A recent study of gamma-ray bursts (that originate from the
collapse of a massive star) finds that spacetime is smoother on
the quantum scale than expected (Image:
Shutterstock)
A recent study of gamma-ray bursts by Professor Robert
Nemiroff and his colleagues at Michigan Technological University
provides the first strong evidence concerning the small-scale
smoothness of spacetime. Oddly, this examination of the very
small is accomplished by measuring a handful of gamma-rays after
they traveled over ten billion light years.
Spacetime is that four-dimensional arena (three of length
(x,y,z) and one of time(t)) in which we play out our lives.
Relativity tells us that spacetime is curved, and that curvature
causes the force of gravity. A very thin rubber sheet is a
pretty good two-dimensional analog to spacetime – if the sheet
is flat, a ping-pong ball on the sheet will stay in place, while
if the sheet is curved, the same ball will move.
On the Earth's surface, spacetime is quite flat, with only a
small curvature associated with the Earth's gravity. Billion
light-year stretches of the Universe also appear to be rather
flat, with only small curvature appearing near galaxy clusters.
However, something strange happens when you try to find out what
spacetime looks like for very small lengths. That something
strange is called quantum foam.
Quantum foam is a conceptual description of how spacetime may
appear on the smallest length scales – far smaller than
elementary particles. From quantum mechanics comes the idea of
virtual particles, which are massive particles that can appear
for a time short enough that the
uncertainty principle prevents our seeing them. In
relativity, however, mass (including virtual mass) curves
spacetime. Accordingly, when a virtual particle appears, the
curvature of spacetime changes.
A highly speculative image suggesting how a universe
composed of quantum foam might appear at the Planck length
(Image:
Shutterstock)
More massive virtual particles are restricted to smaller
volumes of spacetime, as they appear for a time that becomes
smaller as the particles become more massive, and even virtual
particles can't travel faster than light. Eventually, as the
mass of a virtual particle increases, it is located in a region
small enough that it forms a tiny black hole. This occurs for a
mass of about ten micrograms.
At this point, spacetime is filled with fluctuating regions
of curvature that come and go very rapidly. On our scale, all
this activity averages out to appear like a slightly curved
sheet of paper. On the quantum scale (lengths of about one
divided by a thousand quadrillion quadrillion centimeters,
called the Planck length), spacetime (according to this picture)
takes on a constantly changing geometry that has been compared
to a foam. This is the quantum foam.
At present, our instruments can at best examine structures a
hundred thousand trillion times larger than the quantum scale –
a difference so vast that detecting the quantum foam has been
thought to be impossible, or at least impossibly difficult. But
what if the quantum foam has side effects that can be
detected?
The speed of light is held to be a single, constant value
regardless of the color of the light. (This isn't true in some
quantum gravity theories, but that would take us too far
afield.) But can distance between two points change depending on
the color of light used to measure that distance? Here's an
analogy. Let's say that you want to measure the radius of an old
vinyl record. One approach is to take the phonograph needle and
pull it across the surface of the record, keeping track of the
distance the needle tip travels. The accompanying "brrrrrrrip"
sound occurs because the needle tip is traveling up and down
over the grooves of the record as well as radially toward the
center of the record. The radius you measure will be the
distance of going up and down the grooves, in addition to the
horizontal radial distance.
Now perform the same measurement, but replace the needle with
a ping-pong ball. This time the ping-pong ball is barely
affected by the grooves because it is too blunt to get down into
them. As a result, the distance measured this time is almost
equal to the horizontal radius of the record – the "needle"
radius of the record is quite a bit longer than the "ping-pong
ball" radius.
The same idea should hold for measuring distance in
spacetime. With a constant speed of light, the distance between
two points can be found by measuring the time it takes light to
pass between the points. A photon with a very short wavelength
may notice the ripples in spacetime due to the quantum foam,
while a photon with a long wavelength may see a shorter
distance.
This is where the gamma-ray bursts come in. The photons on
which this study is based were detected by the Fermi Gamma-Ray
Space Telescope and selected for having arrived in a cluster of
photons having a wide range of energies, all of which were
emitted in a single gamma-ray burst.
One set of photons in particular, originating in a galaxy
almost seven billion light-years distant, differed in their time
of arrival by a mere 1.55 milliseconds. This limits the
difference in distance between the photons (which ranged in
energy from 1.58 to 24.7 GeV) to less than 500 kilometers, or
about one part in a billion trillion. A similar limit was found
in an earlier study of photons with energies between 30 and 200
keV.
As small as the difference in distance is, it can be compared
with very preliminary quantum gravity calculations to determine
on what size scale the fabric of space may be pixelated.
According to the Michigan Tech group, spacetime is free of major
irregularities on size scales below about 525 Planck lengths in
size. Spacetime appears unexpectedly smooth on the smallest
meaningful distances, taking after foam rubber rather than beer
foam.
Source:
Michigan Technological University (PDF)
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