Starship Asterisk*

neufer wrote:A shadow can transmit information and hence canNOT travel faster than c.

(A shadow travels at exactly the speed of light by definition.)

Yes, depending on how you define "shadow". But I think that the question is not really about shadows, but about certain non-physical projective phenomena (I believe we discussed the intersection of a closing scissors on this forum a few years ago), so I didn't want to get bogged down in the details of shadows specifically. (FWIW, a shadow carries information from the thing projecting the shadow, but it doesn't carry information from one surface it falls upon to another surface it fall upon at a later time.)

Chris Peterson wrote:

outlaw wrote:
I also read that a non thing can travel faster than light such as a shadow?

Of course. Even physical things can travel faster than light, and do so commonly. Indeed, physical things can even travel faster than c under special cases where no information is transmitted at greater than c.

A shadow can transmit information and hence canNOT travel faster than c.

(A shadow travels at exactly the speed of light by definition.)

outlaw wrote:I know that when something travels faster than light it has to deal with time travel backwards in the past and causality,do these things such as shadows travel into the past?

To be clear, it isn't traveling faster than light that is an issue, it is traveling faster than c. The two are not the same. Nothing that travels faster than c does so in a way that results in the transmission of information at faster than c, and therefore there is no backward time travel or problem with causality.

I know that when something travels faster than light it has to deal with time travel backwards in the past and causality,do these things such as shadows travel into the past?

outlaw wrote:I also read that a non thing can travel faster than light such as a shadow?

Of course. Even physical things can travel faster than light, and do so commonly. Indeed, physical things can even travel faster than c under special cases where no information is transmitted at greater than c.

I also read that a non thing can travel faster than light such as a shadow?

neufer wrote:

outlaw wrote:I was reading that the casmir effect can be explained without virtual particles.

Virtual particles were not conceived of after the fact in order to explain an already observed Casimir effect.

[b]Casimir effect[/b] wrote:… Thus it can be interpreted without any reference to the zero-point energy (vacuum energy) or virtual particles of quantum fields.^•

• Casimir effect and the quantum vacuum - RL Jaffe
  • Physical Review D 72 020301 (July 2005) DOI: 10.1103/PhysRevD.72.021301
    arXiv.org > hep-th > arXiv:hep-th/0503158 > 21 Mar 2005

outlaw wrote:
I was reading that the casmir effect can be explained without virtual particles.

Virtual particles were not conceived of after the fact in order to explain an already observed Casimir effect.

Rather, the yet to be observed Casimir effect was predicted to exist (and even
to be of a very specific magnitude) as a necessary consequence of virtual particles.

Phlogiston, aether and the like were once interesting (perhaps even useful) concepts
that have since been clearly proven not to exist.

Virtual particles constitute a very interesting (and extremely useful) concept
and have yet to be proved not to exist.

As far as I can remember, Stephen Hawking's theory that black holes are going to evaporate sooner or later is built on the idea of virtual particles. The idea is that virtual particles always "pop out of the vaccuum" in pairs, one positively charged, one negatively charged. They instantly meet and annihilate each other, which is what happens when, say, an electron and a positron meet. But in the case of the virtual particles near a black hole, the idea is that one of the virtual particles is sucked into the black hole and annihilated before it can annihilate its "virtual particle brother". The surviving virtual particle therefore has no "partner" that is going to annihilate it, and since it doesn't fall into the black hole like it mate (which is purely due to chance), it is going to change from a virtual particle into a real particle. But this change from "virtual" to "real" takes energy, and according to Hawking, this energy can only be taken from the black hole.

Over time, more and more virtual particle pair are going to "pop up" near the black hole in such a position that one particle is going to fall into the black hole and be annihilated, and the other one will become real by "stealing energy" from the black hole. Eventually, the black hole is going to be sufficiently drained of energy by giving "life" to virtual particles that it is going to evaporate.

Ann

Info on virtual particles here. Hope this helps.

neufer wrote:

outlaw wrote:
Hello,Im doing research on virtual particles and was wondering if their are different views on virtual particles,

Are they real or exist or are they not real?

Quantum electrodynamics is our most accurate verified physical theory.

Quantum electrodynamics is based upon the assumption of the temporary existence of virtual particles.

What more can I say?

I was reading that the casmir effect can be explained without virtual particles.

outlaw wrote:
Hello,Im doing research on virtual particles and was wondering if their are different views on virtual particles,

Are they real or exist or are they not real?

Quantum electrodynamics is our most accurate verified physical theory.

Quantum electrodynamics is based upon the assumption of the temporary existence of virtual particles.

What more can I say?

Hello,Im doing research on virtual particles and was wondering if their are different views on virtual particles,Are they real or exist or are they not real?