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Hello! Recently I viewed an article on wired.com titled "A User's Guide to Time Travel," of course I can't say that it was very helpful in demonstrating practical time travel, (these kinds of articles never do that, lol) the ideas that it proposed were interesting (I thought the pictures were nice, too). Anyway, on the second page of the article the author talked about a "Gott Shell". Now from what I understand, it would work by producing a time dilation effect because of the incredible mass of the Shell, "propelling" the traveler into the future. In essence the greater the mass, which produces a greater escape velocity, creates a large time dilation effect. (relative to someone outside the sphere, of course) The author mentioned how it could only work in one direction - toward the future. But, after thinking about it, I wondered: What if you got so much mass together that the escape velocity of your Gott Shell exceeded the speed of light (sort of like a black hole, but thats beside the point)? Wouldnt you, due to the fact that youre in the center of the shell, ("experiencing" the time dilation effects) now "move" back in time? Keep in mind that this is purely a thought experiment, and would most likely never be possible in reality. Why or why not? Thanks for your help!!
Answer 1:

If you were to be in the center of a Gott Shell and kept increasing the mass, you would indeed create a black hole, as you mentioned. However, this would be a very important consequence. Being stuck inside a black hole, the time dilation effects you 'experience' would cause you to see the entire universe play out its history outside of the black hole. So you would still be 'traveling' into the future. However, you would be stuck inside a black hole at this point, and would be unable to ever re-enter the rest of the universe.

Once the universe has progressed to the end of time, there are many speculations that could be made. Perhaps you would enter another universe through a white hole, or you could get tossed out somewhere within our own universe.

Alternatively, you may end up experiencing another big bang at the instant another universe is created, an idea called conformal cyclic cosmology. However, it should be stressed that these are highly speculative theories, and even if they are true, it likely will not affect our lives in any practical way, other than giving us nice thought experiments to think about, as you mention.

Answer 2:

In general relativity, objects in a gravitational potential well experience time running slower than objects out of the well; this is actually important in GPS, which uses relativistic time dilation in order to calculate your position from signals from satellites (otherwise you would be off by over a mile). The deeper the potential well, the more slowed time is for the object inside of the well, and time literally does not pass at a singularity (i.e. the very center of a black hole), even though someone falling into such a singularity would reach it in a finite period of time. I do not know, however, what the dilation effect is at the black hole's event horizon, or whether it varies with the size of the black hole - but the infinity only occurs at the singularity itself.

Answer 3:



It takes two unique types of mirrors working together to see farther back in time and space than ever before, and NASA engineers have just received one of each type. Primary and Secondary Mirror Engineering Design Units (EDUs) have recently arrived at NASA's Goddard Space Flight Center in Greenbelt, MD. from Northrop Grumman Aerospace Systems in Redondo Beach, CA. and are undergoing examination and testing. When used on the James Webb Space Telescope these two types of mirrors will allow scientists to make the observations.

"The Primary Mirror EDU will be used next year to check out optical test equipment developed by Goddard and slated to be used to test the full Flight Primary mirror," said Lee Feinberg, the Optical Telescope Element Manager for the Webb telescope at NASA Goddard. "Following that, the primary and secondary EDUs will actually be assembled onto the Pathfinder telescope. The Pathfinder telescope includes two primary mirror segments (one being the Primary EDU) and the Secondary EDU and allows us to check out all of the assembly and test procedures (that occur both at Goddard and testing at Johnson Space Center, Houston, TX) well in advance of the flight telescope assembly and test."

The primary mirror is actually composed of 18 smaller hexagonal mirrors that are assembled together into what appears to be a giant hexagon that sits atop the Webb telescope's sunshield. Webb Telescope scientists and engineers determined that a primary mirror measuring 6.5 meters (21 feet 4 inches) across is what was needed to measure the light from distant galaxies. Each of these mirrors is constructed from beryllium, a light and strong metal. Each of the 18 mirror segments weighs approximately 20 kilograms (46 pounds).

Why are the mirrors hexagonal shaped? Because a hexagon allows a segmented mirror to fit together without gaps. When Webb's primary mirror is focused on a distant star, for example, that image will appear in all 18 mirror segments. To focus on the star and get one image, the mirror segments can then be tilted to align the 18 separate images into a single image.

Although there are 18 segments, there are three different optical prescriptions for the 18 segments: six segments of each prescription. The segment received is the first of the "A" prescription segments for which a total of seven will be made ? six flight and one spare. A prescription is similar to an eyeglass prescription and specifies a unique mirror curvature. Like eyeglasses, mirrors with the same prescription are interchangeable.

The primary mirror EDU that arrived at Goddard also is a flight spare. That means it can be used on the actual telescope. In fact, it could even be put on the telescope now, if needed.

The primary mirror segment already has been cleaned and coated. Ball Aerospace & Technologies cleaned the mirror segment and Quantum Coating in Moorestown, NJ, coated it. Ball Aerospace then took the mirror segment back, reassembled it with mounts and actuators and conducted final vibration testing.

Afterward, the mirror segment went back to the X-ray and Cryogenic Facility (XRCF) in Huntsville, AL, where Ball performed final cryogenic acceptance testing on the segment before it was delivered to NASA Goddard.

The secondary mirror on the Webb telescope will direct the light from the primary mirror to where it can be collected by the Webb's instruments. The secondary mirror is connected to "arms" that position it in front of the 18 primary mirror segments. It will focus all of the light from the 18 primary mirrors.

The secondary EDU at Goddard is not coated but can be, so it can be a flight spare once coated.

Eventually, the final flight mirrors will all come to NASA Goddard and be assembled on the telescope and the instrument module. Then, as a complete unit, it will undergo acoustic and vibration testing at Goddard.

The James Webb Space Telescope is the world?s next-generation space observatory and successor to the Hubble Space Telescope. The most powerful space telescope ever built, Webb will observe the most distant objects in the universe, provide images of the very first galaxies ever formed and see unexplored planets around distant stars. The Webb Telescope is a joint project of NASA, the European Space Agency and the Canadian Space Agency.

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