The speed of light

What do recent experimental results reveal?


I’ve been pondering this experiment ever since I saw the technical article [L.J. Wang, A. Kuzmich, and A. Dogariu, Gain-assisted superluminal light propagation, Nature, Vol. 406, pp. 277–279] on the Nature website on 19 July. The most puzzling thing to me is how the authors appear to deny the obvious implications of their data. They imply that their results do not suggest that information could be transmitted faster than the speed of light in vacuum, and yet the nearly-raw data in their figure 4 says just the opposite.

The figure shows two pulses, A and B, several microseconds wide and of nearly the same shape. Pulse B has traversed their cesium vapor cell, and pulse A has traversed the same distance (6 cm) in a vacuum, requiring the normal 0.2 nanoseconds to do so. Essentially every point of pulse B has arrived at the detector about 60 nanoseconds ‘earlier’ than the corresponding point of pulse A. The completeness of the advance of pulse B implies we could indeed use it to transmit information faster than the speed of light in a vacuum.

The astute reader may have noticed something even more weird about the above numbers: pulse B seems to have gotten way ahead of itself! A close-up view in the article (Figure 4, inset) shows that the leading edge of pulse B emerges from the cesium cell about 60 nanoseconds before the leading edge of the pulse allegedly causing it enters the cell. The newspapers actually got that point right. This raises the possibility of transmitting information ‘backwards’ in time. That would be astonishing! It would bring into question the standard interpretation of scientific causality; a child might send a warning decades into the past, telling his father not to beget any children. What happens if the father heeds the warning?

Despite the astonishing implications, the authors write: ‘The observed superluminal light pulse propagation is not at odds with causality or special relativity …’ because, they say, the equation on which they based the subsequent analysis ‘itself is based on the causality requirements of electromagnetic responses.’ But if the data itself says clearly that pulse B arrived before pulse A, then the theoretical basis of subsequent analysis is irrelevant. I wonder if the authors put in these mollifying words to help the paper get past the reviewers. But the discordance between the authors’ data and their mollifying words has produced confusion in the media. One critic of evolution and the media wryly commented to me that the news reports combine two discordant themes: ‘This is new and unexpected,’ and ‘This is nothing new and there is nothing to worry about.’

One experimental stumbling-point is that there might have been an undetected low-level precursor pulse which entered the cell before the main pulse, and that the emerging pulse was caused by the precursor. That possibility must be examined carefully, but I would have thought the Nature reviewers already investigated that angle. But a problem with the ‘precursor’ view is how it (or a tiny piece of the leading edge) could tell the cell how to reconstruct an almost-identical copy of the main pulse.

I’ve asked some of my physicist colleagues here at Sandia National Laboratories, ones who are experimentalists and laser-optics experts, to comment on the paper. Their first response was incredulity, but it’s hard to imagine that the reviewers for Nature wouldn’t be just as expert and just as incredulous. The paper makes an extraordinary claim which requires extraordinary proof. We only have the first stage of such proof so far.

Another odd thing is how fast the paper went through peer review. The facts of the article are so counter-establishment that I would have thought the pro-establishment Nature would have taken a year or so to approve it. For example, one paper supporting very rapid reversals of the earth’s magnetic field took several years to appear in the journal. It amazes me that it only took a little over a month for this one.

So either the experiment marks a new epoch in physics (communication faster than light and backwards in time), or it is a colossal blunder by the experimenters, their reviewers, and Nature. It’s pretty hard to make a 60-nanosecond mistake in today’s laboratories. Future experiments by other researchers may resolve the possibility of a mistake.

Let’s look at the other possibility, that Wang, et al., are right. What would happen to physics, then?

First, let me dispose of a wrong impression: relativity by itself does not exclude the possibility of things traveling faster than light. Just look up ‘tachyons’ in the physics literature of a decade ago to convince yourself of that. Tachyons were hypothetical particles which obeyed relativity by ‘always’ traveling faster than light, and never slowing down to that speed.

The idea of things traveling backwards in time is not forbidden, either. Recently I was looking at an old paper by famous physicist Richard Feynmann in which he had proposed that antiparticles (positrons, antiprotons) are just ordinary particles (electrons, protons) traveling backwards in time. Everyone took him seriously; they even gave him a Nobel prize for the theory. Today, fifty years later, it is still a possibility.

The real problem is causality. What happens if the kid’s father decides not to beget the kid? It seems to me that we could wind up seeking a broader interpretation of causality. Science-fiction authors have speculated about this for years. For example, two short stories by Robert Heinlein, ‘By his Bootstraps’ and ‘Elsewhen,’ present two very different possibilities about the nature of time, while still allowing time-travel and resolving the causality problem.

More significantly, for millennia the Bible has been transmitting detailed information to us about the future. I haven’t noticed the world collapsing into non-causal chaos quite yet!

So here’s an opportunity for physicists, philosophers, and theologians to shamelessly speculate about a possibly real phenomenon. But don’t go investing in time machines just yet. I think the odds are high that a mundane, dull, non-exciting explanation will emerge. But there is always the possibility that God has slipped another surprise for us into the world He created. That’s why physics is fun for me.

Editor’s comment: Whatever the outcome, it is a handy reminder that we don’t know as much as we think we know about such things as how light really behaves or propagates in deep space, the nature of elementary particles, and so on, which is all very relevant to issues, such as starlight travel-time, radiometric ‘dating’ processes, etc. The dogmatic certainty with which many researchers and popularizers write about such things as this, and about an alleged ‘big bang’, for instance, is not warranted by the data. There are a number of physicists at the ‘cutting edge’ of such things who prefer a more humble approach. See, for instance, the Creation magazine interview with Professor Keith Wanser.

Published: 28 July 2000 (GMT+10)
Published: 14 February 2006