Gamma-Ray Photon Observations Indicate Space-Time Is Smooth

eldavojohn writes "Seven billion light years away (seven billion years ago), a gamma-ray burst occurred. The observation of four Fermi-detected gamma-ray bursts (GRBs) has led physicists to speculate that space-time is indeed smooth (abstract and a pre-publication PDF both available). A trio of photons were observed to arrive very close together, and the observers believe that these are from the same burst, which means there was nothing diffracting their paths from the gamma-ray burst to Earth. This observation doesn't prove that space-time is infinitesimally smooth like Einstein predicted, but does indicate it's smooth for a range of parameters. Before we can totally discount the theory that space-time is comprised of Planck-scale pixels, we must now establish that the proposed pixels don't disrupt the photons in ways independent of their wavelengths. For example, this observation did not disprove the possibility that the pixels exert a subtler 'quadratic' influence over the photons, nor could it determine the presence of birefringence — an effect that depends on the polarization of the light particles."

Comments

I guess we're not a huge-scale game of Minecraft

[InvisibleClergy]

At least we know for sure that we don't need to deal with Creepers.

Re:I guess we're not a huge-scale game of Minecraf

[Anonymous Coward]

I don't know....here in the U.S. they seem to be campaigning every day...

Re:Size matters...

[maxwell demon]

Probably they refer to the electron's Compton length, which in some sense can be viewed as effective size of the electron. If you try to resolve the electron beyond that size, you inevitably get particle creation.

However if I'm not mistaken, a billionth of a billionth of the electron's Compton wavelength is still about five orders of magnitude larger than the Planck length.

Sensational

[tanujt]

I am not qualified to comment on the accuracy of the findings and their subsequent interpretation of the data. However, as the senior scientist Giovanni Amelino-Camelia suggested, "But the claim that their analysis is proving that space-time is 'smooth with Planck-scale accuracy' is rather naive." (He was the first one to theoretically suggest methods with which one could test for the "discreteness" of space-time)

Is it the artifact of the social media/e-news and the ever growing need for public attention to science (which translates into the elusive funding dollars), that lately a lot of discoveries are being touted as "physics defying", "life altering" etc before they are scrutinized thoroughly? We've already had a faster-than-light and a second-law-of-thermodynamics-broken debacle, and who knows how many more (scour the arXivs and you shall find!). A lot of the stories of scientific discoveries diffuse out of public interest fast, especially now that people are cynical about groundbreaking claims. I wonder if we need to make a conscious effort to not make a big deal out of every discovery, at least not before the data is converted to valuable information. Although, I see the catch-22 here, as the scientific community is trying to break the stereotype of "hard, cold truths presented in a bleak technical manner" or "how does that even remotely affect me", to appease their indirect, impatient employers: the public.

Re:Size matters...

[Antipater]

I thought electrons and all truly elementary particles had no size whatsoever, they were ideal points

Don't worry, there's always more to learn. Before I came to Slashdot, I thought there were gnomes in my computer, riding gnus and drinking wine.

A blow against Quantum Gravity?

[mbone]

If you ask, at what scale do virtual particles (the stuff continually popping in and out of existence) get so massive that they have gravitational effects (i.e., form little mini black holes), you get the Planck mass, and the Planck length and time come from that. It is, however, very hard to see how you can reconcile these experimental results with the notion that mini-black holes really are popping in and out of existence at the Planck scale. That may mean no space-time foam (what is supposed to result from this violent behavior at the Planck scale).

This is not a problem for General Relativity, but it is a problem IMHO for quantum gravity. The old question, at the Planck scale does General Relativity become more like quantum mechanics, or does quantum mechanics become more like General Relativity, may get an answer that the quantum mechanicians do not like.

Re:A blow against Quantum Gravity?

[maxwell demon]

I'm no expert in quantum gravity, but I have sometimes the impression that the pictures of spacetime quantization are often a bit naive; basically the pictures of quantum spacetime look to me more like a classical discrete spacetime. I can't of course exclude the possibility that it's just the presentation.

Think for example of the quantization of the electron spin: It has only two states, up and down. Does that mean that the electron has a certain preferred direction, because, after all, it can only be up and down? Definitely not! You can choose an arbitrary direction, and for each direction you'll find that it is either up or down, and nothing else. But that isn't a contradiction, because the electron isn't just a classical particle whose spin points in a certain direction, and when you measure it, you find out which spin it had. Instead, it's the measurement itself which determines the direction in which you get up or down, and it is the measurement which forces the electron into one of the states. Before it might have been in a superposition. And if you choose another direction, you'll find that the very same state corresponds to another superposition of the up and down states corresponding to that direction. Indeed, for the electron all directions are equal (the current state may be associated with a specific direction, but every direction has an associated state, making no direction fundamentally different than the others).

Now when we come to the Planck length, I can imagine that the very same happens: The spacetime itself is not discrete, just as the directions of the electron spin are not discrete. But if we try to measure it, we can only get discrete values. But those discrete values are not a property of the spacetime itself, because we can make another measurement, and then maybe our discrete values are half a Planck length shifted, just as we can make a measurement of the electron's spin in z direction, and then in x direction, and we will find that the electron's spin after the second measurement is rotated by a right angle, despite the fact that for each measurement individually the only possible values are in opposite directions.

Space/time duration/distance

[wonkey_monkey]

Seven billion light years away (seven billion years ago)

I may not have this right, but due to the expansion of space, wouldn't it have been closer than seven billion light years away at the time of the kaboom? And if the light's taken seven billion light years to get here, space will have expanded further, so the remnants would now be further than seven billion light years away. Right?

Or is this the sort of thing where you can be specific about the distance, or the time, but not both?

Re:Space/time duration/distance

[mbone]

There are multiple distance measures in cosmology - they are all in principle exact (at least, if you know all your cosmological parameters), but they differ significantly once you start getting above about 1 billion light years. Much above that, and they can differ incredibly much. Some of these measures are based on idealized measurements, others on the physics directly.

Some measures used in cosmological work are,

- proper motion distance (the distance a parallax measurement would give you)
- luminosity distance (the distance you would infer from the apparent brightness of a standard candle)
- angular diameter distance (the distance you would infer from the apparent angular size of a standard sized object). The angular diameter distance is notorious for getting smaller if you get far enough away in many cosmologies (including, apparently, the one we live in).
- look back distance (if you imagine that everyone has a clock synchronized at the big band, the difference between your time and the time you would read on the remote clock, if you could read it). This is also called the light travel time.
- proper distance (what some long yardstick would read).
- comoving distance (the proper distance divided by the scale factor - 1 plus the redshift, z - for the remote observer, to get a distance that doesn't change with cosmological time).

And, finally, each cosmological model will have a coordinate distance (the difference between the coordinates of two different places), which need not have a simple relation to any of the above.

It is fair to say that one of the easiest ways to make a fool of yourself in cosmology is to mix up distance scales. (As an additional cause of mixups, only proper distances can be subtracted - for the rest, the distance between A and B is NOT the difference of the distance to A and the distance to B, even if A and B are on a straight line as seen from the Earth.)

In this case, the Gamma Ray Burst 090510A was at a red shift of 0.897. Go to the Cosmology Calculator and you find that that

For Ho = 71, OmegaM = 0.270, Omegavac = 0.730, z = 0.897

It is now 13.666 Gyr since the Big Bang.
The age at redshift z was 6.376 Gyr.
The light travel time was 7.290 Gyr.
The comoving radial distance, which goes into Hubble's law, is 3053.8 Mpc or 9.960 Gly.
The angular size distance DA is 1609.8 Mpc or 5.2505 Gly.
The luminosity distance DL is 5793.1 Mpc or 18.895 Gly.

The proper distance is (1+z) times the comoving distance, or 18.89 Gly.