Faster than Light thought experiment

My friend and I were having this discussion late one night recently. He said that theoretically faster than light travel is possible based on these two observations he heard others talking about.

I’m not sure about either of these experiments as my last physics class was AP Physics (which I got a 5 on, but I digress…) My friend has a PhD in Behavioral Psychology, so, not a physicist either, but I find him to be one of the smartest people I know.

Anyway, what is the SD on these experiments? Let’s assume that it’s physically possible to create these structures. The question is, does this prove the ability to move faster than light, or anything at all? (Please bear with me as it was really late when we were discussing this.)

  1. Construct a very, very long solid, uniform object, like a metal rod, but straightened out, and 186000 + 1***** miles long in space. ***** – +1 unit, like a foot or an inch or even a mile longer than distance traveled by light in a second.

Once constructed, push one end and measure the distance on the other end. Theoretically, since this is one uniform mass, the distance traveled is faster than light emitted from the source.

  1. Construct a tube full of marbles 186000 + 1 miles long. There is no gap between the marbles, and the tube is in a vacuum (like in space). Push the first marble such that the last marble at the end of the tube falls out.

In both cases, if one were to shoot a photon from the starting point (i.e. “the push”), the distance traveled in 1) and the marble falling in 2) should both be faster than the light, i.e. the movements in 1) and 2) are instantaneous.

Now, personally, I don’t believe in this. Practically speaking, there’s the obvious: it’s impossible to create such structures, there’s no amount of force to actually push these things and would collapse/destroy under its own mass (i.e. there would be a bend in the long piece of metal, or one of the marbles would implode thus stopping the transference of energy/information).

My friend did say that either there would be FtL would be practically proved, or there would be a very, very large explosion (infinite energy and all that).

Wouldn’t such structures have infinite mass as well?

We’ve gone over this several times, but I’m too lazy to search. The short answer is: it doesn’t work.

Nope. The “push” will travel as a compression wave down the rod. It will move at the speed of sound in the rod.

This is an old one. I think I even got shot down once myself here for proposing it. Mechanical energy doesn’t transmit instaneously over long distances. Quite to the contrary, it is pretty slow. The types of signals you are talking about will travel at the speed of sound, not light, as dictated by the media in question.

Yep - I’ve seen these threads before, but I want to know what the limitations are on another “faster than light” concept that I’ve wondered about since seeing the beacon on the Palmolive Building many years ago: If that beam is plenty bright/strong, and it can be seen for, say, 50 miles, what happens to the light that is being sent out from the beacon if it spins at, say, 1000 revolutions per second? It seems like something out there on the end of the beam should be moving mighty fast - faster than* c. * Or does it eventually begin to act like a spinning water sprinkler where the beams lag behind the spinning arms?

The “end” of the beam* moves faster than the speed of light. But that’s okay, because the “end” of the beam is not a physical object. Another way to put the reason it’s okay is to point out that the “end” of a beam of light can not carry information from one location to another.


*I put end in quotes because of course there’s no such thing as a unique “end” to a beam of light. It “ends” at the point at which it happens to hit some physical object opaque to it.

This is the general objection I have seen on this before.

And you don’t even have to postulate a rod that’s 186K miles long. You can test this with a 1-foot rod and find that the end of the rod will not move at the exact same moment that you tap on the other end with a hammer.

Ah, now I’m suddenly seeing something - that any particular photon of light sent out from the beam goes straight out at c. And the next photon sent may be sent out in a slightly different direction, but travels straight in that direction, at c. So that if there were an opaque cyclorama positioned out in space, the light would trace a nice little line across it, but each particle of light is traveling only at the speed of light. Yep, finally, it makes some sense.

You said it better than me.

I actually thought about adding a comment advising you to think of what happens if you think of the photons as marbles, but was afraid it would be too confusing! :stuck_out_tongue: Turns out it was almost exactly the right thing to say.


Yes, the beams act like a water sprinkler and will appear “curved” if the arms are rotating fast enough, even though each individual photon emitted will be traveling in a straight line.

But even though the light beam can traverse across a region of space at apparently faster than light, this isn’t faster than light travel…any more than my pointing from one star and then quickly pointing at another is faster than light travel, even though the stars are hundreds of light years apart. The photons emitted by the light beam still travel at the speed of light, and no useful information can be transmitted faster than the speed of light.

As for the classic “very long very stiff metal rod” as a method of FTL communication, it’s easy to understand why it won’t work. What is a metal rod made of? Billions of atoms. When you pull on one end of the rod, what happens? You pull some atoms toward you. Those atoms are electromagnetically bonded to the atoms next to them, so a force is generated on the next section of atoms, and they move over. The next atoms are bonded to the next atoms, and so on and so on. The wave of motion travels as fast as the electromagnetic wave passes along the rod. And that wave doesn’t travel even at the speed of light.

And if you really did have a metal rod hundreds of thousands of miles long, pulling on one end isn’t going to move the other end, it’s just going to stretch the rod ever so slightly. And the electromagnetic forces holding the rod together are soon going to become very small compared to the mass of the whole rod, as it becomes miles and miles long.

Think of it this way. You’ve got a bunch of 4x4 Lego bricks. You start making a rod of lego bricks, one on top of the other. When you’ve got a rod of 10 bricks it is pretty strong, you can wave it around and the bricks stick together. But when you’ve got a rod 100 bricks long, the mass of the rod is 10 times greater, yet the force holding each brick to each brick is the same. In other words, proportionally the rod is now 1/10th as strong. Now make the rod 1000 bricks, then 10,000 bricks. Long before you reach even 1000 bricks the rod is going to break apart under its own weight. Now, if you made that Lego rod in space it wouldn’t collapse under its own weight, but if you pulled on one end of the rod, at some length the rod is going to break apart rather than move as a unit.

You might think that a titanium metal rod is much stronger than a Lego rod, and it is, but only by a few orders of magnitude. If the titanium rod is 1000 times stronger than a Lego rod, it will fail when it is 1000 times longer than the Lego rod. So when the Titanium rod is only a few thousand miles long it’s going to snap. Not to mention, if the rod is anywhere near the sun, the end of the rod closest to the Sun is going to be orbiting faster than the end of the rod farthest from the Sun. Your rod is going to start spinning in orbit.

Former threads on this and related topics:

[thread=430431]What is theoretically wrong with this concept of faster-than-light communication?[/thread]
[thread=435253]What’s wrong with this FTL hypothetical[/thread]
[thread=376157]Relativity and very long sticks.[/thread]
[thread=132058] Physics: Propertys of a Long Solid Rod[/thread]

In brief: all real world materials have a finite modulus of elasticity, and thus will give (or fracture) rather than transmit momentum at faster than the wave propagation speed through the material.


See Superluminal Scissors.

An important note from that link is Special Relativity places an upper bound on the rigidity of any material. No matter what you make it of the scissors will bend (or rod from the OP).

And I believe that even on that page, they gloss over the fact that an imaginary point, such as the place where you see two blades of the scissors intersecting, can move infinitely fast. This is much like the post above that points out that pointing to one star and then pointing to another draws a line potentially billions of miles long in a fraction of an instant. But no *thing * is moving that fast, just your imagination.

Thanks Lemur, Stranger and everyone else for explaining this stuff. Oh, well, I guess FtL travel is still possible in fiction.

It’s not that long a read but here is that part:

It might be possible in real life too…sort of. While no physical object itself can move faster than light we may be able to manage shortcuts via wormholes and such. Currently I believe scientists have come up with potentially workable ways that it could be done in theory assuming things like wormholes (or other possibilities) exist. Unfortunately while theoretically possible (maybe) they are for practical purposes impossible (requiring things like staggering amounts of energy well beyond anything we can dream of producing…one I vaguely recall needed the mass of Jupiter to be converted to energy to run so a wee bit out of our reach yet).

Instead of saying “what if” and then listing a bunch of impossible things like

[ul]Let’s assume that it’s physically possible to create these structures.[/ul]
[ul]Construct a very, very long solid, object, like a metal rod, but straightened out, and with uniform mass along its entire length.[/ul]
[ul]Information travels at no cost (no energy is absorbed by the transmission medium).[/ul]
[ul]Objects can be placed so closely together that there is zero empty space between them.[/ul]

Why not just save a bunch of steps and just assume that one impossible thing is possible:

[ul]Let’s assume FTL travel is possible.[/ul]

Problem solved!

Well, sometimes I’ve believed as many as six impossible things before breakfast.

I believe that we’ll ultimately discover that what we believe to be true about the nature of the universe will turn out to be mistaken. Again. At that point, we’ll discover how FTL speeds are possible.

I believe you’re wrong.

Now science can start out even. :slight_smile: