I Speed of the outside of a wheel faster than the speed of light

  • Thread starter Sciencepoo
  • Start date
Summary
A wheel that can travel ftl
I'd like to know if anybody can tell me either;
How large a wheel would have to be for the outer edge to travel faster than the speed of light if powered by a motor that's around now.
We'll say the wheel is in outer space to eliminate drag and such
 

BvU

Science Advisor
Homework Helper
11,987
2,635
Don't think matter can move faster than light ... Einstein and all that.

Hello Sp, :welcome: !
 

Ibix

Science Advisor
Insights Author
4,901
3,219
Summary: A wheel that can travel ftl

I'd like to know if anybody can tell me either;
How large a wheel would have to be for the outer edge to travel faster than the speed of light
It is not possible to construct such a thing, even in principle. It will necessarily disintegrate or distort so that nothing travels faster than light.

There is no way to trick your way round that speed limit.
 
Hypothetically speak even if we could make a wheel big enough (however big that may be,) I am not sure any motor we have today would have enough torque to turn it due to it's size.
 
27,712
4,171
Summary: A wheel that can travel ftl

How large a wheel would have to be for the outer edge to travel faster than the speed of light if powered by a motor that's around now.
As @Ibix mentioned it will necessarily disintegrate before it reaches c. However, another issue is that it would require an infinite amount of energy. So a motor could run for any amount of time and still not have produced sufficient mechanical energy.
 

Ibix

Science Advisor
Insights Author
4,901
3,219
Hypothetically speak even if we could make a wheel big enough (however big that may be,) I am not sure any motor we have today would have enough torque to turn it due to it's size.
In principle you can turn an arbitrarily large wheel with an arbitrarily small motor. You just put the whole thing in zero g in vacuum and let conservation of angular momentum do its thing. It'll just take an arbitrarily long time.

The problem is that no matter how powerful the motor or how large the wheel, you cannot get the rim to exceed lightspeed even in principle.
 

jbriggs444

Science Advisor
Homework Helper
7,219
2,416
The problem is that no matter how powerful the motor or how large the wheel, you cannot get the rim to exceed lightspeed even in principle.
If you discard the requirement that anything physical is made to exceed light speed, one can accomplish the feat quite conveniently.

Take a laser pointer and assume (for the sake of argument) that its beam is tightly collimated. Point it at the right edge of the moon. Flick the pointer so that it points to the left edge of the moon. Do this in less than 10 milliseconds and the illuminated spot on the moon will have traveled from the right edge to the left edge at a speed faster than light.

 
Yes, it's "information" that cannot travel faster than the speed of light as far as I understand it.
 

russ_watters

Mentor
18,524
4,753
Yes, it's "information" that cannot travel faster than the speed of light as far as I understand it.
Objects are/carry information. Physical objects can't travel faster than light.
 

DaveC426913

Gold Member
17,997
1,592
Take a laser pointer and assume (for the sake of argument) that its beam is tightly collimated. Point it at the right edge of the moon. Flick the pointer so that it points to the left edge of the moon. Do this in less than 10 milliseconds and the illuminated spot on the moon will have traveled from the right edge to the left edge at a speed faster than light.
I'm really not sure what this has to do with the thread topic but it's flawed.

The "beam" of laser light is only conceptualized as a single thing in a simplistic mental model.
It is no more a single object than a spray of bullets from a machine gun.

By analogy to your demo, I could spin the barrel of a machine gun from North to South and claim the spray of bullets is traveling at 20 times the speed of sound.
 

DaveC426913

Gold Member
17,997
1,592
How large a wheel would have to be for the outer edge to travel faster than the speed of light if powered by a motor that's around now.
This answer cuts to the chase of what you're asking. It grants every possible engineering objection - from fuel to speed to material strength*. And yet the edge will never reach c, let alone exceed it.

*Except one: the theory of relativity itself prohibits the existence of infinitely-strong/infinitely-rigid materials.

Q: Why won't it reach c?

A: Like all things, the wheel is subject to relativity. As the edge approaches relativistic speed, it will length contract, just like anything else. Increasing speed will get the edge closer to c but will never exceed it.

Yes, this will cause the wheel to distort in shape, since the non-relativistic inner parts will not likewise get length-contracted. Yes, it will actually disintegrate - but not simply because the material is not strong enough to withstand the forces - it will disintegrate because relativistic effects will distort it.


You'll want to read up on Bell's Spaceship Paradox. That one is two spaceships in a line, but there's a derivative experiment (link, anyone?), set up like so:

Tie a bunch of spaceships into a circle, nose-to-tail, with pieces of string. Space the ships so the string is taut. Set the spaceships chasing each around the circle at relativistic speeds.

Q: We know the spaceships will shrink in length, but will the strings break?

A: Yes. Any rigid loop, when accelerated to a rotation of relativistic velocity, will be contracted. Its rotational speed will approach, but never reach, c.
 
Last edited:

jbriggs444

Science Advisor
Homework Helper
7,219
2,416
By analogy to your demo, I could spin the barrel of a machine gun from North to South and claim the spray of bullets is traveling at 20 times the speed of sound.
Yes, the point of impact of the bullets could be made to move at pretty much any desired speed, including faster-than-light. As you point out, that "point of impact" is not anything physical travelling from point A to point B.
 

Want to reply to this thread?

"Speed of the outside of a wheel faster than the speed of light" You must log in or register to reply here.

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving
Top