Why doesn’t RADAR work underwater?
LINK TO COLUMN: Did the U.S. Navy teleport ships in the Philadelphia Experiment? - The Straight Dope
If there are any impurities, such as salt, in water it becomes a conductor and will prevent AC signals like RADAR from penetrating far. As I recall the “skin depth” of sea water for radar frequencies is a few meters at most. An underwater detection system that’s limited to the depth of an olympic pool does not help much in the open ocean.
Some Googling suggests the relevant parameter is the “Relative dielectric permittivity” of the substance through which you wish to transmit electromagnetic radiation.
A vacuum has an RDP of 1; air is around 1.0003. Fresh water is ~80, salt water ~88 - so no big difference between these.
The original question said the Philadelphia Experiment was “one of the three ‘city projects’” (along with the Manhattan Project). Any idea what the third supposedly was?
There’s a previous thread on this subject, well one post by Bonzer but a good one, that pretty much sinks any notion that a third city project ever existed.
Has anybody ever correlated the popularity of a conspiracy theory with the movie based on it?
I’ve always wondered if the number of people who think we faked the moon landing spiked dramatically after the movie Capricorn One (about a conspiracy to fake the Mars landing) came out.
Similarly, I’d be willing to bet the number of people who believe in the Philadelphia Experiment conspiracy theory spiked after the movie came out in the 80s.
Does anybody know if any research has been done on this? Or if the data even exists?
(I’ve always been fascinated about plot points of films that manage to work their way into the collective unconscious… to the point where something like the “woken a sleeping giant” dialog from Tora, Tora, Tora becomes taught in history books.)
I’ll take a shot at it:
An electromagnetic field (such as radar, light, X-rays) consist of two oscillating fields (as you might guess, electric and magnetic) which are perpendicular to one another. In turn, this results in electromagnetic radiation (which, in turn, is perpendicular from the E and M fields. Maxwell’s equations more or less demonstrate that if you change a magnetic field over time, you will induce an electric field and vice versa. Oversimplifying, once you start that cycle, each component will continue to induce the other forever unless something absorbs it in one fell swoop (like an electron), or is somehow you interfere with one of the field components (in this case sea water). In the latter case, the electric field component is disrupted by the conductivity inherent in sea water, which in turn causes the phase relationship between the magnetic field and electric field to start diverging. This is bad news for the electromagnetic wave because at that point each field is now inducing the opposing field to ultimately destructively interfere with one another, which is ultimately converted to heat. Which turned out to be a distinct advantage to the makers of the Amana Radarange.
But this is just an educated guess–I took electromagnetic fields 30 years ago, and sadly only got a C in the course. Those who find my explanation worthy of the Pit–I say (ala Bush 43) bring it on!
There were, however, radio controlled torpedos. French inventor Gabet demonstrated his “Torpille Radio-Automatique” in 1909:
From 1915 through the 1930s, the United States Navy continued development of the technology with the Hammond Torpedo, which could be guided up to 6 miles by radio:
http://query.nytimes.com/mem/archive-free/pdf?res=FA0F14F7355B17738DDDA00A94D0405B868DF1D3
In the 1940s, screen starlet Hedy Lamarr and composer George Antheil developed the first practical implementation of frequency hopping to secure wireless communications and jam radio signals, including torpedos:
http://www.google.com/patents?vid=USPAT2292387&id=R4BYAAAAEBAJ&dq=patent+2292387
Lamarr’s and Antheil’s invention paved the way for cellular phones and WiFi networks.
Since the US Navy was developing radio guidance technology, presumably they would also research countermeasures.
More information here:
Different parts of the radio spectrum have different behaviors, and there are bands that can handle some amount of water. But, to be useful, RADAR must be up in the microwave area.
And as an add, the size and the shape of an antenna matter quite a for the wavelength of what you are trying to send, and a bit less for receiving. (Source: Antenna Resonance: Radio Aerial Bandwidth » Electronics Notes)
And the length should be around the multipl of the wavelength, and usually the size and shape influences the direction the “lobes” of radiation will head to. It does not work the way that you can just attach wires to any metallic object and yell “I ARE HAVING RADIO STATION, HUR HUR”
Water does not allow very ready transmission of electromagnetic waves, and how well it passes is very related to the wavelength. Extremely low wavelength waves with high precision (frequency is inversely related to wavelength, and directly related to the capability of making out details). Unfortunately, water is relatively opaque to low wavelength (Source: B.P. Fabricand, “A detailed investigation of the absorption by water of electromagnetic radiation”, 1957 for United States Office of Naval Research).
It was known already before this from experiments that radar and radio transmissions do not work very well (with exception of ultra long wavelengths, which have terrible data transfer rates (for radio) and terrible resolution (for finding even something as large as a ship). Magnetic field on the other hand can and does penetrate water moderately well.
But something that was becoming common during the second world war are magnetic influence mines (Source: Yates, Stirling Junior. Fighting submarine mines 1941, Popular Science - Google Books) which recognize the magnetic field. Already then there were experiments on placing coils in ships to demagnetize it’s hull, and make it harder to spot for a magnetic mines. Personally I find it more than likely that Philadelphia Experiment was just a trial for a degaussing system, to make ships invisible for magnetic mines of that time.
How degaussing systems work: 62B-303