July 2000 Sky from Keeble Observatory

Last month we took up the notion of time travel, observing that General and Special Relativity allow one to alter the rate at which time passes. The tenet that "moving clocks run slow" is well known to introductory physics students and space opera aficionados alike. It is also well tested by precise experiments. The general relativistic effect that clocks deep in a gravitational field run slower than those further from the center is also well tested. Indeed, the entire Global Positioning System, which allows you to know your precise location and velocity on Earth, would not work if both of these relativistic effects were not taken into account. But, so far we have only talked of changing the forward pace of time. Is it possible to travel backwards in time?

Consider a videotaped collision between two billiard balls. It matters not one bit whether the tape runs forward or backward through the VCR – the collision looks the same. Now, consider taping a break shot in a game of pool – it’s easy to tell which way time runs in this video, even though it’s made up of nothing more than a large number of collisions just like the one which looked ok running backwards. Apparently our sense of the "arrow of time" has to do with the behavior of large numbers of interactions, any one of which may be, in principle, reversible. This has to do with a principle known as causality: if one event triggers another, their order in time cannot be changed. In fact, even if the events are not causally connected, but happen to lie close enough together in space and time that they could be, their order cannot be changed. Physicists sometimes divide past and future, as measured from a particular point in space and time, as having both absolute past and future, and relative past and future. The absolute past includes all points of space-time (technically called events) which lie close enough to have sent a signal by light (or radio, or x-ray, or any other form of electromagnetic radiation). This part of space-time is also known as the backward light cone. Similarly, the absolute future for a point in space-time includes all events to which that point could send a light signal, hence the forward light cone.

One argument against time travel is known as the grandfather paradox, played out in the popular Back to the Future series of films with Michael J. Fox. Imagine that you were to travel back in time and kill your grandfather before he ever met your grandmother. (Let’s be less violent, and imagine that you simply arranged for them never to meet, which is what the film did.) You would never exist, which is absurd, since you would then be unable to travel back to influence the past. Stephen Hawking has suggested that, if time travel were possible, we should already be flooded with tourists from the future. Others have countered that the tourists all went back to the 60s and 70s, and nobody noticed.

Next month: maybe we can use black holes for time travel.

Lunar phases for July: New Moon, July 1^st, at 3:20 pm, and again on the 30^th at 10:25 pm; First Quarter, July 8^th, at 8:53 am; Full Moon, July 16^th, at 9:55 am; Last Quarter, July 24^th, at 7:02 am. All times are EDT.

There will be a total lunar eclipse on the 16^th – your only chance to really see it is to take a trip to Hawaii, since it will occur during broad daylight here! A partial solar eclipse can be seen from the northern and western reaches of North America on the 30^th – schedule your vacation to Greenland at the end of the month!

Planet watching will be limited to predawn hours again this month. Venus sets less than an hour after sunset, so it will be hard to catch in the early evening. Jupiter and Saturn are visible in the early morning before sunrise. Jupiter is the brightest "morning star" to the east, Saturn is above and to the right less than a hand’s width away. Mercury is very low on the horizon, and Mars is invisible in the Sun’s glare, reaching conjunction on the 1^st. The Delta Aquarid meteor shower peaks on the 28^th between midnight and predawn twilight. In the absence of a moon (approaching New) you have a fair chance to see some "shooting stars" if you can get away from city lights, and have the luck of a clear, haze-free sky.

At about 9:30 in mid-month, bright Arcturus is just to the southwest of zenith (which is the direction directly overhead). About half-way to the southwest horizon is Spica, in the constellation Virgo. Due south we see the brilliant red Antares, the heart of the scorpion in the constellation Scorpio. Vega, Deneb, and Altair mark a triangle to the east. To the west, Leo is setting, with its brightest star, Regulus, marking the heart of the Lion. A sweep overhead with binoculars will find a neat globular cluster in the constellation Hercules, just to the east of zenith. On a moonless night this will look like a faint dandelion head. Further east, near Vega you may also find the Ring Nebula, in Lyra. Both of these objects require a bit of patience, and nearly perfect (for central Virginia) viewing conditions.