## Gamma-Ray Photon Observations Indicate Space-Time Is Smooth 81 81

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."*
## Unit overlap (Score:4, Interesting)

Shouldn't all of the points of space have their own frames of references and not be synced anyway? The planck length units would overlap practically infinitely.

## A blow against Quantum Gravity? (Score:5, Interesting)

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? (Score:4, Interesting)

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

arbitrarydirection, and foreachdirection 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 toanothersuperposition of the up and down states corresponding tothatdirection. Indeed, for the electron all directions are equal (thecurrent statemay be associated with a specific direction, buteverydirection 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

notdiscrete, just as the directions of the electron spin are not discrete. But if we try tomeasureit, 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 (Score:5, Interesting)

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 (Score:3, Interesting)

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?

Wikipedia [wikipedia.org] has an answer, but I think the above is just meant to give the layman some rough understanding of what's going on.

Beware that it is extremely difficult to measure these kinds of distances exactly. The figure may be a few orders of magnitude wrong, so whether you take into account the expanding universe or not may not be that important...

Cosmologists measure everything in gigaparsec. 7b light years is only 0.3 GPc so it may not be that important.

## Re:Space/time duration/distance (Score:4, Interesting)