Can a String Travel Faster than the Speed of Light?

Click For Summary

Discussion Overview

The discussion revolves around the concept of whether a string or rod could theoretically travel faster than the speed of light, exploring various hypothetical scenarios and the implications of physical laws such as relativity. Participants examine the mechanics of rotating strings, energy requirements, and the properties of materials in relation to this question.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Mark proposes a scenario involving a long string and questions why its end could not travel faster than light if it rotates at a certain speed.
  • Some participants argue that accelerating the structure to such speeds would require infinite energy and that the tensile strength of the string would be insufficient.
  • There is a discussion about the correct application of angular velocity and linear velocity, with some participants correcting Mark's calculations.
  • Mark suggests that if a rod were twisted and released, it could potentially achieve high speeds, akin to the crack of a whip, raising questions about energy storage and transfer.
  • Others contend that regardless of the material used, achieving faster-than-light speeds would still require infinite energy and that no known material could withstand the necessary forces.
  • Some participants express skepticism about the feasibility of constructing a 7,000-mile-long string or rod, citing gravitational collapse and material limitations.
  • Mark later questions the implications of placing the string in a negative Kelvin state, suggesting it might allow for faster-than-light travel, which is met with skepticism.

Areas of Agreement / Disagreement

Participants generally agree that faster-than-light travel is not possible according to current physical laws, but there is disagreement on the specifics of the scenarios proposed and the implications of those laws. The discussion remains unresolved regarding the feasibility of the hypothetical constructs and the energy requirements involved.

Contextual Notes

Limitations include assumptions about the properties of materials, the nature of energy transfer in hypothetical scenarios, and the applicability of relativistic physics to the proposed models. The discussion does not resolve the mathematical or physical principles involved.

mausten
Messages
5
Reaction score
0
Fast than speed of light...

Hi guys & girls,

I have a question regarding traveling faster than the speed of light, a small theory, which I am hoping someone can explain to me why this wouldn't work. Note: I'm a total amateur science enthusiast, so please forgive me if this sounds really dumb.

Here's the scenario:

A stone on the end of string 1m in length that takes 1s to travel the circumference is traveling approximately 22mph. If a marker was placed 50cm along the string, it would take the marker 1s to complete a full circle, traveling at 11mph. Correct?

If the string was 7,000 miles long (11,480km) and it took 1s for the end of the string to travel the circumference, by my calculations (probably somewhat wrong), this would enable to end of the string to travel faster than the speed of light.

Hypothetically, if this huge structure was created in Space and a speed of 22mph (1 revolution per second) was achieved 1 meter, why would the end of the string not be traveling at the speed of light?

If you can let me know why I am wrong, that would be extremely helpful! :)

Thanks,

Mark
 
Physics news on Phys.org
You could never accelerate the structure up to full speed because the end of the string would require more and more energy to accelerate as the speed increased.
 
If a marker was placed 50cm along the string, it would take the marker 1s to complete a full circle, traveling at 11mph. Correct?
Correct.

If the string was 7,000 miles long (11,480km) and it took 1s for the end of the string to travel the circumference
You cannot let the string rotate once per second if it has this length.
 
The first part of your reasoning is correct, except for the numbers. The relevant equation here is ##v = \omega r##. If you get the angular velocity to ##2 \pi ## rad/s, then for r = 1 you get a linear velocity of ##v_0 = 2 \pi## m/s which is approximately 22 km/h or 14 mph (miles per hour). Let's assume that you meant the former, then at r = 1/2 from the point of rotation you will find ##v = \pi = \tfrac12 v_0##.

The error in your calculations is probably that you are (implicitly or explicitly) using the same relation in the relativistic case. I'm not sure what the correct formula is (you can derive it using the correct velocity addition formulas and expressions for the acceleration parallel and perpendicular to the direction of motion) but my educated guess is that you would get something like
##v = \tanh(\omega r)## instead (noting that ##\tanh(x) \approx x## if ##|x| \ll 1##).

[EDIT]What mfb says: you cannot let the end rotate with linear velocity greater than c and all points closer will have lower velocity anyway[/EDIT]
 
As Drakkith mentioned, it would take an infinite amount of energy. Additionally, it would require a string with an infinite tensile strength.
 
Thank you for the very quick replies CompuChip, mfb & Drakkith, much appreciated. And also, I apologise for the double posting of this topic.

I understand that theoretically, the end of the string cannot travel faster than the speed of light; E=MC2. But why not? If for example the 'string' was made from a nano tube that was 7,000miles long; hypothetical speaking, what force would be needed?

Also, let's imagine you take a bendy rod and twist it into a circle. Let go of one side and it springs back to its original state (sorry for the ultra layman's terminology!), there is enough kinetic energy/tension (?) in the rod for it to spring back. My head tell me, that the bigger the rode, the higher the energy build up until its release and return to original state. So, IF, the rod was big enough and twisted enough times - the whole 7,000 miles of tube, would there not be enough 'stored'/kinetic/tenstion energy to reach the required velocity to travel faster than the speed of light - much like the 'crack' at the end of a whip?

For this to work the rod would have to be much thinner at the 'cracking' end, than at the start, as to allow the energy to efficiently transfer and travel to the end.

I'm a total novice who likes to day dream.
 
What you don't seem to understand or know is that it simply isn't possible for anything with non-zero mass to travel at FTL speeds – carbon nanotubes or not, it would still imply that an infinite amount of energy would be required to accelerate the mass. It doesn't matter how thick it is or how strong it is, it would still break at some point of the acceleration process.

E=mc2 has little to do with it, try [itex]E=mc^2/\sqrt{1-v^2/c^2}[/itex]. What happens if v=c?
 
mausten said:
If for example the 'string' was made from a nano tube that was 7,000miles long; hypothetical speaking, what force would be needed?
Infinite, as already stated repeatedly.
 
Hmmm... dammit. What if the 'string' was placed into a negative kelvin state - thus meaning that more 'natural' (?) energy would be needed to return to normal state, thus increasing velocity faster than normal...

Believing in God is easier than believing you can't accelerate faster than the speed of light.

Thank you for your clarification dudes & dudettes.

I'll go back to my hole and leave you to answer some proper questions now...
 
  • #10
mausten said:
Believing in God is easier than believing you can't accelerate faster than the speed of light.

Except you can prove the latter from a few simple hypothesis and a sufficient dose of math :-)

(You can try that with the former too, if your name is Isaac Newton, but I would not recommend it).
 
  • Like
Likes   Reactions: 1 person
  • #11
mausten said:
I understand that theoretically, the end of the string cannot travel faster than the speed of light; E=MC2. But why not? If for example the 'string' was made from a nano tube that was 7,000miles long; hypothetical speaking, what force would be needed?

Also, let's imagine you take a bendy rod and twist it into a circle. Let go of one side and it springs back to its original state (sorry for the ultra layman's terminology!), there is enough kinetic energy/tension (?) in the rod for it to spring back. My head tell me, that the bigger the rode, the higher the energy build up until its release and return to original state. So, IF, the rod was big enough and twisted enough times - the whole 7,000 miles of tube, would there not be enough 'stored'/kinetic/tenstion energy to reach the required velocity to travel faster than the speed of light - much like the 'crack' at the end of a whip?

For this to work the rod would have to be much thinner at the 'cracking' end, than at the start, as to allow the energy to efficiently transfer and travel to the end.

I'm a total novice who likes to day dream.
The trouble with your day dreams is that they are still dreams.

You are thinking that it is possible to have a rod/tube/string that is 7000 miles long. Such structures do not exist in our universe. That is almost the diameter of earth. All such structures collapse under their own gravitational mass into spheres. So you can't even start your scenario. You are imagining that their exists a material that is strong enough and light enough to take on the shape of your attached drawing but there isn't and there can't be.
 
  • #12
ghwellsjr said:
The trouble with your day dreams is that they are still dreams.

You are thinking that it is possible to have a rod/tube/string that is 7000 miles long. Such structures do not exist in our universe. That is almost the diameter of earth. All such structures collapse under their own gravitational mass into spheres. So you can't even start your scenario. You are imagining that their exists a material that is strong enough and light enough to take on the shape of your attached drawing but there isn't and there can't be.
Sorry, but I think that explanation is misleading. It looks like it would be just an engineering issue. It is not (and a 7000-miles rod in space can be built).
The speed of light is a very fundamental physical limit. Even if you have a perfect material with infinite tensile strength and whatever, even if you have an arbitrary amount of energy available and put it all into the rod, the resulting speed will be below the speed of light.

mausten said:
Hmmm... dammit. What if the 'string' was placed into a negative kelvin state - thus meaning that more 'natural' (?) energy would be needed to return to normal state, thus increasing velocity faster than normal...
That does not make sense.
 
  • #13
Quote by mausten
Hmmm... dammit. What if the 'string' was placed into a negative kelvin state - thus meaning that more 'natural' (?) energy would be needed to return to normal state, thus increasing velocity faster than normal...
That does not make sense.No, you're right, that doesn't make sense. I read somewhere last week (sorry have forgotten the source - maybe this is simular, they used the same example about making cobustion engines over 100% efficient http://www.chemistry2011.org/news/P...luteTemperatureAreTheHottestSystemsInTheWorld) about atoms being cooled to below absolute zero. The article went on to explain that if atoms are cooled below absolute zero they release more energy in returning to their 'normal state'.

I was thinking that if this FTLD (Faster Than Light Device - Mark2013TM :-) ) was super duper cooled to -x kelvin, that it might just work. Imagine a little transported pod on the end of the whip/rod, that accelerated you to 'X' x faster than the speed of light and let released you onto your desired trajectory.
 
  • #14
Negative temperatures (on the Kelvin scale) are not cold. They are hotter than anything with positive temperature, and you have to put thermal energy into a system to reach a negative temperature.
This has nothing to do with the motion of a rod.
 

Similar threads

High School Let me ask a thing about the Speed of Light
  • · Replies 6 ·
Replies
6
Views
2K
High School Time Travel: Can Humans Go Faster Than Ordinary Speed?
  • · Replies 9 ·
Replies
9
Views
2K
Undergrad Light Speed in Dense Medium: Explained
  • · Replies 1 ·
Replies
1
Views
2K
High School Would We See the Same Light After Traveling Back to Earth Faster Than Light?
  • · Replies 2 ·
Replies
2
Views
2K
High School Can Curved Spacetime Enable Faster-Than-Light Travel?
  • · Replies 12 ·
Replies
12
Views
4K
Undergrad Light speed independent of source
  • · Replies 22 ·
Replies
22
Views
2K
Undergrad Why Can't Objects Travel Faster Than Light?
  • · Replies 18 ·
Replies
18
Views
3K
Undergrad Why is there nothing faster than light?
  • · Replies 16 ·
Replies
16
Views
2K
Undergrad Can Propeller Speeds and Wave Breaks Challenge the Speed of Light?
  • · Replies 13 ·
Replies
13
Views
2K
High School Explanation of light speed limit
  • · Replies 9 ·
Replies
9
Views
2K