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Author Topic: What happens to an object moving in a circle at light speed?  (Read 6799 times)

Offline chris

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When an object moves at light speed it stretches out to the eye of the observer. So what happens when an object follows a circular path at light speed. e.g. a windmill spinning at the speed of light?

C


 

Offline alancalverd

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In order to move in a circle, the object must be subject to a centripetal force.

Since a massive object will have infinite mass at c, there is no force that can constrain it to circular motion - or indeed to change its direction at all. And even if there were an infinitely strong string, what would you attach it to?   
 

Offline PmbPhy

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Quote from: chris
When an object moves at light speed it stretches out to the eye of the observer.
Objects cannot move at the speed of light unless they have zero proper mass such as as photons.

Where did you get the notion that objects stretch out to the eye of the observer? The faster an object moves the shorter it gets in the direction of motion. I.e. objects shrink at high speeds. They don't stretch.
« Last Edit: 13/09/2015 15:47:22 by PmbPhy »
 

Offline evan_au

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If you could whirl a significant mass at relativistic speeds, it would radiate energy in the form of gravitational waves (or so the theory of general relativity and confirming astronomical measurements indicate).

If the object were electrically charged, it would also radiate electromagnetic waves.
 

Offline mathew_orman

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When an object moves at light speed it stretches out to the eye of the observer. So what happens when an object follows a circular path at light speed. e.g. a windmill spinning at the speed of light?

C
In my opinion based on observations there are such cases in reality...
 

Offline PmbPhy

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When an object moves at light speed it stretches out to the eye of the observer. So what happens when an object follows a circular path at light speed. e.g. a windmill spinning at the speed of light?

C
In my opinion based on observations there are such cases in reality...
You're wrong. Nothing that has mass can move at the speed of light. There are no observations which demonstrate otherwise.
 

Offline alancalverd

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In my opinion based on observations there are such cases in reality...


Please cite the observations. There's nothing better than a good experiment to demolish a theory that has been held for the last hundred years.
 

Online jeffreyH

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When an object moves at light speed it stretches out to the eye of the observer. So what happens when an object follows a circular path at light speed. e.g. a windmill spinning at the speed of light?

C
In my opinion based on observations there are such cases in reality...

Which part are you referring to? The stretching out or the circular motion? I agree with Pete on the stretching out part. In the case of circular motion you would need a binding force to keep the object in orbit. If there were a force of such magnitude you wouldn't be posting to this forum. You would be a microscopic speck in a singularity.
 

Offline lightarrow

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In order to move in a circle, the object must be subject to a centripetal force.

Since a massive object will have infinite mass at c, there is no force that can constrain it to circular motion
Which equation are you using here?

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lightarrow
 

Offline lightarrow

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If you could whirl a significant mass at relativistic speeds, it would radiate energy in the form of gravitational waves (or so the theory of general relativity and confirming astronomical measurements indicate).
Excepting when there are simmetries like an homogeneous disk rotating around its principal axis (or other cases).

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lightarrow
 

Offline alancalverd

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In order to move in a circle, the object must be subject to a centripetal force.

Since a massive object will have infinite mass at c, there is no force that can constrain it to circular motion
Which equation are you using here?

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lightarrow

m = m0/√{1-v2/c2} is good enough for CERN and me.
« Last Edit: 12/09/2015 14:20:49 by alancalverd »
 

Offline lightarrow

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In order to move in a circle, the object must be subject to a centripetal force.

Since a massive object will have infinite mass at c, there is no force that can constrain it to circular motion
Which equation are you using here?

--
lightarrow

m = m0/√{1-v2/c2} is good enough for CERN and me.
No, I was referring to what you called "centripetal force".

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lightarrow
 

Offline alancalverd

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The centripetal force required to maintain circular motion of a body moving at relativistic speeds is

F = γm0v2/r

where v is the instantaneous tangential velocity.
 

Offline lightarrow

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The centripetal force required to maintain circular motion of a body moving at relativistic speeds is

F = γm0v2/r

where v is the instantaneous tangential velocity.
Ok.
Now find the expression of the tangential force you need to accelerate the body to relativistic speeds, then I'll show you something interesting.

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lightarrow
 

Offline alancalverd

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Any force will do if you wait long enough - the principle behind solar sails and ion rockets.

The problem is that "long enough" is infinite, whatever the force applied, if you want a massive particle to reach c. How is that "interesting"?
 

Offline PmbPhy

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Use a proton in a particle accelerator as an example. In such accelerators there is a magnetic field that keeps the protons moving in a circle. That force is the required centripetal force. This force is known as the Lorentz force.

The protons are accelerated in the direction of motion by an electric field. As the speed is increased the magnetic field is increased so as to keep the proton moving in the same circle.
 

Offline lightarrow

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Any force will do if you wait long enough - the principle behind solar sails and ion rockets.

The problem is that "long enough" is infinite, whatever the force applied, if you want a massive particle to reach c. How is that "interesting"?
Why do you think to know what I'm referring to?
Write the relation between tangential force and acceleration at relativistic speeds. If you don't know, I can write it for you.

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Offline alancalverd

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We're not talking about "relativistic speeds" but c. Accelerating leptons to 0.99c in a circular path is an engineering problem, but the physics is no big deal. It's the recurring decimal point that distinguishes between difficult and impossible. Your equation is awaited with bated breath.
« Last Edit: 13/09/2015 18:09:34 by alancalverd »
 

Offline lightarrow

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We're not talking about "relativistic speeds" but c.
In this case what you wrote here:
Quote
The centripetal force required to maintain circular motion of a body moving at relativistic speeds is
F = γm0v2/r
where v is the instantaneous tangential velocity
is meaningless since you cannot define γ  :)
Quote
Accelerating leptons to 0.99c in a circular path is an engineering problem, but the physics is no big deal. It's the recurring decimal point that distinguishes between difficult and impossible. Your equation is awaited with bated breath.
If physics "is no big deal" why didn't you write the equation I asked you?
Ok, I'll write it:

F = γ3m0 * a

m0 = proper mass
F = tangential force
a = tangential acceleration

Now compare the two equations:
1) F = γm0 v2/r     (centripetal = radial force).
2) F = γ3m0 * a     (tangential force)

and then tell me which is here the mass that goes to infinity according to the sentence you wrote:
Quote
Since a massive object will have infinite mass at c
I mean, the mass you are referring to is γm0 or is γ3m0?

Is it still meaningful to talk about relativistic mass, in this context? Wouldn't be better to talk about m0 = m only and say that it doesn't change with velocity?

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Offline PmbPhy

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Quote from: alancalverd
]
We're not talking about "relativistic speeds" but c.
I don't understand this sentence. Are you trying to say that the speed of light is not a relativistic speed? If so then you're wrong since the term relativistic speed is defined as follows. From:
https://en.wikipedia.org/wiki/Relativistic_speed
Quote
A relativistic speed is a speed which is a significant proportion of the speed of light.
It's reasonable to say that the speed of light itself is a significant proportion of the speed of light. Saying otherwise without elaborating is confusing.
 

Offline alancalverd

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mv =  m0(1 - v2/c2)−1/2

This works for "relativistic speeds" but clearly raises problems when v = c, as Einstein remarked. And who am I to disagree? 
 

Offline PmbPhy

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mv =  m0(1 - v2/c2)−1/2

This works for "relativistic speeds" but clearly raises problems when v = c, as Einstein remarked. And who am I to disagree?
That didn't help me understand your sentence, i.e. what did you mean when you wrote "We're not talking about "relativistic speeds" but c."?
 

Offline alancalverd

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Pretty much the same as the difference between "green-ish" and "λ = 520 nm". 0.99c is a relativistic speed in most people's language. c is c.

The point is that all and only photons travel at c, whereas any particle with nonzero mass could in principle travel at any speed less than c. That makes c rather special, and 0.999c rather ordinary.
« Last Edit: 16/09/2015 07:05:02 by alancalverd »
 

Offline lightarrow

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Pretty much the same as the difference between "green-ish" and "λ = 520 nm". 0.99c is a relativistic speed in most people's language. c is c.
c is not relativistic speed? I don't know if laughing or crying...
Quote
The point is that all and only photons travel at c, whereas any particle with nonzero mass could in principle travel at any speed less than c. That makes c rather special, and 0.999c rather ordinary.
Your problem is the obstination to want to use  equations which are valid only for velocities different from c. If you used those which are always valid, for all velocities included c, you wouldn't need to "climb on the mirrors"  :)

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Offline evan_au

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Quote from: Chris
what happens when an object follows a circular path at light speed?
Quote from: alancalverd
We're not talking about "relativistic speeds" but c.
Quote from: PmbPhy
what did you mean when you wrote "We're not talking about "relativistic speeds" but c."?
I think we may be talking at cross purposes here.

The OP was asking about swinging an object around at lightspeed=c.
The discussion moved on to discussing what happens at relativistic speeds < c.

Alan was just reminding us that the OP was asking about traveling at speed=c, which is infinitely harder than traveling at "relativistic speeds" < c...
« Last Edit: 16/09/2015 11:28:56 by evan_au »
 

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