitely short duration (multiply some velocity in meters/sec by zero seconds and you get a distance of zero meters traveled), there is no conundrum. There is an infinitely short duration in which the fly has a velocity of zero and the train has some non-zero velocity. Neither the fly nor the train will move in that instant, because nothing can move any distance during a single instance of time, regardless of velocity.

If it's at zero AT any point it's at zero - and that point happens to be when it's in contact with the train - therefore the train should also be at zero (for however short a time - which is not the case since the train does not suffer massive deceleration).

If it's at zero AT any point it's at zero - and that point happens to be when it's in contact with the train - therefore the train should also be at zero (for however short a time - which is not the case since the train does not suffer massive deceleration).

But its only a tiny spot of the train that is 'at zero', not the train itself.

But its only a tiny spot of the train that is 'at zero', not the train itself.

Yup, but when most people are asked they tend to think of "fly = 1 unit" "train = 1 unit" and get confused... :)

I’m trying to not be rude, but it’s reaching the point here where I’m tempted to just say “go take a calculus class” and give up.
If it's at zero AT any point it's at zero - and that point happens to be when it's in contact with the train - therefore the train should also be at zero (for however short a time - which is not the case since the train does not suffer massive deceleration).
No. It is possible for the fly to be at zero velocity and the train to have some non-zero velocity at the same infinitely short instant of time, even thought they are in contact with each other. It would not be possible for that to occur over any non-zero duration of time, but we’re talking about a length of time that is exactly zero seconds long. Your intuition is fooling you here.

Again, the pieces of the fly are theoretically only actually at zero velocity for an infinitely short duration – a single instance of time. If you plot the velocity of the fly (or a single atom of the fly) vs. time on a graph, the fly’s speed will stay constant, then begin to slow down as it begins to hit the train, then pass through zero and continue to go negative as the fly begins to accelerate in the opposite direction. But the plot will only pass through zero at a single value, not a range of values. That single value is the one, infinitely small duration of time at which the fly’s speed is actually zero. It’s not a billionth of a billionth of a second, because that would imply that the fly’s velocity passed through zero over some range of values (ie, x to x+10^-18), rather than at a single value.

You are quite correct. Thanks.


It might seem nitpicky, but it’s the key to understanding why there’s no problem here, even with hypothetical perfectly rigid flies and trains.


Nothing wrong with being nitpicky. The point I was trying to make is there is also no problem because the fly is loaded with acceleration when it reaches that smallest (planck?) moment of zero velocity.

I’m trying to not be rude, but it’s reaching the point here where I’m tempted to just say “go take a calculus class” and give up.

No. It is possible for the fly to be at zero velocity and the train to have some non-zero velocity at the same infinitely short instant of time, even thought they are in contact with each other. It would not be possible for that to occur over any non-zero duration of time, but we’re talking about a length of time that is exactly zero seconds long. Your intuition is fooling you here.

I think you and Oli are pretty much agreeing here though.
The fly and a microscopic part of the train are at zero velocity for an infinitely short duration of time (the moment of inpact). I dont see how you two are in disagreement about this.

I think you and Oli are pretty much agreeing here though.
The fly and a microscopic part of the train are at zero velocity for an infinitely short duration of time (the moment of inpact). I dont see how you two are in disagreement about this.
If you're going to allow the train to deform, you don't have to have an infinitely small duration of time in which the fly is stopped but the train is moving. You can have some non-zero duration during which the fly (and the front of the train) is completely stopped, but the rest of the train is still moving.

In any case, I think we can all agree that the train/fly system that Billy proposed don't pose any serious metaphysical problems for physics or philosophy.

If you're going to allow the train to deform, you don't have to have an infinitely small duration of time in which the fly is stopped but the train is moving. You can have some non-zero duration during which the fly (and the front of the train) is completely stopped, but the rest of the train is still moving.

Ofcourse the deformity would probably be non-existent here but we have to make the reasoning work for heavier objects as well.
A little bit of both happens in reality i guess.


In any case, I think we can all agree that the train/fly system that Billy proposed don't pose any serious metaphysical problems for physics or philosophy.

I think that can be savely said :)

If the fly has stopped then so has the train.
No it hasn't. If you look in detail you'll see the surface of the fly decelerate, stop, and accelerate as it approaches the train (this is caused by repulsive forces between the fly's and train's molecules that build up when the two surfaces come close enough to one another - they're never actually "in contact"). The same thing happens throughout the fly until its entire corpse (by now thoroughly squished by the compression wave) has matched the velocity of the train.

No it hasn't. If you look in detail you'll see the surface of the fly decelerate, stop, and accelerate as it approaches the train (this is caused by repulsive forces between the fly's and train's molecules that build up when the two surfaces come close enough to one another - they're never actually "in contact"). The same thing happens throughout the fly until its entire corpse (by now thoroughly squished by the compression wave) has matched the velocity of the train.

Very good point, never thought of that.. :o

No it hasn't. If you look in detail you'll see the surface of the fly decelerate, stop, and accelerate as it approaches the train (this is caused by repulsive forces between the fly's and train's molecules that build up when the two surfaces come close enough to one another - they're never actually "in contact"). The same thing happens throughout the fly until its entire corpse (by now thoroughly squished by the compression wave) has matched the velocity of the train.

So a fly couldn't stop a train but a table tennis ball could?

:)

i dont understand, i have read it about 8 times and im still stumped. i mean i think i get what is said kinda, but i dont understand how the train was stopped by the train.


peace.

The fly cannot suddenly go from a forwards motion to a backwards motion without anything inbetween. Its a very simple point.
Another very simple point is conservation of momentum. The net momentum of our fly/train system MUST always stay the same, therefore, the train and the fly cannot ever both be stopped.

-Andrew

the question is what speed does the fly has to travel to stop the train from moving...my guess is faster than light

how the fuck does a fly stop the train though?, i really dont understand this atall. the fly cant stop the train, they can both just stop or the train cans plat the fly, but the fly cant stop the train.

thats stupid. ok i will test this theory and if the fly dies then you guys owe me a fly.

peace.

the question is what speed does the fly has to travel to stop the train from moving...my guess is faster than light

so how does the train stop moving in the real world? not lala land?.


peace.

so how does the train stop moving in the real world? not lala land?.


peace.

lala land?...

well we can have this fly to be made out of antimatter....it just looks like a fly

lala land?...

well we can have this fly to be made out of antimatter....it just looks like a fly

yeah lala land, its simular to the place where flys are made out of anti-matter. woo-woo land.

peace.

yeah lala land, its simular to the place where flys are made out of anti-matter. woo-woo land.

peace.

well...we can have this fly launched as a projectile and make sure the fly is within a plasma cloud which lowers the air resistance...and cancels it out. plasma stealth technology on flies.

But if you want not laalaa-woowoo land ideas

than here it is

the train has a photo receptor which causes the train to stop when it gets an image of the fly. The image of the fly...the thermal imagery of the fly causes the photonic receptors to recognize this image and send a signal to microprocessor which sends a signal to stop the train to the main system of the train control.

przyk is the only person who has got the explanation completely correct, I think.

if a fly is flying in the opposite direction to a train speeding at 100 mph..the fly must change direction from oving forwards to moving backwards with the train...therefore there must be a point between the fly changing direction in which the fly has stopped moving. If the fly has stopped then so has the train.

There is nothing wrong with this except for the last sentence. It is not true that because the fly stops, the entire train must stop. In fact, no part of the train, even down to the molecular level, has to stop.

Firstly, the fly only ever stops moving if it would go from forward motion straight to backward motion (reverse). Flies don't do this, they just turn real quick.

Every particle making up the fly must stop at some time in order to change direction. If it goes from having a positive velocity to a negative velocity (for example), then at some time it must have zero velocity.

In fact, not all parts of the fly stop at the same time, because the fly is not a totally rigid object. But all parts stop.

First, for this puzzle to work you would have to assume that the train was a perfectly rigid body. In real life, the very front of the train (probably no more than the outer surface of the paint) would deform slightly as the fly impacted.

That is correct.

It would be rather like gluing a pillow to the front of a car and whacking it with a bat as it drove by. The car never stops moving forward, but the surface of the pillow itself does stop (or even move backward slightly) because it's a compressible material.

It is not necessary for the surface of the pillow to stop. All it needs to do is to slow slightly (relative to the rest of the train), to change the momentum of the fly by the required amount to reverse the fly's motion.

Second, even if you had some hypothetical train that was perfectly rigid and the fly collided with it perfectly elastically, there would be no need for the train to stop just because the fly has stopped, because in your scenario the fly is only stopped for an infinitely small period of time - the exact, infinitely small moment in which the collision occurs. Since we would also expect the distance traveled by the train to be zero over an infinitely small period of time regardless of its velocity, there is no paradox

I'm not sure I like this argument, but it's not really important.

The classic case as given to me 30 years ago was a ball-bearing rather than a fly.
It does stop, while in contact with the train.
The train also, therefore, stops.
The flaw in the thinking is the use of the word "train".
PART of the train stops - the part in contact with the fly/ ball bearing - that's why there's a dent in the front of the train afterwards (although admittedly in the case of the fly a very small dent :)).

I disagree that part of the train stops. All the front of the train needs to do is to exert enough force on the fly/ball bearing. That doesn't require it stopping.

If you look in detail you'll see the surface of the fly decelerate, stop, and accelerate as it approaches the train (this is caused by repulsive forces between the fly's and train's molecules that build up when the two surfaces come close enough to one another - they're never actually "in contact"). The same thing happens throughout the fly until its entire corpse (by now thoroughly squished by the compression wave) has matched the velocity of the train.

This is correct. In effect, the electrons in the molecules at the front of the train repel the molecules in the fly as it approaches the front of the train. That repulsion becomes stronger and stronger as the fly gets closer and closer to the train molecules. Thus, there is a constant repulsive force on the fly's molecules which act to slow them down to rest (for a moment) then accelerate them back in the direction of motion of the train.

By Newton's third law, there is an equal and opposite force on the molecules in the front of the train, which causes them to decelerate and slow slightly with respect to the rest of the train (a slowing that is quickly countered by the molecules behind them pushing them forward again). They DO NOT, however, slow to rest.

Although the fly exerts a force on the train equal to the force exerted by the train on the fly, the acceleration of the fly is much much larger than the deceleration of the train, due to the immense differences in their masses. The train slows hardly at all, while the fly slows to rest, changes direction and almost immediately is propelled to the speed of the train.

Hi James,
Do you remember when we had this argument over at SSSF? I think Chris was involved, too.

Here's a post I made in another forum four years ago this week (not the same discussion as the one just mentioned). It takes a slightly different approach in that it doesn't worry about interatomic forces. It assumes perfectly amorphous, infinitely divisible bodies.

Consider a small, perfectly elastic amorphous object (the fly) colliding with a large, perfectly elastic amorphous object (the train). The train has a much larger modulus of elasticity than the fly.
In my chosen reference frame, the fly is moving slowly to the right, the train is moving quickly to the left.

Consider the fly's rear in relation to the fly's front. An instant after the collision, the fly's rear will still be travelling forward, while the fly's front is travelling backward. The fly is being compressed. The compression wave progresses through the fly faster than the relative speed of the train, accelerating each component of the fly in turn until the whole fly is travelling backward very quickly.

A similar thing happens to the train, expect that it compresses much less, and it's velocity does not reverse but only reduces slightly.

It is clear, therefore, that neither the bulk of the train nor the bulk of the fly undergoes a discontinuous velocity change, and that the bulk of the train does not stop.

Now consider the small chunk of fly and the small chunk of train that actually collide. It seems that at least one of these chunks must undergo a discontinuous velocity change, since immediately afterward, the velocity of the pair must match.

But, we can apply the same reasoning - a compression wave travels through each chunk, accelerating each part in turn.

Taking this line to its limit, it appears that only the zero mass leading 2D surfaces of the two objects undergo a discontinuous velocity change. Every other component in each object will accelerate smoothly, and no finite part of the train need ever be moving at less than the average velocity of fly and train.

Interestingly, if the fly and train have equal densities and leading-edge shapes, the compression wave propagation is symmetrical. The symmetry doesn't break until the compression wave reaches the back of the fly. Up until that time, the leading edges are both travelling at the average velocity of the pre-collision objects. Therefore, unless the train is less dense or softer than the fly, no part of the train will ever travel at less than the average velocity of fly and train.

i dont understand, i have read it about 8 times and im still stumped. i mean i think i get what is said kinda, but i dont understand how the train was stopped by the train.


peace.

Hi Chi,
Billy wasn't real clear in his explanation. He means that the fly hit the train, but that at the moment of impact the fly changed direction because it the train hit the fly (the fly is now stuck to the train, squished).
For the fly to change direction the fly must have been at zero velocity at one point. So if the fly was at zero velocity at one point the train was at zero velocity at that point too.
Above is what Billy meant to say.

Every particle making up the fly must stop at some time in order to change direction. If it goes from having a positive velocity to a negative velocity (for example), then at some time it must have zero velocity.

In fact, not all parts of the fly stop at the same time, because the fly is not a totally rigid object. But all parts stop.


Hi James R,
Yes I know, i didn't understand the first time. The OP didn't state that the fly hit the train (or vice versa). Thats why I wrote that post. :o

It was self evident in the OP.

The fly is neither here nor there, it could be "anything" from a table tennis ball to any light object....Enmos seems to suggest size is an issue...it isn't. As for 'squishing molecules'..again thats another case of "looking too deeply into the situation".

There is a valuable lesson to be learnt from the answer though..I'll tell you exactly what it is later. :)

Substitute Billy Chyld for the fly and you get a much more satifying result. In theory.

So a fly couldn't stop a train but a table tennis ball could?

:)
No, the same thing happens with the table tennis ball: their surface molecules repel when they're a small but nonzero distance apart, so there's nothing to imply that the train's velocity must be the same as the ball's throughout the collision. Of course you might find that some heavier objects do bring some parts of the train to rest momentarily (accompanied by some serious permanent denting or crushing), but this is far from a general conclusion applicable to all collisions.
As for 'squishing molecules'..again thats another case of "looking too deeply into the situation".
It's exactly what happens. In the idealised description of the situation, involving a perfectly rigid train colliding with a perfectly rigid fly which changes velocity instantaneously on impact, the fly simply doesn't have a well defined velocity at the time of the collision since its trajectory isn't differentiable at that point. Handwaving about the fly "going through all the intermediate velocities in an infinitely short time" doesn't allow you to conclude anything about the train, and in any case all you're doing is quibbling around a technicality in what's only an approximate description of a real collision anyway.

As for 'squishing molecules'..again thats another case of "looking too deeply into the situation".

There is a valuable lesson to be learnt from the answer though..I'll tell you exactly what it is later. :)
Are you saying that the correct answer doesn't fit with whatever philosophical point you are planning to make?

Few things on internet forums annoy me more than these “I’ll try to lead you to a deep point gradually, over a series of posts” games. If you have a point to make, just explain it as concisely and clearly as possible in your opening post and let everyone read it.

It was self evident in the OP.

The fly is neither here nor there, it could be "anything" from a table tennis ball to any light object....Enmos seems to suggest size is an issue...it isn't. As for 'squishing molecules'..again thats another case of "looking too deeply into the situation".

There is a valuable lesson to be learnt from the answer though..I'll tell you exactly what it is later. :)

Billy, it was an issue for me, i didnt read in the OP that well. It wasnt so much your fault, it was mine. I was only explaining why i made the error.

Rather than mock and dismiss a theory of such profound consequence and implication, we should explore it.

Since a fly can stop a train for a nanosecond, and light is made of photons traveling at much higher speed (we turn to the Koran fro the exact speed), we should be able to keep the train stopped by continuously bombarding it with trillions of photons, head on.

Each one would only stop it for that nanosecond we have discovered in the jump discontinuity of the accelleration curve. The total bambardment would hold the train stopped as long as the flashlight batteries held out.

So robbing trains would be easy: we stop the train with a powerful flashlight duct taped to a rail, and hold it while we kype the booty.

But in practice this fails - every time I tried it, the train appeared to keep moving. So the photon bombardment was not effective. Clearly, then, there is something wrong with the photon theory of light.

And remarkable thinkers of the past have solved this conundrum:

http://www.jtkdev.com/light.html

Rather than mock and dismiss a theory of such profound consequence and implication, we should explore it.

Since a fly can stop a train for a nanosecond, and light is made of photons traveling at much higher speed (we turn to the Koran fro the exact speed), we should be able to keep the train stopped by continuously bombarding it with trillions of photons, head on.

Each one would only stop it for that nanosecond we have discovered in the jump discontinuity of the accelleration curve. The total bambardment would hold the train stopped as long as the flashlight batteries held out.

So robbing trains would be easy: we stop the train with a powerful flashlight duct taped to a rail, and hold it while we kype the booty.

But in practice this fails - every time I tried it, the train appeared to keep moving. So the photon bombardment was not effective. Clearly, then, there is something wrong with the photon theory of light.

And remarkable thinkers of the past have solved this conundrum:

http://www.jtkdev.com/light.html

Last time I checked photons had zero mass.. :rolleyes:

In response to Enmos, from the locked thread:

The mass of the fly - or the photon - is not, I think, relevant. It's not the mass of the fly that allegedly stops the train (no mass or momentum calculation was put forth) but its reversal of direction, as we are informed. The photons also reverse direction - at least, many do - hence the train must stop for a "nanosecond". Since a sufficiently powerful flashlight will produce billions of photons, the train would be held at a stop for many billions of nanoseconds.

That is a far superior means of stopping a train than placing large obstacles on the tracks or damaging the railway.

Moderator note: Not sure why this thread was locked. Re-opened.

In response to Enmos, from the locked thread:

The mass of the fly - or the photon - is not, I think, relevant. It's not the mass of the fly that allegedly stops the train (no mass or momentum calculation was put forth) but its reversal of direction, as we are informed. The photons also reverse direction - at least, many do - hence the train must stop for a "nanosecond". Since a sufficiently powerful flashlight will produce billions of photons, the train would be held at a stop for many billions of nanoseconds.

That is a far superior means of stopping a train than placing large obstacles on the tracks or damaging the railway.

It IS relevant that the photons have no mass. No mass, no momentum. If there is no momentum there is no effect on the train.

Moderator note: Not sure why this thread was locked. Re-opened.

Maybe it was struck by a fly :)

It IS relevant that the photons have no mass. No mass, no momentum. If there is no momentum there is no effect on the train.
First, he wasn't arguing about stopping the train with momentum - he was arguing that it's impossible for one object (the train) to still be moving when it is in "hard contact" with another object (the fly) if the second object is not moving. The presumption is that since they are in contact, they must either both be stopped or both be moving. But I think we've pretty thoroughly beaten to death why that analysis is wrong.

Second, although photons have no mass, they do have momentum. So you could actually stop a train by shining light on it...but you would need a heck of a lot of light, because the momentum of a photon is very small.

Maybe it was struck by a fly :)
Yeah, but then it should only have been locked for a single, infinitely-short instant of time.

The problem here is: how to launch a fly at such velocity that the flies momentum is greater than that of the train, whereas the fly not burning in an atmosphere before approaching with all its molecules at the train.

if a fly is flying in the opposite direction to a train speeding at 100 mph..the fly must change direction from oving forwards to moving backwards with the train...therefore there must be a point between the fly changing direction in which the fly has stopped moving. If the fly has stopped then so has the train.

If the fly stopped moving, it would fall wouldn't it? by the force of the gravity fulling it to the ground? therefor the fly would never truly stop.

First, he wasn't arguing about stopping the train with momentum - he was arguing that it's impossible for one object (the train) to still be moving when it is in "hard contact" with another object (the fly) if the second object is not moving. The presumption is that since they are in contact, they must either both be stopped or both be moving. But I think we've pretty thoroughly beaten to death why that analysis is wrong.

Second, although photons have no mass, they do have momentum. So you could actually stop a train by shining light on it...but you would need a heck of a lot of light, because the momentum of a photon is very small.

You are right, i forgot about that. :o

The problem here is: how to launch a fly at such velocity that the flies momentum is greater than that of the train, whereas the fly not burning in an atmosphere before approaching with all its molecules at the train.

lol

The problem here is: how to launch a fly at such velocity that the flies momentum is greater than that of the train, whereas the fly not burning in an atmosphere before approaching with all its molecules at the train.

I calculated it for you.
The train weighs 55.000 kg.
The fly weighs 40 mg.
The train travels at 100 mph.
The fly then has to travel at 137.5 * 10^9 mph, or 137.5 billion mph to stop the train.
I think its save to say that will never happen lol ;)

if a fly is flying in the opposite direction to a train speeding at 100 mph..the fly must change direction from oving forwards to moving backwards with the train...therefore there must be a point between the fly changing direction in which the fly has stopped moving. If the fly has stopped then so has the train.

Can a fly stop a train?

Yes.

A fly lights upon a sensitive part of the engineers body and causes a muscular spasm throwing the engineers hand upon the brake lever.

The engineer recovers his senses after the entire train has come to a full halt.

I calculated it for you.
The train weighs 55.000 kg.
The fly weighs 40 mg.
The train travels at 100 mph.
The fly then has to travel at 137.5 * 10^9 mph, or 137.5 billion mph to stop the train.
I think its save to say that will never happen lol ;)

Agreed.

I calculated it for you.
The train weighs 55.000 kg.
The fly weighs 40 mg.
The train travels at 100 mph.
The fly then has to travel at 137.5 * 10^9 mph, or 137.5 billion mph to stop the train.
I think its save to say that will never happen lol ;)

hahahahahaha.

think about the fly's wings, they need to be heavy duty to allow it to accelerate to that hyper light speed :eek:

if you have a very long railtrack and the train is just rolling, without engine on, then a fly can stop it in a finite time. Well, fly should have also a lot of food and time available.

if you have a very long railtrack and the train is just rolling, without engine on, then a fly can stop it in a finite time. Well, fly should have also a lot of food and time available.

its lifespan is about 1 day, so it has only 24 hours to do the work :D

Yes, a fly can stop a train hypothetically. If the scenario was in space, yes. If the fly can reach speeds of light, then yes (more speed = more energy and mass).

Considering the possibility that the fly and train are perfectly stiff...

I believe that at the single point in time that the fly is stopped, so is the train, but as it is a single point in time nothing can be in motion anyway, so it really doesn't make any effect on the train.

I think it's like one of Zeno's paradoxes of an arrow in flight.


(these are just my random thinkings by the way, and they are probably fundamentally flawed. Please point out if so)

How can you say a photon has no mass and yet has momentum? I'm pretty sure momentum = mass * velocity

Considering the possibility that the fly and train are perfectly stiff...I don't think it's needed because the momentum at the fly's atoms is enough to repel the entire train; it needs to crash in a key spot though :D

[QUOTE=Steve100;1622219]I believe that at the single point in time that the fly is stopped, so is the train, but as it is a single point in time nothing can be in motion anyway, so it really doesn't make any effect on the train.[QUOTE]Yes if you think just in position... Think on momentum a lot of energy will be released in form of heat in the next plank's frame.

Poor fly :bawl:

How can you say a photon has no mass and yet has momentum? I'm pretty sure momentum = mass * velocityAs a wave it has no mass, but it behaves like a particle too, right?

How can you say a photon has no mass and yet has momentum? I'm pretty sure momentum = mass * velocity

That's only for things that are moving at slow speeds (i.e., much smaller than c). Since photons move at the speed of light, that equation doesn't apply.

I calculated it for you.
The train weighs 55.000 kg.
The fly weighs 40 mg.
The train travels at 100 mph.
The fly then has to travel at 137.5 * 10^9 mph, or 137.5 billion mph to stop the train.
I think its save to say that will never happen lol ;)

kjkj

The logic with the original statement is inconsistent. If you had two perfectly rigid bodies, then i believe a fly would stop the train. Frankly the impact would produce and infinite force if there were contact.

But the reality is there is never contact bewteen the fly and frankly since the train and fly are flexible, different parts of them will change direction at different times as the pressure waves travel through them.

Back to my point, the fly would never touch the train, it would only approach it and the electrostatic forces would eventually overcome the fly's momemtum and reverse it's direction without contacting the train.

There is no such thing as contact and there is no such thing as infinite stiffness.

The logic with the original statement is inconsistent. If you had two perfectly rigid bodies, then i believe a fly would stop the train. Frankly the impact would produce and infinite force if there were contact.

But the reality is there is never contact bewteen the fly and frankly since the train and fly are flexible, different parts of them will change direction at different times as the pressure waves travel through them.

Back to my point, the fly would never touch the train, it would only approach it and the electrostatic forces would eventually overcome the fly's momemtum and reverse it's direction without contacting the train.

There is no such thing as contact and there is no such thing as infinite stiffness.

All very well, but why do you think the word touch exists... ?
Maybe the everyday definition needs updating ?
Wouldn't you say the electrostatic forces interact ? Can we define this as 'to touch' ?

All very well, but why do you think the word touch exists... ?
Maybe the everyday definition needs updating ?
Wouldn't you say the electrostatic forces interact ? Can we define this as 'to touch' ?

Because in the context of the original question, the original poster is right. A fly would otherwise stop a train or the fly would experience infinite acceleration. Both are impossible which is why he stated the question in the first place.

If you consider that in reality things don't touch such as I stated then there is room for the fly to accelerate in the opposite direction without the train being affected.

Because in the context of the original question, the original poster is right. A fly would otherwise stop a train or the fly would experience infinite acceleration. Both are impossible which is why he stated the question in the first place.

If you consider that in reality things don't touch such as I stated then there is room for the fly to accelerate in the opposite direction without the train being affected.

I agree, but to normal people that is touch.

Because in the context of the original question, the original poster is right. A fly would otherwise stop a train or the fly would experience infinite acceleration. Both are impossible which is why he stated the question in the first place.

If you consider that in reality things don't touch such as I stated then there is room for the fly to accelerate in the opposite direction without the train being affected.

It's not necessary to resort to electrostatic forces providing a buffer against physical contact for this problem. To resolve the suggested paradox, it's only necessary to model the collision with a non-rigid fly.

The simplifications of a rigid train and simple contact forces are good simplifications, I think. They make it much simpler to model the collision, without noticeably altering the result.

No.

The train would need the same proptional value of kinetic energy as the fly.

hey guys, you know how to stop monitoring this thread? it became stupid, gettin spam...

By me?