mistermack

Ok, try looking at it from the reverse angle. Imagine that alongside the matter and energy that we are familiar with, there was today discovered a previously unknown form of matter and energy. It behaves in just the same way as all of the stuff we know at present. The only difference is that the speed of the "new" light is half of c. All of relativity applies to this new stuff in exactly the same way. The only difference is the c/2 figure. There's absolutely no reason to suppose that an equivalent of SR and GR would not apply equally to this new material. And the beings made of this stuff would be claiming exactly the same thing. That no information can travel faster than C/2. And they would argue that if any information COULD be passed at a speed of c, that would break the causation rule, and replies could be received from messages that were never sent. We happen to know that messages CAN be transmitted at c, so where did they go wrong?

It's from your own link. No, all I have so far is assertions.

I didn't start this thread as an argument. I asked for good clear reasons why causality is contravened, if faster than light information can be sent. I'd be perfectly happy for that to happen. I'm just responding to the offerings so far posted.

There's a telling paragraph at the end of the article that Swansont has added the link to, and it goes like this : " There is a “cure” for faster-than-light communication causing causality violations. There isn’t really a problem with signals going back in time, if they only go back in time somewhere else. For example,imagine there was a magic post office in the year 1500 that sent letters from Rome (Rome) to Tenochtitlan (Mexico City) and one week back in time. Since it took 5 weeks to cross the Atlantic, there’s no risk of paradoxes and causality violations (“Dear Ahuitzotl, in a week Giovanni Borgia is going to be killed. Nothing you can do about it, just thought you’d like to know.”). The real problems crop up when you can send instantaneous messages in two or more reference frames. That allows you to bounce signals back and forth, and thus send a message to yourself in the past. So, the fix is to have only one frame with instant communication (magic post offices only send letters in one direction). But this cure; picking a special reference frame (a special speed) in which communication can be instantaneous, isn’t really in keeping with the spirit of relativity or observational evidence; that all speeds are equivalent." Since relativity is BUILT on the assumption that no information can travel faster than light, it's hardly surprising that you can use the principles of relativity to argue for a false result. It's a circular argument. Relativity is right in all frames. Nothing can move faster than light in relativity. If something moves faster than light, you can use relativity to show a break in causality. In fact, if something COULD be transmitted faster than light, then relativity is clearly at fault on that subject. Definitely circular.

Well, that's just stating the obvious. The question was did Swansont's post establish the mechanics of how causality would be broken in his example.

Nope, that adds nothing either. You would need to be a lot more specific to establish anything. That's just hand waving.

That doesn't really add anything. I see how you press, and see the explosion. I see how you cancel, and don't see the explosion. What I don't see established, is the case where you cancel and still see the explosion.

That doesn't sound right at all. I don't see how you have established that you see the explosion in that case.

The experiment I would like to see done, is to measure time dilation at the Earth's surface, for a stationary clock, and an identical clock falling downwards at the Earth's escape velocity. If the "river" interpretation of GR was correct, you would expect the second clock to tick faster, when special relativity would have it ticking slower. Would that be practical, or is there a problem with the concept?

This is the sort of thing I've seen claimed, but I've never seen it properly illustrated or gone through step by step. The ones I've looked at have simply claimed that it can be shown that an observer in another frame can be shown to have OBSERVED events to happen in the wrong order, breaking causality. Which is worth looking at, but it's not the same as breaking causality. I can observe causality being broken, if I press rewind on a video. But it never actually happened.

That sounds like you are transferring energy faster than light.

Thanks, but those don't really do it. I don't want the thread to get sidetracked discussing why matter or energy can't exceed c. I have no problem with that at all. I'm just interested in the claim that causality is violated, if information can be transferred faster than light.

Does anyone have a good illustration, explanation, or link, for how information travelling faster than light breaks causality? I've searched and made an effort to follow various online offerings in the past, and maybe it's me, but never found one that I can go all the way with. It may be beyond my physics grade, which would be a shame, so I thought I would ask is someone could provide or link something that you don't have to be immersed in physics to follow.

I knew you were going to say that.

I think that there is no such thing as good and bad. In the religious sense. I prefer "nice" and "nasty". While I objectively think that it's not intrinsically good or bad to act in a racist manner, and nothing any of us do will count for anything in a million years time, I'm still prepared to go along with my human instinct of approving of what's nice, and disapproving of what's nasty. We humans have both nice and nasty instincts living in us, side by side. Racism is nasty, in the modern world, with people being incredibly mobile. In the world in which we evolved, a world of territorial apes, who needed to defend their territory against their neighbours or starve, or be killed, it wasn't so much nasty as necessary. So we all have dna that gives us racist tendencies. The modern fashion is to work against that, with social education etc. to increase niceness for all, and I'm all for that. But I'm not in any way ashamed of any racist instincts that might crop up in my head, they come from my daddy's sperm and mother's egg. Having said that, I think that Einstein was remarkably non-racist for his time, and would have been in the one percent least racist easily. Being part of a minority obviously gave him a different outlook to the majority, but that didn't apply to every jew at the time. While maybe not matching today's standards of political correctness now and then, you have to realise that nobody did, in those days. Times have changed. He was still one of the least racist of his time.

Sorry, but that's not right at all. It would only be right, if the one point of view was incompatible with the other. If you postulate that the laws of physics apply equally in every frame, which of course I'm not disputing, then they would inevitably apply in a "real" frame in the same way as every other frame. What's the difference between saying that there is no preferred frame, and saying that there is no preferred frame available? My argument for years now has been that the speed of light RESTRICTS us from observing reality, and relativity is just the consolation prize that we can use to recreate a working model. It's not reality, but it's all we can achieve and we're lucky to have it. Imagine that somebody tomorrow discovered a previously undetected form of light, call it L2. Instead of travelling at c, L2 travelled almost infinitely fast. So as far as we could tell, even from billions of light years away, you were seeing the present. What would that do to relativity? What would you consider reality, what you were seeing with L2, or what you observed with ordinary light? Just because there is no way for US to observe simultaneous events, does that mean there ARE no simultaneous events? It's the unfortunate slow speed of light that's stopping us from observing reality. What we CAN see, is reality for us, it's all we've got, but it's not what's actually out there.

The only explanation I can think of for everything being zero inside the shell is along the lines of the river model, where gravity is modelled as a flow of inertial frames into the massive object. Outside of the sphere, there is a whole universe of space time, so that a continuous "river" can flow, constantly being replaced from infinity. Inside the sphere, there is no source of replacement frames, so there can be no equivalent "river" of frames into the material of the sphere. No flow means no gravity.

Yes, but I made it clear I was just referring to what was said in wikipedia. I'm not disputing what you are saying, but I don't think it's a reason not to go there mentally. Nobody knows what's around the corner. Here's another titbit this discussion has thrown up from the web, when I put "river model" into the search : http://www.dailygalaxy.com/my_weblog/2014/06/gravitys-river-new-black-hole-theory-suggests-gravity-is-a-fluid-.html

My own impression would be that inside the sphere, you are under the influence of two parts of the sphere. If you are not the perfect centre, then what's above you is less massive than what's below you. So you are effectively in two gravity wells, which work against each other. In that case, you could subtract one from the other, to work out the effective gravity well, and calculate the time dilation from that. At the perfect centre, both would be equal, and the effective gravity well at that point would be zero. So time dilation is zero. But as soon as you move in any direction, your time dilation starts to increase.

It's the wikipedia author who used the phrase "deeper reality". I don't think that Laughlin is talking about ordinary matter. He's talking about the nature of the vacuum, and the existence of an ether of some sort, that if it existed, must have relativistic symmetry.

OK, but I'm in good company. This again from Robert Laughlin : More recently (in 2005), Robert B. Laughlin (Physics Nobel Laureate, Stanford University), wrote about the nature of space: "It is ironic that Einstein's most creative work, the general theory of relativity, should boil down to conceptualizing space as a medium when his original premise [in special relativity] was that no such medium existed . . . The word 'ether' has extremely negative connotations in theoretical physics because of its past association with opposition to relativity. This is unfortunate because, stripped of these connotations, it rather nicely captures the way most physicists actually think about the vacuum. . . . Relativity actually says nothing about the existence or nonexistence of matter pervading the universe, only that any such matter must have relativistic symmetry." (i.e., as measured)."[35] Of course it's speculation. What's wrong with that? If nobody speculated, everything would stop dead. It's hopefully relevant to the topic of the thread.

Yes of course. Whether that's the permanent case or not remains to be seen.

I think that the point being made in the wikipedia article that I pasted was that relativity certainly works in all frames, but that doesn't eliminate the possibility of one frame representing a deeper reality. It's just that there is no way of identifying that frame. They are saying that while there is no ether that matter travels through, there is no reason to suppose that space time has no substance, and it's looking more and more likely that it has. It's just that it can't be observed or detected in any way. Unless you adhere to the river model which models gravity as a flow of space time. If space time does move about in gravitational fields, then there could be no universal preferred frame, but a free falling object in a gravitational field, that's moving at the escape velocity of that field (in the opposite direction of escape), is motionless in the local preferred frame. The closest you could get to nailing down a universal preferred frame would be to take the entire universe, and compare your motion to that. Maybe that's what the CMBR gives, something independent of local conditions.

If all inertial frames are valid, then the universe must surely be all of those things ?

When I mentioned reality, I was referring to this bit " That interpretation of relativity, which John A. Wheeler calls "ether theory B (length contraction plus time contraction)", did not gain as much traction as Einstein's, which simply disregarded any deeper reality behind the symmetrical measurements across inertial frames. There is no physical test which distinguishes one interpretation from the other.[34] The quote says it better than I could. The point is, if there was a "real" inertial frame, it would be indistinguishable from all others. But what about the total energy of the universe? Is it real or relative? If you take the total energy of the Earth, it depends on what inertial frame you choose. There is only one, which gives the absolute minimum value, and that's the one in which the Earth is stationary. If you scale that up to the Universe, surely there must be an equivalent frame, in which the total energy of the Universe is the minimum. That must be a frame in which the Universe can be considered at rest, as a whole. Any other frame is adding enormous kinetic energy to the universe.