Much of the promise of blazingly fast 5G performance comes from using shorter frequencies to get data rates.

Physics is a cruel mistress, and so millimeter wave 5G will probably never move beyond densely populated urban areas, because the range, and penetration, are inadequate:

5G mobile networks have started arriving but only in very limited areas and amidst misleading claims by wireless carriers.

While all four major nationwide carriers in the United States have overhyped 5G to varying degrees, T-Mobile today made a notable admission about 5G’s key limitation. T-Mobile Chief Technology Officer Neville Ray wrote in a blog post that millimeter-wave spectrum used for 5G “will never materially scale beyond small pockets of 5G hotspots in dense urban environments.” That would seem to rule out the possibility of 5G’s fastest speeds reaching rural areas or perhaps even suburbs.

Ray made his point with this GIF, apparently showing that millimeter-wave frequencies are immediately blocked by a door closing halfway while the lower 600MHz signal is unaffected:

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With 4G, carriers prioritized so-called “beachfront spectrum” below 1GHz in order to cover the entire US, both rural areas and cities.

5G networks will use both low and high frequencies, but they’re supposed to offer their highest speeds on millimeter waves. Millimeter-wave spectrum is usually defined to include frequencies between 30GHz and 300GHz. But in the context of 5G, carriers and regulators have generally targeted frequencies between 24GHz and 90GHz. T-Mobile’s high-frequency spectrum includes licenses in the 28GHz and 39GHz bands.

Millimeter waves generally haven’t been used in cellular networks because they don’t travel far and are easily blocked by walls and other obstacles. This has led us to wonder how extensive higher-speed 5G deployments will be outside major cities, and now T-Mobile’s top technology official is saying explicitly that millimeter-wave 5G deployments will just be for “small pockets” of highly populated areas.

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“Some of this is physics—millimeter wave (mmWave) spectrum has great potential in terms of speed and capacity, but it doesn’t travel far from the cell site and doesn’t penetrate materials at all,” Ray continued. “It will never materially scale beyond small pockets of 5G hotspots in dense urban environments.”

I would also note that if a company is designing a 5G phone, the performance of its wireless modem chip in the millimeter wave band will not be a priority, because the end user will rarely, if ever, encounter the service.

So, 5G will likely be a bit faster, with lower latency, but not the game changer that the phone companies have promised us.