Posted on 02/24/2003 6:22:29 PM PST by NukeMan
Breaking Down Time Struggling scientists try to apply quantum theory to time, but time refuses to be quantized. by Pamela L. Gay
Quantum mechanics was born out of a mathematical leap of faith that wasn't initially thought to describe reality. Now, this still philosophically (and mathematically) difficult theory has grown to encompass most of physics. It successfully defines how large-scale physics breaks down at small scales for energy, momentum, position, and possibly time. It also specifies minimum measurable amounts (quanta) for these phenomena. However, observations by the Hubble Space Telescope have shaken the idea that time is quantized.
The quest of the last century has been to find a theory unifying general relativity with quantum mechanics. The Holy Grail of that quest is a theory of quantum gravity. While no complete theory has been established, scientists had some ideas of what it should include — a particle that carries the force of gravity and a quantum of time, for example. The gravity-carrying particle, dubbed a graviton by theorists, has too high an energy to be detected in current experiments. This leaves quantized time as the only testable aspect of quantum gravity.
If time is quantized like the energy of photons, there should be a certain minimum measurable quanta of time, called a "Planck time" (named after German physicist Max Planck, who originated the idea of quanta). This also means that life, like a movie, may appear to be a continuum of unfolding events but is really nothing more then a series of snapshots that together define the past and future.
One consequence of quantized time is that the speed of light should not be measurable with a precision better than one unit of Planck time, because time becomes undefined or "blurry" at smaller intervals. If light is quantized, it might have minute speed differences that we cannot measure.
If photons emitted from a source at the same instant have tiny speed differences, the light waves will arrive at Earth with different phases. Over short distances, these discrepancies aren't noticeable, just as differences between two cars going 90 and 92 kilometers per hour aren't evident after they've traveled just 10 meters. Over large distances, though, differences in the light become apparent, just as the velocity differences between our cars are obvious after they've traveled 1,000 kilometers.
Astronomers Richard Lieu and Lloyd Hillman at the University of Alabama in Huntsville used this idea to look for speed distribution in light from galaxies billions of light-years away that were imaged by the Hubble Space Telescope. If all photons are traveling from the galaxy at the exact same speed then all the light waves will arrive in phase. When they pass through a telescope's aperture, they interact with each other to create a pattern of bright and dark rings called an Airy disk. However, if the speeds are even the tiniest bit off, the light is not in phase. These light waves align chaotically, and no pattern is formed. Lieu and Hillman suspected that slight differences in the speed of individual photons would cause their light waves to get out of sync over great distances, preventing the Airy disk from forming. Much to their surprise, the light was all in phase and an Airy disk appeared, implying that time is not quantized.
The conclusion that time is not quantized poses large problems for scientists. According to Lieu, "The Big Bang theory supposes that at the instant of creation, the quantum singularity that became the universe would need to have infinite density and temperature. To avoid that sticky problem, theorists invoked the Planck time. They said if the instant of creation was also a quantum event, when space and time were both blurry, then you don't need infinite density and temperature at the start of the Big Bang."
Without quantum time, the universe becomes mathematically uglier at the moment of its inception. Like so many maps that were supposed to lead to the Holy Grail, our notions of quantized time have failed to lead us to a unified quantum theory.
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Learn to catnap between the quanta. That way you won't miss anything.
If the optics are good enough and the magnification is high enough, the Airy disk might be visible. It forms a limit to the resolution of the telescope.
Most telescope don't resolve to the Airy disk limit. If the object is a galaxy, it automatically isn't a point source, and if it is far away the light might have to pass through gravitational fields that cause gravitational lensing. All this will ruin the chance of seeing the Airy disk. However, since the telescope is apparently resolving to the Airy disk limit, the distant galactic light source is acting sufficiently like a point source.
In any case, we are dealing with the useful fiction [model] of photons, which might be wavelike or particlelike depending how they are being perceived. We'll need something more sensitive to detect quantized time.
If time is quantized like the energy of photons, there should be a certain minimum measurable quanta of time, called a "Planck time"
Why does the quantization of gravity imply the quantization of time? I've never heard it stated that way before. My understanding is that the sense in which the Planck time can be said to be a "quantum" of time is different from the sense in which a photon can be said to be a quantum of electromagnetism.
If light is quantized, it might have minute speed differences that we cannot measure.
Light is certainly quantized. But what does "it might have minute speed differences" mean? Differences from what?
If photons emitted from a source at the same instant have tiny speed differences, the light waves will arrive at Earth with different phases.
If the photons have different velocities, they should have different frequencies, but in that case, they can't remain in phase at all. But assuming that two photons from a distant galaxy have exactly the same frequency, why should they be in phase? They weren't emitted in phase.
Without quantum time, the universe becomes mathematically uglier at the moment of its inception.
Without knowing what that mathematical description is, who can say whether it's ugly? The universe is the way it is, and not how we might wish it to be.
An extremely poor description of Planck's results. It would be better to say that QM was born out of an interpolation formula. There was no "leap of faith" only a (much) better explanation.
Planck invented a forumla that interpolates between low and high frequency black-box radiation behavior. The results of the formula were so good, that Planck asked the question, what physical mechanism could describe his formula. The rest is History.
Otherwise the article seems a reasonable description of things.
I agree with Physicist about his misgivings concerning "Planck time". I understand that the Planck distance is probably the minimum distance a photon can travel such that the distance has any meaning. But this isn't the same -- at least to me -- as saying that time is quantized.
Here's another article on this topic, in Nature.
Would this not require finding a galaxy which is exactly normal to the line of sight?...
A 'tilted' galaxy may be 100000 light-years wide; hence the light from the far edge is under no constraint to be 'in phase' with the light from the near edge.
I've always been a little confused as to why "nearby" galaxies such as Andromeda do not look "smeared", since the light from more distant parts does not represent a true 'snapshot' of how the galaxy looked when the light from the nearer parts was emitted. Probably a stupid question, but I ask a lot of stupid questions.
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