Knocking on Heaven's Door - Lisa Randall [188]
[ FIGURE 80 ] Data from the PAMELA experiment, showing how badly experimental data (the crosses) agreed with theoretical predictions (the dotted curve).
Dark matter can also annihilate into protons and antiprotons. In fact, many models predict that this happens most frequently if dark matter particles do indeed find each other and annihilate. However, large numbers of antiprotons lurking in the galaxy due to known astronomical processes can mask the dark matter signal. Still, we might have a chance of seeing such dark matter through antideuterons, which are very weakly bound states of an antiproton and an antineutron, which might also be formed when dark matter annihilates. The Alpha Magnetic Spectrometer (AMS-02), now on the International Space Station, as well as dedicated satellite experiments, such as the General Antiparticle Spectrometer (GAPS), might ultimately find these antideuterons and thereby discover dark matter.
Finally, the uncharged particles called neutrinos that interact only via the weak force could be the key to the indirect detection of dark matter. Dark matter might get trapped in the center of the Sun or the Earth. The only signal that could get out in that case would be neutrinos, since unlike other particles, they won’t be stopped by their interactions as they escape. Detectors named AMANDA, IceCube, and ANTARES are looking for these high-energy neutrinos.
If any of the above signals is observed—or even if they are not—we will learn more about the nature of dark matter—its interactions and its mass. In the meantime, physicists have been thinking about what signal to expect according to predictions from various possible dark matter models. And of course we ask about what any existing measurements might imply. Dark matter is tricky, since it interacts so weakly. But the hope is that with the many different types of dark matter experiments currently in operation, dark matter detection may be within imminent reach, and along with results from the LHC and elsewhere will provide a better sense of what is out there in the universe and how it all fits together.
Part VI:
ROUNDUP
CHAPTER TWENTY-TWO
THINK GLOBALLY AND ACT LOCALLY
This book has presented glimpses of how the human mind can explore to the outer limits of the cosmos as well as into the internal structure of matter. In both pursuits, the late Harvard professor Sidney Coleman was considered one of the wisest physicists around. The story students told was that when Sidney applied for a postdoctoral fellowship after finishing graduate school, all except one of his letters of recommendation described him as the smartest physicist they had known—apart from Richard Feynman. The remaining letter was from Richard Feynman, who wrote that Sidney was the best physicist around—though he wasn’t counting himself.
At Sidney’s sixtieth birthday Festschrift celebration—a conference organized in his honor—many of the most notable physicists of his generation spoke. Howard Georgi, Sidney’s Harvard colleague for many years and a fine particle physicist himself, observed that what struck him in watching the succession of talks by these very successful theoretical physicists was how differently they all think.
He was right. Each speaker had a particular way of approaching science and had made significant contributions through his (indeed they were all male) distinctive skills. Some were visual, some were mathematically gifted, and some simply had a prodigious capacity to absorb and evaluate information. Both top-down and bottom-up styles were represented among those present, whose accomplishments ranged from understanding the strong nuclear force in the interior of matter to the mathematics that could be derived using string theory as a tool.
Pushkin was right when he wrote, “Inspiration is needed in geometry, just as much as in poetry.” Creativity is essential to particle physics, cosmology, and to mathematics, and to other fields of science, just as it is to its more widely acknowledged beneficiaries