Another hypothetical situation.

[Guest liliangrn]

There are two train tracks running parallel (track one and track two). Now imagine observer A is standing on track one. Observer B is travelling on the track toward observer A at a velocity of v = c.

 

A third observer C is travelling in the same direction as observer B at a velocity of v = c/2.

 

Now:

 

Observer B would observe observer A with zero size.

 

(Due length contraction at that speed the mass should all fall outside the body and observer A should have no observable size at all.)

 

It follows that observer A would see that observer B with no size also.

 

Observer C would see both observer A and B about half there usual respective sizes.

 

According observer A and B they should pass straight through each other. But from observer C's POV they should colide with each other.

 

Which would be the correct outcome to this situation?

 

Josephine

[Guest liliangrn]

Oh and observer C is travelling on track 2 so he doesn't collide with observer B.

[C1ay]

According observer A and B they should pass straight through each other. But from observer C's POV they should colide with each other.

 

Which would be the correct outcome to this situation?

 

Josephine

It wouldn't matter if they could see each other or not, they would collide.

[Guest liliangrn]

Hi Clay,

 

Can you explain this any further?

 

According to observer A - observer B doesn't even exist. That is except for a mass orbiting around a massless singularity. If observer B hit the singualrity there is nothing to stop him from going straight through and out the other side. (And vice-versa)

 

Observer C is the only one who can see both the other observers with any mass at all.

 

Josephine

[C1ay]

Hi Clay,

 

Can you explain this any further?

 

According to observer A - observer B doesn't even exist. That is except for a mass orbiting around a massless singularity. If observer B hit the singualrity there is nothing to stop him from going straight through and out the other side. (And vice-versa)

 

Observer C is the only one who can see both the other observers with any mass at all.

 

Josephine

Just because observer A cannot see observer B does not mean that their mass doesn't exist. Neither is massless because of their velocity. Two masses traveling at each other at any speed will collide. It is also important to note that mass increases as it approachs c.

[Guest liliangrn]

Mass is a measure of a body (i.e. how much matter it contains) regardless of gravity. So according to relativity, if observer B is travelling at v = c, he must see observer A as a rather dense singularity, or a mass orbiting a massless singularity (I'm not sure which). Either way observer B will experience observer A as not existing. There would be mass, but there would be no matter, because a singularity has no size. With no size the mass would become empty.

 

If experience is to remain relative to the observer then, although observer A has a mass, observer a doesn't really exist. Therefore it wouldn't make sense for observer B to hit observer A. Does that make COMPLETE sense to you? How can something with no matter collide with something that has matter?

 

 

Josephine

[C1ay]

How can something with no matter collide with something that has matter?

 

 

Josephine

Reword that. How can accelerating a particle to c retain it's mass while stripping it's matter? Are the theories behind particle accelerators and colliders a fallacy? What would be the point in trying to accelerate particles to c if there wouldn't be any collisions?

[Guest liliangrn]

Well if you hit a blackhole at v = c the radius of the black hole will be pushed all the way to the singularity.

 

R = GMm/c^2

 

A zero R would also mean a zero m. Wouldn't it?

All the matter would be pushed all the way to the singularity.

 

So infact observer A would not be hitting a non-existent observer B. Observer B would be hit by a relatively non-existent Observer A.

 

We don't need to discuss this if we can't agree. I was just curious that's all.

 

Josie

[Bo]

The important quantity here is momentum, not mass (and as you probably know photons/stuff that moves with speed c has momentum).

Another thing you -in priniciple- should consider here, is that if a particle becomes pointlike, you should use quantum mechanics, that says that it will never be exactly point like.

 

Bo

[C1ay]

So infact observer A would not be hitting a non-existent observer B. Observer B would be hit by a relatively non-existent Observer A.

 

We don't need to discuss this if we can't agree. I was just curious that's all.

 

Josie

Answering your post in reverse here, I see no reason why we need to agree to discuss this. I don't think the purpose of the discussion is to agree, it is to find the truth. That is what you're looking for, right?

 

I can't particularly see how you conclude that accelerating A to c would avoid a collision with B. Just because B cannot observe A does not mean that B will not be impacted by A. A is still a real entity travelling at c. Even if A were completely converted to photons A would not pass through B unless B was transparent. What form of mass do you think A will become that A could pass thru B?

[Guest liliangrn]

--------------------------------------------------------------------------------

 

The important quantity here is momentum, not mass (and as you probably know photons/stuff that moves with speed c has momentum).

Another thing you -in priniciple- should consider here, is that if a particle becomes pointlike, you should use quantum mechanics, that says that it will never be exactly point like.

 

Bo

 

Hi Bo,

 

I've just learnt about momentum (p = mv) so I'm not sure as to what you are refering to.

 

The second part, I was explained quantum mechanics breifly, I don't understand because the second observer would still only be hitting an object the size of a particle. That's smaller than an atom. At c an observer would already be hitting many, many particles and even atoms. The observer would need to already account for this in order to maintain his momentum at v = c.

 

I am fairly new to this so if I could please ask for you to explain this to me further. I would appreciate it.

 

Josephine

[Guest liliangrn]

Hi Clay,

 

Well that's the whole point of Special Relativety. There is no absolute reference frame.

 

My clocks and rods don't necessarily equate to your clocks and rods if I am static and you are moving. Neither, however, clocks nor rods can be considered incorrect because spacetime has no absolutes (that we know of anyway). Reality for me is not reality for you. If you are travelling v = c and I am stactic, then in your reality I am no larger than a particle (as yourself and Bo had correctly pointed out) and vice-versa. This is one of the known yet unspoken absurdities of Special relativity. I was just curious if there were any other thoughts, that people on this forum had, about this.

 

Josephine

[Bo]

in non-relativistic mechanics indeed the momentum is p=mv

however it turns out that a particle that moves at the speed of light:

has m=0

but NOT p=0.

This is a result coming from relativistic mechanics, where the usul formula p=mv is altered. A result is that for massless particles we actualy have: p=E/c, with E the energy of the particle, and c the speed of light.

 

Quantum mechanics is a difficult subject, so i'll try to keep things simple:

 

QM says that particles sometimes behave more like a wave, then like a pointlike particle. one of the consequaences of this is that the particle can never be exactly located. It always is 'smeared' a bit.

 

Bo

[Guest liliangrn]

Hey Bo,

 

I think I understand what you are saying. I fail to see how this explains why the two bodies will collide.

 

In classical mechanics we use the equation:

 

p = mv

 

But in relativety we use:

 

p = e/v

 

v = c

 

so:

 

p = e/c

 

So what....er..?

 

I'm guessing that a body moving at v = c would have a high level of kinetic Energy. That would be why, with increasing velocity, observer A's mass increases, whilst to another static observer's POV, his size still visibly decreases. Fair enough. So at v = c observer A would appear to be a rather light 'particle-sized' energy with a large amount of energy/mass orbiting around him.

 

What are you trying to say?

 

Aren't these fluctuations, in quantum mechanics, so fast that no currently available clocks can time them. These tiny fluctuations change the (apparent) volume of the particle on a very finite level. The particles are still tiny. That does not appear to be a reason for what you are trying to explain.

 

Help?

 

Josephine