# Disk Rotating Near the Speed of Light

#### Nitefang

I was sitting in Physics class today while we were talking about the speed of light. I know almost every physicist will say that nothing can go faster than the speed of light. While all of these men and women know more than me, one thing they fail to realize is that every single time anyone says something is impossible someone else does it.

Anyway onto my question.
Suppose you were able to get the inside of a disk spinning near the speed of light. Since the outside of the disk has to move faster than the inside what will happen to the outside of the disk? If it does break the speed of light what will happen to it? One suggestion that I was given by a fellow student is that the atoms would begin turning into photons since only light can travel that fast. While this might look amazing I doubt his knowledge on the subject and thought I would ask you guys.

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#### russ_watters

Mentor
Welcome to PF.
While all of these men and women know more than me, one thing they fail to realize is that every single time anyone says something is impossible someone else does it.
That isn't true.
Suppose you were able to get the inside of a disk spinning near the speed of light. Since the outside of the disk has to move faster than the inside what will happen to the outside of the disk? If it does break the speed of light what will happen to it? One suggestion that I was given by a fellow student is that the atoms would begin turning into photons since only light can travel that fast. While this might look amazing I doubt his knowledge on the subject and thought I would ask you guys.
One thing you don't appear to have been exposed to is Einstein's theory of Relativity, just that that one fact was thrown out there empty of explanation. In Relativity, velocieis are not added linear like we do in every day life. It has been found that at high speed that the normal formula stops workinig and you need to use a diferent one. Here's some info on it and the formula:
http://www.weburbia.com/physics/velocity.html

In addition, the reason you can't go the speed of light is it requires an infinite amount of energy. As objects accelerate, they get more massive and their ability to accelerate further is hindered. The closer you get to the speed of light, the harder it is to get any closer to the speed of light.

#### Nitefang

That reminds of something else. Firstly, I am still taking high school physics so I am ignorant to a lot of more advanced physics but here is a thought.

If matter increases as you near the speed of light, would anti-matter also increase? I ask this because I briefly looked into converting matter to energy and vice-verse, according to one of NASA's Astrophysicists when you combine matter and antimatter it becomes energy. So if you infinitely expand as you near the speed of light wouldn't your energy source also increase? Because your infinite supply of matter and anti-matter could be combined into an infinite amount of energy.

#### pallidin

A spinning disk has significant problems well below c.

That is, co-valent bond breakage occurs long before c could be reached. Thus the system, essentially, disintegrates.

#### D H

Staff Emeritus
If matter increases as you near the speed of light, would anti-matter also increase?
Matter (the number of atoms) does not increase as its speed nears that of light. In our everyday world, kinetic energy is equal to 1/2mv2. That isn't the case for very high velocities. Kinetic energy and momentum increase without bound as speeds approach that of light. One way to look at this is that the mass of an object increases as its velocity increases. Another way to look at it is to stop pretending that the equations we love and cherish at low velocities have any relevance at high velocities.

Special relativity isn't all that hard to understand. It is often taught at the high school level nowadays.

A spinning disk has significant problems well below c.
So long as it isn't made of unobtanium, that is.

#### Bob S

If the disk can survive the radial forces at high RPM, some interesting things happen. You probably recall the relation circumference = 2 pi times radius. Well, in this case, the circumference begins to contract due to special relativity, while the radius stays the same. so how good is the formula?

#### DaveC426913

Gold Member
A spinning disk has significant problems well below c.
(True, but it does have the bonus effect of depleting all the manna from the surrounding countryside...)

#### DaveC426913

Gold Member
If matter increases as you near the speed of light, would anti-matter also increase?
You've misunderstood.
It's not "matter" that increases, it's mass (which is a property of matter).

If a lump of antimatter were accelerated to near c, its mass would increase. That doesn't mean you have "more" though....

#### cragar

Special relativity isn't all that hard to understand. It is often taught at the high school level nowadays.
Ok some of the stuff in Relativity is easy to understand but to truly comprehend is almost impossible
Richard Feynman who was a genius among genius’s could not even begin to comprehend how Einstein came up with theory of relativity.

#### jmatejka

You've misunderstood.
It's not "matter" that increases, it's mass (which is a property of matter).

If a lump of antimatter were accelerated to near c, its mass would increase. That doesn't mean you have "more" though....
Interesting...... my undergraduate Modern Physics Text made a point of saying "acts like" mass is increasing. Two of my Prof's said "mass doesnt increase, as you approach "C" in my classroom". My understanding is the whole "mass increasing" concept is that it's a +/-accurate model, (but not actual reality).

Perhaps, I am complicating this thread with higher theory?

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#### D H

Staff Emeritus
Interesting...... my undergraduate Modern Physics Text made a point of saying "acts like" mass is increasing. Two of my Prof's said "mass doesnt increase, as you approach "C" in my classroom".
That's certainly a more modern view. The intrinsic mass (aka rest mass) doesn't increase, but the relativistic mass, E/c2 does. This modern view is that relativistic mass is essentially just an alias energy, so why bother confusing things? Mass is an intrinsic characteristic of some object; energy is observer dependent.

#### pallidin

If the disk can survive the radial forces at high RPM...
That is the question.
The answer is no. There is no known material, composite or otherwise, that can maintain structural integrity under extreme RPM's with respect to a forced c environment.

#### cragar

what if the disk was maintained by a magnetic field.

#### Danger

Gold Member
(True, but it does have the bonus effect of depleting all the manna from the surrounding countryside...)
And the magic goes away. Nice to know that there's another Niven fan aboard.

#### Count Iblis

A millisecond pulsar comes close, though.

#### DaveC426913

Gold Member
And the magic goes away. Nice to know that there's another Niven fan aboard.
Got the whooole set.

#### LuigiHomer

Kind of off the topic,but time slows for ANY AND ALL matter traveling at lightspeed,so the power may last longer,but other people will have found a way to break light speed,so it would be pointless.

#### DaveC426913

Gold Member
Kind of off the topic,but time slows for ANY AND ALL matter traveling at lightspeed,so the power may last longer,but other people will have found a way to break light speed,so it would be pointless.
This is nonsense. Matter cannot travel at lightspeed, so the question of what time does is meaningless.

#### jefswat

So if you infinitely expand as you near the speed of light wouldn't your energy source also increase? Because your infinite supply of matter and anti-matter could be combined into an infinite amount of energy.
I don't think anyone explicitly said this but it is irrelevant because you have to keep putting more and more energy into the system to even make the reletavistic mass increase. So any energy you gain would be offset by all the energy you lost getting said matter and anti-matter to the speed of light. Conservation of energy in action

#### DaveC426913

Gold Member
I don't think anyone explicitly said this but it is irrelevant because you have to keep putting more and more energy into the system to even make the reletavistic mass increase. So any energy you gain would be offset by all the energy you lost getting said matter and anti-matter to the speed of light. Conservation of energy in action
No, the whole argument is faulty. Don't try to rationalize it.

If you take a 1kg lump of antimatter and accelerate it to .9c, it will mass 2.29kg, as measured from an external FoR. The same amonut of matter will mass more, true, but you will not have an "extra" 1.29 kg of antimatter.

Furthermore, in the lump's FoR, its mass is still exactly 1kg - nothing's changed. And it can still only do the same amount of work.

#### jefswat

No, the whole argument is faulty. Don't try to rationalize it.

If you take a 1kg lump of antimatter and accelerate it to .9c, it will mass 2.29kg, as measured from an external FoR. The same amonut of matter will mass more, true, but you will not have an "extra" 1.29 kg of antimatter.

Furthermore, in the lump's FoR, its mass is still exactly 1kg - nothing's changed. And it can still only do the same amount of work.
Thats why I said relativistic mass. It will have more kinetic energy though no? or does kinetic energy disappear when you get near the speed of light? I was meerly pointing out that any benifits like hitting something with said matter and antimatter would be offset by the energy it takes to get the stuff going that fast. energy doesn't just appear from no where

#### Integral

Staff Emeritus
Gold Member
I get very nervous when people start casually throwing around relativistic mass as if they know exactly what they are talking about. It is all good and fine as long as you remember that the following conversation could take place.

Ground control to Major Tom:
"Hey dude ease up on the Twinkies! Our calculations show that your mass is rapidly approaching infinite!"

Major Tom to Ground Control:
"What are you talking about! The food sucks! I have lost 10kg, and have used 50% of my fuel, so the ship mass is way down. It looks to me like YOU are the one on the Twinkies"

The message is be very careful with relativistic mass, it just does not work is the simple minded fashion that is so often found in this type of discussion. No matter what the relative velocity each observer sees himself at rest and cannot observe any effects of his velocity on himself.

#### jtbell

Mentor
If you take a 1kg lump of antimatter and accelerate it to .9c, it will mass 2.29kg, as measured from an external FoR. The same amonut of matter will mass more, true, but you will not have an "extra" 1.29 kg of antimatter.
Not only that, if you accelerate to 0.9c, without touching the lump at all, its "relativistic mass" still becomes 2.29kg, as measured in your new FoR. How do you get extra "matter" or "antimatter" without doing anything to it at all?

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