# Speed of Light vs Speed of Pool Ball Momentum

Picture a perfectly aligned row of pool balls all touching each other that was 200,000 miles long. If the first ball was struck with enough force at the exact center of the ball, would the movement of the last ball occur before a laser beam could move from the beginning to the end of the balls?

Thanks,

Don

Simon Bridge
Homework Helper
No.

That's what you expected right - but what you want to know is "how come?"

Imagine a steel bar 200000 miles long, you hit one end, the other end does not ring right away. This is as perfect a connection as you can get. Similarly, with the newton's cradle thing you have set up, the shock-wave has to travel through all those balls, there is a delay before the last ball leaves, the photon gets there sooner.

Pengwuino
Gold Member
The force pushing down the billiard balls will travel at the speed of sound in the medium (that is, the 'medium' being a line of billiard balls)

You hit a bunch of atoms at the front of the first ball, these then move a tiny distance and transfer their momentum to the atoms behind them, then these atoms move a tiny bit and transfer their momentum and so on until the atoms at the end of the line get knocked in to by the ones in front of them; this will all happen at the speed that the atoms move in that material which is also the speed of sound of that material (by the same reasoning as before).

Thanks for the quick replies. The replies do bring up another question though. Since electrical current moves at just shy of the speed of light with the electrons basically acting like the billiard balls mentioned earlier, wouldn't the billiard balls move at nearly that same speed? I can see the sound of the billiards hitting, moving at the speed of sound but it seems the movement would be seen much more quickly.

Thanks Again!

The reason the force won't travel at the speed of light is because it is the vibration of atoms and not a current, in the same way as you can shake a long rope and watch the force move down its length. The force must move at the speed of sound since sound is just changes in pressure which come from the vibrations of the atoms, if the sound moved faster/slower than the vibrations it would be a paradox.

Pengwuino
Gold Member
Thanks for the quick replies. The replies do bring up another question though. Since electrical current moves at just shy of the speed of light with the electrons basically acting like the billiard balls mentioned earlier, wouldn't the billiard balls move at nearly that same speed? I can see the sound of the billiards hitting, moving at the speed of sound but it seems the movement would be seen much more quickly.

Thanks Again!

Also, electric current does not travel near the speed of light. Electric current actually travels EXTREMELY slow (the current, not the individual electrons).

I have always heard and still find information that contradicts your statements about electricity being slow or close to the speed of sound. As a matter of fact one site compares the speed of electricity to a pipe filled with marbles. When an extra marble is added, the last marble falls out almost instantaneously. It also says the speed of electricity is nearly the speed of light, way faster than the speed of sound.

Simon Bridge
Homework Helper
[some unspecified website] also says the speed of electricity is nearly the speed of light
... oh well, if it is on the internet, it must be true!

Try:
http://en.wikipedia.org/wiki/Speed_of_electricity
... for an overview.

We can put a pulse of electricity in one end of a wire - and have a bit of a wait before it reaches the other end - usually quite degraded. This is a serious issue in the design of computers - where there is a measurable performance advantage in having the components nanometers apart in a microcircuit.

The exact speed of an electrical signal through a conductor depends on quite a lot - it is usually substantially slower than light (60% in coax). If we want to split hairs - electromagnetic waves can travel at exactly the speed of light - that is what light is (sort of).

The answer to your original question though is "no" - the comparison of electricity to a row of billiard balls is just an analogy - the point is to illustrate without actually explaining how the light comes on right away to flip the switch even though it takes the actual electrons quite a long time to get there. There is no reason to believe that the mechanical processes in the balls will yield anything like the totally different electromagnetic processes in the wire just like you would not expect a carriage drawn by a 50hp diesel engine to work anything like a carriage drawn by 50 horses.

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... oh well, if it is on the internet, it must be true!

Try:
http://en.wikipedia.org/wiki/Speed_of_electricity
... for an overview.

We can put a pulse of electricity in one end of a wire - and have a bit of a wait before it reaches the other end - usually quite degraded. This is a serious issue in the design of computers - where there is a measurable performance advantage in having the components nanometers apart in a microcircuit.

The exact speed of an electrical signal through a conductor depends on quite a lot - it is usually substantially slower than light (60% in coax). If we want to split hairs - electromagnetic waves can travel at exactly the speed of light - that is what light is (sort of).

The answer to your original question though is "no" - the comparison of electricity to a row of billiard balls is just an analogy - the point is to illustrate without actually explaining how the light comes on right away to flip the switch even though it takes the actual electrons quite a long time to get there. There is no reason to believe that the mechanical processes in the balls will yield anything like the totally different electromagnetic processes in the wire just like you would not expect a carriage drawn by a 50hp diesel engine to work anything like a carriage drawn by 50 horses.

The speed of electrical current (technically the electromagnetic field) through copper is about 95-97% the speed of light as per Wikipedia and http://lofi.forum.physorg.com/Speed-of-Light-and-Electricity_4992.html [Broken]. I guess I'm comparing the speed of a signal vs light. The movement of the last pool ball would be more like a signal.

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Dale
Mentor
2021 Award
Picture a perfectly aligned row of pool balls all touching each other that was 200,000 miles long. If the first ball was struck with enough force at the exact center of the ball, would the movement of the last ball occur before a laser beam could move from the beginning to the end of the balls?

You may want to check out our closely related FAQ:

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You may want to check out our closely related FAQ:

Thanks Dale. That was the exact idea I was trying to propose. It makes sense that the signal could never travel faster than it slowest part, the electromagnetic field. The signal would theoretically approach the speed of light and therefore infinite mass though correct?

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TumblingDice
Gold Member
This thread took a two-year break before donniet1977 returned today? The original thread was a mash up of billiard balls, light, and then electricity through a medium. After 2 years donniet1977 drops another piece into the fruit salad.

1. DaleSpam's recent reply provided a link to explain the OP question, which he quoted. This is the correct answer to the billiard ball question - signal travels rigid rod at speed of sound for the material it passes through.
2. Way back when the poster changed topic to speed of electrical signal through a wire. Simon Bridge provided accurate real world speed for coax - about 0.6 c. Arguments were made to be almost speed of light. This is totally different than billiard balls as well as basic electromagnetic speed of propagation. For example, although theoretical maximum of EM through copper may be 95-97%, that's not based on a wire or the insulator around the wire.

donniet1977: In your last post you are now mentioning 'infinite mass' of a signal. That sounds like a new point of discussion. What mass is moving at relativistic speed?

I see good answers and attempts to help from forum members, but the target is moving!

1 person
Simon Bridge
Homework Helper
The signal is moving faster than the motions of the individual masses involved. The "relativistic mass increase" idea does not really apply here. It does not seem to be related to any of the other replies either.

It does not require infinite energy to transmit a signal at the speed of light - but it does if you want to use a sound wave to do it. You'd vaporize the medium first. Per your pool balls - the first ball would just plow through the row like it wasn't there and disintegrate pretty much everything well before you get to the kinds of speeds where relativity is a problem.

It looks lie you are trying to reconcile ideas gleaned from pop science shows on TV.

Dale
Mentor
2021 Award
The signal would theoretically approach the speed of light and therefore infinite mass though correct?
No, the signal would be the speed of sound in the chain of billiard balls, which is nowhere near the speed of light.

Now, if you proposed a different material then the speed would be the speed of sound in that material. No material has a speed of sound anywhere close to the speed of light.