# If you had a object one light year long and you pushed on it how long would it take to feel it from the other end?

Asked by talljasperman (21796) February 24th, 2010

would it be instantly or would it take a year to feel the pulse?...could you make a faster than light communication system this way?

Observing members: 0 Composing members: 0

The Vulcan Science Directorate has determined that it would take at least one year, depending on the speed at which the compression shockwave travels trough the object.

ragingloli (43209)

Oh man!

Val123 (12679)

What a terrific question. I’ll ask a physicist friend of mine to weigh in.

Rarebear (25144)

@Rarebear howdy! Been lurking lately? :)

Val123 (12679)

You generate a plane P-wave in the object that travels at a speed determined by the material’s elastic modulus, density, and Poisson’s ratio.

Compare this speed for steel, ~6000 m/s, with the speed of light in vacuum, 299,792,458 m/s. The pulse is going to take much longer to reach the other end than a light beam for any conceivable material.

@hiphiphopflipflapflop That’s what I was afraid was coming! Hence my first reply.

…..so, it would depend on how elastic the material was? Also, if it was stiff, wouldn’t the energy from pushing on it tend to dissipate before it gets to the end?

Val123 (12679)

It would take the length of the object multiplied by the speed of sound in said object until the end of the object moves. Then it would take a year for the light of the end to reach you.

Shuttle128 (2979)

@hiphiphopflipflapflop what if you had something with a higher speed… like diamond or better something much better

talljasperman (21796)

@Shuttle128 The length is one light year long. What would your calculations look like?

Val123 (12679)

@talljasperman Each substance has it’s own P-wave value. It’ll still be slower than the speed of light. (After @hiphiphopflipflapflop mentioned it, I remembered the physics)

Rarebear (25144)

@talljasperman “I leave that as an exercise to the reader”. Seriously, try the math yourself. You should be able to look up the three pertinent parameters for diamond.

@Val123 Heh….should be divide instead of multiply missed that in my edit.

1 lightyear / (speed of sound m/s) x conversion factor from lightyears to meters = time in seconds

Shuttle128 (2979)

@hiphiphopflipflapflop But you wouldn’t be creating a p-wave by just changing the object’s displacement, right?

If you created a p-wave on a piece of rope that was one lightyear long then, yeah, that would take a long time. But if you just have a solid, let’s say rectangular object that you were simply moving forward then theoretically it should move instantly.

squidcake (2634)

@squidcake No, it’s not instantaneous. It’s the same as propagating a sound wave. Let’s say you pound a railroad track with a hammer, and then listen with a very sensitive stethoscope a half a mile away and 30 miles away. The sound will be heard a half a mile away before it’s heard 30 miles away.

Rarebear (25144)

@squidcake You push on the rod, the surface atoms of the rod push on the atoms beneath them, and so on. Pushing on it to generate whole body displacement indeed creates a p-wave.

About a parsec. (give or take half a light year or so)

TheLoneMonk (2892)

I would imagine if it was a solid object and you just pushed it.. you would feel it immediately – the object is constant, but if you just like… slap it or something – the other side probably won’t feel the vibration until later.

It’s not like the length of the object is going to shrink right?

goose756 (655)

@goose756 “Itâ€™s not like the length of the object is going to shrink right?”
Yes indeed. That’s elastic (non-permanent) deformation.

That depends on the ability of the material to conduct compression waves. You can test it out at home with a block of jelly – when you push on one end, it compresses, then expands as the other end starts to feel the force. The distribution of force throughout the jelly makes it wobble. Steel would be just the same – it takes time for the force to travel through the matter, but it takes much less time for more rigid substances. It would never travel even close to the speed of light though, and definitely not over since it relies on the electromagnetic force between particles which itself relies on photon exchange.

No matter what substance you chose, it would take well over a year for the force to traverse the object.

In theory it should be instantaneous (if the object was entirely solid).
But in reality, whatever material it is made of, your imaginary object would have a certain density, and be made of protons and electrons which push each other along the way. Since the speed that this happens can never be faster than the speed of light, it would take more than a year to feel the push on the other side (ie for all the atoms of the material to move into space). Depending on the material, it could take several years in fact.

Jack79 (10984)

@Jack79 By that definition, nothing can be entirely solid. Even protons are not solid, as they are quarks interacting via the nuclear forces. Every particle that may be considered ‘solid’ in that sense is a probability wave, and to apply a force you must use one of the four elementary forces, none of which can act faster than the speed of light.

It is interesting. A millennia ago, no one would have had a problem imagining a really solid object. But today, with the knowledge of atoms, electrons, protons, quarks and strings, you, or at least I, can not imagine an actual solid object because you always have the picture of atoms in the back of your head.

ragingloli (43209)

I use rigid approximations all the time in aerospace engineering; however, just as often I am reminded that this is very unrealistic. A rigid object would instantaneously move but this conceptualization leads to violating special relativity in that information is propagated faster than the speed of light, not to mention rigid objects don’t exist.

Shuttle128 (2979)

Wait a second….how can you measure length by time? I mean….how long is a physical object that’s one minute long?

Val123 (12679)

A lightyear is a unit of distance. It is the distance that light travels in one year.

Shuttle128 (2979)

@Shuttle128 I know what light speed is. But what is it’s physical length? What is the physical length of an hour?

Val123 (12679)

Perhaps you are misunderstanding.

Light speed is the velocity at which light propagates empty space. It does not have a length.

Physical length is the measure of a distance.

Time is not a measure of distance. There is not a “physical length of an hour.”

The idea of a lightyear is that it is a very large unit of length (needed for astronomical purposes) that can be easily conceptualized. Light is used because c is constant and a specific property of the universe. It also allows people to easily conceptualize how long it will take for light from a distant star to reach Earth. If a star is 100 lightyears away it will take the light emitted by the star 100 years for the light to travel that distance.

Shuttle128 (2979)

@Val123 To put it more simply, a light year is the distance that light will travel in a year. A light minute is the distance light will travel in a minute. (In vacuum)

Rarebear (25144)

@Rarebear I know that! I guess I’m being all literal again. The question said, “If you had a object one light year long”.....So I’m trying to imagine a physical object being a light year long, and pin a number on it to get it’s length. But, this is an exercise in theory, only, right?

Val123 (12679)

@Val123 No. A light-year is a measure of distance. You can have an object one light-year long.

Rarebear (25144)

The solidity of physical objects comes about not through the filling of space of matter. Solid matter does not fill space. Solid matter is virtually all empty space. Solidity of solids (and incompressibility of fluids and pressure of gases) is a result of short-range repulsion between the electron clouds surrounding separate nuclei.

We only approached an understanding of the deep nature of these things in the 1920s.

@Rarebear K…so, how long would it be? (I appreciate your patience)

@hiphiphopflipflapflop Very nice to meet you! I’m waiting for someone to through an impossibly complex numerical equation out there, something I can look at so I can get a headache.

And, back to the question….would the energy created by pushing on the object dissipate over such a great length before getting to the end?

Val123 (12679)

@val123 5,878,630,000,000 miles

Rarebear (25144)

@Rarebear Thank you! Now, where’s my impossible equation?

Val123 (12679)

“speed of light”, c = 299,792,458 m/s

1 year = 365 days = 365×24 hrs. = 365×24 x 60 min. = 365×24 x 60×60 sec = 31,560,000 sec

distance traveled by light in one year = (299,792,458 m/s)(31,560,000 sec) = 9.454×10^15 m (approximately) = 9.454×10^12 km = 5.874×10^12 miles

@Val123 go stare here.

299,792,458 m/s—miles per second? Why do I have 186,000 m/s in my head?

Val123 (12679)

Meters. Miles would be abbreviated mi. and are anyway a rather archaic unit of measurement.

Oh….Thank you.
I’m looking at the article now. The only thing I’ve understood so far is “perturbation theory”, because that exactly fits my state of mind right now.
Going back in…..

Val123 (12679)

“predicting the observed frequency dependence of the energy emitted by a black body.” Isn’t that a tad….racist?

Val123 (12679)

(eye roll) Oh for heaven sakes…

Enough of that for now.

@hiphiphopflipflapflop She was kidding.

Rarebear (25144)

Oh, man. There is so much you have to already know to really understand all of this. I get sidetracked by getting into the other links (“black bodies” for example, and from there into “ultraviolet catastrophe” and from there into “equipartition theorem”.....and I can never get back to the beginning….if I ever get the chance, and the money, I swear I’m going to take college courses until I DO fully understand….it is just fascinating.

So…what is the answer to the question? It just depends on the matter the object is made of, but at any rate, it would take longer than the speed of light….? But would it ever actually reach the other end?

Val123 (12679)

@hiphiphopflipflapflop I just went in your “Oh for heaven’s sake” link. :) I did that on my own right after I made the “black body” quip. That’s fairly easy to understand (the picture of the lava sealed it. It’s like, duh!)

Val123 (12679)

@Val123 “And, back to the questionâ€¦.would the energy created by pushing on the object dissipate over such a great length before getting to the end?”

Actually I was wondering the same thing myself. With all the pushing and shoving of atoms in the object eventually the energy imparted on the system should be used up in entropy.

Shuttle128 (2979)

So, one possible answer is….It never would.

Val123 (12679)

From a physicist friend of mine who finally got around to reading the thread:

I pretty much concur with the thread. Meaningless to consider a ‘solid’ object. Speed of sound, compression wave faster than shear wave, dissipation means nothing would actually get to the far end. With perfect mechanics and no dissipation, length divided by p-wave velocity. Not going to be relativistic

Rarebear (25144)

I have no idea what you just said. Shall we move on to algebra?

Val123 (12679)

@Val123 If you take -50 and divide by -2, you get 25. Everybody knows that.

Rarebear (25144)

@Rarebear FINE! What are all the sub sets of (A,B,C)?????? Huh? Huh???!!!

Val123 (12679)

or